Epoxy casting (by alaric)
Inspired by this blog posting I set out to learn how to cast circuits in transparent epoxy.
You see, making decent cases for things is sometimes the hardest bit about an electronics project, and an issue that had been a major roadblock in my interest in wearable computers. What point was there in building something if it wouldn't last long under the wear and tear of being attached to me, and getting rainwater inside it?
The data sheet lists the hardness as "95 Shore D" - I had to look up Shore Hardness on Wikipedia to see what that meant. Turns out it's harder than a hard hat, which is merely 75 Shore D. I look forward to my creations being HARD-HAT HARD.
The first experiment was to just check out the casting process, so we rolled some Plasticine flat onto a sheet of clear plastic from a Fefrero Rocher box, then pressed a diamond-shaped cookie cutter into it and pulled out the plug to leave a diamond-shaped hole. For kicks, we put four of Sarah's Fymo pebbles in, calculated the volume of epoxy required to fill it, and proceeded to follow the instructions.
It takes 2 parts of resin to 1 part hardener, and they recommend putting the hardener into the mixing cup first, so I used a 5ml medicine spoon to measure out 5ml hardener, then 10ml resin. The hardener is like thin cooking oil, while the resin is like honey - so they don't want to mix easily. It takes a lot of mixing to convert it from streaky stuff, consisting of hardener with 'strings' of resin in, into a clear liquid with the consistency of cooking oil; but it didn't take too long.
The resin claims to take 19 hours to cure enough to remove from a mould at 21 degrees C, but it was colder than that where I was, so after a full day it was still jelly-like; I'd poked at the remnants left in my mixing cup to observe the transition from the consistency of oil to the consistency of honey to the consistency of jelly. I moved it somewhere warmer and left it another day, then we carefully removed the Plasticene, leaving the diamond sitting there on the plastic sheet - however, attempting to prize it from the base just made it flex alarmingly; it now had the consistency of soft rubber, and looked like it would tear before coming off of the board. Mindful that the spec sheet claims it takes a long time to reach full hardness at 21 degrees, but that it can be done in only 15 hours at 60 degrees C, I then put it in the oven with a thermometer and carefully regulated it at around 60C for much of a day, whereupon it was then hard enough to prise away from the base.
The result was quite good, but the surfaces where it had been in contact with the Plasticine were bubbly and full of Plasticene, and the surface where it had been on the plastic sheet became cloudy when it tore away from the sheet - so I need to use better moulding materials next time, but nonetheless, I've sanded the sides clear of bubbles, and I gather that with some finer sandpaper I should be able to make it smooth enough for my Dremel's polishing set to make it shiny...
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