Special casting or compression moulding resins (PF, UF, MF and epoxy resins, etc.) are used. In the most simple technique a cold cure resin mix (resin syrup and catalyst) is extended with appropriate fillers and colorants and cast in metal, silicone or hot melt rubber or plaster moulds without heat. In a more sophisticated technique the resinous solution is neutralized with a suitable organic acid, dehydrated under vacuum, and poured into lead moulds which are then oven treated. A third technique using compression moulding is more widely practised in which carefully measured amounts of powder resin or resin pellets are fed into a heated mould and compressed by a heated plunger; the thermosetting plastic is thus first heat softened so that it flows into the crevices of the mould, and is then heat cured to the finished shape determined by the mould and its matching plunger. As thermosetting plastics are more brittle than the thermoplasts, all these techniques are usually reserved for heavier sections.
All types of thermoplastics lend themselves to injection moulding which is the most popular process for high-speed production of moulded structures and components. In this technique the resin chips or powder are conveyed by screw or ram along a heated barrel container which converts them into a viscous syrup or flowable solid; this is then ejected through a nozzle into a closed split mould and allowed to chill. Shell mouldings, i.e. thin-wall hollow mouldings, can be obtained by spinning the mould, thus forcing the plastic against the sides of the mould. Choice of plastic is dictated by both technical and economic factors. Press-in components which have to be compressed slightly to enter a prebored hole are formed by flexible low-density polythene/polyethylene, etc.; open components, i.e. handles, etc. of stiff plastic use polystyrene or polypropylene, or polymethyl methacrylate for transparent components. Polypropylene is also used for plastic hinges in which the actual hinging action is effected by a flexing of the material itself and not by an interlocking knuckle action, and polyamide (nylon) where exceptional strength, toughness and self-lubrication (gliders, hinges, lock components, bushes, bearings, etc.) are required.
Continuous lengths of tube, rod, sheet and various profiles are formed by extrusion processes mainly using thermoplastics, although thermosetting plastics have uses in specific cases. The resin powder or chips are conveyed along a heated barrel by means of an Archimedes' screw action which forces the softened plastic through a nozzle orifice shaped to the required profile. If a mandrel is supported in the centre of the orifice the plastic will be ejected as a hollow tube. Flexible and semi-flexfble sections (wire covering, flexible edging strips, etc.) employ PVC or LD
polythene, rigid sections HD polythene/ polyethylene or polypropylene. Both cellulose acetate and cellulose nitrate are also used, also nylon for strength and toughness, and perspex (polymethyl methacrylate) for transparent sections.
Vacuum forming is the most interesting technique for the furniture-maker as with this process relatively large recessed shapings can be effectively and economically produced with simple apparatus. Integral chair shell structures which are produced by the injection-moulding process require heavy and expensive equipment, but certain plastics which are not sufficiently free flowing for injection moulding lend themselves to vacuum forming. The principles involved are relatively straightforward. A presoftened sheet of suitable plastic is securely clamped round the perimeter of an open-top box containing the shaped mould or former; a vacuum is then applied to the box to pull the flexible sheet down over the former/ mould and held until the sheet has chilled off and fixed the shape. Stiffer plastics may require plunger assistance, and in this system the moulding former is in the shape of a plunger which is forced down into the softened sheet, while for deep drawings a vacuum is employed to assist the deformation by helping to pull the sheet down in advance of the descending plunger. On releasing the vacuum the inherent elasticity of the sheet will pull it back against the plunger. Plastics employed in vacuum forming include polymethyl methacrylate (perspex), polyvinyl chloride (PVC), polypropylene, high-impact polystyrene and various copolymer sheet materials recently developed. In particular acrylonitrile butadiene styrene (ABS) is one of the newest and best for it can be cut, tool shaped, glued, screwed and nailed. Methods of sheet forming without pressure include powder casting with low- and medium-density polythenes/polyethylenes and PVC in which the resin powder is fed against a heated metal mould. Large surfaces can be covered in this way, as the powder softens in contact with the heated metal forming a continuous layer which can be stripped off when cold. Simple bending techniques for one-way curves can also be used as described under Perspex, p. 62.
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