Platicsc Final

Name three methods of reducing the amount of thermal degradation that might occur in a heat-sensitive plastic. How does each reduce thermal degradation?

(1)Reduce the amount of regrind (as a percentage) that is used during processing. This assures that the heat history of the total material will never become large, thus reducing the likelihood that degradation from accumulated thermal processing will occur. (2) Add thermal stabilizers—materials that preferentially accept the heat and, therefore, reduce the amount of heat encountered by the polymer. (3) Add processing aids—materials that facilitate processing, generally by allowing the processing to occur at lower temperatures. Hence, the total amount of thermal energy added to the system is reduced. (4) Use some mechanical energy to melt the plastic material so that all of the energy is not from thermal sources

Describe the bonding in a carbon-carbon double bond. Include in this description an explanation of the mechanism by which the various bonds are formed. What does the existence of a double bond tell about the other atoms bonded to the carbon atoms?

A carbon-carbon double bond assumes that each carbon is bonded to three atoms (the other carbon with the double bond and two additional atoms). All carbons must have four bonds. The approach of the three atoms creates a rearrangement of the carbon atomic orbitals to accommodate the three bonds. This means that the s atomic orbital and two of the p atomic orbitals will be oriented in space at 120o angles from each other in a flat trigonal arrangement. The atomic p orbitals on each of the carbon atoms are dumbbell-shaped and will not be forced to rearrange, but will overlap and create a second bond between the carbons. Each carbon will, therefore, have four bonds. Three of these bonds are sigma bonds. And one of these bonds is a pi bond. One of the sigma bonds and the pi bond are between the carbons.

What is a copolymer and how are copolymers formed?

A copolymer is formed when more monomers are mixed together than are required for normal homopolymer formation. For addition polymerization this means that two different monomer types are required. For condensation polymerization three different monomer types are required. When these monomers are mixed, the resultant polymer has some of each of the monomers within it. These monomers can be arranged in four different patterns. The properties of the copolymer are normally intermediate between the properties of the homopolymers that would have been formed from pure, single-type monomers.

1. Which polymerization method, addition or condensation, is expected to result in branched molecules? Why?

Addition polymerization will give branched polymers. This branching results from the loss of an atom, usually hydrogen, during the polymerization process which causes a free radical to form at the site where the hydrogen atom was previously located. This free radical is then able to bond with other free radicals, as on the end of a growing polymer chain, or with a monomer and begin a chain which grows as a branch off the main backbone. Condensation is less likely to branch because there is no mechanism that will create a reactive site in the middle of a chain.

1. Explain why the properties of a polymer below the glass transition temperature are different from the same properties in the same polymer above the glass transition temperature.

Below the glass transition temperature polymers are hard. This occurs because the polymer structure is relatively fixed in space. Energy input at temperatures below the glass transition temperature can generally only result in small vibrations and rotations which do not lend movement to the polymer structure. Above the glass transition temperature much more movement (vibration, rotation and translation) is present. This increased movement, especially the long-range concerted movements of segments of chains, imparts flexibility to the polymer structure. Hence, above the glass transition temperature, the polymer is pliable and flexible.

Describe the type of bonding between carbon and chlorine.

Carbon and chlorine are both nonmetals and will therefore share electrons to form covalent bonds. Carbon will always form four bonds by sharing its outer four electrons. Since each chlorine needs only one electron to complete its octet, four chlorines are required to give carbon its four bonds. The resulting material is CCl4.

Describe the differences between the carbon and oxygen atoms.

Carbon has six protons in the nucleus and six electrons. (the number of neutrons determines the isotope of carbon but is unimportant for the differences in behavior between carbon and oxygen.) Oxygen has eight protons and eight electrons

1. Describe crosslinking and the resultant properties that it will create.

Crosslinking is the process of connecting polymer chains with covalent bonds. When this occurs, the total length of the polymer chains (or molecular weight) increases tremendously which raises the melting point of the polymer, usually beyond the decomposition temperature. Therefore, polymers that have been crosslinked will decompose before they melt.

You and a friend have just come up with an idea for an attachment for computers that will enhance their performance significantly. You have decided that the part should mount on the inside of the computer box. What steps do you follow through the remainder of the development of your new product? What steps do you need to take and why?

Design, modeling, prototyping, material choice, processing selection, and production development should be some of the first engineering steps taken. The purpose of following a methodical stepwise plan is to eliminate errors and miscalculations that, while easily remedied in the design process, can be costly to fix once a part is in production.

1. Describe the key features of a polymer that would make it electrically conductive.

Electrical conductivity results from the ability of electrons to move freely within the material's structure. This can occur in polymers when the electrons are delocalized. One method to achieve delocalization is when the bonds in the polymer are conjugated double bonds. Another method to achieve conductivity is to add a conductive filler to the plastic material.

1. Explain the flammability of aromatic versus aliphatic molecules from an energy perspective assuming the bond strength of a C==C bond is approximately twice as much as the bond strength of a C—C bond. Also, that the bond strength of a hydrogen attached to a benzene is about 20% higher than the bond strength of hydrogen attached to propylene.

Flammability is a measure of a material's ability to perpetuate a flame. This is done as heat is released when bonds are formed. Typically the bonds that reform are those between hydrogens and oxygens. As the energy in bonds increases, it becomes more difficult to break the bonds (requires higher temperatures) and change it to heat energy. Therefore, the carbon double bonds in the benzene ring of the aromatics will increase the energy required for burn as well as the bond between the hydrogens and the benzene. In order to burn the aromatics, the applied heat to break bonds and begin (and perpetuate) the burn is significantly higher.

1. Explain the relationship between plastic crystallinity and sensitivity to solvent attack.

Highly crystalline polymers have strong secondary bonds that resist the formation of solvent-polymer bonds or, perhaps, require the formation of more or stronger solvent-polymer bonds to achieve solvent penetration into the polymer network. Hence, crystallinity reduces solvent sensitivity.

1. Discuss the implications in MWD when the number average molecular weight and the weight average molecular weight are widely different.

If all the polymers were the same length, the number and weight average molecular weights would be the same. Therefore, when the number and weight averages are vastly different, it implies that the molecular weights are also vastly different and that the MWD is wide.

1. Contrast and discuss the difference between amorphous and crystalline regions in a polymer.

In crystalline regions the polymer chains are tightly packed which results in a high density. The polymers in these crystalline regions have relatively strong intermolecular bonds which increase the stability of the polymer and increase most mechanical and physical properties. Amorphous regions are zones within the polymer structure where no crystallinity exists. Even highly crystalline polymers usually have some amorphous regions. The physical and mechanical properties for largely amorphous polymers are lower than analogous crystalline polymers.

1. Discuss the relationship between melt index and molecular weight.

In general, as molecular weight increases, melt index will decrease. This is not always true from one polymer type to another, but is generally true within a particular polymer type. This relationship exists because the melt index measures the ease with which a polymer is melted. Melting is generally easier (that is more polymer is melted at any temperature) as the molecular weight is reduced.

Contrast the interatomic or intermolecular forces present in solids, liquids and gases. Explain the consequences of these forces and how these forces are normally overcome.

In solids the inter-atomic and intermolecular forces are very strong. The particles (atoms or molecules) are relatively fixed in relation to all the surrounding particles. These materials resist movements of the particles. Hence, solids are rigid. In liquids the inter-particle forces are less strong than in solids, but there is still some association between the particles. The particles can move freely to take the shape of the container but will not fill the entire volume of the container. In gases the inter-particle forces are nearly non-existent. Gases can take both the shape and fill the volume of the container. The inter-particle forces are normally overcome by adding thermal energy to the material. This thermal energy causes the particles to vibrate, rotate, and translate, thus eventually breaking the inter-particle forces.

Why is it harder to make very long polymer chains using the condensation polymerization method than by using the addition polymerization method?

In the condensation reaction every monomer molecule can react with equal probability upon encountering any monomer of the opposite type. This situation favors the formation of many small chains which grow simultaneously. As the chains get larger, their ability to continue reacting is slightly less than small chains because the large chains have less mobility and, therefore, less chance to encounter a monomer with which it can react. Also, in condensation polymerization the byproduct of water is formed as the monomers react. The presence of water molecules in the reaction will hinder chain formation by diluting the reactants. In addition polymerization, encounters between monomers do not form bonds unless one of the monomers has been initiated, that is, reacted with an initiator so that the monomer contains a free radical. Therefore, in addition polymerization, since only a few of the monomers are initiated, the growth of the chains will be largely at the sites that are on the already formed polymers, thus making those polymer chains even longer.

1. Explain in thermodynamic and molecular structure terms why small molecules are more readily solvated than large molecules.

In thermodynamic terms, solvation will occur when the free energy for the solvation is negative, that is, the energy is more favorable for solvation than for staying in the solid state. Either the enthalpy term (DH), the entropy term (TDS), or both, must be negative and dominate whichever of the terms might be positive. The enthalpy term will only be negative when the solvation bonds are stronger than the polymer bonds. This rarely happens. Hence, the entropy term must be more negative than the enthalpy term is positive. This can happen at high temperatures because T becomes very large. It can also happen when the entropy, DS, is large. The entropy will be large when great randomness is associated with the system. Because small polymer molecules can become more random when solvated than can large molecules, small molecules are more favorable to a dominating entropy term and will therefore solvate more easily. (The increased randomness with small molecules arises from the inherent structure that is always associated with large molecules, just because they are large and structurally more defined.) The molecular structure view of solvation is simply that small polymer molecules require fewer solvent secondary bonds to equal the secondary bonds between the polymer molecules. Hence, solvation is easier than for large polymer molecules.

1. Discuss the three major methods of increasing the strength of a polymer. Explain why each method works.

Increase chain length (molecular weight), add pendant groups and crosslink. Increased chain length allows for greater secondary bonding, pendant groups increase entanglement and crosslinking increases force to failure because forces are now applied to primary bonds instead of secondary bonds.

What is an initiator and why is it important in a polymerization reaction?

Initiators are materials that start a chemical reaction. In the case of polymers, the reaction is usually either a polymerization or a crosslinking reaction. The most common initiator is a peroxide which breaks apart, usually when heated or through certain chemical reactions, to form free radicals (chemical groups containing an unshared, and therefore very reactive, electron).

Why is the flexural modulus so important for many plastic applications?

Many plastics are used in applications where the plastic is a cover, a seat, or a board-like structure. The most common type of force that these materials would encounter would be flexural. The modulus is often more important than the strength because these materials rarely fail (crack) but will become unsuitable for the application because they sag too much when the force is applied. A plastic seat is a good example. If the seat deflects too much, it is uncomfortable. The force on a seat is a flexural force.

1. What is the relationship between melt index and viscosity? Explain the relationship using microstructure concepts.

Melt index is a measure of a polymer's ability to flow through an orifice within a given time. Time through the orifice gives insight as to the chain length and structure. Viscosity also measures chain length and structure as an interaction between the measuring device and the viscous fluid.

1. How is modulus likely to be affected when toughness modifiers are added to a plastic? Why?

Modulus will likely be reduced because the toughness modifiers will have longer elongations than will the base polymer and will therefore decrease stiffness (modulus).

1. What is the likely effect on modulus of adding a filler? Why?

Modulus will likely increase by adding most common fillers. The reasons are that the filler is itself a stiff material. Secondly, the filler will likely fill into the spaces between the molecules and thus inhibit the molecules from moving over each other. This resistance to intermolecular motion is seen as an increase in modulus.

1. Discuss how you would expect the glass transition to be affected if a large, pendent group were added to the monomer unit.

Most large pendent groups will require more thermal energy to achieve long-range chain movements. Therefore, the glass transition temperature would be expected to increase.

Can all materials exist as solids, liquids, and gases at various temperatures? Explain.

Most simple, small atoms and molecules can exist in all three states of matter - solids, liquids, and gases. In some cases, especially when the molecules are large, the decomposition temperature of the material is lower than the melting point and therefore, increasing the temperature will cause decomposition before melting. Very large molecules will rarely exist as gases.

1. Achieving a high molecular weight was a major problem in the development of the nylon polymerization process. Why was this so? Suggest a method to overcome the problem.

Nylon is made through the condensation polymerization process. Water is a byproduct of the polymerization. After a certain point in the process, the reaction begins to slow because of the abundance of water. Continued reaction becomes increasingly more difficult and chain growth is halted. This gives shorter chain length and lower molecular weight. One method to overcome the overabundance of water in the reaction vessel is to apply a vacuum to the polymerization chamber. By removing the water from the reaction, the molecular weight can continue to grow.

17. Why does processing of a crystalline material like nylon require more critical thermal control as opposed to an amorphous material like ABS?

Nylon, like all crystalline materials, has a sharp melting point. Processing below this temperature is virtually impossible. Processing much above this temperature can make the melt excessively runny (low viscosity). Amorphous materials (like ABS) have a broad melting range and processing 50 to 75°F below Tmp is possible. Most grades of nylon, for example, do not process below 508°F, the point where it loses crystallinity and is capable of flow. ABS can be processed from 420 to 550°F.

What is the first modern plastic that was synthesized with a specific set of properties in mind, who sponsored the work, and when was it done?

One correct answer would be Celluloid, invented by James Wesley Hyatt in 1868. However, the first truly synthetic plastic that was made to a specification was nylon. The work was sponsored by the DuPont Company (Wallace Carothers was the chief scientist) and the work was done in the 1930s.

1. Give an example of why the functional specification must also include some idea of the production volume.

One of the design criteria that must be met is economics. In most molding processes production volume is critical for determining the cost. For instance, in the injection molding process, the size of the mold can mean a difference of tens of thousands of dollars in mold cost. These mold costs are part of the overhead which must be absorbed in the price of the part.

1. What is the purpose of notching Izod or Charpy samples?

One of the important aspects of impact toughness is the tendency of the material to fail through cracking. Cracks often begin at sharp locations as is created when a notch is installed in the part. Therefore, a key reason to notch the sample is to determine the tendency to fail by cracking. Another, perhaps equally important reason, is to provide a point at which failure is likely to occur so that the failure values from one test to another will be less dependent on the initiation of the failure and more dependent on the crack propagation through the material.

1. Explain why plasticizers increase flexibility and elongation in plastics. What is the relationship between plasticizers and the glass transition temperature?

Plasticizers are solvents that are added to swell and partially solvate the polymer. By swelling the polymer the plasticizer expands the polymer network and, therefore, allows the polymer atoms to move more freely. This greater mobility is seen as increased polymer flexibility and elongation. Raising the temperature above the glass transition temperature has the same general effect. Therefore, adding a plasticizer lowers the glass transition temperature.

Define the term polymer and relate it to the term plastics.

Polymer is a very long molecule made by joining many small molecules together. All plastics are polymers, but some polymers, for instance, natural polymers such as fur and leather, are not plastics.

1. You are molding a toy bridge (6 inches high with a 24 inch span) that is failing (bending) when kids step on the span. The part is currently made of a plastic with a melt index of 10 and a density of 0.94. The market is highly competitive, so ideal solutions should take cost into account. Indicate 4 possible methods of solving the problem, and then choose one of the methods and defend your choice.

Possible answers include thickening the bridge in key areas, finding a material with higher melt index or higher density, or even using mold-in supports. Answer should also include a valid discussion of the chosen solution.

Identify and describe the forms of plastic resins

Resins are usually liquids (like honey), granules (like laundry soap powder), flakes (like uncooked oatmeal), or pellets (like very short spaghetti).

1. Define simultaneous engineering, and point out a problem that might arise if it is not done.

Simultaneous engineering is the concurrent development of the part design and the process design. A problem that could arise if they are not done at the same time is that a part is designed that would be very difficult to make, where with just a minor recognition of the capabilities of a process, the design could be changed and the problem avoided.

1. Why must average molecular weights be used for polymers rather than exact molecular weights based upon the molecular formula as is done for small molecules?

Small molecules have exact formulas which always apply. In polymers, the chains can be of varying length and so a general formula must be used. This general formula represents the number of monomer units by an unknown (usually an n). Hence, if the exact number of units in each of the chains is not known, some statistical measure of the polymer chain length must be used. The average molecular weight is a convenient statistical measure to use.

1. A company has approached you with an idea for a basketball hoop support pole that they would like to make from plastic. What are the most important mechanical aspects of this new part that need to be considered? Why?

Some of the aspects that should be included would be the creep nature of plastics, the impact resistance of both the part and the polymer chosen, the flexural strength of the material and also the torsional strength of material. There may be other considerations that can be validated through appropriate reasoning.

1. Why are strength and hardness of a plastic usually linearly related?

Strength is a measure of the polymer chains' tendency or ability to limit inter-chain movement. Similarly hardness is defined by a polymer chain's ability to move among other chains. Both properties measure similar inter-chain interactions.

1. What is ASTM and why is such an organization important?

The American Society for Testing and Materials (ASTM) is an organization that publishes uniform testing procedures for common tests. It gives one reference for standards that all materials and tests can be compared.

1. What is the limiting oxygen index (LOI)? Which burns more easily, a material with an LOI of 21 or of 17?

The LOI is a test for flammability of a plastic sample. In this test the sample is supported in a candle-like arrangement and ignited. The composition of the gases surrounding the burning sample is controlled and gradually the amount of oxygen is reduced or increased until the sample will just barely burn. That level of oxygen is called the limiting oxygen index. The sample with an LOI of 17 is more easily burned.

1. Low-density polyethylene is normally translucent. If a piece of this material is bent into a tight radius, it translates less light (becomes chalky white). What is this phenomena called? Explain why this happens. Explain why the material often breaks after repeated bends.

The change in light transmission due to stress like bending is called blushing. Blushing forms where areas of crystallinity are created caused by these stresses. As a rule, crystalline areas transmit less light than an amorphous region. The break will come as the induced crystallinity increases and those areas become brittle. Failure will occur when that crystallinity becomes significant.

Identify the type of bond and the product formula expected between magnesium (Mg) and chlorine (Cl), and explain the basic nature of this bond. Show the resulting outer electron configurations of Mg and Cl after the bond has formed.

The compounds will form an ionic bond since one is a metal (loses electrons) and the other is a non-metal (gains electrons). The product formed will be MgCl2. After the bond is formed, Mg will have transferred its outer shell electrons to two Cl atoms. Two Cl atoms are required because Mg wants to lose two electrons to have the stable octet outer electron configuration. Each Cl will accept one electron and so two chlorines are needed for both electrons. The resulting Mg ion will have a charge of +2 and each of the Cl ions will have a charge of -1.

What are three major determining factors in the price of plastics?

The current price of raw materials, the cost efficiency of the processing method and the amount of plastic produced (price-volume relationship) all affect the price of plastics.

1. Indicate why design rules must be followed after a preliminary choice is made of both process and material.

The design rules are both process and material specific. For instance, the shrinkage that is to be allowed would vary depending on the material and might be different if the process chosen does not melt the resin, say with rotomolding instead of a melting process such as injection molding.

1. Identify three tests for thermal properties of plastic materials.

The heat distortion test (HDT), the coefficient of thermal expansion (CTE) and the thermal conductivity test.

Performance tests of valve lifters in an automobile engine showed distortion when the material used was 20% glass-reinforced nylon (6/6). Explain why a shift to a higher crystalline polymer was suggested.

The higher crystalline polymer would have more hydrogen bonding sites per unit of polymer length and would, therefore, have more inter-molecular interactions and a higher performance temperature.

1. A stress-strain experiment was done on a plastic and the following data was noted on the plot of the experiment: ultimate tensile strength = 9000 psi, yield strength = 5000 psi, proportional limit = 4000 psi, strain-to-failure = 0.025, strain at yield = 0.020, stain at proportional limit = 0.015. The original length of the sample was 4 inches. What was the modulus?

The modulus is calculated by examining only the slope of the stress-strain curve before yield, that is before and non-linear behavior is seen. Hence, the slope at the beginning of the experiment is stress at yield (5000 psi) divided by strain at proportional limit (.015).

Discuss how copolymerization would affect crystallinity.

The nature of copolymerization would cause more randomness to occur along the molecular chain than would the corresponding homopolymers. Therefore, crystallinity would be lower.

What is the difference between a number average and a weight average molecular weight?

The number average is dominated by the actual number of molecules and gives equal value to both large and small molecules. The weight average molecular weight gives greater value to the large, heavy molecules.

1. Explain the cause of the differences in structure of LDPE, HDPE, and LLDPE.

The structures are dependent on the amount of branching in the molecules. When the branches are long and frequent, the molecules cannot pack tightly together thus preventing crystallization. This is LDPE. When the branches are short and infrequent, the molecules can pack together and crystals can form. This is HDPE. When the structure is similar to HDPE but with slightly longer and more frequent branches, the polymers cannot pack together and the density is low like LDPE, but other properties are like HDPE, this arrangement gives LLDPE.

1. What is the molecular difference between thermoset and thermoplastic materials?

The thermoset materials are crosslinked and thermoplastics are not. This crosslinking raises the total length (molecular weight) of the thermoset and thereby prevents the material from being remeltable

1. Discuss the physical factors in a plastic that affect permeability.

The tightness or denseness of the polymer structure is one of the most important physical factors affecting permeability. Therefore, crystallinity reduces permeability. Likewise, crosslinking reduces permeability. Other factors affecting permeability would be the similarity of chemical natures between the polymer and the gas passing through. If these are similar, permeability is increased. Other factors affecting permeability include temperature (increases permeability) and size of the permeating molecules (decreases permeability).

What are thermal stresses and how are they caused in plastic materials? How can they be relieved?

Thermal stresses are energy concentrations that are retained in a material resulting from the inability of the material to change shape as would normally occur upon cooling or heating. These thermal stresses can be relieved by slowly heating the material to a moderate temperature (annealing), thus allowing the molecules to move and the stresses to relax.

1. Describe how ultraviolet light degrades plastics and why it does not generally degrade metals.

Ultraviolet (UV) light interacts with the electrons in covalent bonds, often raising the energy of the electrons so they are no longer in a favorable bonding energy level. When this happens, the bond is broken. This rarely occurs in metals because the energy of the sea of electrons can be excited with little effect on any individual bond between the atoms. Hence, the bonds remain intact even when some of the electrons are excited to higher energy levels.

1. Explain what is meant by a viscoelastic material, and relate its response to applied stresses by comparing the material with an elastic solid and a viscous fluid.

Viscoelastic materials have some properties that are similar to elastic solids and some properties that are similar to viscous fluids. When a stress is applied to a viscoelastic material that is a solid, the solid will deform much like an elastic solid but will not immediately recover its original shape when the stress is removed. The viscoelastic material has used up some of the applied stress through internal heating, hence, not all of the energy is available to assist in the shape recovery

1. Using a molecular view, explain why compression strength is generally less than tensile strength in polymers.

When a compressive load is applied to a polymer, the molecules are forced closer together. If the polymer is highly crystalline, this force will simply press on the crystals with little damage, much as would happen in a crystalline solid. However, in an amorphous material, such as most plastics, there is considerable space between the polymer chains, which will allow deformation of the polymers at lower stress levels. Furthermore, the spaces allow the polymers to collapse into the holes, thus reducing the ultimate compressive strength. Hence, the compressive forces are lower for polymers than for highly crystalline materials.

1. Two samples of the same resin are pulled in tension at two different rates. One is pulled at 1 inch per minute and the other is pulled at 10 inches per minute. Discuss the differences in elongation you would expect, and explain why the differences occur.

You would expect to see a higher elongation in the pieces that were pulled at a slower rate. The slower rate allows the polymer chains time necessary for disentanglement. The forces seen in this case are those of the secondary bonding. At a faster rate, the forces that you see are applied to the backbone of the molecule itself and are generally higher.

1. When naming organic compounds, the prefix but- (or buta-) is used to indicate which quantity? a. Four b. Five c. Six Seven

a. four

1. A polymer that is said to have toughness is able to a. Withstand slow moving forces for extended periods of time at elevated temperatures b. Absorb relatively large amounts of energy without rupture c. Leap tall buildings with a single bound

b. absorb relatively large amounts of energy without rupture

10. When surface cracks develop due to exposure to environmental chemical embrittlement this is generally called a. Blushing b. Crazing c. Oxidization d. Charring

b. crazing

17. Which type of material does not generally have a detectable Glass Transition Temperature? a. Amorphous Thermoplastics b. Crystalline Thermoplastics c. Thermosets

b. crystalline thermoplastics

1. When a fluid is said to have a high viscosity that means that it a. Flows quickly, like water b. Flows slowly, like honey c. Is slightly yellow or tan, like motor oil d. Absorbs moisture from the environment

b. flows slowly, like honey

17. For any sample of plastic, it is generally too costly to count the number of repeating units on each of the polymer chains. Instead, for processing reasons, which value is given to designate polymer chain length? a. Sample Density b. Melt Index c. Molecular Weight Distribution d. Parts Per Pound

b. melt index

1. Polyvinylchloride (PVC) is used in industrial applications as a rigid material but is also used in many consumer applications as a soft pliable material called vinyl. The main difference between these two applications is based around the use of a. Fillers b. Plasticizers c. Colorants d. Modified polymerization methods

b. plasticizers

Wallace Carothers was commissioned by DuPont in the early 20th century to develop a new synthetic giant molecule that could be used as a replacement for natural materials such as silk. Carothers' research yielded which synthetic material? a. Polyethylene (PE) b. Polyamide (Nylon) c. Polytetrafluoroethylene (PTFE) d. Polypropylene (PP)

b. polyamide (Nylon)

Every covalent bond is made up of how many electrons? a. One b. Two c. Three d. Eight

b. two

1. Which of the following means basic repeating unit in organic chemistry nomenclature? a. Mono- b. Poly- c. -mer d. -ene

c. -mer

. The name of the testing standard group known as ASTM is a. American Standard Testing Methods b. Association of Standards Test and Measures c. American Society for Testing and Materials d.Association of Systematic Tests and measurements

c. American Society for Testing and Materials

The following are all methods of rapid prototyping in design except: a. Stereolithography b. Ballistic Particle Manufacturing c. Computer Aided Design d. Laminated Object Manufacturing

c. Computer Aided Design

Which of the following would be the least reliable source for data needed for selecting materials: a. Databases b. Suppliers c. Internet search engines d. Manufacturers

c. Internet search engines

1. Step-growth polymerization is also known as which? a. Addition polymerization b. Subtraction polymerization c. Condensation polymerization d. Evaporation polymerization

c. condensation polymerization

Approximately what percent of U.S. petroleum consumption is used directly for the production of plastics? a. 17% b. 62% c. 23% d. 5%

d. 5%

Most polymers in use today are molecules with a backbone consisting primarily of which basic element? a. Silicon b. Nitrogen c. Hydrogen d. Carbon

d. carbon

17. For a typical amorphous thermoplastic material, which temperature designation corresponds to the highest temperature in degrees? a. Glass Transition Temperature b. Melting Temperature c. Heat Distortion Temperature Decomposition Temperature

d. decomposition temperature

1. If a highly polar plastic like Polyvinylalcohol is placed into a polar solvent like water, the plastic will a. Swell or soften b. Do nothing c. React d. Dissolve

d. dissolve

17. Values derived from a melt index test can be used as a valid measurement when a. compared between plastics of differing chemical structures b. compared between plastics of similar chemical structures c. for personal use only d. compared between different grades of the same plastic

d. the most accurate and reliable use for melt index results is in comparing between two different grades of the same resin. The nature of the polymer chains and their interactions make this test less reliable as a comparison between different plastics.

1. What is meant by "blowing the cells open," and how can this event be controlled?

"Blowing the cells open" is the conversion of closed cells to open cells by the rupturing of the cell walls. This can be controlled by controlling the amount and rate of expansion of the gas inside the cells. This control can be achieved by limiting the expansion of the mold in high-pressure foam molding. It can also be controlled by reducing the temperature at which the foaming occurs. Other methods that limit the expansion of a gas, such as increasing the pressure under that expansion occurs, can also prevent blowing the cells open.

1. Briefly describe five different casting techniques.

(1) Mold casting is when the casting resin is poured into a mold and forms the entire part. (2) Embedded is when an article is to be totally encased by the resin. The embedding technique uses a mold, similar to that used in mold casting, and suspends the article in the mold so that the resin fills the mold and completely surrounds the article. (3) Encapsulating is when an article is to be coated with resin but the final shape of the part is to be roughly the same as the coated article. Examples of this are painting and dip coating. (4) Cell casting is when the casting resin is poured between parallel glass plates separately by a gasket to retain the resin and set the thickness of the part. Sheets are typically made by this method or by the closely associated continuous casting method that uses two continuous stainless steel belts to define the surfaces of the sheets. (5) Solvent casting is done by applying a solution of a resin in a volatile solvent onto a belt, causing the solvent to evaporate, and then pulling the solidified resin off the belt.

1. (a) Explain why stretching an elastomer causes an unstable state and, therefore, when the stretching force is removed, why the elastomer recovers. (b) Explain why stretching of some polyethylene molecules results in "blushing" a situation not eliminated when the stretching force is removed.

(a)The entropy of the system decreases when the polymer is stretched because the stretching causes the molecules to become aligned and, therefore, more ordered. Increases in order cause decreases in entropy which causes a net increase in free energy and, therefore, the state is unstable. When the stretching force is removed, the entropy can increase and thus the energy will decrease, causing a return to the original state. (b) In stretching some polymers, the ordering of the molecules causes crystalline bonds to form. These bonds overcome the forces of entropy decrease and result in a net decrease in free energy, thus creating a more stable state. The crystalline structures will persist even when the stretching force is removed. The blushing is caused by light refracting off the crystalline structures.

1. Calculate the size of the press required to mold a part using a multicavity transfer mold in which the part size is 2.5 inches by 2 inches. The mold contains four cavities. There are four runners, each of which has a diameter of 0.375 inches and a length of 1.5 inches. The sprue has a projected area of 0.25 square inches. Assume that a transfer pressure 8,000 psi is needed.

2.5 X 2 X 4 = 20 in^2 for the total projected surface area of the parts. 4 X .375 X 1.5 = 2.25 in^2 for the projected surface area of the runners. 20 + 2.25 + .25 = 22.5 in^2 for the total projected surface area. 8,000 psi X 22.5 in^2 = 180,000 pounds of pressure. 180,000 pounds / 2000 pounds = 90 tons of pressure.

1. What is an MSDS?

A MSDS is a Material Safety Data Sheet that is sent by the manufacturer of the raw material. The MSDS indicates the chemical components in a liquid material and any potential hazards. It also indicates parameters such as boiling point, flash point, chemical reactivity, and toxicity.

1. You have been asked to identify the polymers used in two drinking cups. Both cups are clear. Suggest two identification tests (either physical or chemical) that will distinguish the cups, the probable results, and your rationale. The choices of plastics for the cups are polyethylene and polystyrene.

A burn test will show polystyrene gives off a thick black smoke because of the benzene group. A flexural test will show PS being much more brittle than the PE and the PS will fracture long before the PE. PS will also be much more susceptible to chemicals. Some possible chemicals for the resistance test could be gasoline or acetone. PE will not be susceptible to either.

Define a free radical and explain why it is so reactive

A free radical is an atom or molecule which has at least one electron which is not paired with another electron. It is so reactive because the natural state of matter is for that unpaired electron to form a bond.

7. Define a gel coat and give three reasons for its use.

A gel coat is a coating of resin that is sprayed or brushed into a mold prior to any placement of fiber reinforcement. The gel coat gives a smooth, fiber free surface. The gel coat can be pigmented, thus eliminating the need for painting the part. The gel coat can have additives, such as UV-stabilizers and anti-oxidants that help protect the part from environmental exposure.

Define monomer and polymer. Write typical polymeric repeating unit structures for both addition and condensation polymerization, and explain the various symbols contained therein. [ ─C—C─ ] ethylene mer unit [ ─C—C─ ] propylene mer unit / C

A monomer is a molecule that can be chemically reacted to form a long chain. A polymer is that long chain of monomer units. In a typical repeating unit structure for a polymer made by addition polymerization, the polymer unit is represented without the carbon-carbon double bond and with two bonds extending beyond the basic monomer unit. These bonds indicate that the basic unit bonds to additional units on either side of the unit shown. The entire basic unit is enclosed in parentheses (which normally go through the lateral bonds) to indicate that the basic unit exists as a group that is repeated many times down the chain. A subscripted n follows the second parenthesis and represents the number of times the basic unit is repeated down the chain. For polymers made by the condensation polymerization method, the basic unit is the combination of the two monomers with the monomers joined at the active sites but with the small, condensate molecule removed. All the other symbols are the same as for the addition polymerization case.

1. What is a parting agent, and why is it important in casting?

A parting agent is a mold release. These are important in casting because the complexity of many cast parts makes removal from the mold difficult. Therefore, some materials need to be coated on the mold so that release is facilitated.

. Give five part characteristics that indicate a poorly fused part.

A poorly fused part may have any of the following: bubbles on the inner surface, bubbles inside the walls, pin holes on the outer surface, non-shiny inner surface, and poor color development.

1. The stereoisomer of Polypropylene that is the most common and most useful is: a. Isotactic b.Atactic c. Syndiotactic d.Propyltactic

A. Isotactic

1. What are the differences between adiabatic heating and thermal heating as applied to extruders and why are both important?

Adiabatic heating is caused by the mechanical action of the screw on the resin inside the extruder. Thermal heating is caused by the heaters that surround the extruder barrel. Both are important because they combine to give the internal energy needed for melting. Some especially thermal-sensitive polymers cannot tolerate much thermal heating (such as PVC) and therefore need to have more mechanical energy.

1. If a polyol has four active sites per molecule and an isocyanate also has four active sites per molecule, why isn't a 1:1 mixture by weight a likely ideal concentration mixture? What should be the correct ratio?

Although the number of active sites is in the ratio of 1:1, the differences in molecular weight of the two components must be taken into account. For instance, assume that the polyol is a long-chain polymer with a molecular weight of 10,000 and that the diisocyanate is a small molecule with a molecular weight of only 100. To achieve complete reaction of all the reactive groups, there would need to be the same number of groups, not the same weight. Therefore, the weight of polyol should be 100 times the weight of diisocyanate to give the same number of polymers and the same number of reactive sites, a weight ratio of 100:1.

1. Identify four advantages and two disadvantages of using aluminum rather than tool steel to make cavities for injection molds.

Aluminum advantages: Lower initial cost due to lower machining time Shorter delivery time More even heat distribution Shorter cycle from higher thermal conductivity Lighter weight Easier to cast should that be the preferred fabrication method Aluminum disadvantages: Must be anodized to get hardness and anodizing may wear off Long-term wear us high

1. Why not eliminate the cull and save this extra material when doing transfer molding?

An excess of material is necessary in the transfer chamber to insure that the cavities are filled. If too little material is present or even if just the right amount of material is present, the force on the material in the mold cavity may not be sufficient to give good mold filling. Therefore, excess material is always used. This excess material is the cull.

1. What is an integral skin, and how is it controlled?

An integral skin is a tough outer layer of the foam that is formed when the cells collapse due to contact with the mold, with air, or with any other substance other than the foam. The amount of cellular collapse to create the integral foam can be controlled by adjusting the pressure and temperatures surrounding the foam, the temperature of the foreign material touched by the foam, and the presence of any other foreign substance, such as water or dust that might cause the cells to collapse.

1. Why are sizes and coupling agents generally not generally used for aramid fibers?

Aramid fibers are often used in applications where energy needs to be absorbed. Not forming strong bonds with the matrix allows slippage and friction heating between fiber and matrix and thus increases the amount of energy that is lost.

6. Discuss the relationship between the melt temperature and the location of the frost line in blown film manufacturing.

As the temperature of the melt increases, the frost line rises farther up the bubble, that is, it gets farther away from the die. This is because the frost line is the point of crystallization of the resin and, with a higher melt temperature, the time required for the melt temperature to fall to the crystallization temperature will be longer.

1. Why would fluorocarbon elastomers be so effective in resisting solvents?

As with fluorocarbon molding resins, the presence of fluorine in the elastomer reduces all chemical activity, including solvation effects. This occurs because of the strength of the carbon-fluorine bond and the tightness with which the fluorine electrons are held. These tightly held electrons and the strong, short carbon-fluorine bond mean that the fluorocarbon molecules are repellent to attack from other atoms or molecules. Hence, their solvent resistance is extremely high.

1. Describe the changes in properties that are likely to occur when the temperature is lowered below the Tg for an elastomer. What does this indicate about the usable lower temperature range for elastomers?

As with most plastic materials, when the temperature of an elastomer is lowered below its Tg, the polymer will embrittle. This occurs because the long-range flexibility of the polymer is decreased and the intermolecular forces increase as the polymer motion is reduced with the reduction in temperature. There fore, below Tg, elastomers are not elastomeric but are, rather, hard and brittle. The lower temperature range for elastomers is, therefore, the Tg.

1. What is backdrilling of a vent hole and why is it done?

Backdrilling is cutting a larger hole behind the portion of the vent hole that touches the plastic sheet. This larger hole gives greater air flow and allows faster application of the vacuum.

1. What is the likely cause of the inability of shrink-tubing to shrink sufficiently to bind two wires tightly when it is shrunk onto a connection between the wires?

Because its post-crosslinked pre-stretched condition is larger than what the application calls for

1. Why does casting have more design options than other plastic manufacturing processes?

Because of the ability to use flexible molds in casting, deep undercuts, high surface details and other design features that would be difficult to mold with other processes can be done easily by casting.

1. What constraints are necessary for biaxial rotation to evenly coat the mold in a rotational molding process?

Biaxial rotation must have both axes rotating simultaneously. The frequency of rotation should be different for each axis, and the rotational frequencies should not be even multiples of each other.

Explain bimodal distributions of polymers and their usefulness

Bimodal distributions have two peak areas in the MWD plot. These two peaks indicate that some lower molecular weight material has been mixed with some higher molecular weight material. This mixing is often done to improve the processability of a material with a high, narrow MWD. The lower molecular weight material will melt at lower temperatures and provide lubrication for the higher molecular weight polymers, thus increasing the ease with which they can be processed.

Discuss the differences between blister packaging and skin packaging

Blister packaging forms a pocket of stiff plastic material in a standard thermoforming method. The pocket is filled with the product and then sealed. Skin packs are made with a flexible plastic that is formed over the top of the part and the display card by drawing a vacuum through holes in the card. The skin pack is, therefore, tight against the product and the card whereas the blister pack is not tight against the product, but allows some movement of the product inside the pocket (blister). Normally the blister pack is preferred aesthetically but the skin pack is less expensive.

The compound most commonly used for compression molding is called BMC which stands for a. Bulk Mass Compound b. Bio Machine Cure c. Bulk Molding Compound d. Butyl Monoethylene Compound

C.

1. The pendant group on Polystyrene is: a. A chlorine atom b.An ethane group c. A benzene group d.Large scale branching

C. benzene group

1. The main difference between rigid PVC and vinyl materials is that the vinyl contains: a. Crosslinks b.Higher molecular weight chains c. Plasticizers d.Random copolymerization

C. plasticizers

1. Discuss the advantages of casting over other plastic molding operations in equipment size and cost.

Cell casting does not use high pressures and can, therefore, use a much wider variety of materials for molds and other processing equipment. Hence, the capital costs associated with casting are usually much less than would be required with other plastic processing methods. The absence of a need to withstand high pressures also means that casting can be used for making very large parts.

What is the effect of chlorinating rubber?

Chloroprene rubber or neoprene has many benefits. It has a chlorine bonded to one of the carbons in the carbon-carbon double bond. The effect of the chlorine is to improve oil resistance. Since the chlorine is a larger molecule it will cause intermolecular interference and greater strength and stiffness than polybutadiene, polyisoprene, and natural rubber. The chlorine also gives the material greater thermal stability and also makes it nonflammable

1. Would you expect ABS to be clear or opaque? Why?

Clarity comes only if light transmission is unimpeded in passing through the plastic. ABS is usually multiplastic and so every phase boundary is a potential light-scattering location. Hence, ABS is generally opaque.

1. What is a commodity resin?

Commodity resins are those that are sold in high volumes and are generally similar in properties from one supplier to another.

Identify three advantages of composites over metals for structural applications.

Composites are lighter weight at equivalent strength and stiffness. Composites can be made so that their mechanical properties are directional and can be tailored to a specific application. Composite materials do not corrode.

1. Why does crosslinking increase the resistance of golf ball covers to cuts?

Crosslinking increases the strength of the material, overcomes the inadequacies in knitting the material.

1. What is the effect of crosslinks on an elastomer and what is the structural explanation for their effect? What would happen to the amount of stretch, hardness, strength, and creep in an elastomer if the crosslink density were increased?

Crosslinks prevent the elastomer from creeping at high temperature. Also, crosslinks give some limitations to the range of elasticity for the polymer. These effects arise from the limiting effects of the crosslinks. The crosslinks bind the molecules together and do not permit unlimited stretching. Also, during creep, the crosslinks act as limiting forces to the continuation of creeping motion. If the crosslink density were increased, the amount of stretching would be lower, the strength higher, the hardness and the amount of creep would be lower.

Explain the effects of crosslinking on the glass transition temperature

Crosslinks tie the molecular chains together and make long-range movements more difficult. Therefore, as the amount of crosslinking increases, the glass transition temperature should also increase.

1. What is meant by cryogenic flash removal?

Cryogenic flash removal simply means that the parts are frozen when the flash is removed. This freezing makes the plastic brittle and, because the flash is thin, increases the likelihood that the flash can be broken off the parts when they are tumbled together.

1. How does high crystallinity in a resin affect the way the resin is injection molded, including any post-molding operations that might be done?

Crystallization normally gives materials a sharper melting point. That is favorable in injection molding because it makes the material easier to heat to a uniform viscosity. Furthermore, the crystallinity usually means that the material will cool and solidify more sharply, thus decreasing the time that the material must be cooled. After the part is removed, residual crystallization is common. This continuing crystallization can distort the part. Therefore, cooling fixtures are often needed to ensure that the part will crystallize and complete cooling without part distortion.

6. Indicate at least three problems of most composite processes that are solved in RTM.

Dangerous fumes, it allows for a higher degree of automation, increases repeatability.

Give several reasons for the development and use of plastics.

Demand for highly formable materials with properties that are not easily obtained with natural materials. Plastics permit greater flexibility in design and allow many of the innovative modern products to be made economically.

1. What do the initials DGEBPA stand for?

Diglycidyl ether of bis-phenol A.

Which currently manufactured consumer product does not typically contain plastic components? a. Sewing machines b. Automobiles c. Home furnishings d. Athletic equipment e. None of the above

E. None of the above

Why are ejector pins made of hard yet shock resistant materials?

Ejector pins can meet resistance when ejecting a part that is stuck in the mold. This resistance can cause some impact forces on the pins which, if they were brittle, could cause them to break. In normal use, the ejector pins must be strong enough to push out the part. Furthermore, the constant sliding of the long ejector pins requires certain strength to keep from breaking and excessive wearing.

What are the key polymeric structural features common to most elastomeric materials?

Elastomers are generally linear, aliphatic materials with structures that do not pack together easily and are therefore noncrystalline. These materials have highly random structures in their relaxed state but will become more ordered if stretched.

1. Identify three methods for coating a plastic with a metal.

Electroplating, foil transfer, and vacuum deposition

1. Why does crosslinking increase the environmental stress-crack resistance of polymers?

Environmental stress-cracking is seen in polymers that have high internal orientation, such as extruded polyethylene tubing which has had a high drawdown. The polymer molecules are aligned in such cases and are stressed. When the environmental effects act on these polymers, cracking occurs. By crosslinking the polymers, the chains are held together and cracking is prevented.

1. What are three critical factors that determine price?

Establishing the cost if the product, decide on the profit that is considered appropriate and desired, and examine the market forces to determine what price will create the sale.

1. Identify FEP and discuss its main advantage over PTFE.

FEP is a copolymer of tetrafluoroethylene and hexafluoropropylene. It has the advantage of being melt processable with only minor decreases in many of the excellent physical and chemical properties of PTFE.

. True or False. All casting processes require an outside heating stage for the material to fully cure.

False

1. True or False in order to get the plastic powder to stick to the sides of the mold it is important to mix in adhesive additives into the powder before processing.

False

1. True or False. Other plastic materials are never used as a mold for the molding process.

False

Describe why fillers and other additives may increase or, in some cases, decrease rotomolded part mechanical properties.

Fillers can increase mechanical properties by reducing the degradation that occurs during some heating cycles. Fillers can decrease mechanical properties by decreasing the flowability of the resin and therefore preventing the resin from moving uniformly throughout the mold. Decreased mechanical properties would also occur if the filler interferes with the fusion of the plastic particles.

1. Describe how foamed plastic egg cartons are made.

Foamed egg cartons are often made by thermoforming extruded foam sheets. The sheet is made by the normal extruded foam process. The thickness would be the same as that desired in the finished product. After the foam has stabilized, the foamed sheet can be thermoformed in-line so that the operation will move at the speed of the extrusion. The cartons can be trimmed from the sheet and the remaining material can be chopped and re-extruded.

1. Which type of backbone substituents stiffen the polymer chains? ch 8 # 1 photo

Functional groups (a) and (d) restrict the motion of the polymer chain and therefore result in chain stiffening while (b) and (c) give only minor chain restriction.

Identify gun cotton and indicate why it was important in the development of modern plastics.

Gun cotton or nitrocellulose is made from cellulose fibers that have been treated with nitric acid. This material was, perhaps, the first plastic since it was a substantial modification of a natural polymer and it could be shaped or formed. It served as the model for several other acid-derived cellulose products such as rayon, celluloid and cellophane.

What is a heat history, as applied to polymers, and why is it important in polymer technology?

Heat history is the accumulated total amount of thermal energy input into the polymer system. Hence, if the polymer has been processed twice, the heat history will be roughly twice as much as if the polymer were only processed once. This is important, especially for heat-sensitive polymers, because excessive processing, or processing at very high temperatures, can lead to degradation.

1. What feature in a mold will allow a hollow, cylindrical part to be made?

Hollow parts are made by inserting a core into the mold so that the material will flow around the core, displacing the resin from the volume to be hollow. When the mold is opened and the part is removed, the core is withdrawn and the solid, hollow part is created. Normally the withdrawal of the core occurs simultaneously with the opening of the mold.

1. How does changing the molecular weight distribution (MWD) alter the processability of the plastic?

If the MWD is broad, the short molecules will melt before the long molecules and serve as lubricants for the entire plastic mass, even before complete melting occurs. This results in some residual entanglement among the long molecules which gives tensile strength to the melt. Processes that require melt strength, such as extrusion and blow molding, are thus improved in processability by broad MWD. On the other hand, narrow MWD will result in a sharper melting point and a lower viscosity melt conditions that are favored in injection molding. The cooling of the polymer is also faster with a narrow MWD to further improve injection molding processability.

What is the principle problem when forming a bottle using injection molding?

If the diameter of the mouth of the bottle is smaller than the diameter of the body, some method must be provided to remove the core pin. The core pin will have the diameter of the body and, therefore, must collapse or be reduced in diameter by some other method. This is a difficult mechanical proproblem that is eliminated when blow molding is the method for forming the bottle.

1. Discuss what attributes might make a resin inappropriate for use as a hot-melt casting resin.

If the viscosity of the melted resin were too high, the melt may not flow without external pressure and, therefore, would not work well as a hot-melt casting resin. Another problem could arise if the resin degraded easily at the temperatures required to keep the material liquid.

1. Why can't the cull, sprue, and runners from a transfer molding operation be reprocessed as regrind like runners in an injection molding operation?

In a transfer molding operation the cull, runners, and sprue are made from thermoset materials that cannot be remelted, which is required for reprocessing as regrind material. In injection molding the runners and sprue are made from thermoplastics that can be remelted and reprocessed as regrind

1. Why is the temperature raised in order to achieve crystallization in PET? Is that temperature higher or lower when the PET has been oriented?

In any material, especially one in which the polymer repeating unit is large, energy must be added to the system to allow sufficient molecular motion to align the molecules for crystallization. This is known as the thermodynamic limit of crystallization. If the polymer is oriented, some induced bonding has been created that gives the structure stability. Therefore, to rearrange into crystals, more energy must be added than if the polymer were amorphous. Therefore, the oriented polymers require higher temperatures to crystallize than the nonoriented polymers.

1. Explain how foam-in-place and RIM differ.

In both foam-in-place and RIM the material is squirted into a closed mold until the mold is filled. The foaming usually occurs at the same time as the filling. The major difference is that in RIM the foamed part is removed after it has solidified and the mold is reused. In foam-in-place, the mold is the sides of the part that is to contain the foam and so the foam is not removed.

1. Explain why coining can be used to make more complicated shapes than can conventional sintering.

In conventional sintering the shape of the part is determined by the preforming mold. Since the main purpose of preforming is to eliminate voids and compact the material, high pressures are required and the ability to press the powdered material into complicated shapes is compromised. In coining, the part is placed into a mold when the part is in a softened state. Therefore, pressing into a detailed shape is much easier.

Explain why isotactic polypropylene can crystallize and the other polypropylene stereoisomers cannot crystallize.

In isotactic polypropylene all of the methyl groups are on the same side of the tertiary carbon. This reduces the intermolecular interference between these groups and allows the molecules to pack tightly together, thus forming crystals.

1. Describe the principle similarities and differences between RIM and low-pressure foam molding.

In low-pressure foam molding the mold is only partially filled and then allowed to expand to complete the filling of the mold. In RIM the mold is completely filled but the foaming occurs simultaneously with the filling of the mold. RIM also has the capability of adding a reinforcement inside and of foaming around metal inserts which would be very difficult with low pressure foam molding.

1. Explain the purpose of a preplasticizing unit.

In some operations, chiefly where the melting of the resin is the limiting factor in decreasing the cycle, a second barrel or chamber is added to the injection molding machine. This second chamber is the collection point for the melted resin as it exits from the screw or ram melting unit. The second chamber has a ram injector that can push the melt of resin into the mold even while material is being introduced from the melting unit. The combination of a continuous melting operation and an on-demand injection operation can speed molding significantly in some cases.

1. What is the biggest problem associated with incineration as a waste disposal method?

Incineration has a great potential for air pollution from the waste gases of the burning process.

Identify two advantages of using inserts in injection molds.

Inserts allow a single base to be used for multiple parts. Inserts can be changed and modified easily should they be damaged.

1. What is ionic crosslinking and what properties will it produce?

Ionic crosslinking occurs in ionomers such as EAA and arises when an acid is a pendent group on a polymer. When the acid reacts with a base, the acid loses a hydrogen ion and becomes negatively charged. These negative charges can be attracted to metallic ions that are added to the plastic mixture that are part of the reactive base. The attractions between the metal ions and the acidic groups are ionic. When the acid groups on more than one polymer are all attracted to the same metal ion, the polymers are crosslinked with the metal ion forming a bridge. These ionic crosslinks increase the stiffness, toughness, and strength of the plastic because they increase the energy needed to cause the molecules to slide past each other and because they network the molecules together resulting in a better transfer of impact energy. Melting point and glass transition point are also increased, but the plastic is not a thermoset because heating will cause the ionic crosslinks to break.

Discuss the safety precautions which should be observed when handling isocyanates.

Isocyanates react readily with water and can, therefore, react with the soft mucous membranes in the body. Care should be taken to wear goggles, gloves and a mask when pouring or mixing isocyanates.

6. Assume you are working for a custom extrusion company that uses many dies, many resins, and many colors. To keep costs down, your company generally buys natural (uncolored) resin. You must buy a new extruder and have two choices, one with L/D of 20 and one with L/D of 30. Which machine would you buy? Why? Give two disadvantages of the machine you chose.

L/D 30 for better mixing quality. Disadvantages: heats the material longer, takes longer for the material to get through the machine, more material is wasted in changeovers.

1. Would HDPE, LDPE, or LLDPE be the most strongly affected (crosslinked) by electron beam radiation? Why?

LDPE because it is not crystalline like HDPE, which allows for the free radicals to recombine more easily and can move to allow polymers to join together.

1. Discuss how creep is likely to be affected by aromatic pendant groups.

Large pendant groups such as the aromatics will decrease creep because they cause interference between the molecular chains. This interference raises the energy required to move the chains relative to each other and, as a result, creep is decreased.

1. Identify four advantages of laser cutting plastics over traditional cutting (machining) methods.

Lasers can vaporize the plastic, can make cuts beginning at some location other than the edge of the part, the cutting apparatus does not become dull, and depth precision in cutting can be achieved with changes in focus of the lens on the laser.

1. Identify the four major types of polymeric liquids used for casting, and give one specific example of each type.

Liquid casting resin - acrylic syrup, polyester resin, polyurethane, caprolactam Hot-melt resin - melted nylon, melted polyethylene Plastisols and organisols - PVC plastisol Solvent casting resin - acrylic dissolved in an acetone, nylon in dimethylformamide

17. Given the following data, calculate the number average molecular weight and the weight average molecular weight, and discuss whether the MWD distribution is wide or narrow compared with a normal commercial ratio of 20. Ni (x 1024) 5 7 8 9 10 8 6 3 2 1 Mi (x 104) 1 2 3 4 5 6 7 8 9 10

MWn = 4.10 x 104, MWw = 5.77 x 104, MWw/ MWn = 1.38 (very narrow MWD)

1. Explain why laboratory identification of a plastic so often involves burning the plastic.

Many plastics can be readily distinguished by their burning characteristics and the burning tests are so easy to perform.

1. Describe the four basic methods of creating a foam.

Mechanical—vigorously mix the resin with a gas (usually air) to create a foam; Chemical—create a gas by some chemical process within the mixture of resin and foaming agent so that the gas foams the resin; Physical—create a gas by some change in the physical conditions of the resin and foaming agent mixture; Hollow spheres—mix glass micro-balloons into the resin

Using molecular explanations, discuss the differences in mechanical, physical, and chemical properties of natural rubber and butadiene.

Natural rubber has a methyl group which interferes with movement restricting the bending, twisting, and sliding motions across the molecules and increases stiffness, gives a higher strength and higher temperature stability. Polybutadiene has poor tensile strength, tear resistance, poor resistance to solvents, and tack because of the lack of the methyl group.

Explain how polyethylene is crosslinked in a rotomolding operation.

Normally thermoplastics such as polyethylene cannot be crosslinked. However, if a peroxide or some other free-radical-forming material is added to the polyethylene and the temperature is raised, some hydrogens in the polyethylene can be removed and free radicals formed in their place. These free radicals can then react with crosslinking agents containing carbon-carbon double bonds which have also been added to the mixture. These crosslinking agents bond to the free radical locations on the polyethylene. Because the crosslink agents have at least two reactive carbon-carbon double bond sites, they can react with more than one free radical location and therefore tie together multiple polyethylene chains. Increased heat is needed to give the molecules the mobility to move together to form the crosslink bonds.

1. Compare the water absorptivity of nylon (6/6) to nylon (6/12) and explain the differences that are noted.

Nylon (6/6) will have more polyamide groups per length of chain which will increase the polar nature of the polymer. This will cause the water absorption to be higher than in the (6/12) product.

1. As a design engineer you have begun to receive warranty claims on a part that your company produces. The part is on a conveyor system in a food processing plant. The part that consistently fails is the guide system for the food containers and it only fails in the section of conveyor that is within the steam clean-off area. The runners that you have there begin to abrade and fall apart at unreasonable rates. In the specifications you have chosen nylon because of its excellent abrasion resistance. Why might your parts be failing?

Nylon is polar and therefore swelling when it comes in contact with the steam clean-off system. This swelling causes the nylon to degrade faster than desired. A different non-polar material such as acetal should be considered for this application.

1. Discuss why the compression zone of a nylon-type screw is so much shorter than the compression zone in a polyethylene-type screw.

Nylon melts over a much narrower range than polyethylene. Since the purpose of the compression zone is to melt the polymer, the distance to do the melting need not be as long with the more easily melted nylon.

1. Which of the engineering thermoplastics could reasonably be used as a tire cord material?

Nylon, especially (6/6), PET, rayon, and aramid have all been used extensively as tire cord materials. Each has its advantages and disadvantages. Aramid is the strongest but also the most expensive. Nylon is next in strength, but has a problem with temporary flat-spotting, that is, the tires develop a flat spot if left in one orientation. This flat-spotting disappears after driving for a short distance. Polyester is less expensive than nylon but does not wear as long. Rayon was used extensively in early tires but is less durable than the other tire cords and is now used infrequently.

Describe a technique for forming a part with an outer coating and a different inner material using rotomolding.

One method that will accomplish this formation of an outer shell is to mix two resins that have different melting points. During the rotomolding operation, the lower melting resin will become sticky first and coat the inside of the mold, thus forming the outer layer of the part. Later, the higher melting material will coat onto the shell and form the inner material. Another method is slush molding which one resin is put into the mold and poured out after a thin film has formed on the mold. A second material is then added to complete the part.

1. Discuss two methods that might be employed to make an epoxy resin tougher.

One method would be to add a rubber toughener to the epoxy resin. Another method would be to change the nature of the epoxy so that it is more aliphatic and less aromatic. (This would, of course, mean changing the type of resin being used.)

1. What are two major disadvantages to degradable plastics?

One: The product may degrade before its intended function is completed. Two: The product may degrade into components which are polluting.

Explain why a rubber washer stops sealing tightly after several years.

Over time the rubber is going to continue to crosslink and become harder which will not be as compliant with the other surface. This will also cause it to become more brittle and eventually break. It could also degrade.

1. Explain how both undercuring and overcuring can cause part cracking on removal from the compression molding process.

Overcuring can result in brittle parts that crack when the ejection system pushes the part out of the mold. Overcured parts may also stick to the mold. Undercuring can produce cracked parts because the strength of the part is lower than desired and may not be sufficient to withstand the pressures of ejection.

1. Compare polycarbonate and polystyrene in the following properties and explain your answer on a molecular basis: tensile strength, impact toughness, melting point, abrasion resistance, clarity, solvent resistance, and cost. (Note: This question could be applied to any two polymers.)

PC is higher in tensile impact, melt point, abrasion and cost. Clarity and solvent resistance are closely comparable. Having the benzene in the backbone of the chain allows for forces to be more easily transmitted along the chain. Where the benzene ring is a pendant group those forces are not as easily transferred from one chain to another.

9. In some applications, such as grilles for automobiles, engineering plastics such as PET compete with SMC and also with some metals such as zinc, which can be cast or even injection-molded. What would be the three inherent advantages of each of the material types in this kind of application?

PET: cost, production time, and processability SMC: strength, durability, and equipment costs Zinc: weatherability, appearance, and marketability (personal preference)

1. Discuss the physical, chemical, and mechanical properties of PMMA compared to polystyrene.

PMMA and PS are both clear, noncrystalline materials that are extensively used when clarity is important. PMMA has much better clarity retention and weatherability. PS will yellow readily and embrittle. PMMA and PS are both brittle, although PMMA is somewhat stronger. Both materials are solvent sensitive with PS being much more sensitive to aromatic solvents.

1. What is packing the mold and why is it important in obtaining good injection molded parts?

Packing is the process of continuing to inject resin into the cavity even after the cavity is full. As the plastic shrinks from thermal processes (thermal shrinkage), the part will reduce in size. This reduction can be compensated for by continuing the injection of resin as the cooling and shrinkage occurs. Packing can ensure proper part size and can help eliminate sink marks which can occur in thick sections that shrink more than thinner sections within the part.

1. Compare the overall environmental impact of plastic and paper grocery sacks.

Paper sacks are seen to be more polluting to make and dispose of. They can be biodegradable, although probably not in landfills because the conditions are not correct for degradation. The weight of paper bags and the volume of paper bags are both much higher than an equal number of plastic bags, thus complicating the landfill problem.

1. What are the three major components (material types) of a landfill?

Paper, yard wastes, and plastics.

1. When making bottles by blow molding, you find that about every tenth bottle has a small hole in the wall. Indicate four possible causes for this defect and explain what you would do to correct each possible problem.

Parison wall not uniform, parison not grasped evenly by the mold halves, resin melt index too high, too much regrind, contaminated resin, parison not at the proper temperature, blow-up air enters too slowly or fast.

1. Explain plastic deformation in terms of molecular chains.

Plastic deformation occurs when the molecular chains move relative to each other in a nonrecoverable way. In other words, the changes in shape are permanent because the sliding of the molecules has used up some energy and not enough energy remains to force the molecules into their original positions.

9. Discuss the choice of materials for frying pan handles.

Plastics or ceramics would be better than metals because the metals would conduct the heat and burn your hand. Ceramics are quite brittle, so plastics would be the first choice. Among the plastics, most are good thermal insulators, but the thermoplastics would likely soften or melt when the pan got hot. Therefore, thermosets are the likely group of plastics to consider. Within the thermosets, some properties that might be important are low flammability, low cost, and stiffness. Phenolics seem to be a good choice. The potential drawbacks of phenolics are color (the part will be dark) and filler (the part will need to be filled). Neither of these drawbacks should be a problem for this application.

1. What are plastisols and organisols?

Plastisols are generally over 90% solids where the solid material is a powdered resin (such as PVC) and the nonsolid material is a plasticizer. Organisols are also mixtures of powdered resin and plasticizer but with the addition of up to 40% volatile solvent.

1. Describe poker chipping, and explain how it can be prevented.

Poker chipping occurs in the ram extrusion process when the charges of compacted resin are not well bonded together. The billet made by the ram extrusion process can therefore be in separate charge portions. These are compacted together like discs or poker chips. This problem can be prevented by improving the consolidation between charges, usually done most easily by increasing the fusion temperature.

1. Why is a polyimide film ideal in a very high-temperature electrical application, even one in which the film may be slowly degraded?

Polyimide film has a very high temperature capability. Then as it might degrade, rather than melt, it remains a solid that keeps the electrical conductors separated. The char that is formed may eventually conduct, but char formation is usually a slow process.

1. Explain why polybutadiene is softer than polyisoprene.

Polyisoprene, either cis or trans, has a methyl group that is pendent to the four-carbon backbone repeating unit of the polymer. Polybutadiene has no methyl group attached to its four-carbon repeating unit. This methyl group in polyisoprene interacts with other molecules as the molecules are moved past each other and also causes some restrictions to bending and twisting within each molecule. These intermolecular and intramolecular interactions cause the polyisoprene to have higher energy that is manifested as higher hardness and higher tensile strength.

Discuss the value of a primer coat when painting a plastic part

Primer coats can both protect and preserve the surface of the part on which they are applied and chemically enhance the bonds that are formed. The preservation of the surface can be especially important if the surface is susceptible to chemical change after it has been treated for adhesion. For instance, the surface may oxidize if left open to the air. The primer may also be a coupling agent to assist in forming good bonds between the adhesive and the plastic to be bonded.

1. What is the advantage of a programmable parison device?

Programmable devices allow the parisons to have varying wall thickness to compensate for the longer distances of travel required to fill some parts of the mold. The parts made using programmable parisons are, therefore, more uniform in wall thickness.

1. What method is used in injection blow molding to achieve the type of part wall thickness control that can be obtained in programmable parisons?

Programmable parisons only have meaning in extrusion blow molding because they are made with a movable mandrel in the extrusion die. However, a similar effect can be achieved in injection blow molding if the core pin of the preform mold cavity is shaped. This shaping will create some thick section of the preform which can be placed in the locations that will travel the farthest in the mold.

1. Explain why open-cell structures are created when the mold expansion in high-pressure foam molding is quite rapid and why closed-cell structures occur when the molding is slower.

Rapid expansion of the mold tends to allow the gas inside the cells to expand very rapidly and to expand to a much larger size. This large expansion can rupture the walls of the foam and change closed cells into open cells.

1. Discuss the advantages of rapid prototyping methods versus conventional prototyping.

Rapid prototyping allows a part to be made directly from a CAD rendering. This ensured that the prototype is the same as the drawing. Also, the time to create the prototype is much shorter. In some rapid prototyping methods, the material of the prototype can even be the same as the material of the part, thus allowing mechanical prove-out as well as dimensional verification.

1. What would be the result of a leakage of gas into the acceleration chamber of the electron beam irradiation machine?

Reduction in the total number of electrons passing through the chamber.

1. What is regeneration, and what is its potential value?

Regeneration is the conversion of plastic material back into the chemicals used to make them originally. The advantage of this method is that the chemicals have a higher value than some of the mixed recycle of plastics from which the regenerated chemicals were derived.

1. Why should the concentration of regrind PVC be kept low in any rigid PVC extrusion operation?

Regrind material is rejected product that has been chopped into small particles so that it can be reprocessed to make new parts. This is an advantage of thermoplastics. With PVC, however, the tendency to degrade is so pronounced that each time the PVC is heated, the degrading gets worse. To further complicate the matter, the degradation of PVC is autocatalytic, meaning that the decomposition happens faster as more is formed (like a chain reaction). Therefore, if the concentration of regrind PVC is too high, the accumulation of degrading material will be too high in one place and the material will degrade rapidly.

1. Why is good weldability desirable in a mold cavity material?

Repairs of mold cavities may require the grinding and filling of the cavity. The filling will require welding and this may be difficult with some materials

What is a resin?

Resins are plastic materials that are unformed or require additional forming to be useful. Resins are often liquids, but can be solids in a raw material or intermediate form.

1. Compare the processing differences of resole and novolac resin. What is meant by one-stage and two-stage resins?

Resole and novolac resins are both types of phenolics. The resole resins are linear polymers that are cured simply by heating, while the novolacs must have a hardener added to them and then heated to cure. The resoles are, therefore, one-stage polymers, meaning that only one step is needed to complete the crosslinking, whereas the novolacs are two-stage polymers.

1. You have been placed on a team that is to re-evaluate the material choice for use in plastic milk jugs. Currently your company uses the standard HDPE. The price of HDPE has been rising and the company wants to reconfirm its decision. It is your job to decide if you will continue using HDPE or if there is another material that will fulfill the requirements of the part. List and explain the most important criteria upon which you are basing your decision.

Sanitary, won't react with the milk, stiffness is good so that the container will uphold its shape, easily blow molded and moderately impact resistant.

9. You have been asked to manufacture a wiring junction box for connecting electrical wires in a fighter aircraft. The box sits under the pilot's seat and is part of the seat ejection system. Sufficient forces are exerted on the box that it must be made of a composite material using intermediate to long fibers. What fiber, matrix, and manufacturing method would you use for this part, and why?

Several materials could be chosen but, because of its good electrical and heat insulative capability and its non-flammable nature, phenolic seems to be the appropriate resin. The fibers need to be strong, stiff, and tough enough to withstand the ejection impact. Therefore, aramid is a good choice for the fibers. The manufacturing process could be either compression molding or RTM, probably depending upon whether the fibers need to be longer than those that could conveniently be used in compression molding.

1. Identify three advantages of cell casting over continuous casting. Identify one advantage of continuous casting over cell casting.

Sheets made by cell casting are better optically than continuous cast sheets. The cell cast sheets also have better dimensional control and give more uniform shrinkage when heated. The advantage to continuous casting is that the production rate is much higher than with cell casting.

6. Explain the effect of molecular weight distribution on extrusion.

Short, low molecular weight polymers will melt sooner than long polymers and will therefore act as lubricants for the longer, higher molecular weight polymers in the same resin. The longer polymers, however, give melt strength to the resin. Therefore, extrusion resins would generally have a wide molecular weight distribution so that the resin would flow easily but still have good melt strength.

1. Discuss the use of sizing agents and coupling agents and distinguish between them.

Sizing agents are put on fibers to protect the fibers from mechanical damage during later processing, such as if the fibers are to be woven into a cloth. The coupling agents are placed on fibers to enhance the bonding between the fibers and the matrix.

1. Describe sliding/compression blow molding and indicate its advantage.

Sliding/compression blow molding allows sections to be created on blow molded parts that are undercut or otherwise shaped in some method other than by just blowing outward against the mold cavity. These special sections are created by blowing the parison into a recess in the wall of the mold cavity. The material in this recess is pinched during the molding process, thus compressing the material and giving it form. The pinching is usually done by sliding part of the mold against a fixed part of the mold. The finished part is removed by reversing the sliding and then opening the mold in the normal way for blow molding.

1. Define and describe slush molding.

Slush molding is a process that uses a powdered resin suspended in a nonvolatile solvent as the starting material. The mold and processing equipment can be similar, at least in function, to standard rotomolding equipment. The mold is filled with the liquid material and is then heated and rotated so that the walls are uniformly coated with material that sticks as it becomes hot. When the material stuck to the walls of the mold has reached the desired thickness, the mold rotation is stopped and the excess liquid is poured out of the mold. The mold is then returned to the oven and rotated until fused. The cycle is completed by cooling and then unloading the part.

Discuss the effect of plastic brittleness in snap-fit joints.

Snap-fit joints depend on the ability of the plastic material to deform slightly without breakage or permanent deformation. If the plastic part is so stiff that small amounts of movement will cause it to break, snap fits cannot be used. This breakage might also occur in the mold is a snap fit depends on an undercut in the part that must be stripped out of the mold.

Describe the processes of solvent/solute interaction that lead to swelling and, in some polymers, dissolving of a plastic by a solvent

Solvent molecules form secondary bonds with the polymer molecules, usually at sites on the polymer that are chemically similar to the nature of the solvent. When many of these secondary bonds form, there is sufficient energy in the molecules to allow the penetration of the solvent molecules into the polymeric structures. This causes swelling. If even more of the secondary bonds between solvent molecules and the polymer are formed, or if the secondary bonds are extremely strong, then the total polymer molecules can be separated from the other polymer molecules and total solvation occurs.

List five methods of disposing of municipal solid wastes

Source reduction, recycling, regeneration, degradation, landfill, incineration

1. Why is low specific heat capacity desired in a mold cavity material for some applications and a high specific heat capacity desired in others?

Specific heat determines the amount of heat necessary to raise the temperature of the material. For an injection mold that is cooled, a high specific heat is desired so that the heat from the part that is being molded will not cause the mold to heat. This will minimize the work of the coolant. In a mold that is heated, such as a thermoset mold, a low specific heat will allow the mold to be heated more easily.

Describe a spider die. What adjustments are typical and how are they made?

Spider dies are used to make hallow parts, such as tubes and pipes. The inside diameter of the die decreases gradually and smoothly from the inlet of the die to the land. The land is the final output zone and in this zone the inside diameter of the die is constant. The mandrel inside the die (which forces polymer around it to create the hollow) is supported near the inlet of the die by pins that go through the walls of the die. These pins have a streamlined cross-section and are generally threaded into the die so that they can be moved and adjustments made to the location of the mandrel. The mandrel is cantilevered forward from the adjustment pins without any other support.

1. Explain staking, and explain the effect of its use with large-diameter posts in large-diameter holes.

Staking is used to attach a post inside a hole. It is done by heating the top of the post, often using ultrasonics, and then pressing the softened end of the post so that it spreads out and overlaps the part surrounding the post. On very large parts, the end of the entire post may be difficult to soften all at the same time, thus making staking difficult.

Discuss the role of styrene in the crosslinking of unsaturated polyesters

Styrene is both a solvent and a participating component in the crosslinking. As a solvent it assists in facilitating the reaction by adding mobility to the polymers. As a participating component in the crosslinking it bridges between polyester molecules making the crosslinks easier to form.

10. You are rotomolding a large children's toy with a very complicated shape and you have trouble getting good part definition and complete molding. You have a choice of resins having two melt indices. One is 30 and the other is 2. Which would you choose and why?

The 30 because the higher melt index indicates the material's ability to flow, this increase in flow will allow for better part definition. Processing time will also be shorter by using the 30 rather than the 2.

1. If nylon is sensitive to moisture absorption, why can it still function well as a tow rope for water skiers?

The absorption of cold water is very slow in nylon. Furthermore, tow ropes are usually not totally submerged for long periods of time and therefore do not have high absorptions. Furthermore, nylon ropes are seldom pulled to their limit in this application so the small decrease in tensile strength is not critical.

1. Identify three major advantages of a cast film over an extruded film. Identify one major disadvantage of the cast film.

The advantages of a cast film are clarity, thickness control, and the ability to use higher molecular weight resins and thus achieve improved physical properties. A disadvantage of cast films is that they cannot be produced as quickly as can blown films.

1. A thermoformed part has a total surface area of 1000 cm2 and a required thickness of 1mm. It is formed from a blank sheet with a total usable surface area of 500 cm2 (not counting the part of the sheet used for clamping) and a thickness of 2 mm. When forming the part, the walls often tear. Indicate what might be the problem.

The area ratio of 1:2 indicates that the thickness will be reduced from 2 mm to 1 mm in the final part, assuming that the wall thicknesses are uniform. Uniform wall thicknesses are not likely with thermoforming and so some portions of the walls will be much thinner than the required 1 mm and will be likely to rupture when drawn.

1. In an expansion molding operation, why does fusion occur in the mold and not in the pre-expansion step?

The beads are confined in the molding step and therefore expand against each other. This close contact forces the walls of neighboring beads to meet and, if the temperature is high enough, fusion occurs. During pre-expansion, the beads are in an air flow and therefore do not have intimate contact with each other. To further prevent beads from fusing during pre-expansion, the temperature is often a few degrees lower than during molding. Also, liquids can be added during pre-expansion that tend to keep the beads separated.

1. Calculate the size and thickness of a blank needed to make a part that is 100 x 50 x 10 cm with a finished part thickness of 3 mm. Assume 2 cm per side are required for clamping.

The blank dimensions can be calculated by adding the clamping requirements to the size of the finished part. Therefore, the blank dimensions are 100+4 = 104 cm by 50+4 = 54 cm. The thickness of the blank can be calculated from the area ratio. The area of the blank is (100)(50) = 5000 cm2. (Note that only the thermoformable part of the blank is used in this calculation.) The area of the part is (100)(50)(1 side) = 5000 cm2 plus (100)(10)(2 sides) = 2000 cm2 plus (50)(10)(2 sides) = 1000 cm2 for a total of 8000 cm2. The area ratio is 5000/8000 = 1:0.625. Therefore, if we assume uniform wall thickness throughout, the blank thickness will have to be 3 mm/0.625 = 4.8 mm.

1. Explain why silicones have higher gas permeability than do carbon-based molecules with equivalent pendent groups.

The bonds along the silicone backbone (Si-O-Si) are longer than the bonds in the carbon polymer (C-C-C). This increase in bond length means that the silicone polymer has more space between atoms. This increase in space means that a diffusing atom or molecule can pass through the mass of molecules more easily in silicones than in the equivalent carbon polymer. Of course, is the pendent groups of the polymers are vastly different, the effects from these pendent groups on space between molecules may overwhelm the bond-distance effect. Likewise, crystallinity could overwhelm the bond-distance effect.

1. Explain what may happen to the properties of a rubber material if the carbon black filler is poorly mixed into the batch.

The carbon black serves two main functions in a rubber compound. The first of these is protection against degradation from UV radiation as it depends upon the carbon black to absorb the UV radiation. This function is improved with dispersion of the carbon black, simply because many small and distributed particles can absorb more effectively than fewer, larger particles. However, this effect is not great. The other function of carbon black is to strengthen the elastomer. For carbon black to properly perform this function, the carbon black particles need to be widely distributed and intimately mixed with the elastomeric polymers because this strengthening comes from the interactions between the carbon black particles and the molecules. These interactions occur at the molecular level, rather than at the macro level. Therefore, if the carbon black is poorly distributed, the interactions will not, in general, be at the molecular level and will only be with a few of the molecules. This will have a significant effect on the strengthening of the polymer that would it for from the addition of the carbon black.

What is the origin of the vast differences in physical properties between PVC (rigid) and vinyl? Explain why PE is not treated in the same way to create two very different physical states.

The cause of the differences is due to the plasticizers that are in place in the vinyl. These plasticizers allow for greater movement between the chains and give the feel of being much more flexible. PE is not treated the same way because PE is does not react with or accept plasticizers or solvents of any kind.

1. Why is the cis form of polyisoprene softer than the trans form?

The cis form will not pack together as easily as the trans form. Hence, there are fewer intermolecular attractions in the cis form and the molecules can slide over each other more easily. This ease of sliding and absence of intermolecular attractions means that the cis form is softer than the trans.

1. Assume that you are assigned to determine the minimum clamping force for a part to be molded out of polystyrene. The part cross-sectional area is 10 x 14 inches. What is the clamping force required if as a general rule, 2.5 tons of force are needed for each square inch of cross-sectional area?

The clamping force can be calculated from the equation: Clamping force = (injection pressure) (total cavity projected area). In this case the total cavity projected area is 140 square inches. If we use the assumption of 2.5 tons of pressure (force) for each square inch of area we get a total clamping force by multiplying 140 X 2.5 = 350 tons clamping force.

1. Why is crystal polystyrene clear?

The clarity comes from the ability of light to pass through the material unimpeded by any internal barriers. Crystal polystyrene has no crystalline structure to interfere with the passage of light (it is amorphous) and generally has few additives. It is therefore transparent.

1. What is the chief economic disadvantage of recycling plastics?

The costs associated with recycling raise the cost of recycled materials too close to the value of the original virgin materials.

1. Where is the normal separation point between the material that is removed with each cycle and the material that is in the machine and used in the next cycle?

The dividing line is where the heated resin is cooled. When the resin is cold, it will naturally separate from the hot, molten resin at the sprue bushing. The nozzle is heated and so the resin within the nozzle is hot. The mold is cooled and so the material inside the mold, all the way back to the sprue, is cold. The sprue bushing is the normal separation point.

Discuss three operational steps that can be taken to reduce the thickness of an extruded part.

The drawdown can be increased by speeding up the puller with respect to the speed of the extruder. (Note that this will also increase molecular orientation.) The gap between the outlet of the die and the cooling tank can be increased which will allow some additional drawdown before the part enters the cooling tank. The sizing rings inside the cooling tank can be reduced or the vacuum inside the tank can be reduced to give a smaller diameter.

Explain why a rubber band fails after having been wrapped very tightly around an object for several months.

The elasticity of the chains decreases as they are in constant tension. This consistency in tension causes creep to happen within the material and the elastic ability of the original material degrades and often becomes brittle and breaks.

1. Why is continuous extrusion blow molding not recommended for very large blow molded parts?

The extrusion of the parison for a very large part will often take a considerable amount of time. During this time the parison may begin to sag under its own weight. It could also begin to cool and therefore become more difficult to form.

If non-melted resin particles are noted in the molded part what corrections might be made?

The first step in solving the problem would be to investigate the heating. Are the temperatures too low for this resin or are there cold spots where the heaters are not operating properly? Is the polymer being mixed homogeneously or are dead spots occurring? Is there a combination of effects of heating and mixing? Lastly, is the feed homogeneous? The presence of large and small particles in the feed will complicate the melting process because the large particles take more time and energy to melt than so the small particles. This last problem may require some separation of the particles or a homogenizing step such as re-extrusion.

1. Compare and contrast the following properties of PE and PVC, and explain each property on a molecular basis: flammability, modulus and strength, thermal resistance, effect of plasticizers, acceptability of adhesive bonding. (Note: this question can apply to any two polymers)

The flammability of PVC is largely dependant on the chlorine that is acting as a pendant group. As temperature raises these atoms begin to break off along with some of the carbon and hydrogen atoms. The difference is that the chlorine atoms act as an oxygen suppressant and deny the flame the oxygen it needs for flames to propagate. PE does not have these extinguishing atoms and will have a much greater ability to propagate the flames. The pendant group of chlorine will also increase both the modulus and strength of the PVC over the PE. This is largely due to the increased entanglement caused by the pendant group. The thermal resistance of PVC will also increase with the chlorine group. The interlocking and entanglement between chains will help maintain the thermal stability. Because of the pendant chlorine the crystallinity will decrease and will increase the polymer's susceptibility to plasticizers. This particular pendant group is also a highly polar group. This will increase the polarity of the polymer as a whole. A highly polar polymer will always be more prone to adhesive bonding than a moderately inert polymer like polyethylene.

1. What is the major advantage of a flash-type mold-closure system in compression molding? What are the disadvantages?

The flash type closure system is much simpler than the positive and semipositive systems. Therefore, mold costs are lower. However, the part dimensions are less well defined and, of course, flash must always be removed from the parts.

6. During the operation of an extruder in making thin-walled tubing, several defects have been encountered that appear to be small globules of resin. Investigation of these globules reveals that they will soften but not melt when heated. Explain what these globules might be and suggest two possible remedies for eliminating them.

The globules are likely crosslinked resin that has, for some reason, been created within the extruder. One likely cause is that the extruder may be overheating the resin, thus causing some of the material to crosslink. A solution for this would be to lower the temperature. Another solution would be to add thermal stabilizer additive to the resin which could also prevent this crosslinking from occurring. Another cause of the formation of crosslinked material could be non-streamlined flow in the die. This can be investigated by stopping the extruder and carefully examining the die to see if material seems to be hanging up in some location. If that is the case, the die should be smoothed.

Identify three reasons why plastics have been slower to be recycled than aluminum cans.

The greater variety of plastic materials has led to difficulty in sorting; the greater value of recycled aluminum and the low differential between plastic virgin and recycled materials; the ease of separating aluminum cans (obvious shape) and the corresponding difficulty in separating some types of plastics by shape and type.

List three considerations that must be taken into account in the placement, number and size of the vent holes in a female mold used for fundamental (straight) vacuum forming.

The holes should be as large as possible to allow fast application of the vacuum without showing through onto the surface of the plastic. Soft plastics or those that are heated to high temperatures will probably require smaller holes. The holes should be placed as far apart as is practical while assuring that all areas of the plastic sheet are drawn tightly against the cavity walls. Holes should be placed in any corner or other area where webbing or bridging might be a problem.

1. Explain why copolymers of ethylene and propylene can be elastomers when the pure polyethylene and pure polypropylene are not elastomeric. Discuss whether you would expect a blend (instead of a copolymer) of polyethylene and polypropylene to be elastomeric.

The homopolymers of ethylene and propylene are nonelastomeric because they both have high crystallinity. Even low molecular weight polyethylene has considerable crystalline content. However, when the materials are copolymerized, the ability of the polymers to pack together tightly is significantly reduced when compared with the homopolymers. (the copolymer is much like atactic polypropylene which is very soft and flexible.) A blend of polyethylene and polypropylene would likely have regions of crystallinity and would not, therefore, be elastomeric.

1. Why do plastics heat up when they are irradiated?

The irradiation process bombards the plastic material with much energy. Some of the energy is absorbed in the reactions, such as crosslinking, which occur within the polymer. The remainder of the energy can, therefore, be absorbed by the polymer. This absorption would cause an increase in the temperature of the polymer.

1. Why are injection molding machines not as effective for mixing additives or other resins as are traditional extrusion machines?

The length-to-diameter ratio (L/D), which is a measure of the ability of the machines to mix thoroughly, is much lower for injection molding than for extrusion. Injection molding machines are simply not long enough. Furthermore, the mixing intensity of injection molding is lower than in extruders.

1. Identify five factors that can be varied to optimize the machining of plastic parts.

The machining of plastics can be optimized by varying tool shape (design), feed rate, cutting speed, process conditions (cutting fluid, and so on), and depth of cut.

1. Explain why low molecular weight polyethylene is often added to UHMWPE.

The major problem with UHMWPE is the difficulty in melting and processing the material. By adding a low molecular weight polyethylene, the material will melt and lubricate the higher molecular weight mass, thus allowing the material to be processed even though it is not fully melted.

Two crosslinkable polyesters are being considered for use as the resin in making a fiberglass-reinforced tank for processing various chemicals. One polyester is made from a glycol having four carbons between the alcohol groups and the other is made using the same acid but from a glycol having eight carbons between the alcohols. Compare the physical, chemical, and mechanical properties of the two resins. If the chemicals being processed are water-based, which resin is the best candidate on the basis of chemical resistance?

The material containing the four carbons will have a higher crosslinked density and as such will have a higher shrinkage and stiffness and resistance to chemicals than the material containing the eight carbon bonds. For the same reasons as mentioned, the four carbon bonded polyester will have a better resistance to the water-based chemicals.

What are the three necessary and sufficient criteria that must be satisfied by all plastics materials?

The material must be (a) composed of very large (polymeric) molecules, (b) must be synthetic, and (c) must be formable.

1. What are the structural features in the melamine monomer that lead to the very high hardness of cured melamine resins?

The melamine monomer has three amine groups attached to the ring, with each amine group containing a nitrogen and two hydrogens. Each of these nitrogens is capable of being a crosslinking site. Therefore, the hardness in melamine plastics comes from the very high crosslink density that is possible because of the multiple crosslink sites.

1. Give two possible causes for a thermoformed part sticking to the mold.

The mold could have an undercut on which the part might be hung-up. The vent holes might be so large or the material so soft when entering the mold that it penetrated into the vent holes and these are holding the part in the mold. The part may have shrunk into a male mold. The draft angles may be too small for easy release. The mold surface may be rough so that the part could have mechanically entrained into the mold surface or the surface may be so smooth that the part is electrostatically attached.

1. Describe the defect that will most likely occur if the press used for transfer molding is undersized in clamping pressure.

The molds will likely be separated by the injection pressure and excess flash will result.

1. List three advantages of the moveable system versus the rotary system for continuous extrusion blow molding.

The moveable mold system has fewer molds than the rotary system and therefore less investment. The moveable mold system is less complicated mechanically and is therefore less costly to buy and to maintain. The moveable system need not be synchronized as precisely as the rotary system.

1. Discuss the temperature profile shapes normally used for nylon and polyethylene when using a general-purpose screw. Which is higher in actual value? What are the slopes of each?

The nylon temperature profile is higher because nylon melts at a much higher temperature than polyethylene. However, when nylon reaches its melting temperature, it melts over a narrower range. With a general purpose screw, normally the feed section would be shorter than with the ideal nylon-type screw. Therefore, the nylon temperature in the feed zone would be higher to compensate for the reduction in the adiabatic heating and the time in the feed zone. The compression zone in the general purpose screw is longer than in the nylon-type screw and therefore the heat can be reduced in the compression zone. The net result is a declining heat profile for nylon when a general purpose screw is used. The opposite is true for polyethylene. When using a general purpose screw, the heat profile for polyethylene increases.

6. Explain why outside-diameter dimensional control is easier than inside-diameter dimensional control for extruded pipe.

The outside diameter is usually set by forcing the still-soft extrudate outward against forming rings in the cooling tank. To obtain dimensional control for the inside diameter, a mandrel needs to be extended into the cooling bath for sufficient length that the extrudate will cool while in contact with the mandrel. While this is conceptually simple, actual practice is quite difficult. Some factors that must be allowed for are the thermal contraction of the pipe, difficulty in start-up, and adherence of the material to the cold mandrel.

What key processing considerations must be met in order to use PET to make soft-drink bottles?

The parison must be formed as an amorphous material. Therefore, it is formed from hot resin and quenched quickly. Then, the material is heated to about 30°C above the Tg and stretch blow molded. This stretching gives biaxial orientation and induced crystallization which increases many desirable properties of the bottle.

1. What factors are likely to determine the maximum size part that can be blow molded?

The parison of the part must be able to be formed without excessive deformation under the weight of the parison itself and must be formed within the time that the resin is still hot. This is determined by the melt strength of the resin, the speed of formation of the parison, and the temperature. Also important are the wall thickness and the blow-up ratio.

1. Discuss the differences in resin orientation that would be expected in a part made by transfer molding and a part made by injection molding.

The part made by transfer molding will be made from a relatively low viscosity thermoset resin whereas the part made from injection molding will be made from a much more viscous melted thermoplastic resin. Therefore, the orientation of the molecules that results from flow within the runner system and in the mold is expected to be much higher in the injection molded part. (The high viscous resin resists flow more than the low viscosity resin, thus requiring more pressure to move it and, therefore, more orientation is created.)

1. Why does Deming say that most of the quality problems are in the system?

The performance or quality of any component is based on its contribution to the aim of the system and not for its individual production or profit. Within the system all the components work together to, as supported by upper management, attain the aim of the system. Without the aim there is no system. In order to improve a system the inner components have to be evaluated as they relate to the aim of the system. For this reason most of the quality problems are in the system.

1. What are the purposes of the resin in parts with long fibers at high fiber concentrations?

The plastic resin is the matrix in these composites and its purpose is to bond the fibers together, give shape to the part, and protect the fibers from adverse environmental conditions.

1. Why are most plastics actually beneficial in a landfill?

The plastics are inert and therefore stable. They do not degrade and cannot, therefore, cause pollution of ground waters.

1. Indicate three advantages of clear polycarbonate sheet over clear acrylic sheet.

The polycarbonate is tougher, stronger, and more scratch resistant

6. Why do the cross-sectional shapes of the die orifice and the final (cooled) extruded part sometimes differ?

The polymer material flows unevenly through thick and thin sections. Therefore, if a sharp corner is cut in the orifice, the corner will restrict the flow and the part will have a rounded corner. To obtain a part with a sharp corner, the orifice should have some restriction in the flow of the middle of the part that will push the flow to the corner relatively.

1. Why can blow molding molds be made out of aluminum, whereas injection molding molds are usually made out of tool steel?

The pressures associated with blow molding are far less than the pressures in injection molding. Therefore, deformation is less likely. Another reason for the ability to use aluminum in blow molding is that the material does not flow along the mold surface but rather is pressed against the surface. The flow of the plastic along the surface, as occurs in injection molding, is abrasive and wears out the mold. One further reason that aluminum can be used is the lower temperature of the material when it touches the mold. The high temperatures of injection molding resins when they enter the molds cause thermal shrinkage and contribute to abrasive wear.

Discuss why melt pumps are rarely used with twin-screw extruders.

The purpose of the melt pump is to insure that a specific and dependable amount of material is moved through the equipment downstream of the melt pump. It is a positive displacement pump. The action of a twin screw extruder is also that of a positive displacement pump, although not as positive as a melt pump. Nevertheless, the action of both is similar and, therefore, not needed together.

1. Why do you think cobalt radiation can penetrate farther into plastics than can electron beam radiation?

The radiation from radioactive cobalt is g-particles. The high penetration of these particles, when compared with electrons in an electron beam, is due to the high energy of the g-particles.

What is the major limitation on the distance between walls in a double-walled carrying case made by rotational molding?

The resin powder must be able to flow freely between the walls or non-uniform walls will result.

1. Describe the Izod toughness test.

The sample (cut to the correct dimensions) is held upright in a rigid holder. A pendulum is swung so that it impacts the sample. The energy which was absorbed by the sample is identified by the extent of rotation of the pendulum after the impact.

1. What two measures are used in rating the size of an injection molding machine?

The shot size and the clamping pressure are the important measures of capacity of an injection molding machine. (1) The shot size is the amount of material that can be injected. (2) The clamping force is the amount of pressure available to withstand the opening forces generated from the injection of the resin and the continuing pressure placed upon the part during the molding cycle.

Identify three ways to ensure that an injection-molded part stays on the B side of the mold when the mold opens.

The sprue puller has an undercut that holds the part against the B side. The B side has a smaller taper than the A side and so the part does not pull away from the sides as readily. The B side is cooled and that causes the part to shrink against the B cavity.

1. In your attempt to mold a part in shorter molding cycles, you have been lowering the temperature of the mold using chilled water. You find that the strength of the acetal part has decreased significantly under the cooler mold conditions. What is the cause of the problem?

The strength of an acetal part relies greatly on its crystallinity. The crystalline areas form as the part cools. While still warm the amorphous regions of the part have enough energy that they can move locally and form into crystalline areas. When the speed of cooling is increased, there is less time for these various crystalline areas to form. The parts will maintain an amorphous nature.

Identify one major advantage or use for the following gate types: submarine, edge, and ring?

The submarine gate is used when the part is to be removed automatically from the runner system. The edge is the least expensive gate type. The ring gate is for parts that are cylindrical and it allows flows in the part that are non-turbulent. It may also allow knit lines to be eliminated.

1. Name and contrast four treatment methods for increasing the adhesive capability of a plastic surface.

The surface can be treated with chemicals, plasma, flame, or corona. Chemical etching is not effective on all plastics and so its application is limited. Plasma is almost always effective, but plasma machines are expensive and some experimentation is needed to develop the correct gas and machine parameters to get optimal results. Both corona and flame are inexpensive and usually effective. Corona is somewhat faster than flame, but corona often etches the surface.

1. What is the purpose of a vacuum surge tank?

The surge tank, when evacuated, provides a location for the air to flow when the tank is opened to a closed chamber. The atmospheric pressure pushes the material out of the chamber and into the surge tank. Without the surge tank, the chamber would have to be evacuated by a pump which can be a rather slow process.

1. Studies have found that the rate of crosslinking of a thermoset material can be increased when both electron beam irradiation and thermal heating are used together. Explain why this is so.

The thermal heating gives the molecules more movement and this movement can make the crosslinking reactions more effective and more rapid because there are more collisions between the molecules and, therefore, more reactive occurrences.

Why is there a limit on the thickness of parts that can be reasonably made by rotomolding?

The thickness of the material determines, to some extent, the length of the heating cycle. If too much time is required to properly heat and fuse all of the material, some resin may begin to degrade. Hence, the limit on thickness is imposed by the maximum length of time before degradation occurs.

1. In a blow molded part, where are the thinnest sections likely to occur?

The thinnest sections will occur in the areas where the material must move the farthest to fill the mold. The most common location for these thin areas is therefore the corners.

1. Why is the thickness of a section fixed when it touches a solid object such as the walls of the cavity or the plug?

The thinning process is a movement of material due to stretching. When the movement stops so does the stretching. Expansion of the material into a mold may attempt to draw material that is touching a solid object, but the resistance to movement of that material is much higher than the resistance of free material, so the free material moves and therefore stretches

1. To what does total product life-cycle costing refer?

The total cost of the product is really a function of the costs associated with both making and disposing of the product. These total costs should include the costs of the product on the environment, that is, the pollution potential and costs to eliminate that pollution if it were to be corrected.

1. How can a two-part adhesive be monitored to ensure proper mixing of the two components?

The two parts can be colored differently and when mixed can yield yet a third distinct color. This allows good monitoring of the mixing of the materials.

1. What advantage does a two-part adhesive system have over a one-part system?

The two-part system is stable until the parts are mixed. Therefore shelf life is likely to be much longer than for a one-part system.

1. What is the origin of the variety in the properties of polyurethanes?

The urethane bond is easily formed with little consequence of the groups that are between the active groups on the polymers. Therefore, a wide variety of structural shapes are possible for these groups. These groups determine the basic properties of the polyurethane and, therefore, give rise to the variability.

1. What is a vent in the mold, what problems are prevented by the presence of a vent, and what parameters control its size?

The vent is a small opening from each cavity that allows air to escape as the resin enters. If the vent were not present, hot air could be trapped and burn the resin as it enters, thus causing a discoloration. The air could also build to such a pressure that the resin may be prevented from completely filling the mold. The vents should be large enough that the air will freely leave the cavity with the injection of the resin, but not so large that the resin will flow through them.

1. Discuss why acetals are generally not made into fibers.

The very high crystallinity of acetals makes them brittle and, therefore, not suitable for fibers.

1. Explain why very short fiber composites tend to be made with thermoplastic matrices while longer-fiber composites use thermoset matrices predominately.

The very short fiber composites are made with thermoplastic matrices because of the low cost of manufacture if traditional thermoplastic manufacturing methods are used. Furthermore, the applications for these materials are generally well suited to the thermoplastic composites, with additional strength or stiffness by the fibers. On the other hand, composites with long fibers are made from thermoset because of the difficulty of wetting-out the fibers. The additional stiffness and higher temperature capabilities of thermosets also contribute to their use.

1. Explain how to determine if a packaging material is appropriate for a particular delicate instrument.

The vibrational characteristics, especially the natural vibrational frequencies, are determined for the material by sensing the forces transmitted through the material at various frequencies. The natural frequencies of the material and other frequencies at which high stresses are transmitted are then compared to the natural frequencies of the transportation system that will carry the package. If the frequencies of the transportation system match the natural frequencies of the packaging material, and if the stresses transmitted through the material are greater that the stresses that would be required to damage the delicate part, then that packaging material is probably not appropriate.

Describe why the weight of the resin charge is so important in part performance.

The weight of the resin determines the wall thickness. The wall thickness has a major effect on the length of the heating cycle. Because plastic is not a good thermal conductor, even small increases in thickness required disproportionate times for heating. If the heating is too long, degradation can occur. If the thickness increases without the knowledge of the machine operator, the last material to adhere to the surface may not get hot enough to fully fuse. Therefore, small changes in weight can affect the heat cycle and part physical properties

1. A resin supplier has suggested that a blow molder of polystyrene parts add 20% polyethylene to improve properties. What property differences would you expect? What processing differences would you expect?

There would be an expected increase in toughness and a better chemical resistance. For processing: reduce the screw speed in the extruder because the material will flow better and to ensure the right amount of material entering the mold.

1. What is meant by a stage B resin?

These are thermoset resins which have been polymerized but not crosslinked. The resins are B-staged to improve the handling and control the cure.

. What is the purpose of cold-well extensions?

These extensions of the runners are placed at points where the runners change directions and are a means of capturing the cold, leading front of the resin. By capturing the cold end of the resin flow, some assurance is given that the resin will flow smoothly through the gate without plugging.

1. Why is it important to have the sections of the molded part as uniform in thickness as possible?

Thick sections cool more slowly than thin sections because of the time required for the heat to move through the plastic material. The cooling occurs from the outside toward the middle because the heat is transferred most easily from the outside. Therefore, the inside is liquid after the outside is solid. Because of the natural shrinkage that occurs upon cooling, the liquid can still shrink whereas the outside, which is solid, cannot. This may cause a sink mark over the center section as it cools and shrinks. This problem can be solved by making all the sections of the same thickness.

How can on-time delivery affect profits?

This aspect can greatly increase profits as it is viewed as a basis for customer satisfaction, not only with the product itself, but with the service from the company.

1. Identify three methods for controlling an exotherm in casting.

Three methods for controlling an exotherm are making all sections of a part thin, casting thick parts in multiple pours, and minimizing the heat created by lengthening the curing time.

1. What is the essential reason for safety?

To eliminate the possibility of catastrophe.

A deep cup (20 cm deep x 10 cm diameter) is to be formed. Compare thermoforming, blow molding, and injection molding as processes for making the cup. What technical and economic considerations should be considered in determining which method is best?

To make such a long, narrow part by injection molding the mold must be quite large and the flow to fill the mold would be difficult. Blow molding, on the other hand, would be ideally suited technically because the parison would be long already and forming to the shape indicated would be fairly simple. Thermoforming, too, would be technically simple except for the possible problem of uneven sides where such a deep draw relative to the diameter is required (depth ratio = 2:1). The economic considerations between blow molding and thermoforming are largely based upon the higher capital costs associated with the blow molding and injection molding processes. Therefore, justification for the higher costs must come from higher volume. Specific costs of equipment should be weighed against the number of parts to be made to give some measure of the break-even point.

Milk bottles are sometimes annealed after blow molding. Why is this done?

To normalize the stresses that are induced by the molding process

1. Why is it advisable to have only one or two suppliers?

Too many suppliers complicates getting needed parts or material on time which complicates getting finished products to customers on time. It also adds more individuals in the process if there are any problems with the materials received or products made.

True or False. Both closed or open celled foams can be either flexible or rigid.

True

A submarine driveshaft was made of epoxy and carbon fibers by filament winding. Indicate two reasons why composites have unique and excellent properties for this application. The part was cured in an oven at 250ºF (120ºC). On close examination, the shaft was found to spin irregularly and to cause vibrations. Identify four possible causes of this problem and four possible solutions.

Two advantages: an anisotropic material, easier to design a part in composites that will withstand the torsional forces placed upon it. Four possible causes: unbalanced fiber placement, uneven weight distribution, inadequate strength for the forces applied, insufficient material (dampening of vibrations may not be sufficient for that application).

6. You are responsible for the extrusion of a thin PE rod and find customers are complaining because it is splitting longitudinally after a relatively brief time in service. Indicate two changes you would make in each of the following: resin, extrusion conditions, postextrusion treatment. Explain.

Use LLDPE, use less filler or different fillers, run the material processing speed slower, crosslink the material, stretch as little as possible. Splitting is probably because the molecules are highly oriented with very little intermolecular strength across the alignment so if there is a material with better knitting it will increase the strength. If the process is run at a lower speed, it doesn't induce as highly oriented molecules as it does at higher speeds. Post-extrusion stretching of the material also induces further orientation and decreased cross-directional strength, crosslinking will allow for that strength to be returned.

1. Discuss three ways to alleviate the sticking of a part to the mold.

Use mold release. Use cold air to cause the part to shrink away from the mold and to lift the part away from the mold. Use dry ice to shrink the part. Change the molding conditions to reduce degradation.

1. Which material EVA or PS, would be expected to be a better barrier material against water? Why?

Water is a polar molecule so it would be excluded best by a non-polar polymer. EVA has many polar atoms in the pendent group and would allow water to permeate easily. Polystyrene is nonpolar and would exclude polar molecules like water. The size of the side chain for EVA is large, as is the pendent group for polystyrene. Both plastics would have about the same openness so size would not be a factor in determining the permeation differences between these two plastics. Therefore, polystyrene is a better barrier material for mater than is EVA.

1. What is the curing agent (that is, the material that initiates the reaction) for cyanoacrylates?

Water, usually the moisture in the air.

1. What is the mechanism and thermodynamic explanation for why elastomers recover to their original shape after being elastically stretched?

When an elastomer is stretched the molecules become more ordered. This causes a decrease in entropy. Since entropy naturally tends to increase, when the stretching force is removed the natural tendency of entropy to increase supplies the energy for the molecules to return to their more energetically favored, random configuration.

Why is an octet of electrons a stable configuration?

When both the s and p orbitals of an atom are filled, a certain stability is achieved. There are two electrons which fill the s orbitals and six electrons that fill the p orbitals. Therefore, eight electrons will fill both, giving a stable configuration.

1. The surface of polyethylene after being treated with cold plasma using oxygen as the working gas is similar to the type of surface created when a spark is used to etch the surface of the same polyethylene material. Explain this similarity.

When the plasma reacts with the surface and oxygen is the working gas, the charged oxygen atoms react with the surface of the polyethylene to attach OH groups on the surface. OH groups are also attached to the surface when a spark is used to etch the surface because the high energy of the part breaks apart the oxygen in the air and ionizes it, thus causing an active site which can react with the polyethylene in much the same way that the plasma does.

1. Wire and cable material is often crosslinked by electron beam irradiation. Why is this material crosslinked?

Wire and cable insulation is susceptible to stress cracking. Irradiation to cause crosslinks will tie the molecules together and prevent this stress cracking form occurring.

1. Discuss the pressure capabilities of vacuum forming relative to pressure forming.

With vacuum forming the maximum pressure available is the difference between the pressure in the evacuated chamber and atmospheric pressure. Hence, the pressure is limited to a max of one atmosphere. With pressure forming any pressure can be applied, although normally very high pressures are not needed. The pressures used in pressure forming, however, would always be higher than those used in vacuum forming.

1. You have been assigned to select a polymer for a wall plug. Your choices are PET and Polypropylene. Discuss what you would expect in electrical properties, what physical property requirements would be required, and which polymer might be the best choice.

You would expect the complexity of the PET chain to significantly decrease its conductivity compared to polypropylene. The crystallinity of polypropylene may also slightly increase the conductivity of the PP. The physical requirements would have to do with the forces that are applied on the wall plug. The main forces are applied when an appliance is unplugged from the wall socket. These forces might include tensile, flexural and maybe even torsional forces, all of which would need to have greater cyclic strength than single instance, max load strength. PP would be the most logical choice for the wall plug for reasons such as resistance to stress cracking and lower costs as compared to the PET.

1. Adhesion to polyolefins is often discouraged because of a. Low surface energy b. Lack of pendant groups c. Low carbon content d. Low Density

a.

Cryogenic flash removal consists of a. Freezing the part and then putting it in a rotating tub where the flash is thus broken off through the tumbling motion b. Removing the flash through the use of lasers c. Freezing the part and then dropping it on the floor d. Heating the part until the flash degrades e. Cyclic heating and cooling the part so as to induce internal fatigue

a.

In finishing plastic parts, one of the biggest concerns is flash removal, the amount of flash needed is generally dependent on a. Mold design b. Process choice c. Material choice d. Processing speed

a.

Slush molding refers to a. A mold being filled with a plastisol and then being rotated and heated. b. Another form of rotational molding c. using the "slush" or left over material from previous moldings and using it in a finished product. d. Another term for crystallinity occurring between two recycled materials.

a. A mold being filled with a plastisol and then being rotated and heated.

1. List three critical elements in maintaining a safe workplace.

a. A well-established and agreed-upon set of safety rules and standards b. A clean and neat workplace c. Each employee has a strong attitude of safety in their personal life and in the workplace.

1. All of the following are standard methods for processing elastomers except a. Blow molding b. Dipping c. Preforming d. Molding

a. Blow molding

. Flash is a more common defect in: a. Compression molding b. Transfer molding c. Both d. Neither

a. Compression molding

Products that are blow molded such as milk jugs can also be made by rotational molding, the main reason that they are not however is because: a. Cost, due to cycle time b. Cost, due to equipment overhead c. Appearance of final product d. Colorability

a. Cost, due to cycle time

. The most important parameter to pay attention to while thermoforming is: a. Depth of draw b. Surface area c. Thickness d. Circumference

a. Depth of draw

Which plastic is relatively the most expensive? a. Polytetrafluoroethylene (PTFE) b. Polystyrene (PS) c. Polyethylene (PE) d. Polycarbonate (PC)

a. PTFE

9. When comparing costs, because of part cycle times a. Thermosets are generally more expensive to mold, per part b. Thermosets are generally less expensive to mold, per part c. Thermosets and thermoplastics take about the same time to mold and so they always cost about the same to mold

a. Thermosets are generally more expensive to mold, per part

6. A gear pump is used in extrusion when a Consistent flow is necessary and the extruder isn't precise enough to give you that flow b The extruder can't process enough material fast enough, the pump is used to pull the material out faster c The extrusion screw or die is set up to extrude upward, the pump is used to oppose gravity and get the material to come out of the screw d The extruder is almost worn out and you are trying to get the machine to last long enough to purchase a new machine.

a. consistent flow is necessary and the extruder isn't precise enough to give you that flow.

Typical injection molds are more expensive than the tooling used in other processes. The injection molded parts can be produced and sold cheaper than parts from other processes because a. Injection mold cycle times are significantly shorter b. Resins for the injection molding process are significantly cheaper c. There is never any scrap in the injection molding process like there is in other processes

a. injection mold cycle times are significantly shorter

1. At the molecular level the engineering thermoplastics are generally a. More complex than most commodity resins b. More simple than most commodity resins c. More polar than commodity resins d. More non-polar than commodity resins

a. more complex

2. RTM stands for a. Rotate and Tip Molding b. Resin Transfer Molding c. Resin Treatment Molding d. Rotate Transfer Molding

b.

In filament winding a standard mold is not used but the device that the fibers are wrapped around is called: a. Matched-die b. Mandrel c. Mandable d. Clamshell

b.

Microwave curing is much faster than thermal curing because a. The enclosure is lined different in a microwave than it is for a conventional oven. b. The frequencies are tuned to match the excitation frequency of the polymer molecules c. The timer on the microwave system runs quicker d. The calibration for the microwave is set higher than the conventional oven.

b.

The method of joining that uses ultrasonic energy to soften the end or a post on a plastic part that protrudes through a hole in a part to which it is to be joined is called: a. Stamping b. Staking c. Stacking d. Sintering

b.

The term scission refers to: a. Crosslinking through use of UV rays b. Breaking of the main backbone of the polymer c. Breaking off of large pendant groups from the backbone of the polymer d. The accidental breakdown of materials with radiation exposure.

b.

A significant disadvantage of thermoforming as compared to other plastics processes is: a. Insufficient part formation is a more common problem than with other processes. b. The greater amount of scrap that it generates. c. You can only use HDPE for thermoforming d. Because of toxic gasses, the government only allows thermoforming plants in locations in the southern portion of the United States.

b. The greater amount of scrap that it generates

When high temperatures or direct flame is applied to a plastic part that is made from a thermoset material the typical response is what? a. Bursting into flames b. Blackening and charring c. Melting d. Nothing, thermosets are impervious to heat

b. blackening and charring

9. One aspect of molding thermosets is that a. Thermosets can't be molded in an open mold system b. Thermosets can be molded in either open or closed mold systems c. Thermosets are never molded in the same processes as thermoplastics

b. can be molded in either open or closed mold systems

6. In order to reduce the wall thickness of an extruded part you could do all of the following except a Create a new die b Cool the extruded part sooner c Pull the extrudate through the process at a higher speed d Set up a sizing plate system for the new part

b. cool the extruded part sooner.

All of the following parts are commonly injection molded except a. Garbage receptacles b. Dishwashing soap bottles c. Syringes d. Sunglasses

b. dishwashing soap bottles

To get the best results in rotational molding, the base material should always be: a. liquid b. powder c. pellet d. polymer fibers

b. powder

1. The main difference between extrusion blow molding and injection blow molding is: a. The size of the machinery b. The existence of a preform c. The resin drizzle d. The pressures involved in the process

b. the existence of a preform

1. Elastomeric materials get their name from a. their chemical base which is derived from latex, the first material used in elastic rubber bands b. the fact that their elongation at the ultimate elastic limit is much greater than most materials c. the province in Brazil where rubber was first discovered d. the explorer who first discovered latex rubber was Romeus Elastos

b. the fact that their elongation at the ultimate elastic limit is much greater than most materials.

1. Many of the first elastomeric materials were crosslinked because a. that is what gives the materials their elastic nature b. they are derived from natural products and they didn't have enough strength if they weren't crosslinked c. they would become water soluble if they weren't crosslinked d. crosslinking was a new process that was being used in many industries and the companies working with elastomers wanted to capitalize on the concept

b. they are derived from natural products and they didn't have enough strength if they weren't crosslinked.

3. Composite materials typically show their greatest strength under which loading conditions: a. Shear b. Compression c. Tensile d. Torsion

c.

The most common post-irradiation reactions include all of the following EXCEPT: a. Cracking b. Embrittlement c. Elongation d. Discoloration

c.

When assembling plastic parts using adhesion it is important to identify the material for all of the following reasons EXCEPT a. Material properties, such as flexural strength are taken into consideration b. The adhesive does not degrade/deform the base material c. The adhesive might evaporate before an adequate bond can be formed d. The adhesive bond could potentially be inadequate for the part.

c.

1. Common closed-cell products include all of the following EXCEPT: a. Styrofoam dinnerware b. Life jackets c. Cleaning sponge d. Surf board core

c. Cleaning sponge

What is the main reason for using a plunger in injection blow molding: a. To prevent the material from flowing out the top of the mold. b. Acts as a cooling bar for the material c. Help stretch the material into the proper orientation for strength properties d. To unclog the die as the plastic cools

c. Help stretch the material into the proper orientation for strength properties

1. In general, the more crystalline a sample is the more opaque it appears. Which resin is the main exception to this rule? a. PTFE (Polytetrafluoroethylene) b. PA (Polyamides or Nylons) c. PET (Polyethylene Terephthalate) d. POM (Polyoxymethylenes or Acetals)

c. PET

The process whereby large foam buns are cut into smaller pieces is known as a. Hacking b. Chopping c. Skiving d. Sintering

c. Skiving

In compression molding material is places directly in the mold before the mold is closed, the difference between this and transfer molding is: a. The type of resin used (thermoset vs. thermoplastic) b. There is no difference, terminology is dependent on where you are c. The resin is placed inside a transfer chamber d. Transfer graphics are placed in the mold before molding compounds

c. The resin is placed inside a transfer chamber

Rotational molded parts are typically larger than injection molded parts because: a. Rotational molding requires more pressure so larger machines are required b. Injection molded parts are easier to assemble, so larger parts come from later assembly of separate injected parts. c. The resin used in injection molding needs to be melted in the screw prior to being inserted in the mold. d. It is impossible to remove large parts from an injection molding machine.

c. The resin used in injection molding needs to be melted in the screw prior to being inserted in the mold.

What is drape forming? a. The process of making plastic drapes for household use in the kitchen. b. Forming by hanging sheets from rods. c. Thermoforming using a male mold d. Name of the process for the sag that the sheet makes as it is heated in the thermoforming process.

c. Thermoforming using a male mold

Which of the following are typically blow molded: a. Truck bed liners b. Electronic casings c. Water bottles d. Kiddie swimming pools

c. Water bottles

. The size of an injection molding machine is most often based on the machine's a. Layout footprint in the production facility b. Volume capacity of the hopper system c. Available clamping pressure d. Size of the mold that will fit between the tie bars of the machine

c. available clamping pressure

The four principle steps in rotational molding include: a. heating, remove excess slush, rotate, cooling b. cooling, rotate, heat, cutting c. loading, heating, cooling, unloading d. heating, unloading, remove excess slush, cutting

c. loading, heating, cooling, unloading

1. Which of the following would not typically be a reason for choosing an engineering thermoplastic resin over a commodity thermoplastic resin? a. Strength b. Durability c. Price d. Abrasion resistance

c. price

1. The main difference between silicones and most other elastomers is that a. most other elastomers are derived from natural rubbers or from thermoplastic materials b. silicones are materials that have high filler content, specifically silicone carbide powders to increase the hardness of the elastomers c. silicones have the element silicon along the backbone of the polymer chain instead of carbon

c. silicones have the element silicon along the backbone of the polymer chain instead of carbon.

1. One of the key functions of radiation treatment on plastics is to a. Kill off unwanted bacteria in the parts b. Post process size adjustments c. To prevent shrinkage d. Post-process crosslinking e. To prepare polyolefin parts for printing applications

d.

1. Pultrusion is different from extrusion in all the following EXCEPT a. Type of resin used b. Existence of fibers c. The material is pushed through the machine d. The material is pulled through the machine

d.

Which of the following is NOT an example of a part that is thermoformed a. Truck bed liner b. Trash can c. Clam shell food containers d. Plastic eating utensils

d. Plastic eating utensils

6. Which of the following is not a purpose of the breaker plate and screen assembly in extrusion: a Remove defective materials from the process b Remove particles from the process that can affect the final product c Maintain even pressure and part wall thickness consistency d Keep operators from scratching the screw as they clean out the dies

d. keep operators from scratching the screw as they clean out the dies.

9. One of the main reasons that thermosets are used in composites over thermoplastics is a. Thermosets can be stored in a freezer until it is time to use them b. Thermosets can be colored more easily c. Thermosets are always more UV stable d. Thermosets can be applied to fiber reinforcement with a low viscosity and then crosslinked later to the desired material properties.

d. low viscosity and then crosslinked later.

1. Casting is most often done with a. Polyethylene b. Polyolefins c. Thermoplastics d. Thermosets

d. thermosets

One common use of foams is that of seat cushions, the reason that foam is used instead of other materials is because of the foam's ability to a. Absorb energy b. Reduce friction c. Reduce vibration d. Insulate e. All of the above f. a, c, and d g. c, b, a

f. a, c, and d

17. True or False. All plastics, thermoplastic or thermoset, will degrade at some temperature?

true

Identify the type of bond and the product formula expected between potassium (K) and bromine (Br), and explain the basic nature of this bond. Show the resulting outer electron configurations of K and Br after the bond is formed.

v