PVC Issue

[stevenmon]

We have been manufacturing a sample block housing that is machined from PVC for many years (volume = 10K).

All of the sudden we recently have been getting cracks in the PVC when the housing is placed under internal hydrostatic pressure. The pressure is pretty nominal and the walls are plenty thick. We have been doing this test (100%) since the product was introduced.

Our machine shop says that they have not changed their process at all (the part is machined on a CNC). Could the wrong cutting fluids cause this? The cracks are not jagged and are perfectly straight along the part.

Are there certain types of PVC that are more brittle?

 

[patprimmer]

Yes, yes and yes.

Solvents in cutting oil can cause stress cracks in PVC.

There are many grades of PVC and several aspects of the polymer formulation will effect the performance re stress crack. The most likely cause is the "K" factor of the polymer, but it can also be effected by additive packages.

The processing conditions when manufacturing the block can also have an effect.




Regards

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[Demon3]

As usual Pat is exactly right. Also cleaning chemicals are very good at causing environmental stress cracking. So it may be the cutting oils or even whatever they clean the parts with after cutting. If you absolutely can't prevent this problem with PVC then you might find another polymer that also works but doesn't stress crack.

 

[stevenmon]

Thanks for the responses.

What cutting fluids should I advise our machine shop to avoid?

Also does anyone have a link that describes the different types of PVC?

We have not seen this problem on the first 10K of the products we assembled and tested so I am comforatable with the material choice.

 

[Demon3]

There is no way to predict which liquid types will give environmental stress cracking. That means I can't say what types of fluid to avoid. There are standard tests you can do though. One is that you make normal test bars (ASTM dogbones) and bend them in a metal rig to a specified radius (to stress them). Then you wipe the test fluid on them and see whether they crack over a matter of minutes, hours or days. This could be done to test cutting fluids for suitability.

 

[jdarco]

Have you changed raw material suppliers (for the PVC bar or sheet stock)recently? Sometimes different manufacturers have different stresses in their finished products.

Many people machine parts from PVC. I don't know that you need to do all the testing of different cutting oils. I would ask your material supplier. They may have some ideas for you.

Also, make sure somebody hasn't tried to speed up the machining time. I've seen some problems with cutting too fast.

 

[ZiggyK]

I would have the pvc tested as good verses "bad" and then make a determination as to the grade or quality of pvc needed. Rheology testing or even thermal analysis will predict the difference.