MSE 3190 exam 2
Advantages of injection blow molding over extrusion blow molding
1. No trim scrap 2. High dimensional accuracy 3. Uniform wall thickness 4. No seam lines/pinch marks 5. Better transparencies 6. Good Mechanical properties due to stretch blow molding
Which of the following statements are true about thermoplastics: 1. Thermoplastics are solid materials at room temperature 2. Thermoplastics are viscous liquids at a few hundred degrees 3. Thermoplastics cannot be subjected to repeated heated and cooling cycles 4. Thermoplastics can be shaped in a wide variety of products
1. Thermoplastics are solid materials at room temperature 2. Thermoplastics are viscous liquids at a few hundred degrees 4. Thermoplastics can be shaped in a wide variety of products
Operational steps to reduce thickness of an extruded part
1. increasing drawdown by increasing puller speed 2. gap between the outlet of die and cooling tank can be increased (length increase = additional drawdown) 3. Sizing rings inside cooling tank can be reduced / vacuum inside tank can be reduced = smaller diameter
Steps in conventional injection blow molding
1. injection molded preform formation 2. heating of preform in oven to an appropriate temperature 3. air is injected and preform is stretched length wise 4. cooling and ejection
Difference between injection molding of thermoplastics and thermosets.
1. low temperature of barrel 2. screw has small L/D ratio to avoid heat generation and premature curing 3. high temperature of mold (curing
Which of the following statements are true about thermosets: 1. Thermosets initially soften at elevated temperatures for molding 2. At higher temperatures thermosets undergo a chemical reaction that hardens the material 3. Thermosets can be shaped in a wide variety of products 4. Thermosets can be subjected to repeated heated and cooling cycles
2. At higher temperatures thermosets undergo a chemical reaction that hardens the material 3. Thermosets can be shaped in a wide variety of products
The geometries possible with extrusion is limited to ___ dimensions
2d
Which of the following label indicate the Stress-Strain plot of heavily crosslinked polyisoprene.
A
injection molding
A process during which plastic is heated in a machine and forced into a cavity by a screw or ram. The material solidifies and is then ejected.
tab gate
A small removable tab about the same thickness as the molded item, but usually perpendicular to the part for easy removal.
Blow Molding
A solid bottom hollow tube is placed between two mold halves and heated. The heated tube is then expanded into the sides of the mold with compressed air.
What is the principal problem when forming a bottle using injection molding?
Blow Molding has very low mold cost. Can create parts with narrow openings and broad bodies. Cycles of rotational molding are very long thus used for large hollow parts with short runs. Key cost is material cost. Can be economically used for parts ranging from few ounces to over 120 gallons. Cycles are short
What factors are used to determine the maximum size of part that can be blow molded.
Blow ratio is defined as ratio of diameter of the mold to diameter of the parison. Blow ratios between 1.5 to 3 is common. The materials used for blow molding should have high melt strength
Step-polymerization reactions which involves formation of a small by-product like water is called_
Condensation polymerization
blow molding technique
DOESN'T USE THERMOSETS
What is the sequential change in the mechanical state of an amorphous polymer as we increase the temperature.
Glass, Rubbery Solid, Viscous Liquid
High molecular weight polymers have (high/low) Melt Strength and (high/low) Melt Flow Index.
HIGH, LOW
Which of the following is an indicator of the temperature limit above which the material can't be used for structural applications?
Heat distortion temperature
injection molding advantages
High production rates, design flexibility, repeatability within tolerances, can process a wide range of materials, relatively low labor, little to no finishing of parts, minimum scrap losses
What two measures are used in rating the size of injection molding machine?
Shot Size L/D ratio Clamping Unit (amount of clamping force) is at times also used to rate the Injection Molding machine
In the blow molded part where is the thinnest sections likely to occur?
Wall thickness is usually limited to ½ inch or less. The part is at the corners.
weld line
Where melted material flows together during molding to form a visible line or lines on a finished part that may cause weakening or breaking of the component. Whenever multi-injection points are used mechanical weakness is possible at the place where resin flows meet
die
bolted to the end of the extruder, shapes the product
Injection Molding can be used for :
both thermoplastic and thermosets
A broad molecular weight distribution polymer resin makes it more suitable for processes like _______and narrow molecular weight distribution polymer resin is more suitable for __________
broad = extrusion molding, narrow = injection molding
In tires,_________ is an important additive; it reinforces the rubber, serving to increase tensile strength and resistance to tearing and abrasion.
carbon black
What are the types of twin screw extruders?
corotating and counter rotating screw extruder
stretch blow molding example
making a soda bottle (PET) 1. PET is injection molded at 250-280C). 2. preheated to 95-100C 3. stretch blow molded (stress induced crystallization)
What would you expect a heavily crosslinked polyethylene that has a glass-transition temp of 0C to be at room temp?
thermoset
ring gate
used to make hollow cylindrical parts
die swell can be related to
viscoelasticity
State one advantage and one disadvantage on use of a screw with higher L/D ratio?
Advantage: Helps to have better,mixing ability Disadvantage: heats the material longer, takes longer for the material to get through the machine, more material is wasted in changeovers.
edge gate
An injection method that uses a gate on the parting line of the mold. It typically leaves a vestige on the outside of the part and is sometimes referred to as a tab gate. Major advantage is low cost
What type of defect may be observed in Extrusion Molding when Deborah number if greater than 1? State one way to eliminate this problem.
Deborah number is related to the ratio of relaxation time to process time. If the polymer relaxation time is larger than the process time, then Deborah number if greater than one. If this happens, we observe Melt fracture in polymer processing. When the relative stress on polymer melt exceeds the melt strength, there is local fracture. When Deborah number , De is greater than one there is Melt fracture Use of streamlined die helps to reduce Melt fracture. Eliminate Melt fracture through use of low molecular weight polymer, lowering extruder speed and reduce shear.
What is the purpose of cold well extensions?
Designed to trap the cold end of the molten polymer flowing through the long runner systems. This will prevent clogging or premature solidification of the melt.
What is Die Swell ? How can we reduce die swell?
Die Swell is given by ratio of Diameter of extrudate to Diameter of die orifice. To reduce die swell we can increase Draw down ratio, Increase extrudate speed, Use polymer resin with high molecular weight, Increase the length of Die Land.
spider die
Die where the melt stream is distributed around the mandrel by a cone which is supported by ring of spokes. comes out as hollow tubes
Blown Film Extrusion
Extruding a tube of molten thermoplastic and continuously inflating it to several times the initial diameter to form a thin tubular product that can be used directly, or slit to form a flat film
T/F: Extensional viscosity can be a measure of MFI
F
T/F: shrinkage of amorphous polymers is higher than crystalline polymers.
F
T/F: thermoplastic elastomer must have some crosslinks.
F
Continuous Blow Molding is not suitable for large parts. Why?
For large parts accumulator head system is good. On use of continuous blow molding the parison formation is linked to the extrusion screw speed. As the parison is formed quiet slowly and the parision sags under it own weight resulting in distortion of shape. In accumulator head system the action of ram can be rapid to push the resin and formation of parison. With the mold closed the part is blown before the parison deforms.
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?
Gate
melting point temperature
the temperature at which a solid changes to a liquid
Define the process window in injection molding using a diagram of injection pressure versus holding pressure
If the Injection pressure is too low the resin may not be pushed completely into the cavity resulting in short shots . If the Injection pressure is too high one problem encountered is excessive orientation within the part and also flashing. This is also true if the clamping pressure is insufficient.
Material considerations for Injection Molding:
In extrusion molding we prefer resin with high melt strength ( High molecular weight distribution). In Injection molding we prefer resin with low melt viscosity and low melt strength , narrow molecular weight distribution and low molecular weight. For Thermosets: (i) We require low barrel temperature to prevent premature curing. (ii) L/D ratio is kept low. (iii) Cycles are rapid
How to reduce weld lines
Increase in temperature and injection pressure, injection speed and cycle time can lead to higher mobility of molecules and thus reduce the tendency to form weld lines. Weld lines can also form due to gate position. Thus using a gate position such that the melt flow is such that it does not form weld lines. For example, If you are designing a cup shaped mold and you use a edge gate as noted below it can form weld line or a mechanical line of weakness at the place where the polymer melt meets or joins. Instead now if we use a sprue gate it will help to avoid formation of weld line.
What are the advantages of Stretch blow molding.
It provides biaxial stretching when parison is blown into desired shape and longitudinal stretching due to use of mandrel. Temperature of the preform should be adjusted to correct temperature for orientation. Results in products with higher burst strength and impact strength. PET is amorphous and only stretching helps in orientation.
in extrusion molding it is beneficial to use polymer resin with (L/H) MFI and (narrow/broad) molecular weight distribution.
LOW, BROAD
Extrusion Molding Process :
Long, continuous products. Plastic pellets are poured into a hopper. They enter the feeding section as hard and solid particles, but are softened by plasticizing by a rotating screw that compress and forward them into heated and narrower space in the compression section. In the metering session, the polymer is completely plasticized at right temperatures and pressure for easy flow. The material then exits through a simple die opening at the end, follows the die shape and forms the extrudate. Defects: Melt flow: the irregular surface of an extrudate. This is due to the turbulent flow of plastics due to a sharp change in die. Bambooing: the cyclic defect on an extrudate.This is due to the friction between die and plastic. Ex: Tubing, Plastic Pipe, Roll of film,
Which one property makes PET highly desirable to make soda bottle
Low Gas permeability
Why are injection molding machines not as effective for mixing additives or other resins as extrusion molding machines?
Low L/D ratio of the screws 12:1 to 20:1 and comparatively low compression ratio (2:1 to 5:1) lower than extrusion.
explain newtonian and non-newtonian behavior of polymer melts
Newtonian Fluid: The viscosity of a Newtonian fluid is dependent only on temperature but not on shear rate and time. Non-Newtonian Fluid: The viscosity decreases monotonically as the shear rate is increased.
What is Nylon type screw and how is it different from polyethylene type screw?
Nylon type screw is for resins with sharp melting point and this screw has a very short compression zone. (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. On the other hand the Polyethylene type screw (lightly branched polyethylene) with broad melting range and a longer compression zone.)
What are the significant differences in equipment and operating procedures between injection molding of thermoplastics and injection molding of thermosets?
Reaction Injection Molding (RIM): involves mixing of two highly reactive ingredients and immediately injecting into the mold cavity. Solidification occurs in mold cavity due to chemical reaction (Curing). Advantages of RIM: low mold pressure, low equipment cost, can obtain specific properties. Use of extruder screw (L/D ratio, compression ratio, mold lost (high mold pressure)).
operating diagram between Injection Pressure and Injection Temperature during the Injection Molding cycle. State different problems that can be noticed if operating paramaters are beyond this operating window.
Short shot: at low pressure and low temperature flash: at high pressure thermal degradation: at high temperature
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, post extrusion treatment. Explain.
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. 1.) Change in Resin (Use LLDPE) 2.)Use less filler or different fillers 3.) run the material processing speed slower, crosslink the material, stretch as little as possible. 4.)Run the process at lower speeds ,it doesn't induce as highly oriented molecules as it does at higher speeds. 5.) post-extrusion stretching of the material also induces further orientation and decreased cross-directional strength, so do less of it. crosslinking will allow for that strength to be returned.
Which of these thermoset resins have a higher shelf life?
Stage A resins with no crosslinking (crosslinkings directly proportionate to reactivity. no cross link = no reactivity)
Which of the following is true or helps to increase crystallization ?
Stress induced crystallization
In soft drink bottles every bottle has a small hole at bottom. Why?
The blowing point is often the hole that forms the opening of the part. This can be at the top or bottom of the mold depending on the orientation of the part. Presence of bottom flash which is present when the bottom is pinched off rather than molded. This is found in Extrusion blow molded parts.
corotating screw extruder
The high shear in corotating design makes it popular for compounding operations in which additives are incorporated or multiphase polymer blends created. In twin screw extruder with intermeshed counterrotating screws it lends itself to extrusion of reactive and heat sensitive polymers. The forward motion is like gear pump. The material is propelled forward.
Key processing parameters that must be met to use PET to make soft drink bottles
The melt temperature for PET is typically 480-540 F. Temperature for stretch molding is much lower than melt temperature 200-212 F and Mold temperature 35-40F. PET should be highly pure to avoid degradation thus it is dried before use.
counterrotating screw extruder
The screws are rotating in opposite directions and thus there're is squeezing action and then it increases the pressure in the material as it is forced into the region between the screws. This is more suitable for resins like PVC.
What method is used to achieve the same part wall thickness as in programmable parisions.
The variation in thickness is achieved by timing the movement of the mandrel with the extrusion of the parison.
What type of polymers refer to materials whose elastomeric properties are due to physical connection between soft and hard phases of materials and not due to crosslinks?
Thermoplastic Elastomers
What are the two basic types of clamping units used in Injection Molding?
Toggle Clamps Hydraulic Clamps
plasticizers
additives added to a polymer to make it softer and more flexible, improve its flow characteristics during forming
Which of the following type of Die will you use for Blown Film Extrusion
annular dies
Use of a parison is associated with which one of the following plastic shaping processes:
blow molding
What gate is preferred while designing hollow parts in injection molding machine?
diaphragm gate
Deborah number
dimensionless number, often used in rheology to characterize the fluidity of materials under specific flow conditions. It quantifies the observation that given enough time even a solid-like material might flow, or a fluid-like material can act solid when it is deformed rapidly enough. Materials that have low relaxation times flow easily and as such show relatively rapid stress decay.
which blow molding technique provides more design flexibility and can produce large bottles and provides a wider choice of resins?
extrusion blow molding
What are the sections of a conventional extruder barrel for thermoplastic?
feed section, compression section, metering section
annular die
for solid regular shapes, such as round stock
die swell
hot plastic expands when exiting the die opening (viscoelastic swell). necessary to bring it to the correct size by pulling
addition of plasticizer will
improve flexibility, decrease glass glass transition temperature
Label:
in order from left to right: hopper, screw, heater, extruder, sizing plate, water tank, wind-up machine, cutting/removal
increasing molecular weight of polymer chains
increases melt strength
If you want to design tiger cub figurines which of the following method, you will use.
injection molding
which ones can design automotive fuel tanks?
injection molding, blow molding
phenolic resins
largest volume of thermosets which comprise the market (6%)
LABEL
lowest one: crystalline middle one: semi-crystalline highest one: amorphous singular line: liquid
melt fracture
material experiences random fractures, typically throughout extrudate and not just on surface. It happens when the tensile forces > critical shear stress/ shear rate of the melt. It can also happen if the diameter of the part is too small. Low temperature of melt and high MW can also contribute. To prevent: use streamlined dies, raise melt temperatures, select a resin with lower molecular weight, long land length, lower extruder speed and reduce shear.
shark skin
melt fracture, bambooing (on surface only). To prevent: use a broad molecular weight distribution. slow speed.
breaker plate
placed between the die and the screw tip. increases direct flow along the axis by inhibiting rotation, filters the polymer liquid
Injection Molding Process
polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from the cavity
Extrusion molding
process during which a molten polymer is extruded through a die in continuous form to be cut into lengths or coiled
advnatages of stretch blow molding
produces best mechanical and barrier properties. molecular orientation due to dual action (preform is deformed radially and axially)
Which of the following is true for higher shot sizes, less expensive and used to design larger products in Injection Molding
ram extrusion
Which of the following are related to Injection Molding?
reciprocating screw, ejector pins, ram injection
drawn down
reduces die swell by pulling.
flashing
remedy: decrease injection pressure, check clamping force, decrease fill rate of polymer resin
Submarine gate
separation of part and runner system is automatic
Which of the following are associated with Extrusion Molding.
shark skin
Injection molding defects
short shot, flashing
short shot
solidification occurs before mold cavity fills completely. remedy: increase melt temperature, increase injection pressure
Which type of die is used to design a hollow part like Pipes?
spider die
Voigt model
spring and dashpot in parallel
Maxwell Model
spring and dashpot in series
In injection molding the first channel that leads from the nozzle to the mold (runners) is called the
spruce
How to improve cyrstallinity of polypropylene film?
stress induced crystallization, slow cooling of polypropylene film between Tg and Tm
what is the leading commercial synthetic rubber?
styrene-butadiene rubber
accumulator head / ram extrusion
suitable to design large thick walled hollow parts
extrusion molding defects
surging, melt fracture
Heat distortion temperature
temperature at which plastic sample will deform under specified load
glass transition temperature
temperature where material goes from rubbery to solid
three factors that affect viscosity of polymer melt
temperature, shear rate, molecular weight
