Seal question (Viton oring on metal surface)

[jbguzman]

Hi all, great forum you all have here, I'm hoping to get some advice.

Basic issue is this.

I'm trying to come up with a solution to an ongoing problem with what I'll describe as (oily liquid) from penetrating underneath the seal and damaging the internal electronic components of an assembly.

I believe the major contributor of the the failure is that the metal sealing surface finish is not the correct finish to create a proper seal for the oring. The finish for lack of better terms looks like random micro scratches going in every direction.

The oring is rectangular in shape. I would like to suggest a micro finish that would create a good seal. What type of finish would you suggest and about how many microns/milimeters would you suggest the finish be. The oring size is 3.5mm diameter.

Thanks in advance for any suggestion/advice. Sorry is this seems a bit vague here.

J.G.

 

[flexibox]

3 should do it but can this or better be achieved?. what do you currently have? For 3.5 the groove depth shuld be around 3.1mm from memory.

 

[dgallup]

Most o-ring suppliers suggest a surface roughness Ra 0.8 microns. That is a pretty good number for a random lay. We make a lot of turned parts where the lay is circular and find we can consistently get good sealing with Ra 1.2 microns.

You don't say if it is a static or dynamic seal. That will make a big difference in the recommended squeeze. Parker, Minnesota Rubber & many other suppliers have design guides with everything you need.

 

[DLite30]

First thing I would try myself, would be to use a regular cicrular o-ring, instead of the rectangular one you say you have.

 

[btrueblood]

What specific Viton compound is it, and what temperature does the o-ring have to operate at? Better, what exactly is the oily liquid you are trying to seal against? Reason I ask, some FKM compounds (especially the non-name brand Vitons, but even the original Viton compound) get pretty stiff at cold temperatures, and also take a "set" (google compression set of o-rings) when placed in a groove and squeezed. This will make an o-ring look rectangular when it is removed from the assembly, even if it was round before it went in. The combination of factors means a viton o-ring might leak where a cheaper nitrile o-ring (compounded for low temperatures) might not.

 

[jbguzman]

Thanks all for the prompt response.

I'll try to answer your questions.

Flexibox, the other major problem is that there is currently no groove or channel for the o-ring in the metal flange. The groove is currently on the bottom of the plastic molded heaters. So basically the o-ring lays into the bottom of the plastic molded heaters which then lays on top of bare metal, metal that is not finished correctly. I believe it should be reversed. O-ring grove in the metal flange plate, with no groove on the bottom of the heater assembly.

Dgallup, thanks for the advice on Ra. currently I have no idea what the surface Ra is. I really need to take this part into a good machine shop to get a good gauge on what I'm dealing with. From what I'm used to in my previous line of work (semiconductor) the sealing surface and overall design is terrible.

The seal is static, there is no movement between the parts. Basically it's several small plastic heater assemblies, rectangular in shape that screw down onto the metal flange. We need a good seal to keep out Aliphatic oil fluid that is sprayed down onto the individual heaters.

Dlite, thanks. I agree trying to use a standard circular o-ring versus a custom square gasket/o-ring would be best. However since there is currently no deep o-ring groove in the metal this would be difficult to work with. I think that the groove on the bottom of the plastic molded heaters is not deep enough to work with a standard circular o-ring.

Btrueblood, actually looking back the o-ring is nitrile.

The temperature of the heater itself reaches ~90C / 194F. However the seal does not see that temperature. The heating element is elevated about 1/2" above the o-ring, separated by the plastic heater body.

As for the oily liquid that's used. All I know is that it's an Aliphatic oil, kinda the same consistency and color as extra light olive oil . This oil gives us issues in another application in the use of plastic inline filters with o-ring as it tends to just slowly seep around the seal.


I'd love to show drawing and pics here but the company wouldn't like it much if they found out. Thanks for the advice!!!

 

[keyepitts]

You could also apply a flexible glue like rubber cement to hold the gasket in place during assembly.