Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 31

• Filamentwinding
• Centrifugalreinforcing
• Pultrusion
• Coldstampforming
3.2.1 Extrusion
Other than the production of composite resins made by extrusion (discussed
previously), composites are also made by extruding layers of laminates. This could
take the form of sheet extrusions and layering at multiple stations with the thicknesses
and widths dependent upon the capacities of the machines being used. Hot surfaces
may provide sufficient tackiness for bonding but sometimes adhesives may have to
be used. Colouring or desired surface finishes can be obtained by additives, dyes,
pigments or masterbatches. For cooling the substrates, air or water can be used. If
light weight is desired, foaming agents can also be used.
3.2.2 Matched Die
In this method a 'matched' male and female mould are used to shape-reinforced
resins in the form of compounds. Bulk moulding compounds (BMC) are a 'putty-like'
mixture of resin, fillers, catalysts and short-fibre reinforcements. BMC are isotropic
with fibre lengths usually <0.04 cm. and they are sometimes called 'premix' or
'dough-moulding compounds' (DMC) and, in general, come in rope-like preforms.
They are moulded by compression moulding or transfer moulding. DMC have some
advantages over preform processing. This fibrous putty may be extruded and fed
automatically into a matched-die so that only an exact calculated amount is put into
the mould, thereby preventing seepage and waste due to excess material feed. This
operation can be divided further into 'open' and 'closed' mould processing. Injection,
compression, transfer, stamping and matched die are closed-mould operations. Open-
mould operations include hand layup, sprayup and filament winding. Sometimes, both
operations overlap, whereby open-mould methods are used to reform a part, and
a closed-mould operation is used to complete a moulding and curing a composite.
Sheet-moulding compounds (SMC) are leather-like mixtures of resins, catalysts, fillers
and reinforcements. They are made in sheet form, so fibre reinforcements can be
much longer than in BMC or SMC, thereby enabling greater glass loadings and thus
lighter products, with the longer fibres providing improved mechanical properties.