Ejection Force
Ejection Force
(OP)
Does anyone know of tabels or methods to calculate the ejection force required to expell a part during demolding?
I'm concerned a very fragile part may fracture during demolding. But I'm not sure how to predict it short of extensive testing.
I'm concerned a very fragile part may fracture during demolding. But I'm not sure how to predict it short of extensive testing.






RE: Ejection Force
Then modifying the mould and technique until a part can be reliably de moulded without damage.
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RE: Ejection Force
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
www.injecneering.com (Free software)
RE: Ejection Force
For elastomers you do not polish to a shiny finish, but deliberately leave a mat finish to prevent suction cap effect.
Regards
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RE: Ejection Force
-Eric
RE: Ejection Force
Regards
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Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.