Toast

The first problem was easy enough to diagnose. Since I had rebuilt and rewired the circuit it had to be the thermistor itself. After removing it, I could see the underside was looking a bit toasted. I don't think it was designed for use up to 250°C. The insulation on the wire was not up to it and I suspect it was soldered to the actual device, and that the solder melted. There seems to be some solder on the brass nozzle now where it was mounted.



The remedy was easy: I just replaced it with the recommended glass bead thermistor which had arrived from back order in the meantime. It is rated to 300°C. Its characteristics are different so I had to change my resistor values, but I had anticipated that by mounting them on the connector rather than the board.

Getting nowhere fast

Two weeks ago I had my extruder controller built on breadboard controlling the heater temperature and motor speed. All I had left to do was link it to my main controller and talk to it from my host software. This should have been easy as I already had I²C working to my spindle controller ...

The first thing that went wrong was the temperature reading from the thermistor started to become erratic. I decided this may be due to a bad connection as my breadboard layout was getting a bit messy.



The hot resistance of the thermistor is only about 12Ω so I was willing to think a bad connection could be possible as I had not used the breadboard for over 10 years. I was also getting a lot of noise from the motor so I decided to rebuild the circuit on vero board and shorten all the connections.



I paid careful attention to the layout to keep the high power stuff away from the sensitive inputs and the micro, and route the ground currents sensibly. The connectors on the far left are the outputs for the heater, motor and possibly a fan. Next is the power in connector followed by 3.3V and 5V regulators. The shaft encoder is 5V but the micro is 3.3V, the four resistors handle level shifting. Next are the input connectors for the shaft encoder, thermistor and filament exhausted sensor. The far connectors are for the I²C bus.

I mounted it on the z-axis together with my spindle controller so that the only moving wires are a 12V feed and the I²C bus.



All the wires are now much shorter and screened. I also earthed the casing of the motor. On testing I was very disappointed to find :-

1. The thermistor was still erratic.
2. The I²C bus did not work much at all.
3. The noise around the circuit was just as bad if not worse. Until I added the earth connection to the z-carriage the micro crashed when the motor was running.

Not the result I was hoping for!

Caught in the act

You may remember that I reported a ribbon of swarf coming out of the side of my extruder :-



It wasn't obvious to me how this was formed. When I stripped it down today I caught it in the act :-



It appears that when the threaded rod cuts into the plastic it displaces a corrugated ribbon of material sideways. This remains attached to the filament at the leeward side and the ridges formed by the thread remain joined to each other by very thin webs. As it progresses down the pump it gets separated from the filament, presumably where it enters the barrel, and finds its way out through the side. I think the root cause is that when polymers like HDPE are stretched the long molecules get aligned length ways and it becomes very strong even though it is very thin.