Fabrication Process and Part Properties

Polymer orientation varies through the thickness of the injection-molded part owing to the fountain flow of the melt in the mold cavity. The flow at the center of the cross-section is deformed through extension and the highly stretched flow front rolls up to the cold mold surface, where orientation is frozen in a thin surface layer. The rest of the melt required to fill the cavity flows under this stationary frozen layer in more or less a plug fashion, with minimum orientation. Surface orientation in an injection-molded part can be significantly different from that in the core of the part. 

Some tests are affected by core orientation and some test properties are more influenced by surface orientation. Since orientation is not uniform, but has a gradient through and along the flow path, it is difficult to predict directly the effects of process conditions on part properties, without a complex model of the part geometry and estimation of flow characteristics in the cavity. 

Izod impact strength increases with increasing injection speed and lower melt temperatures. Since the fracture plane is normal to the direction of orientation in the Izod sample, the increased toughness with slower injection speed can be explained by the reinforcing effect of the oriented polymer on the surface. This effect is less noticeable at very high melt temperatures, where the surface can actually be annealed by the temperature of the core and subsequent extended molding cycle time required by the hot melt temperature. 

The compression-molded part, by definition, does not have flow-induced orientation. Comparison of compression-molded part properties with those of an injection-molded part can show the effect of melt temperature on properties. In the compression-molded article without flow-induced orientation, the impact strength remains constant until a certain melt temperature is surpassed and then decreases. This thermal degradation effect can be attributed to the polybutadiene component, which acts as an initiation site for oxidative degradation of the matrices. 

With these considerations and with experience demonstrating that there is no standard design for the injection molding process, one is led to the conclusion that each process must be uniquely optimized. 

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