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This is a schematic diagram for the pultrusion process. The graphite fibers are unwound off the various spools and passed through a bath of resin. They leave the resin bath and pass thru the preform plates to get the fibers in proper alignment for the heated die. As the fiber bundle passes through the heated die the thermal setting resin is cured and emerging from the die is the pultrude spars. This process can be used to make hollow tubing, as it does for kite spars or for most any other cross sectional shape. "I" beams are frequently made this way. Following the pullers that provide the motive force, the continuous pultruded spar is cut to the desired length by the saw. As can be seen this is a continuous process and requires very little labor. It is the most inexpensive method for producing kite spars. Other composite materiel, such as fiberglass, can also be used in this process. The disadvantages of this process is that the spars have a relatively high resin content, which means they will be heavier than wrapped graphite spars, and they have less hoop strength than wrapped graphite spars. When a spar is deflected it generally fails when the hoop cross section deforms. For pultruded spars this failure usually results in delamination of the fibers along the length of the tube. Some manufacturers set the process up to wind some of the fibers spirally. This improves the hoop strength, but not up to the level of wrapped spars.
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