Fiber reinforcement is a known, commonly used method of increasing the mechanical properties of thermoplastic, glass, and glass ceramic materials. Hereafter referred to as molding compounds, these materials are described, for example, in U.S. Pat. Nos. 4,314,852, 4,324,843, and 3,926,904 (incorporated herein by reference). The fibers used for reinforcement vary according to the desired application. These fibers include: carbon base fibers, boron base fibers, glass fibers, silicon carbide fibers, metal fibers, and others. Carbon base fibers are light weight, strong, highly electrically conductive, and are easily oxidized, with a low upper temperature limit in oxidizing atmospheres. Boron base fibers are easily oxidized, similar to carbon fibers, yet have excellent heat-shock resistance and are noncombustible. Glass fibers, good insulators and noncombustible, can lose strength above about 315.degree. C. Silicon carbide fibers can be resistant to oxidation at high temperatures, electrically conductive, noncombustible, and have excellent thermal conductivity. Metal fibers possess extremely high tensile strength, with other properties ranging between noncombustibility to oxidizing at low temperatures. The above mentioned fibers and others will hereafter be referred to as fibers.
Critical factors affecting the composites' mechanical properties include fiber dispersion and orientation. For instance, composites containing unidirectionally oriented fibers possess high tensile strength in the direction of the fibers, and low tensile strength perpendicular to the fiber orientation; while randomly oriented fiber composites display a fairly uniform tensile strength in all directions.
One fabrication method for producing fiber reinforced composites having complex shapes and desired fiber orientation involves hot press molding of glass infiltrating woven and/or non-woven fiber preforms as described in U.S. Pat. No. 4,412,854 (incorporated herein by reference). Related techniques that are suitable for complex shapes, used with matrix transfer molding are described in U.S. Pat. Nos. 4,786,304 and 4,428,763 (incorporated herein by reference). Transfer molding, a process similar to injection molding, consists of softening thermoplastic resin in a chamber with heat and pressure and forcing it by high pressure through an orifice into a closed mold.
An injection molding technique, not requiring fiber pre-arrangement yet obtaining fiber dispersion, consists of injecting a discontinuous fiber/molding compound mixture into a die as disclosed in U.S. Pat. Nos. 4,464,192 and 4,780,432 (incorporated herein by reference). Injection molding, described in the above mentioned patents, consists of feeding a molding compound into a heating chamber and heating it to a viscous liquid, and then, via a plunger, forcing the material into a mold and keeping it under pressure until cool.
Injection molding of fiber reinforced molding compounds, without pre-arranged fibers, fail to fully solve the problem of obtaining a controllable fiber orientation which will produce the optimum mechanical strength. For example, in cylindrical orifice injection molding, as described in U.S. Pat. No. 4,780,432, fibers lie on paraboloidal shaped planes susceptible to crack propagation (FIG. 1A) between such planes.
The object of this invention is to introduce an oscillatory flow of the molding compound into the mold cavity, to produce a shaped composite with desired fiber dispersion and orientation obtaining superior, quasi isotropic, mechanical strength.