Help! Veneer failed to stick

[dadiOH]

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?

[Sonny]

Seems some areas stuck down properly and some areas (some edges) didn't. T= ap 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 stic= k down properly, also. As mentioned, there could be a number of things tha= t 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 prope= rly, also, then I would suspect not enough and/or a mis-mix of glue. In bo= th of the above scenarios, not enough or inconsistent clamping pressure cou= ld contribute to the "non-stuck" symptoms, as well.

Sonny

[Richard]

You have a better vacuum pump that I do. Best mine can do is 14.7 psi...

:)

[tiredofspam]

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.

[Nova]

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

12?25 inches of mercury (6?12.5 psi)."

[tiredofspam]

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 =29.92 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.

[J. Clarke]

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?

[Doug Miller]

"J. Clarke" wrote in news: [e-mail]:

Naaaah. 110 feet is enough. :-)

You are absolutely correct. 14.7 psi is the theoretical maximum achievable with a vacuum bag; in practice, a bit less, maybe 14.5.