NIST Method Measures Polymer Material Properties During 3D Printing

The light-based atomic force microscopy (AFM) technique offers insights into the stereolithography process.

Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a light-based atomic force microscopy (AFM) technique that measures how and where the properties of resins and other materials change when they react under light to form polymers in stereolithography 3D printing. The technique, called sample-coupled-resonance photorheology (SCRPR) has been published in the journal Small Methods.

According to researchers, 3D printing has the disadvantage of introducing microscopic variations in a material’s properties. Because software renders the parts as thin layers and then reconstructs them in 3D before printing, the physical material’s bulk properties no longer match those of the printed parts. Instead, the performance of fabricated parts depends on printing conditions. 

The NIST method measures how materials evolve with submicrometer spatial resolution and submillisecond time resolution—thousands of times smaller-scale and faster than bulk measurement techniques. Researchers can use SCRPR to measure changes throughout a cure, collecting critical data for optimizing processing of materials ranging from biological gels to stiff resins.

In stereolithography 3D printing, where light is used to pattern photo-reactive materials, a print voxel may be uneven due to variations in light intensity of the diffusion of reactive modules, researchers say. AFM can sense rapid, minute changes in surfaces because the AFM probe is continuously in contact with the sample.

The method measures two values at one location in space during a finite timespan. Specifically, it measures the resonance frequency (the frequency of maximum vibration) and quality factor (an indicator of energy dissipation) of the AFM probe, tracking changes in these values throughout the polymerization process. These data can then be analyzed with mathematical models to determine material properties such as stiffness and damping.

The method was demonstrated with two materials. One was a polymer film transformed by light from a rubber into a glass. Researchers found that the curing process and properties depended on exposure power and time and were spatially complex, confirming the need for fast, high-resolution measurements. The second material was a commercial 3D printing resin that changed from liquid into solid form in 12 milliseconds. A rise in resonance frequency seemed to signal polymerization and increased elasticity of the curing resin. Therefore, researchers used the AFM to make topographic images of a single polymerized voxel.