The influence of injection molding conditions and polymer composition on skin thickness and flexural properties of HDPE structural foams
Denis Rodrigue, Simon Leduc
Laval University
Canada
Keywords: structural, foam, injection
Polyethylene foams are used nowadays as structural materials because of their mechanical and thermal properties associated with lower densities. Since the morphology of the foam (dimensions of the cells, cell density, and skin thickness) is the most important parameter affecting the strength of these polymer foams, they must be carefully controlled during processing.
In this study, high density polyethylene was used with azodicarbonamide (ACA) as the chemical blowing agent in concentrations between 0.5 and 1.0 wt.%. The chemical blowing agent/polymer mixtures were injection molded under different operating conditions in order to verify the effect of each parameter on foam morphology. We mainly varied the mold temperature between 30 and 80oC. Some experiments were also performed by adding 0.5% of calcium carbonate as a nucleating agent.
The produced foams were characterized by an average cell dimension, a cell number density, the ratio of skin/sample thickness, and the foam bulk density. As expected, the foam bulk density was not greatly affected, but skin thickness and cell dimension were. Decreasing the mold temperature from 80oC to 30oC increased the skin thickness ratio from 35 to 49% and decreased the average cell size from 75 to 59 microns, while maintaining a similar cell size distribution. The addition of CaCO3 reduced significantly cell sizes (33 microns), produced a narrower distribution, and reduced also the skin thickness ratio (32%). As expected, thicker skins produced foams with higher flexural modulus at equivalent foam densities. These results can be easily explained in terms of viscosity, cell growth time, and nucleation effects. In all cases, skin thickness was found to be symmetric. The flexural properties were then compared with models taken from the literature in order to determine which one represents best our data.