Saw this paper, “Voxelated soft matter via multimaterial multinozzle 3D printing“, pdf. Two or more fluids come together at bend, and static pressure is enough to keep the current printing liquid moving towards the outlet, not backing up into the second material source tube. And the pressure of the current print liquid keeps the other fluids back.
There is effectively no mix chamber, so the change from one fluid to the other is quite quick, and there is little mixing after a switch.
This works because of the size and orientation of the fluid tubes in relation to the viscosity and other properties of the liquids. The authors make the print heads out of plastic and print with silicon and wax.
To use this for 3D printing plastic, the print head should be made out of a material with better heat resistance, such are metal or ceramic.
Make a print head like this out of ceramic (alumina, or similar ‘technical ceramic’). 1) 3D print the flow chamber and nozzle geometry out of a thermoplastic (or wax), then 2) slip cast ceramic around this. 3) When the ceramic is fired, the plastic will melt out or vaporize, leaving the desired nozzle geometry.
The geometry needed is simple, at least for two inputs. The thin join can be a very short segment, a few mm in length. The lead in tube can be drilled 2-3mm wide, then the 0.5 or 0.25 mm join tubes can be drilled out. Drill the outlet from the bottom, then drill the inlets from the bottom of the lead in holes. This would require precision to make the segments join up correctly, but the drill holes would be short.