Binder jetting is an additive manufacturing process in which a part is fabricated layer-by-layer using inkjet technology to selectively dispense binder into powder layers in a designated area. The approach gives this process significant advantages over other additive manufacturing processes such as lower cost, capability to print in a wide range of materials, and little to no heat applied. Although binder jetting has many advantages and has been successful implemented in various industries its overall rate of adoption is slow compared to other processes. This is largely due to poor mechanical properties and consistency in printing which stems from a poor understanding of the interaction between the binder droplets and the powder bed. This is evident as print parameters for new machines and new materials are primarily determined by trial and error. The purpose of this thesis is to report the impact of various inkjet print parameters and humidity on the printing process in binder jetting. The binder/powder interaction is complex and highly dynamic where picoliter-sized droplets impact the powder bed at velocities of 1-10 m/s. Current methods of predicting this interaction assume that it is based only on binder and powder properties. This work studies the impact of inkjet printing parameters that are often overlooked with these assumptions. The impact of droplet velocity, droplet spacing, and droplet inter-arrival time was evaluated based on single line formation and effective saturation levels when printed into various powder material and sizes. Higher droplet velocities were found to decrease effective saturation with larger droplets (92-212 pl). However, droplet velocity had a negligible impact on saturation when printing with smaller droplets from 30 m orifice (29-65 pl). Line formation was dependent on both droplet inter-arrival time and droplet spacing. Max droplet spacing correlated to the square root of inter-arrival time. These results can guide selection of printing parameters that maximize build rates and reduce defects in printed parts. As the binder/powder interaction is difficult to observe and often line formation has been used as a method of observation. However, no report relating line formation to full layer parts exists. Optimal parameters determined in line printing are used for full feature parts. In addition, the impact of ambient humidity on the printing process is studied. The direct use of parameters optimized for line printing in printing a part was shown to be ineffective. When droplet spacing, line spacing, and layer thicknesses are comparable, single and multiple layers can be formed. Over short exposure periods of powder to ambient humidity produces negligible difference however, extended exposure periods significantly reduce the saturation and increase part size. Surface roughness is identified as a possible source of printing defects. Surface roughness increases significantly when printing the first layer but decreases with successive layers. This demonstrates a strong interaction between layers. The surface roughness and effective saturation was insensitive to line and droplet spacing below 60 m. Steam powder conditioning reduces sensitivity of both surface roughness and saturation to printing parameters but causes bleeding beyond the part boundaries. Further research should include improved methods of predicting ideal printing parameters and connecting it based on geometry and parts size. Further research is needed to confirm impact of surface roughness on defects in binder jetting parts. Development of methods to control spread of binder in premoistened powder to take advantage of its potential.
| Identifer | oai:union.ndltd.org:BGMYU2/oai:scholarsarchive.byu.edu:etd-10362 |
| Date | 13 December 2021 |
| Creators | Colton, Trenton Miles |
| Publisher | BYU ScholarsArchive |
| Source Sets | Brigham Young University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | https://lib.byu.edu/about/copyright/ |
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