Large initial extrusion on preheat.
I seem to be getting a large amount of material being extruded during the preheat stage on my MK2S. In all about 150mm of filament oer a 1 minute period.
The nozzle has always extruded a small amount during the heating which is to be expected but this extra extrusion has happed since I changed the nozzle and PTFE tubing. Nozzle was the same 0.4mm.
Filament is standard Prusa PLA and the extruder motor is not moving so it appears to be the 'pressure' retained in the head.
The prints are OK but, unless I wait for this pre-extrusion to finish, during the 9 point bed levelling, I get small amounts of filament deposited on some of the points which I have to wipe off, which can be tricky.
The replacement PTFE tube was cut precisely to the Prusa specifications and there is no leakage from around the nozzle.
Any suggestions please?
Re: Large initial extrusion on preheat.
I have no idea why you are getting so much extrusion with the motor not running. I would look at your end script and see if the printer is somehow pushing filament into the extruder after the temp has dropped, and when it comes back up it comes squishing out.
As far as leaving spots on the bed, during calibration. I have a solution for you. I have changed the "start gcode" in my slicer to do a few things.
First, homes and puts the extruder up from the bed so the PINDA sensor will not be heated by the hotbed. I then heat the extruder to 170C and do the calibration. This is not hot enough to cause the printer to extrude, but hot enough to get a good calibration. The combination of these two steps creates a repeatability that the default does not give you.
Here is an OLDER version of that starter g-code. Since I have the MMU, I use a different version than this, SO STUDY IT and use it carefully. Compare it to the start gcode in your machine, and make sure you are not missing things it provides. I can't promise this code is good for newer firmware versions and/or newer slicers.
M115 U3.0.12 ; tell printer latest fw version
G21 ; set units to millimeters
G90 ; use absolute coordinates
M83 ; extruder relative mode
M104 S170; Extruder to 170
M140 S[first_layer_bed_temperature] ; set bed temp
G28 W ; home all without mesh bed level
G1 X0 Y0 Z125; home X axis Lift to 125
M190 S[first_layer_bed_temperature] ; wait for bed temp
M109 S170 ; wait for extruder temp to hit 170
M104 S[first_layer_temperature] ; set extruder temp
G80 ; mesh bed leveling while rising in temp
G1 Y-3.0 F1000.0 ; go outside printing area
M109 S[first_layer_temperature] ; wait for extruder temp if needed
G92 E0 ; reset extrusion distance
G1 X100.0 E9.0 F1000.0 ; intro line
G1 X200.0 E12.5 F1000.0 ; intro line
Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage or loss. If you solve your problem, please post the solution…
Re: Large initial extrusion on preheat.
Thank you for your suggestions and I will have a look at your script.
I have checked and during preheat the extruder motor is not moving so, whatever is happening, has to be residual pressure between the extruder and the nozzle. After the print finishes the extruder motor reverses (retracts) a few degrees probably around 0.5 mm of filament.
All I have done before this started, was to change the PTFE tube and the nozzle in the hot head after a jam caused by the filament getting trapped on the reel (badly wound). The tube was made to the exact measurements provided by Prusa and is fully homed in the heat-break.
To extrude that amount is equal to around 7 or 8 mm of 1.75 mm filament. It does eventually stop but takes about a minute to do so.