Effect of Weld Lines on the Injection Moulding of Structural Foams. I. Foam Morphology

The effect of weld lines on the morphology of injection moulded structural polymer foams was studied. In this first part, the effect of processing conditions on density and morphology of high density polyethylene foams is reported. In particular, the effect of density reduction and injection speed was investigated. The results indicate that for lower density foams, skin thickness decreases substantially along flow path and cells collapse is present. At the weld line, local reduction of skin thickness and cell size is related to the formation of a V-notch prior the final expansion and to higher pressures generated by the difficulty to expulse the entrapped air inside the mold. As of the effect of injection speed, cell size and skin thickness differences between samples with or without weld lines were observed and explained by time and gravity effects.