Thoughts on an Interesting In-Situ Monitoring Research Paper

A few days ago, I had the opportunity to review a paper on in-situ monitoring for LPBF manufacturing. The paper – expected publication in September 2021 – was authored by eleven contributors from Dublin City University, University College Dublin, and others – and was co-funded under the European Regional Development Fund, I-Form industry partners as well as supported by Science Foundation Ireland (SFI).  

This paper has 153 references… needless to say, it is comprehensive and covers subjects such as: 

• Potential physical defects caused by the powder bed fusion process.
• Different methods of In-situ process sensing including: acoustic, optical, thermal, tomography, and commercially available in-situ sensing systems.  
• Real-time process control, including requirements and machine learning-based process control. 

From my perspective, there are two items of particular interest in the paper: One is the specific mention of Sigma Labs Inc. (NASDAQ: SLGB). The second and more important was a detailed technical discussion of the emissivity effect on the measurement of temperature.

First, the Sigma Labs mention. In Section 3.5 Commercial Systems with In-situ Sensing, after discussing monitoring systems available from machine OEM’s, the authors write:

“Aside from machine OEM’s, companies such as Sigma Labs supply in- situ process monitoring systems that can be retrofitted to L-PBF machines. Sigma Labs technology includes both on-axis and off-axis sensors to monitor the build process. Proprietary quality metrics can provide the user with an indication of the quality of the build.”

Beyond the above statement, the paper references a Sigma Labs whitepaper entitled “PrintRite3D Alerts for Anomaly Detection”, written in 2019 by L. Jacquemetton, S. Betts, and D. Beckett. The paper also lists Sigma Labs modules in the examples of commercially available in-situ monitoring systems in Table 2 of the same section:

What is particularly noteworthy here is that Sigma Labs modules are the only non-OEM in-situ monitoring systems. Perhaps this is one of the reasons Sigma Labs recently won the prestigious AM Innovation Award for Measurement and Analysis at the 2021 ASME AM Tech Forum. 

The second item I found of particular interest in the paper is the detailed technical discussion of the emissivity effect on the measurement of temperature; found in Section 3.4.1 Absolute vs. Radiant Temperature.  The authors noted the variability of emissivity from several factors cause the radiant temperature to vary unpredictably over hundreds of degrees. Furthermore, emissivity must be comprehended for accurate temperature measurement.  They state:

 “This highlights that radiative and real temperature measurements can vary significantly but through diligent modelling or measurement of the emissivity, and knowledge of the process, and calibration of sensors, can ultimately be corrected for.”

They correctly note some of the emissivity issues with the accurate measurement of temperature, but what they fail to elaborate on is the Bichromatic Planck Pyrometry technology employed by Sigma Labs. When considering the choice of wavelengths for intensity measurements and traceable calibration, Bichromatic Planck Pyrometry makes emissivity far less impactful to accurate measurements of temperature.  

When dealing with LPBF, no one can accurately measure temperature without dealing with emissivity, which is highly variable and context-specific, especially when utilizing non-contact measurement platforms.  Sigma Labs technology (PrintRite3d®) along with our expert process knowledge deals with emissivity to provide explicit and actionable data to meet your regulatory and quality requirements.

I hope you find my thoughts helpful. For more information, feel free to reach out to us at info@sigmalabsinc.com. Also, explore the many written and video assets about additive manufacturing quality assurance at the Resources Center.