Hi. Interesting project. What kind of temperature do you need to maintain on the line? There are different electrical heat tapes available, but what range of temperatures you can expect I don't know.
Another thought would be a steam trace-steam has a lot of heat-5 times more than water at 212 degrees! They use it a lot in industry to keep pipes from freezing in the cold. Just copper tubing tied along the length of the plastic tube.
The cool thing about using a screw pump is that it can be a simple on/off unit. Several of the CNC packages allow control of "extra" digital or bit outputs.
What about no pump at all? It occurred to me you would want to purge the lines between colors. How? Blow them out with air. Then it struck me that you could used compressed air to push your molten plastic through the lines.
Control would be via a two-way solenoid valve that would allow pressure to be applied when activated, then vent all pressure off when deactivated. Just some things to think about.
