Abstract
The resistance of short-fiber-reinforced thermoplastic tubes to internal pressure is strongly related to fiber orientation. A new way to increase the degree of orientation in the hoop direction is presented. It consists of injecting a lubricant at the surface of a diverging die. A calculation of flow and fiber orientation in a lubricated die with a divergent final land shows that extensional deformations, which tend to orient the fibers perpendicular to the flow, take place on the entire thickness; in contrast, without lubricant, shear deformations near the surface orient the fibers in the flow direction. Various dies are tested with a 30 wt% short-glass-fiber-reinforced polypropylene. Observations show that, with the lubricated diverging die, the fibers are oriented mainly in the hoopwise direction throughout the tube thickness. Consequently, mechanical properties are increased in this direction. The resistance of the tube is increased by about 40%.