RJ Paste Extruder
The RJ Paste Extruder is one stable example of the numerous Paste Extrusion-type extruders RepRap user-developers are working on. Paste extruders seem to be all the rage these days. They would allow RepRap to utilise a load of new materials to increase the replication count, and also enable us to use a soluble support material. Zach came up with the Frostruder MK2 , and it seems to be the best way of tackling the problem. Zach's idea was to use high air pressure directly to force the paste out of a syringe rather than to use a piston driven by a motor. The real beauty of Zach's design is a relief valve that is able to exhaust the high pressure air from the syringe. This prevents any paste oozing from the nozzle when extrusion needs to stop, a difficulty thats plagues a lot of motor driven systems.
The main difference between this design and others is in the valve. Patrick came up with an idea for a pressure valve based around some reprapped springs and a cam, to clamp a piece of silicon tubing and cut the air pressure. We control the position of this cam by using a simple reprapped tacho, an optoswitch, with the entire mechanism connected to a DC motor.
Secondly, the frostruder design has the high pressure acting directly on the paste. I initially replicated this, but found that for relatively "thin" pastes, the high pressure air was able to bury directly through the paste and come directly out of the nozzle. I employed the use of an intermediate bung between the paste and the pressure. However, the ooze from the nozzle is substantial due to the increased friction unless the bung and the syringe were coated in silicone grease. Adding the grease results its virtually zero ooze.
In addition to the valve, we hacked together a compressor based on a 2L drinks bottle, and a car tyre inflator. What is particularly nice about this is that the tyre inflator is 12V DC i.e. we can control it directly from the MOSFETS on the Extruder controller. As yet we haven't hooked a pressure sensor into the system, but its something we intend to work on(or rather its something we want to replicate)
The above was my first test with the extruder on the machine, attempting to find an appropriate axis speed for a given pressure. The extruder started and stopped with pretty much no lag or ooze. I was manually starting and stopping the extruder by hand with a second optoswitch (We haven't got the firmware sorted yet), which is the reason for any large blobs at the start or finish. I was using the soluble support paste I described in a previous post, I guess each line shown in the picture above dried in under a minute.
All in all, the entire setup cost about £20, I'll be posting some more results next week after we get the entire thing to be controlled by the host software, which should also bring better quality results.