Plastics - EduqasMoulding and forming

It is very often the case that the polymer, for example acrylic, needs to be bent to a shape. Once the acrylic is cut, either by hand or using a laser cutter, it can be bent using a strip heat, often called a line bender.

Acrylic is a thermoplastic and needs to be heated to around 150 to 170°C to bend without cracking, and jigs can be made in a variety of ways so that the bend produced remains the same as the acrylic cools.

Vacuum formers can be used for making moulds and housings for electronic products. High impact polystyrene (HIPS) is often the material used in a vacuum former, as it heats up quickly depending on thickness. The process is as follows:

1. the HIPS is heated
2. once hot, the solid former is pressed into the HIPS from a bed that can rise
3. the vacuum is turned on, removing all the air from around the former
4. the HIPS takes the form of the solid former

Sometimes talc can be dusted on the former so that it drops out of the mould with ease. A good vacuum-formed mould will only be possible if a good former has first been made. The former must have a draft angle, so it doesn’t get stuck in the mould.

Since polymers have a relatively low melting point, they can be shaped easily once molten by either injection moulding or extrusion.

Injection moulding

Injection moulding is used in industry to produce most mass-produced polymer parts using the following process:

1. granular plastic is held in a hopper
2. it is moved via an Archimedean screw along a heated tube, called the heating chamber
3. once the polymer has been melted, it is pushed into a mould with a hydraulic ram - the use of the hydraulic ram ensures just enough material is injected into the mould each time
4. the mould is then cooled so that the moulded plastic can be removed

Extrusion

Extrusion works in a very similar way to injection moulding:

1. granular plastic is held in a hopper
2. it is moved by Archimedean screw along a heated tube, called the heating chamber
3. once the polymer has been melted it is pushed through a die mould, which will form the shape of the extrusion
4. because there is no hydraulic ram in this process, molten plastic can be fed through the die continuously

Common extruded polymer products are plastic pipes, tubes and guttering.

Blow moulding

Blow moulding is used to shape plastics, eg bottles, plastic barrels and similar shapes.

A tube of softened plastic called a parison is extruded into a mould.

1. parison is inserted into a mould
2. the base of the parison is squeezed by the mould
3. air is blown into the parison, which expands to fill the mould
4. the finished product is cooled and released from the mould

Press moulding

Press moulding is used to make electrical fittings, handles, and many other products but can only be used with thermosetting plastics. A mould is made in two halves and sheet plastic is placed between them. They are then heated and pressure applied, forcing the plastics to assume the new shape.