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I've got a secondhand Renkforce RF1000 which I can't seem to get working properly. Initially, I asked about hardware issues that required fixing. After fixing those I still got some inconsistent printing results. These problems got worse with a new nozzle and new filament. I've now narrowed it down to one problem: bad feeding

To summary:

  • I've got a secondhand Renkforce RF1000
  • I print using 2.85 mm PLA
  • I've got a new 0.3 mm nozzle
  • I've got new springs for holding the filament against the extruder

feeding mechanism The feeding mechanism

The problem seems to be that the pressure for extruding filament through the nozzle is too high. This causes the grub screw to keep spinning without extruding filament. So in the picture, the left screw will spin and the right one will remain stationary.

This isn't consistent, it might work for a few millimetres and then stop working for a bit. It can cause the screw to grind into the filament creating a hole that cuts the filament in two.

Here's what I've done to try and fix it:

  • Cleaned the nozzle with new 0.25 mm needles
  • Cleaned the nozzle with acetone against ABS that was stuck
  • Did a few cold pulls to confirm there's no filament stuck in the heater.
    • This wasn't the case and when removing the nozzle I could easily push or pull filament through.
  • Tightened and loosened the screws holding the filament against the grub screw
    • Too tight causes clicking or more grinding. Too loose causes the filament to just not get caught at all

I'm out of ideas of how to fix this. Any help you can give is greatly appreciated.

EDIT: here are some video's extruding 50 mm of filament at 2 mm/s and 10 mm/s

Controls Controls

10 mm/s

2 mm/s (both links to imgur, didn't manage to get the video inline)

EDIT 2:

I replaced the driving gear and replaced the new nozzle with an old one. I'm not sure about the size of the old one but I assume it is 0.5 mm. I calibrated the extrusion and tested at 1 mm filament per second, I now get the exact results. However, when printing two 20mm calibration cubes I get quite bad results. The first cube (left) is printed at a 1.0 extrusion multiplier and the second one (right) is printed with a 0.95 extrusion multiplier. Any idea what these quality problems are caused by?

[Progress of first cube [New calibration cubes [Speed settings [Print settings

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    $\begingroup$ Maximum print speed depends on viscosity and nozzle surface. PLA at 220-230°C can be printed easily at 80 mm/s on 0.4, but you have 0.3, so do not exceed 40-50 mm/s. More likely, stay on 30 mm/s, since your extruder has no gear reduction and the teeth on the filament are pretty short and not very sharp. Try again with at 30-35 mm/s. Also, do E steps calibration while extruding slowly, like "G1 F30" (extrusion 30 mm/minute) followed by "G1 E100" (extrude 100 mm). See mattshub.com/blog/2017/04/19/extruder-calibration but do NOT do the extrusion multiplier calibration, keep it at 100%. $\endgroup$
    – FarO
    May 5, 2020 at 12:55
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    $\begingroup$ Sorry should've specified but this happens when doing a 100 mm calibration test at 2 mm/s. 30 mm/s is quite ambitious. $\endgroup$
    – Stefan
    May 5, 2020 at 13:00
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    $\begingroup$ 2 mm/s of the filament or 2 mm/s of the printing head? Printing that slow always causes problems because the radiating heat of the nozzle melts plastic around it (as indicated by the poor top layers of some other prints from you). Print at at least 20 mm/s! at minimum $\endgroup$
    – FarO
    May 5, 2020 at 14:25
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    $\begingroup$ I know it's not the problem, I just wanted to be sure. So you print at 10 mm/s? it's quite low and you still have the driving gear slipping. Have you cleaned it? have you checked it's sharp enough to grip the filament? I have a much more aggressive gear (sharper and deeper teeth) but if for some reasons I have the filament slip, the teeth get clogged and they slip easily. $\endgroup$
    – FarO
    May 5, 2020 at 15:45
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    $\begingroup$ In the worst case, buy a new driving gear, they are cheap and maybe you can find one with sharper teeth. Have you replaced the nozzle since the previous questions? if not, go for 0.4 which makes things much easier. $\endgroup$
    – FarO
    May 5, 2020 at 15:49

1 Answer 1

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I think you are pushing filament too fast.

Let's go back to the basics: my hotend (old Ubis, ceramic) and extruder can push PLA at 230°C at max 80 mm/s * 0.5 mm line width * 0.2 mm layer height = 8 mm3/s.

At this speed it doesn't extrude very well, there is too much back pressure (see also

). That extrusion speed means (I have 3 mm filament) 1.13 mm/s of filament speed (8 mm3 / 1.52 / pi).

You have 0.3 mm nozzle and you print colder, both of which cause more resistance to the flow. If 8 mm3/s is my absolute max, yours should be about 8 * 9/16 (ratio of the nozzle surfaces) = 4.5 mm3/s (really the max).

Considering the different print temperature, I would start limit to 3.5 mm3/s, which is 0.55 mm/s extrusion speed.

If you print 0.15 mm layers with line width of 0.35 mm, your absolute max printing speed should be 3.5 mm3 / 0.15 /0.35 = 65 mm/s (which is better not to reach, 60 is fine).

Try doing the following tests: after cleaning the driving gear, extrude 50 mm filament at 0.3 mm/s and measure how much filament has been extruded. Then repeat 50 mm with 0.5 mm/s, and measure how much filament has been extruded. Try again at 0.75 mm/s. Obviously don't touch E steps and extrusion multiplier. I'm quite sure that at 0.75 mm/s you will notice a measurably shorter length of filament extruded. At 1 mm/s you will see grinding (but not as much as now).

If you want, try at regular intervals 0.2, 0.3 0.4, 0.5, 0.6, 0.7 mm/s and plot a graph of the actually extruded filament length. It will look like the one in the video (where he extruded a lot more and weighed the filament, which is time consuming and more expensive).

And then switch to 0.4 mm nozzle if it's too slow for your needs.

Additional information

You use 2.85 mm filament with a direct drive, no gears. Judging from your video the radius if the driving gear (teeth) is about 4 mm, meaning 4*2*pi=25.1 mm circumference. The circumference is controlled with 200 steps * 16 microsteps, as result each microstep controls 4*2*pi/200/16 mm filament length, which is 0.05 mm3 and what you extrude along a 0.95 mm length (at 0.15 mm layer height and 0.35 mm line width). Basically your extruder has no control for moves shorter than 0.95 mm, but in fact it's even 4x worse, since you never get a single microstep precision (4 microsteps of tolerance is more reasonable).

You should probably use a much bigger nozzle, or 1.75 mm filament, or a geared extruder, or prints will never be accurate and you will have problems all the time, which you cannot physically solve.

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  • $\begingroup$ Thank you for your answer. The speed and gear did appear to be the problem. However, now I've got quite bad quality problems. Any idea what these are caused by? I'm unsure whether this is caused by the same problem or requires a separate question here. $\endgroup$
    – Stefan
    May 8, 2020 at 10:47
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    $\begingroup$ I'm not sure about the cube, but it appears that the backpressure question has been solved :) I would search for similar issues and/or start a new question. $\endgroup$
    – FarO
    May 8, 2020 at 12:49

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