Plastic forming.ppt

2005 Pearson Education South Asia Pte Ltd
Forming and Shaping Processes and
Equipment
Manufacturing Engineering and Technology
13.Rolling of Metals
14.Forging of Metals
15.Extrusion and Drawing of Metals
16.Sheet-Metal Forming Processes
17.Processing of Metal Powders
18.Processing of Ceramics, Glass and
Superconductors
19.Forming and Shaping Plastics and Composite
Materials
20.Rapid-Prototyping Operations

19. Forming and Shaping Plastics and Composite Materials
Chapter Objectives
• Extrusion as a basic process for producing rods,
tubing, and pellets for subsequent use.
• How discrete plastic parts (such as bottles,
automotive parts, and electrical parts with metal
components) are made.
• Production of plastic sheets and films.
• How reinforced plastics and various composite
materials are produced.
• Characteristics of machinery used, mold design,
and economic considerations.

Chapter Outline
1. Introduction
2. Extrusion
3. Injection Molding
4. Blow Molding
5. Rotational Molding
6. Thermoforming
7. Compression Molding
8. Transfer Molding
9. Casting
10.Foam Molding

Chapter Outline
11.Cold Forming and Solid-Phase Forming
12.Processing Elastomers
13.Processing Polymer-Matrix Composites Processing
Metal-Matrix and Ceramic-Matrix
14.Composites Design
15.Considerations
16.Economics of Processing Plastics and Composite
Materials

19.1 Introduction
• Table 19.1 shows the general characteristics of
forming and shaping processes for plastics and
composite materials.

19.1 Introduction
• Plastics usually are shipped to manufacturing
plants as pellets, granules, or powders and are
melted (for thermoplastics) just before the
shaping process.
• With increasing awareness of our environment,
raw materials also may consist of reground or
chopped plastics obtained from recycling centers.
• Fig 19.1 shows the outline of forming and shaping
processes for plastics, elastomers, and composite
materials.

19.1 Introduction

19.2 Extrusion
• In extrusion, which constitutes the largest volume
of plastics produced, raw materials in the form of
thermoplastic pellets, granules, or powder are
placed into a hopper and fed into the barrel of a
screw extruder.
• Fig 19.2(a) shows the schematic illustration of a
typical screw extruder. (b) Geometry of an
extruder screw. Complex shapes can be extruded
with relatively simple and inexpensive dies.

19.2 Extrusion

19.2 Extrusion
• Screws have three distinct sections:
1. Feed section: Conveys the material from the
hopper into the central region of the barrel.
2. Melt section (also called compression or transition
section): Where the heat generated by the
viscous shearing of the plastic pellets and by the
external heaters causes melting to begin.
3. Metering or pumping section: Where additional
shearing (at a high rate) and melting occur with
pressure building up at the die.

19.2 Extrusion
• Fig 19.3 shows the Common extrusion die
geometries: (a) coat-hanger die for extruding
sheet; (b) round die for producing rods; and (c)
and (d) nonuniform recovery of the part after it
exits the die.

19.2 Extrusion
• The control of processing parameters such as
extruder-screw rotational speed, barrel-wall
temperatures, die design, and rate of cooling and
drawing speeds are important in order to ensure
product integrity and uniform dimensional
accuracy.
• Die shape is important, as it can induce high
stresses in the product, causing it to develop
surface fractures (as also occur in metals).

19.2.1 Miscellaneous extrusion processes
Plastic tubes and pipes
• These are produced in an extruder with a spider
die.
• Fiber or wire reinforcements also may be fed
through specially designed dies in this operation for
the production of reinforced hoses that need to
withstand higher pressures.
• The extrusion of tubes is also a necessary first step
for related processes, such as extrusion blow
molding and blown film.
• Fig 19.4 shows the extrusion of tubes. (a) Extrusion
using a spider die and pressurized air. (b)
Coextrusion for producing a bottle.

Rigid plastic tubing
• Extruded by a process in which the die is rotated,
rigid plastic tubing causes the polymer to be
sheared and biaxially oriented during extrusion.
• As a result, the tube has a higher crushing
strength and a higher strength-to-weight ratio
than conventionally extruded tubes.

Coextrusion
• Coextrusion involves simultaneous extrusion of two
or more polymers through a single die.
• The product cross-section thus contains different
polymers—each with its own characteristics and
function.
• Coextrusion commonly is performed in shapes such
as flat sheets, films, and tubes, and is used
especially in food packaging where different layers
of polymers have different functions, such as (a)
inertness for food, (b) serving as barriers to fluids
such as water or oil, and (c) labeling of the product.

Plastic-coated electrical wire
• Electrical wire cable, and strips also are extruded
and coated with plastic by this process.
• The wire is fed into the die opening at a controlled
rate with the extruded plastic in order to produce a
uniform coating.
• Plasticcoated paper clips also are made by
coextrusion.
• To ensure proper insulation, extruded electrical
wires are checked continuously for their resistance
as they exit the die; they also are marked
automatically with a roller to identify the specific
type of wire.

Polymer sheets and films
• These can be produced by using a specially
designed flat-extrusion die.
• Also known as the coathanger die, it is designed
to distribute the polymer melt evenly throughout
the width.
• The polymer is extruded by forcing it through the
die, after which the extruded sheet is taken up.
• Generally, polymer sheet is considered to be
thicker than 0.5 mm, and film is thinner than 0.5
mm.

Thin polymer films
• Common plastic bags and other thin polymer film
products are made from blown film, which is
made from a thin-walled tube produced by an
extruder.
• In this process, a tube is extruded continuously
vertically upward and then expanded into a
balloon shape by blowing air through the center of
the extrusion die until the desired film thickness is
reached.

Thin polymer films
• Fig 19.5(a) shows the schematic illustration of the
production of thin film and plastic bags from
tube—first produced by an extruder and then
blown by air. (b) A blown-film operation. This
process is well developed, producing inexpensive
and very large quantities of plastic film and
shopping bags.

Plastic films
• Plastic films, especially polytetrafluoroethylene
(PTFE; trade name: Teflon), can be produced by
shaving the circumference of a solid, round
plastic billet using specially designed knives in a
manner similar to producing veneer from a large
piece of round wood.
• The process is called skiving.

Pallets
• Used as raw material for other plastics-
processing methods described in this chapter,
pellets also are made by extrusion.
• A small-diameter, solid rod is extruded
continuously and then chopped into short lengths
(pellets).
• With some modifications, extruders also can be
used as simple melters for other shaping
processes, such as for injection molding and blow
molding.

Example 19.1 Blown film
Assume that a typical plastic shopping bag made by
blown film has a lateral dimension (width) of 400 mm. (a)
What should be the extrusion-die diameter? (b) These
bags are relatively strong in use. How is this strength
achieved?

Solution
a. The perimeter of the flat bag is (2)(400) = 800 mm.
Since the original cross-section of the film is round, the
blown diameter should be (Pi)D = 800, thus D = 225
mm. Recall that in this process, a tube is expanded from
1.5 to 2.5 times the extrusion-die diameter. Taking the
maximum value of 2.5, we calculate the die diameter as
225/2.5 = 100 mm.

Solution
b. Note in Fig. 19.5a that after being extruded, the
balloon is being pulled upward by the pinch rolls. Thus,
in addition to diametral stretching and the attendant
molecular orientation, the film is stretched and oriented
in the longitudinal direction. The resulting biaxial
orientation of the polymer molecules significantly
improves the strength and toughness of the plastic bag.

19.2.2 Production of polymer reinforcing fibers
• Most synthetic fibers used in reinforced plastics
are polymers that are extruded through the tiny
holes of a device called a spinneret (resembling
a shower head) to form continuous filaments of
semi-solid polymer.
• This process of extrusion and solidification of
continuous filaments is called spinning.
• There are four methods of spinning fibers: melt,
wet, dry, and gel spinning.

1. In melt spinning, the polymer is melted for
extrusion through the spinneret and then solidified
directly by cooling. Fig 19.6 shows the melt-
spinning process for producing polymer fibers.
The fibers are then used in a variety of
applications, including fabrics and as
reinforcements for composite materials.
2. Wet spinning is the oldest process for fiber
production and is used for polymers that have
been dissolved in a solvent. Acrylic, rayon, and
aramid fibers can be produced by this process.

3. Dry spinning is used for thermosets carried by a
solvent. Dry spinning may be used for the
production of acetate, triacetate, polyether-based
elastane, and acrylic fibers.
4. Gel spinning is a special process used to obtain
high-strength or special fiber properties. bath.
Some high-strength polyethylene and aramid
fibers are produced by gel spinning.

• A necessary step in the production of most fibers
is the application of significant stretching to
induce orientation of the polymer molecules in the
fiber direction.
• This orientation is the main reason for the high
strength of the fibers compared to the polymer in
bulk form.
• Graphite fibers are produced from different
polymer fibers by pyrolysis.

19.3 Injection Molding
• Injection molding is similar to hot-chamber die
casting.
• The pellets or granules are fed into the heated
cylinder, and the melt is forced into the mold
either by a hydraulic plunger or by the rotating
screw system of an extruder.
• Fig 19.7 shows the schematic illustration of
injection molding with (a) plunger and (b)
reciprocating rotating screw.

• Fig 19.8 shows the sequence of operations in the
injection molding of a part with a reciprocating
screw. This process is used widely for numerous
consumer and commercial products, such as
toys, containers, knobs, and electrical equipment.
• Fig 19.9 shows the typical products made by
injection molding, including examples of insert
molding.

• After the part has cooled sufficiently (for
thermoplastics) or cured (for thermosets), the
molds are opened and the part is removed from
the mold using ejectors.
• Fig 19.10 shows the Illustration of mold features
for injection molding. (a) Two-plate mold with
important features identified. (b) Schematic
illustration of the features in a mold.

• There are three basic types of molds:
1. Cold-runner, two-plate mold: this design is the
simplest and most common, as shown in Fig.
19.11a.
2. Cold-runner, three-plate mold (Fig. 19.11b): the
runner system is separated from the part when
the mold is opened.
3. Hot-runner mold (Fig. 19.11c), also called
runnerless mold: the molten plastic is kept hot in
a heated runner plate.

• Multicomponent injection molding (also called
coinjection or sandwich molding) allows the
forming of parts with a combination of various
colors and shapes.
• Insert molding involves metallic components
(such as screws, pins, and strips) that are placed
in the mold cavity prior to injection and then
become an integral part of the molded product.

Case Study 19.1 EPOCH Hip Replacement
Each year, over one million artificial joints are replaced
world-wide, thus eliminating pain and greatly improving
the quality of life of their recipients. A total hip
replacement usually is comprised of two major
components: (a) a femoral component placed in the leg
and (b) an acetabular component for the hip. The
manufacture of hip stems was described in Example I.5
of the General Introduction; the EPOCH hip stem is a
recent development based on polymer processing
technology.

One of the problems that has been encountered with total hip
replacements is that of bone resorption. To keep it from breaking
down and being reassimilated into the body, healthy bone must be
stressed to maintain its strength and vigor. However, metal
implants that are very stiff tend to transfer loads to the femur
differently than the patient’s natural hip, and much of the bone
towards the top of the femur is subjected to very little stress. The
body reacts to this low-stress state by resorbing the bone. This
loss of bone mass can result in failure of the bone and lead to
fatigue failure of the implant because of the new and harmful
loading environment. This is serious, because repairing or revising
an implant is significantly more painful for the patient and involves
significantly more rehabilitation, and revisions usually are not as
successful as the first implant.

A new and innovative implant design is the EPOCH
system, shown in Fig. 19.12. In this system, the hip
consists of three layers: (1) an outermost layer
comprised of a diffusion-bonded (see Section 31.7)
titanium-alloy mesh for bone in-growth, (2) a
thermoplastic polymer layer intended to provide a
stiffness similar to bone, and (3) a cobalt-alloy core to
provide static and fatigue strength. The polymer used in
this hip is from the polyaryletherketone (PAEK) family,
and the layers are sized to mimic bone stiffness while
maximizing fatigue strength.

The EPOCH hip is manufactured through an insert injection-
molding operation, as shown in Fig. 19.13. A 0.75-MN vertical
injection-molding machine is used (as opposed to a horizontal
injection-molding machine) in order to facilitate locating the
inserts. Two flexible pads are placed into seats on the mold
halves. A machined cobalt-alloy insert is preheated and placed in
the bottom half of the mold. The mold is closed, and the polymer
is injected into the cavity, thus adhering to the insert, partially
permeating into the porous pad, and integrally locking all
components together. After the part is ejected from the mold, the
flash is trimmed, and the hip stem is finished to obtain the desired
surface finish. A cross-section of the resulting structure is shown
in Fig. 19.12(b).

Overmolding
• This is a process for making products (such as
hinge joints and ball-and-socket joints) in one
operation and without the need for post-molding
assembly.
• In ice-cold molding, the same type of plastic is
used to form both components of the joint.

Process capabilities
• Injection molding is a high-rate production
process and permits good dimensional control.
• Injection molding is a versatile process capable of
producing complex shapes with good dimensional
accuracy and at high production rates.
• Methods of avoiding defects consist of the proper
control of temperatures,
• pressures, and mold design modifications usin
simulation software.

Machines
• Injection-molding machines are usually horizontal.
• Vertical machines are used for making small,
close-tolerance parts and for insert molding.
• Fig 19.14 shows a 2.2-MN injection-molding
machine.
• The tonnage is the force applied to keep the dies
closed during the injection of molten plastic into
the mold cavities and hold it there until the parts
are cool and stiff enough to be removed from the
die.

Example 19.2 Force required in injection molding
A 2-MN injection-molding machine is to be used to make
spur gears 110 mm in diameter and 12 mm thick. The
gears have a fine-tooth profile. How many gears can be
injection-molded in one set of molds? Does the
thickness of the gears influence your answer?

Solution
Because of the fine detail involved (fine gear teeth), let’s
assume that the pressures required in the mold cavity
will be on the order of 100 MPa. The cross-sectional
(projected) area of the gear is (Pi)(110)2/4=9500 mm2. If
we assume that the parting plane of the two halves of
the mold is in the middle of the gear, the force required
is (9500)(100)=950 kN.

Solution
Since the capacity of the machine is 2 MN, we have 2
MN of clamping force available. Hence, the mold can
accommodate two cavities and produce two gears
per cycle. Because it does not influence the cross-
sectional area of the gear, the thickness of the gear does
not influence directly the pressures involved, and thus, it
does not change the answer.

19.3.1 Reaction-injection molding
• In the reaction-injection molding (RIM) process, a
monomer and two or more reactive fluids are
forced at high speed into a mixing chamber at a
pressure of 10 to 20 MPa and then into the mold
cavity.
• Fig 19.15 shows the schematic illustration of the
reaction-injection molding process. Typical parts
made are automotive-body panels, water skis,
and thermal insulation for refrigerators and
freezers.

• Major applications of this process include
automotive parts (such as bumpers and fenders,
steering wheels and instrument panels), thermal
insulation for refrigerators and freezers, water
skis, and stiffeners for structural components.

19.4 Blow Molding
• Blow molding is a modified extrusion- and
injection-molding process.
• In extrusion blow molding, a tube or preform
(usually oriented so that it is vertical) is first
extruded.
• Fig 19.16(a) shows the schematic illustrations of
(a) the extrusion blow-molding process for making
plastic beverage bottles; (b) the injection blow-
molding process; and (c) a three-station injection
blow-molding machine for making plastic bottles.

19.4 Blow Molding

19.4 Blow Molding
• In some operations, the extrusion is continuous,
and the molds move with the tubing.
• In injection blow molding, a short tubular piece
(parison) first is injection molded into cool dies
(parisons may be made and stored for later use).
• A related process is stretch blow molding,
where the parison is expanded and elongated
simultaneously, subjecting the polymer to biaxial
stretching and thus enhancing its properties.
• Multilayer blow molding involves the use of
coextruded tubes or parisons and thus permits
the production of a multilayer structure.

19.5 Rotational Molding
• Most thermoplastics and some thermosets can be
formed into large, hollow parts by rotational
molding.
• In this process, a thin-walled metal mold is made
in two pieces (split-female mold) and is designed
to be rotated about two perpendicular axes.
• Fig 19.17 shows the the rotational molding
(rotomolding or rotocasting) process. Trash cans,
buckets, and plastic footballs can be made by this
process.

• A large variety of parts are made by rotational
molding, such as storage tanks of various sizes,
trash cans, boat hulls, buckets, housings, large
hollow toys, carrying cases, and footballs. Various
metallic or plastic inserts or components also may
be molded integrally into the parts being made by
this process.
• In addition to powders, liquid polymers
(plastisols) also can be used in rotational
molding—PVC plastisols being the most common
material.

Process capabilities
• Rotational molding can produce parts with
complex, hollow shapes with wall thicknesses as
small as 0.4 mm.
• Quality-control considerations usually involve
accurate weight of the powder, proper rotational
speed of the mold, and temperature–time
relationships during the oven cycle.

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