WELDLINE STRENGTH IN INJECTION-MOLDED HDPE PA6 BLENDS - INFLUENCE OF INTERFACIAL MODIFICATION/

Most injection molded objects contain defects known as weldlines. This defect may introduce an element of weakness affecting the object's pe rformance. Weldlines are particularly problematic in multiphase materi als where the situation may be exaggerated by component mismatch on th e two sides of the interface that results in additional weakening when the two components do not adhere well to each other. In addition, wel dline behavior is influenced by orientation and morphological effects. This paper deals with relationships between the structure and the mec hanical properties in injection molded high density polyethylene polya mide-6 blends. The weldline effect is investigated in detail. Two mold s were used to generate weldlines: a double-gated tensile bar cavity i n which the weldline results from the meeting of two melt fronts flowi ng into each other from opposite directions, and a film-gated rectangu lar plaque mold with a circular insert that divides the melt front in two. Following the recombination of the fronts, there is additional fl ow as the melt fills the mold cavity. Two preparations containing 75 v ol % of polyamide-6 and 25 vol % of polyethylene with and without comp atibilizer were studied. In the first case, a compatibilizer was incor porated into the polyethylene prior to compounding with the polyamide- 6. In the directly molded tensile bar the minor phase is strongly orie nted parallel to dow. Only in the core, which represents about 10% of the sample thickness, do the dispersed phase particles assume spherica l shape. The morphology of the weldline is closely related to that of the skin: the elongated structures are oriented parallel to the weldli ne plane. The effect of the compatibilizer on the mechanical propertie s (without the weldline) of the directly molded tensile bars is minor: it is overshadowed by the flow-induced morphology. The weldline stren gth loss is about 40% in the noncompatibilized blend. The introduction of the compatibilizer has restored the material's ability to yield an d the properties are close to those measured without the weldline, For the second type mold, the effect of the weldline is less pronounced a nd the effect of the distance from the insert is negligible. The aniso tropy is quite pronounced in the noncompatibilized blend. In compatibi lized blends, all tensile properties are unaffected by the presence of weldline, except for the 2-mm-thick plaque in the position close to t he insert. The properties in the direction parallel to flow are simila r to the type I mold and not affected by the increase of plaque thickn ess. Consequently one may question the utility of the directly molded tensile specimens in studying various aspects of the mechanical behavi or of multiphase materials where the flow-generated structure is very different from that found in ''real'' injection molded parts. (C) 1995 John Wiley & Sons, Inc.