Moulding of Plastics

Thermoplastic Materials and Processing

Classification of Plastics

Thermoplastic Materials Fluidic when subject to heat, harden when

cooled Usually rigid

Thermostatic Materials Fluidic or powder form before processing, cure

permanently when subject to heat or mixed with catalyst

Rigid or elastic

General Purpose Thermoplastic Materials Polyethylene (PE)

Buckets Polypropylene (PP)

Microwavable food containers, electric kettles Polystyrene (PS)

CD cases PolyVinylChloride (PVC)

Sewage pipes, flexible hoists

Engineering Thermoplastic Materials

Polyamides (PA, Nylon) Rope

Polymethylmethacrylate (PMMA, Acrylic) Cups

AcrylonitrileButadieneStyrene (ABS) Product casing

Polycarbonate (PC) Outdoor screenings, Bullet-proof windows, Electric

kettles PolyethyleneTerephthalate (PET)

Soft-drink bottles

Thermostatic Materials

Melamine Formaldehyde, Urea Formaldehyde (MF, UF) Dishes, lamination

Phenolics (PF) Ashtrays, Pot handles, plugs and switches

Polyurethane Shoe heels

Unsaturated Polyester (UP) Fibre-glass installations

Epoxy (EP) Glue, filler

Injection Moulding

The injection moulding machine consists of a heated barrel equipped with a reciprocating screw (usually driven by a hydraulic motor), which feeds the molten polymer into a temperature controlled split mould via a channel system of gates and runners. The screw melts (plasticises) the polymer, and also acts as a ram during the injection phase. The screw action also provides additional heating by virtue of the shearing action on the polymer. The pressure of injection is high, dependant on the material being processed; it can be up to one thousand atmospheres.

Animation: Injection Moulding

Typical Injection Mould Job-shop

Typical Injection Moulding Plant

Rotational Moulding

The Rotational Moulding process is essentially split into four operations:

A pre-determined amount of polymer powder is placed in the mould. With the powder loaded, the mould is closed, locked and loaded into the oven.

Once inside the oven, the mould is rotated around two axis. The ovens are heated by convection, conduction and, in some cases, radiation. As the mould becomes hotter the powder begins to melt and stick to the inner walls of the mould.

When the melt has been consolidated to the desired level, the mould is cooled either by air, water or a combination of both.

When the polymer has cooled sufficiently to retain its shape and be easily handled, the mould is opened and the product removed.

Animation: Rotational Moulding

Adapted from: http://www.bpf.co.uk/bpfindustry/process_plastics_rotational_moulding.cfm

Rotational Moulding

MT3 by Ron Arad

Blow Moulding

The process is divided in to three. The injection blow moulding machine is

based on an extruder barrel and screw assembly which melts the polymer. The molten polymer is fed into a manifold where it is injected through nozzles into a hollow, heated preform mould. The preform mould forms the external shape and is clamped around a mandrel (the core rod) which forms the internal shape of the preform.

The preform mould opens and the core rod is rotated and clamped into the hollow, chilled blow mould. The core rod opens and allows compressed air into the preform, which inflates it to the finished article shape.

After a cooling period the blow mould opens and the core rod is rotated to the ejection position. The finished article is stripped off the core rod and leak-tested prior to packing.

Animation: Extrusion Blow Moulding Animation: Injection Stretch Blow Moulding Animation: Injection Blow Moulding

Compression Moulding

Compound or blend is placed in mould and heated under pressure. When reaction is complete, product is cooled and ejected.

Animation: Compression Moulding

Plastic Resources

http://www.matweb.com http://www.materialconnexion.com

Plastic Resources

http://plastics.bayer.com