Abstract

Binder jetting is an additive manufacturing process that layer by layer builds a 3D model by selectively binding regions of powder using binder deposited though an inkjet printhead. The process offers several advantages over other additive manufacturing processes including fast build rates, vast material selection, decreased cost, and part resolution. The main disadvantage of binder jetting is poor mechanical properties, stemming from a poor understanding of the process physics. Porosity in final parts is not uncommon, but there is little understanding of where the porosity originates. The purpose of this thesis is to report the investigation of increased powder bed cohesion and its effects on part formation, part properties, and printing parameters in binder jetting. The interaction between binder and powder is complex. Binder exiting the printhead impacts the powder bed at speeds up to 10 m/s. The kinetic energy carried by the droplet disperses into the powder bed on impact, causing some powder particles to eject from the bed and other particles to rearrange within the bed. The particle ejection and rearrangement is theorized to be the physical cause of porous regions in binder jetted parts. This work uses a method called pre-wetting to introduce small amounts of moisture into the powder bed to effectively increase the cohesive forces between powder particles. Increased cohesion makes particle ejection and rearrangement during the powder/binder interaction more difficult. A method of accomplishing pre-wetting was developed and achieved successful moisture delivery using water and a water/tri-ethylene glycol mixture. Printed lines were used to characterize moisture content and study its effects on line formation and saturation levels. Low levels of moisture were shown to perform the best. Particle ejection and rearrangement was shown to decrease with moisture addition. Pre-wetting was shown to eliminate the defect known as balling, increasing the parameters known to successfully print lines. Water was identified as a poor substance for pre-wetting due to rapid evaporation, but tri-ethylene glycol/water solutions succeeded in proper moisture delivery. Saturation levels in lines decrease with added moisture and part dimensions increase. high-speed x-ray imaging verified pre-wetting reduction in particle ejection and rearrangement as well as supply some preliminary understanding of void formation during the printing process. The first few layers of the binder jetting process have been shown to increase in surface roughness values when compared to the undisturbed powder bed. This is likely due to a balling-like effect seen in layers. The effects of pre-wetting on layer and multi-layer formation were studied. Pre-wetting reduced the surface roughness levels in printed layers to the levels near the levels seen in undisturbed powder beds. In contrast, saturation levels in layers and multi-layers increased in value above those found in parts printed into dry powder, giving indication that porous regions within bound parts are being eliminated. Layer and multi-layer parts showed increased part dimensions with the addition of moisture. Overall, pre-wetting was shown to greatly reduce the effects of the binder/powder interaction and results strongly suggest that pre-wetting mitigates defect creation during the printing process. Further research should include testing of thicker multi-layer parts to study how saturation trends continue with increased layer numbers. In-process drying should be used in conjunction with pre-wetting in multi-layer parts to determine its effects on saturation levels and part dimensions. Post processing should be done to partially sinter, or infiltrate multi-layer parts created with and without pre-wetting to analyze porosity.

Degree

MS

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

https://lib.byu.edu/about/copyright/

Date Submitted

2022-06-09

Document Type

Thesis

Handle

http://hdl.lib.byu.edu/1877/etd12830

Keywords

binder jetting, additive manufacturing, surface roughness, pre-wetting, saturation

Language

english

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