How to Fix Prusa MK4 Printing in Mid-Air
December 4, 2024
Watching your MK4 start printing in mid-air is more than just frustrating - it's a waste of time and materials. This issue has been discussed on a number of forums*, and we discovered it for ourselves while developing automatic part ejection for the MK4. After lots of testing and head-scratching, we've discovered why it happens and how to fix it reliably. In this blog we'll share how we discovered it, our approach to solving the issue, and give you what you need to fix it on your own MK4 printers.
Contents:
What's Actually Happening?
The MK4 uses a force sensor in the extruder to detect when the nozzle touches the build plate. In theory, this is great for precise homing. In practice, we found that this sensitive sensor can be triggered too early by something seemingly unrelated - tension from your filament spool.
This issue typically shows up as your first layer starting to print in mid-air, because your printer seemingly thinks it has reached the bed when it clearly hasn’t.
Why It Happens
Through our testing, we found this happens most often when:
- You've just finished printing a tall part
- You're using a fresh, spool of filament (especially a 5kg spool)
- Your spool is mounted in certain positions around the printer
We also found that when manually printing, pulling slack into the filament line before the print started would allow the nozzle to properly meet the bed.
The occurrence of the issue combined with our temporary solution confirmed that the issue comes down to filament tension. When the printer needs to move down a long distance to home, or when a heavy spool is pulling on the filament line, it can fool that sensitive force sensor into thinking it's touched the bed when it hasn't.
Finding the Right Fix
We went through several solutions before finding the combination that really works.
Solution Attempt #1
As mentioned above, we tried manually pulling slack into the filament line, but doing this every print would be disruptive and time-consuming. Not to mention it wouldn’t work with automatic part ejection.
Solution Attempt #2
Next, we tried altering the start gcode so it would tell the nozzle to move very close to the bed, then activate the force sensor and home the printer. This didn't always work because when the MK4 is restarted, it doesn't always know how close to the bed it is. It also doesn't relieve the filament from any residual tension that built up when the extruder was pulling filament from the last print.
Solution Attempt #3
Next, we tried a more aggressive approach, adding start gcode that would add a rapid back and forth movement of the nozzle before homing. This would cause the printer to "jerk" the filament forward and create some slack. It worked, but was inconsistent, because the amount of pull required depended on the position of the spool.
The Solution That Works
Since the previous solutions were all unreliable, we kept going, and after extensive testing, we found that combining two simple changes creates a reliable fix:
Final Solution #1
We added start gcode that instructed the printer to retract 50mm of filament before homing, so right before auto leveling, the printer would back up filament that was inside the nozzle to create 50mm of slack. This provided sufficient slack in the filament line to prevent it from tugging on the print head while it was tapping the build plate.
Final Solution #2
To make sure that our previous solution held up regardless of the spool position, we added a fixed length bowden tube to the printer. This bowden tube was fixed between the top of the printer and the extruder, which forced a constant amount of slack to always be present in the filament path and ensured that the filament always entered the machine from a fixed position. This second part of the solution was especially important for big spools, which can't be put on top of the machine. An extra bonus is that the bowden tube would also guide the filament consistently into the extruder, resulting in better quality prints regardless of where the spool is mounted.
Making It Work For You
While we've built the start gcode into AutoFarm3D™ for Mk4, we believe in sharing solutions with the community. To implement this fix yourself, you can use the following start gcode:
We also recommend adding a bowden tube to your printer alongside using this start gcode.
What's Next?
Have you experienced this homing issue with your MK4? We'd love to hear about your experiences or answer any questions about implementing this fix.
*Forum posts about this issue:
Make more with AutoFarm3D
.png){kind=small}
.png){kind=small}
