Mold Concerns

The sprue orifice size, the diameter at the small end, comes standard in odd 1/32s from 5/32 to 11/32 inch.

The volumetric flow rate used during filling largely determines the correct sprue orifice size. Shot size and filling speed, as well as the flow properties of the specific resin, govern the required flow rate.

Large parts and/or parts needing fast filling speeds require large sprue orifice diameters to avoid problems associated with excessive flow shear.
As a general rule, amorphous resins require larger sprues and runners than semicrystalline resins.

The figure below shows typical sprue sizes for Bayer amorphous resins as a function of shot size and filling time. Because the maximum shear rate in a sprue occurs at the orifice and the majority of shear heating and pressure loss takes place in the first two inches, these guidelines should apply to sprues of various lengths.

The diameter at the base of the sprue increases with increasing sprue length. Standard sprue taper, typically one-half inch per foot, leads to large base diameters in long sprues. For example, a 6-inch sprue with a 7/32-inch orifice diameter will have nearly a 0.5-inch diameter at the base. This large base diameter lengthens cooling and cycle times and also leads to regrind problems.

Hot sprue bushings provide one solution to this problem. Hot sprue bushings have a heated flow channel that transports material along its length in molten form, eliminating or shortening the molded cold sprue. Additionally, some molds rely on extension press nozzles that reach deep into the mold to reduce sprue length.

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