Troubleshooting Bowed Hollow-Core Doors

A set of custom door panels made with plywood glued over honeycomb came out with a half-inch hump. What could have gone wrong? March 1, 2006

I needed to make (2) 1 1/8" doors for a pantry unit. They were to be bypass sliders about 30" x 84". I ordered 3/4" honeycomb from a veneer company for the core. I used 3/4" MDF around the perimeter with the infill being the honeycomb. The skins were 1/8" maple ply. I took them to a local millwork shop and they put them in their vacuum bag. The long and short of it is… they are both warped with a 1/2' crown down the length. They were carefully stored, so what went wrong?

Forum Responses
(Cabinetmaking Forum)
From contributor P:
It depends upon what kind of glue they used, and how they applied it. If they used a PVA and applied it to the skins and not the core, then a heck of a lot of water got trapped inside the panel. This can cause all kinds of problems. You can also get trouble if you glue up the skins one side at a time, as then water is trapped on the second face to be glued up. You can also have trouble with improper sticking, drying, and storage procedures of correctly glued up panels. And last but most obvious, were the veneers on both sides exactly the same? Hollow cores, in my experience, are quite tricky to make. Maybe others have had better experiences.

From contributor F:
What you are doing when making a hollow core with a skin on each side of a core is laminating. Laminating can be made straight or curved depending on the forms that are used. The first thing I would check into is what preparations the millwork firm made to insure the doors were registering on a flat and rigid surface when the vacuum was applied.

From contributor T:
I agree with contributor F - the panels will only be as flat as the form it was pressed on. In our shop we made a torsion box made up of a grid of 3/4" ply with faces of same. Used great care to insure it was pressed flat in one plane. We use this to set our vacuum bag on, and we get consistent, flat, warp-free doors. I have it rigged with cables to raise it up to the ceiling when not in use.

From contributor W:
Vacuum veneering on the torsion box assembly table is the way to go. Perfectly flat results very consistently. And, if necessary, the "above and beyond the call" measure is clamping using cauls to hold the whole glue up (in the vacuum bag) flat against the surface of the torsion box. That doesn't add any meaningful clamping value to the pressure of the vacuum bag, but it does insure that he whole works is held tightly against the torsion box, which is perfectly flat.