How to fix bad extrusion that's likely caused by high pressure in the nozzle

Question

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

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

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?

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Answer 1

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.