Melted shit. Thermistor type?

I've been working on my MBot printer. I added a heated bed, but that required me to rebuild the Marlin configuration. I managed a couple of successful prints off of it, and then, it started jamming on me. The tube that holds the hotend melted, and the heater block is sitting at a slight angle, with a large gob of melted plastic squeezed out of the joint between tube and hotend.

I am using the same extruding temperatures as before (240 deg C) to extrude ABS. Previously the firmware was configured to shut the printer off at 250 deg C, which I attributed to the fact that the throat was PEEK, Sometimes when set to 240 deg C it would trigger, so I changed the shutoff to 260 deg C, but I never noticed the temperature going above 245 indicated, but it was obviously hotter.

I'm guessing that I have the temperature sensor incorrectly configured. It must be close. i configured it in Marlin as type '1' (100k thermistor) -- it must be close, or else it wouldn't work at all. the extrusion looked normal, no extra stringiness, and bridging for printing worked okay, plus I had some overhangs, which looked good. Heating time seemed reasonable to nozzle temperature.

Anyone experience something like this?

Anyway, I'm going to redesign with an all metal hotend, since I wanted the ability to print in other materials requiring hotter temperatures, but if my temperature reading is this terrible, I will need to fix it. I'd like to not have to, but I guess I'll switch to a known sensor type.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Printing ABS is about as high a temperature you can go with a thermistor. If you're going to be printing materials like PC at high temperatures you should look into PT100 or PT1000 sensors. They require an interface board for your controller.

When shopping for the sensor, make sure you get one that has fiberglass insulation on the leads coming out of the sensor housing. The cheapies from China @ 2 for $12, the same one's they claim are good to 450C, have teflon insulated leads that don't hold up at even ABS print temperatures (I know from experience). The E3D PT100 sensor (about $20) has fiberglass leads and should work fine.

I recently installed an all-metal hotend from China (with E3D PT100 sensor) and an aluminum Titan extruder (with a small modification) that have been working well.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I was looking at the PT1000 sensor. Truthfully, I will probably not use this printer for much more than ABS. It looks like it's going to be way too much trouble to convert it to handle much higher temperatures. The nozzle mounting setup is pretty restricted for options, and about everything that I can find is stuffed full of PTFE down to the heat block. That pretty much means a usable ceiling of 250 deg C. Well, that's okay, so long as it's actually 250, and not something that's going to melt the PTFE, which will have its own set of problems. The

I have to get something done for my other printer, which will be running on the Duet board. The Duet can have the PT1000 sensor directly connected, which is useful. Since I'm designing it myself, I will be able to do what I want for mounting the hotend. This ought to allow me to go with something that can handle the higher temperatures needed for stuff like nylon or PC. (Hmmm, just checked [www.precisionpiezo.co.uk] and it does say that the wire is "good to 200 deg C" with a glass fiber sleeve. Not much good if the wire itself shorts inside the sleeve. I might have to look further. I was hopeful because the PT1000 sensor is supposed to be able to be directly connected to the Duet.

In the meantime, I'm going to replace the heat block. I think that the temperature that I'm getting reported back is wrong, which means that I must have configured the sensor type incorrectly. I didn't know of anything that would give reasonable "room temperature" values when configured as a 100k thermistor that wasn't a 100k thermistor. I surmise that it must be a different type with a different rating curve at printing temperatures, but of course without the firmware source code, I can only try to do a post-mortem diagnosis.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

The B value of a 100k thermistor can be between about 3900 and 4300. This affects the resistance at higher temperatures. At around 250C the difference in reading from selecting the wrong type in thermistor could be up to about 40C.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I don't know where you're shopping for hot-ends, but the E3D v6 doesn't have teflon down to the heater block. Neither does the XCR3D hot-end from China.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

@dc42: Thanks, that's useful information. It appears that I have misconfigured the thermistor.

@dentist: Yeah, I have an E3D v6 hot end, too, but there's no practical way to mount it to this printer without making big changes to it. That's the hot end that I have planned for the other printer, though I'm going to have to change out the temperature sensor for certain.

This printer has a bar mount for the hot end, and because of the Z axis probe there's a big restriction on how long it can be. I think that I can retrofit a v10 hot end, and I have parts on the way to do exactly that. There's a 7mm opening in the bar with a set screw. The nozzle needs to be 20-25mm below the bar in order for the Z probe and part cooling fan to be in the correct position, which certainly does not leave room for the v6 finned heat sink. The v10 throats all seem to include/need a PTFE liner.

I was hoping for a minimum parts count repair, but the heat block is weird, as it has male threads on it, and the nozzle is integrated in the casting. Any replacement throat that I can find has male threads, and needs a heat break tube to fit into a female threaded heat block, mating to the threaded in nozzle. Therefore I have to replace the whole assembly. I cannot remove the thermistor from the existing nozzle, it's held in with a set screw, but even with the screw right out, it won't budge. the heater cartridge was also stuck hard, but I managed to get it loose. The manufacturer has closed up shop and vanished, of course. It appears that I can get another PEEK throat and nozzle (No heater/thermistor) from aliexpress for the low low price of $54 USD. Meh, for less than half that, I can completely replace everything with easier to replace parts.

I'm not sure what a good solution is to get the v6 hotend with its cooling fins installed underneath this thing, but I'm looking for solutions. My current thinking is to mill a 7mm diameter tube to fit it, and use a 1/4" BSPT die to put male threads on to fit to the heat sink where it fits a pneumatic fitting for the PTFE tube for a Bowden extruder, but that might be ... unweildy. Another option will be to make something to fit a Precision Piezo kit under the mounting bar and just use the groove mount for the hotend. This is probably a solution that will result in a more useable printer, at the cost of 35mm or so of available build height. I'll also need to design a new part cooler fan mount, though that's not so big a deal.

I suspect that all of these kits have some sort of proprietary solution to mounting stuff that's difficult to replace with anything else, no matter how many "generic" solutions exist.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts