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Biscuit expansion in MDF, no water, no glue6/5/14
Any one seen anything like this? Been doing this work for 30+ years and new to me. Glueing solid wood edge to walnut veneered MDF panel. All good, sanded flush looking right, feeling right with finger touch. In the finish shop, once solvent based CV applied, 50% of biscuit joints along the grain swell. Even joints at the field assembly joints where there is no biscuit, no glue and no exposure to any moisture except what is naturally I air. Any thoughts? My veneer supplier is recommending crossbanding all panels, but I'm already getting push back on pricing.
I'm not a biscuit user, but I have heard that they are over-dried to insure good and rapid expansion when hit with water based glue.
If that is true, then perhaps as the biscuit came up to the same MC as the surrounding MDF, they swelled, giving you that nasty little rise. Assuming they were tight fitting.
I prefer biscuits with honey-butter or gravy.
If you used the " ultralite" or lightweight MDF, swelling from relative humdity is probably your trouble; I've seen what you have before and was just as surprised by it. You have to drop the slots to the lower quarter of the thickness and they still might telegraph. Believe it or nuts, even MDF is being made cheaper these days- especially the Asian stuff. Try switching to the regular weight stuff. Of course, you'll have to listen to the installers bitch..........
Peter, you state "Even joints at the field assembly joints where there is no biscuit ". This makes me wonder about the biscuit being the culprit.
Here are a few more possibilities:
1) When the biscuit slot was cut, tension in the MDF was released (yes, there is tension in MDF, especially close to the faces where the material is denser); the material remaining above the slot, now very thin, was able to deflect out of the plane of the panel.
2) After finishing, the thin material above the slot is now unbalanced -- finished on the surface of the panel but not inside the slot -- and therefore warped right at the slot area. This might have been accentuated if a catalyzed finish was used as these have a tendency to "pull" the surface as they cure.
Can't offer a solution for these panels, but it's worth figuring out what happened because if either of these is the case crossbanding in the future is not going to help. Putting the slots further from the show face (as suggested above) could help, and if you use a continuous spline the same problem would likely occur but the distortion might be less noticeable.
Given "Even joints at the field assembly joints where there is no biscuit, no glue and no exposure to any moisture except what is naturally I air." It has to be something other than the biscuits! The only variable that seems likely is the moisture gain from the air. I've seen something similar when parts were cut several days ahead of being banded, when we had high humidity. The edges would swell and then the bander would scalp because the tracers were running inboard from the edge swell. Naturally the swell wasn't even part to part. The cure? Maybe putting a coat of finish on the edges to slow and even-out the process? We've got several jobs shipping to FL soon. Taking a few extra cautions.
Sharpen your cutter.
Where is Dr Gene, that is quite the conundrum...
I think John is on the right track. If you have the time run some 12" square samples through various joining methods and see if you can pinpoint the problem that way.
If the mdf was very dry and the air was at a higher moisture content than the equilibrium of the panel The panel would absorb moisture and swell. Since the biscuit slots make the affective panel thinner there, moisture could get into the mdf faster there. I've got to admit this may be pushing the differential moisture thing pretty hard but wood really only changes size with moisture change. The fact that the field joints w/o biscuits also showed a similar condition, swelling, means the mdf was gaining moisture unrelated to the biscuits. Only conclusion, mdf was at a lower moisture content than what the environmental equilibrium level was. At this point it is probably too late to get an answer with a meter. Lots of meters don't go reliably below 6%.
I just reread your original post. This is happening in the finishing room after the solvent based finish is applied.
I suspect it is likely the penetration of the solvent through the veneer and into the MDF that is causing the expansion problem.
Although changing up on a tried and true finishing process is always risky business I'd suggest trying a flash dry sealer such as shellac prior to the slow dry top coat.
I think BH may have hit it with the solvent being the culprit. Did you lay up the veneer, and what glue did you use?
We delivered the product to air conditioned environment with promise to replace tops after remaking. The problem disappeared once placed in the a.c., so it had to be the air moisture. Thanks for all the feedback.
I didn't know that solvent would cause wood/MDF to swell.
That is cool.
You glued the slots with glue and the problem still disappeared?
Are you in a very humid area?
Dry biscuits in slots just used for alignment. Counter draw bolts below to pull together. Yes, the last 2 weeks in North Texas have been very humid.
This kind of thing makes me want to get out of this business! It's part science, part hocus pocus, part chemistry, part luck...are the stars aligned in the correct constellation, etc. Then you get "I thought you were the expert." Ughhhhh, me too thoughtum that, er.