All-metal hotend, cooling fan failures, and cold-side PTFE

Question

I'm getting a pet bird soon, and I know that off-gassing from heating PTFE above 300 °C creates noxious fumes, which are bird-killer¹. To try to prevent even the chance of that I'm replacing my hotend with an all-metal one. I have an E3D v6 1.75 mm, which I noticed still uses a PTFE tube at the cold side (the 3 mm version apparently doesn't, but 1.75 mm filament is what my printer uses, so there we are).

If I'm reading the instructions correctly, it sounds like the heatsink shouldn't even get warm to the touch, and if that's the case I'm not worried as long as things go well. What I'm still worried about is what happens if the heatsink cooling fan fails. I plan to plug the fan into the always-on 12 V port on my board (SKR 1.3): if I do, then is it possible to set my firmware (currently Marlin 2.0.4.4) to stop a print if the fan fails?

If I can't get the firmware to stop on fan failure, then is there a 1.75 mm all-metal hotend out there that doesn't require the use of PTFE on the cold side?

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1- Here: "at temperatures above 570°F (300°C), Teflon [PTFE] coatings on nonstick cookware start to break down, releasing toxic chemicals into the air (14)"

Answer 1

First of all, we need to discuss the failure mode and what can be done. LEt's do a

Failure mode 1: coolend-fan stops working.

Let's assume the coolend-fan for whatever reason (cut cable, defect fan, burnt board...) stops working. As a result, the coolend starts to rise in temperature, as it doesn't drain as much heat into the room air as before. This directly leads to an increase of the hotend temperature, which results in a case differentiation:

• The hotend does not cope well with the loss of the heatsink and it triggers Thermal Runaway protection as for a given voltage bump the heating gets too high - the print gets aborted before the hotend reaches a temperature above 275 °C.
• The hotend does not trigger Thermal Runaway Protection but the controller alters its heating behavior and works the heater on a lower duty cycle. As long as the hotend temperature is ordered to stay low enough, we will get a lot of extrusion problems from heat-creep, but the PTFE seated into the heatbreak stays below the heater break's temperature, as the thermal mass of the coolend alone draws away thermal energy into the room, even without the fan that keeps it at room temperature. As long as you don't print at above 300 °C and the thermosensor is intact, the failure mode does not release any fumes that could result in Polytetrafluoroethylene Toxicosis.

Failure Mode 2: coolend fan stops Working, no TRP, Thermosensor OK

But what if TRP is off? Let's look at this Double Failure: The hotend does not trigger Thermal Runaway Protection (or it was turned off to begin with) and the temperature increases due to the lack of cooling from the coolend.

As the hotend reaches 275 °C (few printers print that hot, and they use specialized setups), it should trigger the next safety line: a MaxTemp error and cut power. Heating stops before the coolend gets to the dangerous zone of 300 °C, as the coolend always is less hot than the heater block.

Failure Mode 3: no TRP, Thermosensor broken

We are getting desperate and turn off TRP, then break or disconnect the thermosensor to get a static low temperature. NOW we are getting serious, as only with such a failure we can trick our controller to continuously heat the heater cartridge and not trigger any of the error conditions. Only now there is the mere possibility to heat the coolend over 300 °C.

Marlin Firmware is designed to carefully work with checks and balances to keep the heater block in the wanted margin, and it would need a deliberate manipulation of the software to disable all safety features in conjunction with the failure of the thermosensor for the printer to go into Thermal runaway in such a degree that the coolend goes over 300 °C. And then you have different problems: your printer surely is turning into a molten pile in that failure mode. The presence or absence of the coolend fan would just delay the inevitable, should you run such a fire hazard-machine

Conclusion

Configure and install a recent firmware distribution (Marlin 1.1.9 and Marlin 2.x come with TRP enabled by default) and be sure to have MaxTemp enabled at 275 °C and Thermal Runaway Protection on, and you have a 3-layer safety against PTFE-fumes.

Adding more layers surely is possible, but the cost-effect calculation gets worse starting there.

Answer 2

It is doubtful that small PTFE inside hotend could produce that kind of dangerous gas leak. But another thing should be considered: the PTFE tube inside hotend WILL degrade over time and will need replacement.

For last several years I had numerous experiments with all kind of solutions including my own designs made. And recently I found the best and all metal solution: Volcano 20mm heater + M6 stainless throat + normal SHORT E3D nozzle (not the Volcano nozzle but the normal one). Basically this solution put the problematic gap (merge of nozzle with throat) deep into hotter zone, you just need longer throat. So far I have best quality and speed with this approach.

The most important thing for the all metal hotend is to have fast retracts.

For the FAN I recommend to use double deck, that is to put one fan on top of another and connect them in parallel. I found this solution in some server power supplies. And indeed two fans produce really good flow to cool heatsink at higher temperatures (before I was considering water cooling but two fans much cheaper and do the job very good). Also the probability that two fans will fail is very low.