I think I’ve come up with an ideal way to tune the retraction settings on a Bowden-style machine.
Retraction exists to compensate for the “springiness” in the mechanical system between the extruder and the nozzle. It can be a nuissance to tune by trial-and-error, not least because it depends on so many factors.
It occurs to me, though, that it’s not hard to simply measure the amount of retraction your printer requires.
Suppose you’re printing along - steady as she goes - and you suddenly open the jaws of the extruder.
The filament will immediately jump back, relieving all that spring tension I mentioned back in the first paragraph. Until you’re retracting that far, and that fast, you’re never actually pulling on the filament at all - it will still be pushing back on the extruder.
…So I tested it by doing just that. I modelled a simple, 50mm cylinder with 5mm walls, to achieve “steady state” printing. I sliced it with a first layer that just dealt with adhesion, and then varied the parameters for the second and subsequent layers. As it printed steadily, I put a mark on the filament a few mm from the extruder, and then opened the extruder just as the mark reached it. Keeping the extruder clamp open, I aborted the print with my free hand, and then measured the distance by which the filament had sprung back.
Here are the results:
Ender 5 , stock end with magnesium heat break and 0.4mm nozzle. Sakata Fuchsia PETG, 235 celsius, line width 0.4mm.
Layer ht 0.12mm, speed 10mm/sec: Spring was 5.3mm
Layer ht 0.16mm, speed 25mm/sec: Spring was 5.5mm
Layer ht 0.20mm, speed 50mm/sec: Spring was 5.8mm
This should provide an excellent starting point for further tuning.