Method of injection molding articles with selective concentrations or

Abstract

The invention provides an improved process for molding articles having a bulk material and an auxiliary material, such as an additive, a comonomer, or a catalyst, present in the bulk material as a fixed concentration strip or in a concentration gradient in the direction from the surface to the interior. The invention is especially useful for concentrating surface-enhancing auxiliary materials at the surface of an article without wasting the auxiliary material in the interior of the article where it provides minimal value. This invention is also applicable to concentrating an interior-enhancing auxiliary material in the interior of the article without degrading surface sensitive properties. The method for manufacturing an as-molded article having a bulk material and an auxiliary material provided therein generally comprises injecting a moldable bulk material composition into a mold with an essentially laminar flow profile such that the earlier injected material will reside at the surface of the mold (i.e., the surface of the manufactured article) and the later injected material will constitute the interior portion of the article, and controlling the injection of the auxiliary material during filling of the mold with the bulk material to direct the auxiliary material to a desired location within the bulk material.