Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD, USA
Measuring and predicting crystal structure in thermoplastic material extruison
Additive manufacturing (AM) processes differ from formative and subtractive processes as each voxel can have a unique process history. In many materials both soft and hard that unique history can lead to substantial changes in performance. In order to understand the effect of processing conditions on final part performance the critical process parameters need to be identified, characterized, and where possible modeled. For semi-crystalline polymers both the thermal and flow history can induce changes in crystallization rate and crystal morphology, density, size. Further these changes effect mechanical performance, solubility, and degradation. Using a combination of thermography, rheology, polarized light microscopy, and continuum modeling we can predict the size of spherulites in poly(lactic acid) based on the extrusion temperature and flow rate during material extrusion AM. The prediction can be extended to other semi-crystalline systems provided the rheology and crystallization kinetics can be measured or estimated. While this work focuses on material extrusion of semi-crystalline thermoplastics, the combination of modeling and in-situ process measurements are applicable to many materials and AM processes.
Jonathan Seppala is currently the technical lead in the additive manufacturing effort in the Polymer Processing and Rheology Project. His current research focuses on using infrared thermography, rheology, fracture mechanics, and neutron and x-ray reflectivity to study the polymer physics of thermoplastic additive manufacturing processes. Jonathan earned a B.S. in Chemical Engineering from Michigan Technological University and a Ph.D. in Chemical Engineering from Michigan State University studying the rheology and thermodynamics of polymer nanocomposites. Following his Ph.D., Jonathan worked as a Postdoctoral Researcher studying thin film self-assembly of block copolymers and equilibrium dynamics of amphiphilic micelles at the University of Delaware. Prior to joining the Polymer Processing and Rheology project, Jonathan studied ballistic witness materials and shear thickening fluids as part of the Personal Body Armor Project at NIST.