2003 Volume 15 Issue 12 Pages 823-829
During the injection molding process, the surface of the flowing resin that is in contact with the mold surface is cooled first and followed by the inner part. It is reported that these phenomena result in a layered structure through the injection molded part. Particularly, because of the rapid rate of crystallization, poly (butylene terephthalate) injection moldings have a well-defined layered structure. The purpose of this study is to investigate the relationship between the structure and properties of PBT injection moldings in the thickness direction. Tensile tests were carried out using dumbbell specimens that were stamped from the sliced specimens of moldings. The necking start point was delayed with increasing depth from the surface. The tensile modulus and strength increased with increasing depth from the surface and eventually became constant in the inner layer. In order to discuss these results from a structural viewpoint, the microstructure of each layer was evaluated by differential scanning calorimetry, density, WAXD and Fourier Transform Infrared measurements.
The delay in neck propagation with increasing depth from the surface is due to the increased crystallinity of the inner layer compared to the layer near the surface. Both the α and β crystal forms were distributed through the thickness direction in PBT injection moldings. The α form with a lower crystal modulus was preferentially found in the surface layer and the content of the α and the β form became constant in the inner layer.
