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How to make a mould


jpolenek

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Could I get some advice on what materials to use to make a very precise, smooth mould? The idea is to make the shape, drape several layers of fiberglass over it for strength and further smoothing of the surface, and then get Aircraft Windshield Co. to stretch-mould Plexiglas over it to make a canopy. I was thinking of using MDF, but because of the shape I need, some kind of filler would have to be used in various places between the different pieces of MDF. The "sand-ability" if this filler would have to be the same as the MDF in order to make a smooth shape, without bumps where the dissimilar materials meet.

 

Would plaster of paris work?

 

Are there different materials and/or techniques that can be used?

 

Does anybody know what Cory Bird did when he made the mould for his Symmetry canopy?

 

 

Joe Polenek

Joe

Cozy Mk IV #1550

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Joe,

 

Check "YOUTUBE.COM" they have some excellent on-line videos that show you how to make various molds. They also show you how to vacuum bag afterwards to obtain quality results.

 

A couple of the videos are long, but they go through the step-by-step process init's entirety.

 

Ghost

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I do recall a write up from many years back in a CSA newsletter on a chap who made his own canopy as he wanted his own profile. I believe he approached it by lofting some formers, then placing strips over the formers whilst still on the fuse' to ensure he had the desired profile. He then made a large oven box form wood paneling with windows in it.

A number of gas ring burners were used to heat the oven box, and I believe he used a soft fabric over the plug to ensue a smooth finish on the inside of his canopy.

I know it is not a lot of detail, but perhaps Terry could help you out with a back copy of that specific issue of the newsletter.

Hope this is of some help.

 

Bruce.

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Our chapter in Ottawa has 4x4x8 insulated oven in which a sandwich of 3/4 plywood,plexiglass and 3/4 plywood is suspended at the top of the oven. It takes about 3 hours to get the electrical heaters to temperature. At this point the plastic is elastic like a soap bubble. Low air pressure is applied and the bubble is blown. A laser beam is used to set the depth of the canopy. The shape is determined by the perimeter of the canopy marked and sawn out on the two plywood sheets. Imagine a double ended rowboat and that is the primary shape for most canopies.

 

The trick is to hold a constant air pressure to hold the shape until the bubble cools to hold the shape. The optics from the outside in tell you nothing but from the inside looking out is a marvel.

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Our chapter in Ottawa has 4x4x8 insulated oven in which a sandwich of 3/4 plywood,plexiglass and 3/4 plywood is suspended at the top of the oven. It takes about 3 hours to get the electrical heaters to temperature. At this point the plastic is elastic like a soap bubble. Low air pressure is applied and the bubble is blown. A laser beam is used to set the depth of the canopy. The shape is determined by the perimeter of the canopy marked and sawn out on the two plywood sheets. Imagine a double ended rowboat and that is the primary shape for most canopies.

 

The trick is to hold a constant air pressure to hold the shape until the bubble cools to hold the shape. The optics from the outside in tell you nothing but from the inside looking out is a marvel.

This is certainly a fun way to spend time and money, but you'll save a lot of both by paying Todd's Canopies to do it for you.

 

I figured I could try this method since I was building an off-size canopy. The MDF for the forms cost around $80 (the first set wasn't right), the plunge bit for the router another $18. The two gas burners (and the extra one to replace the one that caught on fire) were another $25 each. Then the propane tank, hose attachments, etc... at least $50. Probably $100 to build the 4'x8'x4' oven, out of wood & insulation foam. $20 for the oven thermometers to track temps... So that's only about $350 so far, now for the expensive stuff...

 

I bought two sheets of 1/8" plexiglass, figured I'd learn on the cheap stuff, then move up to 3/16". Layed the plexiglass over the MDF form, layed the other MDF sheet on-top, bolted it down onto the oven, fired up the burners. About 20 minutes in, the sheet cracked where the bolts held it down. (Plexiglass expands a lot as it heats to 275 degrees. You need some type of "floating" clamp system".

 

About $50 more in clamping attachments later, I tried again. This time I got a good bubble, but as the bubble got to about 18" in depth, the air would escape. I'd have to re-clamp the whole thing together, try again... Did this several times before un-even temps in the oven caused distortions in the bubble shape.

 

I'd sunk more than $500 in this venture and was pretty sure it would take another 5 or six tries to get it right, at $50 a pop for the plexiglass, and who knows how much more to upgrade the oven.

 

Called Todd's Canopies and he quoted me $425 for exactly what I wanted, plus shipping. Wish I'd done that from the start.

 

BTW -- he's spent years perfecting the technique. He rebuilt his oven to accomodate the size of my canopy request. Uses quartz heat lamps the works.

 

I usually encourage people to try it themselves, but in this case, don't waste your time and money -- get it done right.

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This is certainly a fun way to spend time and money, but you'll save a lot of both by paying Todd's Canopies to do it for you.

 

I figured I could try this method since I was building an off-size canopy. The MDF for the forms cost around $80 (the first set wasn't right), the plunge bit for the router another $18. The two gas burners (and the extra one to replace the one that caught on fire) were another $25 each. Then the propane tank, hose attachments, etc... at least $50. Probably $100 to build the 4'x8'x4' oven, out of wood & insulation foam. $20 for the oven thermometers to track temps... So that's only about $350 so far, now for the expensive stuff...

 

I bought two sheets of 1/8" plexiglass, figured I'd learn on the cheap stuff, then move up to 3/16". Layed the plexiglass over the MDF form, layed the other MDF sheet on-top, bolted it down onto the oven, fired up the burners. About 20 minutes in, the sheet cracked where the bolts held it down. (Plexiglass expands a lot as it heats to 275 degrees. You need some type of "floating" clamp system".

 

About $50 more in clamping attachments later, I tried again. This time I got a good bubble, but as the bubble got to about 18" in depth, the air would escape. I'd have to re-clamp the whole thing together, try again... Did this several times before un-even temps in the oven caused distortions in the bubble shape.

 

I'd sunk more than $500 in this venture and was pretty sure it would take another 5 or six tries to get it right, at $50 a pop for the plexiglass, and who knows how much more to upgrade the oven.

 

Called Todd's Canopies and he quoted me $425 for exactly what I wanted, plus shipping. Wish I'd done that from the start.

 

BTW -- he's spent years perfecting the technique. He rebuilt his oven to accomodate the size of my canopy request. Uses quartz heat lamps the works.

 

I usually encourage people to try it themselves, but in this case, don't waste your time and money -- get it done right.

If we as builders were driven by dollars, whatever they are these days would do nothing. The original post asked about forming Plexiglas into a form. That would be a true challenge. I think that is why canopies are glassed or framed in metal after they are formed.

 

I don’t think one would want to touch a liquid hot plastic bubble with any thing but a laser light beam.

 

As to clamping the solid plywood, Plexiglas, plywood form sandwich that was done with ¼ inch and one half machine bolts with washers screwed into T nuts spaced every five or six inches. The plastic must be clamped firmly and needs to be air tight to apply air pressure. Yes the holes in the plastic are enlarged to one half inch to allow for the movement.

 

The three boxes of hardware from Home Depot were not dear.

 

It can be done and I am sure Todd welcomes business fortunately we have choices. It is a very rewarding process to observe and work on. As is reading this forum and the skill sets it takes to build our planes.

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