Powder bed non-uniformity investigations via discrete element modelling for binder jetting technology

In the binder jetting (BJ) process, as in most of powder bed additive manufacturing technologies, the powder is periodically recoated onto the substrate layer-by-layer. The elements of the current deposited layer corresponding to the part being manufactured are bonded together using a polymeric binder. In all cases that require a thermal process for sintering, the internal structure of the finished part is defined by the internal structure of the powder bed. This article focuses on the discrete element modelling (DEM) of various powder spreading methods during recoating and their impact on the powder bed structure particularly applied to binder jetting technology. The article demonstrates that despite the thinness of the deposited layers, they typically exhibit porosity and particle and pore size non-uniformities along the build-up direction. These irregularities contribute to the anisotropic sintering shrinkage observed in green BJ bodies during experiments. However, the experiments presented confirmed by modelling show that without binder deposition, the powder bed – except for a narrow surface layer – remains relatively uniform, regardless of the recoating method used. It is the binder injection into the porous structure of the powder bed that disrupts this homogeneity, locks in large surface pores, and exacerbates the effects of powder segregation during spreading. Finally, several strategies, explored via simulation, are proposed to reduce porosity variations during BJ: using a combined roller-wide blade method for powder spreading and a two-hopper approach, where each layer consists of small particles deposited over larger ones.