Plywood Cupping and Moisture Content

      Check panel moisture content when you receive it, advises the Wood Doctor. April 16, 2009

This winter I've had an incredible time with 3/4" veneer panels warping and cupping. Just recently I tried two units of veneer panels on armor core that were rushed from the manufacturer. They arrived to me a day after the mill had completed the work. The panels were still very warm and I sorted through all the sheets only checking for warp(bow across the 8' length), not checking for cupping(bow across the 4' length). None of the panels were warped at all. A week or two later I discovered that almost every sheet of armor core had a cupping issue - anywhere from a 1/16" - 3/8" cup. Does anyone know of the best way to straighten these panels?

This is also the first winter that I've had an additional bay in our manufacturing facility that is strictly for material storage - as of now, it is not heated, but the rest of the shop seems to keep it relatively warm. I am in search of a plywood guru who can understand how these panels are getting bowed, and additionally knows the best way to straighten them. I would also like some advice on how to heat and/ or improve my materials' storage area to keep my material as flat as possible.

Forum Responses
(Cabinetmaking Forum)
From contributor J:
Others may not agree with me, but I have had the same problem with the armor core type of plywood products. At times there by far the best solution for plywood. Sometimes I'll get some that will cup quite a bit. Seems like the sheet will cup both in the 4' and 8'. I believe making plywood with younger trees isn't easy at all, vs. old growth from the past. To be fair I get domestic 7 and 9 ply hardwood plywood that will have a good bow in it. I have several 24" and 12" rips that I can't use because of bowing. Then again the Chinese plywood will do things that plywood should never, ever do. They will warp in such a way that you would think they’re making the stuff out of noodles or something. Most of the armor core products are still the best. Particle board and MDF are the solution if it's an ongoing problem.

From Gene Wengert, forum technical advisor:
The reason it is warping is because the moisture is changing after the panels are manufactured. This is 100% certain. It is my guess that your storage area is very dry, drier than the MC of the wood. For about $25 you can get a digital humidity gage from Radio Shack. Check your RH 3 times during the day. You probably need to humidity the storage area (which can be done by adding moisture or cooling the area to about 25 degrees above outside) to about 30 to 35% RH in the wintertime. If you go much more humid, than the pieces will dry excessively when they reach the customer's home or office and warp at that point.

You should also purchase a pin-type moisture meter and check the incoming MC. In this case, due to thickness variation, you will find that a pin is easier to use than non-pin. Both work well; it is just in your case that a pin will be easier for you.

From the original questioner:
Explain a little more about the MC and how I should treat material differently depending on the reading I get. Is my understanding correct that I would use the pin type MC meter to gauge the MC in the plywood as it enters my shop?

From Gene Wengert, forum technical advisor:
You need to check it when it arrives in your possession. If the MC is wrong, send it back, as warping will occur when the MC changes more than 2% MC. Most houses are around 6.0 to 6.5% MC (30 to 31% RH) right now (a few up north are drier). In the summer they may get to 9% MC (50% RH). Incidentally, it is nearly impossible to flatten much warp.

From contributor J:
Have you tried calling the mill that made the plywood? I've done this in the past and there often very helpful, and rarely get to talk with end users like yourself. Are you storing them as delivered units (flat) or vertical? How do you know they were all flat to begin with unless you open up the units to inspect them? One you get down into a unit the pressure of the sheet on top will let the plywood move to where it wants to move (or cup). The stress may already be in those bottom sheets all along and cold storage may not be the problem at all. The plywood from the mill wasn't made right.

From Gene Wengert, forum technical advisor:
As contributor J indicates, temperature is not the issue. It is 100% RH.

From the original questioner:
Just to quickly answer your question: when the units first delivered I went through both units and checked every sheet for warp but not for bow. I found very little warp in any of the sheets in that original QA. I've gone back and looked at all the sheets and found that they are all relatively straight along the 8' length, and my problem is strictly with cupping. It would have been very helpful and telling to have known if the sheets were cupped as I was pulling out of the unit or not.

From Gene Wengert, forum technical advisor:
The sheet on the top will be exposed to the drier air first, so it will be the first one to warp.

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