Plastic Pressure Components

[TimBow123]

Currently we have plastic injection molded parts that are used in pressure applications. With this being said we qualify the original design / Vendor by pressure testing the component to a 10X safety factor over the working pressure of the part.

Currently our purchasing department is in the midst of sourcing these parts from alternative vendors. How can one ensure that the original material strength is maintained from different vendors other than pressure testing the components all over again?

Also does anyone else use injection molded parts as pressure retaining components? If so how do you handle the large variance in mechnanical strength that can be seen these parts?

 

[Demon3]

I'd say you'll have to test the parts, no way around it. There are too many things that can go wrong otherwise. Factors, each of which can make the part fail include:

Change of resin (new supplier tries to cut costs)
New mold, mold lines (weak points) change position
Change of mold temperature


Chris DeArmitt

 

[Pud]

I am surprised you quote a "large variance" in mechanical strength. Usually once the design is fixed and hence tooled, the moulding process is very consistent. Material (assuming it's specified by Manufacturer and grade ref) is extremely consistent - again assuming it's not some reprocessed "ground up carpets" stuff!

What is the material and what is the variance? Material type may be playing a role here due to inconsistent processing and/or the old chestnut of material pre and post-processing.

For a WAG it's nylon?

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

Are you suggesting the nylon has different moisture uptake at time of testing.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[patprimmer]

Plastics are very commonly used in pressure components from plumbing fittings to bicycle pumps.

PBT is also very susceptible to phantom impact failures which I have attributed to building up due to post moulding differential shrinkage.

You need to tell us a bit more so we an stop making speculative assumptions.

If all things are controlled well you should get results within about 10% variation in most properties.

You need to test every so often no matter who moulds for you.

You need a moulder and some raw material suppliers you CAN trust.

You need to understand what can cause what variables and which are critical for your application.

Are you retaining liquid or gas pressure as it makes a big difference o the safety consequence of a failure

I think you probably need your own test rig It does not need to give accurate quantifiable results, but it does need to give consistent results so you can identify variations from known good results..

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers