Desktop molding machines look awesome, and are way high on my list of things to build. Apparently you can make a hand crank version that works like a bottle capper for a few hundred bucks. It can make dozens of parts an hour from nothing but raw pellets, electricity and elbow grease. Pretty exciting. http://makezine.com/projects/make-41-tinkering-toys/diy-injection-molding/
They got me thinking about a 3D printer hack to replace the print surface with a clamping plate and the print head with some sort of super high flow extruder so you could use 3mm filament as feedstock. You could clamp 2 halves of a small mold into a vice on the bed, then program the printhead to move down to the filling hole (I know next to nothing about injection molding ) and extrude the right amount of plastic at the right time.