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Gasket that seals directly to plastic, not frame?

 
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drcrash
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PostPosted: Sat Oct 21, 2006 4:25 am    Post subject: Gasket that seals directly to plastic, not frame? Reply with quote

(Continued from this thread: http://www.tk560.com/phpBB2/viewtopic.php?t=395)

Another thing I've thought of trying is a gasket positioned inward of the clamping frame, which seals directly to the plastic rather than to the frame.

A lot of people use gaskets that seal to the frame (like the studiocreatons.com guy), or have the plastic mold around the edge of a raised platen.

I've never heard of anybody trying a gasket that seals directly to the plastic, but it seems like a good idea. You could have several gaskets of different sizes for different-sized sheets of plastic, rather than relying on sealing to the outside edge of the platen. I've done that with gaskets that seal to the frame, but sealing directly to the plastic seems like it would be better. (I put the gaskets on a sheet of thin, flexible plastic that goes almost to the edge of the platen, and tape it to the platen to seal it. There's a cutout the size I want in the middle, with a gasket around it that fits my smaller frames.)

If your gasket seals to the frame, you really have two seals to worry about---between the platen (gasket) and the frame, and between the (lower) frame and the plastic.

The studiocreations guy uses fairly wide wooden frames, and tapes the edge of the plastic to the lower frame before clamping the upper frame over it. (The wood frames insulate the tape somewhat, so that it doesn't immediately get cooked.)

That seals the plastic to the lower frame, but it's ugly and a bit messy. The tape has to be removed and re-done for every pull. You can reuse the tape a few times, but only a few because it gets funky.

Silicone rubber seems like a good choice for gasket material, or maybe a layer of silicone over something else. (Maybe aquarium air pump tubing, slit, flattened, and silicone sealered to cheaper rubber foam.)

It wouldn't be difficult to cast custom gaskets out of two-part silicone, or silicone caulk. I'm not sure it would be worth the trouble though; thin silicone rubber glued over something else may work about as well, maybe better.

I'm wondering whether some kind of raised hard edge with thin silicone over it would be ideal. That way, if your plastic is hot enough around the edges, you'd get the plastic forming tightly around it. If not, you'd still have a smooth-rubber-to-smooth-plastic gasket situation, which is pretty good. Even when the plastic does form tightly around it, it might actually seal better than metal edges; I wish I knew.

(By the way, if you do go with platen edge sealing, whatever it's made out of, it should be very smooth. Any little grooves from sanding or whatever provide little channels for air leaks, because the plastic can't form down into them and make a good seal. Tiny leaks don't matter so much when you're using a shop vac; they matter more to a high vac system, where you have a limited supply of vacuum.)
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knightshade
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PostPosted: Sat Oct 21, 2006 7:28 am    Post subject: Reply with quote

I actually purchased some 600 degree silicone to try exactly that...
a thin bead along the outside of the platten to see if I get a better seal (thus still better detail)
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chatterx



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PostPosted: Sat Oct 21, 2006 12:23 pm    Post subject: Reply with quote

I will be trying this as soon as I get the screw holes I filled in sanded down
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drcrash
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PostPosted: Sun Oct 22, 2006 10:30 am    Post subject: silicone gasket issues Reply with quote

Hmmm... I was going to try the trick with slit silicone tubing, but then I realized I have no idea how to glue cured silicone to anything. Almost nothing sticks well to cured silicone, not even uncured silicone. (That's why you can cast glues like epoxy, polyester, and even urethanes in. Urethanes are even more "tenacious"---nastily sticky---than epoxies.)

Maybe cyanoacrylate glue ("superglue") would work, but it might be better to "cast" a gasket, in place, like you do for engine gaskets. (Maybe by putting down a big bead, and flattening it a bit with a greased piece of sheet metal while it cures.)
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drcrash
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PostPosted: Tue Dec 19, 2006 9:00 pm    Post subject: Reply with quote

McMaster-Carr has "Silicone rubber strip, adhesive-backed" that might make a good gasket to seal directly to the plastic. The foam is supposed to be good up to 450 degrees, and the adhesive up to 177 degrees. I'd think you'd be okay with that, because the foam would insulate the adhesive for longer than it takes the plastic to cool. (Unless you're doing something strange.)

3/16" stuff 3/8" wide is $16.75 for 15 feet. (Catalog number 5109K42.) That's more than enough to make a gasket for 24 x 24 inch plastic plus one for 24 x 18 or 24 x 16 plastic. (Or several smaller sizes.) They have other widths and lengths (esp thinner stuff in longer lengths).

Chatterx, you might want to try that. (If you're still out there.)

Paul
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jegner
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PostPosted: Wed Dec 20, 2006 6:59 am    Post subject: Reply with quote

My first vac machine had neoprene type weather stripping to seal around the top of the platen. This approach works fine if you holding frames are not large enough to slip down over the platen the way my current machine does. The hot plastic comes into contact with the outer edge of the platen, making a seal, that I doubt would be made better with a silicone seal.

I leak more air from my fittings, than anywhere else.

Jim
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drcrash
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PostPosted: Wed Dec 20, 2006 8:52 am    Post subject: Reply with quote

jegner wrote:
My first vac machine had neoprene type weather stripping to seal around the top of the platen. This approach works fine if you holding frames are not large enough to slip down over the platen the way my current machine does.


I wonder. My current gasketed platen works fine for low vac (vacuum cleaner only), but leaks more than I'm happy with for high vacuum---I'm losing something like a quart of vacuum a second, so that my little 7-gallon tank only gives me good hard vacuum for a few seconds, and pretty good vacuum for a few seconds more, and then I'm back to vacuum-cleaner-level vacuum. That's enough to let me form plastic twice as thick, or get twice the resolution of detail, but I think it could be better.

The problem may just be the actually lousy gasket, and if so, I know how to fix it. Or it may be that the leakage is happening at the sump pump check valve (for 2-stage a la Walsh), which may not be sealing as well as it should. (If so, my new two-tank/no-vacuum-cleaner two-stage design should work much better. That would be cool.)

But I'm worried that I may be leaking vacuum at the frame. With a frame-to-gasket seal, you really have two seals to worry about---the obvious one, and the seal between the plastic and the lower frame.

(I assume that's why the studiocreations guy uses tape around the edge of the plastic, sealing it to the lower frame, before clamping the upper frame onto it. I don't want to have to do that to get a really good seal.)

If that's the problem, sealing directly to the plastic would be better. (Silicone may not make a better seal, but it can do it directly against hot plastic, over and over.)

Quote:
The hot plastic comes into contact with the outer edge of the platen, making a seal, that I doubt would be made better with a silicone seal.


I don't really expect the silicone to make a better seal than a direct-to-metal platen-edge seal. What I'm looking for is a way to get a comparably good seal without making that kind of platen.

For low vac, I have several gaskets for different sizes and proportions of plastic---I just put the gasket on a removable sheet of plastic, which I tape to the platen. That works great, and it's cheap. I want it to work well for high-vac, too, so that I don't have to actually make platens of different sizes to get a good seal. The silicone stuff would be more expensive than the rubberfoam weatherstripping I'm using right now, but it would still only be a few dollars per sheet size. (About 8 dollars for 2-foot-square, less for smaller sizes.)

Quote:

I leak more air from my fittings, than anywhere else.


I'm sure my fittings (short of the sump pump check valve) aren't leaking more than about a quart an hour, which isn't noticeable when using the system. (If it was, I'd put hose clamps around the hose barbs.) It's definitely leaking orders of magnitude more at the platen during actual forming.

It shouldn't be too hard to get your plumbing to seal essentially perfectly, except for the plastic edge seal. That's the tricky part---it has to be frequently sealed and unsealed, and it's big, and there are heat issues.

(I've managed to thermoform my gaskets with my little over-and-under. The reflective side walls work so well that it heats up the buck and the platen on the table a fair bit, if I don't use my cookie sheet heat shield. That can be good for extending the thermoforming time---the plastic stays hot longer---but if your buck and gaskets can't take it, it's bad. So even if I stick with sealing the gasket to the frame, I may want to switch to silicone so that I can use the oven to heat everything.)

If you can reliably get a really good seal at the plastic, a two-stage design should let you use two small tanks with excellent performance large platens.

The first tank only has to be a bit bigger than the volume of air under the plastic. For a 2-foot-square platen, a 7-gallon tank should usually be fine, and a gallon jug is probably okay for a small platen. The second tank only has to be big enough to stay well ahead of leaks for 10-30 seconds; if there's a very good platen edge seal, a 7-gallon tank should be fine for that, too. Most of the time, even a big platen could get by with a gallon jug or two and a 7-gallon air carry tank. That would fit under the platen with plenty of room for the vacuum pump, plumbing, a pile of plastic, etc. (And no vacuum cleaner.)
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drcrash
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PostPosted: Wed Dec 20, 2006 12:33 pm    Post subject: Reply with quote

Actually, the main reason I'm looking at gaskets that seal directly to the plastic right now is that I've figured out how to make some pretty good adjustable frames, pretty cheaply and easily... only they're not airtight. Life is much easier if the bottom frame doesn't have to be sealed at the corners, and its side bars can have several bolt holes through them.
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jegner
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PostPosted: Wed Dec 20, 2006 2:10 pm    Post subject: Reply with quote

Have you considered a matt to reduce down the size the vaccum can pull though, and mount the seal on the top of the mask?
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drcrash
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PostPosted: Wed Dec 20, 2006 2:41 pm    Post subject: Reply with quote

jegner wrote:
Have you considered a matt to reduce down the size the vaccum can pull though, and mount the seal on the top of the mask?


That's what I do now. Each gasket is mounted on a sheet of thin plastic, with a border left around it to tape down to the platen. (And a big hole in the middle to let the air out through the platen.)

That part works fine, and making a new gasket is pretty easy and cheap. The hard and more costly part is coming up with new frames for each size that won't leak. (Bottom frames, anyway. You can just clip oversized strips to the top edges.)

For small stuff, I do okay with aluminum windowscreen framing stuff. I can mix and match side lengths to make different sizes and proportions. But those frames leak a little.

I don't think that matters much for small stuff and relatively thin plastic, because you don't have to hold the vacuum very long before it cools enough that it doesn't matter anymore. For the bigger, thicker stuff I want to do, I'd like to have much more rigid frames, and either not have them leak, or have it not matter because the plastic doesn't seal to them.
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PostPosted: Wed Dec 20, 2006 4:08 pm    Post subject: Reply with quote

Having different frame sizes would sure cut down on the waste of the plastic. You could try just using the one large holdng frame, but use different size matts with the gasket, to test if this will work well, and if so, make a small, and medium frame with appropriate sizxe matts.

Easy test, and worth a try I suppose.
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