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Cura extrusion path and flow rate are probably mismatched


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Posted · Cura extrusion path and flow rate are probably mismatched



I'm writing this down because I think I found a bug in Cura 4.7.1. I'm not really sure if it's a bug but it might be. I found it when re-calibrating and retuning my 3D Printer, a Tevo Tornado with its original stock hot end but with BMG dual drive extruder.


When calibrating for extrusion width by printing a 20mm cube in Spiralize Outer Contour mode, I found out that the line width was bigger than what I had set. It was 0.5mm instead of 0.4mm. Based on this result I reprinted with 80% flow rate to compensate the over extrusion. It printed exactly 0.4mm. However, when printing a cube with the same settings, the top and bottom layers had gaps on them.


It got me thinking, maybe the flow rate and the extrusion path in Cura were mismatched. Maybe Cura has extrusion path planned for 0.5mm instead of 0.4mm and that caused my gaps. Therefore, I changed my Cura settings to 0.32mm line width and 100% flow rate. The result was great! The line width was exactly 0.4mm and there wasn't any gaps.


I'm not 100% sure that it was Cura's fault, but I think it worth checking. And, for the firmware, I am pretty sure that the X and Y steps/mm are correct because I have calibrated it carefully. I am not really sure with the extruder steps, since the method I used was dependent on the filament, and filaments are curved due to being rolled. Hence, may not be accurate.

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    Posted · Cura extrusion path and flow rate are probably mismatched

    Considering what goes on in the little space under a nozzle reminds me of considering what goes on in the little space of the combustion chamber of an internal combustion engine.  For such small spaces, it's complicated.


    When printing conventionally the stripe of plastic is often trapped on one side by a previous stripe of plastic.  The new stripe can't flow that way because the previous stripe has created a wall and so it pushes the other way.  The line width is the "Index" movement between one stripe and the next so it isn't really "Wall Width".  In Vase mode the width of the plastic stripe is dependent on temperature, the material properties, nozzle wear, first layer vs other layers, flow, etc. and the stripe is not trapped.  You would expect it to be exactly 1/2 line width on either side of the nozzle centerline, but that's not likely to happen although as you found, you can make it so. 

    The area under the nozzle is (usually) defined as a rectangle - "line width wide" x "layer height high".  But the Vase mode stripe isn't a rectangle because it's molten plastic so the sides bulge into a radius making the shape into a flattened oval.  That may be what you are measuring - the additional .05 radius on either side of the Vase mode stripe.  Printing conventionally (at .4 line width), the stripe is trapped on one side by a previous stripe so it's .2+ on one side, and .2- on the other side of the centerline.  At some point you have to make a decision on how to treat the area under the nozzle.  Is it a rectangle, an oval,  or 1/2 of each?  It's a compromise which of course means that it's wrong either way.  I think Simplify3D uses a fudge factor in regards to the size of the rectangle to try to account for the fact that it isn't really a rectangle.  Cura seems to be straight forward in considering that it is a rectangle.


    I've come to the conclusion (opinion really) that trying to calibrate for flow using any method beyond calibrating the Esteps is useless.  Better to use the Mark I Eyeball because as you noted when you "saw the gaps"... when it looks good it's good and when it looks bad it ain't good.  I happen to have a microscope to assist my Mark I Eyeball and I have verified that good is different from bad to my personal satisfaction.

    When printing in Vase mode I usually go to .6 line width with my .4 nozzle.  I like the way it looks.


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    Posted · Cura extrusion path and flow rate are probably mismatched

    Ah, that is really interesting and it explains a lot. I guess I should just quit obsessing with the line width but focus on the visual. Or if I want to calibrate, I should just make sure the extruder's steps are correct.


    For calibrating the extruder steps, is there any accurate way to do it? I mean I did it by measuring the distance travelled by the filament when extruding. However, as I have mentioned before, the filament is a bit curled so measuring it using a caliper will introduce an error.

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    Posted · Cura extrusion path and flow rate are probably mismatched

    Mark a spot on the filament.  Extrude forwards 30mm.  See how far that spot moved - use a ruler.  Don't measure *after* it comes out of the nozzle.  Measure somewhere further back where the filament hasn't melted yet.

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    Posted · Cura extrusion path and flow rate are probably mismatched

    "I guess I should just quit obsessing with the line width"  That's what I advise.  The E numbers you see in a Gcode file are "mm of filament" (some printers use mm^3).  When you calibrate the E steps and tell the printer to extrude 30mm of filament and pretty near exactly 30mm goes through the extruder gear, that's it...you are done.  Move on.


    As @gr5 says, you need to measure back further.  I always calibrate the E steps when I change nozzles because I find it easier that way.  You could pull the bowden tube out of one end and measure there or just measure before the extruder motor.  The photo shows my setup but you'll have to use your imagination because I'm too lazy to pull the nozzle just for this post.  Calibrating all 4 axis needs to be done occasionally as the printer wears in.


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