First of all, I agree with Patprimmer that testing and tweaking may be required until parts are ejected without damage. However, you want to be proactive and minimize the tweaking work by properly designing the part and the tool.
The goal is to design a part and its tool with low frictional forces between the tool and the part’s dragging surfaces.
The part:
-surfaces that present resistance on part ejection must have a generous draft and radii must be added to all sharp edges. If possible, the dragging surfaces must be smooth.
The Mould:
-surfaces that present resistance on part ejection must be well polished.
-Mould design must help on processing the part with low pack pressure, and must be well vented.
-Put ejector pins near all part features in the core side.
- Vacuum may also be an issue. Some mechanisms to break the vacuum can be implemented in the tool.
The formula.
In the book "Injection molding handbook / Dominick V. Rosato, Donald V. Rosato, Marlene G. Rosato. - 3rd ed. ISBN 0-7923-8619-1" there is a formula to calculate ejection force. I have never used it, hence never proved its accuracy. I don’t know if you will find it useful and I doubt many people use it, but if you (or somebody) ever do, share your findings in this thread.
P = St•E•A•µ/(d(d/2t -(d/4t)•?))
(See the file attached)
P = ejection force required (lbf or kgf)
E = elastic modulus (lbf/sq in or kgf/sq cm)
A = total area of contact between molding and mold faces in line of draw (sq in. or sq cm)
µ = coefficient of friction between plastic and steel
d = diameter of circle circumference equal to perimeter of molding surrounding male core (in or cm)
t = thickness of molding (in or cm)
? = Poisson’s ratio of the plastic
St = (thermal contraction of plastic across diameter d) = (coefficient of thermal expansion) x (temperature difference between softening point and ejection temperature) x d (in. or cm)
Mauricio Benavides
(Free software)
