ABSTRACT
I. INTRODUCTION The addition of short glass, carbon, or aramid fibers to polymers such as nylons and polyesters can result in molded parts having increased toughness. temperature resistance, and dimensional stability. Stiffness and tensile strength also increase with increasing fiber content, which can be in excess of 20% by volume: due to cost considerations, glass is the most common reinforcement. The heat-deflection temperature (HDT) of an unreinforced polymer is typically about 20°C below the glass transition temperature: the use of glass reinforcement allows the HDT to easily exceed I 00°C and to approach the polymer melting point, although there may be a reduction in impact strength. Glass-reinforced thennoplastics can be extruded, thermoformed. injection molded. or blow molded in the conventional way. and they are commonly utilized to make gears and other structural parts. Since they are used in engineering applications, these reinforced plastics are known as "engineering polymers." In short-fiber composites, the fiber length is typically 0.2 mm, while the aspect ratio (ratio of length to diameter) is about 15 [ 1]. Note that fibers having a length ranging from 13 to 25 mm are also used to make (thermosetting) sheet molding compounds and glass mats that are compression molded to produce automobile body panels [ 1]. Since the glass reinforcement is long and slender, it can be oriented by flow during processing: an extensional flow field is generally more effective compared to a shear field for this purpose. The fiber orientation gets frozen into the solid composite and makes the mechanical properties of the final part be anisotropic.
