Chapter 13 Textbook Questions
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 (g) sink marks
(d) flash (f) short shots (g) sink marks
Describe structural-foam molding.
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
What does viscoelasticity mean, when applied to a polymer melt?
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
Define die swell in extrusion.
the tendency of the extrudate to expand in cross-sectional dimensions immediately on exiting the die orifice
What is the form of the starting material in thermoforming?
thermoforming starts with a thermoplastic sheet or film
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 (f) shaping section?
(a) compression section (c) feed section (e) metering section
What is the most common type of injection molding machine used in industry?
hydraulic, three-platen system
What is reaction injection molding?
mixing of two highly reactive liquid ingredients and immediately injecting the mixture into a mold cavity where chemical reactions leading to solidification occur
Polymer fibers and filaments are used in several applications; what is the most important application commercially?
textiles
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 (e) vacuum thermoforming using a positive mold
(a) casting
The principal components of an injection molding machine are which two of the following? (a) clamping unit (b) hopper (c) injection unit (d) mold (e) part ejection unit
(a) clamping unit (c) injection unit
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.
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 (f) wire coating
(a) blow molding (d) thermoforming
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
Briefly describe the plastic extrusion process.
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
What is the function of gates in injection molds?
constrict the flow of molten plastic into the cavity, which increases the shear rate and reduces the viscosity of the polymer melt
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 (runnerless mold) 4) insulated runner mold
Name the four types of finishing operations performed on plastics.
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) foamed products 6) discrete formed sheets and films
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
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 (g) transfer molding?
(a) blown-film extrusion process (b) calendering (c) chill-roll extrusion
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 (f) temperature resistance
(a) impact resistance (e) strength-to-weight ratio
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 (e) sandwich molding
(b) blow molding
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 (b) false
(b) false Drag flow is the forward motion of the melt caused by the Archimedian screw principle in the barrel. The resistance to forward flow is called back pressure flow.
In rotational molding, centrifugal force is used to force the polymer melt against the surfaces of the mold cavity where solidification occurs. (a) true (b) false
(b) false It is the force of gravity in the rotating mold that forces the polymer against the mold surfaces.
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 (d) separate the part from the cavity after molding
(d) separate the part from the cavity after molding
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 (d) where the two mold halves come together
(d) where the two mold halves come together
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.
Among the synthetic fiber materials, which are the most important?
Polyester (followed by nylon, acrylics, and rayon)
What are some of the general considerations that product designers must keep in mind when designing components out of plastics?
(1) Plastics should not be used in applications where high stresses will be encountered (2) Impact resistance of plastics is generally good 3) Service temperatures of plastics are limited (4) Thermal expansion is greater for plastics than metals
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 (d) stronger molded parts
(a) automatic separation of parts from runners (b) gating is usually at the base of the part to reduce weld lines
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 (e) vacuum thermoforming
(a) blow molding (d) rotational molding (e) vacuum thermoforming
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 (d) a three-plate mold
(c) a positive mold
The two most common polymer foams are which of the following? (a) polyacetal (b) polyethylene (c) polystyrene (d) polyurethane (e) polyvinylchloride
(c) polystyrene (d) polyurethane
What kinds of products are produced by blow molding?
hollow, seamless containers, such as bottles
Why are the molds generally more costly in mechanical thermoforming than in pressure or vacuum thermoforming?
in mechanical thermoforming, matching mold halves are required in other thermoforming processes, only one mold form is required.
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 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
Discuss some of the defects that can occur in plastic injection molding.
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
What are the various forms of extruded shapes 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) the molecular weight of the polymer affects viscosity
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 viscous 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
What are the four primary elements that influence the injection molding process?
1) the molder 2) the material 3) the injection machine 4) the mold
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.
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.
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 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.
