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Mould Making Methods


A complete account of the very many methods of mould making would be out of place in a book devoted to the causes and cure of faults in injection moulding. There are several excellent articles on the subject to which reference can be made. However,to assist in the understanding of how and why problems occur in connection with the mould,a brief sketch of the main mould-making methods involved will be included.

The chief process used is machining, in which the shape to be generated is cut on a lathe or milling machine to the required dimensions. Cutting of very hard steels is difficult so softer steels are used and subsequently hardened or nitrided. During hardening, the heat treatment which the metal receives may cause stresses which were present in the metal block to be relieved and slight distortion may result. The use of normalized steels help to reduce this,but a number of annealing treatments as the machining proceeds may be necessary with very accurate work. Most metals can be machined and for simple circular shapes the lathe is the most useful tool.

A modification of machining is die-sinking, in which a negative pattern is used which a pantograph tool will follow to generate the appropriate shape. Great accuracy is possible as the pantograph can be arranged to give a reduction of dimension of as much as 10:1 compared with the pattern. Engraving also uses a pantograph and is chiefly used for lettering-

Where repetitive work is done,numerically controlled (N-C) machinery is used. It is expensive and the work load needs to be considerable with a large saving of labour before its use can be justified.

Very hard steels can be worked by either electrochemical machining (ECM) or electric-discharge machining (EDM), often referred to as spark erosion. ECM is very rapid and 1 in3/min of metal can be removed each minute for each 10 000 A of current,which is obviously at very low voltage. Surface finishes are good but electrodes,which are of suitable shape to generate the form required,are expensive. EDM is generally used for smaller jobs and is a substitute for die sinking. Blind holes are quickly generated and sharp angles are possible, dimensions better than 0 001 in being easily obtained. Newest machines employing transistorized pulse generators which produce very rapid sparking give little electrode wear and a good surface finish. Electrodes are reasonably easy and inexpensive to make,graphite and copper being preferred materials.

These methods of mould making involve cutting metal away from a form. Two other methods in use involve the reverse, namely,building up metal to give the desired shape. Electroforming is a process in which metal is electrically deposited on a reverse model of the shape to be formed and the shell of metal thus produced is backed with a fusible casting alloy such as Hoyt metal or Kirksite. Cooling coils of soft copper pipe can be incorporated into the alloy backing. In metal spraying a low melting point alloy is flame sprayed on to a positive model and the shell so formed is backed with a similar alloy. Because the alloys used in both these processes are softer than desired the forms are mounted in a steel bolster and the locking force is taken up on hardened steel pads incorporated in the bolster.

Cast metal moulds of aluminium,zinc,Kirksite and beryllium copper are also used satisfactorily in injection moulding. In common with all metals, their thermal conductivity is relatively good (better than 0-1 cal/cm s0C,Be/Cu being about 0.5 cal/cm s°C) and even with those alloys of very low melting point (c- 160°C) there is no danger of the hot plastic melting the mould. However,care must be taken to minimize flashing of the mould because this is always accompanied by excessive air-venting with its consequent production of heat.

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