Topography-driven fluid distribution for improved strength in binder jetting

Binder jetting (BJT) is an industrially highly relevant additive manufacturing process to create molds and cores for casting applications using sand and a binder liquid. During the layer-wise build-up of volume, a print head deposits a pattern of constant volume binder droplets onto the sand bed. This connects the particles via capillary bridges at low fluid saturation and bonds successive layers. Interlayer connections are widely reported to be weaker than those inside layer planes. In this study, the aim is to improve the layer bonding by placing more binder in deeper locations, which are accessible through the gaps between the grains of sand. To realize this, the sand topography of each recoated layer is captured by a camera, and a manipulated printing bitmap using three different binder volume levels is then fed back into the printer. The new bitmap redistributes the binder toward the observed gaps in the sand bed to increase the penetration depth. For the first time in BJT, closed-loop control is realized, which uses the recoated particle topography. An increase in flexural strength of up to 16% can be reached at the same total binder content. This enables thicker layers for increased productivity and points toward a new approach for BJT processes in the low-saturation regime.