Experimental Investigation of Process Induced Effects on Surface Roughness Characteristics of 3D Printed Parts in a Polyjet AM Setup
Authors: Khapper Vishwanath , More Nitin , Mohan Ram V. 
Published: February 2024

Abstract
Polyjet printing is an additive manufacturing technique for making structures from different photopolymer-based material systems. The technique involves a layer-by-layer deposition process for printing parts. Recent advancements in Polyjet technology include the ability to pause and resume print at specific slices, integrate electronics and mechanical parts in the model, use air as a medium for printing to create voids, and a feature to fabricate parts without substrate. When the print without substrate feature is used, the model material droplets are directly dropped onto the build tray without using the support material. In the print without substrate feature, the model material droplets interact with the hard metallic built tray, unlike the current feature: print with a substrate, where the model material droplets interact with the soft support material. The interaction mechanism governs the structure-property relationship, influencing the quality of the fabricated parts. The quality of a printed product depends on various parameters, including dimensional accuracy, surface finish, and mechanical characteristics of parts. Surface finish is one of the quality characteristics which depends on the process-induced effects. The current study investigates the influence of process-induced effects on the surface roughness characteristics of Vero material system in a Polyjet AM Setup. It evaluates the optimum combination of process parameters for better surface roughness characteristics by applying the Grey Taguchi method. The process parameters considered in the study include high-mix and high-quality mode of printing, build type, and print with-without substrate features available in the current Polyjet AM setup. Taguchi’s L4 (23) orthogonal array was used to develop the set of process parameters with their levels. Taguchi’s method is simple to conduct experiments with minimum repetitions. This method allows for optimizing the process parameters and helps establish a correlation among the variables.
KLA Zeta 20 Optical Profiler was used to measure the surface roughness parameters Ra, Rq, and Rt at five different positions on the base of the printed samples along the direction of the print. Here Ra is the arithmetic average of the roughness values, Rq is the root mean square average of the roughness value, and Rt is the distance between the highest peak and lowest valley. Five one-inch blocks were printed for each set of combinations. Eight combinations of process parameters were considered, and 40 samples were printed. The experiments showed that the parts printed with substrate exhibited higher roughness values, whereas parts fabricated without substrate exhibited lower roughness values. The Grey Relational Grade (GRG) was evaluated for each experiment; the GRG shows the input parameters’ effect on the surface roughness characteristics. The GRG gives the optimal combination of input parameters for better surface roughness characteristics. For the current investigation, the best combination of input parameters for better surface quality is for the parts fabricated using the high-quality printing mode with matte build style and without substrate feature.