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Paste Extrusion

Some designers have proposed using something like the Bowden approach (first popularized by Ultimaker) in which filament is pushed down a long PTFE tube. Following this approach, paste or clay would be plunged into a tube and fed to the nozzle on the print head. This allows the extruder to be mounted off to the side, reducing the weight and complexity of the nozzle carriage, but it adds some friction and thus requires more torque. Nevertheless, with such an approach it’s theoretically possible to use syringes even as large as a 310cc caulking gun cartridge. Another drawback of the Syringe Pump design is that it’s not really a good idea to control your extrusion by pushing your entire volume of material from behind. This approach may work fine in a small 10cc syringe, but it becomes unwieldy when we try to scale up to 100cc and beyond. This is akin to pushing your whole roll of filament from the end of the roll instead of feeding it from the extruder. As we scale up to larger syringes the diameter of the plunger gets larger and it becomes much harder to extrude small amounts with precision.For every millimeter of linear plunger travel, a doubling of the plunger diameter results in a four times the amount of extruded material or a quadratic increase, so any imperfections in the drive mechanism are noticeably multiplied. This also increases the start and stop time, and we see a lot of oozing when scaling past 20cc.

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maintain its shape and not collapse under its own weight. This is especially important in printing taller objects. We experienced frequent mechanical failures while testing RichRap=E2=80=99s Paste Extruder with Precious Metal Clay (an ideal material for the small syringe) and our own 60cc design, which is comparable to the Frostruder MK1. Gears came loose, steppers lost steps, belts broke, and syringe retainers failed under pressure. By now you may be wondering if there’s anything positive to say about the Stepper Driven Syringe Pump. Actually, yes! Foremost, a syringe pump is perfectly volumetric, a vital characteristic of any paste extruder design.

It takes substantial force to push paste through a syringe, and the more viscous the paste the more force it requires. With filament you can use more heat to reduce viscosity, but we can’t do this with paste. Paste needs to have sufficient viscosity at room temperature to be stable for a printed object. Since it takes longer to set, it must be able to Fab@Home - Model_1.1 SyringeTool-

photo by Fab@Home 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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