PREMIXES: (dough moulding compound DMC or bulk moulding compound BMC)
They are the mixtures of polyester resin; a mineral filler (usually CaCO3), clay, silica clay, etc.; reinforcing fibers (glass, nylon etc.); appropriate catalysts; pigments and lubricants.
The resin and filler are mixed to produce a thick pourable liquid or a soft putty like substance. To this material fibers are then carefully added and mixed. Improper or excessive mixing may cause breaking of glass fibers which may result in very rapid degradation of the physical properties of the moulding. The length of the glass fiber have a considerable effect on the final properties of the moulding compound. Shorter fiber result in low material strength and longer fiber do not disperse well in the mixing operation, are apt to break up and may interfere with flow in moulding operation.
Usually the premixed material is wrapped in PE film to minimize the loss of the styrene. The material can be stored for 3-4 weeks at room temperature depending on the catalyst used.
Often, the premix are extruded or rolled into long lengths or cords to obtain preforms of desired length, simply by chopping them.
Polyester premix or BMC are among the cheapest plastic moulding compounds. Epoxy resin based BMC have relatively higher cost.
Advantage:The premixes can be moulded by compression or transfer moulding with fairly low pressure, fast cure cycles and high strengths.