Update: RichRap has finally come up with a workable solution to this, albeit with little volume, using a belt and a common-sized syringe, see http://www.thingiverse.com/thing:20733
Because not everyone has an air compressor lying around, or wants to deal with the issues that stem from trying to control a non-newtonian fluid/suspension/colloid using force, especially with air compressibility effects, http://www.thingiverse.com/thing:1143
The current design is intended to make use of the oft-discarded bit when recycling glass bottles, such as when using this tool: http://www.thingiverse.com/thing:9325
I thought recycled wine bottle-necks would be a good idea instead of some proprietary syringe, because 1. recycling is awesome, 2. it means you can redesign your own nozzle, and 3. the glass is typically low-friction.
I'll give a full part list when the design is somewhat finalised, but for now, the bolts you can see in the picture are M4x40mm, 30mm and 20mm, and bits of M8 smooth bar and studding, plus M3 bolts will be needed to mount a stepper motor.
I have included an executable eDrawings document of the design here so that you can get a better look at it.
It's sort of a mishmash between conceptual design and an actual usable alpha-stage design at the moment. I know it can be done, so please help me iron out the details.
YES I KNOW THE MOTOR IS UPSIDE DOWN. I'm thinking about it. Mounting it from underneath so that you use the 'correct' end of the shaft would mean you need to either raise the mounting surface for the motor, thereby making horrible overhangs in the structure, or lower the nozzle to prevent the bottom of the motor from dragging through your print.
For the latter option I have provided an design for a cylinder clamp that could hopefully bolt underneath a Prusa X-carriage, between the smooth bars.
As it stands now, the height of the motor somewhat restricts the distance that the plunger can travel.
I'm also thinking of putting a groove in the end of the plunger to fit a rubber O-ring to give a better seal. The Inner Diameter I measured on a wine bottle-neck was ~20mm. If anyone has digital calipers, perhaps they could get some better measurements on some very widely available glass bottles.
I have tried printing the nozzle as a test already, and the thread wasn't quite right and got chewed up a bit. Once I get that right I'll release the STL along with a version that's just just the threaded section, so you can mash it up with whatever **** or useful stuff you want to put on top of your used wine bottles. :)
I'm guessing just now that there could be better ways of clamping a slippery glass cylinder, such as with fabric or tape, and that trying to drag the plunger down from one side isn't going to work very well, so better ideas on a postcard please! Perhaps belts and pulleys...