Session: 16-04-03: AM Bench Plenaries III

Paper Number: 173720

Vat Photopolymerization Cure Depth Dynamics: Effects of Light Source Bandwidth and Absorber Type 

As part of the NIST Additive Manufacturing Benchmark Challenge, we investigated the relationship between cure depth and radiant exposure (often called dose) in photopolymer resins with varying monomer functionality and photoabsorber type. The resins were irradiated using either narrow-bandwidth (< 5 nm) or broad-bandwidth (> 10 nm) 385 nm or 405 nm LED light. Both fluorescing and non-fluorescing absorbers were studied. Specimens were fabricated at sizes sufficient to minimize size-dependent effects (> 2 mm x-y).  

Participants were tasked with predicting detailed working curves, including the extracted critical energy (Ec) and depth of light penetration (Dp), across eight experimental conditions: two monomer types, two absorber types, and two light sources. Modelers were provided with comprehensive datasets, including resin reactivity and thermophysical properties, along with radiometric characterizations of the light sources.  

The primary objective was to assess the impact of absorber fluorescence and light bandwidth on cure depth. Experimental data used for model calibration and challenge comparison included real-time Fourier transform infrared spectroscopy (RT-FTIR), ultraviolet-visible (UV-Vis) spectroscopy, and radiometric system characterization. 

All resins were composed of well-studied and widely-used monomers and photoabsorbers with a single, also-ubiquitous photoinitiator for vat photopolymerization additive manufacturing to ensure modelers had an ample body of literature to draw from. The following materials were used in the formulations of the four resins:  

Acrylate Monomers:  

● Trimethylolpropane triacrylate (TMPTA, Sigma Aldrich, CAS# 15625-89-5, Mw = 296.32 g/mol)  

● 1,6-Hexanediol diacrylate (HDDA, Sigma Aldrich, CAS# 13048-33-4, Mw = 226.27) Photoabsorbers:  

● Sudan I (Sigma Aldrich, CAS# 842-07-9, Mw =248.28 g/mol)  

● 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT, Sigma Aldrich, CAS# 7128-64-5, Mw=430.56 g/mol) Photoinitiator:  

● Diphenyl(2,4,6-trimethylbenzoyl)-phosphine Oxide (TPO, Sigma Aldrich, CAS# 75980-60-8, Mw = 348.38 g/mol)  

Many common 3D printers use either a 405 nm and 385 nm light source to initiate photopolymerization so TPO was selected as the photoinitiator for these formulations and BBOT and Sudan 1 were selected as photoabsorbers. Each resin was composed of 30 g base monomer (either TMPTA or HDDA), to which 0.1 wt% and 0.4 wt% of photoabsorber (either BBOT or Sudan 1) and photoinitiator (TPO) were added, respectively.  

A high-power, collimated 405 nm or 385 nm LED was used for all photopatterning (Thorlabs, SOLIS-405C or SOLIS-385C). The light engine intensity and bandwidth were measured using radiometry and photospectroscopy, respectively. 

The following calibration data were supplied to modelers to predict the cure depth and corresponding working curve output of critical cure energy and penetration depth.  

Resin Characterization  

● Fourier transform infrared spectroscopy measurements of polymer conversion vs exposure duration for a constant exposure intensity  

● Ultraviolet-visible spectroscopy was conducted to determine the absorption profile of each resin  

Light Engine Characterization  

● Radiometric measurements of the filtered LED light sources to determine optical power  

● Photospectrometry measurements of the filtered LED light sources to determine optical bandwidth  

The solution dataset comprised of cure depth measurements, which were conducted utilizing an optical coherence tomography system to ensure non-destructive measurement of the samples. 

All experiments were conducted at the National Institute of Standards and Technology and the University of Colorado. Calibration measurements were released for six open-source resins to serve as representative systems within the field. These results aim to advance the understanding and predictive capability of cure behavior in additive photopolymerization processes, ultimately supporting more repeatable and reliable part fabrication. 

Presenting Author: Callie Higgins National Institute of Standards and Technology

Presenting Author Biography: Dr. Callie Higgins serves as the Material Measurement Laboratory Additive Manufacturing Program Coordinator and Co-Project Leader of the Photopolymer Additive Manufacturing (PAM) Project at the National Institute of Standards and Technology (NIST) in Boulder, CO. Additionally, she is an adjunct faculty member at the Colorado School of Mines. Recently, her collaborative work with Co-Project Leader Jason Killgore, investigating the fundamental properties of PAM systems, received the Samuel J. Heyman Service to America Medal (SAMMIES) for Emerging Leaders, one of the Federal Government's highest honors, and is a recipient of the 2025 SME Outstanding Young Manufacturing Engineer Award. She earned her PhD from CU Boulder's Department of Electrical Engineering, specializing in optics and material science, focusing on characterizing photopatterned hydrogels for use in regenerative medicine. Outside of the lab, she loves to adventure around the mountains: skiing, hiking, and picnicking all the way up with her husband, kids, friends, and family.

Authors:

Callie Higgins National Institute of Standards and Technology
Jason Killgore National Institute of Standards and Technology
Thomas Kolibaba National Institute of Standards and Technology
Rion Wendland National Institute of Standards and Technology