ch-19 forming and shaping plastics

Ch-19 Forming and shaping plastics

Thermoplastic (TP) – Polymers that can be shaped when heated and regain original hardness & strength upon cooling– Have a linear or branched structure (weak secondary bonds)– Process is reversible– Acrylics, cellulosics, nylons, polyethylenes, polyvinyl chloride• Thermoset (TS) – Polymers that become permanently set when heated– Have a cross-linked structure (strong secondary bonds)– Process is irreversible– Epoxy, polyester, urethane, phenolics, silicones• Elastomer (Rubber) – Elastic; low elastic modulus– Tires, footwear, gaskets, flooring, weather stripping, hoses

Forming and Shaping Processes for Plastics, Elastomers, and Composite Materials

Figure 19.1 Outline of forming and shaping processes for plastics, elastomers, and composite materials. (TP = Thermoplastics; TS = Thermoset; E = Elastomer.)

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Forming and shaping plastics - Extrusion

• raw materials in form of thermoplastic (TP) pellets, granules, or powder are placed into a hopper and fed into the extruder barrel (F19.2).

• barrel is equipped with a screw that blends pellets & conveys them down the barrel.

• internal friction from the mechanical action of the screw, along with heaters around the extruder’s barrel, heats the pellets and liquefies them.

• Screws have 3 distinct sections:1. Feed section that conveys material from hopper area into central

region of barrel.2. Melt section: heat generated by shearing of plastic and by heaters

causes melting to begin.3. Pumping section: additional shearing and melting occurs, with

pressure buildup at die.

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• Molten plastic or elastomer is forced through a die.• The extruded product is then cooled, either by exposing it to air or by

passing it through a water-filled channel.• The extruded product is then coiled or cut into desired lengths.• Typical products: solid rods, channels, tubing, pipe, window frames,

sheet, Pellets for other plastic processing methods• Plastic-coated electrical wire, and cable are also extruded. The wire is fed

into the die opening at a controlled rate with the extruded plastic.• Extruders can also be used as simple melters for injection molding and

blow molding.• Process parameters:

1. extruder-screw speed2. barrel-wall temp3. die design4. cooling and drawing speeds


Plastic Tubes and Pipes Extrusion

Figure 19.4 Extrusion of tubes. (a) Extrusion using a spider die (see also Fig. 15.8) and pressurized air. (b) Coextrusion for producing a bottle.

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Extrusion - Thin Polymer Films

• Extruders are rated by diameter D of barrel & length to diameter (L/D) ratio of barrel.

• Typical commercial units: D=25-200mm, L/D = 5-30. Extrusion equipment cost: $300,000.

• Thin polymer films and common plastic bags are made from a tube produced by an extruder (F18.4).

• in this process, a thin walled tube is extruded vertically upward.

• then expanded into a balloon shape by blowing air through the center of the extrusion die until desired film thickness is reached.

FIGURE l9.5 (a) Schematic illustration of the production of thin film and plastic bags fromtube-first produced by an extruder and then blown by air. (b) A blown-film operation. Thisprocess is well developed, producing inexpensive and very large quantities of plastic film andshopping bags. Source: (b) Courtesy of Wind Moeller 86 Hoelscher Corp.

Extrusion - Thin Polymer Films

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Injection molding (IM)

• Pellets or granules are fed into the heated cylinder, and the melt is forced into a split die chamber, either by hydraulic plunger or by a rotating screw system of an extruder.

• After part is sufficiently cooled, molds are opened and part is ejected.

• IM pressures: 70-200MPa• Typical products: cups,

containers, housings, tool handles, knobs, electrical components, toys, plumbing fittings.

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• Complex shapes and Good dimensional accuracy.• 3 basic types of molds:

1. cold runner 2-plate mold: solidified plastic in channels that connect the mold cavity to end of barrel must be trimmed.

2. cold-runner 3-plate: runner system is separated from part when mold opens.

3. Hot-runner mold: more expensive, no gates, runners, or sprues attached to molded part. Molten plastic is kept hot in a heated runner plate. Shorter cycle times.

• Insert molding: metallic components such as screws, pins, and strips.

Products Made by Injection Molding

Figure 19.9 Typical products made by injection molding, including examples of insert molding. Source: (a) Courtesy of Plainfield Molding, Inc. (b) Courtesy of Rayco Mold and Mfg. LLC.

(b)(a)

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• Process capabilities:– Good dimensional control.– Cycle times: 5-60sec, several min for thermoset (TS) materials.– Mold materials: tool steels, beryllium, Cu, or Al.– Mold costs: up to $100,000– Mold life: up to 2M cycles for steel molds, 10,000 cycles for Al molds.

• Machines: horizontal or vertical (small close-tolerance parts and for insert molding)

• Injection molding machines are rated according to the capacity of the mold and the clamping force. Force: 0.9-2.2MN.

• Cost of a 100 ton machine: $60,000-90,000.• Cost of a 300 ton machine: $85,000-140,000.• Cost of dies: $20,000-200,000.

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Reaction injection molding (RIM)

• A mixture of resin with two or more reactive fluids is forced into the mold cavity at high speed (F18.8).

• Chemical reactions take place rapidly in the mold, and the mold solidifies into a thermoset part.

• Typical parts: automotive bumpers and fenders, thermal insulation for ref and freezers.

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Structural foam molding (SFM)

• SFM process is used to make plastic products with a solid outer skin and a cellular inner structure.

• Typical products: furniture components, TV cabinets, business machine housings.

• Injection foam molding:• thermoplastics are mixed with a blowing agent (inert gas such

as N2), which expands the material.• The core of part is cellular, and the skin is rigid.• Thickness of skin: up to 2 mm.• Part densities as low as 40% of the density of solid plastic.

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blow molding

• Extrusion blow molding:1. a tube is first extruded or injection

moldod, 2. Then clamped into a mold cavity

much larger than tube diameter.3. Finally blown outward to fill the mold

cavity (F18.9a).4. Blowing is usually done with an air

blast at a pressure of 350-700KPa.5. The molds close around the tubing,

close off both ends (thereby breaking the tube into sections), and then move away as air is injected into the tabular piece.

6. The part is then cooled and ejected.• Typical parts: beverage bottles and

hollow containers.

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rotational molding

• Thin walled metal mold is made of 2 pieces and is designed to be rotated about 2 per axes (F18.10).

• A pre-measured quantity of powdered plastic material is placed inside the worm mold.

• The mold is then heated, while rotated about the 2 axes.

• This action tumbles the powder against the mold, where heating fuses the powder without melting it.

• Typical parts: toys, carrying cases, footballs.

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thermoforming

• a sheet is heated in oven to the sag point.• Sheet is then removed from the oven, placed over a mold, and forced against the

mold through the application of a vacuum.• The mold is usually at RT, hence the shape of the plastic becomes set upon contact

with the mold.• Typical parts: ad signs refrigeration liners, packaging, and appliance housings.• Molds: AL. The holes in molds are usually less than 0.5mm, in order not to leave

any marks on the formed parts.

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compression molding (CM)

• A pre-shaped charge of material, a pre-measured volume of powder, or a viscous mixture of liquid resin and filler material is placed directly into a heated mold cavity.

• Forming is done under pressure from upper half of die (F18.12).

• CM is used mainly with thermosetting plastics, with original material being in a partially polymerized state. X-linking is completed in the heated die.

• Curing times: 0.5-5min.• Typical parts: dishes, handles,

container caps, fittings.• 3 types of compression molds:

1. Flash type: for shallow or flat parts.2. positive: for high density parts3. Semi positive: for quality prod.

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Transfer Molding (TM)

• Uncured TS material is placed in a heated transfer pot or chamber (F18.13).• After material is heated, it’s injected into heated closed molds. Curing takes

place by x-linking.• Typical parts: electrical and electronic comp, rubber and silicon parts.• TM Suitable for intricate shapes with varying wall thicknesses.

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casting

• Typical parts: gears, bearings, wheels, thick sheets.• convention casting of TP: a mixture of monomer, catalyst, and various additives is

heated and poured into the mold. Part forms after polymerization takes place at ambient pressure.

• centrifugal casting• Potting and encapsulation: potting (F18.14b) is done in a housing or case, which is

an integral part of product. In encapsulation (F18.14c), comp is coated with a layer of the solidified plastic

• Foam molding and casting:. Polystyrene beads are placed in a mold with a blowing agent and exposed to heat, usually by steam. As a result, the beads expand to as much as 50 times their original size and take the shape of the mold. Typical products: Styrofoam cups, food containers, insulating blocks, and shaped packaging materials.

• Polyurethane foam processing: it starts with mixing of two or more chemical components, the reaction forms cellular structure which solidifies in the mold.

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cold forming and solid phase forming

• Rolling, deep drawing, extrusion, closed die forging, coining, and rubber forming, can also be used to form many TP at RT.

• Important considerations:1. ductility at RT2. Material’s deformation must be non-recoverable (SB).• Advantages of cold forming1. Strength, toughness, and uniform elongation are increased.2. plastics with high molecular weights can be used3. Forming speeds are not affected by part thickness because there is no heating

or cooling involved.4. Cycle times shorter than molding processes.

• Solid phase forming: carried out at a temp from 10 to 20oC below melting temp of plastic, while it’s still in a solid state.

ch-19 forming and shaping plastics

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