OK, I'm looking at making a canopy. I've done the shortcut methods before, but I don't want to do that this time. No liter bottle canopies or heat, push and hope methods. I've read some articles about production plastic heat/vacuum molding methods. Basically it's making a mold that includes building up a mold from scraps, glassing it., cleaning it, etc. then using that to make a female mold then using that to make a plug using a structural molding material. Then creating your own vacuum box and making the part. This has the advantage that you can pull more than one canopy, though it's a lot of work. I'm thinking of doing this, but have some questions for folks that have done this sort of full bore canopy molding method.

What's the best molding material to cast the plug with? I've looked at several epoxy, latex and polyester based products, but didn't really get a sense of what was best for this sort of project or which was easiest to use.

What's the best material to use to make the female mold. I would have thought maybe silicon, but I would imagine these hard molding materials would more likely cause deformation. Maybe not. But I don't know if a hard molding material like EasyCast or something would work as well.

I've found some sources for PVC. PET and PETg for the actual canopy material. Seems .060 or .050 thickness would work best. Acetates yellow under UV as do many other acrylics, so I wanted to sick with one of these. PETg is a pain to dye, though. Any thoughts on that?

How much vacuum do you need in the vacuum box? Is a regular shopvac enough? The box seems easy enough to construct. A small hole perforated Masonite would probably work fine as a base. Any thoughts on box construction?

Thanks for any input. I'm not convinced to do this yet, but I'm thinking about it.

Randy,
I have in the past succsefully vacu formed canopies, I did not however use the tooling, female, male process. I formed my canopy buck out of hard balsa, glassed it, sanded it, primed it with catalyzed primer,, then used it just like that. My box was about 18 x 18 and about 6 inches deep just made out of pine and a sheet of 3/8 ply for the top. Then I cut an opening in the side to fit the crevice tool into , wrapped the crevice tool with saran wrap and mixed up a slurry of microballoons and epoxy, slathered that around the crevice tool to form a nice guutentight fit. Then I put my canopy mold on a pedastal of 1/2 inch scrap leaving a small reveal around the base of the canopy to draw against. However in hindsight, I figured it would have been better to make it a continuation of the canopy shape with a slight flare at the bottom maybe,,
I then drilled a series of holes around the mold, seems like it was 1/8, or 3/32 inch holes about every 1/4 inch. Heat the plastic up to the correct temp, with everything sitting there waiting, I had better luck with the vacuum running when I put the plastic down. The mold pulls the heat out of the plastic pretty fast. Oh and make sure you leave a LOT of scrap area around your plastic so it can stretch.  Hmm oh yeah, I built a frame out of 1x1 to staple the plastic to.

Randy,
I also recommend having a storage tank for this operation.  I simply vacated an old propane tank from a grill for mine.  I rinsed it out with water several times to clear it of any residual gas, then hooked up a 1" pipe with a ball valve of PVC and used a vacuum pump that we had pitched at work instead of overhauling it.  I leave the pump running during the pull. (I have a wonderful wife who just shakes her head at the contraptions that end up in her kitchen from time to time.) I agree that the shop vac is rather anemic for the job.  My box is 18" x 18" also, and I just used peg board for the top plate.  Drill a few extra holes in the pull area. I braced it every few inches so it won't collapse.
I made my buck out of hard balsa, too.  Put a good finish on it and you're good to go.
Have fun, ya gotta try it!
Will

I found some of the items which I used to form a canopy for a large F-20 Tigershark a couple of years back. Please excuse the crude nature and appearance of my "Redneck Canopy Factory". The vacuum box was built from 1/2" ply and sealed by wrapping it with duct tape. The block on the bottom has a 1 1/4" hole for the nozzle of my shop vac. The top of the box is covered with a pieace of 1/8" thick closed cell foam for a gasket seal to the PET plastic which was stapled to a square frame. The form was carved from a piece of pine 2 x 4 lumber. I sanded the form and applied several coats of water-based polyurethane, then plain old floor wax. The form was fastened onto the box with a couple of drywall screws through the bottom hole. 1/16": holes were drilled all around the form. The entire affair was clamped into a Workmate. I heated the plastic until it sagged quite a bit on the frame, turned on the shop vac and opened the oven and in one smooth motion I slammed the frame onto the top of the box.... and then tried again. My first one looked rough and showed stretch marks from inadequate heat. The neat thing about PET plastic is that it has memory and will return to sheet form, if properly heated. I made several practice runs before putting fresh PET on the frame and pulling three successful canopies. Once you get the hang of it you will be amazed at the details you can form into your canopy such as framing and rivet lines. If your form is not slick enough, you will see wood grain lines in your parts.

Bill,

Well, it really depends on what you are trying to accomplish. For a one off (pull a couple of canopies), a hard balsa plug that has been glasses and polished would probably work fine, unless it's a very detailed unit. I wanted to mold a canopy with frame. The advantage of the other method is, you make the plug, make the female then pull a new plug using mold making material. It's hard as a rock, hold details very well and can be used to make a thousand canopies with no degradation of the plug. But as I said, I'm not all that sold on the idea of going industrial with this. I just wanted to make what would be a fairly large canopy and a one off process would likely work fine. Mostly I wanted to try the mold making material to see how the stuff worked. I could think of a lot of other applications besides a canopy.

Not sure how you would do that without the vac holes also being detailed.   

Ty,

Got the package today. Thanks!

Ty sent an article from Model Builder in the 70s. Outlines a pretty cool system for making your own canopies. I may try it.

Hi Randy,

I know you are on *vacation* so you won't see this for a while.  But I just got a set of plans for Harold Price's Boulton Paul Defiant stunter.  It has an unbelievably detailed canopy, and Harold pulled it from a carved buck.  It has all the framing for everything including the gun turret and it was all one piece.

I have been intrigued by that model since the original article was published.  Don't know if I will build it, but I was blown away by the canopy!

Mongo