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Rulon not working...

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tracetrimble

tracetrimble

Mechanical
May 4, 2004
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I'm looking for an FDA-approved bearing material with high abrasion resistance. This application is for the food industry, similar to a packing seal, subjected to low pressure (<5 psi) water-based product, up to 220ºF. I have tested virgin teflon and two grades of FDA reinforced PTFE, including Rulon-641 ($$$!). These materials are wearing too quickly, and surprisingly, the Rulon really didn't perform any better than virgin.

I beieve the PV is within limits, but is also difficult to control because the loader material is 40 duro flat silicone (non-linear modulus, and somewhat difficult to control size tolerances).

I am considering a new FDA-approved Nylatron material. I am aware of the water-absorption characteristics of nylons, but I believe I can live with the swelling in this case, if the wear rate is reduced. The other problem with harder materials like nylon is that the current design uses the PTFE as a gasket as well, so a harder material will need secondary static seals.

For an idea of similar designs, see and
TIA! Trace
 
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Here is a seal that got us out of trouble on one ocassion. They were very nice to work with for only a couple of seals.
It is the metal faced Duo-Cone. If you can work this in It may do the job.


We used several of the Mecco Seals (sugar seals).

We also use another type that was designed for bottom entering agitators in the food industry. All I can remember that is that it was made in Wisc.
 
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Have you considered UHMPE? It has superior abrasion resistance to PTFE, is available in food grade and would be reasonably pliable at 120F so should make a good gasket as long as the mechanical forces are low.
 
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How good is your mating surface with
the plastic seals? Is it wearing away
because of having too rough a surface?
What kind of material is the seal rubbing
against?
 
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The mating suraface is 303 ss. I have tried several different finishes:
- ~32 RMS raw machined
- 600 grit sandpaper in lathe
- 180, 320, and 600 grit cross hatched
- tumbled
The 180 x-hatch and tumbled surfaces performed best with a cheaper grade reinforced PTFE. Due to time constraints, those are the only two I've tried so far with the Rulon.

The turned finishes seemed to cut into the material pretty quickly, leading me to believe the tangential grooves were a problem. The randomly sanded surfaces generate more heat, but wear more slowly.
 
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Thanks for the input. It looks
like you have already checked out
my concerns. The only other thing
I would try would be to shot peen
the surfaces to see if that would
make any difference. A 32 rms is
too rough for certain. I remember
a 16 rms being the max for similar
applications. No mention of
size in any of your posts as to
diameters, width, whether the seals
are molded, or contained within a
groove. Keep us posted with
your development!
 
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The contact surface is an annular ring, 4.93" OD x 4.19" ID. Have not heard of TFM, is it FDA approved? Got a link? Thanks again for the replies.
 
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There is no reliable P/V for Teflon, especially at higher temperatures. You can boil nylon to impart water into it, but it will loose the water if not submerged. Acetal is another material to check, it is tougher than UHMW. All these materials will expand as they get hot, be sure the Teflon, or any polymer is not expanding and creating a very high P/V. Been there and paid for that, expansion differences between aluminum and Acetal between 30°F and 150°F wasted months of testing and required a material and design change.
 
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Turcite A from Busak + Shamban may be good material experiment. It's Acetal and PTFE, similar to Delrin 500AF, but the PTFE is powderized, not whiskers. I've found that Turcite offers substantially longer wear when compared to AF. The company states it is due to the inclustion of a colorant (Turcite A is turcoise). Turcite A has a lower impact strength compared to AF, but this rubbing application seems suited for it. Upper temperature limit of 220F is problematic. Have you tried Fortron (PPS)?
 
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I'd suggest the metal part be lapped to make it as flat as possible and very smooth; 600 grit or finer abrasive. Lap the sealing face of the plastic, too.

If you have any control over the force between the metal and the plastic, I'd suggest PTFE, virgin (or carbon- filled if available in FDA grade), with a pressure on the sealing face of ~80 psi. Also, if possible, taper the edges of the plastic's sealing face so the contact area increases as the plastic wears.

-Mike-


Mike Halloran
 
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Of the different lapped finishes I've tried, the higher grits generated more heat and sometimes stiction, but the finest I went was 320 grit. I will try 600 and higher.

Rulon-641 is a high-end FDA-approved reinforced PTFE. The fill media is proprietary, but I would assume not carbon because it is white. I have also had Flourosint 207 recommended (mica filled), which I may try, but its stated properties are no better than the Rulon.

I am using Rulon in 1/8" thick sheet; I would think that 80 psi would be enough pressure that any warpage of the sheet would be unimportant, as long as the thickness is consistent. This is a good thing, because the sheet is shipped to our waterjet cutting contractor in a roll, so the parts are curled a bit when I get them.

At 45 rpm, I am calculating 58.08 fpm on the 4.93 OD. With a stated dry PV of 10,000, I have a pretty large tolerance on the face pressure (up to 172 psi) before I approach the limit. Thoughts?

As a side note, our contractor did not do a great job nesting the parts on the sheet, so I have a good bit of scrap. Anyone need some 1/8" thick Rulon? :)

Thanks for all the feedback, this is a great forum. ~Trace
 
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Most of my experience is with very low surface speeds and very low acceptable leak rates, so your mileage may vary.

It appears that your surface speed is fixed. Maybe you can use the scrap to set up a test rig where you can vary the face pressure and material and finish pairings, and measure the leak rate. Or maybe you can divert a few production parts for testing. You'll get answers faster if you can run several test conditions in parallel. With decent sensitivity, you can get a data point in a few hours per test rig.

One sensitive way to measure the leak rate is with a transparent standpipe, of say 1/4" od rigid plastic tubing. Fix it in a vertical position, fill halfway with water, and put a piece of tape on the outside at the meniscus level. Come back later, measure from the new meniscus level to the tape, and you've got a leak rate.

The bottom of the tube is of course connected to a chamber closed by the seal assembly you're testing.

If the static head of the water doesn't provide the pressure you'd like to test the seal with, just pressurize the top of the standpipe with air from a decent pneumatic regulator to any pressure you'd care to use.

For a given static head and material pair, you should find that as you increase the face pressure from zero, the leak rate will go down as the sealing gets better. You will eventually reach a level at which further increases in face pressure don't improve the leak rate, and another level beyond which the wear rate goes up, and maybe the leak rate with it. Your sweet spot of face pressure is somewhere in there.

-Mike-



Mike Halloran
 
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You are being suckered by bogus literature. The max P for Rulon is 1000, and the max V is 400. The V is very low at 1,000 psi, the load is very low a 400 sfm. In my testing PTFE materials P/V should be about 1,000 not 10,000.
 
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