Volumetric flow rate not correct with auto speed

zbrozek commented 6 years ago

Version

1.41.0 beta

Operating system type + version

Windows 10 build 17134

Behavior

Describe the problem Volumetric flow rate computations appear to be incorrect.
Steps needed to reproduce the problem Set all speeds to zero (auto), slice a model, generate gcode, and look at the linear vs volumetric speeds in the preview tab.
Expected Results Volumetric flow should be almost linearly related to linear flow, given that linewidths are the same.
Actual Results Volumetric flow is the same across an almost 4:1 range of linear speeds, which seems implausible.
- Screenshots from Slic3r preview are preferred

Is this a new feature request? No.

STL/Config (.ZIP) where problem occurs

Upload a zipped copy of an STL and your config (File -> Export Config) Config attached config.zip Any STL will do. The pictured file is here: https://github.com/prusa3d/Original-Prusa-i3/blob/MK2/Printed-Parts/stl/x-carriage.stl

bubnikv commented 6 years ago

@FidelCapo Would you please look into this?

Anima1000000 commented 6 years ago

I have the same issues only with Gyroid and 3D Honeycomb infills. Extruder skips steps during those infills, suggesting that the pressure is too high.

bubnikv commented 6 years ago

Set all speeds to zero (auto)

This instructs Slic3r to do exactly what you are seeing: It tries to feed the filament into the extruder at a constant speed, so there will be a constant pressure in the nozzle. It is not an error, it is the intended behavior.

I have the same issues only with Gyroid and 3D Honeycomb infills. Extruder skips steps during those infills, suggesting that the pressure is too high.

That is a completely different issue: These infills are printed with a bridging flow to ensure, that the partially overlapping infill lines support each other. The problem here is, that it does not take into account the layer height (thus the line overlap amount), therefore it causes overfills at thin layer heights. We should fix that.

zbrozek commented 6 years ago

This instructs Slic3r to do exactly what you are seeing: It tries to feed the filament into the extruder at a constant speed, so there will be a constant pressure in the nozzle. It is not an error, it is the intended behavior.

I think you've missed my point. It's exactly what I want, but it's not what I think I'm getting. Compare the two screenshots. One claims the same volumetric flow rate (2.7 mm^3/sec or so?) across the entire part yet the other claims that linear speed varies from ~16 mm/sec to ~60 mm/sec[1]. That's simply too wide a range to make any sense. It's not plausible. It seems like an arithmetic error.

And further, look at the config file. In it you'll find: filament_max_volumetric_speed = 15 15 is a lot bigger than 2.7. If all the speeds are zero, why is the volumetric flow rate 5.5 times slower than the volumetric flow limit?

[1] Naively (which I know isn't what slic3r is doing, or should be doing, but is a good sanity check:

16 mm/sec linear at the nozzle looks like ~2 mm^3/sec. Link.
60 mm/sec linear at the nozzle looks like 7.5 mm^3/sec. Link.

Neither of those numbers is anywhere close to 15 mm^3/sec. Nor are they close to each other. So what's going on?

photodude commented 4 years ago

I also see that the filament_max_volumetric_speed in the configuration files is set really strangely to 15mm^3/s From the tooltip description, I think that's intended to be 1.51mm^3/s See image Capture

For the BIBO printers, I found on the BIBO website that the max volumetric speed for the stock MK8 0.4mm nozzle is 24cc/hour. Assuming this is cm^3/hr converting to mm^3/sec I get 6.67mm^3/s for the max volumetric speed.

If I'm to assume that the filament_max_volumetric_speed value is a filament specific override of the print max_volumetric_speed then 15mm^3/s is out of bounds for what the nozzle could even do (note: all filament_max_volumetric_speed values for the BIBO configurations were copied from the reality and prusa configurations).

This also seems odd to be a print and filament setting with no machine setting for max_volumetric_speed as max_volumetric_speed seems like it would be a value that should be under the machine extruder settings.

bubnikv commented 4 years ago

From http://projects.ttlexceeded.com/3dprinting_prusaslicer_max_volumetric_speed.html

The max volumetric speed you should use depends on your hotend. The E3D V6 hotend that ships with the Prusa i3 Mk3 is advertised with a rate of 15 mm3/s, though in practice 11.5 mm3/s is realistic. If you need to push more filament, the Volcano hotend upgrade increases this to roughly 25 mm3/s but does require some printer and software modifications. Other hotend designs allow even higher flow rates.

so 21. 3. 2020 v 16:08 odesílatel Walt Sorensen notifications@github.com napsal:

I also see that the filament_max_volumetric_speed in the configuration files is set really strangely to 15mm^3/s From the tooltip description, I think that's intended to be 1.51mm^3/s See image [image: Capture] https://user-images.githubusercontent.com/10253980/77229324-e44d8f80-6b52-11ea-8559-23e595070f54.PNG

For the BIBO printers, I found on the BIBO website https://ourbibo.com/product/showproduct.php?id=48 that the max volumetric speed for the stock MK8 0.4mm nozzle is 24cc/hour. Assuming this is cm^3/hr converting to mm^3/sec I get 6.67mm^3/s for the max volumetric speed.

If I'm to assume that the filament_max_volumetric_speed value is a filament specific override of the print max_volumetric_speed then 15mm^3/s is out of bounds for what the nozzle could even do (note: all filament_max_volumetric_speed values for the BIBO configurations were copied from the reality and prusa configurations).

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photodude commented 4 years ago

@bubnikv so from that article am I to assume that the max_volumetric_speed is Max. Volumetric Rate = Layer Height X Extrusion Width X Speed where Extrusion Width = line width and Speed is the printing speed or the extrusion speed (I'm unclear from the article which speed it was meaning) and max_volumetric_speed is not the same as Max. Nozzle Flow Rate

bubnikv commented 4 years ago

There is a beta documentation for PrusaSlicer. Maybe there is also something useful there.

https://help.prusa3d.com/en/article/general-info_1910#_ga=2.252338889.198198072.1584854524-1478025351.1545606326

Volumetric speed at the filament preset is the amount of plastic in mm3 per second. That is roughly equal to layer height x extrusion width x movement speed in XY with the differences that PrusaSlicer calculates the extrusion width using a rounded extrusion model.

https://manual.slic3r.org/advanced/flow-math

The number provided by nozzle manufacturer - I suppose it is the flow rate

is the same as PrusaSlicer's maximum volumetric speed. As long as the units are correct - mm3/sec you cannot be wrong.

so 21. 3. 2020 v 21:18 odesílatel Walt Sorensen notifications@github.com napsal:

@bubnikv https://github.com/bubnikv so from that article am I to assume that the max_volumetric_speed is Max. Volumetric Rate = Layer Height X Extrusion Width X Speed where Extrusion Width = line width and Speed is the printing speed or the extrusion speed (I'm unclear from the article which speed it was meaning) and max_volumetric_speed is not Max. Nozzle Flow Rate

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