SHEET
MOULDING COMPOUND (SMC):
This
class of material, made in sheet form upto ¼” thickness, consists basically
1-2” long, chopped strand glass fiber roving as the reinforcing material, a
polyester resin and a number of additives. The principal additives here
(besides the catalysts) are the thickeners, fillers, thermoplastic polymers and
pigments.
The
function of the thickener (MgO) is to cause a controlled increase in viscosity
of the resin, after thorough fiber wetting has taken place. The thickening of
the resin then insures a uniform flow of reinforcement and resin to allparts of
the mould when heat and pressures are applied. This minimizes the problem of
resin-rich or resin-starved areas found in B-staged material when heated and
pressed.
The
function of approximately 30% by weight of powdered thermoplastic resin is to
decrease the shrinkage of the polyester resin. Such resin formulation are
called low shrinkage or low-profile resin. SMC containing thermoplastic resins
(acrylics, PVC, styrene copolymers, PE, etc.) can be compression moulded to
extremely smooth, high gloss surfaces. Gel coats are not used, but the moulded
parts are usually finished by painting with automotive primers and finished
coats.
Process:
Here the SMC fibers are
saturated with the resin by two doctor blades which meter the polymer on the PE
films. The resin must be very accurately controlled to obtain uniform
impregnation and because of the method of saturation, the fiber layer cannot be
too thick up to ¼” approximately.
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