O-Ring across a parting line - plastic molded part 1

[KevinZ7]

I have seen a number of plastic molded parts that feature a parting line crossing a radial o-ring groove and that are intended to seal against at least 100's of psi.

Does anyone have any experience specifying plastic molded parts with features like these?

How do you specify the size of the parting line? Ra? Rz? mm?

Likewise what equipment do you use to measure against your specification?

How many rejects have you observed after manufacturing has started? (ie parting line grows larger with time due to tool wear)

Really interested to hear peoples feedback on the success of these features.

 

[Compositepro]

It doesn't seem to be a good practice but I must point out that, contrary to your implication, the higher the pressure, the more likely a good seal will be achieved.

 

[KevinZ7]

I was under the impression that higher pressures could exploit defects in the sealing surface that might not fail under lower pressures (while acknowledging that seals are most likely to fail at pressures under 100psi).

Regardless, in the applications I have observed this questionable practice of parting lines crossing o-ring glands, the application calls for extended durations at <100psi.

 

[Compositepro]

O-rings are pressure energized. Higher pressure will make the o-ring conform better to surface irregularities.

 

[Windguru]

Parting lines are defined by thickness and height.Just cut a section and get it measured by QC.

 

[KevinZ7]

What equipment would you use to measure a cut section?

What equipment would you use to measure a parting lines size in a QA/production environment? (assuming your not going to cut up a sample from each production run?)

 

[dgallup]

While I will go to great lengths to avoid parting lines on molded sealing surfaces, I have a few parts where it was unavoidable. My specification allows 0.002" max mold mismatch and flash combined on the sealing surface. This usually works well as long as you have an o-ring cross section of .070" or more and sufficient compression. This usually requires a special deflashing operation post molding. It can be quite difficult in volume production to successfully deflash and not damage the surface.. A brittle plastic like PPS is easier to deflash than a soft plastic. Another important consideration is what sort of leak rate is allowed. Every o-ring leaks if you have a sensitive enough measurement system.

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[KevinZ7]

Thanks for sharing your experience dgallup and the excellent post!

What kind of instrumentation do you use to measure the 0.002" in QA?

 

[dgallup]

We use an optical comparator at 50x mag.

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[dgallup]

If you can't see the parting line in profile, you could use something like a Perthen profilometer to trace across it. Have to carefully choose your stylus to not scratch the surface.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.