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SpoolJoin (MMU2S)

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Last updated a year ago
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If you have spools with just a small amount of filament, and not enough for a print, you do not need to throw them away, or manually change the filament every time it ends. You can use the MMU2S feature of filament depletion, called SpoolJoin.

This feature will automatically switch to the next filament in line, should you run out. In order to activate it, go to LCD Menu - Settings - SpoolJoin [On/Off]. Add each spool that you wish to finish on the MMU2S (do not skip any filament slot in between). Once the filament depletes, the printer will unload it and load the next slot.

The function works sequentially in a loop, switching from filament 1 to 2, then 2 to 3, and so forth. The switching occurs when the FINDA sensor detects the current spool is finished. While the printer is going through filament slots, you can replace the spools on the slots that have run out, and when the filament in slot 5 ends, it will go back to slot 1 and continue.

After an unload, there will be filament on the filament slot: even if you are not replacing the spool once it runs out, you'll have to remove the filament leftover by hand. Alternatively, you can do this after finishing the print, choose in LCD Menu - Eject filament - Eject Filament 1/2/3/4/5. 

We recommend turning the SpoolJoin feature off for multi-material prints. As an example, suppose your multi-material print uses filaments 1 and 2. If filament 1 runs out or run-out is falsely detected, filament 2 will be used in both places, as filament 1 is considered empty.

There is a niche use of SpoolJoin when printing multi-material models. If you have a two-color print and, in PrusaSlicer, set it to use Filament 1 and 3, there is now a spare spool loaded to Filament 2. This will be used when Filament 1 runs out. In addition, Filament 4 and 5 can be loaded before printing with the same intention, substituting for Filament 3.
Filaments are often "secured" to the spool by a bent end (Blue circle). This might make your filament get stuck in the MMU2S PTFE tubes or pull the empty spool. Make sure to cut that end when you are finishing a spool.
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