The Technique of FURNITURE MAKING

Extrusions and extruded sections

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.

Shaped forms and fabrications

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.

Rigid foam plastics

Both polyurethane isocyanate and polystyrene can be foamed with suitable gassing agents to form rigid shell structures. If the foaming is unrestricted then it becomes open celled, but if restricted within shell moulds considerable pressures are created and the foam becomes compacted, with a hard outer skin which will conform to every fine detail in the mould. Self­supporting chair shells, imitation mouldings and wood carvings are made by these methods. The polyurethane foam is the more expensive of the two but will accept staples if upholstered, whereas polystyrene must have tacking strips applied. An example of the open-celled poly­styrene foam is the lightweight ceiling tile.

Recent developments in lightweight sand­wich boards use an open-celled ABS foam with outer layers of compacted foam.

Polyester resin fibre-glass laminates (GRP)

Rapid moulding techniques for the production of rigid plastic shapes or shells (chair seats, etc.) require specialist knowledge and advanced equipment beyond the resources of the small workshop, but glass-fibre laminates, familiarly known as GRP, offer a simple method of forming rigid structures whose only dis­advantage is that one surface, inside or outside according to the type of former used, will be smooth and the other rough. In essence the shaped moulding is composed of laminations of chopped strand glass mat impregnated with polyester resin. The resin must be thrixotropic, i. e. it must be fluid enough for brush coats, but must be capable of building up so that it stays in position and does not creep downhill. Assuming that a simple box shape is to be moulded, then an exact pattern (Figure 27:1) must be made of timber or other suitable material, using water­proof bonding agents {Araldite epoxy resin, etc.), with a generous overhang all round so that the rough edges of the rim can be trimmed up afterwards. The sides of the box should be
corners should be avoided, or the resin mat will tend to bulge over, leaving a void in the structure of the finished mould. The working surfaces of the mould must be filled (resin and talc, Polyfilla, Alabastine, etc.), sanded down smooth, lacquered (polyester wax, poly­urethane, shellac, etc.), again sanded down with 400 grit wet and dry paper, and burnished with cutting-down paste to a high gloss, after which it is given a heavy coat of wax polish and left to harden overnight.

GRP former mould The former mould from which the finished moulding will be struck is now made up as in Figure 27:2. The wax coat is first buffed off the pattern, which is then coated with a polyester emulsion wax release, followed with an application of wax polish buffed to a high gloss, and then a layer of polyvinyl acetate (PVA) release agent applied with a sponge. A gel coat composed of polyester resin, catalyst (setting agent) and a small percentage of colour paste is then brushed on, followed by a second gel coat immediately the first has cured. The second gel coat should be of a contrasting colour, so that adequate warning is given during any subsequent rubbing down, and, when this coat has cured, a coat of catalysed resin is brushed on, and a layer of chopped strand glass worked into it, adding more resin with a stiff stippling-brush until the mat is 'wet out' or saturated. Two or more additional layers of glass mat and resin are then added, and the whole assembly stippled and rolled with a split roller to consolidate the layers and eliminate all air bubbles. The mould is then put aside to cure for at least four hours at normal room temperature, any roughness smoothed out. washed over with warm soapy water, given a thick coat of wax polish and allowed to age for a further 24 hours.

Finished mould

Exactly the same procedure is followed in preparing the finished moulding shown in Figure 27:3. The former-mould is treated with emulsion wax, wax polish and PVA release agent, then a single gel coat followed by the requisite thickness of glass mat (minimum two layers) and resin. After curing the mould is released, washed with soapy water and trimmed to exact size; it should not require any further polishing.

The method described above gives a smooth surface on one side only, marked (A) on the drawings, as the undersides will have been formed by the roller. They can be ground off smooth if necessary but will show the cut ends of the glass fibres and should be painted or otherwise protected.