Question

How does the shear-thinning behavior of thermoplastics in Fused Deposition Modeling affect the relationship between nozzle diameter and extrusion pressure during high-speed printing?

Accepted Answer

Thermoplastics exhibit shear-thinning, which is a physical property where a material&#x27;s viscosity, or resistance to flow, decreases as the speed at which it is deformed increases. In Fused Deposition Modeling, this means that as you force molten plastic through the nozzle faster during high-speed printing, the fluid becomes thinner and easier to push. The relationship between nozzle diameter and extrusion pressure is governed by the Hagen-Poiseuille equation for Newtonian fluids, but because thermoplastics are non-Newtonian shear-thinning fluids, this relationship is modified. If you decrease the nozzle diameter to achieve finer detail, the flow resistance increases significantly. To maintain the same high-speed flow rate, the extrusion pressure must rise proportionally to the cube or fourth power of the diameter decrease, depending on the specific rheological behavior of the polymer. However, because the material shear-thins, the required pressure increase is less than what would be predicted for a standard liquid. At high speeds, the high shear rates within the narrow nozzle diameter trigger a dramatic reduction in viscosity, which helps mitigate the pressure spikes that would otherwise be required to push the plastic through the small orifice. Therefore, the shear-thinning effect effectively lowers the pressure penalty associated with using smaller nozzles, allowing for high-speed printing that would be physically impossible if the plastic maintained its base viscosity.

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How does the shear-thinning behavior of thermoplastics in Fused Deposition Modeling affect the relationship between nozzle diameter and extrusion pressure during high-speed printing?