Erik's Bowden Extruder
The Bowden principle
The Bowden cable 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. See his blog post here 
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.
Several couplings have been suggested to affix the slippery PTFE at either end:
- Glueing it on, not a very good solution, I found - http://www.thingiverse.com/thing:996
- Threading a nut onto the end, using a printed coupling - http://www.thingiverse.com/thing:1899 (Darwin) http://www.thingiverse.com/thing:2221 (Mendel)
- Push-to-fit coupling, you can push in the PTFE and it will have a pretty tight friction fit - http://www.thingiverse.com/thing:3775
- Removing a 'ring' of PTFE and using a Circlip - http://www.thingiverse.com/image:18270 (by Siert Wijnia)
- The Ultimaker uses push fit couplings at both ends, facilitating rapid switches and easy access. See also ultimaker.com.
From left to right: Snap fit Bowden coupling, Captive nut coupling
Bowden hints and tips
- Gives you slightly less control over how much thermoplastics are output where, but:
- It decouples the extruder drive mechanism from the moving mass in the printer.
- By doing this it greatly reduces the mass, allowing much faster motion and less oscillations at the surface of the objects' exteriors.
- It also allows you to use a decent plastic driving motor or more 'space consuming' complex gearing arrangement. This makes sure that you can not only speed up motion, but also output more plastics. So this is especially a good way towards faster printing.
- It can help make printing slightly more reliable. If the nozzle orifice is jammed for just a short while (e.g. too close to a part of the object that curled up a little bit), without the Bowden extruder the drive mechanism will have stripped the filament and not recover because it loses grip when it has stripped it too much. With the Bowden extruder, short blockages are not causing failure of the build. This is useful when you're just starting with printing, because you may have some problems getting large objects to stick to the platform very well.
- Reversing the extruder motor very quickly allows you to have less oozing. The beneficial effect of reversing is more pronounced for the Bowden cable setups than for regular extruders.
- Fast changes in Z-height are helpful if you don't want to get a small seam where it changes layers. So make sure your bot can go fast in that direction too, or optimize it to have zero oozing.
- The Bowden extruder is not as helpful if you have a fixed printing head (or moving in Z-only) and a XY-moving platform. There are still, however, multiple advantages to be gained even in these cases. By physically separating the extruder from the hot end you eliminate potential physical incompatibilities between the two, it becomes much easier to disassemble and fix either component in the event of failure in one part, and you eliminate the problem of the hot end potentially melting the extruder body.