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Paste Extrusion

Another system is the gear pump, commonly found as an oil pump. Two locking gears rotate in a sealed housing and the fluid is pumped on the outside of the gears. While this could work fine for certain self lubricating materials (like oil but may be chocolate too?) it’s terrible when trying to dispense a ceramic clay which is filled with abrasive particles that easily get in between the large contact areas between the static and moving parts and grind the pump down. One of the systems that caught my eye during research was the auger pump, an Archimedes screw extruder, often used for dispensing small shots of solder paste in PCB assembly lines. It’s billed as capable of dispensing high viscosity, abrasive, particle filled materials with ultra precise control. Auger pumps works like this: The material is fed from a continuously pressurized external reservoir into the top of the pump. The pump consists of an auger screw fitting perfectly in a cylindrical housing which at the bottom ends in the nozzle. The pressure on the syringe is just enough to feed the material in the pump where it will hit the top of the auger and stops there because of the increased friction caused by the narrower size of the fluid path along the screw thread and ultimately at the even narrower nozzle end. The screw is actuated by a motor and this rotation forces the material down the screw thread, creating a pressure difference and the material flows out of the nozzle. The flow rate is controlled by the RPM of the motor. I designed a first extruder using this principle nicknamed the Claystruder 2, Auger Paste Extruder which was based on a small 25€ disposable plastic auger extruder from an industrial dispensing system. The first tests with clay were fantastic, this was exactly what was needed. It behaved very much like your standard plastic filament extruder. Turning the stepper on resulted (after priming) in an immediate flow of clay

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proportional to the speed. Stopping the stepper resulted in an immediate stop and no material passed the auger form the continuously pressurized syringe. We did some initial test with clay, chocolate and potato mash and those looked all promising. But after some further extensive testing with porcelain clay the system became less and less reliable and odd symptoms started to occur: flow rate became unreliable during a print and adjustment was needed, material leaked past the auger when the motor was not turning. Closer inspection of the auger confirmed a suspicion, that the delrin plastic augers were being worn down by the ceramic material and this happened already after a few minutes. For 125€ one can get a steel version of the auger but even with that the housing degrades quickly.

Claystruder Auger photo by Unfold

REPRAPMAGAZINE

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RepRap Magazine Issue 3  

The third issue of RepRap Magazine. Interview with Nicholas Seward, Paste Extrusion, Hot-Ends, and much more.

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