Deformation control for mask image projection based stereolithography process

Based on a Digital Micromirror Device (DMD), Mask Image Projection based Stereolithography (MIP-SL) uses an area-processing approach by dynamically projecting mask images onto a resin surface to selectively cure liquid resin into layers of an object. Consequently, the related additive manufacturing process can be much faster with a lower cost than the laser-based Stereolithography Apparatus (SLA) process. However, the part built in MIP-SL process has deformation after building, which may be attributed to several reasons. First, the volumetric shrinkage takes place during the phase change process when liquid monomers are converted to solid polymer. Second, heat will be generated since the photopolymerization is an exothermic process, and thermal shrinkage is resulted when the curing layer cools down. Third, SLA is a layer-by-layer dynamic building process, in which current curing layer is restricted by the layers solidified below, therefore residual stress builds up. Moreover, the material used in MIP-SL process is acrylate resin, which has much larger shrinkage when it is solidified than the epoxy resin widely used in SLA process. As a result, the deformation of built part in MIP-SL is more obvious than that built in traditional SLA process. ❧ In this research, we address the deformation problems in MIP-SL process from two approaches. The first approach is based on using different exposure strategies. The shrinkage related deformation control method has been studied and verified using physical experiments, besides, the curing temperature during the photopolymerization process has been investigated, and exposure strategies have been designed to reduce part deformation. The second approach tries to reduce deformation through reverse compensation of input geometry based on the deformation calculation, which can either be done by using simulation or physical measurement. Finite Element Analysis(FEA) has been adopted to model and simulate the MIP-SL building process based on the curing temperature calibrated, and reverse compensation computational framework based on physical measurements has been investigated and applied to reduce deformation of fabricated part in MIP-SL process. ❧ For the shrinkage related deformation control method, an exposure strategy is investigated based on mask patterns by decomposing the exposure of a large area in one layer into several exposures of smaller areas in several layers, instead of curing the whole layer in one exposure. A mask image planning method and related algorithms have been developed for the MIP-SL process. The planned mask images have been tested by using a commercial MIP-SL machine. The experimental results illustrate that our method can effectively reduce the deformation. ❧ In the curing temperature studies, test cases of curing layers with different shapes, sizes and layer thicknesses have been designed and tested. The experimental results show that the temperature increase of a cured layer is mainly related to the layer thickness, while the layer shapes and sizes have little effect. The curing temperatures of built layers using different exposure strategies including varying exposure time, grayscale levels and mask image patterns have been studied. The curl distortion of a test case based on various exposure strategies have been measured and analyzed. It is shown that, by decreasing the curing temperature of built layers, the exposure strategies using grayscale levels and mask image patterns can effectively reduce the curl distortion. ❧ COMSOL Multiphysics software has been used to simulate the curl distortion of part built in MIP-SL. The dynamic layer-by-layer building process of MIP-SL has been simulated using birth and death technique. The curing temperature calibrated for each layer has been incorporated as thermal load into the FEA model. The curl distortion simulation result has the same trend with that of the physical built part. Based on the simulation result, a reverse compensation method has been applied on a simple test case to show the effectiveness of reducing curl distortion. ❧ A general reverse compensation computation framework based on physical measurements is presented to reduce the complex deformation in additive manufacturing process. A novel method is presented for identifying the optimal correspondence between the deformed shape and the given nominal CAD model. By studying relations of offsets and deformation for each point, the reverse compensated CAD models can be calculated. The intelligently modified CAD model, when used in fabrication, can significantly reduce the part deformation when compared to the nominal CAD model. Two test cases have been designed to demonstrate the effectiveness of the presented computation framework.