Micro-CT Examination of Additively Manufactured Parts

Micro-CT has become widely used to analyze and test additively manufactured parts, particularly for porosity analysis and dimensional measurement. In conventional manufacturing, a weld would always be inspected for voids and inclusions, but in additive manufacturing the entire sample is essentially one large weld. Therefore, to qualify and verify safety-critical aerospace, automotive, and medical components that are additively manufactured, it is essential to know whether voids or inclusions are present, how large they are, where they are, and whether the dimensions of the part are accurate and conform to the design. By providing a full 3D visualization, micro-CT gives the information that is needed to ensure evaluation of each whole part.

With the rapid expansion of additive manufacturing, micro-CT is finding new use in non-destructively examining printed parts and assessing printer performance and function. As part of any manufacturing process, quality assurance measures should be implemented to ensure the final products meet the desired specifications. In the case of 3D printers, especially inexpensive hobby level printers, significant upfront investments of time is generally required to fine tune the assembly and function of the printer to produce accurate products. 

Micro-CT Scan of a 3D Printed Calibration Test Fixture

We designed and 3D printed a small test fixture to assess the performance of a small, hobby scale printer. As can be seen in Figure 1 above, the fixture contained holes, draft angles, raised features, and symmetric features like circles and arcs. All of these features are designed to assess the performance of the 3D printer and its ability to accurately produce files designed in the source computer-aided design (CAD) files.

Figure 2: Rendering of the CAD file used as a source for the print examined in this study. Courtesy of Simpleware software (Synopsys, Inc., Mountain View, USA)

As shown in Figure 2, a custom calibration test fixture was designed in a CAD environment and used as the source file for the 3D print examined in this study. The fixture was designed with useful features such as both circular holes and cylinders to challenge the ability of the printer to produce parts which should be fully symmetric. Raised bars of specified heights were included along with holes incorporating known draft angles to inspect the accuracy of finished parts.

Figure 3: Rendering of micro-CT data obtained from imaging the printed part. Courtesy of Simpleware software (Synopsys, Inc., Mountain View, USA)

Figure 3 demonstrates a 3D volumetric rendering of the scan data for the test fixture imaged in this study. Of note, we can qualitatively see several deviations from the original CAD design. One is the presence of the open mesh on the surface and base of the printed part. As designed, these top and bottom surfaces should be fully closed to form a solid surface. The presence of pores in the top and bottom surfaces suggests the printer was likely under-extruding plastic during the printing process. The operator of the printer should work to calibrate the extruder steps to ensure the correct volume of plastic is passing through the extruder into the hot-end of the printer for deposition onto the build surface. 

Figure 4: 2D planar views through the interior of the printed part showing the hollow core, internal infill structures, and outer wall boundaries. Courtesy of Simpleware software (Synopsys, Inc., Mountain View, USA)


When looking through the individual 2D planar views into the test fixture, Figure 4 highlights the internal hollow volume. To more efficiently utilize resources, additively manufactured parts are typically only partially filled with plastic with the remainder comprised of open air space. While the density of the infill and the infill pattern are customizable, a standard triangular infill at low density was used for this test fixture. This part was produced using a fused deposition modeling printer (FDM), which works by carefully extruding a thin strip of plastic from a hot extruder to build up the structure of the fixture. In examining this planar view, we can see where the individual strips of plastic did not fuse together around each of the features (the square holes, the arch, and the circular hole). Likewise, we can see some thin plastic strips indicating where the hot end traveled between structures without fully ceasing the flow of plastic.

Figure 5: Color map representations of deviation from the printed part compared to the source CAD file. Courtesy of Simpleware software (Synopsys, Inc., Mountain View, USA)

For this work, we also utilized Synopsys Simpleware™ software*  to provide detailed information on differences in the print compared to the CAD file arising during the printing process. In particular, Simpleware software’s CAD module brings the ability to create quantitative color-coded images highlighting the deviation of the finished test fixture from the designed model, as seen in the two different views in Figure 5. We designated our critical deviation values at +/- 0.5mm and have highlighted regions thinner than specified in red and those too thick in blue, as compared to the CAD model.

Conclusion

Utilizing the rapid acquisition times of the high speed detector in the Skyscan 1275, we were able to capture detailed volumetric information to better fine tune our understanding of 3D printer performance in about ten minutes. This rapid turnaround provides critical feedback in the 3D printing process, helping to increase the amount of parts meeting specifications. We hope you found this Image of the Month informative. If you have an Image of the Month sample that you would like us to scan, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com

Scan Specifications

Sample

Calibration Fixture

Voltage (kV)

80

Current (µA)

125

Pixel Size (µm)

40

Rotation Step

0.4

Scan Time (HH:MM:SS)

00:10:54

This scans were completed on our high speed desktop SkyScan 1275 system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using CTVox.

References

https://www.synopsys.com/simpleware/software.html

*Simpleware software enables you to comprehensively process 3D image data (MRI, CT, micro-CT, FIB-SEM…) and export models suitable for CAD, CAE and 3D printing. Use Simpleware’s capabilities to visualize, analyze, and quantify your data, and to export models for design and simulation workflows.

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