TREATMENT

All plastics are polymeric and all polymers may be regarded as potential plastics. Certain polymers occur naturally, i.e. casein (milk solids), cellulose derivatives (wood, cotton),

etc.; all others are produced synthetically and are divided into two types, thermoplastic and thermosetting. In thermoplastic materials, the necessary degree of polymerization having been achieved, the long chain molecular structure can then be activated by heat to allow freer movement between the molecules; thus the plastic can be softened and resoftened without deformation provided the degree of heat is kept below the point of actual degrade of the material. In thermosetting plastics the polymerization has been arrested at a stage which produces relatively short chain molecules. Later application of certain simple chemicals (hardeners or catalysts) or simple heat carries the polymerization a stage further, producing cross linkages which destroy the mobility of the molecules, and the plastic sets into a hard, infusible resin which cannot then be resoftened. This latter type is usually known as 'resin plastics` (synthetic resin glues, etc.). The major thermoplastics are tough, resilient and can be given controlled flexibility either by arranging the molecular structure accordingly or by added chemical plasticizers, while the thermosets are either brittle solids which can be extended with other materials (chopped paper, wood flour, etc.) to form moulding powders, or viscous syrups for use as surfacing materials, impregnating liquids and glues, although here again the addition of plasticizers or softeners will allow a limited degree of flexibility. It should be emphasized that the chemistry of the various types of plastic is very much more subtle than this brief resume might suggest, and readers are referred to the standard works on the subject for a thorough understanding of the principles involved. A list of the more important plastics with their applications in the furniture industry is given opposite.

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Common or chemical names Thermosetting

Phenol formaldehyde (PF) Urea formaldehyde (UF) Melamine formaldehyde (MF) Resorcinol formaldehyde (RF) Phenol resorcinol formaldehyde (RPF) Polyester

Expoxide (epoxy resin)

Thermoplastics (see note below) Acrylonitrile butadiene styrene (ABS) Casein

Cellulose acetate (CA)

Cellulose acetate butyrate (CAB) Cellulose nitrate (CN)

Polyvinyl chloride (PVC) Polyvinyl acetate (PVA) Polyethylene LD (polythene) Polyethylene HD

Polymethyl methacrylate (Perspex) Polyacetal

Polyamide (nylon)

Uses

Wood glues, bonding agents, reinforced laminates, mouldings, etc.

Wood glues, chipboard manufacture, mouldings, etc.

Wood glues, reinforced laminates, mouldings, etc.

Wood glues.

Wood glues.

Finishes, lacquers, laminates (glass fibre, etc.).

Glues (metal, glass, etc.). castings, pottings, etc.

Drawers, doors, knobs, legs, etc.

Wood and paper glues, small moulded components.

Sheet, film, small moulded components.

High impact mouldings (tool handles).

Finishes, lacquers, small moulded components.

Rigid and flexible sheet. Extrusions and coatings.

Wood and paper glues. Contact adhesives.

Low density films, packaging materials.

High density structural mouldings.

Sheet, structural mouldings and components, etc.

Lacquers and finishes. Rigid foam structures.

Flexible foam seating.

Flexible foam seating.

Sheet, film, expanded foam.

High density sheet. Rigid foam structures.

Sheet, structural mouldings and components.

Doorhandles, hinges, etc.

Fibres, sheet, frictionless moulded and shaped components, etc.

Note: Some plastics are also thermosets. Most plastics can now be foamed.

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 com-pression 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 59

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 inter-locking knuckle action, and polyamide (nylon) where exceptional strength, toughness and self-lubrication (gliders, hinges, lock components, bushes, bearings, etc.) are required.

Extrusions and extruded sections Continuous lengths of tube, rod, sheet and various profiles are formed by extrusion pro-cesses 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 tech-nique for the furniture-maker as with this pro-cess relatively large repro-cessed 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 equip-ment, but certain plastics which are not sufficiently free flowing for injection moulding lend themselves to vacuum forming. The principles involved are relatively straight-forward. 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 60

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. Self-supporting 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 poly-styrene foam is the lightweight ceiling tile.

Recent developments in lightweight sand-wich 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 dis-advantage 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 water-proof 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 tapered if possible, as this will facilitate withdrawal, and sharp edges and internal

27 Fibre-glass reinforced plastic moulding

FORMER MOULD

A 2

P A T T E R N

4 A

FINISHED MOULD PATTERN

A

1

LAY UP

A 3

FORMER MOULD

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, poly-urethane, 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.

Perspex

Perspex or acrylic sheet (polymethyl methacry-late) is readily obtainable in clear transparent, pastel shades, full colours, and transfusing and fluorescent colours in thicknesses from V25 in (1 mm) to V2 in (12.5 mm) in colours, and up to 2 in (50 mm) in the clear sheet. It is also supplied in clear rod and tube up to 1 in (25 mm) diameter. Although not as hard as fused glass it has the same clarity and appearance and can be cut very easily with circular saw, band-saw or very fine-toothed handsaw, using low speed and light pressure and feed. It is easily drilled with the normal twist-drill lubricated with a trace of thin oil, and can be bent to simple curves at a temperature just short of boiling water (201° F;

94° C). For accurate bending a wood form should be used, the perspex sheet heated in front of an electric plate or similar source of heat, bent over the form, covered with a cloth and held in position for from one to two minutes to cool and set. Cut edges can be bonded together with simple heat, chloroform or ether, or special perspex cement, and frameless show-cases are now made almost exclusively by this method, with the meeting edges either butted and polished (wet and dry paper, burnishing pastes, etc.) or mitred together, both methods giving invisible joints if well done. There is a growing tendency also to design carcass furniture in flat sheets and simple chair forms in moulded resin, for it is an excellent structural material with outstanding qualities of clarity, strength, rigidity, stability and durability under quite severe conditions of wear, as witness its use in aircraft-work. It is, however, relatively expensive, although no doubt constant research and development, as with all other plastics, will eventually cheapen costs.

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28 Drawer unit with sliders in medium impact polystyrene. (By courtesy of Prestige Caterers)

Nylon

Nylon (polyamide) has universal uses as a fibre material for it can be cold drawn to five times its original strength, thus straightening the chain molecules and imparting great strength and excellent wear resistance. It is also invaluable as a sheet material and for castings, mouldings and extruded sections for it is tough, resilient and provides noiseless and frictionless surfaces invaluable for furniture guides, sliders, rollers, etc. It is also used in knock-down fittings, while barbed or serrated nylon dowel-pegs can be glued into such loose-textured materials as chipboard to provide secure anchorages for screws. As it is a thermoplastic without the hard brittleness of the thermosetting resins it can be cut, shaped and drilled with normal hand-tools.

For particulars, manipulating data and sources of supply of other plastics readers are referred to the manufacturers concerned, lists of whom can be obtained from the various periodic journals devoted to the subject and to the standard textbooks available at most libraries.

Decorative plastic laminates

Familiarly known as Formica, Wilson Art, Consulweld, etc., these are composed of layers of kraft paper impregnated with phenolic resins, while the surface pattern, which can be

purely decorative in an infinite range of designs or an exact simulation of real wood grains, is printed on a cover paper, and coated with a scratch-resistant surface of melamine resin.

Figure 29:1 shows (A) the kraft papers, (B) the cover paper and (C) the melamine coating.

Heat pressing of the assembled pack between stainless-steel platens polished to mirror finish induces a chemical change or polymerization, resulting in a homogeneous sheet possessing outstanding qualities of lightness, toughness, durability and resistance to heat, moisture, acids and alkalis, etc. Standard sheets with the pattern on one face only are usually 1/16 in (0.062 in or 1.58 mm), but a full range of thicknesses is also available, 0.032 in (0.81 mm) for vertical facings, wall claddings, etc., 0.040 in (1.01 mm) for light use in horizontal positions, 0.052 in (1.32 mm) for post forming (heat bending), and 0.10 in (2.54 mm) to 11/2 in (38 mm) built up of separate laminations or with a sandwich core of other materials (ply, etc.).

Thicknesses of 1/8 in (3.17 mm) and over are self-supporting (29:2). A cheap-quality backing or balancing veneer composed of kraft papers only with no surface pattern is available for use on the undersides of core material to counteract the pull.

Cutting procedures

Laminated plastic veneers can be cut with circular saws, using square tipped teeth without set, band-saws with hack-saw-shaped teeth, or fine-toothed backed tenon-saws, in all cases cutting with the face side up. For machine-saws in production-work tungsten-carbide teeth should be used, for the resin content is very hard on cutting edges; while if there is any tendency for the back to chip or 'tear out' with any type of saw the teeth should be resharpened, and the plastic sheet firmly supported with applied top pressure if possible. An alternative hand method which gives no tear is to score the face with a hardened steel scraper or cutting tool—a special blade is available for Stanley trimming-knives—and the sheet will then snap easily with a clean fracture, as in glass cutting. Narrow cross-cuts, corners, etc. can be cut with a fine-toothed hack-saw, while for contour cutting portable electric sabre-saws or fret-saws with 63

5

4

7

3

29 Decorative plastic laminate details

metal-piercing blades can be used, but support must be adequate with strips of waste material, hardboard, etc. clamped to the face to prevent chipping on the upward stroke.

T r i m m i n g edges

Cut edges can be trimmed with a heavy jack-plane, using short, quick strokes and not attempting to follow through; or with steel cabinet-scrapers, or fine-cut saw-files. If the edges require polishing a dead smooth file, or 400 grit wet and dry silicon carbide paper, will ease out the scratches, and a little fine cutting-down paste or metal polish {Brasso, etc.), followed up with a wipe of thin oil, will restore

the lustre. Scratches on the face of the sheet can also be eased out with abrasive paste (p.86), always provided they do not penetrate to the cover paper, and matt black laminates are usually enhanced by rubbing over with finest No. 0000 steel wool, but it must be carefully done. Spilt glue, paint, etc. can be lifted with a

the lustre. Scratches on the face of the sheet can also be eased out with abrasive paste (p.86), always provided they do not penetrate to the cover paper, and matt black laminates are usually enhanced by rubbing over with finest No. 0000 steel wool, but it must be carefully done. Spilt glue, paint, etc. can be lifted with a