Bowing in a Panel Glue-Up

Woodworkers consider why a one-inch thick shelf made up of a 3/4-inch wood layer glued to a 1/4-inch MDF layer came out of the vacuum bag bowed. October 13, 2012

I had a weird little problem this week gluing up some shelf blanks. I glued up about fiveshelves using 3/4" and 1/4" material to get to a 1" shelf. This is something I do regularly, and all the shelves came out of the vac bag bowed! I checked to make sure the platen was flat and the base was solid and everything checked out, so I glued up another batch and they all came out bowed, and all in the same direction.

Then it dawned on me what I was doing differently - the 1/4" material was MDF going onto veneer core ply! Now I've done plenty of shelves over the years and never had this happen. Usually though it's either VC onto VC or MDF onto MDF, and usually it's to get 1-1/4" material. This time I just happened to pull the 1/4" MDF without thinking about it.

I have to say I'm a bit surprised as I would not have thought the MDF would have wanted to move like that. Anybody else ever have this happen? I'll be cutting up some new stock this weekend to get the next batch ready to go, but I'm still surprised these moved as much as they did. They moved a solid 1/8" over 32" in length.

Forum Responses
(Cabinetmaking Forum)
From contributor O:
Call it serendipity and tell the customer that you planned for the eventual sag in the shelves and pre-loaded them for the books they will carry! You are better than you thought.

From the original questioner:
These won't be tossed though. No way, all that nice cherry ply is going to make some mighty fine shelves for the shop. I never seem to have enough time to make things needed for the shop, so that's where all the mistakes end up.

From contributor P:

Not really surprising - different materials gain/lose moisture at different rates. Can a panel made up of two sheets of plywood truly be called balanced? Plywood is always (I think) made up of an odd number of plies. When you glue up two sheets it becomes an even number. Also, looking at the core of 1/4" ply, the center core ply is a different thickness than the plies that make up 3/4" ply. So, I'd expect that would have an effect, too. Maybe you've just been lucky all this time?

From contributor O:
I think you asked for trouble with the dissimilar materials and thicknesses. I have done two layers of ply (for an even number of plies) with no problems, but then I think that after a certain number of plies the total is immaterial, as long as there is balance in each, and each the same material. I'd do 1/4" in between 2 - 1/2" , or 2 -3/4". Better yet, I like thick shelves to be torsion boxes. It always surprises the customer when they realize the shelves are so light and easy to move.

From the original questioner:
Contributor P - you may be right, but if that's the case I've been lucky a lot. I think it has more to do with starting out with two panels that are fairly flat and similar materials. Once glued together I think the thickness helps maintain the flatness and compensates for the even amount of veneers?

Contributor O - you've got my interest up with the torsion box idea. I have to imagine it's quite a bit more time-consuming than just gluing up slabs in the vac bag though. Or do you have some good tricks you wouldn't mind sharing?

From contributor B:
Don't know how Contributor P makes torsion boxes, but in the strawbale housing business, box beams are often used at the top of the bale wall to distribute the weight of the roof over the wall. They are typically the width of the bale wall, or a bit less. (Some builders like to put a wire chase on the inside edge).

Construction is, I think 1/2" OSB for the faces, possibly two layers of 3/8" osb with 2x4's on edge separating them. The cavity is stuffed with insulation. Ikea has a whole line of shelving that is made like a hollow core door. They are light.

I saw one approach to shelving that looked interesting. He took the standard metal track plus the bracket for the shelves, and fastened to a 2x2. At the top he fastened a horizontal chunk that was a shelf width plus a bit long. He tacked a scrap of carpet to the end. Each unit in use looked like an upside down L with the carpet nose against the wall, and the track facing the wall - very much like a one legged scaffold support.

Setting them up went like this: Stack all the shelves next to the wall. Put two frames up 1/4 and 3/4 shelf length apart. Put the top pair of brackets in. Pick up one shelf, put flat to the wall, raise it up, pull the scaffold nose away so that the shelf could fit between the bracket and the wall, and place. Repeat for the other shelves.

Bring up the next scaffold. Put in place. Lift each shelf enough to put the bracket in. Repeat for the other scafflolds.

It was a bit touchy he said until you have one shelf of books on it. At that point the weight of the books is trying to tip the whole thing into the wall. He claimed that he used this for years when he was moving frequently. No marks on the wall from the book case, and they disassembled into enough pieces that they were easy to move.

From contributor U:
It is curious that they came out of the bag bowed. Perhaps there was tension in the 3/4" core material that was used? I do a lot of shelves and panels that are 1-1/2" to 2-1/2" thick. I don't put the assembly in the bag due to the pressure in the bag causing hollows between the ladder core. There are times when I will put the entire assembly in the bag but only if I've used a honeycomb core.

My general work in this area is laying up veneer over MDF, putting it into the bag and then gluing the panels over the core on a flat surface using weights. I'll cut relief slots in the core and drill holes at the extreme to allow breathing. Then there are those situations when I have to do a span of 10' where I'll embed steel or aluminum in the core.

From contributor R:
I'm surprised it hasn't happened until now. I think you have to treat the assembly as a veneered panel and glue something to both sides to balance the stresses created by the glue lines.