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Anders Olsson

Underextrusion when flow is set below 100%

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Strange topic maybe, but here is what I have noticed:

I sometimes use the flow setting to tune the extrusion rate for varying filament diameter.

It is quite convenient and works really well.

However, tonight I had some really odd underextrusion problems.

I am using slightly large Velleman ABS filament so I had the flow set to 97% to compensate for larger diameter (exactly 3 mm)

I tried both changing nozzle and cutting away a part of the filament.

But the filament was moving nicely through the bowden and everything else looked fine.

I had been printing two hours and underextrusion just suddenly occurred in certain parts of the print.

The problems occurred in narrow sections and small parts where the flow is decreased due to the print head changing direction often.

I investigated it further and noticed how the flow almost stops in these sections if the flow setting is below 100%.

It is obvious that it is the feeder motor more or less halts in these sections then.

If I just increase from 99% to 100%, the problem will completely disappear though.

Underextrusion when flow Is below 100 percent

I have not seen this before, but I don't think i have been using oversize filament for complex prints before. So I am not sure I have been printing complex structures with flow setting below 100% before.


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I noticed that the other day when I adjusted it on the controller - but this was a random thing in different places on the print - it would skip a whole line and then carry on like normal with just the infill holding it together! - back to 100% and everything was fine.

If you get to the bottom of it I would love to know!


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I made a very simple test today to confirm this issue:

2014 10 14 4531[1]

What you see is how the extrusion is suddenly not working properly when flow rate is decreased from 100% to 99%

The feeder motor more or less runs on/off, meaning the motor stops when the head is changing direction often and flow should be reduced.

I will put this issue on github


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This could be an issue with retraction. Can you try the test again with retraction off? If the retraction is picking on that flow setting it could start doing some wrong calculations.

No trying to move aside of the issue but did you know you can adjust the diameter setting in the Material menu on the UM2 controller?


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I tried with retraction off and it behaves exactly the same way, so it has to be something else.

As I said, it happens when the flow is reduced because the head changing direction often. It then more or less stops the feeder instead of slowing it (of flow is less than 100%).

I like the flow setting because it can be adjusted on the fly. That way you can optimize the flow for a particular filament first time you print with it and get a really nice result at first try.


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Ohhh I am so glad to see this thread! I have been tearing my hair out (and I dont have much..) - ever since I went to a heated bed, I've been creating lace. Works ok for a while, then goes to junk. Today I put tape back on and tried printing with a cold bed - still created lace. I suspected it might be a problem with a reduction in feed-rate, possily a heat (overheat) issue. I decided to download the latest firmware and lucked on to this post. I didnt expect to find a topic that describes my problem exactly!

I'll be interested to see if someone has a solution - The short term solution will be to re-slice with a print speed that will allow me to run at 100% speed.

IMG 6810


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