Manufacturing processes

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Raw plastics come in the form of liquids or granules. They can be worked in many different ways. Some processes have high set-up costs and are therefore economically viable only if large numbers of the product are made. Others are less expensive at the tooling stage but require more hand finishing. This group of artefacts presents products manufactured by some of the more common processes of both types.

OPEN MOULD CASTING - A polyethylene mould was made from a carved aluminium original. The mould was filled with polyester resin and other materials such as metallic powders to colour the heads, or opaque resins to simulate ivory and gold leaf were added at the appropriate moment to allow them to settle at the right level within the resin. The mould was open at the top and thus the colouring agents could be manipulated with pins. The making of the mould does not require much investment and many moulds can be made from one original. However, once the 'bugs' were removed from their moulds considerable hand finishing was required.

COMPRESSION MOULDING - The radio casing and the cups and saucers are made by compression moulding. A measured amount of the raw material is put into a two-part mould. The mould is closed under pressure and heated. This causes the raw material to melt and take up the form of the mould before setting. In the case of the cups and saucers two moulds were required one for each colour. The tooling costs are not high however on removal from the mould considerable hand finishing is required to achieve such smooth and glossy surfaces.

VACUUM MOULDING - This cake mould was made from a pre-formed plastic sheet, itself made by a process known as calendering, in which plastic granules are fed between a series of heated rollers to form a thin sheet. The pre-formed sheet was attached to a mould and heated until it softened. A vacuum was then introduced between the mould and the sheet so that the sheet was drawn into the shape of the mould. Once cooled, the plastic sheet retained the shape of the mould, and the process could be repeated thousands of times. The mould can be made of any material including wood and plaster and so the tooling costs are low.

EXTRUSION - The polyamide threads in these nylons and socks and also the pencil casing were made by the extrusion process. It involves forcing molten raw plastic through a die. The extruded plastic is then cooled. The die creates the shape of the extrusion, thus it can be quite thick and have a hole in its middle, as does the pencil casing, or very fine, as is the case with thread. It can also have ridges but it is crucial that the profile of the shape extruded is the same all along its length. The pencil casing, once extruded and cooled was cut into pencil lengths and one end sharpened.

CUTTING & WELDING - Ready-made sheets and standard extrusions are often used to make products. This necklace was made from standard extruded plastics welded and strung together. The dark red units of the necklace were made from clear acrylic tubing, bent by the application of heat, and the colour applied by the dripping of pigment down the hole in the middle.The photograph frame was made by a simple process of cutting and welding, the latter achieved by applying heat along the line to be joined.

INJECTION MOULDING - Injection moulding allows the creation of any imaginable form provided, if hollow, its opening is wider than its body diameter. Thus it can produce a hollow simple form as in the case of these beakers or more intricate sold forms, as in the case of the butterfly brooches.Raw molten plastic is injected into a mould. The mould is usually made of metal thus tooling costs are high but the products are produced rapidly allowing low unit costs if produced in sufficient numbers.

BLOW MOULDING - These fruit shapes were made by blowing hot air into a tube of plastic encased in a two part mould made from a model of the fruit and fish. The plastic takes up the form of the mould and cools and sets on contact with the cool mould. Seams from the two part mould are visible. Often the process is combined with injection moulding, the tube and a thread for a lid being thus formed.

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