Thermomechanical Properties of Polyjet Voxel-Printed Parts and the Effect of Percolation

The use of deformable materials in 3D printing has allowed for the fabrication of intricate soft robotics prototypes. Polyjet technology, with its ability to print multiple materials in a single print, has been popular in creating such designs. Vero and Agilus, the commercial materials provided by Polyjet, possess shape memory properties, making Polyjet ideal for high-precision and transformable applications. Voxel printing, where users assign materials to voxels, has allowed for the further expansion of design possibilities by tuning the properties of the jetted material. This study aims to investigate how different compositions of uniformly distributed Vero and Agilus voxels affect the thermomechanical properties of the voxel-printed part. In addition, high stiffness Vero droplets surrounded by a soft matrix of Agilus resemble polymer composites, thus calling for the examination of percolation, which is an important phenomenon in polymer composites. The study explores the presence of percolation in voxel-printed mixtures of Vero and Agilus and its impact on mechanical properties. Using dynamic thermomechanical analysis and thermomechanical analysis, the study characterizes the glass transition temperature ( $T_g$ ), maximum allowable strain, and modulus of the voxel-printed material at different compositions. The study found a highly linear relationship between $T_g$ and maximum yield strain with composition, and maximum yield strain occurs at 7°C above $T_g$ . On the other hand, there is a nonlinear relationship between the modulus and composition, which suggested that the percolation phenomenon might have altered the load distribution, therefore causing this inconsistency. So, in this study we used light microscopy, Monte Carlo simulations, and provided mathematical proofs to reveal the percolation threshold in voxel-printed parts, where Vero droplets suddenly form a single network that spreads across the material, altering the load distribution. This study is the first to highlight the percolation phenomenon in Polyjet voxel-printed parts and provides a useful guide for researchers in selecting suitable materials for their specific applications.