Process fundamentals and quality investigation in extrusion 3D printing of shear thinning materials: extrusion process based on Nishihara model

Due to the complexity of extrusion 3D printing objects and lack of universal parameters, there are few classified statistics or quantitative description for the "extrudability" of material with shear thinning properties. In order to systematically study the rheological properties of extrudable materials and establish a unified criterion or quality evaluation standard from the basic process theory, a Nishihara rheological constitutive model based on visco-elastic and visco-plastic characteristics of materials is explored to predict and investigate the extrusion performance of four typical dispersed-continuous phase binary mixture. The creep model result shows that, expect for the gelatin network-water system with almost complete elastic behavior, the material with shear thinning property based on Nishihara model is in good agreement with the experimental results. In the extrusion process, the shear stress is related to the advancing speed, material viscosity, and nozzle size. The effects of advancing speed and nozzle size on shear stress show antagonistic characteristics in a certain range; that is, the velocity gradient is the dominant factor at lower extrusion speed, and the dynamic viscosity is the dominant factor at higher extrusion speed. In terms of extrusion properties, the material system with smaller yield strain/stress has the least obvious extrusion delay characteristics, and is easier to extrude under the condition of the same material strength.