Process Errors and Aspects for Higher Resolution in Conventional Stereolithography

Due to the rapid development of precision manufacturing technologies, there is a growing market need for appropriate rapid prototyping methods with higher resolution. This paper presents aspects for a general optimization of stereolithography accuracy and gives a deeper analysis of important process errors. Beside a higher precision due to improved optical components, it can be shown that for a better vertical resolution one must mainly reduce the penetration depth of the photopolymer. We found that this is also possible with conventional stereolithography materials by using a different wavelength, achieving cured rugged layers with a thickness of 20 micrometer. The major accuracy aspect lies in the understanding of the layer deposition process. A CFD (computational fluid dynamics) study helps to describe important phenomena of blade based coating techniques. As a result, the inaccuracy of the layer deposition is the general limiting factor in stereolithography. This knowledge can be directly applied to commercial stereolithography systems helping users to achieve higher process accuracy.Mechanical Engineerin

The Approach of Complex Insert Packaging Fabrication in Stereolithography

The approach of complex insert packaging fabrication in stereolithography is studied in the paper. There are many difficulties being overcome, such as the dispensing shadowing problem caused by the geometric shape of insert, the polymer feeding problem caused by greater insert height than the layer thickness, and the positioning problem of insert that leads to instability of packaging. These drawbacks led to unsatisfactory results of the insert packaging in stereolithography. In order to solve the problems, a new method of complex inserts packaging fabrication in stereolithography is proposed in this paper. Based on the geometric information, function and assembly direction of the inserts, the packaging approach is developed. The approach proposed in this paper has been verified by experiments. It brings considerable contributions to the application of insert packaging in stereolithography. It is also favorable to the improvement of insert packaging efficiency and assembly fabricationMechanical Engineerin

Material and Process Parameters that Affect Accuracy in Stereolithography

Experimental real time linear shrinkage rate measurements simulating stereolithography are used in an analysis of shrinkage during line drawing in stereolithography. While the amount of shrinkage depends on the polymerization kinetics, shrinkage kinetics and overall degree of cure, it also depends on the length of time to draw a line of plastic. A line drawn slowly will exhibit less apparent shrinkage than one drawn very quickly because much of the shrinkage is compensated for as the line is drawn. The data also indicates that a typical stereolithography resin in the green state may shrink to only 65% of its maximum, thus retaining considerable potential for shrinkage during post-cure. This infonnation can be used to predict the amount of shrinkage to be expected under certain exposure conditions and to fonnulate overall strategies to reduce shrinkage and subsequent warpage that causes shape distortion.Mechanical Engineerin

Hydrogels in Stereolithography

The use of stereolithography (SL) for fabricating complex three-dimensional (3D) tissue engineered scaffolds of aqueous poly(ethylene glycol) (PEG) hydrogel solutions is described. The primary polymer used in the study was PEG-dimethacrylate (PEG-dma) with an average molecular weight (MW) of 1000 in distilled water with the photoinitiator Irgacure 2959 (I-2959). Successful layered manufacturing (LM) with embedded channel architecture required investigation of the photopolymerization characteristics of the PEG solution (measured as hydrogel thickness or cure depth) as a function of photoinitiator concentration and laser energy dosage for a specific photoinitiator type and polymer concentration in solution. Hydrogel thickness was a strong function of PI concentration and energy dosage. Curves of hydrogel thickness were utilized to successfully plan, perform, and demonstrate layered manufacturing of highly complex hydrogel scaffold structures, including structures with internal channels of various orientations. Successful fabrication of 3D, multi-layered bioactive PEG scaffolds containing cells was accomplished using a slightly modified commercial SL system (with 325 nm wavelength laser) and procedure. Human dermal fibroblast (HDF) cells were encapsulated in PEG hydrogels using small concentrations (~ 5 mg/ml) of acryloyl-PEG-RGDS (MW 3400) added to the photopolymerizable PEG solution to promote cell attachment. HDF cells were combined with the PEG solution, photocrosslinked using SL, and successfully shown to survive the fabrication process. The combined use of SL and photocrosslinkable biomaterials such as PEG makes it possible to fabricate complex 3D scaffolds that provide site-specific and tailored mechanical properties (i.e., multiple polymer materials) with a polymer matrix that allows transport of nutrients and waste at the macroscale and facilitates cellular processes at the microscale through precisely placed bioactive agents.Mechanical Engineerin

Inverse Geometry for Stereolithographic Manufacturing

As parts produced by stereolithography form and cure, they warp and shrink to produce Pitrts that are not quite the san~ ~ as those originally specified. This research attempts to solve the inverse geomptry problem, that is, what shape should be specified initially so that the she pe produced is the desired one. Assuming that 'the process is repeatable, we ~neasure the difference between the ideal and actua). part dimensions. A finite-element based model is built which mirrors the distortion from the ideal geometry. A "pre-deformed" geometry is then built so that it deforms to the ideal geometry under the conditions imposed on the finite element model. This pre-warped geometry is the geometry we seek.Mechanical Engineerin

Process Planning Based on User Preferences

Typical approaches to adaptive slicing in previous literature have typically used surface finish requirements to control the slicing process. As a result, slice schemes improve the part's surface quality, but do not enable explicit trade-offs between finish and build time. The purpose of this article is to present a process planning method that enables the preferences of the user for surface finish, build time, and accuracy to control how trade-offs are made in a process plan. A multiobjective goal formulation is used by this method to evaluate how well user preferences are met by a process plan. This method consists of three modules, for determining part orientation, for slicing the part, and for determining other parameter values. An example with several scenarios representing different user preferences is provided to illustrate the process planning method.Mechanical Engineerin

Development of an Automated Multiple Material Stereolithography Machine

An automated Multiple Material Stereolithography (MMSL) machine was developed by integrating components of a 3D Systems 250/50 stereolithography (SL) machine in a separate stand-alone system and adapting them to function with additional components required for MMSL operation. We previously reported retrofitting a 250/50 SL machine with multiple vats to accommodate multiple material fabrication for building a wide variety of multi-material models (Wicker et al., 2004). In the MMSL retrofit, spatial constraints limited the multiple vats located circumferentially on a vertical rotating vat carousel to cross-sectional areas of approximately 4.5-inches by 4.5-inches. The limited build size of the retrofitted 250/50 motivated the full development of a new system with multiple material build capabilities comparable to the build envelope of the original 250/50 machine. The new MMSL machine required fabrication of a large system frame, incorporating various 250/50 components and software, and adding a variety of new components and software. By using many existing components and software, the previous engineering development of 3D Systems could be directly applied to this new technology. Components that were transferred from an existing 250/50 to the MMSL machine included the complete optical system (including the optics plate with laser, mirrors, beam expander, scanning mirrors, and focusing lens), the rim assembly (including the laser beam profilers), the associated controllers (computer system, scanning mirror controller, power supply-vat controller) and the wiring harness. In addition to the new frame, the MMSL machine required the development of a new rotating vat carousel system, platform assembly, multi-pump filling/leveling system, and a custom LabVIEW® control system to provide automated control over the MMSL process. The overall operation of the MMSL system was managed using the LabVIEW® program, which also included controlling a new vat leveling system and new linear and rotational stages, while the 3D Systems software (Buildstation 4.0) was retained for controlling the laser scanning process. As a demonstration of MMSL technology, simple multi material parts were fabricated with vertically and horizontally oriented interfaces. The fully functional MMSL system offers enormous potential for fabricating a wide variety of multiple material functional devices.Mechanical Engineerin

Dimensional Issues in Stereolithography

New stereolithography photopolymers have recently been introduced that provide a wider range of functional properties similar to those of high-density polyethylene. One of the important criteria for these materials is the dimensional accuracy and stability in end-use applications as mold masters or the actual functional parts. This work investigates the dimensional stability of one of these new materials with varying amounts of exposure during build. The effect of aging on the part dimensions is reported. The result of environmental humidity extremes at ambient temperature on part dimensions is investigated and compared for parts made from two different families of stereolithography resins, namely DuPont Somos® 7100 and Somos® 8100.Mechanical Engineerin

Process Planning to Build Mask Projection Stereolithography Parts with Accurate Vertical Dimensions

Mask Projection Stereolithography (MPSLA) is a high resolution manufacturing process that builds parts layer by layer in a photopolymer. In this paper, we formulate a process planning method to cure MPSLA parts with accurate vertical dimensions. To this effect, we have formulated and validated the “Layer cure” model that models the thickness of a cured layer as a transient phenomenon, in which, the thickness of the layer being cured increases continuously throughout the duration of exposure. We have shown that for longer durations of exposures, such as those common with MPSLA systems, cure depth varies linearly with exposure. We have also quantified the effect of diffusion of radicals on the cure depth when discrete exposure doses, as opposed to a single continuous exposure dose, are used to cure layers. Using this work, we have formulated and validated the “Print through” model that computes the extra curing that would occur when multiple layers are cured over each other. We have implemented the Print through model to simulate the profile of a down facing surface of a test part and validated the simulation result by building the test part on our MPSLA system.Mechanical Engineerin

An Evolutionary approach to microstructure optimisation of stereolithographic models.

Abstract- The aim of this work is to utilize an evolutationary algorithm to evolve the microstructure of an object created by a stereolithography machine. This should be optimised to be able to withstand loads applied to it while at the same time minimizing its overall weight. A two part algorithm is proposed which evolves the topology of the structure with a genetic algorithm, while calculating the details of the shape with a separate, deterministic, iterative process derived from standard principles of structural engineering. The division of the method into two separate processes allows both flexibility to changed design parameters without the need for re-evolution, and scalability of the microstructure to manufacture objects of increasing size. The results show that a structure was evolved that was both light and stable. The overall shape of the evolved lattice resembled a honeycomb structure that also satisfied the restrictions imposed by the stereolithography machine.