I came up with a somewhat odd idea about how to switch filaments. It's a thought experiment at present, but might offer a starting point.

The extruders I'm familiar with shove filament into the hot zone with a cog/gear pressing the filament against a bearing opposite the gear. This lets the extruder shove and retract filament. If the bearing pressure were electronically controlled on and off, perhaps with a solenoid or some such, the filament would be free(ish) to slide. This would let the filament movement be up, down, or free.

So the actual extruder could pull out the filament a bit, then let go of it.

With a manifold of guides/tubes all pointed into the intake of the actual extruder, you could put an up/down/free filament mover at a spool to retract or shove filament into the mouth of the actual extruder, then let the filament slide freely.

The point of all this is to let one actual extruder/hot zone be set up to change filaments semi-automatically. It would eliminate the need for multiple hot extruders, although the filament motion sections would be replicated, but not the heaters, and you could still get full travel without the width lost to alternate extruders.

It would need a "dumping ground" to clear the extruder of the old filament in the melt zone, and probably some program time spent to flush the previous filament/color, so there are obvious issues, as well as unknown flaws to be discovered.

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