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Balancing Glue Volume in Vacuum Press?12/21/15
I work for an architectural millwork company that often laminates custom solid-core doors and presses them in a vacuum press.
We've had large doors (10'+) warp slightly (1/8" to 3/16") occasionally and we've discussed the cause at length. Currently, the owner is emphatic that the glue previous warpage was caused by applying unbalanced/unequal amounts of glue to the front and back of the door core.
The claim is not that the joint is starved, but rather EXCESS glue is applied to one side, causing increased moisture to warp the panel.
I'm unsure of this claim because it seems to me that, under vacuum pressure, the excess would be forced out from the joint, leaving roughly equal amounts on both sides of the core.
We do not have problems with trapped glue and bubbles in our laminations.
Is this claim plausible?
What are you using for adhesive?
In my experience laminating panels with PVA in a vacuum frame press, how I handle the panels after pressing is as important as glue application to ensure flat pressings.
"What glue are you using?"
We're using Titebond I and III to apply paper-backed veneer, 1/4" solid, and G2S sheets to particle-core door cores.
"Don't leave the doors in the press beyond pressing time. Both sides of the door must have equal exposure to air to allow even evaporation of glue moisture. Guard against fans or heaters blowing on one face. If you are going to stack doors, sticker between doors and cover the exposed face with a sheet of ply with sticks for a stand-off."
We follow all of the practices you've listed.
"It wouldn't hurt to ensure you have consistent glue application with a hopper glue roller, I use a 6" Pfohl roller. It speeds up the process and will probably save you glue."
This is exactly my question: would excess glue really affect the process because the press should be eliminating most of the excess, therefore glue cannot be applied in 'unbalanced' amounts.
Typically we use a scraper to spread the glue out broadly while a co-worker follows with a fine bristle roller to even out the coverage.
In my opinion I would think that too much glue will cause problems. A vacuum press is not like a hydraulic press in that the platens of a hydraulic press are massively stiff and flat. As pressure is applied the press has the power to squeeze out excess glue. But a 5/8" sheet of melamine on top of your glue up is flexible and may not squeeze all the excess out.
The too much glue potential problem has been covered.
Cure time is another potential. Tight bond 2 or 3 I would let cure in warm environment 24 hours, then take it out and let it breathe equally on both sides, stickered flat.
Are you guys using a flat tortion box platten kerfed for air evaluation and even pressure distribution? Any breathable mesh? This will help kep the material dead flat during the pressing.
Uniobond 800 is a great adhesive, but more expensive. It's rigid so you will have much less chance of warpage.
If your glue is fully cured, you will be fine in what you describe. Add some heat, and add some cure time...
We do a lot of doors with raw veneers in hot press and the thing causing panels to warp is the fact that one side gets the glue and the veneer first and starts to swell and the panel is unbalanced.
It gets worse if the venner is laid horizontaly on the doors.
The panel will always warp toward the side that gets the veneer and glue first.
Unfortunately I have had this very exact issue. Itís very possible, we had the exact thing happen and tore several laminations apart to see what the exact cause was and it was uneven/assess/ or too little glue. We found that the actual layer of glue is critical to a proper lamination; we started using a glue roller system. They are a bit pricey and I always wondered why anyone would pay that for a silly clue spreader. Now I know. The exact same issue can wreak havoc under veneers.
We did not think it to be possible either until we all had a powwow like you guys and decided to just tear them up. We all agree the issue was as mentioned above, changed to glue spreader or a specialty nap roller but even then with that you much be very careful to be certain you are applying roughly the same amount of glue (millage) to all surfaces. Best of luck.
I'm pretty sure a vacuum press will not squeeze out the excess glue. Therefore both sides need uniform coverage. 1/4" solid is too thick to use as a veneer.