Preheating, transfer moulding

Melamine-based compositions are easily moulded in conventional compression and transfer-moulding equipment. Moulding temperatures are usually in the range 145-165°C and moulding pressures 2-4 ton/in (30-60 MPa). In transfer moulding pressures of 5-lOton/in (75-150MPa) are used. An in thick moulding required about 2 minutes cure at 150°C but shorter times are possible with preheated powder. 

Microwave units are available for both continuous vulcanisation systems, for preheating of moulding blanks for compression and transfer moulding, and for heating large bales of frozen rubber prior to mixing. 

A major disadvantage is that the mould temperature must be kept rela-tivety low so that reaction times are lengthy if the potymer surface in contact with the hot mould cures too quickly (before the rest of the charge is heated and therefore liquified) a poor moulding can result and the mould cavity pressure may then be inhomogenous. These problems are reduced by preheated screw feed and also in the related technique of transfer moulding. 

Transfer moulding differs from eompression moulding in that the rubber is transferred through a hole into the mould cavity. In its simplest form a transfer mould consists of a ram in a cyhnder fed with preheated slugs, and a mould cavity. A piece of the unvulcanized rubber is placed in the cylinder and covered by the ram. When the press is closed the ram forces the rubber through a hole into the actual mould cavity. This allows shorter cure times because of the heat generated as the rubber is forced to flow through the hole. 

Injection moulding is similar to transfer moulding in that the rubber stack is forced into a closed mould cavity through a nozzle. A strip of the rubber is fed into a heated cylinder and mastieated by a screw, which then moves forward like a ram and forces this preheated rubber through a nozzle into the mould cavities. As a result, rubber items ean be vuleanized in very short times, there is better dimensional control, and less serap. 

Transfer moulding can be successfully used for the packaging of resistors, resistor networks, capacitors, diodes, transistors, integrated circuits, coils, reed relays and many other discrete components. The process only becomes economically feasible, however, for mass production operations since the initial high cost of the press, moulds, preheater and preformer can only be amortised over the production of many hundreds of thousands of parts. 

This was the first technique used for forming plastics in production quantities. The mould comprises a matched pair of male and female dies. A measured quantity of partially cross-linked polymer (this technique is not now used for thermoplastic polymers) is placed between the two halves of the mould. The upper die is then lowered and the polymer compressed it is simultaneously heated by heat transfer from the heated mould. Before setting, the hot polymer completely fills the mould. It is then left to cure. The mould is finally opened and the part removed. The metering of the appropriate amount of polymer, and also some preheating, is being increasingly performed by screw machinery. This can increase by up to 400%... 

Molding compound is transferred in the cold state by pressure from the material hopper into the main injection chamber. Here it can be preheated before injection into the heated mould tool. Injection, through a special nozzle, can be either by ram or screw pressure. If screw feed is used, the screw must be of the type designed to process thermosets as opposed to thermoplastics. 

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