Figure 4 - uploaded by Koen Classens
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One-dimensional illustration of the illuminated resin in the domain Ω. The irradiance decays exponentially with respect to the traveled path length in the medium.

One-dimensional illustration of the illuminated resin in the domain Ω. The irradiance decays exponentially with respect to the traveled path length in the medium.

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Article
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Photopolymerization-based Additive Manufacturing (AM), a technique in which a product is built in a layerwise fashion by local curing of a liquid monomer, is increasingly being adopted by the high-tech sector. Nevertheless, industry still faces several challenges to improve the repeatability of product quality, as recognized by several authorities...

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Context 1
... the remainder, a one-dimensional domain Ω is considered as illustrated in Fig. 4. The coordinate z ∈ [0, L] describes the depth coordinate of a material point. The incident light intensity is denoted I in , the top boundary is denoted Ω 0 and the bottom boundary is denoted Ω L . ...
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... the time t ∈ R + , I in (t) ∈ R + and the penetration depth is the depth where the irradiance has reduced to 37% of its initial value [1,2]. Considering the domain Ω, the exponential irradiance attenuation is illustrated schematically in Fig. 4. Note that the incident light intensity, and thus also the actual light intensity, can not be negative. The cumulative energy provided to the resin is described by E = I in (t)dt. The input of this submodel, denoted by Σ I , is the incident light intensity I in (t) and the output the irradiance profile over depth and time I(z, ...
Context 3
... defined in Eq. (35) is chosen as θ R (t) = 1 and ¯ R = 10 −3 . The difference in magnitude of the weights is due to the high magnitude of stresses. To prevent ill-conditioning, the weights are adjusted and stresses are computed in a scaled unit, i.e., MPa in this case. The simulated stress profiles and corresponding control input are depicted in Fig. 14, illustrating that this arbitrary feasible stress profile can be ...

Citations

... Other complex models significantly improved the understanding of the stereolithography process. However, they tend to focus solely on the study of formulation parameters, such as curing time [21], oxygen concentration [22,23], temperature in the resin tank [24], or fillerinduced light scattering in the case of ceramic printing, on numerical approaches that are often static during layer printing [25][26][27][28]. Thus, while existing studies are presented as key tools for understanding, their limited experimental validation and the over-simplification of the resin formulation sometimes makes them restricted predictive tools. ...
Article
Full-text available
Controlling the precision and mechanical cohesion of 3D-printed parts remains a central concern in the development of additive manufacturing. A two-dimensional finite element model of the photopolymerization of a complex sensitive resin, formulated with three monomers to better represent commercial resins, in a stereolithography apparatus is proposed as a tool for predicting and optimizing formulation of photosensitive resin and its printing parameters. By considering light illumination, chemical reaction, and heat transfer in a resin exposed to a moving ultraviolet (UV) laser source, this first approach accounts for monomer-to-polymer conversion and polymerization rate in agreement with experimental results obtained by Fourier-transform infrared spectroscopy (FT-IR) monitoring and the use of semi-empirical models. The temperature gradient along the exposed photosensitive material was also estimated. By varying the photoinitiator content and simulating the addition of an absorbing filler via the molar extinction coefficient, it was shown that a higher photoinitiator concentration and the presence of strongly absorbing fillers lead to a reduction in the light penetration depth, which can result in structural defects without adaptation of the layer thickness to be printed.
... The modern version of the technique was developed in the 1980's [1,2] and since that time has grown rapidly in numerous market sectors including dental, automotive, and medical markets. The process of VP printing involves a complex interplay of different physical processes [3][4][5][6] (chemical kinetics, heat, optics, diffusion, etc.) and a ✩ Contribution of NIST, an agency of the US government; not subject to copyright in the United States. ✩✩ Commercial equipment, instruments, or materials are identified only in order to adequately specify certain procedures. ...
... The modern version of the technique was developed in the 1980's [1,2] and since that time has grown rapidly in numerous market sectors including dental, automotive, and medical markets. The process of VP printing involves a complex interplay of different physical processes [3][4][5][6] (chemical kinetics, heat, optics, diffusion, etc.) and a ✩ Contribution of NIST, an agency of the US government; not subject to copyright in the United States. ✩✩ Commercial equipment, instruments, or materials are identified only in order to adequately specify certain procedures. ...
... The proposed numerical model was validated with the experimental results, which can be used to develop control strategies for liquefier heaters. For the vat photopolymerization process, Classens et al. [501] developed a modular control-oriented model to describe the multiphysical vat photopolymerization process. Moreover, a tracking control strategy based on the proposed model was used to optimize the material properties produced through vat photopolymerization. ...
Article
Lightweight materials and structures have been extensively studied for a wide range of applications in design and manufacturing of more environment-friendly and more sustainable products, such as less materials and lower energy consumption, while maintaining proper mechanical and energy absorption characteristics. Additive manufacturing (AM) or 3D printing techniques offer more freedom to realize some new designs of novel lightweight materials and structures in an efficient way. However, the rational design for desired mechanical properties of these materials and structures remains a demanding topic. This paper provides a comprehensive review on the recent advances in additively manufactured materials and structures as well as their mechanical properties with an emphasis on energy absorption applications. First, the additive manufacturing techniques used for fabricating various materials and structures are briefly reviewed. Then, a variety of lightweight AM materials and structures are discussed, together with their mechanical properties and energy-absorption characteristics. Next, the AM-induced defects, their impacts on mechanical properties and energy absorption, as well as the methods for minimizing the effects are discussed. After that, numerical modeling approaches for AM materials and structures are outlined. Furthermore, design optimization techniques are reviewed, including parametric optimization, topology optimization, and nondeterministic optimization with fabrication-induced uncertainties. Notably, data-driven and machine learning-based techniques exhibit compelling potential in design for additive manufacturing, process-property relations, and in-situ monitoring. Finally, significant challenges and future directions in this area are highlighted. This review is anticipated to provide a deep understanding of the state-of-the-art additively manufactured materials and structures, aiming to improve the future design for desired mechanical properties and energy absorption.
... At present, the studies on the curing mechanism of ceramic vat photopolymerization [41,[90][91][92][93][94][95][96][97][98][99][100][101][102][103] mainly include: 1. adapting the classic resin Jacobs vat photopolymerization model to analyze the relationship between light intensity and curing thickness, and the relationship between light intensity and lateral curing accuracy. 2. using the vat photopolymerization chemical reaction model to study the changes of the chemical compositions in the vat photopolymerization reaction, or using the numerical analysis method [104,105] to analyze the time-space changes of the chemical compositions; 3. using the mechanical finite element analysis to calculate the internal stress and deformation of the green part. ...
... Suitable process parameters were suggested to avoid the warping of the green part. K. Classens [41] proposed a modular multiphysics model for ceramic vat photopolymerization. ...
Article
The developments in ceramic vat photopolymerization technology have the potential to revolutionize the manufacture and application of complex and functional ceramics. At present, there are still defects in parts fabricated by ceramic vat photopolymerization, which are caused by selected materials, layer-by-layer printing, and part processing. These defects restrict the shaping accuracy and performance of ceramic parts and hinder the further application and development of this technology. It is necessary to systematically review the generation and control of defects in ceramic vat photopolymerization. This review summarized the defect types (geometry-related defects and performance-related defects), defect detection methods, theoretical analysis and numerical modeling simulation of defects, defect generation mechanisms, defect control, and innovative methods of ceramic vat photopolymerization. The feasible methods of defect control and performance improvement were proposed, including particle coating, slurry improvement, new green part processing methods, novel printing manners, composite ceramic or precursor, optimized debinding and sintering. The main challenges (caused by both additive printing and traditional processes; caused by the light-curing principle itself; and the right application for various ceramic vat photopolymerization technologies) were discussed, and future research directions for ceramic vat photopolymerization had prospected.
... Over the past several decades, VP has become commonplace and is capable of creating complex parts for a diverse range of applications [3][4][5]. While the original basis for VP was laid out in the early 1980's and is simple in concept [6,7], the process of VP involves a complex interplay of different topics including, but not limited to: optical physics, optical engineering, polymer/chemical formulation, photopolymerization kinetics (involving thermal, chemical, and material property gradients in time and space), and mechanical engineering (i.e., implementing a printer) [8][9][10][11][12][13]. It is a testament to the success and practicality of VP that it has achieved widespread success despite many of these topics not being thoroughly understood in the full context of a realistic/practical VP printer. ...
... Only by having a clear understanding of the spatial heterogeneity in the current generation of light engines can future generations of printers be designed to become more reproducible. In addition, theoretical multiphysics models require more realistic descriptions of the light engines if accurate predictions are to be made [10][11][12][13]. ...
Article
Full-text available
Vat photopolymerization (VP) is a rapidly growing category of additive manufacturing. As VP methods mature the expectation is that the quality of printed parts will be highly reproducible. At present, detailed characterization of the light engines used in liquid crystal display (LCD)-based VP systems is lacking and so it is unclear if they are built to sufficiently tight tolerances to meet the current and/or future needs of additive manufacturing. Herein, we map the irradiance, spectral characteristics, and optical divergence of a nominally 405 nm LCD-based VP light engine. We find that there is notable variation in all of these properties as a function of position on the light engine that cause changes in extent of polymerization and surface texture. We further demonstrate through a derived photon absorption figure of merit and through printed test parts that the spatial heterogeneity observed in the light engine is significant enough to affect part fidelity. These findings help to explain several possible causes of variable part quality and also highlight the need for improved optical performance on LCD-based VP printers.
... In stereolithography process, this conversion rate depends on the laser intensity I received by the slurry during the whole printing and also kinetic variables linked to the paste composition. The model is defined using a m th order reaction kinetic as a function of three spatial coordinates and time [24]: ...
... Where is the maximum achievable degree of conversion ( 1) due to the reduction of reactive species mobility upon the curing, is the initial conversion rate at 0 and b an exponent constant. In order to simplify the simulation model, value is zero, assuming an initial 0% conversion of monomers and the rate constant r is considered temperature independent [24]. This last assumption could induce differences in conversion rate evolution, by not considering the early rise of the reaction rate due to the increasing concentration of radical species during initiation step. ...
... Thermal conductivity of both photopolymerizable and cured paste are measured thanks to the hot-disk method. Density and thermal capacity of the mixture are recalculated using values for pure resin and alumina [19,24]. This theoretical density has been confirmed by measurement on a cured part. ...
Article
To meet industry’s expectations for manufacturing ceramic parts by stereolithography, a better comprehension of the process, in particular laser scattering through the ceramic slurry is mandatory. This knowledge makes it possible to define adapted printing conditions to control the dimensions, homogeneity of the conversion and mechanical properties of the green parts, in order to achieve better resolutions and optimize the properties of sintered parts. This approach is focused on the development of a 3D polymerization modeling for stereolithography process able to predict curing and associated thermal phenomena. First, a design of experiments is carried out to identify material-dependent parameters, calibrate and validate the model, then able to predict monomer conversion rates and dimensions after curing depending on manufacturing parameters. Finally, temperature variation and exposure homogeneity have been evaluated. These results will allow, in future studies, to interpret the differences of deformations and mechanical properties of green parts.
... Its slow measurement rate cannot support the real-time monitoring need. These state-of-the-art methods of AFM and FT-IR are expensive, intrusive, time-consuming, and limited in lab-scale research only; thus not machine-implementable for practical in-situ real-time VPP process monitoring [24][25][26]. Therefore, an efficient and feasible in-situ measurement system is needed to quantify material properties particularly the mechanical properties (such as Young's modulus and viscoelasticity) and the DoC to realize real-time control for VPP process and printed part quality [26]. ...
... These state-of-the-art methods of AFM and FT-IR are expensive, intrusive, time-consuming, and limited in lab-scale research only; thus not machine-implementable for practical in-situ real-time VPP process monitoring [24][25][26]. Therefore, an efficient and feasible in-situ measurement system is needed to quantify material properties particularly the mechanical properties (such as Young's modulus and viscoelasticity) and the DoC to realize real-time control for VPP process and printed part quality [26]. ...
Article
Vat Photopolymerization(VPP) additive manufacturing VPP processes employ photopolymerization reactions to crosslink monomers layer by layer under light exposure schemes corresponding to the cross-sections of a target object. Such processes have been widely used in various applications, from rapid prototyping to biomedical implants, soft robotics, and flexible electronics. In-situ process monitoring is critical for process optimization and control to achieve precise structures and desired properties via VPP. As existing research focuses on the online measurement of part geometry, there lack in-situ monitoring technologies to obtain real-time information about the curing and mechanical properties of VPP printed parts, especially the degree of conversion (DoC) that greatly affects the mechanical properties of as-printed and finished parts. This work demonstrates for the first time an in-situ ultrasonic monitoring (IUM) method, developed based on ultrasonic testing methods for DLP process monitoring. The developed method is simplistic, rapid, versatile, cost-effective, and non-invasive compared to the state-of-the-art methods such as in-situ Fourier-Transform Infrared Spectroscopy and Atomic Force Microscopy that are adapted to monitor VPP. An IUM framework with specific data analytics methods is developed to process the real-time acquired ultrasonic signal for evaluating the evolving Young’s modulus and DoC of the as-printed part during VPP. An experimental study is performed to exemplify that the developed IUM method can vividly reveal the process dynamics under different exposure intensities and probe the part properties evolution with adequate sensitivity and accuracy. This novel IUM method will offer unique insights into process dynamics and process-property relationships for VPP processes modeling and real-time feedback control, facilitating 3D and 4D printing of sophisticated products such as soft robots that require localized manipulation of mechanical properties.
... A precise mathematical description of the chemical-physical phenomena involved in the AM process, whose applicability and generality can be easily appreciated, represents a soundness and scientifically-based alternative to empirical approaches [21][22][23]. In photopolymerization-based AM technologies, the main foundation of the theoretical approach is represented by the mathematical description of the kinetics of the process, typically formulated through a set of partial differential equations providing the evolution of the concentration of reactant variables involved in the curing process [24]. ...
Article
Full-text available
Photopolymerization, based on light-induced radical polymerization, is nowadays exploited in additive manufacturing (AM) technologies enabling to achieve high dimensional quality. The mechanical properties of the obtained material are heavily dependent on the chemistry of the photopolymer and on the way the AM process is performed. Here we study, through experiments and theoretical modeling, how the mechanical properties of liquid crystal shutter (LCD) printed photopolymers depend on the printing process setup, namely UV exposure time and layer thickness. To this end, a multi-physics simulation tool considering the light diffusion, chemical kinetics, and the micro-mechanics at the network level, has been developed. © 2022 The Author(s). Published with license by Taylor and Francis Group, LLC.
... There are some examples of efficient integrations of feedback control loops in the irradiation system in SL equipment (Hafkamp et al., 2019;Hafkamp, 2020;Classens et al., 2021). However, the need for high accuracy and resolution of the sensing techniques for the on-the-fly measuring of the liquid surface topography introduce principal difficulties for the realization of real-time feedback control in the resin deposition process. ...
Article
Purpose This study aims to examine the feasibility of feedforward actuation of the recoater blade position to alleviate the resin surface non-uniformity while moving over deep-to-shallow transitions of submerged (already cured) geometric features. Design/methodology/approach A two-dimensional computational fluid dynamics (CFD) model has been used to determine optimized blade actuation protocols to minimize the resin surface non-uniformity. An experimental setup has been designed to validate the feasibility of the proposed protocol in practice. Findings A developed protocol for the blade height actuation is applied to a rectangular stair-like configuration of the underlying part geometry. The evaluation of the actuation protocol revealed the importance of two physical length scales, the capillary length and the size of the flow recirculation cell below in the liquid resin layer below the blade. They determine, together with the length scales defining the topography (horizontal extent and depth), the optimal blade trajectory. This protocol has also shown its efficiency for application to more complicated shapes (and, potentially, for any arbitrary geometry). Practical implications This study shows that incorporation of a feedforward control scheme in the recoating system might significantly reduce (by up to 80%) the surface unevenness. Moreover, this improvement of performances does not require major modifications of the existing architecture. Originality/value The results presented in this work demonstrate the benefits of the integration of the feedforward control to minimize the leading edge bulges over underlying part geometries in stereolithography.