Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 34

the mould surface, producing a high-density product free of air bubbles. Mould design
and tooling for vacuum-bag processing is costly if large parts are made. Output also
can be slow as compared with high-speed injection moulding. This process involves
moulds having male and female halves. If a smooth surface is required on the exterior
of a boat hull, the female part of the mould would be selected whereas, in the case of
a sink, a male half of the mould would be selected. For easy removal of the finished
composite and for protection of the mould surface, plastic films, waxes, and silicone
resins can be used as release agents.
Popular wet layup resins are epoxies and polyesters. Pre-pregs of polysulfone,
polymide, phenolics, silicones or other resin systems may also be used. Reinforcements
may include honeycomb materials, fabrics, paper, foils or other impregnated forms.
After the mould has been protected with a release wax or film, a peel ply of finely
woven polyester or polyamide (PA) fabric is positioned carefully. The laminate layers
are then placed in a specific design pattern on the peel ply. A second peel ply is placed
on the laminate layers, followed by a release film or fabric such as Teflon fabric.
Because the perforations in the fabric allow air and excess resin to escape, this layer
is sometimes called the 'breather ply'. The vacuum bag can be made of any flexible
material that is airtight and will not dissolve in the matrix.
3.2.7 Pressure-bag Processing
Pressure-bag processing is costly and slow. However, large, dense composite products
with good finishes (inside and outside) are possible. Pressure-bag processing involves
use of a rubber bag to force the laminating compound against the contours of a mould.
In general, pressure (≈35 kPa) is applied to the bag during the heating and curing cycle.
The mould and compounds may be placed in a steam or heated gas autoclave after
layup. Autoclave pressures of ≈350-700 kPa achieve greater glass-fibre loading and
aid in the removal of air. All pressure designs for the mould and pressure bag must
be able to withstand the moulding pressures. This method of processing may suit the
production of long hollow pipes; tubes; tanks; or other objects with parallel walls.
3.2.8 Filament Winding Process
Filament winding processes produce strong parts by winding continuous fibrous
reinforcements on a mould. Long continuous filaments can carry bigger loads than
random, short filaments. For some applications beryllium; PA; polymides; polysulfones;
polyesters; and other polymers are also used. Specially designed winding machines
may lay down these strands in a pre-determined pattern to give maximum strength
in the desired direction. During wet winding , excess resin matrix and entrapped air