Canopies

What material do most of you use when making a mold for your canopies. I have used Bass wood but there seems to always be some grain to be seen in the canopy despite my repeated sandings. Any advice would be great!!!!

I recently made some canopies and used plaster of Paris for the male mold. My plane required a bubble canopy similar to a Sig 8 inch, except it had to be skinnier. So I squeezed the Sig bubble canopy in a fixture to create the female form. Into this I poured the mixed plaster, thus creating the male mold for vacuum forming.
The plaster will take several days to dry. When the plaster is no longer cool to the touch, a majority of the water has evaporated. I baked my mold for several hours at 140°F so that it was bone dry.
Be advised that the surface of the plaster form is very fragile and ANY irregularities will be transferred to your finished part. I store my plaster form in one of my rejected moldings to protect the surface.
Derek Moran

LAST EDITED ON Feb-26-03 AT 01:57 PM (CDT)
 
Might work, but I doubt it. I tried a plaster mold for My Millennium Mustang. It basically exploded when I draped it with hot plastic. I had to dig all of the plaster out, refill the mold with balsa and refinish the mold before I could get usable canopies.

As for plastics, avoid butyrate and styrene. .040 or .030 Acetate (plexiglas) is king but hard to find in thicknesses less than .060 which is too thick. I used .040 Lexan (polycarbonate) on my BBQB Bearcat and Millennium Mustang. Lexan is usable, but hydroscopic, and will form bubbles when heated if not pretreated. It is also very strong and mechanically difficult to form. PTEG is becomming popular for canopies because it is easy to form but it is not as tolerant of heat as Lexan or Plexiglas, it will melt on your airplane with exposure to hot sun or hot automobile storage heat.
Personally, I will only use Plexiglas when I can find it or Lexan for bullet proof canopies.

As for making molds from existing canopies, I've tried many products but find the best material to be Dynalite, a cheap microsphere filled Bondo.

Did I just set a new record for misspelled words in one post?

Al, you're probably right about ordinary casting plaster-way soft and wet. I wouldn't give up without trying Hydrocal casting plaster, or something from a place which sells the plaster used for lost wax jewelry casting. This plaster, when properly burned out, will handle molten gold with no loss of fidelity to the wax original. Look for a lapidary type shop that sells jewelery making tools and supplies. Hope that is clear enough.
Kelvin

Hydrocal source: You could also score this stuff in the model R.R. aisles at the LHS. It's great for making rock castings and such. Sets up rock hard in relatively short order and rings like china (as in plates) when set. Look for the "Woodland Scenics" rack. I think they carry it in milk carton size volumes. It takes detail very well as already stated. Local shop in Port Huron MI use to parcel out Hydrocal by the pound from a 75 pound bag he would get from a local (Detroit area) drywall material supplier. 75lbs would last us all a season or six. Dennis never did say where/who he purchased those 75lbs bags from,..hmmm? Must be some kinda' under ground connection to score the big one .

There is a supply lhouse in St. Louis that I went to two years ago. They carry lots of Hydra Cal. When I walked into the office and told the man behind the counter what I wanted, he said,"You're a model railroader, aren't you?" I asked how he could tell and he said they o nly carried Hydra Cal for model railroaders and sell an average of four 100 pound sacks a month. I have bought five from them and still have two. I have used it to make molds for canopies and cowls. It MUST be dry, and I bake mine in the oven when the wife is at work.
It also makes great rock castings for the layout.

A very timely post. I'm toying with the same idea i.e.; fill a factory canopy with plaster then use this as the master for pulling vac copies off it. Al sounds like your explosion may have been caused by a damp master. Very similar things happen with pottery if it's not fired correctly prior the finishing fire for the glaze application. Same goes for fireplace brick work, or (closer to home) refinery heater refractory. These all must be dried prior to any major application of heat or the moisture in the mixture flashes/expands (into steam as a worse case) and bingo bongo your master is now junk. A slow and lengthy dry out period would be required to achieve a "sound master plaster plug" (say that ten times fast ) As far as wood grain goes: I think someone related that the first pull on the mold should be left on the mold, and then do a second pull over the top of #1. #1 acts as the finish and gives a no grain result. Not sure where I read that one, but may be worth looking into or trying at least.

Gee, you sure can learn a lot by reading these posts.

Al

P.S. My plaster filled mold was about a month old when I tried it. It still had enough moisture to fail.

I respect your experience Al, but my canopies turned out just fine with a plain old plaster of paris (from Home Depot) mold, just as I described. Further, I used .040 Sig butyrate; the ratio of height to width on my mold was approximately 1.5:1. The quality and clarity of the canopies was pretty darn good. I pulled about a dozen parts without any difficuly caused by the mold.
This was my first attempt at vacuum forming. I made my own vacuum stage, used a shop vac as the source and heated the plastic in our electric oven. The next time I do canopies I will probably use .030 material and/or add a little more horsepower to my vacuum source.
Derek Moran

Here is a fool-proof method that I use on all of my airplanes to make custom canopys.

"·Shrink Form Your Own Canopies, RCM, January 1996, Dan Simenson.

How to "Shrink Form Your Canopies" was worked up as a simple way of making canopies for scratch-builders. It utilizes the plastic in 2 liter pop bottles, either clear or green tinted. With essentially free material and your hot air gun, you are ready to try it. It works well for canopies up to about 8-1/2" long and with the Alternate Method up to 11-1/2" long. That seems to the practical limit, as an opened up and rolled out 2 liter bottle gives a piece of material about 13-1/2" long. They are fuelproof and are not affected by acetone, MEK, or dope thinner. RC-56 cement works good for installation. Currently, I have several in use: an 8-1/2" on a .10 sized "Turbulent City," two 5"x3"x2-1/2" deep units on .40 sized models and an 11-1/2" on a .65 sized sport model. This process works for making engine cowlings also, but that's another story.

The canopy pattern or "plug" can be carved from any clear wood: balsa, pine, cedar, poplar, etc.. For best results, the plug should extend about 1/4" beyond the edges of the desired canopy. Final sanding of the plug should be with 220 grit paper. Do not paint.

(1) Mark a trim line and center tics on the plug with a felt tip permanent marker. Remove the label and cut both ends from the 2 liter pop bottle.

(2) Slip the plug and a scrap block of wood into the bottle, positioning the plug in the center of the bottle. Wedge a small block between the plug and block of wood to make a snug fit.

(3) Start shrinking the bottle around large bottom block first. Keep the gun moving. Next shrink the sides, then around the plug at the ends. Avoid too much heat at the very ends or they may curl up.

(4) When almost shaped, go over the whole surface carefully to get the last ripples out.

(5) Trace trim line and center tics on the plastic with a felt marker. Marks can be cleaned off later with acetone or thinner.

(6) Cut canopy along the trim line. After final fitting and cleaning, attach with RC-56.

Alternate Method for Canopies Up To 11-1/2" Long.

(7) Cut ends from bottle as before. Split down one side. Pull Sheet snug from front to back over the plug, placing tacks at each end beyond the trim line. Then pull down at the sides and tack or nail in place. Tacks or small nails about 2" long and can be used and need not be driven all the way in. There will be large "billows" along the sides, but they will shrink up.

(8) Apply heat to the large wrinkles first, alternating sides as you work from the center towards the ends. Finally, go over the entire surface carefully until it is smooth. Trace the trim lines and center tics before cutting out the canopy."

LAST EDITED ON Feb-26-03 AT 08:37 PM (CDT)
 
Plaster of paris will crack/break but usually only does so if you get it too hot. Heat the material to 305 to 325 degrees depending on the room temperature and bring the hot material quickly on to the mold. Suck it down with a shop vac and don't try to remove it for five minues. Then don't do another for at least 15 minutes to allow the mold to cool. The problem with P of P is it is very hard to make correction in it - If you pour an existing canopy full all of the dents/warps etc are transferred to the mold. Because the surface of the plaster is so fragile, any attempt to sand or refinish usually results in gouges - even with the finest sandpaper. Under the surface is a coarse texture that is unuseable it must be covered. There is a much better material for molds than Plaster of Paris. True mahogany slowly hand carved to gradually fit inside an existing canopy. The grain is close so if anything a minimal filling is all that is needed.

I sell canopies of:
Super Duper Zilch
Ringmaster Sportster
Ballerina III
ME109 of the Scientific 1/2A
a 3 inch jet type bubble canopy

I am currently carving the mahogany mold for the Don still Stuka and the Sterling Spitfire. They are both almost done.

In spite of what it says, I am a registered user - I forgot to log in.

Bill did the ME-109 and the 3" bubble for my Scientific scratch models. They are very nice. Can't wait to see the Stuka one.

Oh boy. I'm logged in now. Currell

When my friend Frank Macy recreated the Fireball kit back in the early 90's, he filed the mold for the canopy from a solid block of alumium!! The thing is a work of art. I have made many canopies from it and it works like a charm. Making it must have been like sawing your way out of jail with dental floss!!! Just thought you guys might find this story interesting. Regards, Roy D