Vacuum Frame Press not getting flat panels

[Ian]

Hi I was hoping someone might be able to help.

I have a frame veneering press, which has a suction hole at each end.

The bed is flat but when I veneer a panel and put a ruler across it looks like this at the ends (the ruler is pretty straight).

I do use a breather fabric

My thoughts are that with the suction being at its strongest at the outside the pressure in the centre of the panel would not be as strong.

Is this right?

If so what would be the solution?

I had thought of making a MDF cover with holes and channels to even this out

Underside view

Upper view

Your thoughts would be very much appreciated

Cheers

Ian

 

[jasonB]

Are you veneering one side or two, if one then the moisture in the glue will be swelling the top face of teh ply making it cup . You MUST veneer both sides

If its not cupped but thicker in teh middle then it may help to do as you say or put some strips of similar thickness waste material around the edges so the bag does not pull donw on the xtream edges

Jason

 

[Ian]

It has been balanced by veneering both sides.

I had a thought that if I reduce the pump pressure this might help.

I will have to test that out.

Cheers

 

[yetloh]

I wouldn't reduce the pressure, the most you are going to get is a bit less than atmospheric which is only about 14psi. I have experienced this with vacuum pressing and I believe it is related to the fact that the bag (or frame film in your case) tends to pull down at the edges of the piece first, unlike in a screw or hydraulic veneer press. This means that excess glue and any air between the veneer and the substrate can get trapped. I got around this in two ways; first by keeping the glue film as even (spread it with a roller) and as thin as possible while still having enough to get a proper bond so as to minimise the need for squeeze out. Second place a thick ply caul on top of the veneer between it and the vacuum film. Make sure that the caul has heavily rounded edges on its lower side to minimise early pressure on the edges.

Hope this helps.

Jim

 

[Ian]

Thanks Jim quite interesting.

Yes I agree the pressure is to the outside first and is causing issues. I will try what Jason said regarding some material the same thickness around the outside to so it gets the outer most pressure.

cheers

 

[tomatwark]

Is that WBP ply you are using?

If so try something else as the quality of WBP is not as good as it was with lots of air pockets in it and I have also found it warps after it is cut.

I will no longer by it as it just causes no end of problems.

Tom

 

[Ian]

6mm birch ply

 

[seanybaby]

6mm birch ply

I think that might be the problem. Where i work when we veneer 3, 4or 6 mm MDF,we never use ply, it always seems to bend to some degree. As soon as it comes out the press we stick it under a stack of boards and leave it until the next day. However this is with a heated hydraulic press, so may not be your problem at all.

 

[tomatwark]

6mm birch ply should be fine.

Have you tried putting the veneer with the grain running at 90 degrees to the grain on the ply?

It may the thickness of the ply, I would also try your suggestion of a board either side, but i would have thought you should not need to cut channels in it though.

Tom

 

[Ian]

I do run the veneer 90 degrees to the ply grain.

I have a few things to try and will post my findings

cheers
ian

 

[Ian]

Ok this is what if found out and you were right.

The ply wasn't flat to start of with, I had found that sometimes I was getting flat results and sometimes not. The reason for this was that sometimes I had placed it into the press concave up and sometimes down.

I pressed a piece that was warped with concave down and with the extra forces on the outside of the piece it was never going to be flat, I tried a piece with the concave up and it came out perfect.

I will move to MDF now as it is flatter but at least this clears this up as to why I was getting varying results.

Thanks for alll the replys.

Cheers
Ian