Diaphram post molding distortion 2

[Gins]

We are trying to mold a flexible diaphram approx 4 inches in dia with a thin wall 1/2 inch in from the edge. The thin area (ring) is about 1/4 inch wide. The center is .080 thick and the gate is in the center.

We have tried engage and several other flexible materials but the part is very warpped.

Any suggestions would be appreciated.

Gins

 

[Demon3]

What are the requirements on the use of the part? For example modulus, service temperature, chemical resistance etc.

I not an expert in warpage but how do you get warpage in a symmetrical part? Is the gate symmetrical? Have you tried lower injection speed and longer cooling times in the mold? Have you tried adding filler, e.g. calcium carbonate in order to reduce the shrinkage and warpage?

 

[Gins]

Demon3

The part is symetrical and the gate is in the center the gate dia is about .350 dia at the part.

We have tried engage santoprene etc. The thin ring I mentioned is 0.020 thick and is a semi circle in cross section to provide for a longer stroke. The center section is 0.080 and it has a tubular projection at the center to top and bottom to guide it.

There are no harsh chemicals and it is a low pressure device. Ideally it should have the flexibility of a sheet of rubber 0.020 thick in the thin area.

We have tried long cycle times but it seems to want to revert to a wavy part.

Sorry for the long winded description.

Many thanks

Gins

 

[patprimmer]

The warpage is from different cooling rates in the thin and thick sections.

The slower the part cools, the higher the level of crystallinity in the semi crystalline matrix of the material.

Higher crystalinity means higher shrinkage.

You could try a hotter mould so that the thin parts also crystallise more.

If possible, you might try to heat the part of the mould in contact with the thin part, and cool the thick part, but that will be difficult if not impossible to achieve.

Polypropylene, the matrix material in Santoprene is prone to differential shrinkage due to direction of flow as well as differences in cooling.

Typically elastomer's are also quite sensitive to injection speed and pressure with regards to warpage. The right amount of sheer and pack can help. What is the right amount? I don't know. It seems to be trial and error and feel, otherwise known as a black art. Maybe someone else here has more experience with these specific materials.

Optional materials are EVA, Polyurethane, SEBS or Polyester block co-polymer elastomer's (Hytrel or Arnitel).

Regards

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[Gins]

Is there any chance that the part could be stress relieved after molding perhaps in a fixture??

Gins