All plastics have relatively low elastic moduli and strength, which can be about 100 and 5 times lower, respectively, than those for metals. In general, these properties are lower for thermoplastics than for thermoset resins. While the desired stiffness and strength of a part can be achieved by using mechanical design stiffening principles, such as by using ribs, materials with inherently higher stiffness and strength can expand the design envelope. One successful strategy is to use high-stiffness and high-strength fibers to reinforce plastics. Different types of reinforcing materials and the theoretical background for fiber reinforcement have been discussed in Chapter 8. Fiber fillers are also used to improve thermal stability, reduce shrinkage, and to enhance electromagnetic characteristics.