CONVEX (CONtinuously Varied EXtrusion): A new scale of design for additive manufacturing

This study introduces a new microscale design approach, CONVEX, based on the idea of CONtinuously Varying EXtrusion widths of deposited filaments in material extrusion additive manufacturing (MEAM). The CONVEX design approach breaks free from the traditional 3D printing workflow of filling a CAD-model volume with stacked extruded filaments of constant width and height. Instead, the geometry of each filament is explicitly designed over its entire length. The concept may disrupt a wide range of applications in both the short and long term. In the least ambitious short-term implementation of CONVEX, its principles can be integrated into 3D printing slicing software to allow a geometric form to be fitted by streamlined filaments, with constantly varying widths as required to match the overall geometry (referred to as “streamlined slicing”). The use of continuous streamlined filaments, as opposed to frequently changing the number of filaments, improved the quality of manufactured parts by eradicating voids and defects, which are known to cause critical stress concentrations in specimens for tensile testing. At the other end of the scale, in the most disruptive implementation of the CONVEX design approach, entire new material structures and product types will be enabled, with feature size-scales perhaps an order of magnitude lower than that permitted by present design rules. This will enable new innovative metamaterials and is particularly appropriate for high-value applications such as advanced filtering, tissue engineering, drug delivery, microfluidics or electronics, where the geometries of individual extruded filaments (or deliberate inter-filament pores) form the functional design geometry. To prove the technical feasibility of CONVEX, this study includes rigorous experimental work to characterise how instantaneous changes to MEAM process parameters (e.g. speed, acceleration, extrusion rate and retraction) enable highly controllable and dynamic variation of the width of extruded filaments (from 75 % to 250 % of nozzle diameter). The concept is proven for multiple materials, layer heights, extrusion temperatures, nozzle sizes, and for both Bowden and direct-drive printer types. The Bowden printer was found to be an order of magnitude less responsive to changes in extrusion rate than the direct-drive printer. Case studies demonstrate the CONVEX design approach, which has already enabled break-through micro-mechanical research for MEAM. Industrial implications are discussed along with the potential for translation to other manufacturing processes