Vacuum Pressing: Platen?

[alex grams]

I recently purchased a new vacuum press from Joe Woodworker and will be doing some desk panels for a project. Some of the veneer will go over 3/4" plywood substrate for a desktop. I purchased the breather mesh, but they say you still need a 3/4" platen at the bottom of the project with the mesh for a flat reference. Assuming i do this on my workbench (which is easily the flattest large surface in my shop) would I need a bottom platen for flatness reference if my substrate is 3/4"?

The larger 3/4" substrate panels will be about 28x50".

I will also be doing some smaller panels (24"x24") on 1/2" plywood, and a 3/4" substrate would be advisable for those since the substrate is thinner and less prone to hold flatness.

Thanks in advance.

[john bateman]

Usually the bottom platen has some grooves cut in it to help with evacuation of air. Maybe if you put the mesh both under and over your panels, you can eliminate the plate.

Personally, I like having the platen there, as it give a smooth surface to slide your panels into the bag.

[Jamie Buxton]

I almost never use a platen. Really the only time I use one is when I'm making door panels. Door panels really have to be flat by themselves, because there's no structure around them to hold them flat in the furniture. So for them, I use a platen, and I veneer both sides. And my platen is a 3"-thick torsion box, so I know it will stay flat instead of being distorted by the workpiece. In your case, the veneered panel is going to be held flat by the desk structure, so a little non-flatness will not matter.

I also never use that mesh stuff. Air molecules are teensy teensy little things. They'll find their way to get to the vacuum pump, even without the mesh.

[alex grams]

True, but the mesh will eliminate the need for a grooved bottom platen. Joe woodworker's website still recommends the bottom platen (non-grooved)for a flatness reference when wrapping the entire glue-up assembly in the breather mesh. My question though is if the substrate is 3/4", do you really need a bottom platen for flatness if the breather mesh takes care of air evacuation?

[alex grams]

Jamie,
One of the other advantages of the breather mesh is that it minimizes any bleed through. Some of the veneer I will be working with is going to be oak burl, so bleed through is a concern, so the mesh will also help eliminate any bleed through spreading.

Thanks for the info though. And I concur on the doors requiring the utmost flatness.

[Faust M. Ruggiero]

A lower platen is really a convenience for you as well as a tool to keep the package flat. If you use a bottom platen, you have a surface to do your glue up. Having a piece of flimsy veneer on the bottom will be a pain to deal with when you move from the work bench to the veneer bag. The mesh merely eliminates the need for grooves in the lower platen. Remember this, if your bag is connected to the pump on the under side of the bag, you have to do something to keep the upper portion of the bag from closing off the vacuum hole. That's why many systems have a brass tube that goes first through a fitting on the underside of the bag then into the grooved platen. In fact, the hole in the platen is usually where two groves intersect. I suspect if your system uses mesh on top, the vacuum inlet will also be on top. The early skateboard industry used that system with a hand operated vacuum pump to laminate the boards.
fmr

[alex grams]

I have used some of the skateboard pump kits with the hands pumps before, this is certainly a step up for me in capability.

The veneer will only be on one side of the substrate. I certainly agree If i were to veneer both sides of a piece, the bottom platen makes stacking and moving the stack much easier.

[Thomas S Stockton]

You might want to think about veneering both sides of the panel to keep it from warping. I always using a platen in flat veneering grooved 3/4" for a base and the 1/4 cauls top and bottom I just leave my press set up wit a 4'x5' platen in it and if I need it I have a 4'x8'.
My question is why fight conventional wisdom and the possibility of a major headache down the road, trust me it won't save you any time or money.
Tom

[Chris Padilla]

It is amazing how strong thin veneer is and it will pull a panel out of coplanarity so I concur with Thomas above.

My case in point:

Shop cut 1/16" walnut veneer applied to one side of an MDF sandwich made up of two laminated pieces of 5/8" for a total thickness of 1 1/8". The panel was about 17" wide by about 50" long. After pressing, I took it out and set it on my bench, veneer side up.

The next morning, the veneer had bowed the whole panel a good 1/8" -3/16" (rise in the center). I could easily press it back flat again with my hand but I was quite surprised.

[Frank Drew]

I like a bottom platen, and I'd definitely veneer both sides of a panel; no need to use your best veneer on the side that won't show, but something of roughly equal thickness.

[Van Huskey]

I tend to use a platen just for convenience, makes dealing with getting in and out of the bag easy, but have done some without it when it made more sense. I have never had an issue with veneering one side on 3/4" ply as long as it is 1 ft^2 or less, bigger than that I veneer both sides, most places sell cheap veneers for this task.

[Steve Rowe]

Use a platen. Unless you can get your workbench inside the bag, the atmospheric pressure will not force your substrate to the flat surface and there is no guarantee your plywood substrate will be flat. I use mesh on the top only and a grooved platen on the bottom. Additionally, I use a piece of melamine particleboard material to sandwich the workpiece. The glue will not stick to the melamine and is a lot easier than having to mess with wax paper as a glue release.

I have found that by using Unibond 800 or another plastic resin glue with rigid glue lines keeps any bowing to a minimum (usually undetectable) if you veneer both sides. Veneering only one side is a crap shoot; it may or may not work. I have been both successful and unsucessful veneering only one side.

Note that if you use melamine coated materials, round or chamfer the edges as this stuff is like a knife edge and will quickly cut you and your bag.

[Frank Drew]

Sometimes you simply can't use a platen; veneering a curved table rim is one example -- in one way to do this, the work goes into a long, skinny vacuum bag, like putting your foot into a sock.

[Mike Henderson]

For low cost breather mesh use door screen. Not the metal stuff but the plastic. You can buy it at any borg and you can get it by the roll.

Mike