HydraRaptor was using a knife to remove excess filament from the extruder :-
It always cut the filament OK, but it was random whether the loose bit fell off or stuck to the far side of the nozzle. The soundtrack of a video I saw of a commercial FDM machine said that they use a brush. I thought I would need a wire brush for 200°C but then I reasoned that, if the nozzle passed through fast enough, the high specific heat capacity of plastic might mean that it would not have time to melt. I decide to give it a try with an old electric toothbrush head :-
It does seem to work quite well. Here is a video of it in action :-
The scrap of filament sometimes stays stuck to the brush but subsequent passes eventually knock it off.
When I was using HydraRaptor for milling I had a tray around the table and a plastic skirt to protect the mechanism of the precious XY table from loose plastic chips. When I moved on to FDM I thought these would not be needed because it is a lot less messy. Actually I was wrong as HDPE chips are appearing, presumable from inside the extruder, and the filament offcuts sometimes ping off from the brush. I have therefore refitted the tray and skirt.