Influence of the Printing Direction on the Surface Appearance in Multi-material Fused Filament Fabrication

Multi-material fused filament fabrication (FFF) offers the ability to print 3D objects with very diverse surface appearances. However, control of the surface appearance is largely a matter of trial and error unless the employed materials are very similar and very translucent, so we can think of them as blending together. When the multiple materials are fused into one filament in a diamond hotend extruder but do not blend, the resulting surface appearance depends on the printing direction. We explore how this leads to milli-scale colorations as a function of the printing direction. By having preferable printing directions, it is possible to exploit the limited color blending of this nozzle with multiple inlets and one outlet and further enhance particular color effects, such as goniochromatism. We present a framework based on both experimental and computational fluid dynamics analysis for controlling the extrusion process and the coloration of the surface according to preferable printing directions and mixing ratios with the aim of enabling fused filament fabrication of intricate surface appearances.