Preventing cabinet doors from bending

At what stage of the process are you having a problem ?

Don’t have enough information to advise you

Hi Daniele, you need to tell us how the door is constructed, is it a panelled door or composite , a picture would be worth a thousand words.

-- PJ

tausen,

Like others, I am not sure from your description how the doors fit into the cabinet or how the door is construction. But it sounds like you are speaking of solid wood doors which are either 1 wide board or several boards glued together to form a wide panel. Plywood may behave like solid wood when it comes to bending, but I am not sure. If the doors are plywood, glued on solid wood edge banding can help.

Wood movement is a very powerful force. It is difficult to overcome, even with straightening kits. Simply equalize the moisture on both sides of the panel (in this case the door), the panel should return to flat (in most cases). Keep the rate at which moisture enters and leaves the panel on both sides the same and the panel should stay flat.

Solid wood panels bend because there is more moisture (water) on one face of the panel than on the other face. The face with the most moisture will develop a crown because the wood fibers on this face swell whereas the wood fibers on the opposite face do not swell. If the amount of moisture on both faces remains the same, no bending should occur. Finishes such as polyurethane, urethane, shellac, or even oil finishes help control the rate at which moisture enters a panel. All 6 side of the panel should be sanded and finished the same to manage moisture entering and leaving the panel.

One method used in an effort to keep table tops flat is installing bread board edges. A tongue is cut across the width of the table top on the ends and a groove is cut into a board that slips snuggly over the tongue. The bread board end results in wood grain running perpendicular. Therefore allowance for the expansion and contraction of the top must be engineered. The breadboard can be glued to the top in the center only and pinned with dowels at each end – no glue other than that required to keep the dowels from falling out. The dowels are left free to move in slots cut into the tongue on the end of the table top.

The advantage of the breadboard is that end grain is protected and thus reduces the rate at which moisture enters and leaves the panel. It also provides some physical resistance to cupping of the center panel. However, as the center panel expands and contracts, the width of the center panel will exceed the length of the end board or the width of the center panel becomes less than the length of the end board. This is due to seasonal movement of the center panel.

I hope this answers your question. If not, post more details. By the way, your English is way better than my Italian!

Thank you for the replies.
I try to explain in which case we’d like to prevent the bending attaching 2 pics.

This is how usually we create old wood cabinet doors.
15 mm chipboard inside layers
n° 2 – 5 mm wood – external layers

Does it make sense?

“Raised panel” is my usual prevention – even for wide doors. Raised panel refers to the type of doors I can see in the lower cabinet of your picture. I do not make a wide door of plywood no matter how good the plywood is.

When I have seen the type of construction you show, there are battens added to the back side to prevent warpage. This type of construction with battens where I live is used in very informal settings like a garage or basement. Google “batten door construction” and look at images for examples of what I mean.

Perhaps other guys have more elegant options for you because I don’t think this is much help.

I hope I interpreted your question ok. Your English is good. By coincidence I travel to Bari next week for business which is a long way from you.

-- Dave, New England - “We are made to persist. that's how we find out who we are.” ― Tobias Wolff

The chipboard is not a very stiff and stable material so you need to put all the chances on your side while building the door. What is happening, assuming that the panels were straight when assembled, is that the 5mm wood external layers are not expanding or contracting at the same rate and they pull the door to the side that is retracting the most.

-Before finishing the doors, panels should be stored in a way that exposure to ambiant air is equal on both sides of the panel, stacking them on a flat surface (not the floor) with a cover panel on top for example.

-The 5mm wood external layers should be finished on both sides simultaneously. It is essential that the wood be sealed on both sides of the panel. if not it will absorb moisture at a different rate.

-The 15mm wood end strip that is nailed to the chipboard between the outer layers could be made heavier and from a stiff hardwood to act as a stiffener with no impact on the door appearance.

-- PJ

Thomas, yes that’s better.

-- Dave, New England - “We are made to persist. that's how we find out who we are.” ― Tobias Wolff

To make cabinet doors that don’t bend, requires using wood that doesn’t have internal stress and the moisture content is low. Today it is hard to find good lumber that has been aged properly. When a piece of wood is cut, internal stress causes the wood to twist or bend, usually because of uneven moisture levels. Plywood should be more stable, but even that is subject to moisture level distortion. Marine grade plywood should be best for cabinet doors, but it is expensive. I have used it when I needed it to be flat and stay flat. I have bought plywood from stores that was flat, but by the time I got it home, it was bowed 2” in the center.

I would try making several grooves on the inside lengthwise (with the grain) maybe just deep enough to get through the particle board and 1” wide.

Clamp the door down on something flat and glue some straight, strong wood in the groove.
Use a very stiff drying glue like epoxy maybe.

Does this help?

-- Everything is a prototype thats why its one of a kind!!

The groove idea works for guitar necks. Those get fancy and use square carbon fiber rods but something else should work.

-- Dave, New England - “We are made to persist. that's how we find out who we are.” ― Tobias Wolff

Thank you guys for the involvement!
I know “raised panel” is probably the most stable way to prevent bending.
Unfortunately (or fortunately) trends are changing and in our area is almost impossible to sell this door type.
Modern trend and customers prefer smooth panel. And this is not a cheaper way to build doors.
We use chipboard instead of plywood, because we find it reliable (both bends, but chipboard go back easily).
We’re still searching for the solution!
Be careful. We’d like to PREVENT bending, not repair.

Thank you