Glass-reinforced plastic (GRP) is a composite material or fiber-reinforced plastic made of a plastic reinforced by fine fibers made of glass. Like carbon fiber reinforced plastic, the composite material is commonly referred to by the name of its reinforcing fibers (fiberglass). The plastic is thermosetting, most often polyester or vinylester, but other plastics, like epoxy (GRE), are also used. The manufacturing process for GRP fiber glass uses large furnaces to gradually melt the sand/chemical mix to liquid form, then extrude it through bundles of very small orifices (typically 17-25 micrometres in diameter for E-Glass, 9 micrometres for S-Glass). These filaments are then 'sized' with a chemical solution. The individual filaments are then bundled together in large numbers to provide a 'roving'. The diameter of the filaments, as well as the number of filaments in the roving determine its 'weight'. This is typically expressed in yield-yards per pound (how many yards of fiber in one pound of material, thus a smaller number means a heavier roving, example of standard yields are 225yield, 450yield, 675yield) or in tex-grams per km (how many grams 1 km of roving weighs, this is inverted from yield, thus a smaller number means a lighter roving, examples of standard tex are 750tex, 1100tex, 2200tex). These rovings are then either used directly in a composite application such as pultrusion, filament wounding (pipe), gun roving (automated gun chops the glass into small lengths and drops it into a jet of resin, projected onto the surface of a mold), or used in an intermediary step, to manufacture fabrics such as chopped strand mat (CSM) (made of randomly oriented small cut lengths of fiber all bonded together), woven fabrics, knit fabrics or uni-directional fabrics. An individual structural glass fiber is both stiff and strong in tension and compression -- that is, along its axis. (Although one might intuitively imagine the fiber to be weak in compression, it is actually only the long aspect ratio of the fiber which makes it seem so; i.e., because a typical fiber is long and narrow, it buckles easily.) On the other hand, the glass fiber is unstiff and unstrong in shear -- that is, across its axis. Therefore if a collection of fibers can be arranged permanently in a preferred direction within a material, and if the fibers can be prevented from buckling in compression, then that material will become preferentially strong in that direction. Furthermore, by laying multiple layers of fiber on top of one another, with each layer oriented in various preferred directions, the stiffness and strength properties of the overall material can be controlled in an efficient manner. In the case of glass-reinforced plastic, it is the plastic matrix which permanently constrains the structural glass fibers to directions chosen by the designer. With chopped strand mat, this directionality is essentially an entire two dimensional plane; with woven fabrics or unidirectional layers, directionality of stiffness and strength can be more precisely controlled within the plane. A glass-reinforced plastic component is typically of a thin "shell" construction, sometimes filled on the inside with structural foam, as in the case of surfboards. The component may be of nearly arbitrary shape, limited only by the complexity and tolerances of the mold used for manufacturing the shell.
