A rapid development workflow for binder inks for additive manufacturing with application to polymer and reactive binder ink formulation

Binder jet additive manufacturing (BJAM) enables processing of metals, ceramics, polymers and composites through the deposition of a binding agent onto a powder bed in a layer-by-layer sequence. Most commonly, a polymer binder is utilized to provide temporary strength to the printed component enabling part removal and handling from the printer; however, it is challenging to manage distortion and shrinkage during sintering. Reactive binders, which create solid interparticle bridges upon decomposition, provide additional benefit beyond temporary green strength and can lead to enhanced densification kinetics, control of warping and shrinkage, and tailoring of local composition. This paper presents a process for the development of custom binder inks which includes exploration of binder ink rheology, wetting and infiltration interactions between the binder ink and powder, ink jetting and the effect of drop spacing on line formation, thermal decomposition of the binder, and evaluation of green strength using indentation. The binder ink development process is applied to the synthesis of custom polymer and reactive (metal salt) binder inks, showcasing the capability to control jetting behavior through rheological modifications and the favorable comparability of wetting, infiltration, and green strength of the reactive binders to the studied polymer binders. The viscosity and surface tension of the binder inks vary by 10 cP and 35 mN/m, respectively, with successful jetting and powder binding achieved by polymer and metal salt binder inks, with green strengths measured between 2 and 18 MPa. Through our presented process, rapid sampling of binders can be performed and through the implementation of reactive binders into BJAM in the future, a powerful tool for control of densification and warping is identified.