TWI Frequently asked question
Sheet moulding compound (SMC) consists of polyester resin combined with chopped glass fibres, chemical thickening agents and particulate filler to form a sheet. The glass fibres are aligned parallel to the sheet, and the amount offibre reinforcement can be up to 40% by volume. SMC is processed using a mould and heated press which forces the SMC into the mould and cures (hardens) the resin. The fibre reinforcement is mainly oriented in 2-D, in the plane of thesheet.
Bulk moulding compound (BMC), also known as dough moulding compound (DMC), is usually based on polyester resin, although other resin types are possible. BMC differs from SMC by having a higher concentration of filler and lowerquantities of glass fibres, which are also usually shorter than in SMC. The shorter fibres allow BMC to be processed using injection moulding as well as heated presses, and can also lead to some 3-D orientation and reinforcement in thefinal product.
