Best Sealant for Acetal NPT Pipe Threads

Teflon tape. Three wraps, four maximum.

Now if you want a real answer, give us all the operating parameters and details.

Also, is it Delrin^®or acetal(generic): co-polymer or homo?) You didn't capitalize delrin so I can't tell.

Maybe you should weld it.

LynLynch - Thanks for the suggestions. I have tried Teflon tape and due to the PP fitting seam, the tape thins in this area (At least that is what appears to be happening) and leaking occurs. Note these are super slow leaks at ~100PSI..

Temperature range of coolant running through the manifold is 2-30C, Water with Glycol

PSI 80 max – normal operating PSI is 60 – must pass test at 100PSI

Acetal Co Polymer

I am leaning towards NPT thread cutting as the culprit. The problem is I need a stop gap to get them to stop leaking now until a redesign can be completed and proper validation done on the new design and materials.

We have also tried liquid Teflon. What I really want is an epoxy that will bond Acetal and PP. I could bond them and then replace when the new design is ready.

We have also tried 3M DP-100 epoxy but it fails after a few months of service

Any suggestions?

What PP? You didn't say anything about PP. That changes everything. Male or female threads? What else haven't you told us? Have you considered difference in thermal expansion> Stiffness?

If you are talking about the molding "seam" machine it away or buy better fittings.

McMaster carr sells a mil spec teflon tape that is thicker and more durable than regular tape. I used it on cpvc piping on a glycol system, no problems.

You might want to look at loctite's pipe sealants. their 513 is marked for plastics. Also they have epoxy for plastics that work with Delrin

I spent 25 years in aerospace materials work at Motorola in it's heyday. My experience with Loctite and their application engineers was that they were usually over optimistic about the performance of their materials. (They do make some good products, too)

Unless you flame, plasma or acid etch the surface of the acetals, nothing's going to bond them.

Besides, now it appears that we are dealing with Delrin and PP mated threads, which complicates things.

I've used some of their thread sealants on hydraulic applications. Seem to hold up well with no problems of leaks. But most of those applications were with metal fittings. When dealing with Delrin it is very difficult I have found to get anything to bond to it unless there is some surface treatment.

Most these sealant set up hard filling the voids between the threads to seal leakage. There may be enough of a bond to seal the leaks he is experiencing. Cost is minimal compared to other alternatives in treating the Delrin. Worth a try.

If this is a short term solution while a long term engineering fix is implemented, then maybe a "mastic" material would satisfy your needs.

The relatively thin zone between the threaded parts and gummy nature of those materials would mean they don't migrate.

Since you have access to the threads and can identify the mismatch in form as a possible issue, then have you tried to re-cut the thread to give a smooth finish where teflon tape would work?

Finally, if none of these work, an "O" ring mounted under the fitting flange would also provide a seal.

Remember, (If I understand the original request correctly) the objective is a sealant, not necessarily an adhesive. The material only needs to be able to seal the gap without being displaced by the pressure.

Finding an adhesive might actually be the wrong answer, since this would necessitate destructive actions for dismantling.

The following question may be just academic but how were the threads formed? Were they molded or cut? Are the threads to spec? That could have a lot to do with your problems. I am not extremely familiar with the cutting properties of Acetal but with PVC and CPVC you must use a special die vs. the die used for cutting metal pipe because the die for metal doesn't compensate for the compression of the plastic as you cut it resulting on a shallow cut thread.

I learned this the hard way over 30 years ago and have found very few others that are aware of this.

http://www.ridgid.com/Tools/Hand-Threader-Die-Heads

Good Luck, old salt

Thank you Old Salt - Female threads are cut into the Acetal manifold. The fittings are Polyprop MNPT and off the shelf. the way our model shop macinist explained it to me is Acetal has machining properties very close to steel so using a standard tap should be fine.

Thanks again - All the information I can gather is a great help.

Sorry,

But you are doomed. The coefficient of thermal expansion of acetal is twice that of PVC.

Hi Lynlynch - This has been one of my suspicions all along. However, I am getting some push back from the original design team (Imagine that) as they say the general operating temperature is between 20 and 25c and rarely ever ranges outside of those temperatures. When it does it is for a very short time. I have also requested a change to stainless fittings. The problem is the manifold is feeding part(s) that are sensitive to ionization and thus the reluctance to use any form of steel…My contention is SS (300 series) should not react.

I am stuck here between a rock and a hard place…

I'm going to defer to automationgrunt, who has direct experience with these materials.

Good luck, but keep the differences in thermal expansion in mind. This includes the material temperature during machining. The parts should be machined at the same temperature as the service temperature. It looks like they are in this case.

Good luck.

I'm working on an assembly very similar to this. A manifold that is supplied by the customer (appears to be black Delrin) and a mix of fittings that are metal MNPT, metal with o-rings, and PVC MNPT. This is a very busy assembly that we have built several of and the pressures that you have listed are in the same range. As off the wall as this may sound, just keep in mind..........the tests don't lie.

First,. clean all the threads.

Apply Teflon tape to the male threads; stay about 1/8" from the end of the fitting.

Apply Rectorseal pipe thread sealant (www.rectorseal.com) over the Teflon tape; again staying about 1/8" from the end of the fitting.

With each fitting, let the manifold set flat to allow the Rectorseal to form an EVEN BEAD around the base of the fitting as you install it. DON'T WIPE IT OFF!

Experience has proven that you can omit the tape or the Rectorseal and it will seam like you got it. Do this and you can plan on doing it again.

we also do our pressure testing with the completed unit completely submerged in a tub of water and monitored for 1 hour.

I'd like to know if it works for you. I'm not an engineer, but I've spent the last 20 years building machines and automation equipment with some awesome engineers............well.....some of them;^)

Hi automationgrunt,

Welcome to CR4. Thanks for the help.

Hang around, it sounds like you will be able to contribute much to the party.

In my opinion, knowledge trumps education much of the time.

Excellent Suggestions… I have some of the Rectoseal Tru-Blu on order along with some various other sealants. I have also designed a test fixture manifold with 12 - 3/4" ports to test various combinations of fittings and sealants. Once I have some data I will summarize and post it. The difficulty I have found is that while I may be able to stop leaks for a couple weeks, even months, I cannot say for certain these will never leak again.. Also, as with most projects I have worked on there is no time or funding to do extensive testing to gather statistically valid data. This is not a complaint but just a reality...there is no justification to spend more on testing and validation that the project itself......As, Lynlynch suggested, I may be doomed as the CTE of acetal is 2X that of Polypro...

"we never have enough time to do it right but we always have enough time to do it again"

Here's more on the sealant I'm using; Rectoseal 5, slow dry, soft set. Sorry for missing that detail in my first post.

This stuff is pretty amazing http://www.mcmaster.com/#x-pando-joint-compounds/=auz7of

you could chamfer a groove into the end of the female threads & rig up an oring set up...

or install a catch pan & pipe the leakage into the design teams office :D

Thanks - I will look into that compound as well..

I needed a good laugh! That would be a long pipe!

The spelling messed me up here's the companies Website

Try Vibra-tite VTCS6 or Three-Bond TB-1401. Both work well with hard to bond plastic threads.