Polymer and Materials Science
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Most injection molded objects contain defects known as weldlines. This defect may introduce an element of weakness affecting the object's performance. Weldlines are particularly problematic in multiphase materials where the situation may be exaggerated by component mismatch on the two sides of the interface that results in additional weakening when the two components do not adhere well to each other. In addition, weldline behavior is influenced by orientation and morphological effects. This paper deals with relationships between the structure and the mechanical properties in injection molded high density polyethylene polyamide-6 blends. The weldline effect is investigated in detail. Two molds were used to generate weldlines: a double-gated tensile bar cavity in which the weldline results from the meeting of two melt fronts flowing into each other from opposite directions, and a film-gated rectangular plaque mold with a circular insert that divides the melt front in two. Following the recombination of the fronts, there is additional flow as the melt fills the mold cavity. Two preparations containing 75 vol % of polyamide-6 and 25 vol % of polyethylene with and without compatibilizer were studied. In the first case, a compatibilizer was incorporated into the polyethylene prior to compounding with the polyamide-6. In the directly molded tensile bar the minor phase is strongly oriented parallel to flow. Only in the core, which represents about 10% of the sample thickness, do the dispersed phase particles assume spherical shape. The morphology of the weldline is closely related to that of the skin: the elongated structures are oriented parallel to the weldline plane. The effect of the compatibilizer on the mechanical properties (without the weldline) of the directly molded tensile bars is minor: It is overshadowed by the flow-induced morphology. The weldline strength loss is about 40% in the noncompatibilized blend. The introduction of the compatibilizer has restored the material's ability to yield and the properties are close to those measured without the weldline. For the second type mold, the effect of the weldline is less pronounced and the effect of the distance from the insert is negligible. The anisotropy is quite pronounced in the noncompatibilized blend. In compatibilized blends, all tensile properties are unaffected by the presence of weldline, except for the 2-mm-thick plaque in the position close to the insert. The properties in the direction parallel to flow are similar to the type I mold and not affected by the increase of plaque thickness. Consequently one may question the utility of the directly molded tensile specimens in studying various aspects of the mechanical behavior of multiphase materials where the flow-generated structure is very different from that found in “real” injection molded parts. © 1995 John Wiley & Sons, Inc.
Type of Medium: