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TL;DR: I'm looking to change the steps per mm but I'm not sure what to set it to. Do I decide on a temperature and change the rate based on the percentage for that temp? Or is there a more general setting I can put it to? Or is there a different underlying problem here?

I'm running into an issue with my Ender 3 where when I print PLA at different temperatures I'm getting different extrusion rates. I've done the test where marking it at 120 mm and then extruding using PronterFace 100 mm and measuring the distance and this is my results. I did two tests of each to confirm results

  • 185 °C 80 mm left = 60 % under extrusion

  • 200 °C 31 mm Left = 11 % Under Extrusion

  • 210 °C 32 mm Left = 12 % Under Extrusion

  • 220 °C 28 mm Left = 8 % Under Extrusion

  • 230 °C 25 mm Left = 5 % Under Extrusion

When I feed 100 mm through the Bowden tube without the filament going through the hot end I get exactly 20 mm left meaning:

  • 0% under Extrusion

    echo: Steps per unit:
    echo: M92 X80.00 Y80.00 Z400.00 E93.00
    

I have been having this problem since I started. I normally print at 210 °C at 50 mm/s with a flow rate of 103 % which seems to work well for most prints but on flat walls on prints it under extrudes in the same spot each time it comes around to that point.

Steps I have taken so far:

  • Different PLA filaments. All perform the same

  • Replaced the nozzle with a new one

  • Checked all fittings with the bowen tube system

  • Printed a spring spacer to make the spring tighter on the extruder gear

  • Changed the Vref from 0.75 to 1 for the extruder stepper

The reason I changed the Vref on the extruder stepper is because the system would click back when printing as if the pressure was too great in the tube. Changing this has helped some of the under extrusion a bit.

So. depending on the temperature, it's harder or easier to push the filament by hand through the hot end. Maybe I need a better hot end?

Examples:

My test print to replicate the issue. The ripple bit is brittle and under extruded.

Test print - Image#1

Test print - Image#2

This was a Prototype piece for an up-sized print I was planning. After going around it under extrudes at the same point each time 210 °C

Prototype piece

Most of the print is fine just some parts are different. This is an inner wall.

Inner wall

Heat towers

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  • $\begingroup$ You sure it's not a matter of either acceleration at your "same spot" or a problem with the bed or carriage steppers? Try locating the same part on a different part of the bed, and rotated in X-Y by 45 degrees to see what happens to the under-extruded locations. $\endgroup$ – Carl Witthoft Nov 29 '18 at 15:58
  • $\begingroup$ Yeah its not the carriage. I believe its under extruding because of pressure in the hot end somehow. With the tests that I've been doing today I've got good results printing 40mm/s, 220c, .2mm. I just cant do any cooler or faster then that or i get under extrusion due to pressure I think. $\endgroup$ – Joshua F Nov 29 '18 at 16:11
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Obviously, your extrusion process is troubled by a lot of pressure. This can be seen from the extensive experiment you conducted with PLA extrusion at different temperatures. Please do note that 230 °C is considered pretty high for PLA! Usually it should be in the range of 185 - 205 °C:

In general, PLA filament settings have an optimal printing PLA temperature range from about 185C to about 205C. If you’re using 1.75mm as opposed to thicker 2.85mm (or 3.00mm) your optimal print will be closer to the lower end of this PLA filament temperature range.

The temperature dependency of filament diameter is explained that small diameter filament warms up way faster in the heating zone of the hotend than large diameter filament as the heat travels less far to the filament core. Basically, with 1.75 mm filament you should be able to print at 195 °C. The pressure that the filament exerts on the hotend and extruder is clearly too much and leads to skipped steps.

It is strongly discouraged to create a function of steps per millimeter (or an over-extrusion by specifying a more than 100% flow modifier). This is a mechanical issue that needs to be fixed by addressing the hardware problem. Usually this is done by:

  • fixing the extruder;
    • is it skipping steps?
    • does the filament tension get too high that it skips back pass the extruder gear?
    • does increase the stepper current work?
  • fixing the Bowden tube;
    • is it clean?
    • is there too much friction?
    • are there kinks?
  • fixing the hotend;
    • is the temperature that is reported the correct one? (thermistor problem?)
    • is the heat conducting properly to the nozzle?
    • is there a tolerance issue in the hotend/heatbreak?
    • is the coldend properly cooled?

A few of these you already explored, others you have not. Unfortunately, you have to do a little more troubleshooting the get to the bottom of the actual problem that is causing this pressure preventing the extruder to extrude the proper commanded length.

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  • $\begingroup$ The only thing i can think of at the moment is that my hot end just isnt getting to the temperature its saying it is or it cant maintain it. The only option for that fix then is to just slow the print down even further. 220c - 230c seems to be the best temps to print at for now. $\endgroup$ – Joshua F Nov 29 '18 at 8:59
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    $\begingroup$ Replace the thermistor to see whether that helps, they literally cost a few cents. Personally I would also change the hotend heatbreak and nozzle, also a few cents. Also the cold end should be checked. $\endgroup$ – 0scar Nov 29 '18 at 12:36
  • $\begingroup$ Yeah I'll look into it. I think its just that there is too much pressure in the hot end somehow. I'll pull it apart again and see if I can see whats going on. $\endgroup$ – Joshua F Nov 29 '18 at 16:13
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So I feel that I solved the problem thanks to a few sources. Thanks to 0scar for his quick response and help.

The problem was two fold. Mechanical problem causing blockage and slicer setting causing ripple.

  1. The PTFE tube wasn't pushed all the way in as far as it could go causing too much pressure in the hotend. This seems to be a really common problem with stock Ender 3's Because of the gap the hotend needed to heat up into the heat break to melt the lump in the gap between the hot end and the PTFE tube.
  2. The slicing settings in cura had combing enabled. I set it to not comb on skin but what it was doing is combing just behind the skin and as a result would leak out filament in little blobs that when it printed the outer layer would be pushed out causing a ripple effect. This caused under extrusion once it finished the move because there wasnt enough filament left in the end.

After I fixed these two things I was printing at 190c with no under extrusion, motor slipping, very little stringing and printing much better.

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