I would like someones opinion as to the proper polish for running polypropylene material. We have a mold with lots of thin ribs and have used 320 grit paper to finish these. They are draw polished but the parts tend to stick. We can run polyethylene in this same mold with no problem.

Engis Corporation can provide you with diamond polishing compound specifically designed for mold polishing. Contact Engis at 1-800-323-4069. The Product name is DiaMold diamond compound.

As polishing these areas will take a considerable amount of time to polish, I would like to suggest a mold release such as Slide Products first. The folks there can give you advice as to the product available for this problem. I would try this option first. http://www.slideproducts.com

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Replying to:

I would like someones opinion as to the proper polish for running polypropylene material. We have a mold with lots of thin ribs and have used 320 grit paper to finish these. They are draw polished but the parts tend to stick. We can run polyethylene in this same mold with no problem.

There are always suggestions for buying the next miracle cure.

The real issue is you need to understand your part geometry and the amount of draft to release the part and your injection machine processing parameters.

In case of doubt there is nothing that replaces the labor time of a proper polishing not even sometimes ejectors because these can sometime do more damage than expected and if you're willing to do ejectors then why not just polish.

So I suggest that you go all the way to a 600 paper wet and in the direction of the pull and you'll most likely see you parts come out with consistancy.

I your parts hang in even after this then you should look at part design and where you can put undercuts to pull out the part or modify tooling so you can have air or mechanical ejection.

After you have a good draw polish ont he ribs,
Use a vey fine glass bead ( ie. sand blast ) this
will beark the vacume that is common with this material.

Actually with some PP material, it is better not to have a polished surface. The rougher surface will allow the material to break away by breaking the vacuum. This was told to me by someone who used to work in the contact lens industry and found the high polish to be the cause of his parts sticking.

I am not sure if it will work for your material grade and part geometry, but sometimes you have to step backward to move forward. Good luck.

Dave

I used to mold ATV fenders out of Polypo (polywarp) and we all know PP sucks so you have to keep the finish rough. This will break the vacuum