Temperature Calibration / Extruder Jam

My Problem I will get somewhat random jams during extrusion. By jams I mean that the filament is eaten by the extruder. I ultimately feel that the temperature calibration is off. When I measure the nozzle with an infrared thermometer I get the difference as seen below. This is after I have already changed the Thermistor table in thermistor.h. By changing I mean I have subtracted 20 deg from the current value. What did read as 300 C now reads as 280.

Extruder temp measurements
Reported Measured Difference
270 183 87
225 176 49
200 155 45
180 143 37
150 117 33
125 100 25
100 80 20


What I have tried
Change the Firware to increase the temperature
I extrude at 250, usually with good results
I have tried to extrude the plastic through by hand (with great difficulty)
I have cleaned out the tip and did not find anything out of the ordinary, no burnt plastic
I have disassembled the J-Head and inspected the PTFE liner. The filament slides relatively smoothly through the liner, so no diameter issues.
My print speed is a max of around 30 mm/s before I see problems.

My Setup
Printer: Prusa, kit from MakerFarm
Hot End: J-Head Mk V .35mm nozzle diameter
Thermister: 100k
Material: ABS 3mm diameter
Firmware: Kliment Sprinter
Software: Pronterface and Slic3r


What have others measured for the head temp vs what the software says it is? What are others print speeds with the J-Head? I see 200 m/s for some set ups on youtube. Does your extruder eat the filament very much?

Ok, i just answered my question I think.

What I did
1. I double checked my thermistor settings. I had the wrong one selected, I changed to my modified table.
2. I cleaned out the J-Head nozzle just because and made sure that the filament went through the PTFE tube in the J-Head with little to now resistance
3. I reassembled and tested the unit at extrussion of 200 mm/min. This worked great. I did have the setting at 250 for the extruder, but it seems to work best.

Thanks for the help :-)

An IR thermometer will read low because the nozzle is too small for the aperture of all but the most expensive ones and the emissivity it assumes is higher than brass.

The only way to measure it accurately is to put a thermocouple inside the nozzle as that is where the plastic is.

Having it accurately calibrated is good for comparing setting with other people or if you have more than one machine but every colour of plastic likes a different temperature, so each new ones needs some trial and error.

---

[www.hydraraptor.blogspot.com]