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Purge Calibration

Calibrating the Purge/Wipe Volumes

One of the more tedious procedures in multi- color/material prints is calibrating the purge volumes. As I've described in the Prusa/Super Slicer set up, you'll need to set up your purging volumes correctly, in order to avoid the so called "color bleeding" issue.
This is tedious because: It not only depends on which color follows which but also what type and brand of filaments you use. Needless to say, that you have do this purge volume calibration each time you swap out either a filament color or brand.

Sure, you could set the purge volume for each tool to an insanely high value, say 500 mm³, and you'll never experience color bleeding but you'll also waste a ton of material. So, calibrating the volumes is more or less a must.

To make this calibration process somewhat easier, I came up with this procedure:

First, download and unpack the Purge Calibration ZIP file, which contains the 3MF and STL files, which can be loaded directly into the Prusa/Super slicer. This model is made for a 5 tools SMuFF, which covers all possible transitions. In this case that's 20 transitions. Expressed as a formula the number of transitions is calculated by: n²-n, whereas n represents the number of tools/materials you use.

Keep in mind: The more tools you use, the more permutations there are! For example, by using 12 tools/materials this equals to 12²-12 = 132 possible color transitions. By using 20 tools, this amount would jump to 380!
Each of the transitions needs to be calibrated separately.

The "Purge Calibration" model consists of 3 columns, each column built up by 21 square shapes, each of them 20 x 20 x 3 mm in size. Printed with a 0.4 mm nozzle using 4 perimeters at 0.2 mm layer height, each square will eat up roughly 20 mm³ of filament per layer. Since three columns are being printed in one go, each layer on the build plate consumes about 60 mm³ of filament.

Before you slice this model, make sure you have assigned all the colors to each tool according to the colors in your SMuFF. This will make it easier to distinguish them later on.

Then, make sure you have set your layer height to 0.2 mm and also 4 perimeters for the walls, in order to ensure no infill will be used.

Please notice: Prusa slicer might complain because the biqu_convert_p24.exe wasn't found. Go into your Print Settings -> Output options -> Post-processing scripts and remove the biqu_convert_p24.exe entry, then slice again.

For a starting point, set up the "Purging volumes" to the default 140 mm³. Now slice the model, export the GCode and print it.

After the print has finished, mark the columns from left to right as "1, 2, 3" or "20, 40, 60" (which denotes the amount of filament used per layer in mm³), before you remove them from the build plate. Inspect each color transition closely.
Whenever you spot "color bleeding" on a transition of colors on the first column, move over to the 2nd column and check whether it's still bleeding. If it's gone on the 2nd column, simply add 20 mm³ (to the value, i.e. 140 + 20 mm³) to the purge volume in Prusa/Super Slicer.
If it's still bleeding, move over to the 3rd column and check this one out. If it's gone on the 3rd column, add 40 mm³ to the purge volume. If it's still there, return to the 1st column and move up one layer.

Repeat this process until you find the sweet spot and you're fully satisfied with the color transitions.
For each column add 20 mm³ of purge (or 60 mm³ for each layer printed) and write those values into the according input fields in Prusa/Super Slicer (as seen in the picture on the left).

After you've processed all color transitions and transferred the new values into Prusa/Super Slicer, re-slice the model and print it once again. There might still be one or two transitions that need some fine tuning.

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