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Retraction length vs material flow

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For a few different reasons I have tried relatively high material flow factors recently. I have made two observations with higher-than-normal factors:

1) When changing filament, the material flow factor is used both when ejecting the old filament as well as forwarding the new filament. At the point of forwarding the new filament, the system doesn't yet know the properties of the filament, but it uses the material flow factor from old filament. In practice, when you switch Flex PLA into normal PLA, the latter filament will be forwarded too far as it has much lower material flow factor. When making the switch the other way around, the Flex PLA will be stopped quite far from the printing head.

2) I have noticed I need to use shorter retraction length the higher the material flow factor is. Actually, if I print an object with plenty of thin pillars, I might see the filament escaping from the printing head - if the material flow factor is very high. To me it looks like a bug, but I haven't tested it enough to be certain. Has anybody else noticed this?

I'm using Cura 14.03-TEST2 and latest firmware on UM2.


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Sounds like a Marlin bug for both issues. Marlin should set flow back to 100% when changing filament (incidentally I don't use this anymore - I just pull it out by hand and shove in the new one - Robert taught me that trick).

Bug #2 is the G codes for retraction (G11, G10 or something like that) should take into account the current flow.


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