British ceramicist Jonathan Keep, for example, has shown that a DAPE with a 310cc caulking gun syringe can be installed directly on a DeltaBot XYZ carriage.
static speed of your Stepper Driven Extruder which regardless of the type or viscosity of paste gives a predictable and continuous flow rate.
As you might expect, DAPEs tend to perform very poorly in the controllability and predictability of the extrusion process, and in their compatibility with RepRap electronics and slicing software. The first problem is that there are so many variables involved in getting a repeatable and predictable flow-rate. The most obvious variables are air pressure and material viscosity, which combine to produce a certain flow rate. Then you must consider plunger friction, the changing material level in the syringe, etc.
Many ideas are being considered to address this chaotic system. Firmware could be updated to work with the parameters of air pressure systems and compensate for some known effects. But this may require replacing syringes with each successive print because a plunger acts differently in a used syringe, and generally the consistency will never be exactly the same on each print and throughout the entire process. Today we can control for these variables with an industrial setup, but not in RepRap style.
Flow rate = 3D Material Viscosity + Air Pressure.
A possible fix is to create a closed control loop with an electronically-regulated pressure valve monitoring flow at the nozzle and adjusting pressure to achieve the desired flow-rate. The components required to do this aren’t the cheapest parts available. Alternatively it could adjust the print speed dynamically based on the flow rate within a certain “workable” range, not too fast/slow.
So if your material’s viscosity changes only slightly you need to compensate for that by raising or lowering pressure to keep a steady flow. A static input of air pressure gives a wide range of flow rates depending on the material. Compare that to the photo by Unfold