Retraction Test: Tuning Length to Stop Stringing

What the test measures

Retraction pulls filament back before a travel move so the nozzle does not ooze on the way. Too little leaves strings; more than needed costs time and can cause other problems (see Retraction). A retraction tower prints a vertical structure with notches, each section at a different retraction length, so one print shows where stringing stops. OrcaSlicer's rule: the best length is the shortest one that produces the cleanest tower.

Prerequisites

• Temperature first. Temperature changes stringing on its own; see Temperature Tower.
• Pressure advance and flow before retraction. OrcaSlicer: default retraction values usually work well, but if you calibrate them, do it after pressure advance and flow ratio, as both change the pressure in the nozzle. Klipper says a well-tuned pressure advance reduces the retraction needed. See Pressure Advance and Flow Calibration.
• Dry filament and a sealed nozzle. Wet filament strings no matter what retraction does; see Wet Filament Symptoms.

How to run it

OrcaSlicer

Calibration, Retraction test. Choose a start length, end length and step. OrcaSlicer's defaults and its Bowden suggestion:

Extruder	Start	End	Step
Direct drive (dialog defaults) Starting point	0 mm	2 mm	0.1 mm
Bowden (OrcaSlicer's suggestion) Starting point	1 mm	6 mm	0.2 mm

Bambu Studio

Bambu Lab documents a Retraction Test in its calibration guide for third-party printers (Developer Mode required). It generates the same kind of tower; Bambu Lab says to note where quality degrades and how the gloss changes to find the shortest length that gives the neatest tower.

Klipper firmware retraction

If your Klipper config has a [firmware_retraction] section and your slicer is set to use firmware retraction, retraction comes from G10 and G11, and SET_RETRACTION RETRACT_LENGTH= changes the length. In that setup TUNING_TOWER COMMAND=SET_RETRACTION PARAMETER=RETRACT_LENGTH with a START and a FACTOR (or STEP_DELTA and STEP_HEIGHT) steps the length up the height of any tower model, using the same formulas as the temperature tower. Adding [firmware_retraction] is a printer.cfg change: back up the config first. Klipper notes SET_RETRACTION is commonly set per filament in the slicer.

PrusaSlicer

We did not find a built-in retraction test in Prusa's documentation. Use one of the methods above, or a retraction tower model together with firmware retraction.

Reading the result

Look at each section for strings, hairs and blobs between the two towers. In OrcaSlicer, measure the height of the cleanest section, then search the G-code preview for the Calib_Retraction_tower comment at that height to read the length. OrcaSlicer warns not to read the ; retract comment instead, because it does not always show the real length (it leaves out values like the amount retracted before wipe).

Tower looks like	What to do
Clean from the very bottom	OrcaSlicer and Bambu Lab: common with low-ooze filaments like PLA or ABS; a length of 0.2 to 0.4 mm should suffice Starting point
Strings fade out partway up	Use the length where it first becomes clean
Still a lot of stringing at the top	Do not go longer. Dry the filament and check the nozzle is installed without leaks

What to change

Set the retraction length you found in the profile your slicer uses for retraction (in OrcaSlicer, the printer's extruder settings or the filament's override). If you use Klipper firmware retraction, put it in [firmware_retraction] or the filament's start G-code. After pressure advance is tuned, Klipper suggests a small slicer retraction (its example is 0.75 mm Starting point) with wipe on retract, and disabling z-lift on retract; the Retraction lesson explains what z-lift protects against.

Leave these alone during the test

• Nozzle temperature. Pick it first; the tower is only valid at that temperature.
• Retraction speed, travel speed, wipe and z-hop. Change length only, then revisit the others one at a time.
• Pressure advance and flow.

For every other cause of strings, see Stringing and Oozing.