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

In this study, HDPE structural foams were 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 were also produced by setting independently the temperature of the fixed and moving plate of the mould to detect the sensibility of foam structure to a temperature gradient in processing. The resulting foams were also characterized in terms of mechanical properties including impact and flexural tests. It was found that for homogeneous mold temperatures, symmetrical skin thicknesses were obtained, which increased 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 were obtained. The degree of asymmetry was found to increase as the temperature difference between both molds increased. Furthermore, decreasing mold temperature produced a small increase in average cell sizes and reduced cell density. In general, both impact strength and flexural moduli of the structural foams increased with increasing skin thickness. For the particular case of asymmetric foams, the moduli were slightly dependent on the direction of the applied force (surface on which the stress is applied). Higher impact strength was obtained when the falling weight stroke the samples on the face having the smaller skin thickness, whereas for flexural tests, the reverse was observed