Re: Fire Hazard July 03, 2014 02:08PM |
Registered: 10 years ago Posts: 814 |
Re: Fire Hazard July 03, 2014 02:13PM |
Registered: 10 years ago Posts: 24 |
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Hazer
And all of this does not cover the electrical fire potential. I have seen some people place extra thermal cutoffs around the electronics, but they would not trip out until the fire has already ignited. So what does cutting power do when the flames are already there?
Re: Fire Hazard July 03, 2014 02:57PM |
Registered: 10 years ago Posts: 553 |
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Hazer
The point I have been trying to make is that you need more than just temperature-cutoff devices in order to eliminate the potential for fire. As mentioned, if you do not have the thermal cut-off directly on your heat block, then you run the risk of your hotend dropping out of its plastic fixture before the thermal cut-off does its job. And even then, there is no guarantee that the thermal cutoff will trip BEFORE the ignition occurs (and if your already on fire, who cares about disbaling the power to yuor hotend). You can only solve thermal runaway if your trip device is designed for the media you are heating in the first place.
In the case of thermal cut-offs in appliances, you forget that the high voltage heater element is attached through metal fixtures that wick away the heat by design and that is what the thermal cut-off is attached to. It is directly coupled to the secondary heated material. Now you could say by attaching a thermal cutoff to the top of your all-metal hotend would work, except 99% of those designs have a fan blowing on that portion of the hotend, and thermal runaway would still occur.
And all of this does not cover the electrical fire potential. I have seen some people place extra thermal cutoffs around the electronics, but they would not trip out until the fire has already ignited. So what does cutting power do when the flames are already there?
Re: Fire Hazard July 03, 2014 03:10PM |
Registered: 10 years ago Posts: 790 |
Re: Fire Hazard July 03, 2014 04:49PM |
Registered: 10 years ago Posts: 172 |
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As mentioned, if you do not have the thermal cut-off directly on your heat block, then you run the risk of your hotend dropping out of its plastic fixture before the thermal cut-off does its job.
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And all of this does not cover the electrical fire potential. I have seen some people place extra thermal cutoffs around the electronics, but they would not trip out until the fire has already ignited. So what does cutting power do when the flames are already there?
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you forget that the high voltage heater element is attached through metal fixtures that wick away the heat by design and that is what the thermal cut-off is attached to.
Re: Fire Hazard July 03, 2014 05:30PM |
Admin Registered: 17 years ago Posts: 7,879 |
Re: Fire Hazard July 03, 2014 07:16PM |
Registered: 10 years ago Posts: 172 |
Re: Fire Hazard July 03, 2014 07:58PM |
Registered: 9 years ago Posts: 34 |
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nophead
The simple solution is to use a 20W heater instead of a 40W heater. If it is 100% on the hot end gets to about 300C and smokes a bit. The insulator might be damaged. If it has PTFE it might kill your pet fish or give you flue like symptoms. Other than that it is fail safe.
Re: Fire Hazard July 03, 2014 09:11PM |
Registered: 10 years ago Posts: 67 |
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umdpru
I guess you missed the part about the automatic fire bottle above your work bench? I have MUCH greater peace of mind since I installed one along with fire alarms right above my printing bench.
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umdpru
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you forget that the high voltage heater element is attached through metal fixtures that wick away the heat by design and that is what the thermal cut-off is attached to.
THAT is entirely inaccurate! You're telling me that my stove's metal heating element is connected to structure on the stove that is DESIGNED to wick heat away from the element into the stove? That's an absolutely assinine assumption and absolutely wrong. The efficiency of heating appliances would suffer catastrophically if they were required to heat the appliance AND the medium needing heating (my food). Sorry to be so blunt but you are way out in left field on that one.
Re: Fire Hazard July 03, 2014 10:53PM |
Registered: 10 years ago Posts: 790 |
Re: Fire Hazard July 04, 2014 12:28AM |
Registered: 10 years ago Posts: 1,381 |
Re: Fire Hazard July 04, 2014 04:26AM |
Registered: 11 years ago Posts: 172 |
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umdpru
I like that solution, Nophead. A 40W cartridge is certainly overkill. My J-heads come to 235C in a a few minutes at most. I can wait 10 or 15 for them to heat up. the heatbed takes that long anyways.
Re: Fire Hazard July 06, 2014 04:01AM |
Registered: 10 years ago Posts: 16 |
Re: Fire Hazard July 08, 2014 08:45PM |
Registered: 9 years ago Posts: 5 |
Re: Fire Hazard July 10, 2014 10:47AM |
Registered: 10 years ago Posts: 790 |
Re: Fire Hazard July 16, 2014 01:12PM |
Registered: 10 years ago Posts: 814 |
Re: Fire Hazard July 23, 2014 02:03PM |
Registered: 9 years ago Posts: 41 |
Re: Fire Hazard July 23, 2014 09:13PM |
Admin Registered: 12 years ago Posts: 1,063 |
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ElectroWomble
Has anybody tested one of the all metal hot ends (which presumably conduct away a bit more heat than a PEEK hotend) with a 20W heater as @nophead suggests? If we can still get to 315C then this just seams like a no brainer. I would like to be able to print at 310C to print PC but I would very much like not to be able to print at 600C! Intrinsically safe just seems like a good idea.
Re: Fire Hazard July 24, 2014 06:14AM |
Registered: 10 years ago Posts: 903 |
Re: Fire Hazard July 24, 2014 07:07AM |
Admin Registered: 12 years ago Posts: 1,063 |
Re: Fire Hazard July 24, 2014 07:29AM |
Registered: 10 years ago Posts: 903 |
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thejollygrimreaper
the resistors they sell are 6.8ohms so each one is a 21watt heater on it's own, with two you've got 40watts , ultimately i wouldn't expect much different heating behaviour
Re: Fire Hazard July 24, 2014 08:08AM |
Admin Registered: 12 years ago Posts: 1,063 |
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vreihen
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thejollygrimreaper
the resistors they sell are 6.8ohms so each one is a 21watt heater on it's own, with two you've got 40watts , ultimately i wouldn't expect much different heating behaviour
Would there be any safety benefit to modified electronics and firmware that have separate circuits for each resistor, and only use one of the two for warming up? Ie: shutting the second resistor down once the hotend is in the PID loop?????
Re: Fire Hazard July 28, 2014 03:38PM |
Registered: 11 years ago Posts: 1,171 |
Re: Fire Hazard July 28, 2014 06:31PM |
Admin Registered: 12 years ago Posts: 1,063 |
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tmorris9
I have been thinking about this since this thread started.
What about putting a second thermister in the heater block (just drill another hole) run it to an Arduino that is powered seperatly (wall wart) and then set it up to turn on a relay for the mains power to the machine as long as the temperature is below a set point (say 300c). This would assure that if something happens to the heat or to the Arduino that the power would be cut to the printer.
I checked with several manufacturers of thermal fuses but none have high enough temperatures for our use.
So for the cost of an Arduino, thermister, relay and wall wart (say $50 or less) you could have a way to power off the machine in case of thermal runaway.
I am not a programmer but I would think the program for the Arduino would be dead simple.
Re: Fire Hazard August 04, 2014 05:14PM |
Registered: 11 years ago Posts: 1,171 |
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thejollygrimreaper
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tmorris9
I have been thinking about this since this thread started.
What about putting a second thermister in the heater block (just drill another hole) run it to an Arduino that is powered seperatly (wall wart) and then set it up to turn on a relay for the mains power to the machine as long as the temperature is below a set point (say 300c). This would assure that if something happens to the heat or to the Arduino that the power would be cut to the printer.
I checked with several manufacturers of thermal fuses but none have high enough temperatures for our use.
So for the cost of an Arduino, thermister, relay and wall wart (say $50 or less) you could have a way to power off the machine in case of thermal runaway.
I am not a programmer but I would think the program for the Arduino would be dead simple.
the program would be very easy and quick to do, i would do it on a small pic chip or a small avr chip as opposed to a full blown arduino
i've been looking at putting very specifically chosen fuses in line with the hotend so that the large current draw blows the fuse when the mosfet fails but not during normal operation, what is interesting though is that most of the automotive fuses are only rated for a service life of 100 hours at 100% of their rating but for 45c a piece it's a cheap disposable option
Re: Fire Hazard August 08, 2014 02:48PM |
Registered: 10 years ago Posts: 469 |
Re: Fire Hazard August 08, 2014 09:23PM |
Registered: 10 years ago Posts: 128 |
Re: Fire Hazard August 08, 2014 11:51PM |
Registered: 11 years ago Posts: 1,171 |
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chngyian
Have a simple idea that I'd love to try out, but would like to get some comments first.
If a falling out or faulty thermistor is the top reason in causing fire, can I just add a second thermistor to the heating block of the hotend and monitor the temperature? If the temperature exceeds a preset (indicating a thermal runaway), heating and printing would stop. Would like to some of your comments on the feasibility and effort to do so.
Re: Fire Hazard August 08, 2014 11:52PM |
Registered: 11 years ago Posts: 1,171 |
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jaguarking11
I have been reading about this topic and doing my own thinking about using a proper thermal fuse on the hot end. As of now I have a j-head hot end with peek. I will be designing a larger all metal hot end soon enough. One of the things that interests me is the thermal fuse solution. I have seen this applied many times before. I think some testing is necessary to assure the right measures. The idea is to use a fairly poor conductor as a sandwich for the thermal fuse. Stainless steel sheet seems right. Then wrap the fuse in the stainless on one end and have it stick out like a blade on the other. The stainless should be insulated to avoid environmental effects. This can control the heat reaching to the fuse. We can experiment with a spare cartridges and some thermal fuses of different ratings until a sweet spot is reached.
1 - Thermal fuse option (100-150c) seems appropriate.
2 - Stainless steel of set dimensions
3 - flame proof insulator
4 - thermal fuse should be connected directly to the hot end. No signal wires going out through a moving head. This way if a wire breaks it just kills the hot end power.
Anyone care to comment on this view?
Re: Fire Hazard August 09, 2014 02:22AM |
Registered: 10 years ago Posts: 469 |