Hot moulding processes


Hot Moulding applies to processes that utilise pressure and temperature to shape and cure the finished parts. These often involve hydraulic presses to close the heated matched dies required to shape and cure the parts. Cycle times vary from 30 seconds to 4 minutes. Hot moulding processes are used for high volume production will relatively high dimensionally accuracy.

Sheet Moulding Compound (SMC)

This is a mixture of resin (usually polyester), filler, catalyst and chopped reinforcing fibres (usually E glass) in 20-50 mm lengths, in the form of a compounded sheet. In SMC manufacture the glass strands are sandwiched between two layers of film, onto which the resin mixture has already been applied. This is then compacted and stored for several days to reach optimum properties. The sheet formed by this process is malleable but non-tacky.

Dough Moulding (DMC) & Bulk Moulding (BMC) Compound

This is a mixture of resin (usually polyester), filler and 15-20% chopped reinforcing fibres (usually E glass) 3-12mm in length. Most moulding compounds are un-thickened, although in the past BMC contained a chemical thickener. The terms DMC and BMC are now used interchangeably.

Dry reinforcement, catalysed resin, fillers and additives are mixed into a dough like consistency, before being placed into the mould.

Both SMC & DMC/BMC are shaped & cured by hot moulding

  1. In the case of SMC, the sheet is cut to the required dimensions, then placed in the heated mould.  For DMC/BMC the measured quantity is placed between heated matched moulds.
  2. A hydraulic press brings the moulds together under pressure at temperatures around 100-170°C.
  3. The pressure and elevated temperature cause the compound to flow and completely fill the mould cavity.
  4. Once cured, the pressure is then released and the part removed from the mould.  Cycle times vary from 30 seconds to 4 minutes. Hot moulding processes are used for high volume production.