NIST probes SLA 3D printing to improve quality by the voxel

High resolution 3D printing is reaching new levels of quality thanks to a technique developed at the U.S. National Institute of Standards and Technology (NIST).

Using a very-high-resolution scanning probe, the NIST researchers were able to measure the exact moment when material cures in an SLA 3D printer. Using data gathered in this process, 3D printing conditions can then be modified to achieve optimal material properties in a printed part.

Results of a study concerning NIST SLA probe methodology have recently been published in Small Methods journal.

Innovation and competitiveness in additive manufacturing

In its mission to promote innovation and the industrial competitiveness of the U.S., NIST has demonstrated a great deal of interest in additive manufacturing as an emerging technology.

In May 2014, the institute launched its Consortium for Additive Manufacturing Materials (CAMM) to focus on the development “metals, polymers, and ceramics for additive manufacturing uses in the aerospace, biomedical, energy, and electronics industries.”

Following this, speaking broadly about the institute’s aims with additive, Shawn Moylan, NIST mechanical engineer and project leader, told 3D Printing Industry that NIST wanted to “Get in at the ground level” to “let us do a lot of high impact work in a relatively short amount of time.”

Over the years, NIST has published countless studies relating to its developments in 3D printing including, but not limited to, research relating to the quality of parts made using laser powder bed fusion, and powder recycling.

This latest method of measuring SLA 3D printing to be developed by NIST is termed Sample‐Coupled‐Resonance Photorheology (SCRP).

SLA measured by the voxel

Seeking ways to improve the photocuring process in SLA 3D printers, SCRP measures material solidification at the volumetric-pixel (voxel) level. To do so, researchers introduce an atomic force microscopy (AFM) probe to the material vat of an SLA 3D printer.

Experimental set up of the AFM probe in a photopolymeric material. Image via NIST
Experimental set up of the AFM probe in a photopolymeric material. Image via NIST

In constant contact with the transforming media, the AFM rapidly senses changes (i.e. solidification) in the surface of a photopolymer.

Throughout the 3D printing process, the AFM constantly tracks resonance frequency (the frequency of maximum vibration) and quality factor (an indicator of energy dissipation) of the cured material.

By analyzing these data points, NIST scientists then determine the exact material properties of a 3D printed voxel, including any surface variations due to light intensity or diffusion.

A 3D topogrA 3D topographic image of a single voxel of polymerized resin, surrounded by liquid resin measured by sample-coupled-resonance photo-rheology (SCRPR). Image via NISTaphic image of a single voxel of polymerized resin, surrounded by liquid resin measured by sample-coupled-resonance photo-rheology (SCRPR). Image via NIST
A 3D topographic image of a single voxel of polymerized resin, surrounded by liquid resin as measured by AFM. Image via NIST

Probing experimentation 

In one NIST experiment, a commercial 3D printing resin is measured transforming from a liquid to a solid at a rate of 12 milliseconds.

Though cured under teh same conditions, some of the cured resin voxels exhibited high elasticity than others. According to AFM readings, these voxels experienced a rise in resonance frequency when 3D printed, which seemed to signal polymerization and increase elasticity.

In another example, exposure power and time influenced a polymer’s transformation from a rubber into a glass.

The next step for SCRP is to work with the information to get the most out of photopolymers, and tune the SLA process to instill greater stiffness or flexibility in 3D printed parts.

Since publishing the method, the team have also received a surprising amount of commercial interest in the method from companies in additive manufacturing and wider industry, such as coatings and optics.

Monitoring Fast, Voxel‐Scale Cure Kinetics via Sample‐Coupled‐Resonance Photorheology” is published online in Small Methods journal. It is co-authored by Callie I. Fiedler‐Higgins, Lewis M. Cox, Frank W. DelRio and Jason P. Killgore.

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Featured image shows a 3D topographic image of a single voxel of polymerized resin, surrounded by liquid resin as measured in the NIST study. Image via NIST

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