I built recently built a printer based on a Anet A8 design with an MK7/8 style extruder. I had the printer up and running and have run over 100 hours of prints on it, but have not been happy with the quality of my prints and have been working to improve my calibration. One of the biggest issues I have had in all my prints has been with under extrusion. I attempted to calibrate the extruder by measuring out 120 mm of filament above the intake for the extruder, extruding 100mm of filament at 120mm/min and measuring the difference and calculating a new steps per mm for the extruder. I originally did the test at 210 degrees C with the hot end in place and the extruded distance did not get better even though I updated the steps per mm in the EEPROM on my controller. With some tests the extruded distance even got worse, but changes were inconsistent. I repeated the test again with the hot end removed, so I was just extruding filament through a hole without a heating element or nozzle. When I did this test I got consistent extrusion and believe was able to calibrate the extruder. I then replaced my entire hot end (nozzle, heating block, barrel, PTFE tubing, heating element and thermistor) and reconnected with brand new parts. I recalibrated the temperatures and PID and attempted to extrude again and consistently only extruded ~85% as much as should have. When I attempted the same calibration again I had the same issues as previously with inconsistent extrusion. I don’t believe my motor is stalling and the filament is not visibly slipping in the extruder, but it seems like those can be the only remaining reasons for my problems. Any suggestions on how to better calibrate the extruder would be greatly appreciated.

If it helps my setup is controlled by an arduino MEGA with a ramps1.4 shield and pololu A4988 stepper driver. My motor is KS42SH40-1204A stepper motor. I am running Repetier firmware using Repetier Host.