Shaping by deforming: plastics

Bending

Thermoplastics cannot be bent when cold. They are bent using a strip heater or line bender which will heat the plastic in a straight line, so it can be bent by hand, as shown below.

Sheet of thermoplastic held in a strip heater. Heater bars heat the thermoplastic in a straight line, prior to bending.

Press or compression moulding

A mould is made in two halves - top and bottom. Sheet plastic is placed between the two halves, which are then heated and pressure applied. The heat from the mould causes a chemical reaction in the plastic which enables it to assume the new shape. Press moulding is used to make electrical fittings, handles, and many other products. It can only be used with thermosetting plastics [thermosetting plastics: types of plastic that can only be melted once, and then become very hard ] such as phenol and melamine formaldehyde.

Blow moulding

Blow moulding is used to shape plastics - mainly for making bottles, plastic buckets and similar shapes. A tube of softened plastic called a parison is extruded [extruded: shaped by being forced through an opening ] into a mould, which then clamps one end closed. Air is then blown under pressure into the interior of the parison, which expands to fill the mould and create the desired shape. PVC, polythene and polypropylene are commonly used in blow-moulding.

Vacuum forming

This process uses a wooden mould [mould: a hollow form into which a softened material is placed. The material hardens and the mould is then removed to produce a product of the desired shape ] or formform: A style of poem, with a particular pattern of rhythm, rhyme pattern or regular line length (eg sonnet, couplets or ballad).. A sheet of thermoplastic is heated, then shaped by creating a vacuum [vacuum: A volume that contains no matter - space is almost a vacuum. ] underneath it. Air pressure then forces the plastic over the mould. Vacuum forming is used to for food and confectionary packaging, trays, shop fittings and baths. Suitable thermoplastics include acrylic, polystyrene and PVC.