Removal of Stair-Step Effects in Binder Jetting Additive Manufacturing Using Grayscale and Dithering-Based Droplet Distribution

Abstract

Binder jetting is a layer-based additive manufacturing process for three-dimensional parts in which a print head selectively deposits binder onto a thin layer of powder. After the deposition of the binder, a new layer of powder is applied. This process repeats to create three-dimensional parts. The binder jetting principle can be adapted to many different materials. Its advantages are the high productivity and the high degree of freedom of design without the need for support structures. In this work, the combination of binder jetting and casting is utilized to fabricate metal parts. However, the achieved properties of binder jetting parts limit the potential of this technology, specifically regarding surface quality. The most apparent surface phenomenon is the so-called stair-step effect. It is considered an inherent feature of the process and only treatable by post-processing. This paper presents a method to remove the stair-step effect entirely in a binder jetting process. The result is achieved by controlling the binder saturation of the individual voxel volumes by either over or underfilling them. The saturation is controlled by droplet size variation as well as dithering, creating a controlled migration of the binder between powder particles. This work applies the approach to silica sand particle material with an organic binder for casting molds and cores. The results prove the effectiveness of this approach and outline a field of research not identified previously.

Simulation of Binder Infiltration in Additive Manufacturing of Sand Molds

“…The binder jetting process was realized in a fully automated test setup (Figure 2) by applying the following process steps iteratively until the desired build height was reached: 1) adjustment [22] Copyright 2022, Hartmann et al…”

“…With this approach, a near‐net‐shape geometry without stair steps could be demonstrated. [ 22 ] To the knowledge of the authors, there exists no prior study that focuses on the 3D infiltration of different droplet patterns at a size scale present in industrial sand inkjet printing while implementing random packings that are realistically reproduced from spheres according to their PSD.…”

“…Binder jetting test setup. Reproduced under terms of the CC-BY license [22]. Copyright 2022, Hartmann et al…”

Using adaptive slicing method and variable binder amount algorithm in binder jetting

“…Many studies investigate surface quality for binder jetting (Myers et al , 2019; Miyanaji et al , 2018; Song and Nur, 2004; Huang et al , 2019; Rishmawi et al , 2018; Zhao et al , 2020; Hodder and Chalaturnyk, 2019; Ur Rehman and Sglavo, 2022; Sadeghi Borujeni et al , 2022; Zhao et al , 2022; Marczyk et al , 2022). Hartmann et al (2022) tried removing the staircase effect using the greyscale method. They controlled the saturation by droplet size variation.…”

Technical and financial feasibility assessment of producing an impeller sand mould by three-dimensional printing using a Voxeljet VX 1000 machine

“…A phenomenon related to 3D printing which occurs when layering patterns become visible on the surface of a printed object, creating the visual impression of a staircase-like texture [12]. This effect cannot be eliminated by a certain printing orientation but by binder concentration, where the binder saturation is controlled in the individual voxel volume either by over filling or underfilling them through variations in droplet size and the application of dithering and grey scaling [13]. This setback can also be eliminated by post processing or evaluation of certain printing parameters including the layer thickness, the build direction or scan strategies, ongoing research efforts are focused on uncovering proactive measures or strategies to get rid of the stair-step effect before it even shows in 3D printed objects [14].…”