Energy control and material deposition methods for fast fabrication with high surface quality in additive manufacturing using photo-polymerization

2014-06-23More and more research prototypes and commercial systems have been developed to pursue faster build speed and better part quality in different manufacturing scale levels. Compared to traditional prototyping approaches that take days, additive manufacturing (AM) can build physical objects with any complicated structures in hours. Many industries have profited from AM and now AM is considered as an alternative to traditional manufacturing processes when the existing manufacturing methods cannot create a product practically, efficiently or affordably. However, to meet such high expectations, many challenges still remain. The primary challenge to overcome is the conflict between the build speed and the surface quality. Since the surface finish and approximation error depend on the layer thickness used in AM, the dominant approach to achieve a high surface quality is to reduce the layer thickness. However it would significantly slow down the building process. On the contrary, if a faster build speed is desired, a bigger layer thickness should be adopted and hence the surface quality would be worse. A tradeoff between the surface quality and building time is usually needed in AM processes. ❧ The thrust of this research is to contribute to the advancement of AM by addressing such a dilemma of the surface quality and build speed, to develop novel approaches to achieve both the high surface quality and the high build speed goals in photo‐polymerization based AM processes. In this research work, photo‐polymerization AM processes from meso‐scale to micro‐scale were investigated. Novel approaches and strategies were developed to deliver energy and polymerize material at the target position quickly and accurately, thus reaching high build speed and high surface quality simultaneously. ❧ CNC accumulation process and Mask Image Projection based Stereolithography (MIP-SL) process, are two typical photo‐polymerization AM processes: the non‐layer based and the layer based photo‐polymerization AM, respectively. Studies of these two processes were performed to show the feasibility of applying the developed approaches in any photo‐polymerization AM systems. ❧ In CNC accumulation process which is non‐layer based, an accumulation tool head is made of optic fibers and the light is delivered by the optic fibers to polymerize liquid resin. Multiple accumulation tools were designed for fabrications with different resolutions and speeds. The accumulation tools are driven by multiple stages and thus are able to move or rotate along multiple axes. To control the energy power of each accumulation tool in the multi‐tool and multi‐axis CNC accumulation system, statistical methods were applied to optimize the manufacturing process settings and force analysis were performed, thus to achieve the optimal build speed and part quality. In addition, in order to successfully fulfill a manufacturing task, another challenge related to energy control has to be overcome. In CNC accumulation, it is critical to accurately deliver the energy to the exact position with a good normal direction. Otherwise, the fabricated result may not be able to reach the accepted geometry accuracy, or the material may attach on the tool tip instead of the base surface, which would cause failures to the build task. To address this problem, a novel dual‐axis 3D scanning unit was developed and integrated. A point processing method based on the Algebraic Point Set Surface (APSS) fitting and Layered Depth‐normal Image (LDNI) representation was developed for converting the scanning points into a 3D surface model. Based on the constructed surface model, a multi‐axis tool path could be generated for building tasks of repairing or remanufacturing of any geometry using the developed system. With those developed energy control approaches, the developed multi‐tool and multi‐axis CNC accumulation system is able to fabricate the part accurately and rapidly. ❧ In MIP-SL system which is a layer based photo‐polymerization AM system, a DMD chip is usually used to dynamically generate a mask image. The mask image is used to define the light pattern hence control the energy distribution. As a layer based photo‐polymerization AM process, MIP-SL is much more complicated on material deposition than CNC accumulation process. Related challenges include ultra‐thin layer recoating, liquid surface profile, and so on. Accordingly, methods both on energy control and material deposition aspects were developed to achieve the two syngenetic goals of fast build speed and high surface quality. In order to address the notorious stair case effect in AM which damages the surface quality significantly, on energy control aspect, a gray scale image approach was developed by modeling and controlling the cure depth with different light intensities. On material deposition aspect, a meniscus equilibrium approach was developed by modeling and controlling the liquid profile with different liquid/gas/solid equilibrium conditions. With the two approaches, extreme high surface finish could be achieved without sacrificing build speed or even with higher build speed. In addition, a novel fast material recoating approach was proposed and implemented in MIP-SL system. With the developed material recoating approach, our system is able to realize an order of magnitude faster build speed comparing to the existing additive manufacturing systems in the market. ❧ Different building scales share some common physics in the fluid flow and polymerization process but also differ in many fields, such as separation force effects and optic system requirements. When it comes to micro‐scale fabrication, the optical systems and machine designs were modified, and the corresponding optimal process parameter settings were investigated to build micro features in both CNC accumulation system and MIP-SL system. Besides, when the scale comes down to micro, the fast recoating technique used in meso‐scale cannot be copied directly. Different recoating mechanisms should be applied due to the change in separation forces. The meniscus equilibrium approach developed in meso‐scale systems was also investigated and tuned to overcome the stair‐case effect in micro‐scale fabrication. ❧ Testbeds were developed and experiments were performed to verify the effectiveness and efficiency of the proposed approaches in the two photo‐polymerization AM systems. The test results demonstrated that the proposed approach could achieve an order of magnitude faster build speed than any other AM systems while the surface finish could be improved by ~80%. Unlike other technologies which can only accomplish one goal at the cost of the other, the approaches proposed in this research can improve the building speed and surface finish to the largest extent at the same time in both meso‐scale and micro‐scale fabrications. The build speed and surface finish are not conflicting goals any more as they were in the conventional AM systems. This work would be meaningful in advancing AM from ""rapid prototyping"" to truly ""rapid"" manufacturing technology for not only prototypes but also end‐use products