Three binders, polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) are studied to elucidate the principal fundamental features giving rise to an excellent binder in a polymer-stainless steel particle array, used in the context of binder jetting. Using an out-of-printer screening approach, samples were compared based on compression tests, specifically the 0.2 % yield strength as a function of void volume fill factor, 𝜙, leading to a quasi-sigmoidal generalized curve for green body strength development. PVA demonstrated a strong tendency to form inter-particle bridges at low 𝜙 and the highest rise in strength, while PAA samples exhibited a lower tendency to form inter-particle bridges and a weak exponential rise in strength. This paper relates these behaviors to polymer strength, capillary properties and polymer-metal interactions and ultimately provides a rigorous selection process for binder efficacy.

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Advances in binder jet printing (BJP) require the development of new binder–powder systems, for example, to increase compatibility with better performance metal alloys or to increase the strength of parts using stronger binders. The dynamics of binder absorption are principally understood through capillary models. However, validation of these models in BJP has focused on variation of powder properties. Using a design-of-experiments approach and an optical observation method to track absorption of droplets, this study tests the influence of fluid properties on absorption time against the predictions of capillary models. Properties specific to polymeric binders, such as molecular weight and entanglement state, are also considered. Capillary models are found to be generally accurate in predicting absorption time in dilute systems; however, these predictions are not accurate for highly concentrated binder solutions. The effect of polymer entanglement becomes prevalent as the solution concentration increases, which can also potentially occur as a result of increased evaporation due to powder bed heating. Specifically, concentrated solutions close to the onset of entanglement will absorb much more slowly than predicted. Future models of BJP systems must account for the possibility of polymer entanglement throughout the absorption process. Improved models will provide a more accurate understanding of the flow and solidification of the binder in the powder, allowing faster development of new binders for improved performance in printing.

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