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Large custom vacum bag

9/7/17       
Pete

I am currently trying to price a bed and need some guidance- It is a king size and 100% veneered. The question is around the headboard. It is roughly 60" tall and the outside edges are curved in plan view so it wraps the mattress or follows the slight curve to the mattress. My initial plan is to laminate the headboard over a form in a vac bag with bending ply and layers of veneer to get to a thickness of around 1 1/2" to 1 3/4" roughly. THe inside and outside face of this headboard is veneered. I do not have a vac bag large enough for this and was wondering if anyone has ever built their own to accommodate this size. Source for wide vinyl? how would you join the pieces? Any input is much appreciated.

9/7/17       #2: Large custom vacum bag ...
KEit

Pete, I have built lots of special purpose bags. There is a wholesale automotive upholstery supply near me that keeps an assortment of big rolls of vinyl thickness. I can buy whatever length I want, in 54" width. They also sell a special contact cement, so I just make a big envelope with one end open.

I almost never use the form inside the bag. If you can easily bend the bundle to the shape, it's much easier to just put everything in the bag, then bend the bag around a much simpler form that doesn't have to resist collapse if it were inside.

I also use epoxy for the adhesive, which is a wonderful lubricant between the parts while making the bend, with plenty of open time so there is no pressure to rush it.
Before switching to epoxy, I had some of the inside faces wading up rather than slipping.

I've been doing it this way for about 30 years, so have plenty of experience if you have more specific questions.

9/7/17       #3: Large custom vacum bag ...
Jared

I second what KEit says. I build molds from CNCd MDF parts and skin the surface with any high gloss laminate laying around. Make the mold surface 5 or 6 inches larger than the part you want, so you can attach a bag directly to the gloss surface. I use 1/8" butyl tape for this. It's essentially a one-sided bag, and as KEit says, the mold now doesn't have to resist collapsing from vacuum. Not sure I'd try laminating 1.75" thickness in one go though. I'd probably do the core in 2 stages, correcting the curves and re-fairing in between. For the final veneer, I'd take the core off the mold and bag directly to it. Lots of ways to skin this cat.

9/7/17       #4: Large custom vacum bag ...
Pete

I attached an image here to give you an idea of what is being planned.


View higher quality, full size image (800 X 610)

9/7/17       #5: Large custom vacum bag ...
rich c.

I think you have more issues that just getting the bag. Burl veneer is not going to like going around the curve, and loading that headboard into a bag is probably going to take 4 guys. That's a lot of bending plywood to make up 1 1/2 to 1 3/4".

9/8/17       #7: Large custom vacum bag ...
Keith Newton

Nothing says you have to stick with vinyl for these one off bags. You can use the heaviest construction poly film that you can find. I wouldn't use black, because I always want to see through it.

For your project, if your shorthanded or just have one helper, I'd set my form up on saw horses or a work table where I had the bottom half of the bag laying on top, then start with the inside face laying on top of a sheet of post-form laminate for a platen, then thin plastic film, then face veneer.

I use the 1-1 mix ratio epoxy from FGCI which I then mix colloidal silica into to get the viscosity up a little thicker than latex paint. For a project this size, I like to measure out the resin into separate mixing cups, then mix in the silica ahead of mixing parts A and B, unless you have a third person just mixing while you and another are rolling out the epoxy.

Rather than pouring the epoxy into a pan, I like to just pour out a puddle onto the substrate, spread it out with a scrap of wood, then work it outward with the roller. By mixing small batches it never spends enough time in a pot with depth to start heating up, so there is plenty of open glue time unless it's really hot in your shop. Unlike most other adhesives, temperature is your friend or foe. Cooler will add more time.

As you spread glue on one side of the bending ply core sheets, just flip them over onto the growing stack until they are all built up right in place on the form.

Rather using the rope or string caulk or butyl roll like Jared suggested, while that will work, you can just use the butyl in the caulking gun tube. it's a little thinner, and very sticky, so run a heavy bead around the outside, then bring the top sheet over and mash them together.

Most tutorials show using a bleeder fabric over the top of the pack, but I usually just use the thin plastic drop cloth from the hardware store between the platen and face. In this case post-form laminate would be about the right stiffness. I've even used it as the second ply under the face, since your probably going to have staggered seams in your bendy stuff. Sand the face with really coarse abrasive to give the epoxy tooth to grab onto.

When you make your form, make the ends run a bit longer, so you have some leverage past the bend. You can never bend any wood all the way to the end. It takes at least 8 times the thickness longer.

While you may be tempted to use 3/8" bending ply, I'd consider using 1/4" since it would probably be about 50% easier to bend, and less prone to want to spring back a little on the very end when you trim back to the spring line at the curve when your done.

I usually run around the whole perimeter with something like a cheap ski rope or parachute cord, then connect to the hose. I tape an old shirt sleeve onto the end of the hose, and make a big wad or caulk where it enters the bag. As your drawing the vacuum, once all the extra air is out of the bag, what your doing, is slowly stretching the air within the wood in the core, which will travel out to the edges faster than the face, especially if you've used plastic lam for the platen and maybe second ply of your sandwich.

Oh, one last thing, when building the form for the bag outside, you will need to clamp some straight 2x4s along the edges of the straight part to keep it from buckling up, so maybe cut some holes for the clamps.

When actually making the final bend with everything epoxied, take it nice and slow. The inside face is going to be getting compressed end to end, while the outside it going to be getting stretched. It may need to slide sideways about 2", so take it slow and let it flow.

Here is a link to the activator that I buy, and use with their standard resin. You can probably get by with the 1/2 gallon kit, but once you use it, you probably will want to always have some around, and the price is about half what some of the other consumer brands charge.

https://www.fgci.com/item/125231/Non-Corrosive-Epoxy-Curing-Agent-1-1-Ratio-EA-075B-Gallon-Jug/

9/8/17       #8: Large custom vacum bag ...
Pete

Keith,

Your feed back is hugely appreciated. My thought was to do the core with bending ply with maybe 1/16 mahogany b/w each layer, alternating grain direction. Maybe get more rigidity and less spring back? I have always had the form and material in the bag. No experience forming it in the bag outside of the form. Would you say it is easier, knowing the form does not need to be built to with stand the pressure... I am thinking I would not do the entire pressing-face, cores, and backside face in one shot. Maybe attempt the core first then press the face and back. Doing the entire thing in one shot scares me honestly but I know its faster. call me chicken.....again, much appreciated.

9/8/17       #9: Large custom vacum bag ...
Keith Newton

I would do it all in one layup, but would want at least another person, or better two.

No, I wouldn't bother using 1/16 veneers, that would end up being twenty something layers, instead of 7. Besides you'll have more problem bending that many layers across the grain on that radius.

What is your face veneer going to be?
Rich was concerned about a burl bending around that R. However, if you made your faces up as 2 ply before bagging the whole bundle, that wouldn't be as big a concern as just edge glued single thickness. And I have bent Carpathian Elm Burl around the end of a long narrow ellipse where the end was about 1.5" radius, so I know it can be done.

There are plenty of ways to go at this, but I wouldn't be the least bit concerned about how tough it would be with that many layers of bending ply. If you are, just add a horizontal layer just under both faces of something tough for it's structural property.

9/8/17       #10: Large custom vacum bag ...
Pete

Face might be Macassar. So if I have a bending ply core, we have not solidified the overall height of the headboard. There may be a joint on that bending ply. I could arrange it to be on the lower area. How would you handle a joint like that on the core below the face veneer? I could tape up a horizontal grain direction thick mahogany veneer to go right under the face and on the core. cross band basically. Thanks for helping.

9/8/17       #11: Large custom vacum bag ...
Keith Newton

Oh, I meant to mention that bending ply comes in 4x8 as well as 8x4, so check with your supplier to see if they have it both ways.

The ebony sounds like a great choice, if you can find some good stuff. last time I looked though most was pretty narrow. You might want to look at zircote as an alternate.

Zircote link


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