Step 11: Potting the Watch
To make the watch suitable for everyday use it needed a case. I visited AFF Materials ( http://www.aff-materials.com/ ) to buy polyester resin. A nice guy there suggested that I use a clear epoxy instead. According to him, the polyester resin shrinks ~5% which might fracture connections on the PCB. The clear epoxy only shrinks ~2%. He also suggested that gases from the polyester might damage components while it cured.
Having never worked with a clear epoxy before, I did some test castings. I started by casting some samples in an ice cube tray. Sunflower seed oil, silicone lubricant, and silicone bicycle lubricant were tested as release agents. One sample was done with no release agent. The silicone lubricants beaded in the bottom of the mold and left pock marks on the epoxy. The control suck to the bottom of the mold. The oil worked pretty well, but left a slight residue in the epoxy.
Next, I needed to know how to do a multi-layer casting with this material. A polyester resin is usually poured in layers. A first layer is allowed to set (about 15 minutes) to a gel. An object is placed on the first layer and a second layer of fresh resin is poured on top. The working time of my epoxy is about 60 minutes. I poured a first layer and checked it after 30 minutes - still soft. After about 1 hour and 15 minutes the first layer had stiffened enough to place an object on it. For this test I put the LED test board seen in step 2 face down on the first layer, and covered with a layer of fresh epoxy.
This worked great, the LEDs didn't pop off the board. I concluded here that absent a proper mold, the clearest surface I can make is the air/epoxy interface. The 'top' of the casting has a significant miscus. The miscus is limited to the very edge of the casing and is easily removed with a grinder.
For the first real test I needed a rectangular plastic mold. The best option I found was a 'smeer kaas' container. It wasn't perfect, so I made it smaller with a few layers of tape-wrapped foamcore. This wasn't a stellar mold, but choosing the top as the display surface gave me some leeway. The mold was lightly wiped with oil on a paper towel.
I ditched the multi-layer pour procedure from above. I soldered leads from the coin cell battery holder to the PCB. The cell holder was hot-glued (ok, stickie-tacked) to the bottom of the PCB. The battery holder was filled with stickie-tack, and the programming header protected with yet more stickie tack (plasticine would also work great). This was then placed, face up, in the mold. The stickie tack protecting the battery and header was pressed firmly into the bottom of the mold, anchoring the watch in place. Clear epoxy was poured into the mold until it covered the watch. The pin-headers were still quite long, but can be cut after the epoxy dries.
The watch released from the mold after about 36 hours. The protective putty was removed with a screw-driver. The edges were smoothed with a drill-press grinder bit. The watch was cast a little large to be worn as a wrist watch. I may try to cut it down if I can find a band saw. For the time being, it will be a pocket watch. The tape-over-foamcore gave a cool texture and ultra-clear surface. Next time I will try to make the entire mold using this material, something more in the neighborhood of wrist watch size.