Chapter 6B: Forming and shaping polymer

• 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.

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.

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• 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.

• 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.

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• 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.

• 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).

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.

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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.

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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.

• 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.

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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.

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• 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.

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.

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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.

• 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.

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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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Thermoforming • Thermoforming is a process for forming thermoplastic sheets or films

over a mold by means of the application of heat and pressure.• In this process, a sheet is (a) clamped and heated to the sag point

(above the glass-transition temperature, of the polymer), usually by radiant heating, and (b) forced against the mold surfaces through the application of a vacuum or air pressure.

• Fig 19.18 shows the various thermoforming processes for a thermoplastic sheet. These processes commonly are used in making advertising signs, cookie and candy trays, panels for shower stalls, and packaging.

Process capabilities• Typical parts made by thermoforming are packaging, trays for cookies

and candy, advertising signs, refrigerator liners, appliance housings, and panels for shower stalls.

• Molds for thermoforming usually are made of aluminum because high strength is not required, hence tooling is relatively inexpensive.

• Quality considerations include tearing of the sheet during forming, nonuniform wall thickness, improperly filled molds, poor part definition, and lack of surface details.

Compressing Molding• In compression molding, a preshaped charge of material, premeasured

volume of powder, or viscous mixture of liquid-resin and filler material is placed directly into a heated mold cavity that typically is around 200°C but can be much higher.

• Fig 19.19 shows the types of compression molding—a process similar to forging: (a) positive, (b) semipositive, and (c) flash, which is later

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trimmed off. (d) Die design for making a compression-molded part with external undercuts.

• Compression molding is used mainly with thermosetting plastics with the original material being in a partially polymerized state.

Process Capabilities• Three types of compression molds are available:1. Flash type: for shallow or flat parts2. Positive type: for high-density parts3. Semipositive type: for quality production

Transfer Molding• Transfer molding represents a further development of the process of

compression molding. • The uncured thermosetting resin is placed in a heated transfer pot or

chamber and after the material is heated, it is injected into heated closed molds.

• Fig 19.20 shows the sequence of operations in transfer molding for thermosetting plastics. This process is suitable particularly for intricate parts with varying wall thickness.

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Process capabilities• Typical parts made by transfer molding are electrical connectors and

electronic components, rubber and silicone parts, and the encapsulation of microelectronic devices.

• The process is suitable particularly for intricate shapes with varying wall thicknesses.

• The molds tend to be more expensive than those for compression molding, and some excess material is left in the channels of the mold during filling, which is later removed.

Casting• Some thermoplastics (such as nylons and acrylics) and thermosetting

plastics (epoxies, phenolics, polyurethanes, and polyester) can be cast into a variety of shapes using either rigid or flexible molds.

• Fig 19.21 shows the schematic illustration of (1) casting, (2) potting, and (3) encapsulation processes for plastics and electrical assemblies, where the surrounding plastic serves as a dielectric.

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