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mahogany veneer not bonding to substrate1/12
I have tried 3 different softening and flattening methods, and two different types of glue, ultra cat and better bond cold press in my vacuum press. The samples come out great, the project is a loss of more than 700 to me at this point. I can't figure out what is going on. Bubbling, edge pull up, and it gets worse the longer it sets. I have never had this happen on smaller veneer projects. Does anyone think my pressure is not sufficient for larger items at 9,000 ft elevation? Pulling approx. 19-20 hg. 5.5'x3.5' half crotch mahogany veneer over mdf substrate.
Has the mahogany crotch been "two plied", or are you using the single sheet veneers? It is advisable to have the crotch two plied by a veneer face maker. This will take dimensional stability out of the equation.
I am only using single layer veneer, all wood, as I have always done with my smaller projects. Do you think my larger project has to much tensile forces for the glues to bond correctly without a double layered veneer?
I feel your pain, Kerry. Can you be very specific about the flattening/softening techniques you have used? I suspect the problem might be there.
What substrate are you gluing to.
Ezra -- OP Kerry states MDF substrate.
Thanks for the suggestions everyone. My softening method I tried first was exactly down to the tee of Scott Groves Advanced veneering. 3 parts distilled h2o, 2 parts pva glue, 1 part glycerin, 1/2 part alcohol, 1/2 part acetone. Second try was just straight up super soft 2. Third try was a combination of the 2. All the platens were pressed in a vacuum bag. Next go around I am leaving them open air with platens sand bags. None of my methods left the sheets super flat or leather soft, still pretty brittle to work and tape at this point. Thanks, I got to get more veneer on the way soon...
I haven't read Scott's book (though I have seen some of the pictures -- Wow!) so I don't know what procedure he gives; also, it's been about twenty years since I've used the homebrew softener. We use GF-20 from Veneer Systems, but I'm guessing that Super Soft is the same stuff or pretty similar. In any case, I think they all rely on glycerine for the softening action.
When we use GF-20, we thoroughly wet each veneer leaf and allow the liquid to flash off (about 1/2 hour, I believe). If using a cold press (such as a vac press), you would then want to sandwich the veneer between two good flat cauls (3/4" MDF minimum) and press overnight. Forget about "a couple of cinder blocks and some plywood" for pressing -- you need more force. We don't usually put paper between the veneer leaves, but with the homebrew I think you would need to use paper to keep the PVA from gluing the leaves together. After flattening the leaves you usually have a day or two to press your veneer before the effect wears off. Sometimes it never wears off, go figure!
Is that what you are doing?
Is the Super Soft diluted properly? GF-20 has to be diluted and if it is not it can interfere with bonding.
Is the veneer dry enough after flattening? Too much moisture left in it and it isn't going to bond.
Is your Ultra Cat fresh? Again, we don't use this product but it looks like a powdered urea resin glue which is a good choice for this project. However, that type of glue has a relatively short shelf life. This is personal preference, but I like Unibond for a cold-pressed urea. I would not use a PVA like the Better Bond for a crotch -- not rigid enough (buy me a beer and you can ask me how I know this...).
Are you leaving the panels in the press long enough and at a high enough temperature? Even cold press glues have minimum working temperatures, and urea needs a baseline to kick off the chemical reaction.
Lastly, you are using a platen or caul on the veneer, right? You can't have the bag push directly on the veneer and expect this to work.
If you answer these questions I and the others on this forum will try to help you work through this.
I'll ask the obvious - What is the temperature of the shop, materials and glues? At 9,000' and Winter, that may be a problem. I can't use any PUR type glues unless we are 72 degrees or higher. Even 65 degrees will give failed bonds.
Just a couple of reminders:
You need to dilute GF20; if you mix your own softener, you don't want to add too much Glycerin.
You need to dry the treated material to 6-8% MC under pressure. We use a hot press to do this and check with a moisture meter. You can use paper to absorb moisture, some people use flake board in between each sheet.
Try a sample before doing the whole project.
I warm my shop up in the mornings of glue ups to 72 to start the acclimation process. After cutting and taping the veneers, a minimum of 4 hours elapses before I start to mix glue. Any other day or night the veneer room is a constant 64. I am trying some theory's on another small test piece this weekend with the ucat.
Adding to what David said...
During the winter months, in addition to bringing the shop temperature up to above 70F, I also use an electric blanket on top of the workpiece while it is curing. If it is really cold, I add a comforter on top of the blanket to keep the warmth from escaping into the shop air. It can get quite warm (90F+) in there. This also accelerates the cure time for the Pro-Glue that I use: a win-win.
I have a flip-top press, so I also sometimes heat up the table with the electric blanket ahead of time. If I was using a bag I might consider wrapping the entire bag with the blanket once it was under vacuum. (I'm not sure about this, though. I have never used a vacuum bag.)
I have been taught that once an attempt is made to cure the glue when the pieces are too cold, there is no recovery. The glue will never cure no matter how long one waits.
Also, note what David asks. "What is the temperature of the shop, materials and glues?" It is not enough that the shop and glues are warm enough. If the materials are too cold, the glue will get cold once it contacts the materials. It may be overkill, but I acquired an infrared thermometer just for checking my veneering projects. (I have use the IR thermometer for a variety of other things since as well.) The cost of one destroyed project was much more than the thermometer. There are some online for less than $20.
Charles and I are thinking alike. I also have an IR thermometer to see what temp things really are. It is handy for all sorts of things - bearing temps, motor temps, the temperature of door finished surfaces - and glues.
I agree that if the glue gets chilled-short term cooling for just a few minutes, it will affect whether it cures/bonds.
And ditto on the electric blanket. we use a bag and platen and laying the blanket over the layup in the bag, and then a packing quilt or two really accelerates the Pro-bond, with no shortcomings.
I believe any veneer bond issue I have ever had is due to temperature.
You gave us an idea what the air temp is in your shop. What are the material temps? MDF core temp doesn't come up that quickly.
Thanks guys, it is tricky veneering when my wood shop is -10f when not working in it, and my veneering room kept at a modest 64f. I took your recommendations and ordered an electric blanket and a ir thermometer. I was using a digital thermometer placed right on the substrate, but this may have been the missing link. I can not bring myself to keeping a 75 degree room temp., to costly. Going to try some experiments in the next couple of days. ps. a $700 loss is big, but that would be about two months expense of bringing one room up from 64 to 75.... I will let everyone know how my tests came out.
I have the advantage of coming in later and reading the responses. Sounds like glue starving. The substrate is eating up the glue or not enough to begin with. MDF is okay for the substrate if you haven’t removed any of the original hardened surface. Wiggle wood is notorious for soaking up the adhesive.
I'd consider covering the materials the night before with a tarp and leaving a small heater under and then covering again with the heater while curing. In that way you wouldn't have to heat your whole shop as warm. We use a large canvas painters tarp with the glue up on saw horses and the heater. Works quite well.
It sounds like a bonding issue. Typical causes are low/uneven pressure on the veneer, not enough glue, or the glue not having enough time or temperature to set properly. Veneer doesn't need to be real flat and the size of the panel shouldn't matter. What else is different between the sample and full size panels?
I use flip-top vacuum presses. I press with the veneer face up using a 1/4" MDF cover sheet. I figure this is the only way to insure even pressure on the whole veneer face (especially when veneering torsion boxes). No substrate is perfectly flat (run a flat file over a sheet of MDF) and ~10 psi isn't enough to perfectly mate a 3/4" substrate to the table or another piece of 3/4" (veneer between). It sounds like it works, but I'd think the glue line would have to be relatively thick (I've never had squeeze-out run down the sides). I use Titebond and would rather minimize the the moisture, glue line, bleed through and set time. Pretty much all the purported PVA negatives only apply to excessively thick glue lines. The all direction porosity of the MDF cover sheet also provides more efficient edge to center evacuation of higher pressure (lower HG) air.
The OP says he is also at 9,000 ft altitude. That means the atmospheric pressure is about 2/3 of what I have at 900 ft altitude.
10 psi vs 14 psi - more than I would have guessed. I think he would need about 28-29hg or more to get up to 14 psi.
Good point, David, but he can't ever get more than atmospheric pressure using a vacuum system -- that's the nature of the beast. So if one atmosphere at 9K feet exerts 10 psi, 10 psi is the maximum pressure he can get. This also assumes a perfect vacuum, which none of us can really get. In other words, down here at sea level I don't actually get 14.7 psi -- though I believe it's close.
Re: the other Dave's comment that "veneer doesn't need to be real flat" I guess I would respectfully differ after 25 years and a few square miles of the stuff. Especially crotch veneer, which I have seen make grown men cry!
IIRC 21HG/~10psi is recommended. The OP is a little low at 19-20HG... I couldn't find the exact number, but HG x .491 is pretty close to PSI.
Titebond has a claimed strength of > 1,500psi at 150 degrees. If the veneer is well and properly bonded, the MDF (< 900psi sheer strength) should fail before the glue line. I could have disasters waiting to happen, but I cannot imagine that pressing wavy/bumpy veneer will be the root cause any failed bonds.
I only have a handful of years veneering and one immediate failure (a quick and dirty sample w/ too little glue or press time). I've never veneered crotch and I'm curious, are we talking immediate glue line failures, time based failures, or what?