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Molding problem in EPDM part

Molding problem in EPDM part

Molding problem in EPDM part

Hello All.

My company is having an Diaphragm Compression Molded in EPDM. The part is made in Taiwan. Material is a Nordel 4570 base. Diaphragm is 60mm diameter. The Outer bead is round in section.
The diaphragm acts as a dump valve. It operates 50 cycles per year with a life of 10 years.

I have received the first samples off the tool and they all show a slight discoloration. The discoloration occurs in a 3 armed pattern ( think peace sign or mercedes symbol) with the 3 arms starting in the middle of the part and emanating outwards, not quite reaching the outer diameter. Each 'arm' is between 5-8mm wide. The arms are not straight but look like flow patterns. The orientation of the 3 arms is always different so its not something in the mould. The Discoloration occurs right through the part onto the opposite side so not just a surface imperfection. The color change is very subtle and only slightly lighter than the base part.

The supplier is of course telling me not to worry as 'it will not effect the mechanical properties'
They are telling me the discoloration is caused by contact with the hot tool surface.

Can anyone help me here?
What is causing the discoloration?
What effect will it have on the part?

Thanks for your help,


RE: Molding problem in EPDM part

I've seen this sort of "defect" many times, although usually they've been injection moulded parts. The flow marks might be the result of using a slug of rubber compound (rather than a sheet) to fill the mould.

I doubt it will cause service problems but the only way to find out for definite is to try it out on a test rig.

RE: Molding problem in EPDM part

Thanks Graham.
I've since heard back from the supplier that the pattern is caused by the slug that they pre-cut and place into the tool. It seems that they use a 3 armed slug rather than a round slug. I am still unclear on why there would be a colour change across the surface and what effect this will have.

I've set up multiple test rigs but it will be some time before we have any meaningful data from them.

thanks Again

RE: Molding problem in EPDM part

I have to admit to never ever hearing of this type of slug shape. Normally slugs are cut out of compound sheet in circles, squares or rectangles depending on the shape of the cavity(ies).

RE: Molding problem in EPDM part


I used to design rubber seals from concept through production including the molds and over the course of 14 years never saw a problem caused by the discoloration you are seeing.

We used a large variety of shapes for the preforms going into the molds. The shapes included the typical round, rings, square, and rectangle along with dumbbells, roundish rectangles, crosses with circles on the end and so forth. The shape is only limited by your imagination and ability to make a die for the extruder. The shape was determined by a trial and error process when debugging a new mold. The shape that causes the fewest defects was chosen for production.

Never dug into the reason for the discoloration as it never caused a problem, but it often takes the form of the preform and is probably a resultant of the compound's components not getting fully and evenly distributed along with demolding spray getting redistributed during the closing of the mold.

I hope this is helpful.

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