Is there a tool that is able to transform a multi-band image to single band rasters? I usually use the raster calculator within the modeler to do that.
You could use
Raster > Conversion > Translate (Convert format)...
Then click the pencil icon to edit the console and use something like the following to export band 1:
gdal_translate "path/to/raster.tif" -b 1 "path/to/result.tif"
To get band 2, replace
-b 1 with
-b 2 etc.
Note: you may need to enable the GdalTools plugin.