to get the material to feed into the extruder, which will take over from there and control the ﬂow rate. Let’s look at a couple of positive displacement pump principles that are often suggested for a paste extruder.
From wikipedia: A positive displacement pump makes a ﬂuid move by trapping a ﬁxed amount and forcing (displacing) that trapped volume into the discharge pipe.
So the key here in the deﬁnition is the ‘trapping a ﬁxed amount’ part. Each rotation of the pump mechanism traps a speciﬁc volume of material and sends it to the exit of the pump, the nozzle. If a stepper motor drives this, we have a system in which we know that X amount of motor steps results in Y amount of extruded volume. You place this pump as close as possible to the nozzle and let it ‘pull in’ the material from a feed line in exactly the same way your ﬁlament extruder pulls in the ﬁlament right before the nozzle.
The peristaltic pump is a type of pump where the pumped medium travels through a tube and never comes into contact with the pump mechanism. The pump consists of a set of rollers that squeeze the tube and push the medium forward. It shows a lot of pulsation in the output at each contact point between roller and tube. This can be solved by having multiple tubes with alternating wheels. Ioan Festeu designed a very elegant, compact peristaltic extruder design with dual channel feed lines. Unfortunately neither Ioan or me managed to get it to work with thicker pastes and literature will tell you that the peristaltic pump is only suitable for low viscosity ﬂuids and there are several other issues with it too.
The only difference is that we don’t have a nice roll of ﬁlament but a messy blob of paste in a reservoir. So there is the need for an external reservoir. In case of a runny ﬂuid like a molten chocolate one could use a gravity feed system (the material ﬂows in the pump by gravity when the reservoir is above the extruder) but more often than not we will need to pressurize the reservoir. The easiest way is by air pressure. Yes, that uncontrollable thing we rejected above. In this case we apply more than enough pressure Ioan Festeu Peristaltic Paste Extruder 2 photo by Ioan Festeu
The third issue of RepRap Magazine. Interview with Nicholas Seward, Paste Extrusion, Hot-Ends, and much more.