Have you ever experienced stringing, blobs, zits, and other imperfections on the surfaces of your prints?
This happens because the extruder moves with no load, leaving behind excess material.
But don't worry, you won't have to sand your prints every time, to solve this problem you can set the retraction by setting some parameters on your slicer.
What is retraction?
Retraction is a function of the slicer that is used to eliminate strings, burrs, drops, and other defects on the surface of the piece that are created during the movement of the extruder.
Setting the retraction of the 3D printer means that the motor makes a reverse movement during printing, to withdraw the filament inside the extruder.
With retraction, we create a "pause" moment in which the filament is no longer free to flow outside. This time is needed to get the extruder moving to the next printing point without leaving a trail of material behind.
How is retraction set?
Retraction is enabled and controlled from the slicer, and generally consists of three main parameters:
- the retraction length
- the retraction speed
- the minimum distance for retraction
Retraction length
How much filament should be retracted into the extruder?
The retraction length indicates the amount of filament (expressed in mm) that is retracted upwards each time there is a retraction.
We suggest you use a retraction length between 2 and 5 mm for direct-drive extruders and between 4 and 7 mm for bowdens. But be careful: each filament has a different behavior inside the nozzle extrusion chamber, so you will have to find the perfect value for your printer and for each material you use.
The higher the value of the retraction length, the more the nozzle will be "unloaded" of material at the moment of displacement. We suggest that you test this, bearing in mind that higher values are not always more effective: a longer length requires a longer retraction time, which will increase the printing time. Furthermore, if the nozzle is emptied and filled too frequently, it is more prone to clog.
Retraction speed
How fast does the filament retract into the extruder?
Setting the retraction speed, which is measured in mm/s or mm/min, means setting how fast the filament is retracted into the nozzle. A high retraction speed allows you to use shorter retraction lengths and guarantees good results without lengthening the printing time too much.
We suggest setting a retraction speed between 30 and 60 mm/s, again the perfect value depends on the printer/nozzle/filament configuration.
In general, it is preferable to increase the speed and reduce the retraction length, but be careful: too high retraction speeds can lead to filament slippage on the extruder knurling wheel or, worse, filament breakage inside the hot end.
Minimum retraction distance
Correctly setting the retractions is essential to minimize the risk of stringing and clogging, and to do this you will also need to take into account another parameter: the minimum distance for activating the retract function.
If you are printing a very articulated part, for example, it is better to avoid a lot of retractions in a small area, because then you risk damaging the filament and increase the possibility of extruder clogging. For this reason, it is possible to set a limit to the number of consecutive retractions the printer can make.
Each slicer has a slightly different method of operation: in Cura slicer, for example, this tool is defined in the maximum number of retractions per area. By default it is set to 100, but if the distance and speed are set correctly, we recommend lowering the number to 10, a sufficient value for most prints.
Other slicers, such as IdeaMaker or Simplify3d, prefer to consider the minimum extrusion length between retractions. In this case, we suggest you use a minimum distance for the activation of retractions between 0.5 and 1 mm. The printer will then extrude at least 1 mm of filament after a retraction before giving the green light to the next one.
Z Seam and retractions
An important aspect that you must not overlook when setting up retractions is the Z Seam, the imperfection that is created at the beginning of each layer when the print bed is lowered. Slicers usually try to hide the joint between layers in the corners of objects, and in more complete slicers such as PrusaSlicer or Cura, the positioning of the Z Seam can be customized by manually choosing the points where the layer starts.
In the case of very curved objects with few edges, however, unsightly lines can be created between the faces. To avoid these defects, a quick method is to set the "random seam", i.e. a casual starting point for each layer.
However, if the retraction settings are correct, the visibility of the Z seam will also be greatly reduced, so it is important to understand what you need to change to make these defects less noticeable.
Other parameters
And that's not all: there are other advanced settings for retractions such as Coast at end, Wiping, Combing, and Priming, which are useful to avoid start/end extrusion inaccuracies. However, we recommend that you first find the right combination of length/speed/minimum distance and then concentrate on these parameters, which will be the subject of a separate article.
