Prusa Slicer and XL MMU

Working with the MMU (multi material unit) in the Slicer has a learning curve. Here are random tips and questions that we can better organize as we figure it out.

Prusa documentation on this: Prusa article

Extruder Painting

In the Slicer when you select an object you can paint the different extruders on it. Then, if the extruders are loaded with different colors or materials your finished object will have these.

Tip 1: It doesn't work well on a flat surface. For example, in Fusion 360 you can create an extrusion that is equal to a surface. Normally this extrusion would then be joined to the existing body and disappear. However, you can select to have the extrusion be a "new body". Then when you export the mesh you will have two bodies that are at the same height - a flat surface. The Prusa Slicer will let you paint a different extruder on each area. However, when you then slice the model your different bodies might get merged together - or might not. I've found it to be a bit random.

Tip 2: Make sure your extrusions are taller than your layer height. If not you will still be able to paint an extruder on the feature but when you slice the model the feature might be merged into one layer arbitrarily and you will lose your intent to have different extruders for each feature. For example, if your feature extrudes 0.1mm from a surface but you are printing with 0.2mm layers, then you might not see your features in the sliced model.

See Colors in the Slicer

In the filament list on the right side of the slicer is one drop down for each extruder. The drop downs each have a little color block: one big, one small. The large color swatch is the filament type default color. E.g. PETG is orange. You set this by clicking the config gear to the right of the extruder drop down. The smaller swatch is the color configured for that media for display in the plater views. You set this by clicking on the color swatch itself. Colors configured this way are saved in your .3mf file.

Tip: You can customize the painted colors in the paint tool on the Plater view, but they are NOT saved. So next time you open your .3mf file you will get the default colors again.

Multiple Bodies in Fusion 360

To facilitate extruder painting, you might want to have several bodies in your Fusion file. You would normally create one body and only have separate bodies for truly different components in Fusion.

Bodies that are not shown in fusion will not be saved when the parent component is saved. This is useful for saving all bodies at one time but excluding bodies you don’t want to print.

If a file has multiple bodies then when it is imported into the slicer, the slicer asks if they should be treated as one big part. This keeps them aligned to each other properly. If you say no, each body is moved down to touch the print bed and can be dealt with individually. If you answer "treat as one" you can change your mind after import by right clicking on the object and select Split / To Objects.