Duplicating the clear plastic parts
This method uses sheet plastic and a vacuum former.
- A sheet of clear plastic such as the Vivak plastic made by K&S and sold by Micro-Mark and Hobbylinc (I am using 0.015" thickness)
- Plaster of Paris (or similar plaster product)
- Rubber mold making materials (optional)
- Vacuum cleaner (a standard household vacuum cleaner is adequate)
- Heat source (electric burner, oven, etc)
- Vacuum former (build your own or buy one from Shorty's Basement - they are made by Chris Boehm)
- Heat resistant gloves
Hints and Tips
- Remove the dust bag/filter from your vacuum cleaner for a stronger vacuum source
- Make sure the plaster mold is as smooth as possible and free of dust or it will show up in the new canopy. I used liquid mask to protect the canopy, which put brush strokes into the plaster mold, which became brush strokes in the plastic of the new canopy. A few iterations of sealing and sanding would have prevented this.
Vacuum Forming a New Canopy:
- There are two ways to make a plaster mold of the clear part. The first is to line the inside of the canopy (or other part) with liquid mask then pour it full of plaster. The other is to make a rubber mold of the outside of the canopy as described in Method #1 then fill it with plaster. It will be helpful to insert a dowel or thick balsa stick into the plaster to give a handle for demolding. After about 1 hour carefully remove the plaster mold and set it aside for a day or three to fully dry. Take the canopy to the sink and clean it up, removing the liquid mask and plaster residue. The rubber mold produces a plaster mold that is oversized by the thickness of the canopy plastic but is the smoothest. Casting the plaster directly into the canopy makes the mold the correct size but the plaster can take on brush strokes in the liquid mask. For this example I used the plaster mold that was cast directly into the canopy. Shown below is the method that uses a rubber mold of the canopy.
Update: I have successfully cast a plaster mold directly into the canopy without any sort of release agent. The resulting plaster mold is the correct size and has no brush strokes showing in it. Just mix the plaster, pour it in, let sit for an hour, carefully remove from the canopy, and allow to dry.
- Cut a piece of sheet plastic the size of the holder (6" x 8" for my vacuum former) and clamp the frame together as shown below. I found out that these clamps need to be slid out to the edge or they will get in the way and the metal frame will not seal well against the vacuum former box.
- You can heat the plastic by holding it about 2" above an electric stove burner (or hot plate) or in a 350F (177C) oven. I chose to use a small toaster oven that I got from a local store for $30. Since plastic sags when hot you need to raise the holder. The picture below shows a drip pan that came with the toaster oven and four sockets being used as spacers.
- Set up the plaster mold on the vacuum former base as shown below. Use something thin to hold the mold slightly off the base. About half of what you see in my example probably would have been better. I have read that the thickness of a nickel should be used. Plug your vacuum cleaner into the box and set it all up as close to your heat source as you can.
- Preheat the oven to 350F (177C) then insert the plastic sheet. If using a burner then hold the plastic about 2" above the burner while wearing gloves. The plastic will start to wrinkle a bit then start to sag down. I waited till I had a little over 1/2" of sag before removing it from the oven.
- The next series of steps can be taken in any order you like, except that you must have the vacuum running before removing the plastic from heat so that you can immediately place the hot plastic onto the vacuum former. When the plastic is starting to sag I flip on the vacuum cleaner, then when the plastic has sagged sufficiently I remove the holder from the oven (while wearing gloves) and quickly set it down over the box, making a seal against it. Then I turn off the vacuum cleaner and the oven. When removed from the frame, the plastic sheet and plaster mold will look something like what is shown below.
- Carefully trim the plastic part around the edge of the plaster mold and remove the mold from the plastic. Shown below, from left to right, are the original canopy (in need of cleaning), the vacuum formed canopy, and a plunge molded canopy. It is hard to see, but the plunge molded canopy does not have very good detail and concave surfaces are not duplicated. The vacuum formed canopy is nearly identical to the original, the edges of the canopy frames are not quite as sharp but are very similar. With a little practice a vacuum molded canopy can be made that is an excellent duplicate.
- Shown below is a canopy made using a blue dinner plate. Keep in mind the plaster mold was still rough and could have been made a lot smoother, but at least you can see the level of detail that was captured.
- Trim the canopy to fit your plane and install.
I have never been happy with the smoothness of the canopy finish. Everything I applied to the plug then sanded would either bubble on the plug or stick to the hot plastic or both. The only coating that would not stick was the blue plastic dinner plate like is shown above. Recently I read a two part posting by legliderman over on RC Groups showing how he makes his canopy plugs from scratch. I asked him for some details on his process and he replied that his process has changed a bit. He used to make the plug from plaster then after it was shaped he filled the holes with bondo, then sealed the whole thing with automotive primer, sanded the primer, then used wax (presumably as a mold release agent). He informed me that his process has changed and now just uses straight bondo and wet sands it with 2000 grit and dusts it with Pledge every 3-4 canopies. It occured to me later that he could have meant the whole plug was made of bondo, but I think it is still plaster and it's just that he skips the primer step now. You can read his original posts on canopy plug making - part 1 and part 2 .