Flow Visualization in the Extrusion of Highly Filled Polymer Composites for Additive Manufacturing

There is an increasing interest in the application of additive manufacturing techniques such as material extrusion to highly filled granular composites in the form of pastes. This category of material is generally understood to include cements, various foodstuffs, and uncured energetic materials such as polymer-bonded explosives and composite propellants. Pastes generally comprise a volume fraction of solid particles in excess of 0.5 dispersed in a liquid carrier. At these volume fractions of solid, particle-particle interactions such as lubricated contacts and jamming are understood to greatly affect the bulk rheology of the material, giving rise to complex properties such as yield stress and pseudoplasticity. As such, accurately modeling the behavior of these materials, which is an important part of the design and optimization of an additive manufacturing process, can be extremely difficult. Experimental investigations of the extrusion of pastes on the laboratory scale have been carried out using bespoke apparatus at the University of Cambridge with the aim of optimizing additive manufacturing processes. As part of this work, a novel approach to visualizing the flow of pastes through ram extruders has been developed using colored tracer formulations of glass spheres in thermally cured silicone resin. Following extrusion experiments, in which material can be extruded at a defined rate while monitoring the overall pressure, the material can be cured to a rubbery solid then sectioned. A custom application can then be used to analyze the sections, producing quantitative information about the material extension and static zones preceding the nozzle entry as well as flow through the nozzle. This paper describes a series of paste extrusion experiments designed to investigate the relationship between the dimensions of the extension zone, the volume fraction of solid in the formulation, and the extrusion pressure. By directly measuring the dimensions of the extension and static zones across a range of conditions and comparing them with measured extrusion pressures, the mechanism associated with steady-state extrusion can be elucidated along with various unstable regimes. This understanding of the extrusion mechanism and how it changes as a function of solid volume fraction will provide critical input for the modeling of additive manufacturing of granular composites via material extrusion, supporting the safe, efficient, and rapid development of the technology.