Processing of Polymers

• Increasing use of polymers– Light, tough, corrosion resistant and cheap– Price: Raw material + Processing

• Processing of “Solid” Polymers:– Polymer heated to the melt state, shaped under high

pressure and cooled down to room temperature (below Tgor Tm) to preserve its shape.

– Shaping involves shear, bulk and elongational deformations of the polymer melt, which have different viscoelastic characteristics.

From: Principles of Polymer Engineering (McCrum et al. 1995)

Processing of Polymers

• Extrusion– Films and sheets– Fibers and filaments– Pipes, tubing and profiles– Wire coating

• Injection Molding• Thermoforming• Blow Molding• Compression and Transfer Molding

Extrusion

• Most common polymer processing (60% of world production).

• Manufactures an endless product of constant cross-section that is cut, sawed, chopped, rolled or reduced to specific length.

• Two-part process including “plastication” and “shaping”

– Extruder converts a solid feedstock into a homogeneous melt and pumps it through a die at a uniform rate.

– Following the die, a train of equipment ensures the extrudatecools to the right shape with the required molecular orientation.

Extrusion:• Extruder: Plastication

– Cold polymer granules fed into a hopper and supplied to the screw by gravity– Granules advance between the flights of the screw and the hot walls of the barrel– As the polymer advances along the extruder, it becomes liquid

(by melting or by passing through the glass transition)– The polymer becomes pressurized at it moves along the extruder,

so it can easily exit the extruder through a die.

Extrusion:

Screw Parameters:

1) Zones (number, lengths, types)

2) Helix angle (φ)

3) Flight width (W)

4) Channel depth (H)

5) Screw Length to Diameter ratioA.) Conventional; B.) For “fragile” polymers; C.) PVC-type; D.) Nylon-type

Extrusion:

• Shaping: Die shape dictates the nature of the polymer product. – Circular die: Melt spinning of fibers– Slit die: Sheet casting– Annular die:

• Film blowing • Pipes (φ > 12 mm) and tubing (φ > 12 mm)• Wire coating

Extrusion:

• The Die-swell Issue: A polymer extrusion problem…– Phenomenon: Swelling of the extrudate occurs at the die exit.– Explanation: Polymer liquid is stressed as the cross section decreases– Swelling is due to unrelaxed stresses (rubber elasticity).– Swell ratio (DS/DC) depends on residence time in constrained region (die). – Parameters which affect the swell ratio: LC, γ’, MW, T

DC DSDB

Extrusion:

• Film Blowing

Extrusion:

• Pipes and Tubing

Extrusion:

• Wire Coating

Extrusion: Die-swell IssueSolution : 1) Keep die swelling phenomenon. Why?

2) Design die profile to account for die swelling.2’) Use pull rolls to draw down extrudate.

Injection Molding:

• Injection + Molding

Reciprocating-screw injection molding machines are the most common (thermoplastics and thermosets). Do we need to apply pressure during molding ? Why ?

Injection Molding:

• Injection + Multiple Moldings

Cycle time: several seconds to several minutes.

Injection Molding:• Gate Designs: Cause work to be done to make the liquid flow

through a small constriction, so T increases (η decreases).– Important for homogenization– Important for control of orientation– Important for control of surfaces

Injection Molding:• Pressure, Temperature and Velocity Controls:

Thermoforming• Application for large formings, thin wall packaging, short-run or

prototype products. Much lower initial investment than injection molding.

With matched molds Pressure-bubble vacuum-snapback technique

Blow Molding

• Blow molding is widely used for producing hollow containers in vast numbers and cheaply.

• Three major variants:– Extrusion-blow molding– Injection-blow molding– Stretch-blow molding

• Common features– Formation of a precursor (hollow tube called a parison).– One end of parison is closed, so it can be inflated in the heated, softened state.– Inflation continues until parison touches the walls of the cooled mold.– Mold is opened and bottle released.

• Four-step Process– A) Extrusion of parison in open mold– B) Mold closes and parison is inflated– C) When parison reaches the walls of the mold, it is cooled– D) The mold is open and the bottle released

Extrusion-Blow Molding

Injection-Blow Molding

• Three-steps at three stations– A) Parison is injection molded on to a

steel rod.– B) Parison is inflated by air and take

the shape of the mold.– C) The mold opens and the bottle is

released.

Compression and Transfer Molding

• Compression Molding– Historically, the first technique for mass production– Used currently only for cross-linked polymers– Partially cross-linked polymer between two-halves of mold

Upper part of mold is lowered and polymer is compressedCross-linking reaction is completed under controlled P&TMold is open and product released

– Advantages:Flow over shorter distances, thus less frozen-in stressesCheaper and easier mold designCheaper mold maintenance

Compression and Transfer Molding

• Transfer Molding– Measured charge of partially

cross-linked polymer is heated in a pot, from which it is rammed into a heated mold.

– Forcing polymer through a gate leads to homogenization and pressure increase and heating.

– Cross-linking is completed and solidified sample released.

– Use for the production of precision shapes (electronic industry).