Ejection Force

[EricatNordic]

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.

 

[patprimmer]

Testing is the only method I know.

Then modifying the mould and technique until a part can be reliably de moulded without damage.

Regards

eng-tips, by professional engineers for professional engineers
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[mauriciob]

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

http://www.injecneering.com

(Free software)

 

[patprimmer]

When polishing surfaces resisting ejection, ALWAYS polish in line of draw. This in my opinion is the single most important factor.

For elastomers you do not polish to a shiny finish, but deliberately leave a mat finish to prevent suction cap effect.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[EricatNordic]

Great thank you to everyone who responded. Interesting that draft angle is not included in the force equation.

-Eric

 

[patprimmer]

Direction of polish is much more important than draft angle as the part holds onto the microscopic undercuts. Draft only helps after the part has started to move.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.