Need quick answer to veneering question - it's in the bag now

[Jim Underwood]

How long before I can take a veneered panel out of the vacuum bag?

I can't let this thing run all night, because it costs too much money to run this 25hp pump... (long story, I'll post pix later if you like.)

These 18 panels been under vacuum for three hours now, and I may leave them in for another hour, but I'd like to go home and shut the pump off soon....

I used Franklins Tightbond on the veneer. On regular wood panels we can usually handle and machine them a minimum of two hours in the clamps.

Can I release the vacuum on this veneer job with no fear after 3 or 4 hours?

[John Harden]

If you have a tight seal, the bag will hold a vacuum long after you turn off the pump. Since you're leaving it running, I'll assume you don't have a tight seal on the bag.

What's the temp there? If it's 70 or above, it should set up in 4-6 hours without much of a problem, though its better to leave in in overnight.

Depending on the "value" of the veneer press you're doing, you might be able to shut it off after 4-5 hours and be okay.

Just my .02

[Thomas S Stockton]

Titebond is usually good after an hour unless it is real cold, I put mine in for about 2 hours.
Tom

[Jim Underwood]

It's probably around 70 degrees right now... (well it probably was, it's now 62 as the sun sets.)

And no it's not a tight seal. Even though I used 6 mil plastic, the duct tape around the edges doesn't make for the best seal...
But the high CFM from the CNC router table's 25 Hp vacuum pump makes up for it. :D

I'm down to .9 Hg right now...

The ominous popping noise has me bothered though. I need to see what's going on under there.

I think I'll turn it off at the 3:45 mark and take a look..
It's gotta be cured pretty well by now.

[David DeCristoforo]

At 70 degrees, TB is gone after an hour. Not "dry" or "cured", but "set". The thing is, it will never really dry in a vacuum bag anyway so I pull mine after two hours. Also, FWIW, I have never been able to bring myself to trust a bag not have some minuscule leak that will inflate the bag if the vacuum is removed. So I leave my pump running the whole time.

[Jim Underwood]

Well I just checked it. It's dry enough it's not coming off. But I was afraid the new veneer would pull the substrate, and it has, even thought there was veneer on the other side. I just hope it balances out over the weekend... Maybe I should have used conact cement instead.:confused:

At least the layup and adhesion went well. The veneer layed nice and flat on the substrate even if the substrate cupped.

[David DeCristoforo]

What was the substrate? When you say it "had veneer on the other side" do you mean that it was veneered on both sides and you added an "extra" layer on one side? If so, you have just learned, firsthand, the primary rule of veneering which is to always "balance" both sides. This is called "gaining experience" AKA "reinventing the wheel", depending on how you want to look at it...

[Jim Underwood]

Well I knew the rule... I just didn't know whether it applied in this situation...

The deal was that I sanded off the veneer on one side of some 1/4 veneered MDF making it about .195" down from .230". I figured there was a pretty good chance that the the veneer left on the other side of the MDF would balance what I put on. Apparently not. :o

The customer didn't like the C part of the A/C veneered MDF, and it would require buying a minimum of 24 sheets plus another week of time to get some A/B veneered MDF. So we ordered some veneer and tried it out.

I suppose I should have used a contact cement....:(

[Mike Henderson]

I suppose I should have used a contact cement....:(
If you're using standard raw wood veneer, and not paper backed veneer, I wouldn't recommend contact cement.

I leave titebond glued panels in for an hour. In a pinch, I've taken them out after 30 minutes and they held okay.

Mike

[Frank Drew]

I've never left anything in the press more that a couple of hours; after that point the glue bond is much, much stronger than anything but the most contrary veneer can do to come unglued (assuming you don't have a starved glue joint.) A piece of wood even 1/16" thick -- a thickness almost unheard of these days in veneers -- has very little strength.

[Blake Holton]

The deal was that I sanded off the veneer on one side of some 1/4 veneered MDF making it about .195" down from .230". I figured there was a pretty good chance that the the veneer left on the other side of the MDF would balance what I put on. Apparently not. :o



The issue may also be that, in sanding the thin MDF, you removed the high density face layer of the MDF. That will create an unballanced composite that will be prone to differential shrinking ans swelling.

[Jim Underwood]

It was a paper backed veneer which is about .032 thick.

I think I shoulda used contact cement. I doubt very much whether the weekend has improved the tension.

There was tension created after sanding the veneer and several thousands off the panels, but it was very slight. I figured the newly applied paper backed veneer would pull it back.

Unfortunately, it seemed not so much to pull, but to "push". The the panel was cupped away from the new veneer. Now explain that.:confused:

I think I figured out what the ominous popping sound was while the whole rig was under vacuum.. The panels were held down very tightly, but not tightly enough that the curing glue and the veneer couldn't move. When it did move, I think it created the "pop" noises I heard.

I'm dreading Monday morning... Gotta figure out a solution to the problem... and none of them look attractive.

[Jim Underwood]

Well, I'm glad I didn't worry anyone else in the shop about this. When I came in this morning all the panels were very nearly flat.....:cool:

And now I know we can do veneering on a mass scale.... All kinds of possibilities open up.

But I won't veneer just one side of a panel anymore.. that's for sure!