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Chapter 10
Properties and Processing of
Polymers and Reinforced Plastics;
Rapid Prototyping and Rapid Tooling
Questions
10.1 Summarize the most important mechanical and
physical properties of plastics in engineering ap-
maries of mechanical properties, make compar-
10.2 What are the major differences between the
properties of plastics and of metals?
There are several major differences that can be
enumerated, as described throughout the chap-
ter. Some examples are:
(a) Plastics are much less stiff than metals.
(b) They have lower strength than metals and
are lighter.
(c) The thermal and electrical conductivities
of metals are much higher than those for
10.3 What properties are influenced by the degree of
polymerization?
polymerization will lead to higher strength and
10.4 Give applications for which flammability of
plastics would be a major concern.
By the student. There are several applica-
tions where flammability of plastics is a major
concern. These include aircraft, home insula-
tion (thermal and electrical), cookware, cloth-
ing, and components for ovens and stoves (in-
cluding components such as handles and dials).
Students should be encouraged to describe ad-
ditional applications.
10.5 What properties do elastomers have that ther-
moplastics, in general, do not have?
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10.6 Is it possible for a material to have a hystere-
sis behavior that is the opposite of that shown
in Fig. 10.14, whereby the arrows are counter-
clockwise? Explain.
10.7 Observe the behavior of the tension-test spec-
Recall that the mvalue indicates the strain
Fig. 10.13 on p. 598 elongates extensively by
orientation of the polymer molecules, thus it
10.8 Why would we want to synthesize a polymer
with a high degree of crystallinity?
swered in several ways. Students may rely upon
Fig. 10.4, and note that a high degree of crys-
(b) Functional and decorative: electrical out-
By the student. The glass-transition tempera-
ture is the temperature where a thermoplastic
behaves in a manner that is hard, brittle and
glassy below this temperature, and rubbery or
sition temperature higher than the maximum
10.11 Why does cross-linking improve the strength of
polymers?
Cross-linked polymers have additional bonds
linking adjacent chains together (see Fig. 10.3
resent additional bonds that must be broken
10.12 Describe the methods by which optical proper-
which can alter the color or translucence of
oped. Based on the contents of this chapter, are
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10.14 Give several examples of plastic products or
components for which creep and stress relax-
important. As an example of the importance
of creep, consider polymers as pot handles in
cookware. As an example of stress relaxation,
10.15 Describe your opinions regarding recycling of
plastics versus developing plastics that are
in the intended time frame, and they may have
more failures in service. They can be signifi-
10.16 Explain how you would go about determining
the hardness of the plastics described in this
tion 2.6 (see also Fig. 2.22 on p. 52) are not
10.17 Distinguish between composites and alloys.
Give several examples.
By the student. Consider the following:
•With composites, the reinforcement is
10.18 Describe the functions of the matrix and the
reinforcing fibers in reinforced plastics. What
fundamental differences are there in the char-
acteristics of the two materials?
fibers are generally stronger and/or stiffer than
the polymer matrix. The function of the rein-
are made of reinforced plastics? How can you
tell that they are reinforced?
tion and students can develop a wide variety of
answers. Some suggestions are tennis rackets,
fiber reinforcements can be seen directly on the
By the student. See Table 16.1 on p. 956. A
typical comparison is given below:
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10.21 Compare the relative advantages and limita-
tions of metal-matrix composites, reinforced
10.22 This chapter has described the many advan-
tages of composite materials. What limita-
tions or disadvantages do these materials have?
What suggestions would you make to overcome
these limitations?
By the student. Consider, for example, two dis-
advantages as anisotropic properties and pos-
sible environmental attack of the fibers (espe-
cially water adsorption). Anisotropy, though
not always undesirable, can be reduced by hav-
ing a random orientation of reinforcing ma-
terials. Environmental attack of the fibers
would cause loss of fiber strength and possibly
10.23 A hybrid composite is defined as a material con-
taining two or more different types of reinforc-
ing fibers. What advantages would such a com-
posite have over other composites?
level could be obtained at a lower cost by using
a combination of fibers, rather than just one
load.
10.25 Assume that you are manufacturing a product
in which all the gears are made of metal. A
salesperson visits you and asks you to consider
(a) Will the plastic gear retain its required
strength, stiffness, and tolerances if the
temperature changes during its use?
(b) How acceptable is the wear resistance and
fatigue life of the plastic gears?
(c) Is it compatible with meshing metal gears
and other components in the gear train?
(d) Are there any backlash problems?
(e) What are its frictional characteristics?
(f) Is the lighter weight of the plastic gear sig-
nificant?
(g) Is noise a problem?
bricants present?
(i) Will the supplier be able to meet the quan-
tity demanded?
(j) How much cost savings are involved? (See
10.26 Review the three curves in Fig. 10.8, and de-
scribe some applications for each type of be-
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Fig. 10.10.
food containers with impact strength at
low temperatures, such as in freezers.
will not crack at a wide range of tempera-
or impact resistant, it must be limited to
be used in passenger cars? If so, which compo-
nents? Explain.
tions concerning automobiles: (1) Radiators
with copper honeycomb structure to improve
10.29 Other than those described in this chapter,
what materials can you think of that can be
regarded as composite materials?
•Walls: consists of a plaster matrix with
wood stud or metal reinforcements.
10.30 What applications for composite materials can
you think of in which high thermal conductivity
would be desirable? Explain.
and.3.9.5. Composite materials with high ther-
ment, and identify the components that are
weight of these materials, combined with high
stiffness and strength, thus resulting in superior
10.32 We have described several material combina-
tions and structures in this chapter. In relative
sponses with appropriate rationale. For exam-
ple, very low temperature applications require
(1) considerations of the polymer or matrix me-
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10.33 Explain how you would go about determin-
ing the hardness of the reinforced plastics and
composite materials described in this chapter.
What type of tests would you use? Are hard-
ness measurements for these types of materials
meaningful? Does the size of the indentation
make a difference in your answer? Explain.
By the student. The important consideration
here is the fact that the smaller the indentation,
the more localized the hardness measurement
will be (see Section 2.6). Consequently, one
can then distinguish the hardness of the ma-
trix and the reinforcements separately by using
small indentations. A large indentation, such
metalworking techniques to the forming and
shaping of plastics.
uous extrusion process. Injection molding of
composites is difficult because the fluidity of
the material is essential to ensure proper filling
of the die, but characteristics and presence of
the fibers interferes with this process. Plastics
which are produced through reaction molding
are difficult to produce through other means,
and other processes are not readily adaptable
to allow sufficient mixing of the two ingredi-
ents. These difficulties should, however, be re-
garded as challenges and thus novel approaches
are encouraged. The students are encouraged
to develop additional answers.
10.37 Would you use thermosetting plastics in injec-
tion molding? Explain.
cross-linking of the material. The major draw-
back associated with this change is that, be-
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10.39 Give examples of several parts that are suitable
for insert molding. How would you manufacture
these parts if insert molding were not available?
Fig. 10.30 on p. 628. Some common parts are
screw drivers with polymer handles, electrical
volved in making a metal beverage container
versus a plastic one?
beverage can must have sufficient strength to
prevent from rupturing under internal pressure
10.41 Inspect several electrical components, such as
light switches, outlets, and circuit breakers, and
describe the process or processes used in mak-
electrical components may be potted.
10.42 Inspect several similar products that are made
of metals and plastics, such as a metal bucket
explain the reasons for their differences, if any.
much lower elastic modulus, as well as in-
volve designs with higher section modulus.
forced plastics. Identify which of the following
fiber orientation and arrangement capabilities
Sheet-molding compound X
Tape lay-up X X
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10.44 As you may have observed, some plastic prod-
ucts have lids with integral hinges; that is, no
other material or part is used at the junction
section, which can bend easily, thus acting like
a hinge. It should be noted that there are signif-
10.45 Explain why operations such as blow molding
and film-bag making are done vertically and
why buildings that house equipment for these
10.46 Consider the case of a coffee mug being pro-
duced by rapid prototyping. Describe how the
top of the handle can be manufactured, since
Fig. 10.49b on p. 649 would have to be used.
Other processes such as selective laser sintering
10.47 Make a list of the advantages and disadvantages
By the student. As examples, the students
could investigate (a) cost (where FDM, 3DP,
STL have advantages over SLS of metals, for ex-
10.48 Explain why finishing operations generally are
needed for rapid-prototyping operations. If you
the part has to be cured in order to fully de-
velop its mechanical properties (the laser does
10.49 A current topic of research involves producing
parts from rapid-prototyping operations and
then using them in experimental stress analysis,
ally manufactured parts, especially, for exam-
10.50 Because of relief of residual stresses during cur-
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10.51 One of the major advantages of stereolithog-
raphy and cyberjet is that semi- and fully-
transparent polymers can be used, so that in-
ship-in-the-bottle type of part; d (d) market-
ing models to explain the internal features of a
10.52 Based on the processes used to make fibers
as described in this chapter, explain how you
would produce carbon foam. How would you
make a metal foam?
erature search. One approach is to produce a
polymer foam followed by a carburization pro-
10.53 Die swell in extrusion is radially uniform for cir-
cular cross-sections, but is not uniform for other
cross-sections. Recognizing this fact, make
a qualitative sketch of a die profile that will
produce (a) square and (b) triangular cross-
sections of extruded polymer, respectively.
10.54 What are the advantages of using whiskers as
a reinforcing material? Are there any limita-
tions?
than other fibers because of their small size
and lack of defects (see pp. 105 and 463).
Whiskers will yield composite materials with
10.55 By incorporating small amounts of blowing
agent, it is possible to produce polymer fibers
with gas cores. List some applications for such
10.56 With injection-molding operations, it is com-
mon practice to remove the part from its run-
ner and then to place the runner into a shredder
and recycle into pellets. List the concerns you
would have in using such recycled pellets as op-
posed to so-called virgin pellets.
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10.57 What characteristics make polymers attractive
for applications such as gears? What character-
10.58 Can polymers be used to conduct electricity?
If a polymer’s chemistry can be identified, then
a polymer product can be cut into small pieces
preferred orientation, similar to the effect of
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or warp in the presence of bodily fluids;
support the loads developed during nor-
mal walking, sitting, and standing; not
portant, as well as impact strength and
toughness. The polymer needs to have a
stiffness consistent with typical golf balls,
demanding) shapes. It also has to be
available in a range of desired colors,
cloth and withstand normal wear and tear.
The polymer must have low elastic modu-
cleanable.
10.63 How can you tell whether a part is made of a
thermoplastic or a thermoset? Explain.
10.64 Describe the features of an extruder screw and
comment on their specific functions.
and depth is larger than elsewhere on the
screw.
the pressure and flow rate needed for the
extrusion operation.
10.65 An injection-molded nylon gear is found to con-
tain small pores. It is recommended that the
10.66 What determines the cycle time for (a) injection
molding, (b) thermoforming, and (c) compres-
sion molding?
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ing processes? Explain.
The type of defect shown in Fig. 10.57 also oc-
casting, and on p. 5.64 the same features are
10.68 List the differences between the barrel section
of an extruder and that on an injection-molding
heating elements and less from friction,
so there will be more (or larger capacity)
10.69 Identify processes that are suitable for making
10.70 Review the Case Study to this chapter and ex-
plain why aligners cannot be made directly by
rapid prototyping operations.
10.71 Explain why rapid prototyping approaches are
for producing parts (2 hours or so for a small
part is rapid when only one part is required.
10.72 List and explain methods for quickly manufac-
turing tooling for injection molding.
•An RTV molding/urethane casting opera-
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used, using an RTV mold, filled with pow-
10.73 Careful analysis of a rapid-prototyped part in-
dicates that it is made up of layers with a white
filament outline visible on each layer. Is the ma-
terial a thermoset or a thermoplastic? Explain.
process requires adjacent layers to fuse after
10.74 List the advantages of using a room-
temperature vulcanized (RTV) rubber mold
in injection molding.
10.75 What are the similarities and differences be-
tween stereolithography and cyberjet?
both processes rely on the same layer-creation
technique, namely liquid-layer curing, (b) both
use a principle of using a photopolymer to cre-
10.76 Explain how color can be incorporated into
•The ZCorp (see Fig. 10.52 on p. 651)
versions of three-dimensional printing ma-
curve (toughness) for the material in Fig. 10.9,
plot them as a function of temperature, and de-
(◦C) (MJ/m3)
-25 140
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Temperature (°C)
-50 0 50 100
800
600
10.78 Note in Fig. 10.9 that, as expected, the elastic
modulus of the polymer decreases as temper-
strain. The following table can be constructed:
-25 70 1.75
0 60 1.5
10.79 Calculate the percentage increase in mechani-
cal properties of reinforced nylon from the data
p. 585 as 1.4 GPa.
(J/m) 295b321
80c14
10.80 A rectangular cantilever beam 75-mm high, 25-
mm wide, and 1-m long is subjected to a con-
and steel, and for the same beam dimensions,
calculate the maximum deflection. Compare
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