Why high retraction values in single extruder mode?
I was wondering (did a quick search but didn't find any results) why even in single extruder mode these 4-6mm retraction distances are still needed?
Re: Why high retraction values in single extruder mode?
Bowden tubes and the viscoelasticity of the filament.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…
Re: Why high retraction values in single extruder mode?
Because of the slack in the tubes?
Re: Why high retraction values in single extruder mode?
There are 2 things which determine retraction:
1. Extruder geometry
2. Viscoelasticity of the filament
The extruder geometry is broken down further:
Nozzle diameter (default 0.4mm)
Nozzle bore length (default 0.6mm)
Drive length (MMU default 535mm(
And the filament:
Viscosity of the melted filament (runniness)
Elasticity of the cold filament (stringiness)
All you need to do when retracting is to remove a very small amount of filament from the actual bore of the nozzle (default 0.4mm diameter x 0.6mm length); to do this you have to remove all the pressure in the nozzle built up due to the elasticity of the filament and the distance between the filament drive pulleys and the nozzle, while taking into account the viscosity of the meted filament in the hot end.
This is why linear advance requires a much higher K value when printing with the MMU. This K value is an approximation which is determined solely by test prints and differs according to the filament in use.
KISS, uses a slightly different approach with a per filament "Preload VE" value in combination with the extruder geometry.
Currently both methods have pros and cons.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…