The ability to accurately assess the structural integrity of 3D printed parts is a critical step in advancing additive manufacturing (AM). Oak Ridge National Laboratory has identified fault detection and overall part quality as one of the fundamental challenges that is currently limiting the use and applications of polymer-based additive manufacturing. As a result, quality assurance and inspection methodologies for additive manufacturing are being pursued relentlessly in both academia and industry.
|The image shows a vial of gold nanoparticles in solution on the left. In the background on the right are the shredded plastic-containing gold nanoparticles (maroon in color) and pure PLA (white) used to extrude the filament for 3D printing. The gold nanoparticle-functionalized filament is in the foreground. Researchers used the filament to 3D print the Vanderbilt logos. In the background is the “filabot” extrusion system used to manufacture the filament itself. (Image source: Vanderbilt University)|
In particular, research is being conducted on multiple fronts to establish and outline metrics for AM processes and the resulting parts and materials that are created. One of the first steps in this process is ensuring that 3D-printed parts are manufactured exactly as they were designed and ensuring that there are no material defects in the final product that could lead to failure. By establishing quality assurance and control metrics throughout the manufacturing process, researchers can speed the adoption of AM across multiple industries and even enable new applications.
Researchers at Vanderbilt University have developed a non-destructive testing method for 3D printed parts that uses gold nanoparticles to visually identify defects, such as missing print layers that occur during the manufacturing process. These defects might be caused by a clogged print nozzle, poor material extrusion, or other factors. If the flaws remain undetected, they can reduce the overall strength and performance of as-printed parts and materials and result in premature material failure. What’s revolutionary about the new process is that the means of uncovering flaws is essentially embedded within the material itself.
Here’s how it works: Researchers create functionalized, thermoplastic-based filament with the gold nanoparticle already added through a process of dissolving, mixing, drying, and shredding. The gold nanoparticle-based filament, which is compatible with commercially available 3D printing systems, is then extruded and dried and pressed into gold nanoparticle-filled polymer filaments, or thin tubing, that can be used in standard 3D printers. Once the material is used to 3D-print a part or structure, the embedded gold nanoparticles provide a signature—they show up as a deep maroon color under a UV-Vis spectrophotometer—that can be used to detect defects based on changes in optical properties and color alone, according to Cole Brubaker, civil engineering graduate student and lead author of the study.
“Using a single scan that takes a matter of seconds, embedded gold nanoparticles are able to provide an indication of material state in real time,” Brubaker told Design News. “This inspection methodology and approach helps minimize the reliance on large-scale sensor networks to effectively monitor a material. One of the most relevant uses for this technology is in quality control assurance, ensuring that parts are manufactured exactly as they were designed, and with no defects present in the final structure.”
Relying on an Extension of Beer’s Law
Under normal circumstances, visual inspection of a part generally cannot detect missing print layers. But print materials with the gold nanoparticles offer a visual signal, according to Brubaker, who says that the process is an extension of Beer’s Law.
“Beer’s Law states that, for any nanoparticle solution, there is a linear relationship between absorbance intensity (think color intensity) and concentration of nanoparticles for a given path length,” he said. “Thus, when you increase the concentration of nanoparticles, absorbance intensity also increases. We are using a similar idea for 3D-printed parts. In our case, though, instead of changing the concentration of nanoparticles, we monitor changes in ‘path length’ or overall material thickness.”
As the concentration of gold nanoparticles is constant for the filament used to manufacture the part, researchers can obtain a direct relationship between absorbance intensity and overall material thickness. As more print layers are added to the part and material thickness increases, a linear relationship with absorbance is achieved. Vanderbilt researchers are using this relationship between the absorbance/optical properties of the embedded nanoparticles and physical dimensions of the 3D-printed part to detect and monitor for the presence of defects.
The flexibility of the new process to tailor-design the material response is what gives the research so much potential. The new method has been shown to detect defects as small as 0.2mm in thickness, approaching the size of a single missing print layer. The researchers’ goal is to detect defects and discontinuities along individual print beads to provide a full image of 3D-printed parts for quality control and assurance.
The Number of Potential Applications Is Immense
“By looking at a range of material parameters and processing considerations, we have, in a sense, developed a protocol to be able to design an array of smart material systems and functionalized composites compatible with additive manufacturing technologies,” Brubaker told Design News. “Continuing upon our recent progress and findings, we are very interested in and excited about developing materials responsive to a variety of external stimuli, including mechanical, chemical, and thermal insults, through changes in optical properties alone.”
The technology is also commercially feasible and scalable to industrial applications as other nanoparticle-based technologies, such as the use of quantum dots in consumer electronics, have shown.
“The possibility of materials design for additive manufacturing will only continue to grow as additional fields and industrial processes begin to adopt the technology for larger scale production of functional, end-use products,” said Brubaker.
The research paper, “Nondestructive Evaluation and Detection of Defects in 3D Printed Materials Using the Optical Properties of Gold Nanoparticles,” appears in the American Chemical Society’s journal Applied Nano Materials.
Tracey Schelmetic graduated from Fairfield University in Fairfield, Conn. and began her long career as a technology and science writer and editor at Appleton & Lange. Later, as the editorial director of telecom trade journal Customer Interaction Solutions (today Customer magazine), she became a well-recognized voice in the contact center industry. Today, she is a freelance writer specializing in manufacturing and technology, telecommunications, and enterprise software.
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