A valve on the feedline would switch between extrude and refill. With some cleverness and two alternating pistons it could even print continuously. Such an approach could retain the straightforward volumetric principle while keeping the size of the print-head small and the control precision high.
2) Direct Air Pressure Extruder The Direct Air Pressure Extruder (let’s call it DAPE) is almost the exact opposite of the Stepper Driven Syringe Pump. Instead of using a mechanical method to push down a plunger DAPE applies air pressure directly to the material. This extruder design is radically simpler and lighter than the method previously described. The top of the syringe is sealed with a pressure cap which connects via a narrow tube to an air compressor. Air flow is then controlled using a solenoid valve (a fluid or gas valve controlled by an electromagnet) to start and stop the pressure. Using this method the extrusion can be started and stopped almost instantaneously. While the extruder design is light and simple, it does require a supply of compressed air. Most homes unfortunately are not equipped with central air pressure, so an air compressor needs to sit next to the machine. This approach was first proposed by Zach Hoeken for Makerbot Frostruder MK2 and in all fairness (and to stick with our digging into RepRap history for traces of paste): Adrian Bowyer designed a very crude paste extruder that uses a balloon filled with material in a PET bottle pressurized with a bicycle pump! You can find this rather amusing 2007 Support Extruder 1.0 on the RepRap wiki. So that’s the major benefit, a simple and lean extruder. At Unfold we still use DAPEs all the time because of their relatively straightforward simplicity. We designed a couple of iterations of the extruder for the Bits from
RepRap Support Extruder 1.0 photo by Adrian Bowyer
Bytes “Rapman” which became very popular with paste extrusion experimenters because its print-bed is limited to Z movement. You can find these designs on Thingiverse; just search for “Claystruder.” Mechanically this design is also far easier to scale up. Just extend the syringe or go for a larger syringe and cap.