During my first steps with 3D printing I was completely lost with the question how to set the printing speed. I was facing irregularities in the print and clicking in the extruder as the major problems.
With a look at the theory it all becomes quite easy.
The irregularities at the print are mostly depending on the velocity delay, assuming that the extrusion parameters are properly set. In the original firmware from Vellemann the parameter steps/mm for the extruders is set to 200, in reality form the hardware design a theoretical factor of 154.4 has to be set, by some finetuning of the extruder a value around this point will be settled.
Otherwise a clicking in the extruder will occur. This can have three different causes:
1) the print is done with the original firmware settings and too much material should be provided. Since there is no room under the nozzle for this material. the pressure in the nozzle will build up until the force in the extruder is too high and the filament will slip back with a click.
2) the distance between nozzle and print bed is incorrect and not adopted in the slicer. There are two possible results: the first layer is too thick and is not building a closed layer or the first layer is too thin, generating the clicking in the extruder. Once again the extruder tries to deliver more material than it can be processed, the same mechanism like before.
3) the extrusion velocity is too high. As it was shown in the theory section, the extruder has to provide two forces: one for the pressure build up of the filament to be deposited on the print, the second for the sliding friction of the seal. When the sum of these two forces are exceeding the limit of the extrusion force, the filament will slip back in the extruder, generating a click.
In this chapter we will not discuss the velocity delay, but the printing speed. When clicking the of the extruder appears, something is going wrong. In case of reason 3) we will loose material, because the slip back will never be compensated, the material is missing somewhere in the printout, degrading the part.
So we have to find the pressure limit for our hardware. With the theory in mind this is an easy task: we have to extrude a sufficient long filament at the maximum speed without clicking.
For the K8400 you can apply the following steps after the properly selected temperatur is reached:
1) Positioning of the plate
G1 X100 Y100 Z100
2) set the relative mode
3) and finally do the extrusion
G1 Te E50 F50
The last gcode will push 5cm filament through the nozzle. You have to select the active extruder e (0 or 1), for the fine tuning you need to vary the speed value: F50 ... F100.
Having in mind the theory you will achieve the highest velocity with the fan on, because the seal length is minimised, minimising the fraction of the force which is needed to push the filament.
Having the maximum value for the extrusion speed, we have to divide the F-Number by 60 and multiply with the cross section of the filament to get the limit for the volume flow. e.g. for 1.75 mm filament (A = pi * d² / 4 = 2.4 mm²)
F60 -> 60 / 60 * 2.4 = 2.4mm³/s
This is the limit for the extrusion speed in any cases. We have taken this value without material deposition, it is the upper limit of the extrusion speed. Since we have to apply some additional pressure to spread the extruded material on the printout, this additional force has to be provided by the extruder too. As a practical approach you can reduce the speed by 20% to be on the safe side.
This final value can be taken into the slicer as maximum volumetric flow, which will ensure a proper extrusion flow throughout the whole printing. Otherwise you can calculate the maximum speed depending on the extrusion width and layer height.