X-ray Microscopic Examination of Fiber-Reinforced Polymers for Additive Manufacturing

Figure 1: Rendered cut view highlighting carbon fibers within the polylactic acid composite starting material

Additive manufacturing continues to grow for both prototyping and production of end use parts, and many scientists are working on developing the next generation of advanced materials to take better advantage of the benefits from these new techniques. One common addition to the base polymer relies on incorporating fibers, either glass or organic, into the matrix to increase structural properties of the final products.

X-Ray Microscopic Imaging of Fiber-Reinforced Polymers

This month we imaged carbon fiber composite 3D-printing filament using the SkyScan 1272 micro-CT. For inspecting microscopic features within polymers, the high-resolution capabilities of the SkyScan 1272 are highlighted by their ability to capture the internal structure and defects.

A commercially available polylactic acid (PLA) composite filament with carbon fibers added was purchased from a local retailer and imaged as received. The filament was also used to print a few small test pieces using a standard hobby level FDM printer and the fiber orientation was examined for each sample.

Figure 2: Planar 2D views within the 1.75 mm PLA filament

As shown in Figure 2, the SkyScan 1272 micro-CT captured fine details both on the surface of the sample as well as throughout the volume. In particular, the fibers are oriented along the length of the filament. Our imaging also highlights a significant number of air voids within the PLA filament. While these may be acceptable for a hobby-level product, the presence of these voids in the starting material may lead to material defects in finished products intended for commercial applications via additive manufacturing.

Figure 3: Volumetric 3D rendering of the surface of the PLA filament

When looking through the 3D view of the sample, a high degree of roughness is clearly visible on the surface of the filament (Figure 3). This roughness will lead to additional wear on the printer components such as the extruder, hot end, and nozzle. For this reason, printing with fiber supported filaments is typically coupled with upgrading from a standard brass nozzle to a hardened steel nozzle to prolong the lifetime of the printing components.

Figure 4: Volumetric 3D rendering of the isolated carbon fibers within the PLA filament

Bruker CTAnalyzer allows us to isolate the fibers and quantitatively organize them by orientation in 3D space (Figure 4). For the portion of fiber imaged, CTAn determined the fibers comprised about 1.7% of the total analyzed volume of polymer. At the filament level, the fibers are generally located along the length of the fiber, and few are present in unintended directions. CTVox allows us to view the extracted fiber data in an interactive 3D process.

Figure 5: Planar 2D views of the reconstructed dataset for small cylinder printed using the carbon fiber PLA composite filament

While examining the PLA filament is a good example of quality control, more interest may lie in the assessment of how the filament performs in actual use. Figure 5 presents the imaging results of a small cylinder manufactured using the carbon fiber PLA composite filament. In fused deposition modeling, the filament is extruded from a heated nozzle onto the build plate to sequentially build up parts in a layer-by-layer fashion.

Figure 6: Volumetric 3D renderings of the fibers arranged in the circumference of the printed cylinder

Within CTVox we can observe the radial distribution of the carbon fibers around the circumference of the printed cylinder (Figure 6). This deposition pattern matches the path of travel of the print head and suggests limited migration of the carbon fiber within the polymer when moving through the melting transition.

Figure 7: Planar 2D views of the reconstructed dataset for small rectangular rod printed using the carbon fiber PLA composite filament

Figure 7 demonstrates the more varied location of fibers within the larger printed rectangle as well as highlights some regions of under-extrusion in the part arising from the printing process.

Figure 8: Volumetric 3D renderings of the fibers arranged throughout the printed rod

In the case of the larger printed rod, we see the fiber distribution still generally tracks along the path of the extruder, but more dispersion in the fiber direction is present since the extruder moves in varying directions while building larger parts (Figure 8).

Conclusion

The fine resolution of the SkyScan 1272 micro-CT was a great match in finding both defects in the PLA carbon fiber composite as well as tracking the orientation of the carbon fiber both in the raw material and finished products.

We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our Image of the Month series next month.

Scan Specifications

Sample Filament and Rod Cylinder
Voltage (kV) 40 40
Current (µA) 100 100
Filter None None
Pixel Size (µm) 1 1.5
Rotation Step 0.15 0.18
Exposure Time (ms) 1800 1800
Scan Time (HH:MM:SS) 06:38:12 05:23:30

 

These scans were completed on our SkyScan 1272 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon while visualization and volumetric inspection of the 2D and 3D results were completed using DataViewer and CTVox. CTAn was utilized to isolate and quantify the fibers within the polymer matrix.

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com.

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