ESTIMATION AND MEASUREMENT OF THE DISTRIBUTION OF TENSILE MODULI IN INJECTION MOLDED THERMOPLASTICS

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Abstract
A variety of techniques have been employed to study anisotropic behavior, as a result of processing, in injection molded parts. The techniques include density, sonic modulus, birefringence, wide-angle X-ray diffraction, infrared dichroism and mechanical testing. Generally, maximum density is observed in the core of the molding and near the gate, while minimum density is observed near the surface. Maximum orientation occurs at or near the surface, as indicated by both birefringence and sonic measurements. X-ray diffraction suggests a complex pattern of orientation for the three cyslallographic axes. The crystallographic a-axis tends to be oriented in the flow direction as shown by both the X-ray and infrared techniques. In general, raising the molding temperature has only a marginal influence on crystallinity and orientation. However, the two resins included in the study exhibited substantial differences in the distribution of these properties. Both elongation at break and at yield exhibit maxima near, but not at, the surface. On the other hand, tensile stress at yield tends to increase monotonically from the surface towards the centre. The results of crystallinity and orientation are combined to predict the distribution of tensile moduli using available theories. Good agreement is obtained between the calculated and measured values.