Powder moulding techniques

A compression moulding technique [94] has been used to prepare composites of polystyrene with layered perovskites C12Mn polystyrene powder (Mn = 145 000) and finely powdered C12Mn (average particle size 150 pm) are dryblended and subsequently compression moulded at 160 °C and a pressure of 50 kg/cm2. 

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited. 

This technique is used in free radical polymerisation of methyl methacrylate or styrene to obtain transparent moulding powders, etc. 

Plastic explosives contain one or more of the explosives listed above, moulded in an inert, flexible binder. Because powders do not readily hold a shape and TNT is the only common melt-castable explosive, most of the explosive powders (RDX, HMX, PETN, 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)) are plasticized to make a mouldable material, for example, C-4, Semtex H, PE4, sheet explosive. A variety of plasticizers are added, but the maximum level is usually 10-15% because most plasticizers are inert and would degrade explosive output. Plastic explosives were originally developed for convenient use in military demolitions but have since been widely used in terrorist bombs. For detection techniques that rely on vapour signatures, such as canine olfaction, it is worth considering that the plasticizer is much more volatile than the explosive component. 

Compression moulding is the simplest technique to transform a raw material from the solid state into an end-product. The material is, as granules or as a powder, brought between the two heated halves of the mould, which are then being pressed together. 

Dispersed in an infrared transparent powder. Pellets are made with I to 2% by weight of the sample, usually in dry potassium bromide. A pellet of approximately 300 mg is prepared in a mould called a matrix using a laboratory press. This method produces pellets 1 to 2 mm thick for a diameter of approximately 13 mm. This technique can be used for quantitative studies but the spectra may sometimes differ ftom those obtained via dispersion in a liquid. This is because, under the effect of the high pressure to which the powder is subjected, the solid may undergo certain modifications. 

The work briefly described above was specifically concerned with transfer moulding resins for monolithic packaging but we have since shown that the technique of inhibitor addition is also applicable to other encapsulation methods and materials e.g. powder coating. Work is continuing in this area. 

Dip moulding is a technique for making very thin walled, flexible products. Metal forms with identical dimensions to the final product are heated to the film forming temperature of the plastic being processed. The forms or mandrels are dipped rapidly into a bath of plastic paste or fluidized powder and withdrawn to allow excess material to drip. The cooled film is peeled from the forms to produce balloons and protective and medical gloves. 

U. Attia and J. Alcock, A review of micro-powder injection moulding as a microfabrication technique. Journal of Micromechanics and Microengineering, 21(4), 043001, 2011. 

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