| Abstract: | The advantageous use of short-fiber-reinforced thermoplastic (SFRTP) resins in the manufacture of pipe requires that the fiber orientation be controlled and that the surface finish not be adversely affected by the presence of the fibers. It is proposed here that a coextrusion process be combined with the use of a Specially designed, expanding channel die to achieve these objectives. Conventional pipe dies tend to promote the axial orientation of fibers, which has an undesirable effect on the hoop properties of pipe. The use of an expanding channel die promotes fiber orientation in the hoop direction. However, even in this case, there is still a strong component of the shear field near the wall of the die, and in this region, axial orientation still predominates, The use of a three-layer, coextrusion process in which unreinforced resin is used for both the inner and outer layers makes it possible to minimize the shear effects on the fiber orientation in the middle SFRTP layer. At the same time, it provides a smooth, fiber-free pipe surface. A theoretical analysis of this process is presented. The analysis shows that the presence of unreinforced surface layers increases the level and uniformity of fiber orientation. The results of an experimental study of fiber orientation in the middle SFRTP layer are found to be in qualitative agreement with the predictions of the theory. |
