Importance

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Abstract

Optimum sprue design requires suitably large cross-sectional area of the open aperture during mold filling and packing yet fast heat removal from the molten sprue during solidification. This is accomplished by a 'hollow' sprue configuration, wherein an enlarged female sprue bushing cavity of large draft angles is axially aligned with a 'spreader die' type of male member having a generally conical-tipped or parabolic-tipped torpedo shape, creating a cross-sectional open aperture consisting of an annular ring along at least partway up from the base to the tip. Both male and female may be fitted with internal mass-transport cooling. Since the cooling time of the sprue is directly proportional to the wall thickness of the molten plastic sprue being formed, and cooling time is further inversely proportional to the plastic/mold surface area, the resulting 'hollow sprue' provides superior cooling without impaired fill and pack function. Further gains are made by varying the relative axial distance of the male and female sprue bushing members within each molding cycle. Before molten plastic injection is started, the male and female members of the sprue bushing assembly are axially coaligned but in a first position of relatively large separation distance. After plastic flow into the disk moldset is done, the male and female members may be urged together to a second position with a smaller separation distance.

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