Glue using a Vac Bag

[Giff]

A bit of a random question. I use a Vac Bag and have good results. Even very tight formed curves over a Poly Former. The question is how does an "air drying" glue work without air ? I use a D4 waterproof PVA type and haven't had problems, but I have just laminated a large batch of 3mm veneers on to ply and notice the glue has not gone off when it comes out of the press and is lifting at the corners on some panels. How is it supposed to work without any air to cure it ? Probably a simple answer but I would be grateful for an explanation. Thanks Geoff

 

[MusicMan]

Unless you are using a specialised vacuum pump, the vacuum you pull in a shop (e.g. with a HPLV shop vac) does nothing like get rid of all the air; it just reduces its pressure by about 4 psi. This still gives 4 psi to push on veneer, or over 500 pounds/square foot. But as there is somewhat less air, and of course the glue is harder for it to get to, curing might take longer.

Keith

 

[Giff]

Thanks Keith. I am using a proper set-up. Bag-Press. The question was really if the glue needs air to cure how can it cure in a vacuum which has no air ? Geoff

 

[MusicMan]

I realise that was your question. My point was that it is not a complete vacuum, but REDUCED pressure not ZERO pressure, thus there is still some air. Much less with your setup than with a shop vac, though. I checked the Bag-Press site (which is very good) and they claim an 85% reduction in pressure, which means that there is still 15% air pressure (2.25 psi instead of 15), enough to set off an air cured glue but probably will slow it down (as you found). Try leaving it in a lot longer.

If you have the Bag-Press high vacuum version (oil lubricated high vacuum pump) then indeed there might not be enough air to cure the adhesive effectively. Though pressure may also initiate curing, indeed I thought most PVAs were of that type. Anyway, a pressure-curing glue might be a better choice.

Indeed air-cured glue will not set in an outer-space vacuum but the Bag-Press is very far from that!

Keith