View Full Version : Plastic injection mold allowances

01-09-2013, 09:21 PM
I have some down time waiting for the ejectors, to vent the cone mold. The PS didn’t have much shrink and I got the fit of the cones about right. Now the cap for this project will be made out of HDPE (unless there is a better choice) it will slip over the plastic tube with a friction fit. And hold the ignition device in place. So it needs to be somewhat stretchy but strong to. Liger-Zero has found a reference for shrinkage for the HDPE in the .015 to .040” range per linear inch. Will my plastic supplier be able to help me better find a number for their plastic?

Assuming that my plastic shrinks .040” per liner inch. Will I take the circumference or my cap X .040 and add that to the total circumference of my cap. And that will provide me with the right amount of allowance in the mold for shrinkage.

This is a cap I did on my hand operated press. The new cap will be very similar.

01-09-2013, 10:02 PM
If you're cap is 1.000" in diameter you simply multiply your diameter by 1.040".