Chapter 13 Review

According to the injection molding video, name the four types of mold design most common in industry

(1) Cold runner two-plate mold (2) Cold runner three-plate mold (3) Hot runner mold, also known as runnerless mold (4) Insulated runner mold

Name the four types of finishing operations performed on plastics, according to the plastics finishing video.

(1) Degating (2) Deflashing (3) Cleaning (4) Decorating

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

(1) Extrusion (2) Molding (3) Forming of continuous sheets and films (4) Fibers (5) Foaming products (6) Discrete formed sheets and films

List the four most common blow-molding processes according to the video on blow-molding.

(1) Extrusion blow molding (2) Injection blow molding (3) Biaxial stretch blow molding (4) Co-extrusion blow molding

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

(1) Filter dirt and lumps (2) Build pressure (3) Straighten the flow and remove memory of the polymer melt

According to the blow molding video, what materials are used in blow molding? Name three.

(1) High density polyethylene (HDPE) (2) Medium and low density polyethylene (MDPE, LDPE) (3)Polyethylene terephthalate (PET) (4) Polypropylene (PP) (5) Polybinyl chloride (PVC) (6) Thermoplastic elastomers (TPE) (7) Polystyrene (PS) (8) Polycarbonate (PC) (9) Fluropolymers (PTFE) (10) Polyimide/Nylon

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

(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 (5) Painting and other finishing processes are generally not required

What are the processes by which polymer foams are produced?

(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 (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 are the two basic types of clamping units?

(1) Mechanical toggle clamp (2) Hydraulic

What are the different processes that can be used to apply decorations to plastics parts according to the plastics finishing video?

(1) Painting (2) Plating (3) Vacuum metallization (4) Pad printing (5) Hot stamping (6) Silk screening (7) Fill and wipe

List the stages of extrusion blow molding according to the video.

(1) Plasticizing the resin (2) Parison production (3) Parison or preform inflation and cooling (4) Ejection from the blow mold (5) Finishing and trimming

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.

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.

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.

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

Textiles

According to the injection molding videos, what are the four primary elements that influence the injection molding process?

The four primary elements that influence the injection molding process are the molder, material, injection machine, and the mold.

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 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.

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

(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.

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

(1) Short shots, in which he 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 (2) Sink marks, in which the surface is drawn into the molding by contraction of internal material (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 various forms of extruded shaped and corresponding dies?

(1) Solid profiles, such as rounds and L-shapes (2) Hollow profiles, such as tubes (3) Wire and cable coating (4) Sheet and film (5) Filaments (continuous fibers)

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

(1) Temperature (2) Shear rate (3) Molecular wight of the polymer

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

(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 vicsous fluid (3) The metering section, in which pressure is developed to pump the plastic through the die orifice

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

(1) The injection unit (2) The clamping unit

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.

According to the injection molding video, what is the most common type of injection molding machine used in industry?

A hydraulic, three-platen system

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.

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 produces by blow molding?

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

Describe the calendaring 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.

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.

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

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

What is reaction injection molding?

Reaction injection molding involved 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 mixed-activiated thermoset systems.

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

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.

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 0.5 mm, while film stock is less than 0.5 mm thick.

What is the form the starting material in thermoforming?

Thermoforming starts with a thermoplastic sheet or film.