Numerous articles of commerce are manufactured by subjecting a moldable composite sheet to a molding process. The moldable composite sheets usually comprise a thermoplastic resin such as a polypropylene resin which is reinforced with reinforcement fibers such as a glass fiber mat. These composite sheets are commonly referred to as glass mat thermoplastics or GMT. These composite sheets can be heated to soften the resin and molded under pressure to make various articles such as parts for automobiles, cabinetry for computers and other electronic equipment, components for small appliances, and recreational, agricultural and other durable consumer and industrial goods. A typical molding process for these composite sheets is compression molding in which the moldable composite sheet is heated, pressed into a mold, and subjected to pressure in a relatively cold mold to form the molded part. The resin and reinforcement material flow during the molding process to fill out the entire volume of the mold.
A compression molding technique is disclosed in U.S. Pat. No. 4,970,044 to Kim et al., which is hereby incorporated by reference. Kim et al. teach that a deformable plastic film can be placed on the surface of a thermoplastic charge prior to compression molding. The film acts as an insulator to prevent the relatively cold mold from cooling the thermoplastic charge so fast that the surface of the charge becomes moldable. Kim et al. further teach that the film can be compatible with the thermoplastic charge so that the molding process bonds the film to the charge, thereby producing a molded composite product.
Moldable composite sheets and molded composite products often contain additives that are nonresinous but are used to enhance the molded product. An example of an additive is a colorant. Various pigments are added to the resin so that the molded product has the desired color. One of the problems with using additives in molded composite sheets is the cost of the additives. Frequently, the cost of the additive is a substantial fraction of the total raw material cost of the molded product. It would be advantageous if the cost of molded composite products containing additives could be reduced.