Effect of Molding Temperature on Structures and Properties of PBT Injection Moldings
Miaki SHIBAYA, Naoto OHKUBO, Hideaki ISHIHARA, Katsuhisa YAMASHITA
Division of Advanced Fibro Science, Kyoto Institute of Technology
Japan
Keywords: skin-core morphology, PBT injection moldings, thickness direction
There is the skin-core morphology in the general injection moldings. During injection molding process, at first the surface of the flowing resins and then the inner resins is cooled by the mold. Therefore, the skin-core morphology, the difference of microstructure appears in the moldings. Particularly, because of the rapid rate of crystallization, poly(butylene terephthalate) injection moldings have definite skin and core layers. The purpose of this study is to investigate the effect of molding temperature on structures and properties of PBT injection moldings in the thickness direction. At first, specimens with 4mm thickness were sliced at every 0.1mm thickness. Tensile tests were carried out using dumbbell specimens which were stamped by the sliced specimens, simultaneously temperature rise were measured by an infrared camera. As a result, the necking start point delayed into high strain region with increasing the depth from surface and temperature rise occurred at the necking start point. Tensile modulus and strength increased with increasing the depth from surface. In order o discuss these results from structural view points, the crystallinity were evaluated by density measurement, the crystal orientation were examined by X-ray diffraction (WAXD) and Fourier transform infrared measurements were conducted to clarify the crystal modification of PBT. Low crystallinity and crystal orientation were observed near the surface layer. With increasing the depth from surface, the crystallinity increased and molecular orientation decreased. According to the infrared absorption spectra, it is clear that the layers near the surface have more alpha-form PBT crystals than inner layer. Consequently, they are obviously that the delays of necking point and high tensile modulus and yield strength in inner layer mainly depend not on the orientation and the low existence of the alpha-form but on the crystallinity.