Search for: All records

PurposePowder bed density is a key parameter in powder bed additive manufacturing (AM) processes but is not easily monitored. This research evaluates the possibility of non-invasively estimating the density of an AM powder bed via its thermal properties measured using flash thermography (FT). Design/methodology/approachThe thermal diffusivity and conductivity of the samples were found by fitting an analytical model to the measured surface temperature after flash of the powder on a polymer substrate, enabling the estimation of the powder bed density. FindingsFT estimated powder bed was within 8% of weight-based density measurements and the inferred thermal properties are consistent with literature findings. However, multiple flashes were necessary to ensure precise measurements due to noise in the experimental data and the similarity of thermal properties between the powder and substrate. Originality/valueThis paper emphasizes the capability of Flash Thermography (FT) for non-contact measurement of SS 316 L powder bed density, offering a pathway to in-situ monitoring for powder bed AM methods including binder jetting (BJ) and powder bed fusion. Despite the limitations of the current approach, the density knowledge and thermal properties measurements have the potential to enhance process development and thermal modeling powder bed AM processes, aiding in understanding the powder packing and thermal behavior. 

Binder Jet (BJ) additive manufacturing creates parts by binding powder particles together with inkjet-printed droplets. BJ shows promise as an industrial process, but poor final part properties often limit applications. Prior work has shown that there is significant powder rearrangement from the kinetic impact of binder droplets that may contribute to the formation of defects in the final parts. This study builds upon previous research by studying the effects of print parameters, including droplet spacing and inter-arrival time, and droplet parameters, including droplet volume, velocity, and satellite formation, on the formation of lines. A new method, using an adhesive film, for extracting single-layer parts is described which allows for study of smaller, more sensitive primitives. The results show that pre-wetting the powder bed expands the feasible design space and allows printing with larger droplet spacings, smaller inter-arrival times, and slower droplet velocities. This enables up to 50 % faster print rates and the potential for reduced powder relocation due to droplet impact. Results from this work can be used to inform the selection of optimal process parameters and the design of new BJ systems to produce higher quality parts. 

Binder jetting (BJ) is an additive manufacturing process that uses a powder feedstock in a layer wise process to print parts by selectively depositing a liquid binder into the powder bed using inkjet technology. This study presents findings from high-speed synchrotron imaging of binder droplet-interaction during the BJ printing process. A custom laboratory-scale BJ test platform was used for testing which enabled control of relevant process parameters including powder material, print geometry, spacing between droplets, powder bed density, and powder moisture content. Powder ejection was observed above the powder bed surface and powder relocation due to droplet impact was observed below the powder bed surface. Powder relocation was observed to be sensitive to powder material, powder bed density, powder bed moisture, droplet spacing, and print geometry. Increasing powder bed density was found to increase particle ejection velocity but reduce the total number of particles ejected. Process parameters that increase binder / moisture content in the powder bed were found to reduce powder ejection. The number of ejected powder particles was reduced for lower droplet spacings. Both powder ejection and powder relocation below the powder bed were reduced by treating the surface of the powder bed with a water/triethylene glycol (TEG) mixture before printing. Results from this study help to build understanding of the physical mechanisms in the BJ printing process that may contribute to formation of defects observed in final parts. 

Although commercial binder jetting (BJ) printers are available, they typically do not allow sufficient control over process parameters needed to study fundamental process characteristics. This work presents an overview of the design and construction of a custom BJ system used to observe fundamental phenomena in the BJ process. CAD models for the design and information on the software of this system is also given. This system will help elucidate the mechanisms that introduce part defects and other challenges unique to the BJ process. The BJ system was designed for both laboratory-scale experiments with a 100 x 100 mm build box and high-speed synchrotron X-ray imaging with a 500 μm thick powder bed, requiring high-accuracy motion stages and a controller with precise timing. The printer includes functionality for depositing and rolling powder, printing multi-layer parts, and direct observation of the jetting nozzle. This BJ system has enabled experiments that provide insight into the printing process that will aid future efforts to mitigate challenges associated with BJ. 

The Binder Jetting (BJ) process is capable of producing parts at high speeds from a variety of materials, but performance is limited by defects in the final parts. An improved understanding of fundamental phenomena in the printing process is needed to understand the source of these defects. This work presents initial findings from high-speed imaging of the BJ process using synchrotron X-rays. High-speed X-ray imaging allows for direct observation of key physical mechanisms in the printing process that may introduce defects including binder droplet impact on the powder bed, powder rearrangement below and above the powder bed surface, and balling formation. Testing was performed with multiple materials and droplet spacings to compare the effect on observed phenomena. Multiple lines were printed on packed and loose powder beds to further explore factors that affect defect formation and to better simulate industrially relevant conditions. 

Binder Jetting (BJ) has increased in popularity and capability since its development at MIT as it offers advantages such as fast build rates, integrated overhang support, low-power requirements, and versatility in materials. However, defects arise during layer spreading and printing that are difficult to remove during post-processing. Many of these defects are caused by particle rearrangement/ejection during binder deposition. This study explores methods of reducing particle rearrangement and ejection by applying small amounts of moisture to increase the cohesive forces between powder particles. A moisture application system was built using a piezo-electric disk to atomize water to apply a desired liquid to the BJ powder bed without disruption. The moisture is applied after spreading a new layer. Lines of binder were printed using varying droplet spacings and moisture levels. Results show that the moisture delivery system applied moisture levels across the entire application area with a standard deviation under 23%. The moisture levels delivered also had a single position test-to-test uniformity standard deviation under 21%. All tested levels of moisture addition showed mitigation of the balling defects observed in lines printed using dry powder under the same parameters. Moisture addition decreased effective saturation and increased line dimensions (height and width), but lines printed using the smallest amount of moisture tested, showed similar saturation levels and line widths to lines printed in dry powder while still partially mitigating balling. 

Binder Jetting (BJ) is a low-cost Additive Manufacturing (AM) process that uses inkjet technology to selectively bind particles in a powder bed. BJ relies on the ability to control, not only the placement of binder on the surface but also its imbibition into the powder bed. This is a complex process in which picoliter-sized droplets impact powder beds at velocities of 1–10 m/s. However, the effects of printing parameters such as droplet velocity, size, spacing, and inter-arrival time on saturation level (fraction of pore space filled with binder) and line formation (merging of droplets to form a line) are unknown. Prior attempts to predict saturation levels with simple measurements of droplet primitives and capillary pressure assume that droplet/powder interactions are dominated by static equilibrium and neglect the impact of printing parameters. This study analyzes the influence of these parameters on the effective saturation level and conditions for line formation when printing single lines into powder beds of varied materials (316 stainless steel, 420 stainless steel, and alumina) and varied particle size (d50=10–47 µm). Results show that increasing droplet velocity or droplet spacing decreases effective saturation while droplet spacing, velocity, and inter-arrival time affect line formation. At constant printing velocity, the conditions for successful line printing are shown to be a function of droplet spacing and square root of the droplet inter-arrival time analogous to the Washburn model for infiltration into a porous media. The results have implications to maximizing build rates and improving quality of small features in BJ.