Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 33

and designs use preforms, cloth, or matt materials for additional strength in selected
areas of a part. Among the main advantages of hand layup composites are low-cost
tooling, minimal requirement of equipment, and the ability to mould large parts. The
disadvantages are that the process is labour-intensive, the production rates are low, and
the quality is dependent largely on the skill of the operator. Also, the operator must
take safety measures to counter exposure to hazardous chemicals.
3.2.4 Spray-up Processing
Spray-up is similar to hand layup but it can be accomplished by hand or machine.
A mixture of resin, catalyst and chopped reinforcements are sprayed simultaneously
into a mould shape. After the gel coat has been applied, spray-up of the resin and
chopped fibres begins. Care must be taken not to damage the gel coat. Roll out helps
to 'densify' (eliminating air pockets and aiding in the wetting action) the composite.
Poor roll out can induce structural weakness by leaving air bubbles, dislocating the
fibres, or in poor coating of reinforcement. Heat may be applied to accelerate curing
of the composite and increasing the production rate. This process allows the creation
of very complex shapes, and production rates can be much higher than those seen for
layup methods. To ensure uniform mechanical properties throughout the composite,
care must be taken to apply uniform layers of materials. In complex composite
parts or stressed areas, additional thickness, metallic stiffeners or other reinforcing
components can be placed and sprayed over.
3.2.5 Rigid Vacuum Forming
In a process sometimes called rigidised shell spray-up , a thermoplastic (softens on
heating) sheet is thermoformed into the desired shape, eliminating the gel coat.
Some commonly polymers used in the process are polyvinyl chloride; acrylonitrile-
butadiene-styrene; polycarbonate; and high-impact polystyrene (HIPS). These shells
are reinforced by hand layup or spray-up for added strength (if necessary) to obtain
strong composites. Bath tubs, sinks, small boats, refrigerator shells, exterior signs,
car-top carriers or other similar products are made from these composites. Final
products with great finishes and in colours can be achieved with these composites.
3.2.6 Vacuum-bag Process
In vacuum-bag processing, plastic films such as polyvinyl alcohol, neoprene, PE or
polyester is placed over a layup and drawn between the film and the mould using
vacuum suction of ≈85 kPa. The plastic film forces the reinforcing material against