Polymer Processing

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Processing Method– Choice is dictated by the product desired and

the quantity desired.• Fiber, film, sheet, tube• Cup, bucket, car bumper, chair.

– Fiber manufacture is different, it is continuous.– Large quantities usually use extrusion or

injection molding– Smaller quantities use compression molding or

transfer molding

Processes for Thermoplastics:• Extrusion• Blow Molding• Injection Molding• Thermoforming• Calendaring• Spinning

Forming Processes for Thermosetting polymers:• Compression Molding• Transfer Molding

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Thermoplastic: the resin is heated to make a viscous liquid and then processed into a usable object without much additional chemistry. Example: polyethylene, polystyrene.

Thermoset: upon heating, further reaction occurs to make molecules “set up” into a useful product. Chemistry occurs, so these are sometimes called “reactive polymers”. The resin may be provided as either small molecules or “prepregs”—partially polymerized stuff. Example: polyurethanes, phenol-formaldehyde, melamine-formaldehyde, epoxy glue.

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Compression Molding

Platen

Mold Plunger

Guide Pins

Mold Cavity

Platen

Hydraulic Plunger

Heat and Cooling

Heat and Cooling

Hydraulic Pressure

Compound to be molded

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Injection Molding

Hydraulic Pressure

Nozzle Feed hopper,contains polymer pellets

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• Polymer is heated, mixed, the then forced to flow into a mold cavity

• Similar to extrusion– Hopper, barrel, screw

• Screw rotation is the principal motion only in one part of the cycle– Mixes, compacts, plasticizes, and heats

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• In the injecting stage, the screw is driven axially by a piston to generate the working pressure– 150 – 250 MPa (21,756 – 36,260 psi)

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Injection Molding Sequences

(1) Close the mold (2) Inject the melt

(3) Retract the screw (4) Open mold – eject part

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Two Plate Mold• The mold here is closed• The mold is position

between two platens– One stationary– One moveable

• Note the water channels for quickly cooling the mold and its polymer load.

Sprue: channel from die nozzle

Into the mold

Runner: channel from SprueInto the cavity

Gates: restrict the polymer

Flow into the cavity

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Two Plate Mold2• The mold here is open• The ejector pins push the

rather fragile plastic from the mold cavity

• The sprue and runners are waste..

Blow Molding—e.g. milk bottle

Extruded Parison-Mold Open

Mold Closed and Bottle Blown

Finished Bottle Removed from Mold

Plastic

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Four-roll calender Wad of plastic

To conditioningequipment

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Extrusion

• This process is fundamental to both metals and ceramics as well as polymers.

• Definition– Extrusion is a compression process

• Material is forced to flow through a die orifice• Cross-sectional shape determined by the shape of

the orifice• Product is long and continuous

A Plastics Extruder

Feed hopper

HeatersCores for

cooling water

Die

Screw

Drive shaft

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• Rarely used for thermosetting polymers

• Products• Tubing, pipes, and hose• Window and door moldings• Sheet and film• Continuous filaments (as we saw in module 3A)• Coated electrical wire and cable.

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• The extruder consists basically of– a hopper and – a barrel and – a screw.

Fiber Drawing

Drive roll

Heater (Optional)

Drawn yarn to bobbin

Undrawn pretwisted yarn

Control rolls

Snubbing pin

Skewed idler roll1

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(2 > 1)

Stretching Zone

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Dry Spinning: From solution

Melt Spinning: From Melt

Nylon 6,6 & PETECellulose Acetate

Wet Spinning: From solution into solution

Kevlar, rayon, acrylics, Aramids, spandex

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Mel

t Spi

nner

Bobbin

Extruded Fiber Coolsand Solidifies Here

Metered Extrusion(controlled flow)

Melting Zone

Polymer Chips/Beads

Pump

Filter and Spinneret

Air Diffuser

Heating Grid

Pool

Lubrication by oil disk and trough

Packaging

Bobbin drive

Yarn driver

Feed rolls

Moisture Conditioning Steam Chamber

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Melt spinning

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Feed

Filtered polymer solution

Metered extrusion Pump

Filter and spinneret

Solidif ication by solvent evaporation

Heated chamber

Lubrication

Air inlet

Feed roll and guide

Yarn driving

Balloon guide

Packaging

Ring and traveler

Bobbin transverse

Spindle

Dry Spinning

Dry

Spi

nnin

g of

Fib

ers

from

a S

olut

ion

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Wet Spinning (e.g. Kevlar)

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Thermoforming

• Three major types of thermoforming

– Vacuum• Pressure limit of 1 atmosphere

– Pressure • Higher allowable pressures

– Mechanical

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Vacuum Thermoforming

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Pressure Thermoforming

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Mechanical Thermoforming

• In (1) the polymer is pre stretched• In (2) the polymer is draped over the positive mold and pressure applied to

force it in place29

Mechanical Thermoforming

• In (1) the polymer is pre heated• In (2) the polymer is forced into place in the negative mold.

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