fundamental priciples of a vacuum press

Does a vacuum press improve a glue joint between a veneer and a flat, smooth substrate (say.... MDF)? If so, why? What fundamental physical properties of the veneer, substrate, or glue are changed by the vacuum that improve the surface joint?
Or is a vacuum press just effective for gluing veneer to curved or irregular surfaces?
Thanks a heap, -jbb
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The vacuum isn't doing anything magical - it's just a convenient way to get very even pressure over a large surface. Large, even pressure gives a thin glue line with intimate contact between the veneer and the substrate. You can do the same with cauls and a boatload of clamps but a vacuum press is a whole lot faster and easier. A vacuum press is also cheaper than the gazillion clamps you'd need for anything larger than a breadbox.
--
Scott Post snipped-for-privacy@insightbb.com http://home.insightbb.com/~sepost /

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In rec.woodworking

Equal distribution of pressure over the entire surface area. Pressure with clamps, even using flat cauls still leaves room for hot spots to form under the clamps and uneven distribution due to surface variations, etc.
When the bag is empty, there is an even 14.8 psi (1 atmosphere) uniformly distributed over the entire surface. Imagine diving to the bottom of a pool. Every part of your body is under the same pressure as long as you are at the same depth. Eardrums too ;)
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On Wed, 30 Jun 2004 01:52:56 GMT, snipped-for-privacy@nospam.com (Bruce) vaguely proposed a theory ......and in reply I say!:
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...and schchchwwp....you are stuck firmly to the bottom of the pool....<G>

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Probably should have mentioned turning off the pump...huh? ;~)
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Bruce wrote:

One other subtle feature is that the glue is absorbed deeper into the substrate and the veneer.
R.
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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email
HeyMan! You is a universal fish cooker! Wotchuno?
<G>

Seriously. How? I can see the more even theory. But pressure is pressure.
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Ah there you have it, it is not pressure is it , When you apply glue everything is at atmospheric pressure . When the veneer is placed on the substrate it also is at atmospheric pressure . As the vacuum is applied the veneer and the substrate both being to some degree porus air is pulled from these and the remaing voids will be sucked into the voids until the pressure [ vacuum] at the glue line is the same as the common pressure in the bag Anyway thats what I think.....mjh
-- http://members.tripod.com/mikehide2

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vaguely proposed a theory ......and in reply I say!:
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OK. I can see that up to a point..
...no I can't. The glue is also under the vacuum, and therefore will no more be forced into the wood than by sheer pressure on the substrate/veneer. Pressure on its own would force the glue into the somewhat porous material. The vacuum will "draw" in both directions, unless you carefully sealed the edges of the board under work.
I can see that it may make _some_ difference, with a shorter, wider path through the wood than out the edges. But I would wonder at how difference it makes.

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Old Nick wrote:

You're neglecting the effect of dissolved gases and outgassing.

--
--John
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On Thu, 01 Jul 2004 16:17:18 -0400, "J. Clarke"
remove ns from my header address to reply via email

And they would be...? Serious. What sort of effect is this, and how difference does it make?
Thanks
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Old Nick wrote:

Pull a vacuum on _anything_ and gases come out of it. What quantity depends on the composition and porosity of the substance. Depending on how the vacuum bag is arranged, the gases coming out of the wood or the expansion of gas bubbles and evaporation of volatiles (water is a volatile in this context, the organic solvents even more so for the most part) entrained in the glue should drive the glue into the grain to some extent. What extent is going to depend on the particular composition of the glue and the type of wood being used, so there's no way to generalize.

--
--John
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Old Nick wrote:

Open a see-through bottle of your favorite carbonated beverage & watch the gas un-dissolve.
-- Mark
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Why not neglect them ? This is only woodworking after all. Our vacuums are pretty rough - they're just in the "mechanical" region, not the "chemistry" or "physics" regions.
Woodworking has two uses for vacuum; clamping and de-bubbling. For clamping, then you could equally well put it all in a hyperbaric chamber and apply a little excess pressure to the outside. So long as you have a few psi difference, then that's all you need.
For de-bubbling (such as casting thick epoxy or polyurethane resins) you only need to drop the pressure below the pressure at which the resin was mixed, which is presumably atmospheric (some commercial systems mix under pressure to avoid the need for vacuum afterwards). For most of my "potting" work I use a Vac-U-Vin wine storer and a hand pump to get down to about 1/4 atmosphere - this de-bubbles 2" cylinders fine. You _don't_ need to "lower the pressure to the vapour pressure of the resin", as is sometimes claimed.
And yes, I _have_ built steel hard-vacuum systems with heaters in the walls to outgass them. I used to be a laser fizzicist.
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On 2 Jul 2004 03:59:53 -0700, snipped-for-privacy@codesmiths.com (Andy Dingley) vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email
Sorry if this was already a play on words by you, but a laser fizzicist that deals with outgassing just had to be commented on....<G>
feel free to take full credit....

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On Wed, 30 Jun 2004 01:26:07 GMT, "J.B. Bobbitt"

you don't say what you want to compare it to.
vacuum presses apply VERY even pressure over the whole surface....
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