Composite Technology - Introduction to Polymeric Composites with Rice Hulls - page 35

will be forced out between the strands. Depending on the end product designed, the
filament winding tension can be varied.
In dry winding , pre-impregnated reinforcements help to insure consistency in design
of the resin-to-reinforcement content. These pre-impregnated reinforcements can
be machine- or hand-wound on the tooling. Curing can be accelerated by heated
mandrels (tooling), ambient ovens, chemical hardeners or other energy sources. Many
cylindrical laminated forms are produced by this method. The collapsible mandrel must
have the desired shape of the finished product. The advantage of filament winding is
that it allows the designer of the composite to place the reinforcement in the areas
subject to the greatest stress. Containers made by this process usually have a higher
strength:mass ratio than those made by other methods. Filament-wound applications
include pressure vessels, underwater buoys, nose cones, storage tanks, automotive
parts, helicopter blades, fuselage and many aerospace parts.
3.2.9 Centrifugal Reinforcing
In centrifugal reinforcement, the resin matrix and reinforcing materials are forced
against a pre-designed mould surface by centrifugal forces generated through rotation
of the mould. During this rotation process, the resin is distributed uniformly through
the reinforcement by centrifugal forces. Heat is then applied to help polymerise the
resin. This method is similar to the rotational moulding process. Large tanks and
tubing can be made this way.
3.2.10 Pultrusion
Pultrusion is a method in which composites can be made by pulling a resin-soaked
matting with fillers and other additives through a long die heated to 121.1-176.6 °C.
The product is shaped and the resin is polymerised because it is drawn through the die.
Additional heating will also help accelerate production rates. This process is similar
to the standard extrusion process, in which a hot-melt resin is pushed through a die
orifice. In pultrusion, resin-soaked reinforcements are pulled through a heated die
and the resin is cured. Pultruison dies are, in general, 60-159 cm in length and heated
to aid polymerisation. Cure must be uniform by careful control to prevent cracking,
delamination, incomplete cure or sticking to the die surface.
Depending on the equipment used, production output will vary but, in general, will be
≈3 m/min. Common resins used for this process are vinyl esters, polyesters and epoxies.
Fibrous glass is used widely for this process (though graphite; asbestos; carbon; boron;
polyester; and PA fibres may also be used successfully). Extra reinforcements can also