Method for improving knit line strength in polymeric materials

Abstract

The knit line strength and other packing related properties both aesthetic and functional of injection molded polymeric products is improved by equipping the mold with packing pins. The packing pins are similar in construction to mold ejector pins and may also be mold ejector pins depending on mold configuration. The packing pins are sequentially actuated during or after filling of the mold to cause flow of plastic either transverse or parallel to the direction of flow in a to and from manner. The result is a kneading action in the viscous flow and an oriented layering effect in the solidified plastic much like a scarf joint at the knit line location. The irregular shape of the layers enhances the mechanical strength at the knit line in addition to increasing the likelihood that long chain molecules will re-entangle between the two melt flows at the knit line. The packing of parts both with and without knit lines can be optimized in localized areas requiring more packing such as thick sections or in areas adjacent to insert molding locator pins which are pulled out of mold cavities during cooling causing voids which must be filled. Packing pins can also be used to cure gateblush by retracting the blush area adjacent the gate during fill and packing it forward into proper part location after the packing is complete. Packing pins can also be used in concert with gates both into and out of a molded part to enhance venting, provide gate shutoff and localize packing at the end of fill in the gate area.

Dependencies are inferred from the claim text.