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The Bowden tube lets you reduce the moving mass of the extruder and thus allowing faster controlled motion, less shaking of your machine, less energy use and importantly: faster printing! Normally the mechanism that drives the filament into the hot end (where it melts) is directly on top of the extruder. This creates problems of balance and oscillations with faster motion which you can see in your printing results and hear and feel when your machine is shaking.
If the filament drive mechanism is placed on a non-moving part of the 3D printer, it can be pushed into a tube. PTFE (Teflon™) is useful because it is slippery: it has little friction with the plastic. This limits wear and loss of energy. The tube's other end is connected to the extruder's hot-end.
The concept of using a Bowden cable as guide was suggested and pioneered by Ed Sells.
The lighter the extruder is, the quicker you can move the toolhead(s), e.g. when you're not extruding. The machine will need less current to run the motors and it reduces stringing when not extruding. Even more so, it reduces the moving mass of the X-axis so much that a much simpler and lighter Cartesian bot is possible. The steel rods wouldn't necessarily be needed, allowing a much larger ratio of self-replication. The Bowden cable itself could be printed in several modules that snap together. This guide could be lined with telfon-tape (available in every hardware store) that can be replaced when worn.
The Bowden cable system has one major drawback: Hysteresis. The plastic filament will compress in any extruder, but putting pressure on such a long length of filament will multiply the effects of this compression, leading to springiness. The flexibility of the PFTE tube exacerbates this problem.
It is potentially possible to control this problem, perhaps with an encoder just above the hot end. Software might also be able to reduce this problem, but would require (maybe lengthy) calibration.
A stepper-based extruder is required if no encoder is used.
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