Advice needed about getting a new hotend

[reprapper54]

I have a RepRapPro Mendel Mono, and it’s been working OK for over a decade – I have printed over 57km of filament; but last week I managed to destroy the hotend.

I think what has happened is that, it was the longest print I’ve ever done – it all started going wrong after 5 hours; and I suspect the cold end of the hot end throat got too hot, the PLA filament started to melt, created too much friction, or possibly the PTFE liner melted or something and well, it’s all gone to pot. Taking the hotend apart after the event, the fan and heatsink were all filled with cobwebs and dust so that probably didn’t help either.

The problem is, the Mendel is completely non-standard and the bits are no longer available. e.g the throat and nozzle are all a single piece threaded M5 steel, the heater is 5mm dia and it appears as if the end has burnt through. On the plus side I have managed to salvage the thermistor (RS 198-961) which is obsolete and no longer made.

So I have to get a new hotend, and when I look to buying a new hotend, they are all for specific printers, and although I could just buy anything and then figure out how to mount it on my machine, the next problem is that I am running Marlin software, and need to know the thermistor details. There seems to be very little or no information about the thermistors being used in the commercially available hotends.

So if I do not buy a new hotend for a specific machine, and instead try and get something more generic, e.g. Mk8 bits, but use a thermistor I do know about, then I have problems finding a heater block with the right size holes, e.g. my thermistor needs a 2mm dia hole the full width of the block as it is an axial device.

I would be surprised if I was the first person to have this problem, and I am wondering what other people do, or suggest.

[Antipodean]

Well, the details for the RS thermistor is available.
https://docs.rs-online.com/71ea/0900766b8157ea0c.pdf

It is the last one in the table, and gives all the info needed to characterise it.

[CrazyIvan]

I agree it might make sense to use the same thermistor or a thermistor with the same characteristics, but that's only the beginning of matching a hot-end to the firmware. The thermal characteristics or the hot-end are included in the Marlin settings for PID control – a closed loop predictive control mechanism for the heater so that a constant temperature is maintained.

The PID parameters can be determined and set through G-code, and stored in EEPROM, but it would be better if they became factory settings... if you have access to your control board firmware.

However, these are fiddling small details compared with the main task of replacing the hot-end with a different one. All hot-ends do basically the same thing though, so I don't consider it undoable, especially if there is plenty of room.

This is not the voice of been-there-done-that, and I am not hopeful that this thread will come to the attention of somebody with practical experience. It is something I have contemplated though.

[reprapper54]

Thanks for the replies.

As fate would have it, I have a lathe & milling machine, so can nake something up from scratch. I also have a lot of electronic test equipment, and I have upgraded the Marlin software several times over the years, so should be able to fix this.

OK, well I guess it's just a matter of cracking on and doing something.

[CrazyIvan]

As fate would have it, I have a lathe & milling machine, so can nake something up from scratch.

I also have a lot of electronic test equipment, and I have upgraded the Marlin software several times over the years, so should be able to fix this.

OMG! Dead right you can!

Do you want Bowden or direct drive? Not that it matters, I think this is a case of selecting your parts (from commonly-available ones) and making up the housing for them to fit your gantry. If I were doing this I think I would go for a combined nozzle and heat break.

OK, well I guess it's just a matter of cracking on and doing something.

All you need is a round tuit. We're all short of round tuits, but there's nothing like necessity to bring them out of the woodwork.

[reprapper54]

Thanks for the reply.

It is a Bowden thing. I did start to upgrade the printer to multicolour, so I have 3 of the Bowden feeder things, but never got around to adding the extra hotends which I regret now - if only to canabalise them for spares.

I think the Gods must be smiling on me as I have just had an email from Dr Adrian Bowyer - who I believe is responsible for the whole RepRap open source project, and he's going to send me an old used part that still works.

I think for insurance purposes I should still investigate building my own hotend from scratch. After all, that's why I've got a 3D printer - I like making stuff.