O-ring not sealing after temperature cycle testing

[etang858]

Hello,

I’m having an issue with sealing plastic parts after temperature cycling.

I have two handle halves which come together using an “18-8 Stainless Steel Torx Thread-Forming Screw for Plastic” (McMaster P/N 96001A164). The bottom half has a through-hole boss, and the top half boss is screwed into. An o-ring sits between the two halves to seal the assembly.

Handle Material: Wellman 42L-XE-N (

http://www.wellmaner.com/42l-xe-n.html)

O-ring material: black silicone rubber, Shore 40A
Bottom Boss OD: 0.080”
Bottom Boss: 0.096”
Top Boss OD: 0.163”
Top Boss ID: 0.070”
Torque: 2.5 in-lbs

At room temperature the handle is fully sealed and no water ingress occurs when submerged under 1m for 30mins. After temperature cycling, the handle leaks. The temperature profile goes from -45⁰C to 85⁰C at 2⁰C/min and, when reached, holds at -40⁰C and 85⁰C for 1 hour each. This cycle runs for 4 days straight. The handle is then allowed to settle at room temperature and re-tested for leaking.

The o-ring is designed to have 3-4% stretch in the ID and 16-34% vertical compression. If I take the post-cycled units, torque the screws back to 2.5 in-lbs, the leaking stops. Compression is getting lost due to thermal cycling.

Attached image shows cross section of handle.

Any thoughts on how to keep the handle sealed after temperature cycling?

Thanks

 

[Compositepro]

All of the seal compression is provided by two screws. The heat deflection temperature of your nylon is 55C and you are cycling to 85C. It is hardly surprising you are loosing your clamping force. The plastic around the screw threads is creeping. Compression set of your gasket is another factor you should consider. It really is a gasket and not an o-ring (the sealing principles are different). I would also be concerned about the parts warping due to stress relaxation at 85C.

 

[btrueblood]

What cpro said (I was surprised that the heat deflection temp. for the nylon you chose for the handle is only 55 C). You could try a different material, or a filled version of the one you have, to push the heat deflection temperature up well above your upper cycle temperature. And choose a less creepy material than silicone for the gasket (nitrile rubber would be a likely choice, in a Shore 70 durometer).

 

[etang858]

Thanks guys. This was a project I got thrown into. Parts were basically made and shown to fail, I was then tasked with fixing the problem.

Both your inputs were extremely helpful and gives me a good starting point. Much appreciated.

 

[etang858]

Hi all,

I did do a test using another material with a HDT of 200C. Spec. sheet attached.

The same characteristics were shown:
Pre-temp. cycle- no leak
Post-temp. cycle- leak
Post-temp cycle tightening- no leak

With this material there should be minimal creep around the screws at 85C, correct?

Seems like I need to focus on a new gasket material as btrueblood suggested.

 

[btrueblood]

Etang,

Do you see any warping/deformation of the parts across the thermal cycling?

It's not really clear from the picture you posted, but is the seal around the screw fully encapsulated, like the o-ring, or is the inside edge of the ears allowed to "pooch" into the internal cavity? If it were possible, having seperate o-rings, one for case in its existing grooves, and seperate small o-rings in their own grooves around the screws (or a captive seal on the head of the screw, see the trademarked "seelscrew" design) would be preferrable, since each seal would have its own engineered capture groove.

 

[btrueblood]

Sorry, one last thing. Are you using a spring washer or similar compliant device under the screw heads? The differential expansion from steel to plastic is fairly high, and you may be seeing yielding of the plastic parts near the screw bosses.

 

[etang858]

Hi b,

I had a bezel (flat piece with a hole cut out form the center and 4 smaller screw holes near each corner) made from the nylon which shows clear warping. I made the bezel out of the higher HDT material and did not see any clear warping.

For the handle parts I did not see any clear warping when made from either material. That's not to say it did not happen though. I'll need to look into this more.

The seal around the screws are merely the ears shown on the gasket. The handle halves seal via the gasket and water ingress is stopped from the screw bosses via the gasket "ears". The ears are not encapsulated at all. They merely compress between the two bosses where the screw is. So the ears do "pooch" into the internal cavity.

There is no spring washer or anything else to help retain compression around the screw head.

At this point I think i'll keep with the higher HDT material. Order new o-ring material and spec some spring washers. I'll also look into those seelscrews.

Thanks for all your help thus far!

 

[Tmoose]

How many turns does it take to tighten a heat cycled assembly?

Are gaps visible between the flange faces after heat cycling, and before re-tightening?

Did you mention if the "o-ring" width dimension changes after heat cycling ?

 

[etang858]

Tmoose,

I didn't check how many turns exactly to re-tighten after heat cycling. I took my torque driver and re-tightened it to the 2.5 in-lbs I originally tightened it with. I have a few samples I didn't tighten yet, I'll check on that tomorrow.

Everything is black on black so its hard to see gaps. I'll do more up close inspection.

"O-ring" width- that is a good point. I'll have to re-measure and see tomorrow as well. I didn't take a before measurement though but it should be within spec if no dimensional changes occurred.

Thanks for the insight.

 

[etang858]

Just wanted to give you guys a quick update. I found out I was given incorrect information regarding the original handle material. The HDT is actually 195C. Well above the 85C being tested.

I've tested two materials, MRGF30/10 42H-NBK1 and PBT 3030-104.

Looks like the handle materials are OK after all. Time to look into other factors.

Again, thanks for all the support.

 

[dgallup]

Do you have a spec on your "black silicone rubber, Shore 40A"? It may be taking a compression set. However, I think you are getting creep, probably around the screw threads since you can re-torque them and it seals. You may need a bigger diameter screw or put thread inserts into the top half.

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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.

 

[etang858]

The silicone we are using is rbb-2881-40(40A). No compression set data available according to data sheet

I did measure the gasket best I could pre- and post- temperature cycling and the measurements were pretty spot on, +/-.001" in vertical thickness. It doesn't seem like I'm getting compression set.

Tmoose,
After temperature cycling I cannot see any gaps between the plastic. It is mainly because everything is black. The handle top is black, oring/gasket is black, and handle bottom is black. They are just different colors in the images.

I temperature cycled the assembly and took before and after measurements. The lip on the handle bottom which serve as the stop for the handle top and determines the compression of the gasket did not change. Measurements did change radially though. Although the vertical measurements did not change, I am certain the material is changing dimensions...just not in the obvious direction which would reduce compression. Although since overall radial dimensions are changing. the same effects could be happening within the screw boss, thereby allowing the screw to loosen.

Thoughts?

Thanks

 

[Compositepro]

If you can always get your seal back by re-tightening the screws, it is clearly the screws that are losing their clamping force.