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Is a Stack of MDF Heavy Enough to Press Veneer?

In a word, no. Here are some common-sense explanations of why not. April 18, 2009

Question
I found on the internet a claim that a cabinetmaker placed his larger than his press veneer glue ups on a flat surface and then hand bombed a dozen five by ten sheets of MDF on top, let抯 call it a gravity press. Before I ask my question I ask anyone extolling the virtues of contact or hot hide please refrain from responding it is not about the glue, it is about the mechanics and if a dozen isn't enough you don't even have to turn on the fork lift to drop on twice the mass. My question, why wouldn't I have heard of this before?

Forum Responses
(Veneer Forum)
From contributor C:
It抯 about PSI, a lift of 40 pcs of 4 x 8 flk is roughly 80 Cubic Ft x 38 lbs per CF or 3040 lbs. If you put that on a 4 x 8 sheet (4608 sq in) it is a whopping .66 lbs per sq in. We regularly press at 60 to 80 lbs per sq in. That would require 276480 lbs of board sitting on your panel which amounts to 90 lifts of flake board to make 60 lbs per sq in. Hydraulics are a beautiful thing.



From contributor J:
Vacuum presses simply cause the weight of the atmosphere to bear on the piece - something less than 15 PSI - but even that is many times as much pressure as a stack of MDF would likely produce. There's also the problem of uneven distribution of that pressure; if the pressed piece were significantly smaller than the stack of MDF then, even though the average psi would increase, the stack could curl down and put more pressure on some of the edges while lifting in the middle. I'm sure that someone could get away with this technique now and then, in very limited circumstances and with a lot of luck, but as a general practice it's a really bad idea.


From contributor V:
Uniform pressure is the key, no matter how you press. Don't forget the veneer can be of uneven thickness which can influence the quality of the pressing.


From contributor Y:
Because a full sheet of MDF is heavy, it seems as if it would be applying a lot of pressure. But if you scale it down it is easier to see, for example, would you try to press a 1x1 square of veneer by putting a dozen 1x1 pieces of MDF on top of it? It's easier to visualize on a smaller scale that it would not be enough pressure.


From the original questioner:
Thanks for the input, but I am still not convinced it is a bad idea, anyone ever tried it? I haven't worked on a hot press since the late seventies and if I had one now I would still use veneered stock on MDF. For the most part, I can't justify the cost of a press for the few times I need one and use a company that that custom presses when I need it. It costs a lot and takes two weeks. As I recall the veneer thickness issue was a problem on the press I used, but I would only use it for Brookside type composite paper backed uniform thick veneer, it is the type that is sometimes called for by designers and I can't buy it off the shelf. I also know the importance of filling the press with same thickness product, I saw the result when a young fellow on the night shift tried to squash a tape ball in the centre of the press. My thought was to use a uf 109 type glue, good gap filling properties and leave it in the stack overnight.

I have had some good luck building platen type decks for huge work benches by half lapping and creating a honey comb and cladding the broad faces with MDF. A hoist and straps for the top platen with a lift of sheet goods on top, glue it up and drop it on for the night. The fact of the matter is it would mainly be used for flat storage of sheet goods, on lift in and one lift on top. Now let抯 assume I do a good job of constructing this and the hoist and support are twice as strong as they need to be, are we still ready to abandon this. By the way I know this isn't even close to a proper veneer press but some people use contact cement in this application.


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