Temperature control issue

Temperature control is the key for 3D printing. Unfortunately it seems that the printer has a problem with measuring the temperature reliably and repetitively. The regulated 5V of the PCB are taken as reference and then divided by two resistors. On of them is the NTC either on the extruder or underneath the headed bed. As NTC stands for negative temperature coefficient, the NTC reduces its resistance when getting warm. Therefore the microcontroller gets a lower voltage when the NTC is getting hot. However this is also the case when the voltage of the controller board gets low or when some parasite resistance appears in parallel to the NTC resistor (as dirt humidity, bad isolation toward ground, ... )

Checking the temperature of the extruder is important when the extruder motor shutters or the printing results not are very promising as when the two layers to not attach well. A good sign is if PLA makes sizzling noise and flows easily and quickly out of the extruder.

It appears that the sensor should be clean and free from melted filament. Too cold filament will put stress on the extruder motor and might produce shuttering. The temperature check can be done using a thermocouple

Figure 6.6. Extruder temperature check

Thermo extruder

or an infrared thermometer (as MLX90614)

Figure 6.7. Extruder temperature check

Thermo extruder

The extruder NTC is very small and exposed to the high extruder temperatures. The manufacturer is Futura Electronic 7350-NTC100K. It has 100K at 25°C and and a temperature coefficient of 4.5%/C.

Figure 6.8. Magnified Extruder NTC

NTC resistor

The temperature curve is not linear. Considering absolute resistance changes, it is very sensitive at low temperatures but no more sensitive at filament melting temperatures. The chart below shows an old "broken" NTC a new one and a mathematically calculated curve with 4.5% resistance decrease per °C.

Figure 6.9. NTC characteristics

NTC resistor

It appears that this tiny NTC can get bad values ("damaged") easily and has no more the correct resistance temperature behavior. A weak point might be the contacting point between wire and chip NTC resistor. It should be rated up to 250°C so even higher temperature as necessary for ABS should be supported.

Conclusion choosing such an NTC resistor characteristics for the extruder temperature regulation is a bad choice. Spending more money having a thermocouple or spending less money and having a PT1000 would be better.

An other issue is that the marlin SW Configuration.h has configured an other NTC resistor the ATC Semitec 104GT-2 that seems to have a slightly different curve. Unfortunately this device has reached end of life production. However marlin uses a temperature voltage lookup table produced using the python script createTemperatureLookup Marlin.py. Those tables end up in thermistortables.h.

Temperature check of the heated bed is done best using an infrared thermometer.

Figure 6.10. Bed temperature check

Heated bed

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