Mold Design for compression molded EPDM part 2

[jaquez]

I have to design a mold to be used in compression molding an EPDM part. My dilemma right now is figuring out how to adjust the mold cavity and core dimensions to account for both the mold material (Aluminum) CTE and part material (EPDM) shrink rate.

I have read various posts and literature online that say you simply scale your dimensions by 1+S, where S is the shrink rate in decimal format. However, I don't know if this takes into account the thermal expansion of the mold material. Also, can someone nail down for me what the definition of Shrink Rate is, I have seen a couple of different definitions such as :

S=(D_MA-D_PA)/D_MA , where D_MA=Mold dimension at Ambient Temp and D_PA=Part dimension at Ambient Temp.

The other is:

S=(D_PH-D_PA)/D_PH , where D_PH=Part dimension at Highest Temp and D_PA=Part dimension at Ambient Temp.

Any help or advice would be greatly appreaciated, thank you.

 

[Compositepro]

I've dealt with similar issues in the past. The answers to your questions depend on the specific test method used to test shrinkage. Typically, rubber is cured in a test mold that has known dimensions at room temperature. This mold will have a different dimension at cure temperature but this is not measured. A test sample is molded and measured after it cools to room temperature. Shrinkage is the difference in length between the mold cavity at room temp. and the test specimen at room temperature. As a result, you will generally get higher shrinkage with higher cure temperature. But what is actually being measured is the result of CTE of the sample cooling from a higher temperature.

You will also get different numbers based on whether your mold is steel or aluminum.

 

[GrahamBennett]

To make matters of shrinkage even worse is EPDM from one supplier might have different mould shrinkage rates compared with the same grade from another supplier.

 

[jaquez]

Thank you both for your replies. Although, I have Shrink Rate values from the EPDM manufaturer, and they're the only one we'll be using, I guess my best best bet would be to make a test mold using the actual mold material and EPDM we'll be using a measure shrink rate from that, then adjust my dimensions on the actual mold.

Thanks again for your help.

 

[btrueblood]

I'm glad the parts I make molds for don't have very tight tolerances. Then again, I designed the parts so that the tolerances wouldn't be critical. Are you really looking for a few parts in 1000 tolerances?

 

[GrahamBennett]

I used to have a customer (a metal-bashing company) that insisted on rubber parts being moulded to tolerances of +/- 0.0127mm (5/10,000") - about 10 times tighter than M1 tolerances. I lost count of the number of times they complained and we countered by quoting the relevant BS/ISO standard. In the end, they paid us a visit and we showed them all of the research and development work we had carried out to reduce the scrap rate from 90+% to about 25% before the parts even left our premises. The outcome was they changed their drawings .