unscientific ply/mdf movement experiment

[AndyT]

Looking around the house, I can see four relevant projects where I have used a lightweight softwood frame with a plywood panel glued in. I glued the panels all round so as to get extra stiffness - bear in mind that the framing was only 2x1 PAR and I was using 6mm ply, with dowels or biscuits at the corners. Three of these projects were built in cupboard doors in our unheated, well ventilated basement. The other was for doors on the airing cupboard - continuously warmed by the hot water cylinder and pipes inside.
All the doors have remained square and flat. The difference in overall stiffness gained by gluing the panels was significant - and meant that I could make fairly large doors from lightweight materials quickly and easily.
If I had been using solid wood panels, I would have left them free to move, but I would also have used thicker framing and made mortice and tenon joints on the corners. That would have taken longer and cost more. For a quick job to tidy up the basement I'm satisfied with my results.

 

[Jacob]

Looking around the house, I can see four relevant projects where I have used a lightweight softwood frame with a plywood panel glued in. I glued the panels all round so as to get extra stiffness - bear in mind that the framing was only 2x1 PAR and I was using 6mm ply, with dowels or biscuits at the corners. Three of these projects were built in cupboard doors in our unheated, well ventilated basement. The other was for doors on the airing cupboard - continuously warmed by the hot water cylinder and pipes inside.
All the doors have remained square and flat. The difference in overall stiffness gained by gluing the panels was significant - and meant that I could make fairly large doors from lightweight materials quickly and easily.
If I had been using solid wood panels, I would have left them free to move, but I would also have used thicker framing and made mortice and tenon joints on the corners. That would have taken longer and cost more. For a quick job to tidy up the basement I'm satisfied with my results.

Yebbut your construction sounds like a hybrid, false panelling, neither one thing nor the other.

A frame and panel construction relies entirely on the frame for strength, the panels merely filling the spaces in between. The panels may or may not contribute something to the strength by bracing against distortion, but this is secondary and not desirable; they have to be loose to absorb rather than resisting distortion due to changes in temp, humidity etc .

Your construction is basically a sheet material (used as door etc) stiffened by the addition of trim, which you have slotted on, but it could have been simply stuck on the surface (2" x 3/8") to the same effect. If you are doing this for door with more than one false panel, it'd be better to use the whole sheet of ply uncut with stuck on false framing, rather than slotting it into the trim as you have done.

 

[AndyT]

Jacob, I think we are agreeing about construction, but using different labels for the methods in use.

I don't disagree with what you have said about frame and panel construction.

It could be helpful to distinguish "stiff frame with loose panel" from "lightweight frame reinforced by glued panel". Both work, and have their uses.

But the point I was trying to make - which I think is where this thread started - is to offer some evidence that if you glue plywood all round (regardless of whether it is necessary or useful to glue it) you won't find that expansion or contraction of the plywood breaks the surrounding wood that it is glued to.