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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.

Processing of Polymers (chapter 10)

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Forming and

Shaping Processes

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

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Extruder

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Die geometry (coat-hanger die) for extruding sheet.

Schematic illustration of the production of thin film and plastic bags from tube first produced by an extruder and then blown by air.

Sheet and Film Extrusion

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(c)

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.

Injection Molding

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Typical products made by injection molding, including examples of insert molding.

Examples of Injection Molding

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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.

Injection-Molding Machine

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Reaction-Injection Molding

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Schematic illustrations of (a) the blow-molding process for making plastic beverage bottles, and (b) a three-station injection blow-molding machine.

Blow Molding

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

Rotational Molding

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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.

Thermoforming Processes

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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.

Compression Molding

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

Transfer Molding

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Schematic illustration of (a) casting, (b) potting, (c) encapsulation of plastics.

Casting, Potting and Encapsulation

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Schematic illustration of calendering. Sheets produced by this process are subsequently used in thermoforming.

Reinforced- plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swingarm, a wheel, and brake disks.

Calendering and Examples of Reinforced Plastics

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(a) Manufacturing process for polymer-matrix composite. (b) Boron-epoxy prepreg tape.

(b)(a)

Prepegs

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(a) Single-ply layup of boron-epoxy tape for the horizontal stabilizer for F-14 fighter aircraft. (b) A 10 axis computer-numerical-controlled tape-laying system. This machine is capable of laying up 75 mm and 150 mm wide tapes, on contours of up to ±30° and at speeds of up to 0.5 m/s

(a)

(b)

Tape Laying

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Sheet Molding

The manufacturing process for producing reinforced-plastic sheets. The sheet is still viscous at this stage; it can later be shaped into various products.

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Examples of Molding Processes(a) Vacuum-bag forming. (b) Pressure-bag forming..

Manual methods of processing reinforced plastics: (a) hand lay-up and (b) spray-up. These methods are also called open-moldprocessing.

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Filament Winding

(b)(a)

(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.

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Pultrusion

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Design Modifications to Minimize Distortion

Examples of design modifications to eliminate or minimize distortion of plastic parts. (a) Suggested design changes to minimize distortion. (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.

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Comparative Costs and Production Volumes for Processing of Plastics

Typical production volume, number of parts Equipment

capital cost Production

rate Tooling

cost

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2

10

3

10

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10

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10

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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 *Continuous process.

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Economic Production Quantities for Various Molding Methods

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