July 14, 2022

Estimated ejection force formula

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From the book “Injection Mold Design Engineering – David O. Kazmer”, Estimated ejection force is calculated by using formula:

Feject = µfric . cos (draft) . E . Ctermoexpand . (Tsolid – Tejection) . Aefect .

The same could be used for slide calculation force.
From this you can see that big contribution is delta T or cooling time. If you apply this to a basic ABS plastic with 0.45 coefficient of friction µ and E=2.3GPa, Cterm.exp=8.8E-5 m/m.°C and 1° draft then David Young roughly calculation – about 2.5kg per 1 square centimeter is about the quick answer. Vacuum is one big factor that is depending on surface finishing and polishing of the tool. One big contributor is also the holding pressure. Poission’s Ratio on you equation didn’t affect the force calculation because of the small ratio between the thickness of the wall.

I have just used a rule of thumb of 3Kg per square centimeter. OK probably overkill but have never had to use very large cylinders certainly no more that 50mm. To avoid a large overhang with a long stroke cylinder use 2 cylinders one each side of the mould. The design I prefer is again Philip’s suggestion. If you need to be economic just 2 cavity plates with a chain to pull the cross head come stripper plate which lifts the part out of the core. The core can be supported at both ends by the half cavity shutting of round the core so the part travels the length of the shut off before starting to strip. As the cavity shuts off over the core at the other end it is centralized and held central very strongly and you do not need heel blocks as no plastic acting on the end to push the core back. The force acting to push the core out is only due to the draft angle so the power of the hydraulic cylinders should be enough.

That may be a little beyond my expertise, but I think you calculated about 800 lbs. But you have to consider the surface finish, any slip coating you may add to the pins and draft—— if you have ANY draft, the force will only be a peak force at the instant of the core break away. Typically if we had a 4 cavity mold- we put one cylinder on center. This would be for the larger size nozzle. If we made 8 cavities we’d go with 2 cylinders out board near the guide pins. I can’t ever remember building any of these with larger than 2.5″- 3″ dia. cylinders. You might find some of these products at builders supply stores that sell building adhesives and epoxies for setting anchor bolts in concrete.

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