NIST technique that precisely measures polymer pro
Nov.22, 2018 There are many types of 3D printers, from the ever-popular FDM (fused deposition modeling) variants that adorn many a maker’s desktop to the ultra-precise SLA (stereolithography) machines that cost tens of thousands of dollars, and those fancy SLA machines just got an upgrade through a NIST (National Institute of Standards and Technology) technique that precisely measures how polymerization occurs during the photocuring process of resins and gels in 3D printers.
SLA 3D printers cure resins into solids very quickly, and from the human perspective, the parts that come out are nearly perfect. But on the molecular level, tiny inconsistencies in the curing process can affect the physical properties of a 3D print, making it more brittle or less dense. A voxel is a 3D unit of volume similar to a pixel on a 2D display, and this new method from NIST can observe and analyze subtle changes of a single voxel of resin as it undergoes the curing process.
The technique is called sample-coupled-resonance photorheology (SCRPR) and it’s a type of light-based atomic force microscopy (AFM); it “measures how and where a material’s properties change in real time,” the report states. The scale is submicrometer spatial resolution and submillisecond time resolution, magnitudes smaller than conventional bulk measurement methods. Data gathered from observing various substrates polymerize will provide insights into optimizing the physical and chemical properties of resins as well as improving cure time, which is already as short as 12 milliseconds (to fully transition from liquid to solid) for the commercial resin tested in the NIST study.
A commercial AFM probe was modified to use an ultraviolet laser to cure the polymer (resin) right where the probe contacted the sample. Two values are tracked: resonance frequency and energy dissipation. Mathematical models can be applied to the value changes to determine stiffness and other mechanical properties. Polymerization seemed to be signaled by an increase in resonance frequency, and a topographic map of the polymerization of a single voxel was created to visualize the value changes. NIST materials research engineer Jason Killgore added, “We are also working on modeling the probe sample interaction to allow absolute quantification of material properties during the liquid to solid polymerization.”
This information is not valuable only to the 3D printing industry as optics and coating companies have reached out to NIST to collaborate and investigate materials properties. “We have had a ton of interest in the method from industry, just as a result of a few conference talks,” Killgore said.
Some 3D printing firms spend oodles of money on R&D to make their machines and resins cure as fast and accurately as possible, and SLA technology is one of (if not the) fastest types of 3D printing, so technological and methodological improvements implemented as a result of the NIST SCRPR research technique will push 3D printing speeds into unknown boundaries thus far reserved for science fiction.