Real vacuum-newbie question - Why not air pressure versus vacuum?

I have long read here Q&As about vacuum setups for veneering. Took me some time just to get a handle on the basics. Question: What if you wanted to veneer some 1" thick wood, maybe 6" x 12". So, you build an 18x18 frame of sorts with a study piece of ply at the bottom and the same on the top about 10" above the bottom one -- there are uprights at each corner and maybe an upright halfway along each side. Sorta like the top piece of bread of a sandwhich being suspended above the bottom piece with nothing in between. You put the newly veneered piece of wood on the bottom piece of ply and on top of it, and below the top piece of ply in the frame, you place a bladder - maybe a big ol' section of inner tube that has been sealed at each end. Then use a compressor to pump up the bladder to exert the force you need to press down on the veneer. As best as I can tell, the reason to use a vacuum press does not involve sucking out air pockets that may exist between the veer and the underlying wood -- if that IS part of the process, then I suppose a vacuum is the way to go. But, if it is just a matter of even pressure across the top of the project wood, it SEEMS that my scenario would work. Just wondering. Comments appreciated. TIA. -- Igor
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pressure or vacuum- either will work.
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In theory, yes. But consider that the bladder is flexible (i.e., it is subject to stretching). With air pressure, you are stretching it, thinning out the material, making it weaker. With vacuum, you are compressing the material, thickening it, making it stronger. Of course, this is just based on how I've seen vacuum used for veneering...I'm sure you could come up with a set-up that would flip-flop the advantages/disadvantages of vacuum vs air pressure.
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igor wrote:

It will work, just as long as you don't blow out your inner tube--while an inner tube in a tire will take a lot of pressure, without the tire they tend to blow out at surprisingly low pressures.
Nice thing about vacuum is that you don't need any structure for it to press against.
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Seems to me without an elaborate setup it would not work. If you simply put the piece into a airtight comtainer and pressurized it the overall pressures on both sides of the veneer would equalize and effectively there would be no differential pressure on the veneer.
If on the other hand you apply pressure to a secondary bag then the inner bag pressure would increase effectively doing the same as the previous example . The only way it would work is if the bag is constrained by an external structure even then for non flat surfaces it would still be hit and miss. you would probably end up with the old type veneer press with profiled cauls....mjh
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I have made some small laminated bends by packing the former, glued bend material and inner tube inside a very strong box. Hole in side for the valve.
Tricky to get it right.
Jeff G
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Either way will work except for the following. Let's assume that 12 pounds of vacuum is enough for your 6 by 12 inch piece. With vacuum the air pressure on the outside pushes against the vacuum on the inside from both directions and balances out. That is why a simple and flexible bag can work, no frame needed.
With air pressure pushing down from inside a frame as you describe then the total force on the frame is 12 pounds per square inch times 6 inches times 12 inches or 864 pounds. Your frame has to be strong enough to hold that as it trys to blow apart the frame. remember that your home is designed with floor loads of 40 pounds per square foot. or 20 pounds on a 6 by 12 area. A simple frame with a plywood "floor" won't work. Now compute the force on say a dresser top of 18 by 42 inches at even 5 pounds 9not 12) that is 3,780 pounds or just under two tons to hold back pressure with a frame.
Howard

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Howard wrote:

First, why would one need 12 psi, which is 1728 PSF and well above the recommended clamping force for any common woodworking adhesive?
Second, houses are typically built with 12 foot or more spans. The same framing on an 18" span will support a far greater loads.
Third, 2 tons _sounds_ impressive but you can achieve that much force with four clamps--Bessey K-bodies are rated to apply 1000 pounds of clamping force, so 4 of them give you 2 tons. Designing a frame that can't be torn apart with 4 K-bodies is not all that hard.
Certainly this is a valid concern, but it's not as difficult to deal with as you seem to be suggesting. The _real_ concern is that if you screw up the support frame then you can end up with pieces flying.

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John -- Thanks to you and the others for the comments. All of the caveats, as usual, are mostly on the mark. Vacuum bags do not explode - though the pumps can overheat and start a fire ...
It seems to me that a big advantage that a pressure versus vacuum setup has is that everything but the bladder is in the usual woodworker's bag of tricks/supplies/tools. No need for scrounging vacuum pumps and cut-off switches, etc. What IS needed is a bladder. If someone would produce those for the market, that could be a faster route for woodworkers. (LV are you listeneing? Let me know and I'll tell you where to send the royalty checks.) Then again, I would not be surprised to find that a bladder like this already exists in some other field. For a small piece, using a piece of mdf under it (to avoid a strange indent pattern), one could even use a basketball which is designed for 8 PSI.
As for making sure that the frame does not blow apart, at most I would think that a series of pairs of 1" aluminium Ls bolted with sheet goods in between could be OK. FAIK, 1x lumber on edge might be enough. (Note: This is not engineering advice. You are all on your own.. End of legal disclaimer.) Again, the point is that this approach, or a version of it, seems much easier to cobble together. That being said, I have often not taken the easier route myself. Again, thanks to all for the comments. -- Igor
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Pressure bladders are hard to seal. Vacuum bags are pretty much self-sealing, if you can keep a reasonable seal long enough to get a vacuum across their overlap. OTOH, a vacuum bag does need a seal that can be opened and re-sealed to get the workpiece in.
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Vacuum or air will both need some support structure for the piece as well. With vacuum, this support or "buck" is in compression, with air pressure it has to be an enclosing box that's hard to load the pieces into and is then placed into tension. It's easier to make an internal buck that resists compression, then an external tension frame.
The extra air pressure is a red herring. 15 psi is a huge pressure for clamping and you don't need anything like as much. Running your vacuum switch at maybe -5psi will give you all the clamping you need, but makes the buck easier to build and reduces the risk of "starved dog" - the effect where an outer skin gets pressed so hard over the ribs of the buck that it retains the skeleton shape afterwards.
For outgassing epoxy on small pieces, I often use a hand pump (Vac-U-Vin coffee jar) that has a pressure differential of only 1/4 atmosphere.
For a convenient vacuum gauge, find a mil-surplus aircraft cabin pressure gauge. This is a self-contained aneroid mechanism reading from sea level to 40,000 feet altitude (normal atmospheric pressure to bugger all). Just throw it into the vac bag somewhere where you can read the dial through the polythene.
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