Stacking Stave-Core Glue-Ups

Stacking and gluing multiple layers at once reduces the effective clamping pressure per lamination. September 19, 2009

The company I work for currently has 90 stile cores/skins to glue up. The engineered cores are poplar with fir skins. We have two 4 x 10 VacuPress bags. We are playing with the idea of attempting to glue up 18 stiles at a time in one bag (three layers of six). First, is this a bad idea? Second, in the past we have used Titebond II for our stiles but have never used the vacuum bags for this process. Should we stick with the Titebond or move onto a urea or other type of glue? More importantly, we don't want to have to spend a month just gluing up the stiles (hence the 18 stiles per batch). We are hoping to leave the stiles in the bag for 24 hours until the next batch. Is this enough set time?

Forum Responses
(Architectural Woodworking Forum)
From contributor A:
Think about the surface area of all of your stiles, times 6 if there are two faces, times three layers, and divide that by the negative pressure of the air in the bag (14-16 psi), and you have several ounces per sq/inch. Not much. Is there a question as to whether water based glues cure in a (partial) vacuum? I think there is.

Stiles like you propose are typically ganged on clamp racks with air pressure that will accommodate squeeze out and still maintain steady pressure. Moving up the chain is the use of heated or RF curing presses. Moving down the chain is clamps and cauls and such.

From contributor B:
Titebond 2 (I prefer Titebond 3) in the vacuum bag is fine. Use individual top cauls, not one large one across the tops of all parts. We use top and bottom cauls, shrink wrap the ends, then slide in the bag on our main scored bottom sheet. Obviously, they need to be good and flat so adjust your scored caul for width and length accordingly. Actually 14 psi equates to over 2000 lbs per square foot. I would roll 2 loads a day personally, but in-first-thing and out-at-end-of-shift is plenty. Good machining is paramount, of course.

From contributor C:
To contributor A: Can you explain the negative pressure? I am not quite following that since the press evacuates the air. I am not questioning your comments since I know you are far, far more experienced than I.

To the original questioner: I have done up to six staves at a time in a bag. I am by no means the most experienced making stave/core, so take from this what you wish.

I assume you are planning to use PVA. If so, I think one problem with that quantity may be getting all the parts glued and in the bag before the glue starts to kick. If you use a Urea F then you will have time. I don't see any need to keep them in the bag 24 hours if you are using PVA, only 2-3 hours and put them aside (stickered). If you are using Urea, 3-5 hours will be sufficient, assuming you mix appropriately.

What is the skin thickness? If your skins are 1/8" or 3/16", another problem with stacking is that the parts may shift (particularly with PVA) when you put them in the bag, even if you tape the skins. One can oversize the skins, but this creates another problem. The bag will roll the edges over and crack them off if you don't use stiff enough cauls. I would reduce the time in the bag and stick with a single layer.

From contributor A:
Negative pressure or vacuum is the same thing as I see it. The math is there to do since the layers diminish the effective lbs per s/f as layers increase. My quick recalculation says about 2 lbs per sq/inch with 6 faces. You should also factor in the pressure the glue likes.

Experience has born this out. I'd stay at one layer (two faces, with caul), rather than worry over two layers. Glue with a lot of water in it - yellow - and 1/8" faces or thinner? I'd think that one through also. Lower water and glue that sets well in a vacuum is where I'd steer.

From contributor D:
Our system for doing this has evolved over the years. We do a lot of skins for interior doors and 3 ply same specie glue-ups for window and thick door parts. The vacuum bag works but it does take longer to cure. You should get 2 glue-ups per day as contributor B describes. Tight bond does cure slower in the bag, at least the squeeze out does. The other problem with the bag is keeping the skins from moving. We used to pin-nail the ends and tape the skins going in the bag. The bag does not get the joint as tight as our hydraulic press.

Our preferred method now is using our hydraulic frame press with torsion box culls. We can stack ten at a time and get 3 loads per day with real tight joints. We also use a JLT clamp rack with high jaws for overflow to get more per day if needed. The ends of the clamps on this do not line up flush so a torsion box cull is needed or a slight bow will be clamped in.

Your glue application method can be the constraint of quantity per load. We use a Pizzi pressurized system and roller. The systems where boards can be fed through are faster. I like the Pizzi because cleanup is easy for small batches.

Some companies have a clamp made for this purpose that will take a lot of stiles, 25 or so I would guess. Iíve seen some of these shop-made, tightened with an impact wrench. The Europeans have Hydraulic presses for this that apply side pressure to keep everything flush. Hydraulic hot presses are another method.

From the original questioner:
Thank you for all of your responses. You have all helped for future projects that may require gluing up 90 stiles.