Manufacturing Processes & Materials II, ME200/201
Dr. Zadeh
HW # 4
SHAPING PROCESSES FOR PLASTICS
Review Questions
1 Identify the main categories of plastics shaping processes, as classified by the resulting product
geometry.
2 Viscosity is an important property of a polymer melt in plastics shaping processes. Upon what
parameters does viscosity depend?
3 How does the viscosity of a polymer melt differ from most fluids that are Newtonian.
4 What does viscoelasticity mean, when applied to a polymer melt?
5 Define die swell in extrusion.
6 Briefly describe the plastic extrusion process.
7 The barrel and screw of an extruder are generally divided into three sections; identify the sections.
8 What are the functions of the screen pack and breaker plate at the die end of the extruder barrel?
9 What are the various forms of extruded shapes and corresponding dies?
10 What is the distinction between plastic sheet and film?
11 What is the blown-film process for producing film stock?
12 Describe the calendering process.
13 Polymer fibers and filaments are used in several applications; what is the most important application
commercially?
14 Technically, what is the difference between a fiber and a filament?
15 Among the synthetic fiber materials, which are the most important?
16 Briefly describe the injection molding process.
17 An injection-molding machine is divided into two principal components. Name them.
18 What are the two basic types of clamping units?
19 What is the function of gates in injection molds?
20 What are the advantages of a three-plate mold over a two-plate mold in injection molding?
21 Discuss some of the defects that can occur in plastic injection molding.
22 Describe structural-foam molding.
23 What are the significant differences in the equipment and operating procedures between injection
molding of thermoplastics and injection molding of thermosets?
24 What is reaction injection molding?
25 What kinds of products are produced by blow molding?
26 What is the form of the starting material in thermoforming?
27 What is the difference between a positive mold and a negative mold in thermoforming?
28 Why are the molds generally more costly in mechanical thermoforming than in pressure or vacuum
thermoforming?
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Manufacturing Processes & Materials II, ME200/201
Dr. Zadeh
29 What are the processes by which polymer foams are produced?
30 What are some of the general considerations that product designers must keep in mind when
designing components out of plastics?
Problems
Extrusion
1
An extruder has a diameter of 5.0 in and a length to diameter ratio of 26. The barrel heats the
polypropylene melt to 450°F, which provides a melt viscosity of 0.0025 lb-s/in2. The pitch of the
screw is 4.2 in and the channel depth is 0.15 in. In operation the screw rotates at 50 rev/min and a
head pressure of 450 lb/in2 is generated. What is the volume flow rate of polypropylene from the die
at the end of the barrel?
2
Determine the helix angle A such that the screw pitch p is equal to the screw diameter D. This is
called the "square" angle in plastics extrusion - the angle that provides a flight advance equal to one
diameter for each rotation of the screw (in class).
3
An extruder barrel has a diameter of 2.5 in. The screw rotates at 60 rev/min; its channel depth =
0.20 in, and its flight angle = 17.5. The head pressure at the die end of the barrel is 800 lb/in2 and
the length of the barrel is 50 in. The viscosity of the polymer melt is 122 x 10-4 lb-sec/in2.
Determine the volume flow rate of the plastic in the barrel (in class).
Injection Molding
4
The part dimension for a certain injection molded part made of polycarbonate is specified as 3.75
in. Compute the corresponding dimension to which the mold cavity should be machined, using the
value of shrinkage given in the following table (Table 13.1), (in class).
5
An injection molded polyethylene part has a dimension of 2.500 in. A new material, polycarbonate,
is used in the same mold. What is the expected corresponding dimension of the polycarbonate
molding?
Other Molding Operations and Thermoforming
6
A parison is extruded from a die with outside diameter = 11.5 mm and inside diameter = 7.5 mm.
The observed die swell is 1.25. The parison is used to blow mold a beverage container whose
outside diameter = 112 mm (a standard size 2-liter soda bottle). What is the corresponding wall
thickness of the container (in class)?
7
A blow-molding operation is used to produce a bottle with a diameter of 2.250 in and a wall
thickness of 0.045 in. The parison has a thickness of 0.290 in. The observed die swell ratio is 1.30.
(a) What is the required diameter of the parison? (b) What is the diameter of the die?
Plastic
Shrinkage, mm/mm, or in/in
ABS
0.006
Nylon-6,6
0.020
Polycarbonate
0.007
Polyethylene
0.025
Polystyrene
0.004
PVC
0.005
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