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In FlashPrint there is an Extrusion Ratio setting. It's set to 97 % for the stock PLA profile and 107 % for the stock PETG profile.

I would've thought these would be 100 % by default (and the same for all materials). Why is it that these defaults make sense and they differ? Why would you need to push more PETG for the same print than PLA if they're the same diameter filament?

FlashPrint Extrusion Ratio setting

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  • $\begingroup$ I think that it may be a better resource to ask this question generally, ie "Why would you use different extrusion ratios for different materials" rather than being slicer-specific $\endgroup$
    – craftxbox
    Aug 16 at 12:35
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    $\begingroup$ @craftxbox perhaps, but as far as I can tell this might be specific to FlashPrint. I can't find any other recommentations for doing this (eg. help.prusa3d.com/en/article/… says default to 1 for PLA and PETG). $\endgroup$ Aug 16 at 12:37
  • $\begingroup$ Yes, it's true that most other slicers don't configure this setting, Flashprint is unique here, I just mean that it may be better to phrase it in such a way that it comes like "Why would I change it for myself?" instead of "Why does this software do it differently?" $\endgroup$
    – craftxbox
    Aug 16 at 12:39
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    $\begingroup$ @craftxbox I understand why I might change it myself, but I'm specifically interested in why FlashPrint has these defaults. How did they pick them? Doesn't a value < 100% suggest there will be less material than required? And why would pushing 5mm at the extruder result in something other than 5mm x diameter out the nozzle? Does it expect to slip? What's different about this slicer/printer to others? $\endgroup$ Aug 16 at 12:44
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    $\begingroup$ @craftxbox but if you push 5mm and the diameter of the roll is consistent and there's no slipping (and the filament has no air in it?), the amount of plastic coming out of the nozzle should be known exactly too. It feels to me like this adjustment is more likely to differ based on the brand/accuracy of a roll rather than the type of filament? 🤔 $\endgroup$ Aug 16 at 12:55
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If your extruder uses spring tension to hold the filament against the hob, the effective diameter of the filament at the point the hob contacts, which determines the amount moved, is a function of the compressibility of the material. Thus, it makes sense to use a higher flow ratio for more compressible filaments. However, this is very much a matter of what type of extruder your machine has, and doesn't apply if you have a tension arm holding the idler bearing at fixed distance from the hob.

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  • $\begingroup$ Ah, interesting! It's a Flashforge Creator 3 - I can't actually find the answer to this (it hasn't arrived yet). Although for PLA the value is set to 97% which still seems strange, since surely this explanation would only result in less filament being pushed and never more? $\endgroup$ Aug 17 at 8:00
  • $\begingroup$ Maybe it's assuming the 100% point is set based on slightly more compression? I don't know. Or it could all just be magical fairy dust. The idea of using a different extrusion ratio for the first layer certainly is; there's no physical justification for doing that, and it's just a compensation for having your bed height wrong, but that's not material-specific. $\endgroup$ Aug 17 at 16:13
  • $\begingroup$ It seems weird to pick based on some compression though.. To me it'd make more sense to assume none (or at least, the lease compressable material) than some abitrary point higher. It's almost like software is making up for something being wrong in the hardware 😁 I think I agree about the first layer - it shouldn't be needed, but since it's very hard to get things perfect (esp. as it seems the levelling only advises to move in 45 degree increments), a slight overextrusion in the first layer might be prefereable to it being much harder to get the print to stick :-) $\endgroup$ Aug 17 at 17:46
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I may be entirely out of my league here, But I think it's related to Viscosity, or how thin or thick the fluid plastic is while hot and Temperature Expansion, how much larger in volume the fluid plastic will be hot compared to cold.

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    $\begingroup$ I wondered that too, but I couldn't find anything online about this. For example this Prusa help page says "Basic materials (PLA, ABS, PETG) Extrusion Multiplier = 1" and "This calibration is not necessary with Prusament". I've also never modified this on my previous printer for PLA, so not sure why even PLA is 97% here. $\endgroup$ Aug 16 at 12:35

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