Optimising moulding conditions to improve the quality of injection moulded parts: A design of experiments approach

The performance of an injection-molded part is the result of a complex interaction of the inherent material properties and selected processing conditions. To increase stiffness and strength and reduce thermal expansion, short fibers are often incorporated in the resin. However, a key aspect of these fiber-reinforced materials is the complicated fiber orientation distribution produced during injection molding. The mixture of glass fibers and molten polymer is injected under high pressure into a mold cavity, which fills in a matter of seconds. The resulting velocity fields, which are generated during this mold-filling process, have a profound influence on the fiber orientation structure and hence on the composite mechanical properties. Typically, a layered structure is found throughout the thickness of the molding, and the orientation of each is highly dependent on the fiber characteristics, the melt flow pattern within the mold and the conditions used in the molding process. The number and depth of each layer has been the topic of a number of studies. In general, most agree that five layers exist, forming a skin-shell-core structure. However, some researchers have discovered the formation of up to nine layers from surface to surface.