Help! Veneer failed to stick

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Hi All,
I've just veneered a sheet of birch ply (one side ash, one side ash burr) and when trimming the edges in places the veneer flaked off the glued surface. I think that I could probably peel chunks off, a bit like a nasty bit of far eastern ply.
Inevitably I'm wondering what I did wrong.
The panel is ~700 x 250 mm. Glued with cascamite, applied with a toothed spreader then went over it with a sponge roller (apparently it's important not to have too much glue as it can push through the veneer). With the veneer positioned I placed it on a piece of MDF and then stacked a pile of MDF sheets on top.
My guess is that there wasn't enough pressure to force a thin glue line: so either (i) more pressure; and/or (ii) mix the glue a bit runnier.
Am I on the right track? If so how much pressure am I going to need? Curved cauls + cramps? Vacuum bag?? Is there something fundamental I've missed??
And am I right in thinking that this piece isn't usable :( as the veneers could delaminate in the future?
g.
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graham wrote:

How long was it under weight and how much flaked off when trimming? Flaked off cross or with grain?
--

dadiOH
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dadiOH wrote on 18/11/12 15:54:

12 hours
> and how much flaked off when trimming?
a couple of corners; one a triangle with sides 2cm x 2cm along the panel edges (this the burr veneer)
> Flaked off cross or with grain? Can't really tell on the burr; on the face veneer the flaking is predominantly with the veneer grain, having lifted from the edge grain end. But that's just because my cutting wasn't as gentle as it could have been - I can lift the edges protruding over the sides (ie. the veneer long grain) and there's a degree of lifting too (if I continued the veneer would snap before much came off)
g.
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graham wrote:

Obviously, the veneer isn't glued solidly. There could be lots of reasons but if I had to guess I'd guess insufficient glue.
If it were me, I'd try checking that thought by gluing a small piece of veneer - 2" x 6", eg - on each of two substrates; glue the first as you did originally, the second with a much more generous amount of glue.
Another though is in the mixing of the glue. The only one of that type I've used is a dry powder that one mixes with water to a creamy consistency. "Creamy consistency" is pretty subjective...perhaps you used too much or little water? I'd check that thought too by mixing up some glue using varying amounts of water.
Other thoughts include the fact that the glue has a finite shelf life, that it needs to be at least 65 degrees to cure and that 12 hours is at the low end of cure time - 18 to 24 is safer.
--

dadiOH
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dadiOH wrote on 19/11/12 12:42:

Good ideas - I shall experiment. I've used this glue lots, and it was mixed as per instructions and how I would use it for normal joints. But now that I think about it I recall mixing it a bit runnier when I was butt jointing 4x2 for the bench top, I think because I had some notion of the water being absorbed overly quickly, though it may have just increased working time before all the clamps were in place.

New glue. The tub specifies 6 hours @15 degrees (C), and it was more like 20 degrees (C) (~68F?).
I shall experiment (and also work out the best route to more pressure cost effectively)
thanks, g.
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graham wrote:

I've never done this but think it should work well and I'll probably do it someday...
The peoblem in gettring enough pressure is not at the edges - that is easy - but at the center. Cauls help that, vacuum press is ideal but can be pricey.
My idea involves dense foam rubber. By "dense" I don't mean rigid, but squishy, just not real squishy. The sequence, bottom to top...
1. 3/4 ply
2. subststrate
3. veneer
4. cover sheet (brown wrapping paper would serve
5. 3"-4" thick foam
6. 3/4 ply
Clamp along all edges to bring top & bottom ply close together. That will squish down the foam in the central areas and although the pressure there will be less it will be even and should be sufficient as long as the width isn't enough to seriously bow the top/bottom ply.
--

dadiOH
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Am I missing something, here?
His project is 250mm X 700mm, or about 10" X 27". There shouldn't be a great problem properly clamping the veneer to the substrate without going through extraordinary means, i.e., special clamping, extending/extended cauls, etc.
Or is he trying to veneer a whole/half sheet of ply, then later cut that sheet into one or more 250mm X 700mm units?
Initialy he says sheet of ply (implying whole/half sheet?), then he uses the term panel (only the 250mm X 700mm unit?).
Sonny
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dadiOH wrote on 20/11/12 17:39: > > <pressure distribution with foam layer> >
Interesting idea. Strikes me as quite tricky to get foam of the right density.
Sonny wrote on 20/11/12 18:10:

problem properly clamping the veneer to the substrate without going through extraordinary means, i.e., special clamping, extending/extended cauls, etc.

I didn't mean to imply the entire "sheet". So panels cut to size then veneered.
So yes the panel being veneered was 250x700, the next (and largest) is 600x700.
What would you suggest clamp-wise in order to distribute the force evenly?

g.
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graham wrote:

Things of those sizes should present zero problems in clamping, any type will do. Probably even little spring clamps if you used a bunch of them.
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On 11/18/2012 7:23 AM, graham wrote: ...

Indeed. You need either a press or at a minimum cauls/clamping. You need something at a minimum of 50psi which translates to a lot of total force over a larger surface.
A useful general paper on using-- <http://www.djmarks.com/pdf/ureaglue.pdf
--
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dpb wrote on 18/11/12 16:12:

Thanks for the reference.
I followed up on vacuum presses (expediency vs accruing hide glue & hammering skills).
I read up at http://www.joewoodworker.com/veneering/welcome.htm but the price of equivalent looking pumps here in the UK seems ridiculous (>400), unless I'm looking in the wrong places.
Does anyone know where to get a dry running vacuum pump in the *UK* at a sensible price?!!
Also, isn't 50psi more than an atmosphere?
rgds, g.
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On Monday, November 19, 2012 5:38:28 PM UTC-6, graham wrote:

1 atmosphere = 14.6959488 pounds per square inch
Sonny
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Sonny wrote on 20/11/12 01:24:

yes, which is far less than the 50psi dpb suggested. 50psi doesn't seem unreasonable compared to normal jointing pressure, but is completely unachievable with a vacuum press. So my point really was what is the required pressure for glueing veneer? (as vacuum presses clearly work!)
g.
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On 11/20/2012 6:13 PM, graham wrote:

Are you kidding. 50psi is easily achievable in a vacuum bag.
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tiredofspam wrote:

I've never used one but the word that comes to mind is, "How?". Suck out every iota of air from the bag and all you have is our standard one atmosphere pressing, no?
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Seems some areas stuck down properly and some areas (some edges) didn't. Tap your fingernail on the interior areas to see if you can hear a different sound, a hollow sound, to see if there are interior areas that didn't stick down properly, also. As mentioned, there could be a number of things that have contributed to the problem.
If only those reported edge areas didn't stick down properly, and the rest of the interior areas did, then I would suspect not enough glue along those lifting edges. If there are interior areas that have not stuck down properly, also, then I would suspect not enough and/or a mis-mix of glue. In both of the above scenarios, not enough or inconsistent clamping pressure could contribute to the "non-stuck" symptoms, as well.
Sonny
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On 11/21/2012 7:10 AM, dadiOH wrote:

No not even close.
Take a look at some of the web sites on vacuum bagging. I have a medical grade unit. Small, and it can crush styrofoam foam easily and some forms of pink depending on the density. 1 atmosphere won't do that.
I used to use it for model airplanes wings, now I use it for veneering. I have used it to solve some interesting clamping issues too.
Do I need it? No.. not real often, but since I have it, I would not get rid of it.
I still have some stock on some nice veneers, just haven't figured out what I am going to do with them. Some Birds eye maple, quilted maple, tiger maple , more etimoe, some beautiful walnut burl...
I think when I am up at 30hg I am in the hundreds of pounds per square inch or more. I don't remember anymore as I don't do it that often.
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On Wed, 21 Nov 2012 18:16:59 -0500, tiredofspam <nospam.nospam.com> wrote:

From West System Epoxy's web site:
"Vacuum bagging uses atmospheric pressure as a clamp to hold laminate plies together. The laminate is sealed within an airtight envelope. The envelope may be an airtight mold on one side and an airtight bag on the other. When the bag is sealed to the mold, pressure on the outside and inside of this envelope is equal to atmospheric pressure: approximately 29 inches of mercury (Hg), or 14.7 psi. As a vacuum pump evacuates air from the inside of the envelope, air pressure inside of the envelope is reduced while air pressure outside of the envelope remains at 14.7 psi. Atmospheric pressure forces the sides of the envelope and everything within the envelope together, putting equal and even pressure over the surface of the envelope.
The pressure differential between the inside and outside of the envelope determines the amount of clamping force on the laminate. Theoretically, the maximum possible pressure that can be exerted on the laminate, if it were possible to achieve a perfect vacuum and remove all of the air from the envelope, is one atmosphere, or 14.7 psi. A realistic pressure differential (clamping pressure) will be 1225 inches of mercury (612.5 psi)."
http://www.westsystem.com/ss/assets/HowTo-Publications/Vacuum-Bagging-Techniques.pdf
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Jack Novak
Buffalo, NY - USA
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On 11/21/2012 7:21 PM, Nova wrote:

http://www.westsystem.com/ss/assets/HowTo-Publications/Vacuum-Bagging-Techniques.pdf
I stand corrected. You are correct as was Graham and dadioh. Somehow I remembered it differently, I rememberd very high clamping pressures. I will add to this from that site:
2.1.1 Vacuum pressure The Hg maximum level is the maximum vacuum level (measured in inches of mercury) recommended for the pump. This vacuum level translates to the maximum amount of work effect or clamping pressure that can be generated. Two inches of mercury (2" Hg) equals about one pound per square inch (1 psi) of air pressure. (Remember that 1 atmosphere ).92 inches Hg = 14.7 psi) If you are vacuum bagging a one square foot laminate, a 20"Hg vacuum will yield 10 psi clamping force or a total of 1440 pounds of clamping force over the entire laminate. If you are laminating a 4'8' panel, the same 20" Hg (10 psi) will yield over 46,000 pounds of clamping force spread evenly over the entire panel.
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says...

Yeah, if you happen to have it under 150 feet of water.
How, specifically, do you propose to get 50 psi using a vacuum bag when the surrounding atmosphere only provided a maximul of less than 15?
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