Variations in the skin-core morphologies of injection-molded poly(butylene terephthalate) samples have been characterized as a function of molding conditions by optical microscopy, x-ray diffraction, and density measurments. Samples molded from 450 and 510°F melts at mold temperatures ranging from 70 to 250°F were found to have nonspherulitic skins whose thicknesses varied inversely with processing temperature. Very little orientation was observed in the skin or core regions of the molded bars. Density profiles are presented.
Impact data obtained on bars with molded-in notches demonstrate that significant increases in impact strength are observed with increasing skin thickness, although the maximum skin thickness remains less than a few hundred microns. This behavior persists through the ductile brittle transition which is centered at ∼50°C.
Fracture surface micrographs and high-speed stress-strain curves are presented and used to give a qualitative description of the fracture process. At room temperature the samples appear to deform elastically up to the point of fracture, while at higher temperatures extensive shear deformation is observed. In each case failure ultimately occurs via a craze–crack mechanism.