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Wednesday, March 12, 2014

Premix for Press Moulding

BULK MOLDING COMPOUND (BMC) / DOUGH MOULDING COMPOUND (DMC)
BMC/DMC has been defined as ‘a fiber reinforced thermoset molding compound not requiring advancement of cure, drying of volatile, or other processing after mixing to make it ready for use at the molding press. BMC can be molded without reaction by products under only enough pressure to flow and compact the material. BMC is usually manufactured by combining all the ingredients in an intensive mixing process.

Recent advances in BMC technology dictate that both the dry ingredients and wet ingredients be batch mixed separately and then combined together in an intensive mixer. The BMC is usually in a fibrous putty form when it comes out of the mixer and resembles ’sauerkraut’. It is usually compacted and extruded into bars or ’logs’ of simple cross section. 

The earliest BMC’s were probably made by employing a process of impregnating roving strands with blend of resin, filler, etc. and chopping them to a length in the wet stage. Since wetting glass fibers with a resin containing much filler is difficult and slow, these premixes had a high glass content. The first high volume commercial BMC was made with sisal fibers and used in molding automobile heater housings. Improvement in the binder chemistry of glass fibers, development of a chemical thickening system and thermoplastic low profile additives help BMC to attain strength, chemical resistance and to overcome surface irregularities. 

Consequently, BMC was accepted for use in the electrical, chemical and appliance industries. Today, BMCs are accepted as high performance engineering thermoset molding compounds and used extensively in the electrical, automotive and consumer goods industries. BMC is increasingly injection molded to take advantage of the automation and reproducibility afforded by the process, although it is also both transfer molded and compression molded. 

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