Most newcomers to 3D printing don’t think about retraction settings until they’ve succumbed to stringing. This is a common printing problem during which the nozzle leaks excess filament over the printed part, resulting in a mess of plastic strands that look like cobwebs.
Stringing can be caused by several factors. Too much heat in the hot end is a common culprit, while some materials — PETG, for instance — are more prone to stringing than others. When reducing the printing temperature doesn’t fix the problem, a great way to counter stringing is to adjust the retraction settings found in software like Cura, the popular slicer owned by 3D printer company Ultimaker.
There are several Cura retraction settings to play with, including retraction distance, retraction speed, and minimum travel distance, and knowing how these parameters affect the printing process is a great weapon in the fight against stringing.
This article lists all of the Cura retraction settings available to users, explaining what they do and how they can be optimized to suit different prints.
Recommended reading: 3D printer stringing: How to fix it
During FDM 3D printing, the printer’s extruder uses gears to move plastic filament to the hot end, where the filament is melted and pushed through a nozzle onto the build area below. Meanwhile the print head is constantly moving around in order to deposit the molten material in the right places.
Every now and again, the print head needs to move from one area to another without depositing any material. In 3D printing terminology, this type of movement is called travel. However, since there is always a bit of melted filament in the printer’s hot end, the plastic can sometimes accidentally ooze out of the nozzle and onto the build during travel. This is a problem for printer users, because that oozed material can cause an ugly, stringy mess over the printed parts.
One way to counter oozing and stringing is retraction. This is when the extruder stops pushing the filament towards the hot end and pulls it back instead. It can do this by reversing the direction in which the extruder gears move.
Retraction cannot magically eliminate oozing, because the already-melted section of the filament cannot itself be retracted. However, by retracting the solid section of filament immediately above the melt zone, pressure is relieved on the molten material, making it less likely to ooze out from the nozzle during travel movements.
By adjusting Cura retraction settings, the slicer can adjust the g-code sent to the printer, altering the retraction process in various ways. Users can specify when retraction should happen, how much filament should get retracted, how quickly it should get retracted, and more.
Cura’s retraction settings can be found in Print Setup. Most are grouped under Material settings, while some others are grouped under Travel.
The first two retraction settings, which can be checked or unchecked, are fairly self-explanatory: “Enable retraction” allows users to toggle whether retraction takes place at all, while “Retract at layer change” instructs the printer to perform retraction once a 2D layer is complete.
The remaining Cura retraction settings, most of which are defined using numerical values, may be less intuitive to beginners. Related settings like print speed and coasting are not discussed here, but may be adjusted in conjunction with retraction settings to deliver the best results.
Retraction distance refers to the amount of filament pulled back by the extruder. If the value is greater, then a longer length of filament is retracted.
The retraction distance is usually set in the range of 0.5–15 mm, depending on the extruder and material type. A greater retraction distance can help to prevent stringing, but this can cause stress on the filament and potentially cause clogs, as well as resulting in longer print times.
Bowden and direct-drive extruders require different retraction distances. Bowden extruders can require a retraction distance of up to 15 mm, while direct-drive systems generally only need a few millimeters.
Retraction speed is the rate at which the extruder retracts the filament. In Cura, retraction speed settings are divided into two values:
Retraction retract speed is the speed (in mm/s) at which the extruder pulls back the filament
Retraction prime speed is the speed (in mms) at which the extruder returns the filament back to its original position at the tip of the nozzle
Faster retractions can help to prevent oozing but run the risk of damaging the section of filament by grinding it. Most users set their retraction speed in the range of 30–80 mm/s.
During retraction, priming is when the extruder returns the filament to its original position — by pushing it forwards after it has temporarily pulled it backwards. However, since a bit of material may have oozed during the travel movement, it can be helpful to extrude extra material to compensate for this loss.
The retraction extra prime amount is defined in cubic millimeters. A greater value helps to build pressure in the nozzle, which can prevent issues like under-extrusion. However, the required amount depends on several factors, including nozzle size and travel distance.
Cura users can download plugins that set the retraction extra prime amount automatically based on travel distance.
During printing, a travel move is when the print head moves from one area of the build to another without depositing material. Sometimes the print head travels just a few millimeters, but sometimes it moves all the way across the build.
The retraction minimum travel distance setting allows users to define whether the printer should perform retraction even during short travel moves, or just for very large jumps. A small minimum travel distance (in mm) can help to prevent stringing but can cause issues like filament grinding.
Though not specifically a retraction setting, users can also adjust parameters like travel speed, with faster travel moves potentially reducing oozing and stringing.
This print setting lets users set a maximum number of retractions on a given length of material (defined in the “Minimum extrusion distance window” box).
Setting a maximum retraction count can be useful, because repeatedly retracting the same piece of filament makes it more susceptible to grinding, which can ultimately reduce print quality. However, capping retractions can potentially make the print more vulnerable to oozing and stringing.
Cura can instruct the 3D printer to carry out a retraction when the nozzle is put in standby mode. It offers two adjustable parameters for this situation:
Nozzle switch retraction distance (mm) works in the same way as general retraction distance, but users can set a higher value as there is less negative impact on overall print times
Nozzle switch retraction speed (mm/s) works in the same way as general retraction speed
Found under Travel settings, combing is a feature that instructs the printer to avoid any travel moves that stray beyond the perimeter of the build.
Combing can be useful for several reasons. Although it can result in greater travel distances, it allows for a degree of looseness during printing, since any oozing or stringing can take place inside the part and therefore be invisible. This makes retraction less of a necessity and allows users to print faster, with fewer retractions.
Users can select from the following combing modes:
Not on outer surface
Not in skin
Sometimes called vertical travel, Z-Hop is a feature of Cura that instructs the print head to move upwards (or the build platform to move downwards) while retracting and making a travel move. This can prevent the nozzle from dragging along the top of the build and causing scratches or blobs.
If Z-Hop is enabled, users can adjust the following parameters:
Z-hop only over printed parts
Z-hop after extruder switch
With so many Cura retraction settings to play with, it can be useful to carry out test prints after adjusting certain parameters in small increments. A good retraction test print is a stringing tower, a simple model purposely designed to be susceptible to stringing. If the stringing tower exhibits a large amount of stringing, then calibration has not been carried out successfully.
It is best to use Cura’s default settings as a starting point, then make small adjustments and test the new retraction settings with the stringing tower.
Finding the best retraction settings depends on different factors, like the type of material, the purpose of the printed part, and the dimensions of the printed part. However, Cura’s range of adjustable parameters gives users a great degree of control over their prints, meaning that there is a suitable set of retraction settings for any project.
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