A Micro-cutting Test for Checking Weld-line and Skin-core Structure of Injection Molded Materials

概要

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The formation of weld-line or skin-core structure in injection molded materials reduces their strength. A new technique which is called "micro-cutting test" has been performed on injection molded specimens of crystalline polymers. This method can give quantitative information on shear yield strength in the region near weld-line or in the subsurface layer so that inhomogenous mechanical properties of the weld-line and the skin-core structure can be evaluated. The following results have been obtained:(1) There is a significant difference in shear yield strength between the flow and transverse directions in a liquid crystalline polymer (LCP) specimen. This difference attained a minimum value at the weld-line.(2) The shear yield strength obtained from cutting along the weld-interface decreased with approach to the weld-line. The region which was affected by the presence of the weld-line was within 6-8mm from the center of the weld line.(3) The fracture toughness of the LCP weld specimen was about 25% of that of the non-weld material. The reduction of fracture toughness has also been observed for the other crystalline polymers.(4) The skin-core structure of HDPE consisted of three different layers. The shear yield strength of the skin-layer increased with increasing mold temperature.
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