Short Fiber orientation and its Effects on the Properties of Thermoplastic Composite Materials

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Abstract
While it is true that preform processes involving the use of long or continuous fibers are known and used in the manufacture of reinforced thermoplastic articles–Azdel [1] or STX sheet [2], for example–it is generally the case that such articles are formed by injection molding. Both the feedstock requirements for this process and the occurrence of high melt shear during it ensure that only short fibers will be present in the finished article. Although the use of slow screw speeds, slow injection rates, low back pressure, wide sprues, runners, and gates, and large radii of curvature avoids fiber breakage during molding, such conditions are not often found in practice. Furthermore, the necessity of incorporating reground material into the feedstock also ensures short fiber lengths in the final part, lengths not greatly in excess of the critical length required for effective stress transfer from polymer matrix to reinforcing fiber. In a practical part, design uncertainties caused by fiber length attrition are further compounded by the effects of fiber orientation. Although length distribution effects have been studied by a number of workers, both experimentally [3] and theoretically [4], relatively little has been reported on orientation effects in short fiber reinforced thermoplastics.