Working with clear resin for canopies and other sublime forms of torture.

Saturday, February 25, 2017

A number of people have asked about the process of making model canopies with clear  resin recently, so finding myself with a spare hour I have decided to write up a blog post on how I do it.

The initial shape, or plug must be made from a hard material, preferably non-porous.Wood is lovely to carve and finish, but ultimately will cause more trouble than it's worth, so it's best to use a synthetic wood like Ren-Shape, or a block of hardened Bondo or a similar filler. The form must be somewhat undersized, as you're going to smash-mold or vac-form heated plastic over it to get the wall thickness you want. Plan ahead! Make the master longer and deeper than the finishsed part so you can trim the edges perfectly square and clean.If your plug is net to the size of the final part at the ends the plastic will roll over the edges and you're back to square one.

 Any scratches or tool marks that are on the plug will transfer right over to your future master, so be prepared to spend a long time smoothing and finishing the plug.Soothing music and the occasional adult beverage will help. I prefer Blues and a good gin & tonic.

Once you have a clean plastic shell to work with, you must polish it to a very high finish. I use very fine wet sandpaper ( 600 grit ) for the first pass, with a drop of dish soap in the water to prevent loading up of the sandpaper, which will cause nasty scratches and another beverage, perhaps two.

I then work the surfaces of the canopy master to a better  polished finish with varying grits of Micro-Mesh, and then cast a prototype in a silicone rubber mold from which I can make opaque resin castings.There are two reasons for doing this: Firstly, the urethane resin part will be harder than the plastic one in most cases, and will therefore take a higher polish, and secondly because you're going to ruin a few masters scribing lines in them before you get one you're happy with.Trust me on this, it may seem like a pain, but it will pay for itself many times over.

Final polishing is best done with a small felt bob and a Dremel tool.Use fine rubbing compound and then swirl removing compound from the auto pats store.A gentle touch is required here or you will get the part hot and it will deform.

The nifty resin master will need the transparent parts outlined, preferably with fine scribed lines.I use the 3M vinly tape sold in auto parts stores to outline the frames and a sewing needle held in a pin-vise to do the scribing.Black electrical tape is also very good for this.Start out gently, and make a number of passes so as to create a smooth line, then burnish it with a toothpick.These scribed lines are going to be very important later on, so if you muff one, scrap the part and start over.

Once you have a shiny master with good outlines of the transparent bits, you can clean the part with Windex and give it a dip in Future, or Johnson's Klear, Pledge with Future Shine or a similar acrylic floor wax. If all goes well, you will have an extrememly glossy, perfect master from which to make a mold. If you like, you can take Scotch lo-tack painters tape and use that toothpick to burnish it down into the scribed outlines and cut out the frame.This allows you to spray a bit of matte primer onto the frames which will  make them more visible  later on for painting.Remeber to mask off the interior of the master, ask me how I know!

 

The finished masters are then molded for casting in clear resin.Most clear resins do not cast well in tin-cure RTV rubber, although it can be done.Platinum cure rubber is best as the resin will not react to it and it will not shrink during cure.

In the photo above you can see the primered frames have a different texture than the polished areas that will remain clear.

There are quite a few clear casting resins available today, my weapon of choice is BJB Industries WC-784 which has proven to be fairly reliable.The resin must be cast into the mold after mixing, and then the mold put into a pressure pot  with 35 to 40 pounds pressure piped in to crush any small bubbles.With this resin you must also heat the pressure pot to 120 degrees f and hold it there for three hours or so to get a perfect cure. I do this with an elctric oil heater under the pot, and the whole affair covered with an old blanket to hold in the heat.

As you can see, it's a long, involved process.You can probably find a resin to work with that does not require the heating for a good cure, but either pressure casting or vacuum casting is a must to avoid those pesky bubbles.

It took me a long time to get a reasonbly reliable production technique worked out for this stuff, if you're game to try it I say have at it and good luck! If it was easy, everyone would do it.

Best, 

Paul

 

 

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