Question about vacuum veneering before I ruin this table

I recently bought and assembled the V2 Venturi Vacuum pump kit from Joe Woodworker. I haven't used it yet, but I tested it with just the bag and it seems to work exactly as it is supposed to, so we're ready for the maiden voyage - a 3x3 table top. I'm able to detach this table top easily from the apron and base, so what I've got is just the top, a solid core with face and bottom veneer.

My question: do I need platens at all if I have enough breather mesh to cover both sides of this table? I haven't made any platens yet, and due to time constraints, I probably won't make any until I need them.

I was thinking that I would just wrap the table top in the mesh, then place it into the bag and proceed with nothing else, but I've never done this before and although I understand the purpose of the platens from a physics standpoint, I can't seem to wrap my head around their concept from a logical standpoint. In other words, if this were road engineering class, I understand the purpose of a stop sign, but for some reason, I'm having trouble selecting logical locations to place them.:o

Thanks!

I highly recommend doing at least one test piece before attempting your table. There are things that you'll learn through experience, that you may not get right the first time.
Personally, I always use a platen, but I've never tried using just breather mesh. A platen imo gives a better chance of success.

I do lots of veneering. For instance, a three weeks ago I applied about 150 sq ft of veneer in a bookcase project. I use a vacuum bag. I never use a breather mesh, and don't use a platen inside the bag either. Air atoms are tiny little things, and will find a way to get over to the bag exit, even without the mesh. The atmospheric pressure squeezes them out, and squeezes the veneer tight to the substrate. There's no need to get fancy.

Using the mesh without a platen will create small indents in the veneer from the intersecting points in the mesh. System works great.

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Originally Posted by Gerry Grzadzinski

I highly recommend doing at least one test piece before attempting your table. There are things that you'll learn through experience, that you may not get right the first time.

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This.

I've done both with and without a platen. I don't use a platen anymore. I have not found it value-adding for the effort it takes to use it (or make it). I don't use much breather mesh, either -- just a tiny strip from the vacuum inlet to the edge.

Jamie's right - air will make it's way over as needed without anything. The only reason I use a little strip of mesh is to make it evacuate just a little quicker.

Dan's right, too - the mesh can leave a faint pattern on the surface but you usually need to scrape or sand the surface before finishing anyway, so that's never been a problem for me.

First I went the platen route but my inexperience cause a lack of support one time so things broke on me. :)

Then I went the breather mesh and that has worked great although can be a pain to use trying to slide it in (tape it down if possible...not always possible). It can leave marks but depends on the wood species and your glue but I, like Jason, never found it a problem.

I may just try nothing since it sounds like a few folks have done just find that way.

One thing you will want, however, is a good flat surface to set your bag/project on.

As to how fast air escapes, usually with flat projects, faster is better. However, if you have curved objects, slower is usually better to give you time to smooth the bag out as air escapes. Creases can form and they can be bad if too large or a void develops where it isn't fully pressing on the veneer. Definitely do a dry run to get the feel for how your system performs.

My bag gets used for lots of things, some involve thick skins on door core, which leaves little dimples in the poly bag, something's I have little bits of squeeze out left in the bag....so so there is always something between any delicate veneer and the bag. Sometimes it's just an 1/8" skin, or 1/4", or whatever drops I have that fit the work. I've used breather mesh, pretty good stuff for lots of applications, not always fun to place a large panel in the bag with breather mesh in play, and I don't trust it touching delicate veneers that are thin either. It always wants to slip of the work but clings to the bag, I finally cut the welded end of my bag off and bought a second closure so I could just pull it through over larger panels, got sick of "bag diving".

The bag is a clamp. I doubt many here would start clamping delicate assemblies with solids without putting pads between clamp heads and work....why do the same with a bag? At the very least I use a criss cross bottom board to let the air escape quickly and a flat platen on that so my show face is dead flat. One of the best ideas I picked up was to ditch the expensive breather mesh and just use fiberglass window screen material. Cheaper, just as effective at evacuation, easier to tape to the work and keep it there, you can even staple it to the edges if required to keep it put. If the work is highly porous like veneer on vc plywood a bag alone mAy evacuate well, or it may leave voids that are a great joy to fix, I've watched a bag leave inevacusted spots on less porous stuff like phenolic backed veneer.

I started with a grooved bottom platen but found it unnecessary for reasons others have stated. Now I use a solid bottom platen. A sheet of plastic goes on that. The veneer/substate/veneer package goes on top of the plastic, then another layer of plastic. I put breather mesh on top of the that and extend it under the vacuum port. Window screen would work just as well or better as Peter said, and would be cheaper, too, but I have the mesh and old habits die hard. I don't find it hard to use. I use 1/16" shop sawn veneer almost exclusively so there's no worry of the breather mesh leaving indentations.

One improvement I need to implement is to get some thin foam rubber of the correct durometer to act as a pressure equalizer between the bottom platen and veneer. If my shop sawn veneer is not sawn/drum sanded perfectly it won't be the same thickness everywhere. When that happens the pressure won't be even against the rigid bottom platen and the bond might not be good in those low pressure zones. I've had a couple of seams open up days/weeks after pressing; not good. A thin piece of foam should fix that problem.

John

I use the breather mesh alone (no platen).

I use a 1" platen.

My laminations are > platen/1/4" MDF caul/plastic/veneer/core/veneer/plastic/1/4" MDF caul...

The plastic will keep the interior of the bag clean, keep the lamination from sticking to the cauls :) and a top caul produces a "flatter" surface than a bare bag, especially with curly veneers.

No need for breather mesh and I get similar results using the grooved surface or the flat surface of the platen. Perhaps a couple seconds quicker to reach vacuum using the grooved side.

The platen is necessary to give you a flat reference point.

Attachment 322819

Cheers, Don

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Originally Posted by John TenEyck

One improvement I need to implement is to get some thin foam rubber of the correct durometer to act as a pressure equalizer between the bottom platen and veneer. If my shop sawn veneer is not sawn/drum sanded perfectly it won't be the same thickness everywhere. When that happens the pressure won't be even against the rigid bottom platen and the bond might not be good in those low pressure zones. I've had a couple of seams open up days/weeks after pressing; not good. A thin piece of foam should fix that problem.

John

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Try some cloth canvas from the local fabric store where the women tend to shop. I learned this trick from Paul Schurch in a class I took from him up in Santa Rosa...at David Marks' shop. :)

If you're using commercial veneer, it is so thin that air goes right thru it. But shop sawn is a different matter; your inlet port is likely to seal itself off without mesh. A small piece of window screen will do the trick. I do nothing but very small parts and always use screen as there is no point in risking failure. Besides, it is very easy to do. All the mesh needs to do is channel air to the port from one side or edge of work piece. Nothing more.