I use vacuum for veneering shop-sawn veneers and laminating thicker stock for door stiles, etc., and believe this is common practice in most shops.

A quick survey of clamping pressures for PVA's and PUR's suggests 100-300 psi is recommended , depending on wood density. Fairly high vacuum, say 25+ hg/in, is only about 13-14 psi, yet seems to bond extremely well. I've done a fair bit of looking around on this site and otherwise, and don't find anything that speaks to this seeming contradiction. Can anyone shed light on this?

Like you, I have always wondered where those specs came from. The bottom line is that the glued surfaces need to be in "intimate contact" and pressures much lower than you refer to do achieve that in practice.

I also see a large disconnect between what the technologists say and practice - successful practice, that is - in the shop.

I have little idea of what a 1" pipe clamp is capable of putting out in psi, or a bar clamp in the same use, nor a 12" handscrew or deep throat slip clamp.

When the technologist says a glue line of .00x0x is desired, how can I measure that? As for psi, how can one know what we are doing? Do we even need to know?

I recently have had failures with Western Red Cedar and Titebond 3 as well as Titebond 3 letting loose at temperatures of 180 degrees. In the years I have used this glue, I never heard of these problems. Yet I pay close attention and massage the details to what I sometimes think is obsessive, yet these things somehow remain unknowable.

If you dive into the Titebond website you will find that Titebond 3 loses considerable strength at elevated temperatures http://www.titebond.com/product.aspx?id=e8d40b45-0ab3-49f7-8a9c-b53970f736af

The reason that so much less per square inch pressure is required in vacuum pressing is that atmospheric pressure is being applied to the entire surface. If you take the 13 lbs. per square inch and multiply it over the entire surface being clamped you'll find the actual overall pressure being applied is quite large. A 10" x 10" sandwich of two boards on a vacuum press has 100 square inches of surface. 13 psi x 100 is 1300 psi pressing down on that 10" x 10" surface.

If you were to put 8 standard C-style hand clamps around the edge of that 10" x 10" glue up, and each clamp applied 150 lbs of pressure, you would have a total of 1200 lbs of pressure. While similar to the 1300 psi being applied by the vacuum system the pressure of the hand clamps would be directed at individual points around the edge of the part, instead of evenly across the whole surface.

That is why vacuum pressing works.

As to TB2 on Western Red Cedar my experience is that it is unreliable. Even if you do the recommended wipe down with Acetone it may not hold well.

Years ago I did a number of glue tests on Western Red Cedar. I took 4" x 3' long boards and did a face to face 12" overlap glue up using various adhesives. When dry I used the boards as pry levers around a post. Surprisingly polyurethane glue (I like the Franklin brand) did the best. Most adhesives simply failed. Only West System epoxy and polyurethane glue resulted in split wood vs. failed glue joint.

These results don't indicated that other adhesives I didn't test won't work but it did rule out Titebond, Titebond 2 and one or two other brands and types I don't recall now. So while polyurethane glue would be my last adhesive of choice on most projects it is our go to glue for Western Red Cedar.

BH Davis

Bernie - I am just on my first cup of coffee, but I think you may want to rethink your math in the last sentence of your first paragraph. The 10 x 10 will have a TOTAL of 1300 psi, but it will still only have 13 pounds per square inch.

As for the major label glue insufficiencies that you, I and everyone else deal with, I wonder why we have to play this game of discovery as to precisely when and where the glue is appropriate. I like to think I know which end is up, but both the WRC failures and the temp strength loss surprised me.

In the example Mr. Davis gives, the total pressure applied to the glueup is 1300 pounds, not psi. 13 lbs/sq. in. x 100 sq. in. =1300 lbs. Not trying to pick nits, just to be clear about units.

Kevin,

Thank you !

I reread that sentence 3 times and didn't pick up that I'd put "psi" there instead of "lbs". I thought David needed to go back and get that 2nd cup of coffee !

I appreciate the correction.......both from David and yourself.

BH Davis

Here is a specific question I have been unable to reason out on my own, or get an answer from others.

Using that same 10 x 10 square with 13 psi on it, what if I put two 10 x10 squares on top of each other? Does that make for 13 psi on each panel? Or does it reduce to 6.5 psi per panel?

I can argue it either way, and have some vague answers from otherwise good sources. No general agreement on the answer, though.

The practical application is that if I am veneering a batch of said 10x10 panels, I can stack up 2 or 4 ( or more?) and need only one caul on top. Laying them out as singles, I need a caul to cover each panel.

David, I would say that the pressure on the surface of the stack remains the same at 13 psi (that may be a bit high in practical terms depending on where your pressure switch is set), but the available force of 1300 lbs is distributed among however many gluelines it is pushing together, so the effective psi per glueline would be halved with double stacking. I have had good results in stacking parts in the vacuum press with a gap-filling glue (epoxy) but would be less confident with pva adhesives that like a thinner glue line.

When stacking panels it's also necessary to maintain alignment in the stack and to consider the extra time loading the press vs. the glue's assembly time. I'm sure you have had the experience of workpieces/ cauls moving in the vacuum press if the bag is stretched in an unbalanced way. One way to increase the throughput is to load separate panels in one layer with spacers between them such as door bumpers and use one caul over the lot This is especially useful when veneering a stack of drawer fronts or other parts that require grain alignment through the stack, using one sheet of veneer that is cut apart after layup.

Kevin & David,

I look at this layered pressure issue from the other perspective.

I see it this way. If you were to put a 10" x 10" scale on a vac table (as vs. a vac bag for simplicity in this discusion) with nothing on top it would read a theoretical 1300 lbs. If you put 1, 2 or more 10" x 10" boards on top it would read 1300 lbs plus the weight of the boards. It wouldn't read less just because there were some boards on top.

Maybe this is oversimplifying and I'm completely missing something but this seems to indicate to me that the force on multiple layers is the same as on a single layer.

BH Davis

Ok, so if I have it right, then if have a 10 x10 panel with crossbands and face veneers both sides, then there will be 4 glue lines. If glue tech says 13 psi is recommended for the glue, and the pump/bag/table pulls 13 psi, will my panel be good?

Can I stack 2 or 3?

I see those 48" high opening presses that will take 20 or 30 panels in one go and think that the pressure they have must read throughout the stack, or they would have to put out 1.5 million psi to achieve 13 psi on each glue line.

I appreciate the thought and expertise.

I think the weight of the boards in the stack can be disregarded. The total force on the assembly is limited by the area exposed to atmospheric pressure, and it is dispersed among however many gluelines are in the stack.

We use clamps, including vacuum presses, to force our components close enough together that the adhesive can make a bond. If the components are flat enough, or flexible enough, a limited force can bond multiple gluelines at once, but there are practical limits.

I have layed up curved panels with 6 or 8 layers of 1/8" bending ply and pva glue successfully, but I would not attempt 6 layers of 3/4" hardwood even freshly surfaced, though 2 layers would not pose a problem in a vacuum press. As I see it,
the more gluelines and the stiffer the material, the more force is necessary to close the joints.

A lively thread. David, I can't give a definitive answer, except that probably given flat substrate and mild veneers you would get a good bond in a vacuum press with that layup.

To get back to the original question though, most glues have a recommended pressure much higher than we can achieve in a vacuum press, so we have to rely on experience and common sense to guide us.

My guess is that the 100-300 psi referenced by the original poster came from furniture factories clamping up panels with multiple narrow strips produced by methods resulting in less than perfect workpieces. Carefully and freshly milled lumber or thin veneers require substantially less force to achieve tight joints.

I am not familiar with industrial plywood production, but they must have extremely high pressure to do what you are talking about. They certainly are not limited to atmospheric pressure.

I hope I am not being too argumentative, just trying to think through the issue.
I very much appreciate all that I have learned from you and B.H. Davis through your posts here and am glad to be able to discuss technical issues with you. Also Previous Mel.

From an engineering point of view, it wouldn't matter how many perfectly uniform, flat panels you stacked. The pressure would be the same through out. Problem is the likelihood of getting all those wooden parts to be perfectly uniform and flat diminishes as the stack gets deeper. If you are doing solid lumber stacks, any variations in flatness adds up, just like a truck leaf spring. Those big hydraulic presses are distributing their tons of pressure over a large area. If I did the math correctly and you wanted to have 100 psi for your glue line on a 50X98"stack of plywood, you would need 245 tons on the press!!! To get to 300 psi you would need 735 tons, serious!
Black Bros pod presses are available from 20 to 80 psi. Conclusion: seems highly unlikely that 100 to 300 psi would ever be used in practice. Wonder how those #s were arrived @??

Just for grins I did a little research on those industrial presses. They are typically rated in the hundreds of tons. One company supplies 4'x8' presses rated between 640 and 1500 tons. The 1500 ton unit works out to about 600 psi and has openings for 25 sheets.

http://www.srichakraengineering.com/plywood-press.html