[mechdesignplast]

Can anyone provide me the formula or assistance in calculating torque(in/lb) required to start initail rotation of a cylindrical steel core in a plastic injected mold for part ejection. The core is a threaded core material S7 or H-13. The molded plastic is PE or PP. The Diameter of part is approx 5 inches. Threaded surface width is 1.5 inches. Part thickness is about .200 inches. I want to eject molded part by rotating core in opposite direction of thread to push out part off core/cavity. I am attempting to apply a 1 hydrualic motor with chain drive to demold two parts. If I get any help in this calculation I would appreciate it.

[Scleros]

I know very little about injection molding, but reflecting on the question it would seem to be rather complicated to accurately account for just some of the factors that could influence the torque - part temperature at time of ejection, core surface finish, effects due to presence of mold release, etc. Have you considered obtaining the value empirically by measuring it in a test setup with a torque wrench?

[mechdesignplast]

The part only exist in CAD and one SLA Prototype. To realistically replicate shrink of plastic of molded part would be difficult, aside from manufacturing the threaded core, but that would be option once I exhaust any theoretical attempt before kicking off mold design. I have calculated internally using parameters of coefficient of static friction for plastic (.35 worst case), Shrink percentage of 1% to calculate radial forces acting on core. Came up with theoretical break torque of 300 foot pounds or 3600 inch pounds required. Another co-worker came up with computer simulation for theoretical break torque needed at 8000 inch pounds which seems excessive. Just want a couple other approaches to compare. Thanks