Plastic Extrusion

Question 1

net shape process

Answer 1

Further shaping is not needed.

Question 2

Plastic moulding

Answer 2

Almost unlimited variety of part geometries.
Less energy is required than for metals due to much lower processing temperatures.
Handling of product is simplified during production because of lower temperatures.
Painting or plating is usually not required.

Question 3

Thermoplastics

Answer 3

Chemical structure remains unchanged during heating and shaping.
More important commercially, comprising more than 80% of total plastics tonnage.

Question 4

Thermosets

Answer 4

Undergo a curing process during heating and shaping, causing a permanent change in molecular structure.
Once cured, they cannot be re-melted.

Question 5

Extrusion

Answer 5

Polymer pellets or powders are fed through a hopper into a barrel chamber of a screw extruder. Polymer gets heated, homogenised, compressed before being forced through a die opening by a rotating screw. The extrudate (extruded product) is cooled and hardened by jets of air or sprays of water while passing along a conveyor belt before being cut to length. Generally used for thermoplastics and elastomers.

Barrel length to internal diameter ratio between 10-30.
Initial heating of solid pellets; subsequent mixing and mechanical working generates additional heat.
Extruder screw speed ~60rev/min.

Question 6

Extruder Screw

Answer 6

 Spiraled flights with channels between them through which the polymer melt is moved.
 Pressure applied to the polymer is largely determined by the depth.

Question 7

Die Zone (Metering Section)

Answer 7

Melt passes through a screen pack (series of wire meshes) filtering contaminants and hard lumps, build up pressure, and straighten the flow of the polymer melt (removing its “memory” of the circular motion imposed by the screw).

Question 8

Viscoelasticity and die swell

Answer 8

 Hot plastic expands when exiting the die opening
 Extruded material “remembers” its former shape and attempts to return to it.
 The compressive stresses acting on the material as it enters the small die opening do not relax immediately.

Question 9

Solid Profiles

Answer 9

Polymers with high melt viscosities are best as they hold shape better during cooling. For shapes other than round the die opening is designed with a cross-section slightly different from desired profile.

Question 10

Hollow Profiles

Answer 10

Polymers melt flows around the legs supporting the mandrel to reunite into a monolithic tube wall.
Mandrels can include air channels through which air is blown to maintain the hollow form of the extrudate.

Question 11

Wire and Cable Coating

Answer 11

Polymer melt is applied to bare wire as it is pulled at high speed through a die.
A slight vacuum is drawn between wire and polymer to promote adhesion of coating.

Question 12

Sheet and Film

Answer 12

Die with manifold which spreads the polymer melt laterally before it flow through the die orifice.
High production rates require an effective cooling and collecting of film mechanisms integrated in the process.
Cooling by directing the extrudate into a quenching bath of water.
Cooling by using chill rolls to quickly quench and solidify the extrudate (chill-roll extrusion).

Question 13

Blown-Film

Answer 13

Combination of extrusion and blowing to produce a tube of thin film.
Upward extruded molten tube expands in size as simultaneously inflated with air through the mandrel die.
Rolls restrain the tube dimension and guide it to the pinch rolls.

Question 14

Melt fracture

Answer 14

 Stresses acting on the melt (immediately before and during its flow through the die) are high and cause irregular surface finish.
 Caused by turbulent flow of the melt through a sharply reduced die entrance that breaks up the melt.

Question 15

Sharkskin and bambooing

Answer 15

 Roughened surface of the melt upon exiting the die due to friction at the interface resulting in a velocity profile.
 Developing tensile stresses can cause minor ruptures and prominent marks on the surface.

Question 16

Compression Moulding

Answer 16

Solid granules or preformed tablets of unpolymerised plastic are introduced into an open, heated cavity. A heated plunger applies pressure to the plastics, melting it and making it turn into a fluid. The pressure in the cavity is maintained until the material is set using hydraulic or pneumatic presses.

 Long cycle times 1-20min depending on the rate of heat transfer and curing time of the polymer.
 Compression equipment 10-150MPa.
 Polished tool steel dies (often chrome plated).
 Mould temperature between 150-200°C (650°C possible).
 High surface finish.
 Low shrinkage, good dimensional accuracy.
 Simple shapes only due to limited flow of material.
 Recent developments in forming fibre-reinforced plastics, both thermoplastics and thermosets.

Question 17

Transfer Moulding

Answer 17

Unpolymerised charge is loaded into a chamber immediately ahead of the mould cavity and heated until molten. A plunger forces the soft polymer to flow through a runner system into a heated mould where it cures. Pressure and temperature in the cavity are maintained until the thermosetting resin has completely cured.
2 variants:
 Pot transfer moulding - charge is injected from a “pot” through a vertical sprue channel into cavity.
 Plunger transfer moulding – plunger injects charge from a heated well through channels into cavity.

 Less turbulence and less uneven flow than compression moulding.
 Transfer moulding equipment up to 300MPa.
 Good for small and complex parts.
 Excellent detail and good dimensional accuracy.
 Good for incorporating inserts.
 Main limitation is scrap from sprue and runner system.