polymer processes (technical principles)

Vacuum forming

A polymer process used to make lightweight 3D objects e.g. trays. Thermoplastic sheets used (HIPS). Mould place on platen (base) - polymer sheet clamped and heated. Platen then brought up and vacuum sucks polymer onto mould. Small production (mould from MDF) large scale production (mould from aluminium)

Key aspects in vacuum forming

• Ensure draft angle to let polymer off mould

• No undercuts in mould as plastic will get stuck

• Vent holes to allow vacuum to suck plastic onto mould

• Rounded edges of mould to stop polymer sheet splitting

Calendaring

Thermoplastic pellets heated to dough consistency and extruded through heated rollers which squash and stretch it. Cooled by cooling rollers and chopped into stock sizes. Makes thin film. Typically continuous production

Themoforming

A polymer process used to mould additional detail on polymer surfaces. Positive mould pressed between as well as vacuum

Usually for logos and product names

Faster than vacuum forming

Line bending

A polymer process used to product bends in sheet thermoplastic (e.g. acrylic). Strip heater heats the middle of a polymer allowing it to bend in half

Creates boxes and shelves. Usually batch or one off production

Laminating (lay up)

A polymer process used to make 3D products from fibre based composites (CFRP and GRP).

Mould is in shape of product (timber or manufactured board). This is coated in wax to aid release

Coat of coloured polyester resin gives the mould colour. Fibreglass matting cut to size and layered on mould and polyester resin is rolled on to ensure air bubbles gone. Repeated until desired thickness

Injection moulding

A polymer process used to manufacture complex items from thermoplastics. Thermoplastic granules fed into hopper, Archimedean screw moves these along chamber and melt. Once needed amount collates at the end of screws hydraulic ram forces polymer into mould. Mould water cooled and ejector pins push item out - excess polymer trimmed.

Large scale / continuous production

Injection moulding pros / cons

• can make complex shapes

• Can make products in a range of sizes

• Undercuts and screwthreads can be made using this method

• Fast process ( 10-60 seconds)

• Expensive to set up

Blow moulding

A polymer process used to manufacture water bottles (HDPE,PP) polymer fed into hopper, Archimedean screw pulls polymer melting it - then extruded as tube (parison), mould slices around tube and air is injected into mould forcing polymer to sides. Polymer cooled and removed and excess trimmed.

Continuous production

Blow moulding pros/ cons

• low labour costs

• Large manufacturing times (100-2500 per hour)

• High machinery and tooling costs

Injection blow moulding

Used for fizzy drink bottles. Preform injection moulded to stretch polymer - gives it high tensile strength

Rotational moulding

A polymer process used to produce heavy duty, seamless hollow objects (HDPE, PP). Polymer powder put into mould which is clamped shut. Mould heated 260-370˚ and rotates slowly making polymer coat mould on a 2 axis plane. Once thickness desired reached, polymer cools (which causes it to shrink) and is removed

Extrusion

A polymer process used to produce solid rod, hollow tube and channel stock forms (PVC). Polymer granules into hopper -Archimedean screw moved these along melting them. Once certain amount collates, hydraulic ram pushes screw forcing polymer into steel die. Extruded polymer can also be cooled by rollers and cut to desired length

Compression moulding

A polymer process used to mould thermoset polymers (urea formaldehyde). Pre weighed polymer (slug) placed between preheated moulds. Hydraulic press presses moulds together ensuring polymer takes shape. Moulds held to ensure thermoset cures. Ejector pins remove polymer from mould. Edges trimmed