Friday, March 12, 2010


Bowden Paste Extruder

As you may recall, the paste extruder I’ve been developing is based on the makerbot design which utilises air pressure to drive the paste from the nozzle. The reason for choosing this route was mainly that it allowed the extruder itself to be very compact, and thus make our lives easier when we get around to the head changer. However, as has been pointed out by various people, this does not offer the same control as say, a stepper motor driven extruder.

Last week, Adrian had an awesome idea to get the best of both worlds….A Bowden paste extruder. I had a stab at making one yesterday. Basically, I used Araldite to set some PLA filament into the cavity of a bung, and used the standard approach of routing this filament through some PTFE tube to whatever drive mechanism we choose. Here I am testing it by hand, hopefully I’ll get around to driving it with a stepper motor soon, but it looks promising.


Also inspired by the bowden extruder, I was thinking of using bicycle breaking cables and use steel cable tension instead of compression (that you seem to be using). You could push in a stiff piston by pulling the wire. A screw driven by a threaded rod could produce ample force over a short length.
cool idea,

I think you would need to geardown the stepper to get more torque.

I imagine that paste would require some force.
Yes - we have a geared extruder almost finished based on Wade's design which was based on mine (open source _will_ take over the Universe...).

I'll blog it as soon as it's working. It will drive this device, or be an extruder directly mounted on the Mendel X carriage, or drive a Bowden extruder like Erik's development of Ed's idea.

If you want to see what it looks like the files are here:
Erik, you might want to explore using indexed-shifting cables (that means no Sturmey cables), instead of bowden brake cables. The difference is the shift cable uses a very shallow spiral with many parallel small steel cables, which is much more precise, particularly when bending the cable (changing the bend). The disadvantage is that it can't handle quite the pressure that regular brake cables can (eventually it disintegrates, which the tight spiral doesn't), and of course it's slightly more expensive.
Thanks for the hint jasper!

Bicycle parts are free in low quantities if you have a nice bike repair shop down the road, which is fairly common in the Netherlands where I live.

Forces on the paste are going to be lower than in braking applications, so if gearshifting proves to be functional, great. Otherwise braking cable is still going to be an option.
"don't push on rope"
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