Material extrusion additive manufacturing of high particle loaded suspensions: a review of materials, processes and challenges

Material extrusion additive manufacturing is of increased interest in producing materials with very high loadings of particles, specifically through the use of the direct ink write (DIW), or robocasting, technique and the use of highly loaded particle suspensions (HLS). Applications from biomedical composites to solid rocket propellants to powder metallurgy green bodies would benefit from the complex parts enabled by additive manufacturing but require very high particle contents during processing. This leads to very high viscosity fluids and challenges in flowing and curing the inks. In this comprehensive review, we examine the main components of designing an ink formulation and a DIW process: the ink rheology, the print mechanics and the solidification/post-processing. Our expanded discussion of these elements includes an introduction to the basics as well as the latest research in the field, so serves to both introduce a new practitioner and generate new ideas for those already working in the area. We finish with a discussion of two important applications and a perspective on the future directions of DIW for highly loaded particle materials.