Changing Hot End Help

I want to change the hot end to extrude at around 45 C. I will be changing the entire hot end setup, which will entail purchasing a new heater (and I think a new thermistor).

Heater - I want to change from the provided cartridge heater to different type (Kapton or Silicone Rubber). For the new heater, what requirements/limitations do I have? The cartridge heater I have now is 12v 3ohms, so it is rated around 50W. I assume the new heater will have to run on 12v at <50W, but I could be mistaken.

Thermistor - Based on the reprap documentation, the 100k thermistor provided is very accurate in the >200 C range, but can vary quite a bit at low temperatures. If I want to operate around 45 C, I think I would need a thermistor with lower resistance. I've found that selecting a thermistor is dependent on the temperature controller bias current, does anyone know what this is for the Duet (Ormerod 2)?


I am terrible with electronics and still have a lot to learn about what is compatible at certain voltages/resistances and what can be changed without damaging the system. Any advice is much appreciated.

Thanks

Edited 1 time(s). Last edit at 04/30/2015 12:48PM by kkozel.

The older Duet boards have 1K thermistor series resistors. The new ones have 4.7K series resistors. Either way, at 45C a 10K thermistor will give you more accurate readings than a 100K thermistor.

For the heater, I suggest you aim for somewhere between 10W and 40W @ 12V.

---

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].

Quote
kkozel
I want to change the hot end to extrude at around 45 C. I will be changing the entire hot end setup, which will entail purchasing a new heater (and I think a new thermistor).

45 C? As in 45 Celsius? I think you must be mistaken. You mention 45 C twice in your opening post and I think if you mean 245 for ABS then that is likely too high for a majority of ABS plastics. Are you using specialty stuff?

Quote
Chris Rap

45 C? As in 45 Celsius? I think you must be mistaken. You mention 45 C twice in your opening post and I think if you mean 245 for ABS then that is likely too high for a majority of ABS plastics. Are you using specialty stuff?

Yes 45 degrees Celsius. Much much lower than plastic extrusion temps. It will be attached to a syringe extruder and the material is liquidous above ~45 C and solidifies at temps lower than that.

Quote
dc42
The older Duet boards have 1K thermistor series resistors. The new ones have 4.7K series resistors. Either way, at 45C a 10K thermistor will give you more accurate readings than a 100K thermistor.

For the heater, I suggest you aim for somewhere between 10W and 40W @ 12V.

From the reprap website:
Each thermistor circuit (bed and hot end) is a simple voltage divider, with the thermistor on one side, and a resistor on the other. The part of the thermistor circuit that was changed is the value of the voltage divider resistor.

I have a new one with the 4.7K resistor. Why do you say a 10K thermistor will be more accurate (as opposed to a 5K or 50K)?

And why the 10-40W range? I am curious what your reasoning is so I can figure this out by myself if I need to change it in the future.

Thanks

I suggested 10K because 10K and 100K thermistors are widely available, and a 10K thermistor will gibe you a good accuracy at 5C whereas a 100K one won't if your Duet had a 1K series resistor.

The temperature resolution provided by the ADC is at its best when the thermistor resistance is about the same as the series resistor. At 45C, a 10K thermistor will have a resistance of about 4.3K, which is a good match for the 4.7K series resistor on your Duet.

The 10-40W range I suggest was based on gut feeling, informed by experience. To a first approximation, the temperature rise you can get above ambient is proportional to heater power. 3D printers typically use 40W heaters to achieve around 245C for printing ABS (225C temperature rise above a 20C ambient temperature). If we assume your worst case ambient temperature is 10C, then you are looking for a 35C temperature rise, so by the proportional rule the power you need is about 6W. But there ar4 other factors that may require more power, such as the rate a which you will be extruding paste. So my feeling is still that you should plan on at least 10W to be safe. 40W is much more than you need, but you can reduce the maximum PWM to tame it.

---

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].