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

Paper Number: 172889

Reproducubility, Benchmarking and Understanding Photopolymerization-Based Additive Manufacturing 

With the advent of photopolymerization-based additive manufacturing, numerous resins, printers and software have been developed, each with the aspiration of improving various aspects of the printing process including speed, resolution, and material properties, among other print qualities.  In particular, the potential for additive manufacturing has spawned development of a large variety of printers frequently with different light sources that have different emission spectra, intensities and even sample geometries.  While these differences often seem minor, the interactions of the printer light source with the resin are critical and dictate the ultimate performance of the printed part.  Herein, we will focus on three different elements related to these systems:  (i) the interrelation of reaction kinetics, diffusion and material properties during the material’s curing; (ii) the shortfalls of the classical working curve approach used to  describe these systems, and (iii) the importance of accurate and thorough descriptions of the curing conditions due to the sensitive nature of the curing to those conditions. We utilize a comprehensive 4D model of the photopolymerization process to demonstrate how different aspects of the printer-resin interaction change the polymerization kinetics, depth of cure, resolution, temperature profile and, as a result, the final material properties.

 

Jacob’s working function is a valuable yet simplified model that utilizes macroscopically measurable parameters (i.e., penetration depth and critical dose) to correlate cure depth with light dose. This model offers a practical framework for translating photocuring behavior across different printers, especially when light intensity and spectral output vary. However, this approach does not account for several important factors such as the basic reaction mechanism which is not linear with intensity, spatial and temporal temperature variations, oxygen inhibition and diffusion-related reaction kinetics. For instance, deviations from Jacob’s prediction arise for even classical bimolecular termination processes, under diffusion-limited conditions, or at the boundary of light pattern where gradients in radical and monomer concentrations—driven by non-uniform light intensity—significantly alter the reaction profile. These deviations have been thoroughly captured and explained by comprehensive kinetic models of photopolymerization. Therefore, Jacob’s equation serves as a useful starting point; however, print and printer specific considerations are essential for successfully adapting a given formulation across different printing platforms.

 

These fundamental limitations further highlight the critical nature of proper reporting of conditions for photo-induced processes. Unlike thermal processes, where a temperature and duration suffice to fully describe most systems, light-based processes necessitate many conditions to be properly documented and reproduced. These conditions and considerations include, but are not limited to, light spectrum, incident intensity, exposure duration, sample thickness, sample geometry, sample boundary conditions, chromophore absorption, and chromophore concentration. A study of 150 papers in the areas of polymer science, chemistry, and physics assessed the state of reproducibility in the literature via these categories and demonstrated the substantial lack in reporting of these key variables.

 

Presenting Author: Christopher Bowman University of Colorado

Presenting Author Biography: Professor Christopher N. Bowman is currently the James and Catherine Patten Endowed Chair and a Distiguished Professor of the Department of Chemical and Biological Engineering at the University of Colorado with additional appointments in the Materials Science and Engineering Program, the Department of Chemistry and Biochemistry, and the Dental School. He received his B.S. and Ph.D. in Chemical Engineering from Purdue University in 1988 and 1991, respectively. After receiving his Ph.D., he began his academic career at the University of Colorado in January of 1992. Since that time Professor Bowman has built a program focused on the fundamentals and applications of crosslinked polymers formed via photopolymerizations and click reactions.

Authors:

Christopher Bowman University of Colorado