Little progress on the imote as of late because I was busy hammering out a massive PCB order for imotion, a cool Christmas wizmo (more on that later), and a new product called Logic. I managed to get that out the door at about 5AM Saturday morning.
Then it was back to the imote, and imote adhesive, the current bane of my existence. The adhesive is a two-part epoxy that hardens up in about 10 minutes. You mix it with a cool glue gun (see below). The trigger presses a plunger which ejects glue from the cartridge. The two parts are mixed in a long “mixer” which swirls the two together as they travel down the tube. Then a luer-lock attached syringe needle caps it all off, giving you a nice controllable output. That is, until the glue starts to harden up. Then you take off the needle push a bunch of glue through to clear it and put the needle back on. Yes, it’s as messy as it sounds. Not great for small quantities. I went through at least 20 disposable gloves trying to keep the parts free of glue…
Anyway the main takeaway so far is that this isn’t going to be easy. Basically the issue with gluing in the polycarbonate/plexy/glass is as follows: too little glue, you have a cosmetic problem because you can see the difference between where the glue is and isn’t. To much, and it squeezes out the sides, becomes a cosmetic problem. So that has it’s own class of potential solutions.
The other issue is that (stupidly) I figured the PCB could be installed with adhesive as well and so didn’t design a mounting solution. So one solution is to design one up and through away all the parts I have (~800$) but let’s call that a last resort. The first good solution is to combine the problem with that of the glass (not really glass but that’s what I’m going to call it from now on) — install the glass so that some glue overflows. Then dump a bunch more glue into the assembly on top of the glass, and stick the PCB in on top of that, allowing the glue to seep up through the apertures in the PCB and around the edge. Solution part two, is then to fill up the rest of the enclosure with glue (!) and install the top glass piece at the same time, squeezing out and cleaning up the extra at the same time. Luckily the adhesive I’m using is very well behaved and doesn’t expand or contract noticeably otherwise obviously this would be out. The problem, though, is that 1) it looks like a fish bowl (index of refraction), it increases – probably doubles — the weight, and most importantly makes little bubbles on the surface of the PCB/components where it reacts (presumably) with the residue on the surface. This last thing makes it a cosmetic issue.
To sum up, what looks like the best option now is to float the pcb in the adhesive but not fill up the part with adhesive. Then, install the other glass piece by 1) applying glue along the ridge and then 2) using a custom tool to essentially scrape out all but a controlled distribution of adhesive, such that when you put the glass in it’s the perfect amount — perfectly covering the ridge surface but not spilling out.
Ok, on to the pictures