Cure depth control for complex 3D microstructure fabrication in dynamic mask projection microstereolithography

Abstract

Purpose -The paper's aim is to explore a method using light absorption for improving manufacturing of complex, three-dimensional (3D) micro-parts with a previously developed dynamic mask projection microstereolithography (MSL) system. A common issue with stereolithography systems and especially important in MSL is uncontrolled penetration of the ultraviolet light source into the photocrosslinkable resin when fabricating down-facing surfaces. To accurately fabricate complex 3D parts with down-facing surfaces, a chemical light absorber, Tinuvin 327e was mixed in different concentrations into an acrylate-based photocurable resin, and the solutions were tested for cure depths and successful micro-part fabrication. Design/methodology/approach -Tinuvin 327 was selected as the light absorber based on its high absorption characteristics (, 0.4) at 365 nm (the filtered light wavelength used in the MSL system). Four concentrations of Tinuvin 327 in resin were used (0.00, 0.05, 0.10, and 0.15 percent (w/w)), and cure depth experiments were performed. To investigate the effects of different concentrations of Tinuvin 327 on complex 3D microstructure fabrication, several microstructures with overhanging features such as a fan and spring were fabricated. Findings -Results showed that higher concentrations of Tinuvin 327 reduced penetration depths and thus cure depths. For the resin with 0.15 percent (w/w) of the Tinuvin 327, a cure depth of ,30 mm was achieved as compared to ,200 mm without the light absorber. The four resin solutions were used to fabricate complex 3D microstructures, and different concentrations of Tinuvin 327 at a given irradiance and exposure energy were required for successful fabrication depending on the geometry of the micro-part (concentrations of 0.05 and 0.1 percent (w/w) provided the most accurate builds for the fan and spring, respectively). Research limitations/implications -Although two different concentrations of light absorber in solution were required to demonstrate successful fabrication for two different micro-part geometries (a fan and spring), the experiments were performed using a single irradiance and exposure energy. A single solution with the light absorber could have possibly been used to fabricate these micro-parts by varying irradiance and/or exposure energy, although the effects of varying these parameters on geometric accuracy, mechanical strength, overall manufacturing time, and other variables were not explored. Originality/value -This work systematically investigated 3D microstructure fabrication using different concentrations of a light absorber in solution, and demonstrated that different light absorption characteristics were required for different down-facing micro-features.

Mitigation and control of the overcuring effect in mask projection micro-stereolithography

“…This presents a new approach to solving this problem while building on previous research which focused mainly on improvements in hardware [8,9] and resin chemistry [6,7]. Future work will include the creation of an STL file modification program.…”

“…Most recently Tumbleston et al [9] made improvements to SL DLP hardware by introducing an oxygen permeable build platform which inhibits polymerization and enables 'layerless' fabrication at faster build speeds. A second more widely researched approach involves altering the SL resin material with a chemical light absorber [6][7][8]. In this work we develop a third approach in mitigating for the overcuring effect by simulating the process and predicting the level of overcuring likely to occur.…”

“…This effect is most prevalent in channels or features oriented horizontally (in a parallel plane to that of the build platform). Currently there are two main approaches in controlling the cure depth; 1. the chemical approach, which involves doping the resin material with a chemical light absorber [6][7][8]; and 2. by improving the system's hardware and optical elements to improve the homogeneity of the light dosage and control the cure depth [9]. Here we investigate a third approach through modification of the 3D CAD file prior to printing to mitigate for UV light leakage from successive build layers.…”

Combined micro and macro additive manufacturing of a swirling flow coaxial phacoemulsifier sleeve with internal micro-vanes

“…Vector-by-vector µSL, also referred to as scanning µSL, uses a scanning focused laser beam (Ikuta and Kirowatari 1993;Zissi et al 1996;Sun and Zhang 2002;Lee et al 2007. Integral µSL, also referred to as projection µSL, projects a 2D image onto the resin from a dynamic mask (Bertsch et al 1997(Bertsch et al , 2001(Bertsch et al , 2004Sun et al 2005;Choi et al 2006Choi et al , 2009aLimaye and Rosen 2007;Ha et al 2008;Han et al 2008).…”

“…In this work, a Digital Micromirror Device (DMD)-based µSL system was used to produce the sleeve with the micro-vanes (Choi et al 2009a(Choi et al , b, 2010. Figure 1 shows the µSL system, which consists of an OmniCure™ S2000 as a light source with an output of 200 W, a collimating lens, and an optical fiber (EXFO Co., Canada); a DMD with ∼780,000 micromirrors with the size of 13.68 µm (DMD Discovery™ 1100 Controller board and Starter Kit, Texas Instruments, USA); a tube lens with a focal length of 120 mm (Achromat doublet lens, MellesGriot Co. USA); an aluminum-coated reflecting mirror; a focusing unit (IM-4, Nikon Co., Japan); an objective lens (CFI Plan Flour 4×, Nikon Co., USA); a motor-driven linear Z-stage with a resolution of 1 µm (ATS100-050, Aerotech, USA); and a stainless steel resin vat.…”