In addition, the mechanical properties of the composites are relatively uniform in all directions. They have also been evaluated successfully in gears, bearings, compressor vanes, and pump impellers. In the past, two drawbacks have made aramid-reinforced thermoplastic composites impractical. This and the unusually low bulk density of commercially available aramid fiber did not allow processing in conventional compounding equipment. A second problem was the fiber damage that occurred, both during mixing and injection molding.
Damage is usually severe because of the low compressive strength of aramid fiber.
The first of these problems has now been minimized, and fibers can be incorporated uniformly into a resin matrix. Consequences of the fiber damage show up, of course, as reduced properties which are similar to, or lower than, those of their glass-reinforced counterparts. There is a silver lining, however, to the dark cloud of disappointing mechanical properties. Conventional fiber-reinforced composites have lower shrinkage and higher strength and modulus in the flow direction. Not so with the aramid-reinforced materials.