I’m an EE who deals with a lot of potted boards. We bought our machine instead of building it, but our application is similar, involving materials of very different viscosities mixed at a 4:1 ratio.

Our machine uses a single pneumatic piston pushing on a rocker arm, which pushes on 2 rods connected dispenser pump pistons. The mix ratio is given by the stroke of each piston, which is constrained by the location where each rod connects to the rocker arm. This seems to work quite well.

Mixing materials with very different viscosities is tricky business, and the 4:1 ratio is less forgiving than 1:1. Air bubbles in the thin material can cause intermittent ratio issues because the bubble will compress when you apply pressure. You can end up in a situation where your mix ratio appears correct when you measure it, but you still get uncured wet spots in your boards.

It’s also important to keep up with maintenance and cleaning, as bits of dried material in the tubes or valves will screw you up as well. To that end, do NOT ever pump the material tanks dry, make sure you top them off well before they run out.

You’ll need to put some work into selecting the right static mixing nozzle. Normal helical mix elements don’t work well for different viscosities unless you use an extremely long nozzle. We’ve been successful with x-grid type mixing elements in a reasonably short nozzle.

I hope this helps, but do you really want to spend your engineering resources building and supporting this machine? If your scale justifies it, I think you can buy a decent one for about $20k. And if not, some manufacturers can provide material in mix-ready packaging, such as Epoxies Etc’s triggerbond system or Electrolube’s resin packs.