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During printing, the extruder pushes the filament into the hotend, where it will be melted and extruded through the nozzle orifice. During this process, pressure is created inside the nozzle which forces the plastic out through the nozzle.
However, once the extruder stops pulling the filament, the pressure in the nozzle is maintained, causing a small amount of molten plastic to continue to flow out until the pressure inside and outside the nozzle equalises.
This phenomenon causes a small amount of material to continue to come out during displacement movements in which no material should be extruded. This causes small droplets and fine threads to appear on the surface of the part.
To avoid this, FFF 3D printers employ a mechanism known as retraction. Before performing a displacement movement, the extruder retracts a small amount of filament, releasing the pressure inside the nozzle. When the displacement movement ends and an extrusion movement begins, the extruder pushes back another small amount of filament until the nozzle is primed and the pressure inside the nozzle is restored.
In order to optimise the printing quality to the maximum, it is very important to set the retraction parameters for each material.
The basic retraction parameters are:
In addition to these basic parameters, there are others that influence the quality of the piece:
Not all software includes all these parameters, and some of these parameters may behave differently from one software to another. Before modifying values of the retraction parameters, the software manual should be consulted to understand how each of them behaves.
Because there are multiple parameters, it is complex to obtain a suitable combination in a straightforward manner. For this, it will be necessary to carry out multiple tests in stages until the right values are obtained. It will be necessary to adjust the retraction parameters independently for each material, and for this we must have previously determined the appropriate printing temperature for each material.
First of all, the appropriate retraction distance and retraction speed must be obtained, so the rest of the parameters must be left deactivated or with their pre-set values.
The optimum values for retraction speed and retraction distance are the lowest values that avoid the appearance of threads during the displacement movements. The following steps shall be taken to determine these values:
In any case, distances of no more than 3 mm should be used for direct extruders and no more than 5% of the pipe length for Bowden systems. If it is not possible to obtain an acceptable result within this range, the retraction rate should be increased by 5 mm/s and the process repeated. It is not recommended to use retraction speeds above 40 mm/s, so if a satisfactory result is still not achieved once this has been reached, the combination that gives the best results should be selected.
With some printers and filaments it may not be possible to completely remove the threads, even with the best combination of speed and retraction distance. In these cases it is advisable to activate other functions such as "Z-lift", "Clean while retracting", "Run-in" or "Nozzle cleaning", which will help to minimise or eliminate yarns.
The consequences of excessively high or low values of the retraction parameters are very different. While too low values of retraction speed and retraction distance have purely aesthetic consequences (appearance of threads and droplets on the parts), setting too high values often leads to jamming inside the heatbreak. This is because if the retraction is too long, the molten tip of the filament will reach the cold zone of the heatbreak, expanding and blocking it.
If, after setting the retractions, jamming occurs, especially on long prints, the retraction distance should be reduced slightly.
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