WELDLINES IN INJECTION-MOLDED PARTS - A REVIEW

It is a well-known fact that weldlines are unavoidable in most injecti on-molded products of even moderate complexity. While there are many s ituations where they are barely perceptible, weldlines represent a pot ential source of weakness in molded parts. In injection molding weldli nes are generated when two separate melt streams join either in multig ated molds or as a consequence of flow around obstacles. The developme nt of many interesting materials has been hampered by poor weldline st rength. Among such materials are plastics reinforced with fibers or pl atelets, liquid crystal polymers, and a number of multiphase polymer b lends. Weldlines have ever been called the ''Achilles' heel'' of these multiphase materials. This article is a review of the literature publ ished on weldlines in injected parts. It deals primarily with the aspe cts related to the mechanical behavior of weldline-containing parts. I t begins with a brief description of the phenomena important for the p art formation in the mold, including those leading to weldlines, in ad dition to the techniques used to characterize weldline-containing part s. The following three sections consider the structure and properties of weldlines in neat amorphous and semicrystalline polymers, filled an d reinforced plastics, and finally in polymer blends and alloys. In th e last section methods developed for increasing the weldline strength are discussed. (C) 1995 John Wiley and Sons, Inc.