Warping Plywood Cabinet Doors
Cabinetmakers agree that plywood is not suitable for making doors because of its tendency to warp. For flatness and stability, MDF-core material is a better choice. May 13, 2008
I've made some slab doors out of fir plywood for a small kitchen and I'm concerned about the doors warping, as some have started to cup slightly. The doors haven't been sprayed yet, and have been in my shop for several days since making them. They are going on frameless cabinets. The good news is that the majority of the doors are small (approximately 15" x 24") with only 6 larger doors. Also, I have the option of storing them flat for awhile as the install is a month off. Short of remaking the doors, what's the best way to counter the warping? I'll add an extra hinge to the larger doors, which may take some of the curse out.
From contributor R:
You said you built the doors from fir plywood. Do you mean construction grade fir? That stuff is not for cabinets and you can expect it to warp. The primary use for construction grade plywood is roof and wall sheathing where warping isn't an issue.
The sheet goods that I have had decent luck with are Classicore or MDF core. Even PB core is reasonably stable. The way that you store your plywood can also be a factor. Best is to lay it flat, as you mentioned. Make sure that both sides of the plywood have the same veneer thickness.
The question of how to stop the warping is a good one. But there isn't an answer that I know of. Of course, finish it completely to help prevent more warping, but you knew that already, didn't you?
From the original questioner:
The plywood is A1, purchased as vertical grain fir veneer with no additional layup - cabinet grade. I'm thinking I may do 1x bracing on the back of the doors, in either an x or z pattern? My only other thought was to use a free swinging hinge and some type of catch for the top and bottom of the door, though I haven't figured out the type of catch to use. I'm thinking I may add a hinge to the smaller doors to help flex them somewhat if needed.
From contributor H:
In my experience, never, ever use plywood for cabinet doors. If there are not that many, then don't waste your time finishing them, if you can then order the same veneer on an MDF core and the customer will not know the difference after edgebanding. There is no plywood today at any price that will stay flat, guaranteed. Lay them flat with weight on top and they will still cup again. If you have no choice, then the third hinge may help, along with bumpers at install, but try to avoid replacing after installation as it brings all the rest of the work into suspect in the consumer's eyes and you will regret it. I talk from experience.
From contributor C:
My maxim is to always use three hinges when in doubt. Cheap insurance.
From contributor T:
Skip making the X or Z. Buy some veneered MDF. Save lots of future trouble. It will finish better and stay flat better.
From contributor S:
I agree. I would replace everything with MDF core. As a general rule, I use MDF core for doors and carcass parts that need to be dead flat. Any attempt to compensate for the warping plywood is bound to lead to callbacks. Don't waste your time buying and installing more hinges or trying to apply different amounts of finishes to each face. Lots of us have tried those solutions and they just prolong the inevitable. Count your losses now and replace them.
From contributor L:
I agree with the rest: never use plywood for doors! They will always make you wish you hadn't. Don't try the sticks on the back - it won't work.
From contributor W:
I make MDF doors on my CNC machine. Make no mistake, MDF can warp! I've found that it's usually heat, not moisture, that causes it to "pringle" (potato chips). The good thing about MDF is that you can simply turn it over and let the heat get on the other side and it will flatten out. Once hung, MDF tends to flatten out since the heat inside a house is fairly consistent. MDF doors are cheap too. You can't make doors out of plywood cheaper than I can make them for you. Shipping them to you may be costly, though.
Take the other advice and use veneer over MDF core.