Other moulding methods

Dynamic densification by means of shock waves produces an extremely dense material, which in its turn reduces the sintering temperature required. The result of a shock wave is regroup-ing, plastic deformation and breaking of the particles. 

The use of shock waves was imitated from nature, more specifically from meteorite craters. Among other things, unique forms of diamonds were found in there, i.e. hexagonal diamonds formed out of graphite. Furthermore the mineral stishovite was also found there. 

Stishovite (density 4.28 g/cm3) is a modification of quartz (density 2.65 g/cm3) and can be made from the latter at a temperature of 1000-4000 °C and a pressure of 120-160 kbar. 

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The process is attractive for a number of reasons. Firstly, since it is a low pressure process the moulds are generally simple and relatively inexpensive. Also the moulded articles can have a very uniform thickness, can contain reinforcement, are virtually strain free and their surface can be textured if desired. The use of this moulding method is growing steadily because although the cycle times are slow compared with injection or blow moulding, it can produce very large, thick walled articles which could not be produced economically by any other technique. Wall thicknesses of 10 mm are not a problem for rotationally moulded articles. 

The biological method thus only serves for the preparation of that isomer which does not occur in nature, since the mould or yeast or enzyme destroys the naturally occurring form, leaving the other isomer untouched, or according to Marckwald and Mackenzie, it acts upon the natural isomer more rapidly than upon the other. The method therefore does not lead to the synthesis of the naturally occurring amino acid. 

N-propinylazoles of the BAY d 9603 type have a close structural relationship to the tritylazoles. They have a wide range of activity, particularly effective against mould fungi. Starting material here is the readily available diphenyl-propinol obtained by ethinyla-tion of benzophenone. The method for the conversion of this carbinol into the corresponding N-substituted imidazole is the reaction with carbonyl or thionyl-bis-imidazole, or preferably with the tris-imidazolide of phosphoric acid in a polar solvent. All other alkylation methods in which carbocations occur as intermediates completely fail. 

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. 

Low capital investment costs. Because the method of heat transfer is integrated into the mould die, the need for an autoclave is eliminated. The initial capital investment costs of RTM are low when compared with the many other moulding processes. 

A brief introduetion to injection moulding has now been given. Before moving on to consider it in greater depth, a quick summary of other commercial methods of polymer processing will now be considered for eomparison. 

A number of different resins are available and the ultimate choice will depend on the end use and proposed method of fabrication. For example, one resin will be recommended for maximum strength and fastest cures whilst another will have the best electrical properties. Some may be suitable for low-pressure laminating whilst others will require a moulding pressure of lOOOlbf/in (7 MPa). 

The other method is to use a transfer mould where the blank is placed in a pot portion of the mould and then picks up further heat as it passes into the cavity. This method is frequently adopted for rubber-to-metal bonded, large and complex components. 

The method is based on the fact that certain bacteria, fungi, mould or yeast when allowed to grow in a racemic solution, assimilate or consume one of the enantiomers faster than the other. This is why the method is also known as selective assimilation or preferential decomposition. Thus Penicillium glaucum a species of green mould when allowed to grow in ammonium racemate solution consumes the d 0 tartaric acid and leaves the l form, but in a racemic lactic acid it assimilates the l form leaving behind the d form. 

Enzymes exhibit specificity as regards their behaviour towards stereo-isomerides. Pasteur s biological resolution of racemic acid with moulds depends on this fact, and such moulds may also be used for the resolution of racemic mandelic acid, for the partial digestion of racemic polypeptides according to E. Fischer s method, and in numerous other processes. 

ISO 7246 1984 Pipes and fittings of acrylonitrile/styrene/acrylester (ASA) - General specification for moulding and extrusion materials ISO 12092 2000 Fittings, valves and other piping system components made of unplasticized poly(vinyl chloride) (PVC-U), chlorinated poly(vinyl chloride) (PVC-C), acrylonitrile-butadiene-styrene (ABS) and acrylonitrile-styrene-acrylester (ASA) for pipes under pressure - Resistance to internal pressure - Test method... 

The method of structural foam moulding permits the manufacture of foams with a compact skin and a cellular core. The properties of such mouldings depend, among other parameters, on the structure of the foam. This article reports on a study of the influence of density, specimen thickness, orientation and ambient temp, on mechanical properties and chemical resistance of injection monlded structural foams made from PP. Results are discussed with reference to relationships between structnre and properties. 5 refs. 

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