Critical Design Considerations for Sprue Bushings

Last month I discussed how to reduce the massive area where the sprue, runner, and cold well intersect at the parting line. This month let’s take a look at some additional sprue-bushing design considerations to further improve the cycle time and to help avoid several processing problems.

Cooling: The sprue doesn’t need to completely solidify before opening the mold. It only needs to be able to shrink enough to negate the taper lock and have sufficient physical integrity to remain adjoined to the part or runner. But that doesn’t mean cooling the sprue bushing isn’t critical, because it is.

Do you have any molds that eject ice-cold parts, but the sprue is still as soft as butter? For maximum efficiency, parts should be ejected at, or slightly below, the material manufacturer’s recommended ejection temperature. That’s often a lot hotter than many molders eject at, and is an area for enormous savings potential. Putting a molded part against your forearm, or counting how many seconds you can hold it before it burns your fingers, is not the most accurate method of measuring the temperature of a molded part, which is one requirement for establishing the best cycle time. Click here to read more about plastic mold sprue bushings