Large Area Maskless Photopolymerization (LAMP) is a disruptive additive manufacturing technology developed in the Direct Digital Manufacturing Laboratory at Georgia Tech. Due to polymerization shrinkage during the layer-by-layer curing process, stresses are accumulated that can give rise to cracks and delaminations along the interfaces between adjacent layers. The objective of this doctoral dissertation is to investigate the mechanisms of stress evolution and cracking/delamination during the LAMP manufacturing process through theoretical modeling and experimental characterization methods. The evolving conversion degree in a layer was characterized through Fourier Transform Infrared Spectroscopy and this leads to a so-called print-through curve. The polymerization shrinkage strain in each exposed layer was calculated on the basis of the theoretical relationship between the volumetric shrinkage and the degree of conversion. Furthermore, the material’s elastic modulus, which also evolves with the degree of conversion, was characterized by three-point bending tests. With the degree of conversion, cure-dependent modulus and shrinkage strain as the three primary inputs, finite element modeling was conducted to dynamically simulate the layer-by-layer manufacturing process and to predict the process-induced stresses. To investigate the fracture process, Mode I and Mode II interlaminar fracture toughness of the LAMP-built laminates was characterized, using the double cantilever beam (DCB) test and the end notched flexure (ENF) test, respectively. In order to predict the crack initiation and propagation occurring in a LAMP-built part, a mixed-mode cohesive element model was developed. The Mode I and Mode II cohesive parameters, which are used to describe the bilinear constitutive behavior of the cohesive elements, were determined by matching the numerical load-deflection curves to the experimental ones obtained from the DCB tests and the ENF tests, respectively. Using this model, the fracture of a hollow-cylinder part was analyzed and the simulation results were compared with experiments. Finally, several possible strategies for mitigating the shrinkage related defects were investigated. Reducing the overall polymerization shrinkage, optimizing the print-through curve and delaying the gel point of resin composite were demonstrated to be effective in reducing stresses and cracks.

Large Area Maskless Photopolymerization (LAMP) is a direct digital manufacturing technology being developed at Georgia Tech to produce ceramic molds for investment casting of turbine airfoils. In LAMP, UV light incident on a spatial light modulator is projected in the form of a structured black and white bitmap image onto a platform supporting slurry comprising a ceramic particle loaded photocurable resin. Curing of the resin is completed rapidly with exposures lasting 20~160ms. Three-dimensional parts are built layer-by-layer by sequentially applying and selectively curing resin layers of 25-100 micron thickness. In LAMP, diacrylate-based ceramic particle-loaded resins with photoinitiators sensitive in the range of spectral characteristics of the UV source form the basis for an ultra-fast photopolymerization reaction. At the start of the reaction, the monomer molecules are separated by van der Waals distance (~10⁴Å). As the reaction proceeds, these monomer molecules form a closely packed network thereby reducing their separation to covalent bond lengths (~ 1 Å). This results in bulk contraction in the cured resin, which accumulates as the part is fabricated layer-by-layer. The degree of shrinkage is a direct measure of the number of covalent bonds formed. Thus, shrinkage in LAMP is characterized by estimating the number of covalent bonds formed during the photopolymerization reaction. Polymerization shrinkage and accompanying stresses developed during photopolymerization of ceramic particle-loaded resins in LAMP can cause deviations from the desired geometry. The extent of deviations depends on the photoinitiator concentration, the filler loading, the degree of monomer conversion, and the operating parameters such as energy dose. An understanding of shrinkage and stresses built up in a part can assist in developing source geometry compensation algorithms and exposure strategies to alleviate these effects. In this thesis, an attempt has been made to understand the curing kinetics of the reaction and its relation to the polymerization shrinkage. Realtime Fourier Transform Infrared Spectroscopy (RTFTIR) is used to determine the conversion of monomers into polymer networks by analyzing the changes in the chemical bonds of the participating species of molecules. The conversion data can further be used to estimate the curing kinetics of the reaction and the relative volumetric shrinkage strain due to polymerization.