Two-Dimensional Real-Time Interferometric Monitoring System for Exposure Controlled Projection Lithography

Abstract

Stereolithography is an additive manufacturing process in which liquid photopolymer resin is cross-linked and converted to solid polymer with an ultraviolet light source. Exposure Controlled Projection Lithography (ECPL) is a stereolithographic process in which incident radiation, patterned by a dynamic mask, passes through a transparent substrate to cure a photopolymer layer that grows progressively from the substrate surface. In contrast to existing stereolithography techniques, this technique uses a gray-scale projected image, or alternatively a series of binary bit-map images, to produce a three-dimensional polymer object with the desired shape, and it can be used on either flat or curved substrates. Like most stereolithographic technologies, ECPL works in a unidirectional fashion. Calibration constants derived experimentally are fed to the software used to control the system. This unidirectional fabrication method does not, by itself, allow the system to compensate for minor variations, thereby limiting the overall accuracy of the process. We present here a simple, real-time monitoring system based on interferometry, which can be used to provide feedback control to the ECPL process, thus making it more robust and increasing system accuracy. The results obtained from this monitoring system provide a means to better visualize and understand the various phenomena occurring during the photopolymerization of transparent photopolymers.

A feasibility study on process monitoring and control in vat photopolymerization of ceramics

“…Laser interferometric techniques that rely on refractive index changes upon polymerization have been employed by several researchers to monitor the photopolymerization process, using both Michelson [65,[104][105][106][107][108] and Mach-Zehnder [109,110] configurations. The goals of these systems range from monitoring shrinkage-induced deformations [104,108] to monitoring the degree of cure [107].…”

A simple, inexpensive, real‐time interferometric cure monitoring system for optically cured polymers

“…In addition, because the software associated with the video camera allows any individual pixel within the image area to be selected and displayed over time, the evolution of the interference pattern for any part of the sample can be monitored, as we have previously reported in part. 16 In practice, when using a shaped bitmap to cure only a portion of the region covered by the DMD TM image, it is useful to use full-aperture ICM to visualize exactly where the polymerized shape appears within the frame and to select a pixel within the cured region; the sample chamber can then be translated to bring a fresh portion of resin into the beam from the DMD TM , the aperture can be closed down to a small beam which is positioned over the selected pixel, and the irradiation repeated with the time evolution of the fringes monitored by the video camera. Figures 10 and 11 show excellent examples of the information provided by this approach.…”

Non-destructive quality control methods in additive manufacturing: a survey

“…The cured height was calculated via an interferometer, filtered by a Kalman filter. Additionally, in Jariwala et al (2012) a real-time monitoring system based on interferometry applied in the stereolithographic process has been introduced. Various phenomena aroused during the photopolymerization of liquid resin were captured and visualized.…”