Experimental investigations and theoretical modeling of large area maskless photopolymerization with grayscale exposure

Abstract

Large Area Maskless Photopolymerization (LAMP) is a technology being developed to fabricate integrally-cored ceramic molds for the investment casting of turbine airfoils. In LAMP, ultraviolet (UV) light in the form of bitmap images is projected from a spatial light modulator (SLM) onto a photocurable ceramic material system (PCMS). Exposed and unexposed regions are determined through black and white portions of the bitmaps, respectively. UV light induces photopolymerization and the formation of an insoluble solidified network. Three-dimensional structures are built layer-by-layer through sequential application and curing of PCMS layers of 100 micron thickness. To date, ceramic molds fabricated using LAMP have been successfully implemented in investment casting of single-crystal turbine airfoils without internal cooling schemes. Two particularly important challenges for the fabrication of airfoil molds with internal cooling passages are: (a) fabrication of unsupported structures in the mold geometry and; (b) mitigation of internal stresses that arise during layer-by-layer build-up due to volumetric shrinkage during photopolymerization. Unsupported geometries arise in nearly every cored airfoil mold and often in a location where support structures cannot be easily removed after fabrication. Internal stresses generated by volumetric shrinkage can lead to cracking during binder burnout (BBO), sintering and casting. This thesis aims to simultaneously address these challenges through the investigation of grayscale exposure to control the degree of monomer conversion during photopolymerization of single and multiple layers. The effective intensity of the UV light incident on the monomer system can be reduced by selectively turning off pixels within the nominally "white" or "on" regions of the projected bitmaps, effectively producing an exposure with a lower light intensity. In an effort to reduce internal stresses in the mold, the grayscale exposure can be tuned to create regions of uncured or partially cured monomer within the mold geometry to reduce the connectivity between cured regions and thus reduce the net effect of volumetric shrinkage. Grayscale exposure can also be used to generate support structures with a low degree of polymerization to create a gel state beneath and surrounding the unsupported segments of the mold, which can be washed away after completion of mold fabrication. In order to successfully utilize grayscale techniques in LAMP, the cure depth must be predicted. This is accomplished through cure depth measurements at different exposure times to develop a "working curve." In addition, the degree of monomer conversion and its relation to cure depths resulting from grayscale exposure must be understood. Measurements of the degree of conversion are obtained through Fourier Transform Infrared spectroscopy (FTIR). Empirical models are developed and compared to theoretical predictions. Also, the scattering length pixelation model is introduced as a technique to predict the light intensity distribution within the PCMS for exposure patterns at multiple length scales. Results from these grayscale investigations are then applied to LAMP and the effectiveness of grayscale to fabricate unsupported geometries and internal stresses from volumetric shrinkage is discussed.