Ph.D. Dissertation Defense in Plastics Engineering: Karun Kalia 4/7

Brief Abstract:

Polymer foams have applications in thermal insulation, protection gear, vibration/damping/noise control, footwear, sports equipment, tissue engineering, etc. Cellular morphologies inside foams can be created using physical or chemical blowing agent as well as incorporation of thermally expandable microspheres (TEMs). Successful Integration of foaming and 3D-printing processes could offer several advantages: a) low material usage, b) microstructure control, c) shrinkage and inter-bead void mitigation in 3D-printed parts and, d) additional design freedom. However, there are limited reports on foam 3D printing processes. Here, a facile manufacturing method to enable the in-situ foam 3D printing of thermoplastic materials is being explored. The first phase of the study includes material formulation and process design exploring various polylactic acid (PLA) grades that can suppress the expansion during filament fabrication process. It also includes the characterization of mechanical, thermal and microstructure evolution of foam 3D-printed parts w.r.t various TEM loadings. The second phase focuses on the identification of the process-structure-property relationships in foam 3D-printed parts. The effect of print process parameters towards in-situ foam 3D printing process is investigated in detail as there are certain key factors which control the expansion of the extruded beads. The expansion and cooling kinetics of the extrudate will ultimately control the cellular morphologies as well as the bulk density, thereby, affecting the mechanical properties of printed foams. The third phase includes the printing and characterization of functionally graded foams (FGFs) based on the material formulation and the process established in phase 1 and 2. Exploiting the printing process control, a methodology is established to design and print graded structures with controllable microcellular morphologies, densities, and mechanical properties within a single print. Another innovative method of creating FGFs is also investigated, which is based on the in-situ control of blowing agent content using a mixer nozzle and multiple filaments. Dual filaments with preset blowing agent contents are fed into the mixer nozzle at controlled mixing ratios. Both methods could successfully produce FGFs with desired density gradient. The mechanical performance of FGFs were assessed using compression, impact and bending tests and the advantages of FGFs are elaborated against parts with uniform densities. Overall, functionally graded structures provide enhancement in properties such as energy absorption, efficient use of material usage, and customized design for applications such as knee pads, helmets, tissues, drug delivery, etc.