183A Chp 13

The term encapsulation refers to which one of the following plastics shaping processes: (a) casting, (b) compression molding, (c) extrusion of hollow forms, (d) injection molding in which a metal insert is encased in the molded part, or (e) vacuum thermoforming using a positive mold?

casting

The two most common polymer foams are which of the following: (a) polyacetal, (b) polyethylene, (c) polystyrene, (d) polyurethane, and (e) polyvinylchloride?

polystyrene polyurethane

A thermoforming mold with a convex form is called which one of the following: (a) a die, (b) a negative mold, (c) a positive mold, or (d) a three-plate mold?

positive mold

The parting line in injection molding is which one of the following: (a) the lines formed where polymer melt meets after flowing around a core in the mold, (b) the narrow gate sections where the parts are separated from the runner, (c) where the clamping unit is joined to the injection unit in the molding machine, or (d) where the two mold halves come together?

where the two mold halves come together

Which of the following defects or problems is associated with injection molding (three correct answers): (a) bambooing, (b) die swell, (c) drag flow, (d) flash, (e) melt fracture, (f) short shots, or (g) sink marks?

flash short shots sink marks

In which of the following properties do plastic parts often compare favorably with metals (two best answers): (a) impact resistance, (b) resistance to ultraviolet radiation, (c) stiffness, (d) strength, (e) strength-to-weight ratio, and (f) temperature resistance?

impact resistance strength to weight ratio

What does viscoelasticity mean, when applied to a polymer melt?

Viscoelasticity is a combination of viscous and elastic properties which cause the melt to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in extrusion.

A three-plate mold offers which of the following advantages when compared to a two-plate mold (two best answers): (a) automatic separation of parts from runners, (b) gating is usually at the base of the part to reduce weld lines, (c) sprue does not solidify, and (d) stronger molded parts?

automatic separation of the parts from runners gating is usually at the base of the part to reduce weld lines

What is the difference between a positive mold and a negative mold in thermoforming?

A positive mold has a convex shape; a negative mold has a concave cavity.

What are the advantages of a three-plate mold over a two-plate mold in injection molding?

As the mold opens, the three-plate mold automatically separates the molded part(s) from the runner system.

What kinds of products are produced by blow molding?

Blow molding is used to produce hollow, seamless containers, such as bottles.

Describe the calendering process.

Calendering is a process for producing sheet and film stock out of rubber or rubbery thermoplastics such as plasticized PVC. In the process, the initial feedstock is passed through a series of rolls to work the material and reduce its thickness to the desired gage.

Which of the following processes would be applicable to produce hulls for small boats (three best answers): (a) blow molding, (b) compression molding, (c) injection molding, (d) rotational molding, and (e) vacuum thermoforming?

blow molding rotational molding vacuum thermoforming

Which of the following processes are generally limited to thermoplastic polymers (two best answers): (a) blow molding, (b) compression molding, (c) reaction injection molding, (d) thermoforming, (e) transfer molding, and (f) wire coating?

blow molding thermoforming

The principal components of an injection molding machine are which two of the following: (a) clamping unit, (b) hopper, (c) injection unit, (d) mold, and (e) part ejection unit?

clamping unit injection unit

In rotational molding, centrifugal force is used to force the polymer melt against the surfaces of the mold cavity where solidification occurs: (a) true or (b) false?

false It is the force of gravity in the doubly rotating mold that forces the polymer against the mold surfaces.

Briefly describe the plastic extrusion process.

. In plastic extrusion, a polymer melt is compressed to flow through a die orifice and thus the continuous length of the plastic assumes a cross-sectional shape that is approximately the same as that of the orifice.

Briefly describe the injection molding process.

. Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. The molding is then removed from the cavity.

Viscosity is an important property of a polymer melt in plastics shaping processes. Upon what parameters does viscosity depend?

Viscosity of a polymer melt depends on (1) temperature and (2) shear rate. Also, (3) the molecular weight of the polymer affects viscosity

Why are the molds generally more costly in mechanical thermoforming than in pressure or vacuum thermoforming?

In mechanical thermoforming, matching mold halves are required; while in other thermoforming processes, only one mold form is required.

Among the synthetic fiber materials, which are the most important?

Polyester is the most important commercially, followed by nylon, acrylics, and rayon.

Use of a parison is associated with which one of the following plastic shaping processes: (a) bi-injection molding, (b) blow molding, (c) compression molding, (d) pressure thermoforming, or (e) sandwich molding?

blow molding

Which of the following are sections of a conventional extruder barrel for thermoplastics (three best answers): (a) compression section, (b) die section, (c) feed section, (d) heating section, (e) metering section, and (f) shaping section?

compression section feed section metering section

What are the various forms of extruded shapes and corresponding dies?

The shapes are (1) solid profiles, such as rounds and L-shapes; (2) hollow profiles, such as tubes; (3) wire and cable coating; (4) sheet and film; and (5) filaments (continuous fibers)

What is the blown-film process for producing film stock?

r. The blown-film process is a widely used process for making thin polyethylene film for packaging. It combines extrusion and blowing to produce a tube of thin film. The process begins with the extrusion of a tube that is immediately drawn upward while still molten and simultaneously expanded in size by air inflated into it through the die mandrel.

The function of the ejection system is which one of the following: (a) move polymer melt into the mold cavity, (b) open the mold halves after the cavity is filled, (c) remove the molded parts from the runner system after molding, or (d) separate the part from the cavity after molding?

separate the part from the cavity after molding

Polymer fibers and filaments are used in several applications; what is the most important application commercially?

Textiles

Technically, what is the difference between a fiber and a filament?

A fiber is a long, thin strand of material whose length is at least 100 times its diameter; a filament is a fiber of continuous length.

How does the viscosity of a polymer melt differ from most fluids that are Newtonian.

A polymer melt exhibits pseudoplasticity, which means that its value decreases with increasing shear rate

Define die swell in extrusion.

Die swell is the tendency of the extrudate to expand in cross-sectional dimensions immediately on exiting the die orifice. It results from the viscoelastic properties of the polymer melt.

The forward movement of polymer melt in an extruder barrel is resisted by drag flow, which is caused by the resistance to flow through the die orifice: (a) true or (b) false?

False Drag flow is the forward motion of the melt caused by the Archimedean screw principle in the barrel. The resistance to forward flow is called back pressure flow.

What is reaction injection molding?

Reaction injection molding involves the mixing of two highly reactive liquid ingredients and immediately injecting the mixture into a mold cavity where chemical reactions leading to solidification occur. The two ingredients form the components used in catalyst-activated or mixing-activated thermoset systems.

What are some of the general considerations that product designers must keep in mind when designing components out of plastics?

Some of the general considerations are the following: (1) Plastics are not as strong or stiff as metals and should not be used in applications where high stresses will be encountered. (2) Impact resistance of plastics is general good, better than many ceramics. (3) Service temperatures of plastics are limited relative to engineering metals and ceramics. (4) Thermal expansion is greater for plastics than metals; so dimensional changes due to temperature variations are much more significant than for metals. (5) Many types of plastics degrade from sunlight and certain other forms of radiation. Also, some plastics degrade in oxygen and ozone atmospheres. Finally, plastics are soluble in many common solvents.

Describe structural-foam molding

Structural-foam molding is an injection molding process in which a gas or gas-producing ingredient is mixed with the polymer melt prior to injection into the mold cavity; this results in the part having a tough outer skin surrounded by a foam core.

Identify the main categories of plastics shaping processes, as classified by the resulting product geometry

The categories are (1) extrusion, (2) molding, (3) forming of continuous sheets and films, (4) fibers, (5) foamed products, and (6) discrete formed sheets and films.

What are the two basic types of clamping units?

The clamping units are: (1) mechanical toggle clamp and (2) hydraulic. In addition, there are hydromechanical units which combine hydraulic and mechanical actuations.

An injection-molding machine is divided into two principal components. Name them

The components of an injection-molding machine are (1) the injection unit and (2) the clamping unit.

Discuss some of the defects that can occur in plastic injection molding.

The defects include (1) short shots, in which the polymer melt solidifies before filling the cavity; (2) flashing, in which the polymer melt is squeezed into the parting surfaces between the mold halves and around ejection pins; (3) sink marks, in which the surface is drawn into the molding by contraction of internal material; and (4) weld lines where the melt has flowed around a core or other convex detail in the mold cavity and met from opposite directions, thus resulting in mechanical properties that are inferior to those in the rest of the part.

What are the significant differences in the equipment and operating procedures between injection molding of thermoplastics and injection molding of thermosets?

The differences in injection molding of thermosets are (1) shorter barrel length, (2) lower temperatures in the barrel, these first two reasons to prevent premature curing; and (3) use of a heated mold to cause cross-linking of the TS polymer.

What is the distinction between plastic sheet and film?

The distinction is based on thickness. Sheet stock has a thickness greater than 0.020 in (0.5 mm), while film stock is less than 0.020 in (0.5 mm) thick.

What is the function of gates in injection molds?

The function of gates in an injection mold is to constrict the flow of molten plastic into the cavity, which increases the shear rate and reduces the viscosity of the polymer melt.

What are the functions of the screen pack and breaker plate at the die end of the extruder barrel?

The functions are to (1) filter dirt and lumps, (2) build pressure, (3) straighten the flow and remove memory of the polymer melt

What are some of the reasons why plastic shaping processes are important?

The reasons include (1) many of the processes are net shape processes; (2) in general, less energy is employed than in metalworking processes; (3) lower temperatures are required to process plastics than metals or ceramics; (4) there is great flexibility in geometry; and (5) painting and other finishing processes are generally not required

The barrel and screw of an extruder are generally divided into three sections; identify the sections.

The sections are (1) the feed section, in which the feed stock is fed from the hopper and heated; (2) the compression section, in which the polymer changes to a viscous fluid; and (3) the metering section, in which pressure is developed to pump the plastic through the die orifice.

What are the processes by which polymer foams are produced?

There are several foaming processes: (1) mechanical agitation - mixing a liquid resin with air, then hardening the polymer by means of heat or chemical reaction; (2) mixing a physical blowing agent with the polymer - a gas such as nitrogen (N2) or pentane (C5H12) which can be dissolved in the polymer melt under pressure, so that the gas comes out of solution and expands when the pressure is subsequently reduced; and (3) mixing the polymer with chemical compounds, called chemical blowing agents, that decompose at elevated temperatures to liberate gases such as CO2 or N2 within the melt.

What is the form of the starting material in thermoforming?

Thermoforming starts with a thermoplastic sheet or film.

Which of the following processes are associated with the production of plastic sheet and film (three correct answers): (a) blown-film extrusion process, (b) calendering, (c) chill-roll extrusion, (d) doctor blade method, (e) spinning, (f) thermoforming, and (g) transfer molding?

blown-film extrusion process calendering chill-roll extrusion