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.
Process:
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.
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