what's the gcode to retract filament?

[barnett]

Reading mhackney's calibration tests, I'm reminded of a question I keep forgetting to ask.

On my Rostock Max, after a job finishes and the machine cools down, I can't get the filament out if I want to change it... like it's frozen in the hot end someplace. Maybe I'm just being wimpy about pulling on it, but my process has been to heat it back up to get it out.

Instead, I'd like to add some gcode to the post routine section of the slicer software so that when a job finishes, it will retract out maybe 30mm. I just don't know what exactly to type in there. G1 E-30? Do I have to fuss with the exact vs relative positioning (which I don't completely understand)?

Michael's comment about the string from the printed object back to the nozzle when a job finishes reminded me about this question... because I suppose it might resolve that too.

[mhackney]

Hey barnett, you go it right. G1 will do it and you can specify the feed rate with the F parameter like:

G1 E-30 F100 (to retract at 100mm/s)

On the extruder, moves are always relative. So if you say E-30 it will retract 30mm. For X, Y and Z movement you could use absolute mode which means "go to this coordinate from where you are" or incremental mode which means "move this many units from where you are" - see the difference?

Doing a fast retract at the end of print should eliminate the stringing but you'll need to make sure to advance the filament for the next print!

[Polygonhell]

Extruder moves are not always relative, most firmwares default to Absolute E in fact.
You probably want to add a G92 before that incase the Slicer isn't

G92 E0
G1 E-30 F100

The G92 just resets the E coordinates to 0, if the Slicer isn't adding it then the G1 would retract to 30mm before the last G92 E0, which is for most slicers would be the start of the last layer.

[mhackney]

I use slic3r and it does do a G92 E0 for the extruder. But I agree, it does not hurt to be explicit!

[barnett]

Thanks guys! I'll do that.

And my habit has been to manually prime the extruder for a bit after everything's heated up just before starting a job. That way, I know there's fresh filament ready.