Inducing inter-filament fusion during embedded 3D printing of silicones

Abstract

Embedded 3D printing (EMB3D) is an additive manufacturing technique that enables fabrication of soft materials including silicones and biological tissues. In EMB3D, a nozzle is embedded in a viscoelastic support bath and writes continuous filaments. Because the bath takes care of form holding, inks that are not printable using conventional extrusion methods can be printed using EMB3D. However, EMB3D can experience defects including positioning errors, poor inter-filament fusion, shrinkage, and rupture. Here, we use in-situ imaging experiments to investigate how defects evolve while disturbing pairs of filaments and writing triplets of filaments. By comparing written triplets to disturbed pairs, we isolate whether defects come from writing of subsequent filaments or from nozzle movement alone. Using silicone-based inks printed into water-based supports with varying concentrations of rheological modifier and surfactant, we probe how the balance between interfacial tension and viscous dissipation governs both inter-filament fusion and capillary instabilities. Critically, higher interfacial tensions and lower support viscosities lead to more rupture, more shrinkage, and more inter-filament fusion. Running the nozzle next to unfused pairs of filaments can cause fusion, but it can also cause rupture. The microstructure of the ink can also influence defects, as phase separation can inhibit both inter-filament fusion and rupture. In addition to material properties, the toolpath also controls defect formation. Fusion quality varies depending on whether filaments are horizontal or vertical in the bath, and whether filaments are stacked on top of each other or next to each other. Moreover, considerable overlap is necessary to achieve inter-filament fusion, leading to over-extrusion. Toolpaths should consider the positioning defects that come from this over-extrusion, particularly vertical displacement of horizontal lines. This work provides key guidelines to achieve prints with better shape fidelity, better resolution, and improved mechanical properties.