A. Definition of Weld LinesIn the broadest sense, “weld” or “knit” lines occur whenever two or more molten polymer interfaces are brought into contact. Actually the regions of different morphology and properties thus formed are not lines at all but are rather extremely complex three-dimensional areas of finite dimensions. For the purposes of our discussion we will use this rather broad interpretation of the term “weld line” in order to discuss the wide occurrence of the phenomenon in commercial fabrication processes.