Problem with leakage of welded Nylon components

[PLCRookie]

I am currently trying to establish the root cause of some leakage problems that we are experiencing with welded plastic components. I have been working on this part for the last year, and am really at a stand still now. The part is a small canister (approx. 3" diameter)that must not leak after welding, and be able to withstand pressure up to 300 psi. When I initially was assigned to this project, the burst pressure was the major defect that we saw. I had wild variation in the burst pressure and leakage was manageable (1% max). All the parts are 100%leak tested (bubble emission @ 80 psi), so we were o.k. there. I have changed the material to a prime Dupont Zytel 70G33LBK031, to try to correct the burst strength issue, and have had excellent results. Burst pressures since the change have not fallen below 450 psi, and are much, much more consistent. Part appearance is greatly improved, but I am now seeing huge leakage rates at times (up to 60%). It seems to be lot specific, as one lot that we just finished was .25% failure due to leak, and the previous was 45%. I have made multiple visits to the molder, but still cannot find the root cause of the problem. Now for the question....I would like to have some recommendations for testing (laboratory)that could give me some better information. We have tried some labs in the past, but we really didn't get anything we could use (other than "you need to do more testing...that will be $$$$$"). The parts are ultrasonically welded, and I am quite confident it is a material issue. What specific test could be performed on these parts?, and who can do it?
Any ideas or reccomendations would be greatly appreciated..

 

[unclesyd]

You need to carry Demon3's advice back through to your supplier and molder.

Even though operators are not supposed to use any silicone around the die heads it can and will be used if the operator has any problems.

The mention of furniture polish brings up an incident where car polish was the culprit at a local molding company had when parts went for further processing

 

[PLCRookie]

NO chemicals, cleaners, etc. at the weld operation. I am sure it is possible at the molder, although they are firm in their insistence that this is not possible.

Thanks again for the advice.

 

[patprimmer]

As Chris says many lubricants or mould release type sprays can cause the poor weld problem and may very well come from obscure sources. It may even be a mould release agent built into the compound.

Due to the new information that the leak occurs at a bubble or void in the weld, I am more inclined to think it is moisture in the nylon when welded or a lack of resin at the weld joint due to higher glass fibre levels. Tensile strength significantly improving while weld strength significantly drops fits well with more glass and/or higher molecular weight (tougher but poorer low) base resin.

I have seen so called vibration welding do better than so called ultrasonic welding in 35% G 6.6 (the combined water tank and chassis for a steam iron).

They are both friction via vibration at the surface to be welded type welds, but the frequency and amplitude are different.

Regards
Pat
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