Effect of Mixing Pressure in Injection Molding on Interfacial Shear Stress of GF/PP Short-Fiber Reinforced Plastics

Abstract

In polypropylene resin matrix glass fiber injection molding composite materials, surface of fibers is usually treated by coupling and/or sizing agent and resin is modified by malefic acid because mean fiber length is shorter than their critical length, and interfacial adhesion strength between fibers and resin is low. Therefore, fiber strength has not been utilized completely in these composite materials. In the present study, we propose a method to improve the interfacial adhesion strength by increasing shear strength between fibers and resin, which can be attained by increasing mixing pressure in kneading machine. Since stress-strain relation of composite materials is affected by surface treatment of fibers, interfacial adhesion strength was evaluated by the average interface shear stress through a visco-elastic model. The effect of mixing pressure was discussed based on the shear stress. For a composite material with no surface treatment of fibers and no resin modification, the interfacial shear strength was higher for higher mixing pressure. On the other hand, for a composite material with the surface treatment and the modification, the effect of mixing pressure on the shear strength was not observed. For these materials, however, higher mixing pressure also contributes the adhesion strength between fibers and resin because fiber length in the materials is longer for higher mixing pressure.