THERMOPLASTIC COMPONENT PART CRACK 5

[skengg]

hello to all,
i am attaching image below which showing thermoplastic transparent cap part thread jointed with a stainless steel body, product name is pressure reducing valve.
so in the image there is visible crack marked with blue.
this crack appear very later after assembling with stainless steel body. like, after few hours of assembly the crack starts appearing and then slowly propagate through the part at various location in the part.
we are using rubber O-ring between joint of thermoplastic part and stainless steel part.
so my question is what are the possible reasons behind this and any solution?
i haven't much expertise in plastic industry, so your opinion would be very helpful.
THERMOPLASTIC COMPONENT EXACT MATERIAL GRADE IS POLYCARBONATE 122.
Thanks.

 

[Brian Malone]

Your polycarbonate part is most likely suffering from solvent sensitivity cracking/crazing from a plasticizer in the o-ring or lubricant on the o-ring. You can confirm this by assembling without the o-ring or with a substitute such as fiber cord stock of the same/similar diameter and relative density. Mainly, just take the elastomer out of the system and observe if the cracking/crazing occurs. If the o-ring is confirmed as the initiator of the cracking then you have to consider changing seal compound to get the reactive chemical out of the environment. Review plastic guidance for polycarbonate, such as:

https://acrobat.adobe.com/id/urn:aaid:sc:VA6C2:088bde83-5f7b-40df-85f0-595f7de828eb

https://www.sciencedirect.com/science/article/abs/pii/S014139100300199X

If you continue to use polycarbonate for the bowl (polycarbonate has many pluses for use as a bowl: clarity, impact resistance, etc.), you probably should consider placing a large and prominent warning label on the product to give guidance for a user to avoid environments that have incompatible chemistry, and avoid cleaning the bowl with incompatible fluids.

If the cracking is not from solvent sensitivity then your mold gating/venting or mold process may be inducing residual stress concentrations that are increased by the additive stress of the assembly operation (threads). Also ensure your molding process is properly drying the resin per the manufacturer's guidance, etc.

 

[Compositepro]

Constant stress above a certain level will alone cause cracking. I've seen lock washers between a screw head and washer cause radial cracks around screw holes in polycarbonate. The 0-ring acts as a spring.
From the shape of the crack, I bet the the plastic flange is not perfectly square with the metal part and that is causing one side of the flange to see much more stress at this location after assembly.

 

[IRstuff]

There's something that seems to like to craze polycarbonate; my wife has a small PC jar that she put some lotion into and it resulted in multiple cracks.

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[skengg]

Brian Malone, thanks for responding in detail form,
actually that plastic part we do outsource, i have talked to the manufacturer regarding this issue, they suggested to assemble it with a lubricant at the joint along with O-ring, we are working on it and also along with it, i will do one more trial of assembling without the O-ring as u suggested.
let's see if any of it work.

 

[skengg]

Compositepro , thanks for responding,
i get what are u saying,
the line u said "I bet the the plastic flange is not perfectly square with the metal part and that is causing one side of the flange to see much more stress at this location after assembly."
what's this exact interpret? is that we aren't tightening it uniformly, which is generating non-uniform stress at joint?

 

[skengg]

yea IRstuff,
we are looking into it, the reasons behind crazing of polycarbonate.

 

[TugboatEng]

PC will crack at constant stress levels. Polycarbonate will crack if exposed to methacrylate ester adhesives. Polycarbonate is highly sensitized by temperature. PC is a very challenging material to use correctly.

 

[skengg]

TugboatEng, the product we are using is pressure reducing valve for plumbing industry,
can u suggest any other material equivalent to this or superior in terms of fluid sensitivity handling?

 

[Pud]

Have a look at transparent amorphous Nylon 12. It's used for bowls for air line oil seperators etc.
Doesn't suffer from ESC. Excellent chemical resistance.

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[Compositepro]

The threads in the casting may not have been machined to be exactly square to the O-ring groove or flats. There could also be dirt or a shaving under the O-ring. Check that gap is uniform as you screw in the plastic when it firsts contacts the O-ring, before tightening any further.

 

[skengg]

thanks Pud, for recommending it,
i will study on this.

 

[skengg]

Thanks compositepro,
yea it could be one of the reasons, i will talk to production department about this and observe in detail to see what's really going there.