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Mold design for plastic gears

magneplanar

New member
Fellow Pro E users,

anyone built a mold for plastic gears? I am trying to explain to a coworker that the tooth profile may have a different shrinkage rate due to its concentration of material. That applying the nominal material shrinkage rate across the entire part may not be producing the tooth profile as intended. I read something where they say you should use the standard shrinkage as depicted in the material spec and then correct the cavity after measurement of the off tool part.

just wondered what experience there is...

we are working with small gears, less than 50mm diameter, with a gear and pinion on the same part. We also want to add in some phase shifting to compensate for the involute undercut on the pinions.

thanks...

M
 
This may not be helpful but your manufacturer should design their mold to compensate for plastic shrinkage.
 
fiebigc,

thanks for the comment...we are the manufacturer. The plastic shrinkage is based on the material specification. However, standard shrinkage rates do not apply in the same way when the tooth goes from a thick base to a thin tip.

cheers,

M
 
Hello,

The only thing I know of that is (often?) used instead of measuring the part is to do a filling simulation for the part, also calulating the shrinkage and warpage.
The results will indicate which areas need to be thickened compared to the nominal geometry, maybe you can even determine how much. I'm not sure how accurate the simulations have become nowadays.

Calculating/determining compensation for shrinkage and warpage is very complicated, because -anything- you change in the process will change this behaviour, like e.g. melt temperature, mold temperature, injection speed and pressure and 'after presure' time and height, material properties, and so on. (Sometimes even the cooling channels are placed specifically to influence this behaviour.)

To some extent the shrinkage and warpage can be optimized by optimizing these parameters, but at some point the only way to further improve the part is to compensate the tool itself for shrinkage and warpage.


Edited by: Zestje
 
i have such experience.

i had designed and developed one gear out of acetal co-polymer. normal shrinkage for POM is 1.75 - 2.25.
we applied 2% all other dimensions were correct except the diameter of the gear teeth.

we revers worked the shrinkage achieved and recut the profile, the shrinkage on the gear profile diameter was 2.9%. then we got it approved.
 

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