Making strong rubber/silicone diaphragms

[Tom Booth]

There have been times in the past that I've wanted to use a diaphragm piston. And since seeing the engines by TK Motors, with internal diaphragm "displacer" (I guess, for want of a better name) running so fantastic, diaphragm displacers now, as well.

My most recent idea for a project being an LTD engine with fairly large diaphragm displacer and stationary regenerator, I've been hunting for suitable material to make a STRONG, lasting, heat resistant diaphragm.

Silicone seems to be one of the best materials, but finding thin sheets of silicone can be difficult.

I found a few sources online that I sent away for, but it seemed awfuly expensive.

So I've also been experimenting with making my own from the relatively cheap tubes of silicone available from auto parts, hardware or dollar stores.

I tried a few different methods, just spreading some silicone caulk out on glass with a spackle knife, squirting the silicone into soapy water and trying to kneed it and flatten it out that way.

The most promising looking method I just came across is this, use some paper, and spacers to get the desired thickness and a couple sheets of glass. Flat metal plates should work just as well. Mash it down, then wash off the paper after it dries.

Getting dry silicone off glass without damage can be almost impossible. Just washing off the paper is probably easier.

Anyway, I'll be trying this above described method using paper next, seen in this video:


https://youtu.be/4-t5pCPfOm0


I don't see balloons and latex gloves as a viable long term solution, though they will work for a while, they often end up tearing or breaking or melting.

Having to wash the paper off the dried silicone is a bit of a nuisance but perhaps using wax paper would work better?

If anyone with more experience at making silicone diaphragms has any tips or advice, that would be appreciated. When I figure out something easy and inexpensive that works reliably, (without streaks, or bubbles, or tears), I'll post it here, and probably make a video.

[Tom Booth]

This video is very interesting for a couple of reasons.

First, it says, and seems to show, that some hydrogen peroxide in hot water helps silicone cure rapidly. Though this guy seems to be unsure how necessary it is, ordinarily silicone can take DAYS to fully cure, maybe longer if very thick, but in this video the silicone cures rather quickly.


https://youtu.be/KlNXHEqKLdg


Second, for a very long time I've known that some Stirling engines used to use a special kind of seal for the piston. A kind of hollow tube, gasket thing. I believe there is actually a name for it, but can't think what at the moment.

It is seen in this "thermal Lag" engine patent here:

https://patents.google.com/patent/US5414997A/en

Number 112 in the patent image:

Screenshot_20211202-231527.jpg (114.2 KiB) Viewed 8976 times

How on earth to make such a piston or displacer seal that can roll in the cylinder, the right shape length and size? Or where to buy such a thing? No idea, for the last ten years...

Now I know!

Make a simple extruder. Take a caulking gun. Tube of silicone. Hot water and peroxide, Voilá!!!

Not sure of the best way to get the ends of such a tubular seal joined together though.

Apparently, according to several comments on Quora, silicone caulk can be used to bond cured silicone to silicone. So, just cut to size and join the ends with the same stuff. I'll have to see if this actually works.

Edit: The patent refers to this seal: " an internally pressurized annular rolling seal 112 having an inflation valve 114"

So, like a bicycle inner tube. Sort of. Except I'm not planning on building an engine with any kind of piston with a diameter as big as a bicycle wheel.

[airpower]

Wenn time comes and this covid junk is done with i will simply use inner tubes from tires. Inner tubes are made with butyl or sometimes latex.
The closer to the limit of the rubber temperature range, the faster the material will degrade. Silicone and butyl have the same maximum elongation 800%, change in length before rupture/failure property. Silicone has much better tensile strength and better tear resistance.
For testing this will work just fine, as i see it.
The lower ranges dont matter for this type of use (in celsius)

[Nobody]

I've seen it called a rolling sock seal.

I would think it needs to be initially a toroid or donut shape, with a circular crossection. Then, when cured, pressed into service into the oval crossection that is commonly shown.

There is another seal I've seen, that is called a sock seal. It is commonly used to provide the expansion seal for air shock absorbers. The best, IMHO, is the one used for Gabriel Air Shocks. It is similar but has an open internal/pressurized end, and longer.

Think of putting a piston in a sock. Take the cuff of the sock and fold it back over itself. Attach the cuff to a thin walled cylinder. Pressure inside the cuff keeps it inflated away from the piston. The sock rolls and unrolls on the piston as it moves in and out of the cylinder. I've never seen one on an actual Stirling. It does require a minimum pressure.

A flat diaphragm could be made by spreading some RTV onto a thin cloth using a putty or drywall, knife. The sock seal could be made the same way using a cloth cylinder. The cloths would strengthen the rubber.

The fatigue life of different compounds and cloths would be interesting to measure.

[Tom Booth]

... i will simply use inner tubes from tires. Inner tubes are made with butyl or sometimes latex.
...

I was able to get unlimited amounts of inner tube from a local tire repair shop. I just asked and the owner pointed me to a huge pile behind the building, eventually to be picked up for recycling. He said to take whatever I needed. Also some nice sheets of rubber roofing material from a construction site. Possibly great for a bigger engine. Right now I'm trying to make a diaphragm for a small model LTD though, so heavy stuff is out.

I did find this video that might be helpful for shaping big heavy diaphragms.


https://youtu.be/ctH0noEYh2A


My first attempt at just spreading out the silicone, more or less worked OK, as far as getting a paper thin layer of silicone, but now that it's cured, it's basically impossible to remove from the glass. I just used strips of tape to gauge the thickness.

IMG_20211203_102037564.jpg (129.74 KiB) Viewed 8957 times

I had imagined that when dry it might just peel off the glass easily, but even a razor blade can hardly get it loose.

[Nobody]

Try a coating of wax on the glass first. Then smear the silicone.

[skypupbob]

The trick will be to get a smooth coating of wax. Try buffing the wax. Warm up the glass and it should lift off easy.

[Tom Booth]

I got some silicone sheet material in the mail yesterday, but at first impression, I'm not really pleased with it. It seems kind of soft and gummy and the surface almost seems to be not fully cured, everything sticks to it; dust, lint, fingerprints. It seems pretty strong, stretchy, but almost too much so. and did tear a little as I was trying to get it out of the plastic wrap with a dull knife. I do like though that it is perfectly clear, but it was also rather expensive, about $20 for 1square foot of the stuff.



Today, though, just a short while ago, I was at the Big Lots store looking for something in the kitchen utensils aisle and happened upon these silicone microwave mats for keeping the microwave oven turntable clean.



They are good and thin, not sticky at all, seem very tough, stretch, but not so easy as to seem almost flabby like the clear stuff and cost just $6 for a 12" disk.

These microwave mats are available online as well. They seem about as perfect as anything could be for this project, though there doesn't seem to be any clear.

I see there are some online that are flat and some with dimples or bumps. I think the flat ones would be preferential, advertised as being able to "cut to fit" or "cut to size".

Anyway at just $6 at the local store I bought a handful.

I'm literally stretching this thing with all my strength, and it springs right back instantly without any permanent deformation

[tibsim]

I’ve tried paper and plastic accordions before, and it worked out very well, for example, and it lasted for several years.:
to make:
https://www.youtube.com/watch?v=b_SdiDrFIwk
to use:
https://www.youtube.com/watch?v=WCApVnpDkNU
https://www.youtube.com/watch?v=f_8GNURkVGY&t=40s
or paper diaphragm for displacer:
(it worked but did not achieve the effectiveness of latex, but see if it could be done better.)
to make:
https://www.youtube.com/watch?v=b6_VuJ4V9wc
to use:
https://www.youtube.com/watch?v=Fx5Cl5QCac0
https://www.youtube.com/watch?v=BRTPXHtjjDM

[airpower]

...........
https://www.youtube.com/watch?v=Fx5Cl5QCac0
https://www.youtube.com/watch?v=BRTPXHtjjDM

Nice, diaphragm is the go, but scalability is though to deal with and the heating cooling battle...

[Bumpkin]

Hey airpower; I’m pretty sure the temperature chart you posted is in Fahrenheit not Celsius. Anyway, I’m happy to see a conversation on home-made diaphragms. I believe diaphragms properly applied can take Stirlings into some useful areas. An issue as Tom mentions; is the material being too flexible where you don’t want it and wasting/absorbing some of the expansion/contraction cycles. I’ve messed around a bit with trying to properly orient fiberglass fibers, (fine ones from insulation,) into a thin RTV band around a metal disc “piston” so as to limit movement to where you want it. So far no joy but I haven’t been taking it seriously enough. Retirement and more spare time looms though, and I might run out of excuses to the point that someday I’ll actually have to build something of practical use. Who knows where the next great inspiration will come from, and thanks to all of those throwing their learned experience/looney ideas into the arena. Bumpkin

[Tom Booth]

I got some silicone sheet material in the mail ..., at first impression, I'm not really pleased with it. It seems kind of soft and gummy and the surface almost seems to be not fully cured, ...

To update, the clear silicone sheets I sent away for, after being removed from the plastic protective film and given a few days to vent, seem to be toughening up considerably and loosing the tackiness.

My guess is, either the demand is very high, or the stuff is produced, more or less on demand, so it was very freshly manufactured.

Very likely it is what I will end up using, because I like that it is perfectly transparent, so in a video, it will be possible to see exactly what is going on inside the engine. (No hidden magnets/motors inside).

The way the blood is going out of my fingers in the photo, is some indication of how hard I'm stretching it.

I'm guessing this is manufactured using a vacuum of some sort, to eliminate air bubbles, which might make it tougher/better than DIY stuff made from caulking, but that remains to be seen.

It still has some of that acrid odor of fresh silicone out of a caulking tube, even after several days of exposure to the air, so, I think it is pretty obviously very freshly made.

I'm going to try another DIY method of rolling out the silicone with a kitchen rolling pin, between sheets of ordinary paper, wax paper and plastic wrap.

I'm not entirely sure, but I think, maybe this stuff was manufactured by some similar method of rolling it out between the sheets of plastic film. Does silicone stick to plastic? Guess I'll find out, But that would explain the initial tackiness, if it had never been exposed to the air before.

IMG_20211212_051845128.jpg (202.22 KiB) Viewed 8821 times

IMG_20211212_051517822.jpg (120.71 KiB) Viewed 8821 times

[Tom Booth]

One more possibility I came across. It seems that thin heat resistant silicone sheets are used for a lot of things in the kitchen, including to line cookie baking sheets.

This video has nothing to do with Stirling engines, or baking cookies, but watching it (while looking for information on plasma torches) made me aware that there are such things as cookie sheet size silicone mats.


https://youtu.be/12nBBE4dEDs


As far as I can gather, at least some or these are about ,030 in (30 thousandths of an inch) which is about the thickness of a matchbook cover.

There are dozens of different types of these silicone baking sheets. Apparently some are woven, some impregnated with fiberglass for extra strength, some with raised edges, thicker, thinner, stronger, softer, more flexible, don't know where to begin, but if at least some are good for gasket material, as shown in the video, there is likely some that would be suitable, if a larger silicone mat is needed for a Stirling.

I will probably send for some at some time, or just keep an eye out, next time I'm in a store that sells kitchenware.

Many of the online sources seem to want more for shipping than for the silicone mat itself. I can't imagine postage for such a thing actually costs $20, so I'll likely check locally in stores before resorting to that.

Screenshot_20220205-174606.jpg (215.71 KiB) Viewed 8577 times

[Goofy]

Hi Tom,
Why not try with exercise band ?
They come in several different thickness and "strongness" and are relative cheap . . .
BR

[Tom Booth]

Hi Tom,
Why not try with exercise band ?
They come in several different thickness and "strongness" and are relative cheap . . .
BR

You mean, like these?

Resize_20220213_062237_7825.jpg (139.59 KiB) Viewed 8535 times

Not a bad idea, and not too expensive for the thin light ones.

https://www.prohealthcareproducts.com/6 ... ance-band/

About $14 for a six yard roll? Fantastic!

Thanks!, I think I'll give it a try. Not particularly intended for very high temperatures afaik but who knows?

Well, it says these ones at least are latex. Latex can withstand up to about 320° F

Are there any made of some more heat resistant material?

Could be a good choice, depending on the operating temperature of the engine.