Effect of mold temperature on morphology and mechanical properties of injection molded HDPE structural foams

In this study, HDPE structural foams are produced by injection molding under different mold temperatures to study the effect of this variable on average cell dimension, cell density, and skin thickness ratio. Samples are also produced by setting independently the temperature of the fixed and moving plate of the mold to detect the sensitivity of foam structure to a temperature gradient in processing. The resulting foams are also characterized in terms of mechanical properties including impact and flexural tests. It has been found that for homogeneous mold temperatures, symmetrical skin thicknesses are obtained, which increase with decreasing mold temperature. On the other hand, by keeping one mold face at a constant temperature and varying the second one, asymmetric skin thicknesses are obtained. The degree of asymmetry is found to increase as the temperature difference between both molds increased. Furthermore, decreasing mold temperature produces a small increase in average cell sizes and reduced cell density. In general, both impact strength and flexural moduli of the structural foams increase with increasing skin thickness. For the particular case of asymmetric foams, the flexural moduli are slightly higher when the load is applied on the thicker skin; while much higher impact strength is obtained when the falling weight strikes the samples on the face having the smaller skin thickness.