Hello!
The Problem
A direct extruder (with the extruder motor at the nozzle) makes the end effector heavy. Due to the limited torque of the axis-motors and the limited stability of the linear bearings and the frame the printing speed is limited.
A bowden extruder (with the extruder motor at the frame, feeding the nozzle trough a tube) does not have these issues. The end effector is very light an can move with high speeds and high acceleration. However the bowden principle introduces many problems. This is mainly friction in the tube and a hysteresis due to the elasticity of the tube and play of the filament in it.
To fight stringing retraction is used. But because of this hysteresis you need more retraction. You'll also need more retraction if you are trying to print faster, because the pressure in the nozzle is higher and this pressure has to be eliminated before moving the nozzle to another position (otherwise you get strings). More retraction takes time an you get small blobs during the time the nozzle stays at one position.
Long story short: The fastest I could print with a decent quality was around 25mm/s with 5mm retraction.
The Solution
The Flying Extruder!
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The extruder is suspended by rubber bands. These are running around several rolls to increase the length (each around 2m relaxed) to have more or less the same force over the whole vertical range. The extruder is connected with a very short tube (around 75mm) to the nozzle. This tube decouples the high frequencies between the nozzle and the extruder.
Long story short: I'm printing with no problems with 75m/s and 3mm retraction and still with the same acceleration of 800mm/s². But this was only a short test and I think I can bring it up with a bit optimization.
Another advantage is, that it's now easier to build an enclosure and that the printer is much quieter, because the extruder is decoupled.
(Cabling is not finished yet.)
Additional tips
Its important to cool the cold end even with ABS, because retraction is then more effective and reproducible.
I added some shrinking tube (with internal glue) to the tube and used cable ties around the tube fittings (to keep the plastic part away from the metal part) to minimize the play and make the retraction more effective.
Kind regards,
MK
The Problem
A direct extruder (with the extruder motor at the nozzle) makes the end effector heavy. Due to the limited torque of the axis-motors and the limited stability of the linear bearings and the frame the printing speed is limited.
A bowden extruder (with the extruder motor at the frame, feeding the nozzle trough a tube) does not have these issues. The end effector is very light an can move with high speeds and high acceleration. However the bowden principle introduces many problems. This is mainly friction in the tube and a hysteresis due to the elasticity of the tube and play of the filament in it.
To fight stringing retraction is used. But because of this hysteresis you need more retraction. You'll also need more retraction if you are trying to print faster, because the pressure in the nozzle is higher and this pressure has to be eliminated before moving the nozzle to another position (otherwise you get strings). More retraction takes time an you get small blobs during the time the nozzle stays at one position.
Long story short: The fastest I could print with a decent quality was around 25mm/s with 5mm retraction.
The Solution
The Flying Extruder!
The extruder is suspended by rubber bands. These are running around several rolls to increase the length (each around 2m relaxed) to have more or less the same force over the whole vertical range. The extruder is connected with a very short tube (around 75mm) to the nozzle. This tube decouples the high frequencies between the nozzle and the extruder.
Long story short: I'm printing with no problems with 75m/s and 3mm retraction and still with the same acceleration of 800mm/s². But this was only a short test and I think I can bring it up with a bit optimization.
Another advantage is, that it's now easier to build an enclosure and that the printer is much quieter, because the extruder is decoupled.
(Cabling is not finished yet.)
Additional tips
Its important to cool the cold end even with ABS, because retraction is then more effective and reproducible.
I added some shrinking tube (with internal glue) to the tube and used cable ties around the tube fittings (to keep the plastic part away from the metal part) to minimize the play and make the retraction more effective.
Kind regards,
MK