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Veneer bubbles 5 years later...help4/28
The photos show the concave side of a curved face (back) mdf audio speaker. I have pressed veneers with Unibond 800 and the Vacupress for the last 15 years. This is the first job I have had come back to me. These speakers have been kept in a climate controlled room without excessive heat or sunlight. (I understand the room is filled with about 750K of audio equipment which makes me believe that excessive environmental variables are not the problem.) This condition occurs only on the concave face of both speakers. The convex faces, veneered at the same time, same glue, same species, same finish show no problems. Nor does a flat veneer job on a nearby woofer box. (veneer direct to mdf) I paper taped and pressed the Pau Ferro veneer to the brown paper backing by Neenah (http://www.neenah.com/technical/heirloomfurniturecomponents/veneerbacker.aspx)and
How did you glue the paper backing to the veneer? Did you use raw paper back or glue one side paper back. Most paper back is pressed at 150psi with a min. temperature of 250F. These aren't bubbles but surface checks in the veneer. 99% of the time these surface checks are moisture related. What type of finish did you use? Water based or solvent?
Why the paper? What do you expect from this added step that you won't get from bonding directly to the MDF? Is that the seam I see where it is slightly curled over the entire length? Sure does look like moisture, but maybe some of that is knife cracks from the veneering process? Maybe consider that knife checking when choosing the side to face the MDF.
Not sure what Unibond is made of, but with hard veneers like Pau Ferro, a resin glue never moves. Heat can effect PVA glue even after hot pressing/ catalyzing. Paper moves more than you might think. Movement creates stress, stress creates fissures.
From the Neenah link: "Compatible with - Contact glues - water-based emulsion glues - hot-melt adhesives"
"I paper taped and pressed the Pau Ferro veneer to the brown paper backing"
If you flat pressed them together with Unibond 800, that's the cause. I also don't understand the use of paper, but I would definitely use PVA for the paper to veneer joint.
Thank you for your thoughtful responses. We pressed the pau ferro to a paper backing with Unibond 800, a urea resin cold press glue. The paper adds strength to the veneer when pressing over a curved substrate, especially the concave substrate. We did not use the paper on a nearby square, flat woofer box. It shows no problems. But the convex curved faces of both speakers we used the paper backing and also show no problems. Either by chance I treated the two 2-ply (paper and veneer) differently that I used on the front from those I used for the backs. I think I glued all four 2 plies at the same time so it would be just chance that two bad ones ended up on the backs and two good ones ended up on the front. We used a conversion varnish finish on all faces, applied at the same time, same manner. I am requesting better pictures from the customer and will repost if I learn something. It looks environmental to me but I am certainly open to another explanation. Thanks
When the kitchen cabinet door manufacturers first started membrane pressing the raised panel inserts there were a lot of issues when using straight UF resins. The solution was to blend in up to 35% PVA and 5% Melamine into the UF glue. You retain the thermoset glue line with the 65% UF content, the PVA makes the glue line more flexible and the Melamine gives a higher water resistance.
The stresses between the paper and the veneer could be causing your glue line to crack and this could be transferring to the wood.
And what's between the paper and veneer appears to be a rigid glue line that was put into compression when the flat 2 ply lamination was applied to a concave shape. Plastic resin is great for bent laminations that you don't want to flex, its not a good choice for flat laminations that you plan on bending.
This is just a wild idea, but I thought I would through it into the mix.
The damage to the veneer really looks like moisture damage to me. That damage occurs because the veneer swells with moisture and pushes up like a mountain range.
So why the difference between the convex and concave. The concave is already compressed and that leaves no room for the expanding veneer to go. The convex side is in tension, so there is some space for the veneer to move to when it expands.
We had some bending ply core baseboard delaminate a couple years ago on the concave side of of a curved wall. The convex side was fine.
We had a double layer of 1/8" thick white oak veneer applied to the face of a double layer of 3/8" bending ply. As the oak shrunk with moisture change it remained stable on the convex side because shrinkage just pulled it tighter to the curve. On the concave side though shrinkage allowed the oak to move away from the wall surface thus putting pressure on the bending ply layer structure. The grain direction was going length wise down the wall so that the only shrinkage involved was over the 4" width of the material. In effect the oak was able to curl away from the wall.
One could consider that the same was taking place here. If the wood had been glued direct to the MDF substrate there would have been no way for it to move. My guess is the paper backer was the weak link much as our bending ply. It acted as an weak barrier between veneer and MDF allowing for the veneer to surface to move and fail.
Think of the paper barrier's strength in terms of paper being used in a joint where the surfaces are going to have to be separated later..... like when mounting blanks for turning on a lathe. Paper is put between the two because it will split on center when the two wood parts are split apart with a chisel.
Even in a climate controlled space there will be some moisture variation over a 5 year period. Perhaps a different veneer wood species would have been more forgiving in this situation. I suspect the paper allowed for enough wood movement to stress the grain structure, thus resulting in surface failure.
We had some paper backed Maple veneer on a flat surface do this same thing. The company that supplied the veneer tried blaming the finish. It wasn't the finish.