Vacuum Bag Versus Clamp Jig for Curved Moldings

For a small molding laid up from laminated plies, keep it simple: a curved jig and clamps works as well as vacuum baggin, or better. June 23, 2006

We are bending 2" w x 3.5 mm ash to approx 1'6" radius, using West Systems epoxy. My cobbled vac system works great holding 22Hg no problem. Using shower curtain for bag and a perfectly routed template for formů Problem! I'm frustrated with the compression we're getting. Too many voids and some are just too big to pass. Is the shower curtain too pliable so that it is stretching instead of pulling the material? Is there some other element we're missing? I want a no-seam end product. Is this possible with epoxy?

Forum Responses
(Architectural Woodworking Forum)
From contributor R:
I find if there are voids in bent laminations, it is usually a sign that the plies are too thick. More plies, thinner. I have used epoxy in the past, but find Unibond 800 ideal for vacuum bag work.

From contributor B:
If I am reading your post correctly, I envision a number of plies 1/8" + thick at 2" tall bent around a radius of 18" for how big a portion of a circle. That can be done without a bag at all, by wrapping around the form and pulling in with a web clamp or a steel strap and clamp. We have done a ton of these laminations for casings, jambs and other stuff. Forms made out of MDF and screwed down to a sheet of anything. Can do bigger pieces with blocks laid out on a pattern line and pull the plies into and around the outside of the curve. The plies will bridge the space between the blocks and still give a fair and accurate form.

From contributor H:
That is right - if your laminations are 1/8" x 2", you are making this more complicated than it needs to be. Either a strap clamp or a form with holes every 4" just inside the curved edge for clamp heads is the way to go. As to the shower curtain, it is not pulling the laminations together. The atmospheric pressure is pushing on them. The material and how much it stretches has nothing to do with it. If you're pulling 22" and not getting tight seams, then the laminations are too thick for the radius (as stated above).

From the original questioner:
My shop help often accuses me of over-engineering... They were right on this one. We went to the strap clamp and all is well. Thanks for the help! By the way, any sources for bag material you like? Expect to be laminating 20 table tops next month.

From contributor B:
Vinyl from McMaster-Carr. 54" wide by any length.