Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 32

SMC are fed to moulds and go through phases of pressing, curing and de-moulding
in one continuous cycle. SMC need a processing pressure of ≈500-2,000 psi, with
processing temperatures varying with product design and polymer contents.
Composite product designs that require thick walls or sections need thick moulding
compounds with strength being provided by the dominant fibre direction. Some are
filament-wound on a mandrel with a protective film wrapped over the final layer, and
the preform is removed and stored for moulding later. A variation of this process is
called macerated reinforcing processing . Macerated parts are produced by chopping
the reinforcements into small pieces of length ≈2-100 mm to be processed in the
matched moulds. Reinforced resin products made from matched die moulds are
strong and may have a superb surface finish, both inside and out.
Major markets for BMC- or SMC-moulded parts are in the transportation and
appliance industries. For example, shower and bath mouldings as well as appliance
housings are made of BMC. As the name implies, SMC are used for large parts such
as automotive body panels, hoods, boat hulls, furniture and appliance components.
3.2.3 Hand Layup Processing
For hand layup processing, in general, thermosetting (hardens on heating) resins are
used. Only atmospheric pressure is used in applying the saturated reinforcing material
to the mould, so this can hardly be called a 'moulding' process. Open moulding
may be a more descriptive term. In this method, a mould half is coated lightly with
a release agent, a layer of catalysed resin is applied, which is allowed to polymerise
and gel to a tacky state. This first coat/layer is a specially formulated gel-coat resin
used in industry to: improve flexibility; be blister-resistant; and to provide a surface
finish and weatherability. Gel coats based on neopentyl glycol, trimethylpentanediol
glycol and propylene glycol provide major advantages as surface treatments for
reinforced-polyester products.
The gel coat forms a protective surface layer through which fibrous reinforcements
cannot penetrate. A prime cause of deterioration of fibre-reinforced plastics is penetration
by water, which can take place if fibres protrude at the surface. Once the gel coat has
partially set, reinforcement is applied. Then, more catalysed resin is poured, brushed or
sprayed over the reinforcement. This sequence can be repeated until the desired thickness
is achieved. In each layer the mixture is worked to the mould shape with hand rollers,
and then the reinforced-laminate is allowed to cure and harden. Application of external
heat may also be used to accelerate polymerisation and faster curing. Hand layup and
spray operations are often used alternatively to obtain a resin-rich superior finish, or
a matt finish. Coarser reinforcements are then placed over this layer. Some operations