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it has been a couple years since i last veneered but i'm having problems getting corners to stick.
vacuum bagging. -18 - -21 pressure (auto cycling). walnut (wood-on-wood) veneer going over solid walnut (test piece) and baltic birch. 80 g scuff both the substrate and veneer. better bond x press glue (purchase 2 months ago). using their glue spreader/roller to apply to the substrate only. bagging it for an hour (per directions on glue bottle). using top and bottom cauls.
it seems to stick fine in the field but not at the corners. we get glue squeeze out everywhere and are spreading it to touch all surfaces on the substrate. peeling it back makes it seem like we missed gluing the corners but this isn't the case. you can see the squeeze out all around the corners.
I think you have too large or an overhang on your cauls. This effectively cantilevers over the edge and causes glue bond problems when vacuum veneering. Aim for a 1/4" or so. I am not familiar with the glue but it appears from your photo to be glue starved at the edges and too much back from the edge.
Perhaps just because I do it differently, but I cut the substrate oversize to what I need as a finished size, and cut the veneer and the caul (in my case, a piece of 1/8" masonite) to the same size as the substrate. I also only press one side at a time, that side being against the membrane material of the press.
If the caul was larger than the substrate, the force of the press may have caused it to buckle past the substrate, lifting it off the edge of the substrate, which would leave the area at the edge of the substrate with less pressure on it, and the pressure on the rest of the caul would remain once the buckle had straightened out over the center of the piece. This may have even been the case with just the veneer being oversize, as shown in your photo.
I had this happen to me years ago on a time consuming sketch face that I taped up, and was heartbroken when that happened. I have pressed a lot of veneer in the manner I described since then, and have had no issues.
Hope this helps.
thanks for the ideas.
i'll have to double-check things tomorrow but i'm pretty sure we're getting on both side. top and bottom cauls are 3/4" melamine. top caul was 3/4" over-sized on all sides. we reduced this to 1/2" on our latest test.
with veneering one side at a time - i really didn't think this was possible to do without affecting the flatness of the substrate. i'd much prefer to do this for the table glue up (4x7.5 x 2 pieces). what would be the procedure?
For flat work I made a frame press that will accommodate a 4x8 sheet; I would veneer the bottom of a table, and then veneer the top shortly afterwards on the same day. My vacuum pump runs constant and is set on a timer for 3 hours. I use Franklin Cold Press glue (similar to the Better Bond) and a much thinner caul (1/8" masonite); if the veneer is pourous I will place wax paper between the veneer and the caul, but I veneer directly without a crossband, so this would not be a problem for you in this application.
So far, this has not been an issue for me, and I have pressed a variety of veneers including burls, some that I have flattened myself.
Perhaps others will weigh in, but what jumps out at me is the thickness and rigidity of the cauls, as I think the idea is to draw the veneer down to the substrate, and the rigidity of 3/4" melamine might not pull down tight against the veneer, and unless everything makes contact everywhere, there will be areas that have no pressure applied to the veneer. This is also why I veneer one side at a time, as the rigidity of the substrate may replicate this condition on the bottom veneer as well, if you are veneering top and bottom in one pressing.
As I have no formal veneer training, but instead lots of trial and some error, others may well disagree with what I am saying, and weigh in with their view and experience.
Just trying to be helpful.
tony - thanks for your reply. i appreciate you sharing your approach and suggestions. my lead guy is out today but i'm planning on testing some sample pieces out shortly.
we seem to get getting a slightly better bond at the corners on the bottom side than the top side but all are still not as good as the sides. our test piece caul was maybe too big and provided too great an overhang.
with you putting your veneered side down - does it really matter about the caul? i would think the caul, and don't top and bottom sides at one time, would matter, but be irrelevant if only veneering the bottom side in one pressing.
Perhaps I did not state it clearly.
The face that gets veneered, whether it is the back or the front of the panel, is always face up in my press, such that there is the substrate, the glue, the veneer, and the caul, and that is face up against the membrane that is being sucked down onto the face being glued. In this manner, the vacuum only needs to overcome the resistance of the veneer and the 1/8" caul, and can provide maximum consistent pressure to the face being glued.
Perhaps the confusion was in stating that I glued the bottom and then the top of a table. The bottom face of a tabletop would be glued first, face up, in the manner described above. After that was dry, I would flip it over and do the top face of the tabletop, face up, in the manner described above. Both faces would be glued on the same day, so that it manages to stabilize the substrate and minimizes any warpage.
One other thing. With adhesives more is not better when veneering. Excess glue may lead to extended cure times, bleed through marring the surface, and too much squeeze out will gum up tooling. I aim for just barely opaque glue surface and small - less than 1mm beads of glue squeeze out.
We pretty much do the same as TonyF. We have a 5 X 10 vacuum table that is very flat, has grooves in the bed. A hinged frame with the membrane. Much easier to use than the bags we started with. We also use 1/8" hard board cut very close to the work size. In the press less than one hour, flip and do front side. Our membrane is urethane, wish it was silicone so we could contour press solid surface w/o some form of insulation.
I know you said the directions said an hour for glue up but that sound short to me . Also are you meeting your open time because you can get dry edge if the open time is really short. I use cold press from franklin and had that happen because dry edge. I press for 2-3 hour depending on conditions and open time is 15 -20 min. Try a differant glue because it sounds like everything else is fine .