Why does the priming line over-extrude material?
I was looking at the start G-code for my i3 MK3S+, and I noticed that the priming line extrudes way more plastic than normal extrusions:
G1 X60.0 E9.0 F1000.0
With a 0.4mm nozzle, 0.2mm layer height and 1.75mm filament, for a normal 60mm extrusion line you need 4.8mm^3 of plastic, which is ~2mm of filament. The command above extrudes 9mm of filament, which is 21.6mm^3. The only place for that extra plastic to go is line width, so we get a priming line that's ~1.8mm wide.
Looking at the priming lines for my printer, indeed they are ~1.8mm wide. Why does the start G-code over-extrude like that? I've seen similar over-extrusion used in printers other than Prusa, but nobody seems to have an explanation, everyone's just copying everyone else's G-code when configuring a new printer...
Anyone know the reason for priming like this, instead of using a longer line with less over-extrusion (or none) ?
RE: Why does the priming line over-extrude material?
The purpose of the prime line is to initialize flow and trap any oozing filament on the nozzle, so a bit of over-extrusion is beneficial. You can alter your startup gcode if you prefer a thinner line. It's not for any cosmetic use, so you'll notice the prime line routine is not adjusted based on nozzle size. You could certainly do so, but I don't see a real benefit. FYI, the prime line is printed with a 0.15mm nozzle height.
I personally like to extrude a bit (2mm) without movement to trap any oozing filament, then print wide line to get flow going in case the last print job was stopped in the middle of a retraction. A thin line gives me a visual confirmation that flow is good, and a bit wider at the end with a little "wipe" wiggle reduces the odds of the prime line itself coming loose or causing stringing. Some people do exotic patterns or write their names.
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