Steam moulding

Whilst it is possible to purchase standard equipment for the steam moulding process, attempts continue to be made to make sweeping modifications to the process. These include the use of dielectric and microwave heating and the development of semicontinuous and continuous processes. 

The outstanding features of steam moulded polystrene foam are its low density and low thermal conductivity. These are compared with other important insulating materials in Table 16.10. 

One alternative approach to the two-stage steam moulding process is that in which impregnated beads are fed directly to an injection moulding machine or extruder so that expansion and consolidation occur simultaneously. This approach has been used to produce expanded polystyrene sheet and paper by a tubular process reminiscent of that used with polyethylene. Bubble nucleating... 

The single most important factor responsible for the rapid commercial growth of expandable PS is its ability to be steam-moulded into lightweight, closed-cell, low-cost foams suitable for beverage cups, packages, ice buckets, picnic chests, insulation board, etc [11]. (Figure 1.10). 

PTFE is used for lining chutes and coating other metal objects where low coefficients of friction or non-adhesive characteristics are required. Because of its excellent flexing resistance, inner linings made from dispersion polymer are used in flexible steam hose. A variety of mouldings are used in aircraft and missiles and also in other applications where use at elevated temperatures is required. 

The copolymers have been used in the manufacture of extruded pipe, moulded fittings and for other items of chemical plant. They are, however, rarely used in Europe for this purpose because of cost and the low maximum service temperature. Processing conditions are adjusted to give a high amount of crystallinity, for example by the use of moulds at about 90°C. Heated parts of injection cylinders and extruder barrels which come into contact with the molten polymer should be made of special materials which do not cause decomposition of the polymer. Iron, steel and copper must be avoided. The danger of thermal decomposition may be reduced by streamlining the interior of the cylinder or barrel to avoid dead-spots and by careful temperature control. Steam heating is frequently employed. 

Other, more recently developed, uses include microwave oven parts, transparent pipelines, chemical plant pumps and coffee machine hot water dispensers. One exceptional use has been to produce, by an extrusion moulding process, very large rollers for textile finishing for use where cast nylons cannot meet the specification. Also of growing interest are medical equipment applications that may be repeatedly steam-sterilised at 134°C, filtration membranes and cartridges for ink-jet printers. 

The sheets may be formed to some extent by first softening in hot water or steam and then pressing in moulds at pressures of 200-500 Ibf/in (1.5-3.5 MPa). Machining, using high-speed tools, may be carried out on conventional metal-working machinery. 

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

Alternative proeesses for the reeyeling of fibre-reinforeed plastie (FRP), and their applieation in Japan, are briefly reviewed. Pulverised waste has been used in plastie mouldings for automotive applieations, and in eement roof tiles. FRP may be burned in ineinerators and used to heat water, or as an additive to eement kilns, where the resin aets as a fuel and the glass and filler beeome eement raw materials. Pyrolysis, in eonjunetion with metal eatalysts, has been used to reduee the waste to oils or gases, and treatment with steam or supereritieal water has also been sueeessfully applied. 26 refs. 

A pressure vessel constructed of steel, used for the vulcanisation of rubber products by the open steam method. The autoclave may be horizontal (vulcanisation of footwear, cables, hose), or vertical and fitted with a hydraulic ram (e.g., moulding pneumatic tyres). 

A sheet of rubber or of rubber-textile composite moulded, if desired, to a particular shape and used in a wide variety of applications in industry, e.g., diaphragm packings, diaphragm-operated valves for steam, air or liquids. 

A method of manufacturing latex foam. Compounded latex is foamed and gelling agent added the mould is filled and the gelled foam vulcanised by steam at 100 °C the product is stripped from the mould, washed and dried. See Talalay Process. 

The method of vulcanising those rubber products which do not require the precision of shape given by a mould. The heat for curing is supplied by steam circulating openly in an autoclave, the products being supported on mandrels or embedded in chalk. See Moulding. 

A preliminary or pre-cure given to a product or part of a product, e.g., a moulding may be given a semi-cure in a press and the cure completed in open steam a thick section of a product may be semi-cured before assembly to the product a component may be semi-cured to reduce the flow during the subsequent vulcanisation of the product. 

An obsolete process for reheading pneumatic tyres. The vulcanisation of the new head rubber is not done in a mould but by the open steam method in an autoclave, and the deshed pattern is cut on the head after cure. 

Typically this sort of product would be made using a spiral mixer with a specially formulated bread improver. The other requirements are the moulds to shape the dough and an oven with steam. 

After initial development the dough is fermented for hour or an hour and a half at around 24—28°C. The dough is then knocked back three-quarters of the way through proving. Next the buns are moulded to size and proved in a cool prover with steam to prevent skinning. The products are then baked, possibly at 210 220°C. 

The expanded polystyrene can be made by impregnating small porous beads of the polymer with pentane and heating in steam. The volume increases many a times. Expanded beads are kept in warm air for 24-36 hours and moulded with steam into blocks. 

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