Woolmer Forest Composites
Technical Information

The Pultrusion Process

Pultrusion is an automated manufacturing process for the production of constant cross-sectionally shaped profiles of fibre reinforced composites. The profiles produced with this process can compete with traditional metal profiles such as steel and aluminium for strength and weight.

The pultrusion process is one of the most cost effective methods for the production of composite materials. It is a continuous process whereby fibre reinforcement is pulled through a resin impregnation zone to coat and impregnate the reinforcement with resin, through perform plates to begin to shape the fibre/resin bundle, and through a heated die to cure the resin. A cured part in the desired shape that requires no further processing exits from the die.

Although the process appears to be simple, numerous process variables such as pull speed, die temperatures, quality of fibre/resin wet-out, and fibre volume can affect the quality of the pultruded composite. In special constructions some of the reinforcement is either spirally wound or braided into the fibre perform just prior to entering the die.

Fibre reinforcements can be glass, carbon or aramid (Kevlar). Generally a high strength grade of carbon fibre is used but for exceptional stiffness intermediate or high modulus grades are available to special order. Combinations of glass and carbon fibre can result in useful cost effective methods to optimise performance and aramid fibre will add to toughness

To achieve optimum performance the resin matrix is usually an epoxy although vinyl ester resins are suitable for certain combinations.

Wrapped tube

G-Force tube is manufactured by an entirely different process. First  bundles of carbon fibre filaments in the form of tows are aligned and pre-impregnated with an epoxy resin to produce a continuous thin sheet which is then wrapped around a stainless steel mandrel together with a final tape layer of heat shrinkable plastic film. The mandrel is then placed in an oven to cure and solidify the resin and after cooling the finished tube can be slid off the mandrel and the plastic tape removed.

This technique allows the type of carbon fibre to be selected for the application. By controlling the alignment of each layer of the carbon fibre sheet as it is wrapped onto the mandrel it is possible to adjust the stiffness and optimise the weight/performance of the final tube.

Although partially automated this manufacturing process inherently results in higher labour costs and significantly more expensive tubes. However, the improved performance means that virtually all competition grade kites are framed in hand-wrapped tube.