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From what I've seen, you can take a typical extruder heater, apply the designated supply voltage without temperature control, and as long as the heater isn't contacting something with a flashpoint below the temperature the heater reaches, the heater will not catch on fire. Thus, unless one catches the filament on fire, it seems that thermal runaway of the extruder heater wouldn't normally start a fire.

I'm not sure what would happen if someone installed wrong components, such as a 12V heater to a 24V supply. What are situations that could cause a 3D print to catch fire?

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  • $\begingroup$ I know it's a bit out-there, but for the interest of people who like science and a laugh, a spillage of FOOF over the printer would cause it to catch fire: Things I Won’t Work With: Dioxygen Difluoride. "Catch fire" includes the filament, the aluminium, the circuit boards, and anything else. $\endgroup$ Jan 14 at 17:14
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In short:

  • The control board does not have thermal runaway installed
  • The heated bed connectors are loose and begin to heat up until the wires catch fire
  • The power supply or it's connectors begin to heat up until they catch fire
  • The modifications that you made to your printer were not well thought out resulting in a capacitor to explode (raises hand; that's happened to me.)
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  • $\begingroup$ Note some connectors, definitely not all, tend to be self fusing. See 3dprinting.stackexchange.com/questions/10695/… $\endgroup$
    – Perry Webb
    Jan 13 at 15:45
  • $\begingroup$ That essentially says when modifications are made to the printer, or with a newly designed printer, it should not go unattended until one is reasonably sure of the modification or design. It's also good to check cases that could lead to failure. $\endgroup$
    – Perry Webb
    Jan 13 at 15:56
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A normal hotend will not melt or cause fires, usually, see first video. However, if the power regulating unit fails as well and higher voltage is supplied (19 V on 12 V cartridge are enough) it can happen, see second video.

Fires are more likely caused by overheating wires, especially where joints are presents.

Hotend runaway does not cause fires from the hotend itself, but it could cause fire related to its wiring. Also, the uncontrolled temperature may make the hot end fan fail, worsening the situation.

Issues canbe caused also by a runaway on the bed and overheating MOSFETs.

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  • $\begingroup$ Too much current on a wire can definitely cause a fire if a fuse doesn't blow and current control fails. In such a case, shorted wires could cause a fire. $\endgroup$
    – Perry Webb
    Jan 13 at 15:50
  • $\begingroup$ Aluminum will burn at its Melting Point 660.32 °C. That appeared to be the fire with thermal runaway test 2. $\endgroup$
    – Perry Webb
    Jan 14 at 2:04
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Another source of fire comes from connections, not the heaters themselves.

As the connection and connector ages, the electrical resistance of the connection increases. Up to a point, this is not a problem. A portion of the energy goes to heating the connection, and less energy goes to the heated element. These are in closed loops, so energy is applied longer to compensate.

The connection is using the lost energy to heat itself. This increases the corrosion rate, and also can cause plastic in the connector to flow, which relaxes the connection pressure, which further increases the corrodion, resistance, and heating.

This situation is unstable, and ends in one of three ways:

  1. the extrusion hot-end or the bed don't get as hot, with implication on the print quality,
  2. the connection opens, and the heated system simply fails,
  3. or the worst case, where the temperature increases to the point where the connector housing, wire, heater PCB, or dust and fur on the connector catches fire.

The only real mitigation of this risk is to periodically check power connections for warming. If a connection is warm, it should be watched more carefully. If it is hot, you already have a problem and should fix it.

If you are concerned, focus on preventative maintenance, and on being sure that there are no flammable materials near the printer. Don't put your printer in a simple foam case. Consider the flammability of any enclosure components.

You might also consider adding a smoke and high-temperature detector connected through a relay to disconnect main power. If a connection starts sputtering, you might detect it before the fire can transfer to your house.

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  • $\begingroup$ Would you suggest re-making connections occasionally so that the oxide layer/dirt buildup is broken? Might contact cleaner products help? $\endgroup$ Jan 14 at 17:19
  • $\begingroup$ I would not gratuitous reseat the connector. If the connection is sound, there is already a gas-tight metal-to-metal connection. Breaking that connection will increase the wear, and decrease the chance that the repeated connection is sound. I would only remake the connection if it feels warm, or if the warmth increases. Contact cleaner used at that time could be help. If it continues, I would plan to replace both sides of the connector soon. This is my opinion based on reading connector specifications and failure modes. I don't speak for experts, or any manufacturer. $\endgroup$
    – cmm
    Jan 14 at 18:03

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