Vacuum Veneer pros - question

Once I get these dining table halves flat again(long story, and also in another thread), I'll be using my vacuum bag to veneer the tops. On any other project I've does this on(all have been smaller), I've never used a bottom platen. I've always just wrapped the panel in the mesh, and stuck it in the bag. I was reading on joewoodworker's website that you need to use a flat bottom platen in the bag. Do you all use the platen, or do you just throw it in the bag and be done with it?

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Originally Posted by Mike Dowell

Once I get these dining table halves flat again(long story, and also in another thread), I'll be using my vacuum bag to veneer the tops. On any other project I've does this on(all have been smaller), I've never used a bottom platen. I've always just wrapped the panel in the mesh, and stuck it in the bag. I was reading on joewoodworker's website that you need to use a flat bottom platen in the bag. Do you all use the platen, or do you just throw it in the bag and be done with it?

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You can shift your vacuum bag around on your bench just as easily after you have drawn a vacuum as you could have before you drew a vacuum. Clamping pressure only works within the bag. So if you expect the vacuum veneering to help at all with your out-of-flat table tops, you will have to use an in-bag platen.

If you are using breather mesh, there is no need to groove the bottom platen. I use a piece of melamine-coated particle board with all of its corners rounded. Some wax on the top helps to ensure that nothing sticks.

I've always used a platen. Never failed me yet.

Here's a link to my Bedrails I laminate for stability when doing beds. I buy 4x8 sheets of melemine (PC) Cheers

https://flic.kr/p/cBfFYd

I stopped using an internal platen years ago. I also don't use that breather mesh; air molecules are teeny little things that find their way to the exit quite well.

I generally don't use a platen but I do use breather mesh.

Mike

I rarely use a platen unless creating shape ... but use peel ply and a breather (old socks and sweatshirts work just as well as the poly blanket). But I was thinking perhaps I should be using a platten. For those that do, especially sometimes, what are the factors influencing your choice?

i always use a bottom and top platen - grid cut in at 1" spacing - and use a little piece of breather mesh right under the nipple - just to make sure
jerry

To clarify my answer, I consider a platen to be a large sheet of something like plywood that covers most of the vacuum bag, and usually has grooves cut into it. You put your work on that.

I don't use that but I use what I call "cauls" on the top and bottom of a glue-up. Cauls are pieces of a flat substance, such as plywood or MDF, that is cut to fit the work. I make them a bit larger than the work in case it shifts when you put it in the bag (maybe 1/8 to 1/4 inch larger). I also often round the top edges of the caul to avoid a sharp edge for the bag.

So my answer, modified, is that I rarely use a platen but I generally always use cauls. I also usually use breather mesh of some kind, including window screen.

Since you are only veneering one side of the leaves there is no need for a platen. Veneer on top, breather mesh over the top, and into the bag. Any curve in the table leaf will remain unstressed, which would not be the case if you put a flat, rigid panel against either side. Do that and you risk cracking it.

John

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Originally Posted by Bill Adamsen

I rarely use a platen unless creating shape ... but use peel ply and a breather (old socks and sweatshirts work just as well as the poly blanket). But I was thinking perhaps I should be using a platten. For those that do, especially sometimes, what are the factors influencing your choice?

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I see two primary reasons for using a platen (by which I mean any flat surface at least slightly larger than the workpiece) when veneering flat panels. The obvious reason is that the panel gets clamped to a flat substrate, which can help prevent warping. The other reason is that it is easier on the bag: the corners and edges of the platen can all be rounded, which you might not want to do with your workpiece. Beyond this, using a platen underneath and a slightly oversized caul on top leaves a somewhat larger margin for error in trimming the veneer - if the veneers are a bit bigger than the panel, you don't have to worry about the edges getting snapped off & can simply plane/trim everything flush afterwards.

If you are forced to re-veneer the tops without flattening them, John's advice is valid, although I would use a caul over the veneer myself if the curve is fair. I always use a caul for veneer in order to distribute pressure smoothly, ensuring that any bag puckers are compensated for and any ripples in the veneer are forced out. I use 1/4" melamine coated particleboard for flat work and p-lam or waxed 1/8" mdf or bending birch ply for curves.

I generally use a 3/4" melamine grooved platen to distribute air and provide a reliably flat surface within the bag. It makes it easy to slide layups in and out on a 1/4"-1/2" melamine "cookie sheet" cut slightly larger than the workpiece.

This thread is a good reminder of the need for a balancing veneer to keep things flat.

I use a bottom platen, but just breather mesh on the top. With the tons of pressure that the vacuum is creating, I don't see how that helps when the massive pressure is squeezing the board flat against the bottom platen.

Well I decided to put a 4x4 piece of melamine in the bag, before I read all the suggestions that it wasn't critical! It ended up veneering just fine - thank you all.
http://i.imgur.com/wcfjX5U.jpg

Looks like it came out very nice. Nice looking veneer, also.

Mike