15 - Polymer processes

Question 1

Vacuum forming

Answer 1

Thermoplastic sheet up to 6mm thick is clamped, heated, and draped on to the mould. A vaccum then sucks the sheet tight to the mould, before the sheet is left to cool and then ejected from the mould

• Mould must have air holes and rounded corners
• Small-scale batch and mass production

Uses: Trays, takeaway coffee cups, yoghurt pots

Question 2

Thermoforming

Answer 2

Similar to vacuum forming, but an additional ‘positive’ mould is pressed on to the polymer sheet as the vacuum is applied

-Good for fine detail such as moulded in logos
-Slow process, used in industrial production

Uses: Food packaging

Question 3

Calendering

Answer 3

Thermoplastic pellets

• Heated rollers squash and stretch the polymer to make it thinner
• Industrial production method, carried out by specialist manufacturers
• Used for continuous production

Uses: Thin polymer sheet and film

Question 4

Line bending

Answer 4

Thermoplastic sheet

• Uses a heating element which provides heat along a straight line, to soften the polymer so it can be bent
• Mainly used in schools and college workshops as it is slow and labout intensive
• Used for one-off or limited batch production

Uses: Acrylic boxes and shelves, or one-off displays for school etc.

Question 5

Lamination (lay-up)

Answer 5

Fibre-based composite sheets (e.g CRFP)

The bonding of two or more layers of materials using pressure, heat and a polymer adhesive (e.g polyester resin)

• Requires a former to mould around

Question 6

Injection moulding

Answer 6

Therrmoplastic granules travel down Hopper to the Archimedian Screw

Granules are heated and melted as they are pushed through by the Archimedian screw into a mould

• Complex 3D shapes can be easily made
• Fast industrial production method
• High start-up cost

Used for large scale mass or continuous production

Question 7

Blow moulding

Answer 7

Parison (tube) of polymer is placed in a mould.

Hot air is then blown into the parison, so that it expands to fill the mould

-It is used to create hollow, thin-walled components

-High start-up cost
-Used for contunuous production

Question 8

Rotational moulding

Answer 8

Thermoplastic powder or granules placed in a mould

Mould is heated and rotated, spreading a layer of the polymer around the mould

Mould is still spinning whilst it cools

-Large scale mass or batch production

Question 9

Extrusion

Answer 9

Similar to injection moulding

-Instead of being pushed into a mould the molten plastic is pushed through a die, and carried away on a conveyor belt

-This forms a thin, extended length of plastic

Question 10

Compression moulding

Answer 10

Preheated ‘slug’ of polymer (block) placed into the lower mould.

Hydraulic press the reverse shape of the mould is lowered on to the slug, to form the polymer into the shape of the mould

Both halves of the mould are heated during the process, and the polymer shape is removed while it is still warm

• Large-scale batch production