Plastic is everywhere. It’s used in the production of an enormous range of products, from spacebars to spaceships. This is thanks to its low cost, versatility and ease of manufacture. Here we take a look at various processes used in the manufacture of plastic goods.
Uses: hollow products where the opening is smaller than the body, such as bottles and other containers.
Blow moulding involves blowing hot air into a pre-formed tube of semi-molten plastic, referred to in the plastics industry as a parison. This causes the tube to expand to meet the sides of a mould. The mould usually consists of two parts and is made of metal.
Uses: sheets, tubes, rods and other preformed shapes; designer furniture, radio housings, paperweights.
With casting, plastic, in a liquid form, is poured into an open mould. Often the mould itself is made from plastic. A disadvantage is that this process often results in trapped air bubbles, which can threaten the integrity of the cast object.
Uses: plugs, sockets, tableware; products that have visible “mould lines”.
Compression moulding involves placing a measured amount of solid plastic, referred to as the “charge”, between an upper and a lower mould, forcing the upper mould down onto the charge, closing the mould, and subjecting it to high heat and pressure. This melts the plastic and forms it into the desired shape.
Uses: objects with constant cross-sections; fibres, tubing and pipes, curtain rails, window frames.
With extrusion, plastic pellets are fed into a heated cylinder, where they are driven forward by a turning screw, compacting and melting them through a die at the end. This creates continuous lengths of material, which may then be cut into the desired lengths.
Uses: packaging, sponges, steering wheels, soles of shoes, foam furniture, insulation.
Plastic foam is manufactured in a number of different ways. Generally this involves releasing gas or air into the plastic so that it fills with bubbles and foams out to fill a two-part mould, usually made of metal.
Uses: precision plastic parts produced in large volumes, such as LEGO, plastic cutlery, machine housings.
Injection moulding is similar to plastic extrusion. Plastic granules are fed into a tube. They’re then heated and pushed out into a mould, where the plastic forms the desired shape.
Uses: products with uniform wall thickness, such as storage tanks.
Rotational moulding involves feeding a measured amount of solid plastic into a mould that’s rotated by two axes at low speed. The plastic melts and then covers and adheres to the inner surface of the mould. The mould is then cooled so that the plastic hardens while still rotation, and the product is finally released.
Uses: wide, shallow open-ended objects, such as baths, boat hulls, margarine tubs.
Thermofoaming involves warming preformed sheets of plastic and then using a vacuum to suck the plastic into a mould. Neither high heat nor pressure is necessary for this process, making it possible to use cheap materials, such as aluminium or medium-density fibreboard (MDF), for the mould.