Forming and shaping plastics and composite materials

Kalpakjian • Schmid
Manufacturing Engineering and Technology © 2001 Prentice-Hall Page 18-1
CHAPTER 18
Forming and Shaping Plastics and
Composite Materials

Characteristics of Forming and Shaping
Processes for Plastics and Composite Materials
TABLE 18.1
Process Characteristics
Extrusion Long, uniform, solid or hollow complex cross-sections; high production rates;
low tooling costs; wide tolerances.
Injection molding Complex shapes of various sizes, eliminating assembly; high production rates;
costly tooling; good dimensional accuracy.
Structural foam molding Large parts with high stiffness-to-weight ratio; less expensive tooling than in
injection molding; low production rates.
Blow molding Hollow thin-walled parts of various sizes; high production rates and low cost for
making containers.
Rotational molding Large hollow shapes of relatively simple shape; low tooling cost; low production
rates.
Thermoforming Shallow or relatively deep cavities; low tooling costs; medium production rates.
Compression molding Parts similar to impression-die forging; relatively inexpensive tooling; medium
production rates.
Transfer molding More complex parts than compression molding and higher production rates; some
scrap loss; medium tooling cost.
Casting Simple or intricate shapes made with flexible molds; low production rates.
Processing of composite materials Long cycle times; tolerances and tooling cost depend on process.

Forming
and
Shaping
Processes
Figure 18.1 Outline of forming and shaping processes for plastics, elastomers, and
composite materials. (TP, Thermoplastic; TS, Thermoset; E, Elastomer.)

Extruder
Figure 18.2 Schematic illustration of a typical extruder. Source: Encyclopedia of Polymer Science
and Engineering (2nd ed.). Copyright © 1985. Reprinted by permission of John Wiley & Sons, Inc.

Sheet and Film Extrusion
Figure 18.3 Die geometry (coat-hanger die) for extruding sheet.
Source: Encyclopedia of Polymer Science and Engineering (2d ed.).
Copyright © 1985. Reprinted by permission of John Wiley & Sons, Inc.
Figure 18.4 Schematic illustration of the production of thin film and
plastic bags from tube first produced by an extruder and then blown by
air. Source: D.C. Miles and J.H. Briston, Polymer Technology, 1979.
Reproduced by permission of Chemical Publishing Co., Inc.

Injection Molding
(c)
Figure 18.5 Injection molding with (a) plunger, (b) reciprocating rotating screw, (c) a typical part made from an
injection molding machine cavity, showing a number of parts made from one shot; note also mold features such as
sprues, runners, and gates.

Examples of Injection Molding
Figure 18.6 Typical products made by
injection molding, including examples
of insert molding. Source: Plainfield
Molding Inc.

Injection-Molding Machine
Figure 18.7 A 2.2-MN (250-ton) injection-molding machine. The tonnage is the force
applied to keep the dies closed during injection of molten plastic into the mold cavities.
Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.

Reaction-Injection Molding
Figure 18.8 Schematic
illustration of the
reaction-injection
molding process.
Source: Modern Plastics
Encyclopedia.

Blow
Molding
Figure 18.9 Schematic illustrations of (a) the blow-molding process for making
plastic beverage bottles, and (b) a three-station injection blow-molding machine.
Source: Encyclopedia of Polymer Science and Engineering (2d ed.). Copyright
©1985. Reprinted by permission of John Wiley & Sons, Inc.

Rotational Molding
Figure 18.10 The
rotational molding
(rotomolding or
rotocasting) process.
Trash cans, buckets, and
plastic footballs can be
made by this process.

Thermoforming Processes
Figure 18.11 Various thermoforming processes for thermoplastic sheet. These processes are
commonly used in making advertising signs, cookie and candy trays, panels for shower stalls, and
packaging.

Compression Molding
Figure 18.12 Types of compression
molding, a process similar to forging: (a)
positive, (b) semipositive, and (c) flash.
The flash in part (c) has to be trimmed off.
(d) Die design for making a compression-
molded part with undercuts.

Transfer Molding
Figure 18.13 Sequence of operations in transfer molding for thermosetting plastics. This process
is particularly suitable for intricate parts with varying wall thickness.

Casting, Potting and Encapsulation
illustration of (a) casting, (b)
potting, (c) encapsulation of
plastics.

Calendering and Examples of Reinforced
Plastics
Figure 18.15 Schematic illustration
of calendering. Sheets produced by
this process are subsequently used
in thermoforming.
Figure 18.16 Reinforced- plastic
components for a Honda motorcycle.
The parts shown are front and rear
forks, a rear swingarm, a wheel, and
brake disks.

Prepegs
Figure 18.17 (a) Manufacturing process for polymer-matrix composite. Source: T.W. Chou, R.L.
McCullough, and R.B. Pipes. (b) Boron-epoxy prepreg tape. Source: Avco Specialty
Materials/Textron.
(b)(a)

Tape Laying
Figure 18.18 (a) Single-ply layup of
boron-epoxy tape for the horizontal
stabilizer for F-14 fighter aircraft. Source:
Grumman Aircraft Corporation. (b) A 10
axis computer-numerical-controlled tape-
laying system. This machine is capable of
laying up 75 mm and 150 mm (3 in. and 6
in.) wide tapes, on contours of up to ±30°
and at speeds of up to 0.5 m/s (1.7 ft/s).
Source: Courtesy of The Ingersoll Milling
Machine Company.
(a)
(b)

Sheet Molding
Figure 18.19 The manufacturing
process for producing reinforced-
plastic sheets. The sheet is still
viscous at this stage; it can later be
shaped into various products. Source:
T. W. Chou, R. L. McCullough, and R.
B. Pipes.

Examples of Molding Processes
Figure 18.20 (a) Vacuum-bag forming. (b) Pressure-bag forming. Source: T. H. Meister.
Figure 18.21 Manual
methods of processing
reinforced plastics: (a)
hand lay-up and (b)
spray-up. These
methods are also
called open-mold
processing.

Filament Winding
(b)(a)
Figure 18.22 (a) Schematic illustration of the filament-winding process. (b) Fiberglass
being wound over aluminum liners, for slide-raft inflation vessels for the Boeing 767
aircraft. Source: Brunswick Corporation, Defense Division.

Pultrusion
illustration of the pultrusion
process.

Design Modifications to Minimize Distortion
Figure 18.24 Examples of design
modifications to eliminate or
minimize distortion of plastic parts.
(a) Suggested design changes to
minimize distortion. Source: F.
Strasser. (b) Die design
(exaggerated) for extrusion of
square sections. Without this
design, product cross-sections
swell because of the recovery of
the material; this effect is known as
die swell. (c) Design change in a
rib, to minimize pull-in caused by
shrinkage during cooling. (d)
Stiffening the bottoms of thin
plastic containers by the bottoms of
thin plastic containers by doming-
this technique is similar to the
process used to make the bottoms
of aluminum beverage cans.

Comparative Costs and Production Volumes
for Processing of Plastics
TABLE 18.2
Typical production volume, number of parts
Equipment
capital cost
Production
rate
Tooling
cost 10 10
2
10
3
10
4
10
5
10
6
10
7
Machining Medium Medium Low
Compression molding High Medium High
Transfer molding High Medium High
Injection molding High High High
Extrusion Medium High Low *
Rotational molding Low Low Low
Blow molding Medium Medium Medium
Thermoforming Low Low Low
Casting Low Very low Low
Forging High Low Medium
Foam molding High Medium Medium
Source: After R.L.E. Brown, Design and Manufacture of Plastic Parts. Copyright (c) 1980 by John Wiley & Sons, Inc.
Reprinted by permission of John Wiley & Sons, Inc.
*Continuous process.

Economic Production Quantities for Various
Molding Methods
TABLE 18.3
Relative investment
required Relative Economic
Molding method Equipment Tooling
production
rate
production
quantity
Hand lay-up
Spray-up
Casting
Vacuum-bag molding
Compression-molded BMC
SIVIC and preform
Pressure-bag molding
Centrifugal casting
Filament winding
Pultrusion
Rotational molding
Injection molding
VL
L
M
M
H
H
H
H
H
H
H
VH
L
L
L
L
VH
VH
H
H
H
H
H
VH
L
L
L
VL
H
H
L
M
L
H
L
VH
VL
L
L
VL
H
H
L
M
L
H
M
VH

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