Vacuum Bag Glue-Ups with the Form Outside the Bag

For large, bulky curved laminations, keeping the form outside the vacuum bag simplifies the process. September 19, 2009

For years we've been building forms for our curved work out of solid particle board. This is done to withstand the pressure of the vacuum bag, and it generally just seems to make the form faster to build. On smaller projects like radius vanity doors, it hasn't been an issue. But now we've got a huge radius veneered panel that can't be broken up into smaller parts. We want to build a form for the vacuum bag, and if we build it per our normal methods it will take 12 sheets of particle board cut into curved ribs and result in an unmanageable solid slab of particle board 40" wide, 112" long and up to 8" thick in the middle. I'm contemplating using rigid insulation foam for our form. I'm wondering if anyone else has experimented with it, and if so what the results were.

Forum Responses
(Architectural Woodworking Forum)
From contributor A:
Can you please describe your current project with more details?

From the original questioner:
I need to make a huge form to go inside a vacuum bag, and I'm wondering if anyone has had success with rigid insulation.

From contributor K:
I make ribbed forms with 5/8" or 3/4" MDF or plywood drops for the ribs and base, and 3/8" bending plywood for a skin, stapled together. 3" spacing for a single thickness skin, 6" for two plies. Ribbed forms need to be braced laterally between the ribs every 12" minimum to prevent implosion. Sometimes I will stick a layer of p-lam on top to smooth out the wacky wood surface and make for easier waxing for release.

Even this construction can get ridiculously heavy. I haven't used it, but vacuum pressing systems sells a bladder to inflate inside lightly built hollow forms to prevent their implosion. Also, for forms too large or unwieldy to go in a bag, a sheet of poly can be sealed to the surface of a form over the laminates, as in boatbuilding. I haven't used foamboard for forms- I think it would be problematic to build a ribbed form with it and relatively expensive to make a large solid form.

From contributor J:
I have made much smaller forms from rigid insulation board, and liked them. I did it by using water-based contact cement to glue up a solid chunk, and then put matched masonite templates on each end of the chunk and cut the curve with a home-made hot wire cutter. Overall length was about 3'. For the scale you're working with, you'd have to do build it in shorter sections.

From contributor N:
The form doesn't have to be inside the bag. When you take it out, the form only has to be strong enough to resist the bending force when you lay the bag of glued up parts on it before you draw the vacuum. Since most single axis bends tend to get rigid and straight along the axis of bend, you don't need a lot of ribs, they could be as thin as 1/4" at that. I normally use epoxy for my adhesive, which is really slippery, so a project as you describe, would likely just sag onto that form under its own weight, however, you can have a pattern cut to the arc of the face side to check, I normally just have a stiff batten to clamp along both of the long straight sides.

Normally, just clamping a batten on the arc ends, will take care of the arc, but try to set it up so you can look under form to make sure it is against the form if it is that critical. As you get more comfortable, you may find that you start seeing just how simple you can make it. Some small parts that don't have much spring-back, I will just hold against the form while the vacuum is being drawn. Once it reaches maximum, you can lift it off the form, and it will hold its shape, however if you had a leak or a lot of spring-back it might creep, but it sure doesn't take much clamp pressure on the outside to just hold it there. Life is hard enough without making it harder than it needs to be. Give it a try.

From contributor B:
Another potential problem with rigid insulation is it will compress. At the rated capacities (20psi, 40psi) some do compress up to 10%. The 40,60 psi is crazy expensive. Personally, I would use the bladder trick or use the bag on the outside.

From the original questioner:
I'm certainly open to exploring the idea of doing a project like this outside the bag in the future. But right now time is of the essence and I need to get it going today. The cost of the material is not a huge consideration; it's primarily labor time and weight. For those of you that have 3" to 6" between your ribs with cross braces and wiggle wood top sheets, don't you find that the labor is higher with this method than just having a router cut out a gazillion ribs of which you just glue them all together? No need for top sheets, no need for cross bracing. I'll probably just go buy a sheet of rigid, stick it in the bag and see if it crushes.

From contributor B:
Since you are talking about a large form, I suggest you try stacking several pieces for your test. The thickness of a 1.5" piece of pink (20psi) will only reduce in reduce by about 0.11" under 15psi.

From contributor M:
We have built ribbed type forms with either 1/4" ply lams or two layers of wiggle wood on top. However I have found that labor to wood cost and the 12 sheets of particle board are probably cheaper and ultimately more accurate. I am hoping, and perhaps assuming, you have a CNC to cut the material with, but we rout a couple small notches in the bottom of the form for an alignment piece made of hardwood. From there staple them together and run with it. Talk to your lumber supplier for "cover sheets" if you want to save a few more bucks.

From contributor N:
Since you are going to cut the ribs the same either way, why don't you just set up six of them and lay your work on top, and see how well it conforms. Beyond that, you do understand that the bag will press equally on both sides of the lay-up, providing the clamp pressure, just the same as if the form was in the bag, so what is the risk? I thought you want to save time!

From the original questioner:
So what you're suggesting is that I place the ribs up on a table, lay the bag over the top relying on gravity to give me my bend, and then the bag with its contents of seven layers of bendy ply and two layers of veneer will all laminate together perfectly smooth and still maintain the bend under vacuum? Can anyone else claim success using this method?

From contributor M:
Contributor N is correct, but I do find there is more difficulty pulling the very ends down in that method as opposed to form and material in the bag. If your material lays right down over the form with minimal clamping pressure use that method, but if it takes a little more effort to wrap over the form then go with it in the bag. We build lots of base and jambs in the fashion contributor N described.

From contributor N:
To the original questioner: you will still need to do a bit of a frame to keep the sawn curved parts aligned, but your glue-up laminates all go into a bag which is sealed with a hose running into it. You then lay it on top of the form, and use as many clamps as needed to counter the spring-back of the parts before you draw the vacuum. However, if you have an arc with a chord length of 40" and depth of 8" using bending ply, it shouldn't take much clamping pressure to hold it down to the form, before you pull the vacuum. Once the vacuum is drawn, it will hold that shape, but the form can just be so simple and weak in comparison to one which needs to go inside the bag.

If you only use a bag which has a table as the bottom half, that won't work. You just need a big bag which is all vinyl or whatever material you choose. I have been doing it this way since the mid eighties. I would not think of going back to wasting all of the time it takes to make forms that will withstand that much pressure. It is just a waste of time and material. If you have a bag, just put some parts in it dry, and drape it over the first few parts you are going to make anyway. Once the vacuum is drawn, you can even take it off of the form and feel how stiff it is. Let me know if you still don't understand and want to give it a try, and need more information. I just can't believe there are not more out there bagging this way.

From contributor R:

Contributor N is on the right track for this glue up. Stair builders regularly slip the vacuum bag over a stringer glue up, then wrap the entire thing around the "drum" that forms the stair. It takes less than 1/10th the number of clamps. For ease and speed, set up your bench, and lay full length cleats along both sides at the proper distance from the edges of the form. Do your glue up, lay one edge down between the form and cleat, and then move to the other side, overbend the edge and slip that into the cleat. Now the entire glue up should be in place and ready to evacuate.

One more idea I've used with success is to make a vacuum table out of your CNC bed. Set the form, drape heavy plastic over the glue up, and have a frame made to seal the edges of the plastic. Works surprisingly well. If you are running a 5x10 table with a good pump, not only will it provide adequate clamping pressure, but will also be very forgiving of leakage. Obviously, this is not feasible unless you are using a fairly quick setting adhesive. Overnight clamping is not practical.

From contributor L:
We do lots of glue ups with the form outside of the bag. We usually clamp-cull lightly on top of the bag near the center and near each end. You will be making your parts a little big so you can trim to size later so the little bit of odd shape on the very edge is no big deal. Do a dry run first as a test. We've got several bags and can keep two going at once with a valve manifold and 3/4hp pump. We have also added a 2"PVC pipe to our CNC vacuum system. It doesn't seem to affect the CNC at all to pull a little bit of vacuum for the bags. That way we can run four bags at a time. You'd be amazed how fast a bag pulls down with a 40hp pump! If the bag still needs vacuum after the CNC is shut down we just turn the valves to use the 3/4hp pump to maintain vacuum longer. My employees have played around making skateboards using the blue extruded foam shaped for compound curves and many layers of veneer on the vacuum table. They came out strong enough to stand a lot of flexing as a skateboard. The compound curves look neat too. They used Titebond 3.