The rate of cooling in the molding cycle is determined by a variety of factors including local part thickness, the presence of ribs and the mold wall temperature. Thicker sections cool more slowly and develop more crystallinity. The degree of anisotropy is greatest for perfectly aligned long fibers. But with perfectly aligned fibers, the material is isotropic in a plane, and termed transversely isotropic. This feature simplifies design and analysis to a great extent.
Longer fibers are also more easily oriented in shear flows.
Effect of Fiber Length on Properties Experimentally measured values of tensile modulus and tensile strength for two different Polybutylene terephthalate molding compounds are presented. Both the modulus and the strength are significantly higher for the molding compound with longer fibers on the average. There is a distribution of fiber lengths in any molding compound. Typical distributions of fiber length for three different compounds are shown here. It is particularly important to understand trends in tensile modulus because it is a good indicator of trends in other properties such as fatigue strength.