Effects of Time-Dependent Rheological Properties of Cementitious Materials on the Print Quality of Extrusion-Based 3D Printing

Cementitious materials exhibit time-dependent rheological properties. This behaviour is especially important in 3D printing, which has been gaining momentum in recent years.
The increase in apparent viscosity over time results in changes in the viscous stress term in the Navier-Stokes equation which governs the flow of fluids and may drive changes to the morphology of extruded filaments. This potential morphological change to filaments is yet to be fully studied but it is known that any changes can lead to an increase in the porosity of the final printed product, which is one of the main defects in the 3D printing of cementitious materials.
In this study, the time-dependent rheological properties of cementitious materials for 3D printing were characterised, single filaments were printed with a stable flow rate output and their cross-sectional morphologies were prepared and analysed. We found that for the first two hours after mixing, the extrusion pressure increased linearly over time while a stable cross section was attained. However, the filament’s cross-sectional morphology changed significantly from two hours onwards. . This occurred even though the open time was determined to be up to four hours using conventional measures. The experimental results were further corroborated with numerical simulations.
For industrial applications, where screw pumps are widely adopted, the effects of decreasing volumetric flow rate after a threshold time, as reported by Chen et al. [1] and Lee et al. [2], can induce more noticeable morphological changes, even filament thinning. By taking into consideration the pattern in which filament morphology is changing, engineers can design printed structures in a way to reduce porosity and improve the quality of the print.
This study deepens the understanding of the evolution of filament morphology in 3D printing and its dependence on time-dependent rheological properties to further pave the way towards 3D printing with improved quality and repeatability.