EFFECT OF MOLDING CONDITIONS ON RESIDUAL-STRESSES IN POWDER INJECTION-MOLDING

The layer removal method was used to assess residual stress distributi ons in 10 mm thick plates injection molded from a 56v/o alumina suspen sion in polypropylene. Three different molding techniques were examine d. Conventional moldings present a center-tensile, surface compressive distribution with the magnitude of the tensile stress strongly influe nced by hold pressure. Heated sprue moldings show a distribution which is within the limits of experimental error and is effectively negligi ble. Modulated pressure moldings present an unbalanced distribution wh ich indicated that oscillatory flow influenced the elastic modulus of the polypropylene, probably as a result of orientation effects. The re sults show that stress distributions are strongly influenced by contro l of sprue solidification time and confirm that it is steep changes in pressure in molding during the course of solidification which widen t he residual stress distribution.