Troubleshooting Bubbles in a Vacuum-Press Veneer Glue-Up

Platen characteristics, veneer issue, or glue quality might be to blame for small bubbles appearing under the veneer. April 7, 2008

I recently built a vacuum pump and bag to do some kitchen door panels. I'm getting about 20lbs of neg pressure from the pump and a good seal in the bag. I'm using various figured veneers and cold press veneer glue onto 1/4" MDF between 3/4 melamine platens. I keep getting bubbles in the veneer - some big, some small, but I'm consistently screwing these panels up. Anyone ever have this happen? I originally thought maybe I was rolling on too much glue, but now am baffled. Maybe bad glue?

Forum Responses
(Adhesive Forum)
From contributor F:
My take is that there no bad glues if it's a PVA and if it flows. What is cold press veneer glue? Urea, PVA, PVA II, or Fast Set as in FSV? Zero in on moisture, is my bet. Urea and FSV work pretty darned good in a bag.

From contributor C:
Bubbles towards the edges can result from the top platen being too big. 3/4 platens shouldn't overhang any edge by more than 1/4". I go for 1/8 to 3/16.

From Jeff Pitcher, forum technical advisor:
I'd be curious to know what the bubbles look like if you slice them open. You can tell a lot by looking at the failed glue area. It may be that you've applied too much glue or it could be contamination or too little glue in those spots. I doubt you're getting 20lbs of pressure, as a vacuum press is only capable of drawing 1 atmosphere of pressure (~14.6 lb/square inch).

From contributor W:
Are your veneers flat and pliable or wrinkled and dry? Raw veneer or prepared on paper or veneer?

From the original questioner:
The glue is not a PVA. The bubbles aren't on the edges, they are exclusively toward the middle. They feel hard but after some time, soften up. The veneer is raw and is figured as well, some are a little lumpy, but not too bad. I've used much worse. As far as the pressure, I don't know the science behind it, but my gauge reads 20 and it cycles back on when it drops to 18 (I have a slight leak at a valve). In my tests it broke melamine and MDF panels that weren't laid flat.

From contributor T:
Are your platens grooved on the outside surfaces? This is to draw out all air. Are your platens bigger than the work piece? If the pressure is forcing an overhanging edge down, it is also not pressing the middle of the panel. If you have an overhanging platen setup and are using filler blocks to fill the open voids, make absolutely sure they are exactly the same thickness as the work piece. Are you taking too long getting it in the bag, allowing the adhesive to skin over?

From contributor T:
I just want to add that when I'm pressing panels I make the platens exactly the same size as the work piece, then pin two of the corners so it doesn't shift. It is guaranteed to press properly this way.

From contributor G:
The bubbles could be a result of the veneer not being uniform in thickness. A 3/4" thick caul will not conform to minor variations. I suggest either using a sheet of rubber between the caul and the veneer, or using a thinner caul. 1/4" thick material will be fine.

From the original questioner:
Well, I pressed up a bunch of panels this weekend, and they all came out fine. I did two things differently than before - I bought some new glue (essentially the same stuff, just new) and used less of it. What was the problem? I suspect it was settled, old glue, but who knows.