The Technique of FURNITURE MAKING

PLASTICS APPLICATIONS

Plastics applications fall into three main categories: cast and moulded structures and components; extrusions and extruded sections; shaped forms and fabrications. The appropriate method of manipulation is determined by the plastic itself and whether it is thermosetting or thermoplastic, and the nature of the required component.

Cast and moulded structures Thermosetting plastics

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.

Thermoplastics

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 self­lubrication (gliders, hinges, lock components, bushes, bearings, etc.) are required.