Vacuum Forming Curved Panels
Making curved panels in a vacuum press requires a reliable bag and evacuation system. JoeWoodworker's vacuum press plans have certainly made that affordable and fool proof, so I offered to provide a FAQ on the methods I use to make curved panels. The most challenging parts I make are harp shells. These have a tight bending radius, and a book matched veneer seam that must be aligned with the centerline of the panel. You may not need to use all of the techniques for your project.They are presented as a set of tools you can adapt to your particular needs.
I strongly suggest builders start by vacuum veneering a few flat panels to learn how their adhesive behaves (open times, cure times), the importance of aligning parts, getting a good bag closure etc.
Curved panels are usually built in one of two ways. The simplest approach uses a bending form outside the bag. A sandwich of plywood veneer and adhesive is put inside the bag and clamped over boards cut to a curve while the vacuum is drawn.
Here the builder can use the evenly distributed pressure inside the bag to bend the veneer and thin ply over the form. It is amazing to watch the bag suck the veneer and plywood into a tight, uniform curve over the mold. With the proper technique, the parts are come out very smooth and fair - a few strokes with 220 grit paper, and they are ready for a finish.
Q1: How many layers do I need to use and how thick should they be?
One rule of thumb I use is to incorporate at least three layers of material. Panels made from only two layers of material near their bending limit are going to have significant "spring back". For thin substrates, that third layer can be a layer of veneer (harp shells I use two layers of 1/8" plywood and an outer layer of veneer). A layer of 6 oz. fiberglass between the veneer and plywood can serve as the third layer in lightweight epoxy lays-ups.
Substrate materials can be selected by experiment. I saw off a narrow (one to two inch) strip of candidate plywood and bend it to see if it can conform to the tightest radius in the panel. The grain orientation can dramatically affect the substrate's stiffness and minimum bending radius, so you might want to try two strips, one that is cross cut from your sheet of plywood and one that is ripped.
Once they are glued together, curved panels are surprisingly stiff. In most applications, bracing along the edge or the inside the piece can significantly add rigidity to the part too.
Q2: What is the tightest curve I can pull in a vacuum bag?
Tighter curves are made by using a thinner substrate. These are the bending radius limits, costs and sources for four kinds of thin ply I have used.
Q3: What do you use for forms in a vacuum bag?
When I have tried to use hollow lath forms, the vacuum press deforms or even crushes the mold. Solid wood can withstand the pressure, but can be difficult to wrestle into a vacuum bag, so I use foam molds for harp shells.
Large foam molds can be built up using the 2" pink foam insulation sold at your local home improvement emporium. I have cut the foam to shape with a band saw and faired the surface using sanding boards. Big blocks of blue foam can be bought from outfits that cater to home-built aircraft builders – one is www.aircraft-spruce.com. I cut the molds for my harp shells using a hot (electrified) nichrome wire - a technique used by model airplane builders for making light, but strong parts.
Q4: What adhesive do you use?
I use BetterBond Ultra-CAT PPR mixed at an 8:3 ratio (by volume). I have used epoxy for applications where I need a better strength and I am willing to put up with veneer bleed through. Things went a lot better after I learned how to spread a consistent, thin coat of adhesive on my plywood substrates.
Q5: How much adhesive do you use? How do you spread it?
I use a thin nap 4" foam roller to spread the glue, and mix about 1 fluid oz of adhesive per square foot. The coverage should not be a thick, glistening wet layer (too much), but a slightly duller, uniform sheen – kind of like newly applied eggshell semi-gloss latex. Wood does not absorb adhesives evenly, so three or four minutes after the initial application, I go back to the sheet and play Robin Hood, redistributing the epoxy from the rich areas to the starving patches.
Q6: I get wrinkly veneer and adhesive pooling in spots on my panel. How can I stop this?
First, make sure you are getting good vacuum over the entire surface of your panel. I use a breather layer to channel air from the shell to the connection to the vacuum pump. Some builders use bubble wrap as a breather layer. I use nylon window screen. This is the very top layer of the "sandwich" I place in the press.
In flat veneering, the builder can use a stiff platen or caul (usually a ¾” MDF or ply) to ensure the veneer is pressed flat to the substrate or core. For curved panels, I rely on a third piece of ply as the platen. The platen should be placed right under the breather, and is made of the same material as the substrate. When the substrate and platen have the same flexibility they will keep the veneer from wrinkling or the adhesive from pooling. If you are making several identical parts, a single platen can be used over and over again.
The surface of the finished panel will be not be any flatter or smoother than the platen, so I pick the nicest piece for the platen, turning the A side (smoothest) towards the veneer.
Q7: How do I keep the panel from gluing itself to the mold, platen or vacuum bag?
Excess glue can bleed though pinholes in the top layer of veneer, along the seams and at the edges of the panel. I have tried waxing the mold and platen with limited success. To keep from gluing the panel to the platen, mold, vacuum bag or breather, wrap the outside of the veneer in a barrier layer. I have tried plastic, velum, wax paper as barrier layers. I now use freezer paper. I found it next to waxed paper at the super market. It is like butcher paper but has a thin, shiny plastic coating on one side. The shiny side is placed towards the veneer and glue and the edges of the barrier sheet are folded over onto the innermost layer of ply and taped into place.
For larger panels, I tape several strips of freezer paper together using packing tape on the matte side . If you do a lot of panels you can get a 1,000 foot roll (24” wide) for about $40 from www.pospaper.com.
Q8: How do you keep the veneer and panels aligned?
We got the idea from sandwich shops that hold tall sandwiches together with oversized toothpicks. For harp shells, I use two #6x 1¼ inch screws at opposite ends of the center seam to hold the ply, veneer and platen in alignment during assembly and in the press. Wood blocks are epoxied into the mold (and reinforced with fiberglass), and the screws are driven trough the platen, veneer, and substrate down into the mold.
On projects with a larger bending radius, the veneer and panels are less apt to slide around and I simply use tape to keep things aligned.
Here is a "sandwich" ready to go showing alignment screws, barrier layer (wax paper) and the platen. Note the tape, and breather (window screen) at the ready in the background. The barrier still needs to be taped to the bottom of the stack before it goes into the vacuum bag.
Q9: How do I remove the alignment screws/nails/pins?
If you wax them before driving them into place they are less likely to stick. If you have trouble removing them, heat them with a soldering iron which will soften the epoxy or PPR so you can back them out.
Q10: Whew, that is a lot to keep track of! Can you outline the process you use for assembling a veneer sandwich?
It really isn't that bad. After my substrate is cut to size, and the veneers are taped up, it takes about 15 minutes to get the platen, breather, and barrier set up and aligned. 15 minutes after that the glue has been mixed, spread and the sandwich is in the press. All it takes is a little organization.
Here is the procedure I use to put a sandwich together for a harp shell.
Q11: I have trouble with the bag "sucking up" between the sandwich and the mold. Is there any way to stop this?
As the vacuum is applied, an oversized bag will tend to do that. It prevents the sandwich from being pressed firmly to the sides of the mold by the top of the bag. On small panels, you can coax the sandwich into a curve and/or pull back on the edges of the bag. On larger projects, I use a backer board.
The backer board is made from ½" MDF and extends 4-6" beyond the footprint of the mold. The bag needs to be large enough extend another 3-4" beyond the bottom plate. These allowances may need to be greater (or smaller) depending on the height of your mold.
Q12: The plywood keeps poking holes in my vacuum bag as the vacuum is drawn. Is there any way to prevent this?
Round the corners of the backing plate, substrate plywood and platen to a ¼ ” or greater radius. After I switched to a nice heavy (30 mil) polyurethane vacuumbag, I never had any problems.
Credit where credit is due:
Lee Gayman who has been an inspirational mentor over the years as I improved my harp making skills. Lee does meticulous top notch work, and was generous and kind enough to show me many tricks of the trade including how to make birch bending ply into harp shells.
Vacuum Forming Curved Panels