Page 216 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Synthesis, Design, and Projects I 95

7.38. How can you tell whether a part is made of a thermo- 7.41. In injection-molding operations, it is common practice
plastic or a thermoset? to remove the part from its runner, to place the runner into a
7.39. As you know, there are plastic paper clips available in shredder, and to recycle the resultant pellets. List the concerns
various colors. Why are there no plastic staples? you would have in using such recycled pellets as opposed to
so-called virgin pellets.
7.40. By incorporating small amounts of a blowing agent, it
is possible to manufacture hollow polymer fibers with gas
cores. List possible applications for such fibers.

QUANTITATIVE PROBLEMS

|l7.42. Calculate the areas under the stress-strain curve same beam dimensions, calculate the maximum deflection,
(toughness) for the materials shown in Fig. 7.11, plot them as and compare the results.
a function of temperature, and describe your observations. 7.45. Estimate the number of molecules in a typical auto-
7.43. Note in Fig. 7.11 that, as expected, the elastic modu- mobile tire. Estimate the number of atoms in the tire.
lus of the polymer decreases as temperature increases. Using |]7.46. Using strength and density data, determine the min-
the stress-strain curves in the figure, make a plot of the mod- imum weight of a 2-m-long tension member that must support
ulus of elasticity versus the temperature. Comment on the a load of 5000 N if it is manufactured from (a) high-molecular-
shape of the curve. weight polyethylene, (b) polyester, (c) rigid PVC, (d) ABS,
ll 7.44. A rectangular cantilever beam 100 mm high, 20 mm (e) polystyrene, and (e) reinforced nylon.
wide, and 1 rn long is subjected to a concentrated load of 50 kg
D 7.47. Plot the following for any five polymers described
at its end. From Table 7.1, select three unreinforced and three
in this chapter: (a) UTS vs. density and (b) elastic modulus vs.
reinforced materials and calculate the maximum deflection of UTS. Where appropriate, plot a range of values.
the beam in each case. Then select aluminum and steel for the

SYNTHESIS, DESIGN, AND PROIECTS

7.48. Describe the design considerations involved in replac- 7.53. Repeat Problem 7.52 for the curves shown in
ing a metal beverage container with one made of plastic. Fig. 7.12.
7.49. Assume that you are manufacturing a product in 7.54. In order to use a steel or aluminum container for an
which all of the gears are made of metal. A salesperson visits acidic liquid, such as tomato sauce, a polymeric barrier is
you and asks you to consider replacing some of these metal usually placed between the container and its contents.
gears with plastic ones. Make a list of the questions that you Describe some methods of producing such a barrier.
would raise before making a decision. 7.55. Perform a study of plastics used for some products.
7.50. Sections 7.6 and 7.7 list several plastics and their appli- Measure the hardness and stiffness of these plastics. (For ex-
cations. Rearrange this information by making a table of prod- ample, dog chew toys use plastics with a range of properties.)
ucts (gears, helmets, luggage, electrical parts, etc.) which shows 7.56. Add a column to Table 7.1 which describes the ap-
the types of plastic that can be used to make these products. pearance of these plastics, including available colors and
7.5l. Make a list of products or parts that currently are not opaqueness.
made of plastics and offer possible reasons why they are not.
7.57. With Table 7.3 as a guide, inspect various products
7.52. Review the three curves shown in Fig. 7.10 and give both in a typical kitchen and in an automobile, and describe
some applications for each type of behavior. Explain your the types of plastics that could be used in making their indi-
choices. vidual components.