Your Me2 extruder is equipped with a temperature sensor called a thermistor (a portmanteau of ‘thermal' and ‘resistor'). You can read about them on Wikipedia.
The thermistor is part of a feedback loop that maintains the print nozzle at the right temperature to melt and extrude plastic filament. This feedback is called a ‘thermostat’ loop, the same as you might use to control the heating in your home.
Your Me2 constantly monitors extruder temperature and will automatically shut down as a safety precaution if something has gone wrong with the thermostat. The most likely causes are:
- Over temperature warning: Me2 is intentionally limited to a maximum temperature of 250°C. Unless you are printing very specialised materials requiring the addition of a heated build tray you should not need to go above ~240°C. If your printer is reporting absurdly high temperatures (999°C for example) your thermistor may be damaged.
- Under temperature warning: If the printer is connected but reporting 0°C it is assumed your thermistor may be damaged or disconnected. We don’t recommend trying to print at freezing ambient temperatures, though it is possible. If you need to print at sub-zero, please contact email@example.com to find out how to modify your firmware.
- Thermal runaway: If it appears the extruder is much hotter or colder than it should be for the amount of energy put in, the printer will trigger a ‘thermal runaway’ error. This type of error is common to any thermostat control, but in the case of Me2 it may mean that something is wrong with your thermistor circuit.
When any of these errors are triggered, your Me2 will immediately stop what it is doing. Because this is a hard fault, the machine will require a manual reset to continue.
To manually reset your Me2
- Gently support the weight of the extruder arm with your hand, being careful not to touch the underside of the extruder head as it may be hot. You can alternately hold the back end of the extruder arm if there is enough space for you to hold.
- With your other hand, turn off the power switch on the back of Me2. Be careful: you are now supporting the full weight of the extruder arm!
- Avoiding contact between the hot end and the print bed or any printed parts on the tray return the extruder and print bed to the home corner position and gently let the extruder down.
- In Mattercontrol, disconnect from the printer (or disconnect the USB cable), then reconnect. IF you are familiar with the terminal, you can send M999 to the printer instead.
- Turn the printer power back on.
If you are routinely suffering thermal runaway and both over and under temperature warnings, your thermistor may be incorrectly installed or damaged. In this case you should contact firstname.lastname@example.org for assistance and to make a warranty claim if your extruder is still within the warranty period.
Diagnosing your issue
These failures can occur for a variety of reasons when the extruder temperature doesn’t match the amount of power delivered to the extruder heater. Some of the most common reasons are listed below, along with advice on how to fix the issue.
Heater wire terminal connection
The extruder heater is connected to the mainboard with two wires set in a screw terminal near the power switch at the back of the Me2. The terminal block is labelled HEATER1 (see below)
If these wires are loose, poorly seated, or wire insulation is preventing good contact you may see thermal runaway issues. To inspect and correct, you’ll need a flat head screw driver no wider than 4mm.
- Unscrew both the HEATER1 terminals. The wires should come free. If they were already loose, then that’s most likely where your problem has come from! Make sure the terminals are then screwed all the way out to aid in reseating the wires later.
- Inspect the wires - they should have 3-4 mm of exposed wire at the free ends. Too little wire exposed and the insulation is getting in the way of the terminals making good contact. Too much wire and the heater wire is potentially exposed to short circuits (very bad!).
- One at a time, place the exposed wire inside the terminal jaws and tighten the terminal until the wire is securely held. Do not push the wire in so far that the plastic wire insulation is inside the terminal block.
Thermistor mounting screw too tight
A common failure can occur when the screw that pins the thermistor to the hot end block is overtightened and digs into the wires of the thermistor, causing a short. You may be advised that you have the option of attempting a quick repair on your own to fix this. The quick repair is as follows:
- Make sure the extruder is cold.
- Prop the extruder up on a book, box or other supporting object over the build tray (a filament box works quite nicely), or use the jog controls in MatterControl to lift the extruder up to a comfortable working height. (You may alternately remove the extruder assembly to work on it separately)
- Approaching the extruder from the rear, use a sharp blade to put a single horizontal slice through the kapton tape and insulation on the rear face of the hot end block next to where the heater and thermistor wires enter. Make sure to cut completely through the insulation.
- Through this cut you will expose the mounting screw for the thermistor. You can gently prise the split open with tweezers to see the screw head.
- Using a screwdriver, turn the screw anticlockwise by half a turn.
- Reset your printer and try running a test print or heat cycle by using Mattercontrol to turn on the fan, clicking the preheat button, and extruding some filament once the nozzle is heated up, retracting the filament and turning off the heater before allowing the extruder to cool).
- If you’re still experiencing errors, you can try to loosen the screw further. If the thermistor wires are loose and mobile then the error is likely elsewhere and may require further investigation.