Vac-Forming

For my ambitious Sopwith Dolphin project, there was no avoiding the fact that I had to make some vac-formed bits. I had never tried vac-forming before, but it turned out to be quite easy. All you need is the ability to make a box, and recourse to a vacuum cleaner. And a grill.

Vac-forming box

The procedure is as follows:

1. Make a box. I made mine out of some offcuts of MDF. Using a circular saw, I cut a long strip the right height. Then I added rebates for the top and bottom using a router. Then I cut it into nicely mitred sections using a mitre saw.
2. Drill a hole in one side just big enough for the appropriate vacuum attachment. My vacuum comes with a little plastic tube attachment, so I made it to fit this.
3. Join the sections together with wood glue and panel pins.
4. Cut the bottom of the box from plywood or whatever's handy. It just drops into the rebates. Wood glue and panel pins again. Before adding the top, seal round the inside joints with flexible decorating filler.
5. Add the top from perforated hardboard. I went down to B&Q to get some perforated hardboard and believe it or not, they didn't have any. So I had to drill lots of holes in a piece of plywood, with the rather untidy results you see above.
6. Seal with more decorating filler.
7. Make a frame that will hold a sheet of plastic and just fit over the box. Mine was made from two pieces of plywood; and maybe this wasn't the ideal solution, or maybe I allowed my quality control to lapse drastically here, but it does look a bit crude, I have to admit. On the other hand, it does fulfil its purpose of holding the sheet of plastic and just fitting over the box; so mockery, though tempting, would be unfair I feel. 

The frame. "Rude" is the word that someone like Defoe might have used, and I imagine that Robinson Crusoe, for example, might have achieved a neater finish. But then he would have been a bit stuck for styrene sheet, wouldn't he?

 The next step is to carve the wooden moulds for the project you are undertaking: they might represent the decking for a Sopwith Dolphin for example. I am sure there are other applications.

Now you get to the fun bit of actually doing some vac-forming. What you do is as follows:

1. Set up the vacuum and box close to the oven.
2. Put a piece of styrene sheet into your frame and put it under a high grill. Keep an eye on it.
3. The styrene will start sagging and then become taut again.
4. Very quickly, take the frame out and put it over the box.
5. Switch on the vacuum, and the shapes appear:

 It's all quite easy once you've got the basic equipment sorted out, and I got some usable mouldings first time; but when trying to get some duplicate mouldings later on, as is often the way, I struggled a bit. Here are some tips for best results:

• The styrene hardens very quickly, so set up everything very close to the oven.
• Make sure that you can fit the frame over the box very quickly. If you spend a second or so getting it into place, you will get rather imprecise results. My frame could certainly do with some improvement in this respect.
• There's nothing to stop you cutting out the mouldings you have just made and then reusing the same sheet of plastic, but this does reduce the suction slightly because there's a lot of air being sucked in straight through the exposed holes. It makes a big difference if you cover up the exposed holes using masking tape or whatever comes to hand.
• It probably helps to seal up the all joints as far as possible: some sort of seal where the vacuum attachment goes would probably help.
• The thinner the styrene sheet, the more accurate the moulding.
• You will not get a nice sharp right-angle at the base of the moulding, so to avoid the moulding flaring out, make sure that the wooden mould is taller than the piece you want to mould.

The next post will show the results of applying these techniques to a scratch-built Sopwith Dolphin.