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Here are a few pics I found explaining how an extruder works. These pictures help to de-mistify the extruder and its functions.
A worm drive can drive a gear, the other way around meets a *lot* of friction?! I thought typically the filamentis driven by a hobbled or grooved bolt, the filament is pushed against that with a spring with a wheel on it.
The diagram that you are referring to is for pellets going through a hopper. Its kinds of hard to see but the hopper is outlined in blue. Sorry for the confusion. These are just meant to give an idea of how the concept works. Thanks for your comments.