Stacking Laminations in a Vacuum Bag

Stacking multiple pieces in one vacuum bag to save time increases the likelihood that pieces will not be under sufficient pressure. March 16, 2015

Question (WOODWEB Member) :
This is basic to vacuum veneering, but I have never found an answer. When putting several laminations of the same size in the vacuum bag, one can place the assemblies around the bag so each assembly gets maximum pressure - the way I have always done this. Recently I had a discussion where I was informed that all the laminations can be stacked on top of each other and the pressure will still be the same.

Forum Responses
(Veneer Forum)
From contributor C:
Yes, you can stack your laminations and the pressure will be the same if everything you are laminating is dead flat. If the atmospheric pressure exerted where the bag (or table if it is a fliptop) touches the work has to work against warped or out-of-flat layers it will diminish the amount of pressure in the middle of the stack. Another important thing to remember when stacking is how this whole vacuum thing works. It's not the bag that exerts pressure on the work. It's the differential between the pressure of the atmosphere outside of the bag (always exerting 14.7 pounds per square inch on those of us who live near sea level) and the near-vacuum inside the bag (0 psi if we could pull a perfect vacuum). The bag (and/or table) just allow us to create and make use of the differential.

Why is this relevant? The force created by this differential exists only at the barrier (bag/table), and it is transferred to our work only where the bag/table actually touch our work. Therefore, it is important to make sure that the bag touches the work in as many places as possible. When stacking there is more likelihood of tenting (bag stretching and missing areas of lower, larger areas); there will be no force exerted on these tented areas where the bag does not touch. It might not be a big deal when you are laminating thicker material because the material itself might distribute the force sufficiently, but it can be a problem if you are pressing thin veneers.

From contributor B:
The only time I've ever had veneering failures is when I stacked components to save time. It might work fine for a stack of two but didn't for me. I do one layer at a time and have no problems.

From contributor V:
I would advise against stack laminating with veneers, the reason being that veneer can vary in thickness across the cross section. You will get good pressure on the high spots and only good pressure on the panel that is being pressed by the bag or membrane. So if you stack five panels you could potentially have areas of no pressure on the panels below the top one. Stack laminations with melamines and vinyls are fine because the material is uniform thickness.