Study of printing parameters and optimization of nozzle designs for cement paste 3D printing (CP3DP)

Abstract

In general, the mechanical performance of cement paste manufactured by 3D printing technology largely depends on the strength over the interface between two successive layers. If the interlayer strength is low, cracks tend to emerge and penetrate quickly through the interface. It is thus necessary to produce higher interlayer strength, such that the printed layers perform homogeneously. However, in the early stage of cement paste 3D printing (CP3DP), interlayer defects commonly exist, and thus the interlayer strength is usually low. Therefore, it is crucial to figure out the parameters that influence the interlayer strength significantly, and thus to design and optimize novel nozzles that achieve higher interlayer strength. In this thesis, the first achievement is made for a deep understanding of the influence of printing parameters on the interlayer strength through experiment and theoretical simulation. The experimental work is conducted with an emphasis on the effects of three printing parameters, namely the nozzle design, nozzle height, and printing speed. In particular, the influences of the interlayer notch and the shear stress are investigated respectively on the interlayer strength, in order to further understand the fundamental mechanism of the interlayer strength of 3D printed cement paste. It is observed experimentally that the cement paste printed by the circular nozzle generally achieves higher interlayer strength, compared with that printed by the rectangular nozzle. A smaller nozzle distance may impair the interlayer strength if the interlayer notch is not treated properly. Besides, a higher printing speed marginally increases the interlayer strength, showing the promise of higher productivity. Theoretically, the experimental phenomena are explained innovatively by the interlayer notch coupled with interfacial shear stress, through computational-fluid-dynamics (CFD) and finite-element (FE) simulations, such that eliminating the interlayer notch and increasing the interlayer shear stress become crucial in the CP3DP process. The second achievement is made for the design and optimization of novel nozzles with various outlet shapes and other important components via experiment and theoretical simulation. It is found that larger outlet area results in smaller notch depth, and the circular outlet with a proper diameter generates average interlayer strength of 2.24 MPa, higher than those by other outlet shapes in SC3DP. Meanwhile, the interface shaper and the side trowel as important nozzle components increase the interlayer strength of 3D printed cement paste up to 2.85 MPa and 3.88 MPa respectively, by enhancing the interfacial adhesion and reducing the interlayer notch.