Fiber reinforced plastic (FRP) parts are finding increasing acceptance in the marketplace. FRP articles, also known as composites, generally include a resin mixed with reinforcing fibers and various fillers. The part is typically made by molding a charge into the shape of the desired part under heat and pressure. The charge spreads to fill the mold cavity and the charge is maintained in the mold for a sufficient period of time to allow the resin to cure to form the finished part.
Among the attributes of composite articles is that they are generally lightweight but exhibit relatively high strength characteristics. The strength of these parts is due in large part to the reinforcing fibers in the resin.
The charge composition varies considerably from application to application. One of the most commonly used charges is known as sheet molding compound (SMC). SMC is typically made by a continuous process whereby reinforcing fibers are applied to a paste containing a thermosetting resin. The following patents disclose such a method of making an SMC-like charge:
U.S. Pat. No. 4,294,490 to Woelfel;
U.S. Pat. No. 4,294,639 to Woelfel et al; and
U.S. Pat. No. 4,376,749 to Woelfel.
These patents, along with U.S. Pat. Nos. 4,220,496 to Carley et al and 4,220,497 to Carley, also disclose a method of making a charge known as XMC, a trademark of P.P.G. Industries, Inc. Briefly, XMC compounds are made by winding continuous fibers around a drum so as to achieve a generally helical shaped configuration of the fibers.
While XMC and SMC charges have generally performed satisfactorily for most applications, the processes for making them do not readily lend themselves to precise control of the fiber orientation therein. This is especially true when it is desired to orient the fibers in a multiplicity of different, but precisely controlled, orientations.
Bulk molding compound (BMC) and thick molding compound (TMC) are conventionally used in transfer or injection molding processes. It is often desirable to use fiber reinforcement in BMC and TMC charges. Fiber reinforced BMC is generally made by mixing the resin, filler and cut raw fiber strands or bundles together in a mixer. Typically, the mixing is accomplished by using two blades that rotate in opposite directions thereby providing a kneading action. Unfortunately, this mixing action can separate the individual fibers in the strands unless the mixing is carefully controlled. Such degradation of the strand integrity can lead to a loss of physical properties of the molded part since the fibers provide a more efficient reinforcement if they remain integrated as strands instead of being separated into individual fiber filaments.