Thermal runaway protection is exactly what it sounds like. It’s a protection system against a situation where your printer heats up uncontrollably. To understand this risk, it’s useful to understand how a 3D printer is supposed to manage its temperature. We highly advise that you test if thermal runaway protection is enabled and functioning on your printer.
When a 3D printer is being heated up for use, the control board sends an instruction to the heating element to heat up. The heating element is turned off again when a thermistor registers the heating cartridge to reach the desired temperature. Unfortunately, if there is an issue with the thermistor, this process doesn’t work.
Unless there’s a failsafe protection system in place, the heating element will continue to heat up unchecked. Eventually, something will melt and generally catch fire or otherwise cause a fire. This is what the thermal runaway protection system prevents.
The printer knows roughly how long it should take for the heating element to reach the desired temperature. If the thermistor doesn’t register the heating block as reaching this temperature in the expected timeframe. Then the thermal runaway protection system will trigger. Some extra headroom is left for weaker heaters.
When triggered, thermal runaway protection cuts all power to the heating element and stops printing. A message is also displayed on a screen, if present, to explain the reason for the shutdown. To continue after a thermal runaway protection shutdown, you will need to restart and reset your printer.
Note: A thermistor is a resistor where the resistance varies based on its temperature. Measuring the resistance allows the temperature to be calculated. This process will be automatically managed by the mainboard.
What Causes Thermal Runaway?
The cause of thermal runaway is almost always an issue with the printer hardware. The thermistor itself could be faulty or not making good or any contact with the heating block. Faulty or partially broken connectors or wires can also be to blame. Finally, your heater cartridge itself could also be in the wrong place or have fallen out.
If any of these things happen, the heating element will continue to be powered until something changes. Ideally, the thermal runaway protection system kicks in. If not, the heating element will continue to get hotter until it breaks. While the Nichrome wire in the heating element melts at around 1,400 °C, the aluminum of the heating block melts well before that at around 660°C. You then have incredibly hot aluminum falling onto what is likely at least a partial print which will burn. Of course, some nearby plastic may melt or ignite even before this.
What Should You Do?
It is an excellent idea to plan for what to do in the eventuality of a fire before the risk becomes a problem. You should invest in proper fire safety equipment. Verify that thermal runaway protection is on. At a minimum, when dealing with fire hazards such as a 3D printer, you should ensure that you have suitable fire preparations.
You should ensure that your printer is placed so that it does not block any exits, that you’ve installed a smoke detector directly above it. Also, make sure you’ve installed a CO2 fire extinguisher within arm’s reach of the door to the room.
To enable thermal runaway protection for your printer, you will need to go into the printer configuration files. While the exact location will vary between firmware providers and versions, you need to look for the thermal runaway protection section. Check that the settings are as recommended by the firmware provider.
Many printer manufacturers and third-party firmware providers now enable thermal runaway protection by default. Some versions do not, however, or don’t even include the option. For the verification step, it is helpful if you check the length of the protection timer. If you don’t check this, you don’t know how long to wait for the system to activate.
Testing Thermal Runaway Protection
Once you know it’s on, there are two relatively easy ways that you can verify that the protection is functional. You can disconnect the power cables for the heating element. Then instruct the printer to heat up until the thermal runaway protection mechanism triggers. Alternatively, you can instruct the printer to heat up normally and then deliberately cool the heater cartridge until the protection system triggers a shutdown.
If you want to disconnect the element, make sure that the printer is cold and unplugged before disconnecting or reconnecting anything. For the cooling method, you’ll need a fair amount of cooling power. A hairdryer with the heating disabled or compressed air can work well. In either case, you’ll need to give the printer time to realize that something is wrong and shut down automatically. If this happens, then the thermal runaway protection is working as intended.
If your printer doesn’t automatically shut down, then either your printer’s firmware doesn’t support the protection system or something else is wrong. Try checking for and installing the latest version of the firmware. Using a third-party firmware solution that offers the feature is also an option. Depending on your confidence in diagnosing the issue, you could also try repairing it yourself. If the problem persists, you may want to replace the printer or repair it.
Safety is always important, especially when working with boiling components that can start fires. Do you have any tips for testing the thermal runaway protection of a 3D printer? Let us know down below.