We are molding an ABS flange and installing a stamped SS ring which is held on by two "ears" on either side of the flange.  We have had a recent problem with the ears fracturing and want to eliminate the SS ring stamping oil as a suspect.  
We have been unable to recreate the fracture problem by adjusting machine parameters, so we're looking for other ideas.

Any help or suggestions would be greatly appreciated!

ABS has reasonable chemical resistance but I have seen it suffer from environmental stress cracking when under stress and exposed to oil residues on touching metal parts. One easy thing to try is to replace the ABS with ASA. It will process in the same mold. Some grades of ASA (those with highest acrylonitrile content) have chemical resistance that is better than ABS. ASA also will not go yellow over time or degrade (even years in the sun). ABS on the other hand will lose properties quite fast especially in the sun or when warm. Of course ASA is more expensive than ABS but you are paying for a higher quality product. One product I can think of is Luran S (www.lurans.com). There are other ASA on the market but I can't remember the names.


See www.rtpcompany.com for a chart comparing chemical resistance for many thermoplastics.

My experience with ASA is that it is better than ABS to UV, but more susceptible to environmental stress crack.

Regards

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As with any product there are differences between manufacturer so ABS and ASA each describe a range of materials with different properties. Let me put it this way, from real results I have seen I would say that the best ASA is better than the best ABS in environmental stress cracking resistance. The reason is that the best ASA has a higher acrylonitrile content and also the rubber is different than ABS and that helps. Basically though it's worth a try because ASA will work in the same mold and meet all the mechanical requirements of the ABS.

Thank you for the suggestion to try ASA. However, because of plumbing codes (this is a closet flange) we must stay with ABS. There are no UV issues - only stress cracks. How much oil does it take to cause a problem? Or could the box be packed too tight, causing stress at the ear which could be aggrivated by any oil present?

Hello,

ASA is almost exactly like ABS chemically, are you sure you cannot use it? ABS is SAN (polystyrene-co-acrylonitile) plus butadiene rubber particles to give impact resistance. ASA is the same polymer but with polybutylacrylate rubber particles. That means they are really close in performance except for the better rubber and some ASA grades have high acrylonitrile content for better chemical resistance.

To answer your question, it only take a minute trace of oil to give the problem. It will be worse the more concentrated the stress is. Try to get an ABS with as high an acrylonitrile content as you can, that has a big effect on the chemical resistance of the ABS. Cheaper ABS has about 19% AN whereas 25% is common and you can get 30% I believe. Suppliers such as GE Plastics and BASF can advise.