Compression and transfer moulding

Urea-formaldehyde moulding powders may be moulded without difficulty on conventional compression and transfer moulding equipment. The powders, however, have limited storage life. They should thus be stored in a cool place and, where possible, used within a few months of manufacture. 

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. 

In the UK these materials are processed mainly in the same manner as urea materials (compression and transfer moulding) although some grades can be injection moulded on suitable machines. Care must be taken to establish the optimum moulding condition for each application as cure times can affect properties. ... 

DAP compounds can be processed by compression and transfer moulding at temperatures in the range 135-165°C and pressures of 14-70 MN/m. Their hot strength is inferior to that of phenolic moulding powders and care must be taken when extracting these materials from a mould cavity. Typical physical and electrical properties are given in Table 14. 

One factor which affects the cost of both compression and transfer moulded parts is the fact that a certain amount of material may flash between the two parts of the mould while the molten material is under pressure. The amount of flash depends on the mould design but if it is appreciable it must be removed. In the case of small objects a simple tumbling process is usually sufficient but larger parts may have to be treated on a linisher or similar device. 

Whilst the machinery, or presses, that are required for both compression and transfer moulding are basically the same, the latter type has the addition of an integral transfer ram mechanism. It follows therefore that the purchase of a press specifically for moulding by the compression process will show a small cost advantage. 

Figure 4.3 Selection of compression and transfer moulded products made from DevulC02 rubber. Reproduced with permission from the DevulC02 Project Consortium, Smithers Rapra, Shawbury, UK (Lead Partner). 2012, DevulC02 Research Project Consortium...

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