![]() Various sensors are used to quantify the pressure, temperature, and deformation of a mold or an injection machine. The injection molding process for optical products requires sophisticated optimization, such as strict control of the injection process and monitoring of the molding process. Annealing treatment can reduce the residual stresses in a finished product. The process parameters that strongly influence the residual stress are the melt temperature, injection time, packing time, and mold temperature. Moreover, the optimization of process parameters through a Taguchi experiment can effectively reduce the amount of residual stress generated. For example, the residual stress can be effectively reduced by optimizing the mold gate design to improve the flow of molten glue and by improving the water circuit design to make the cooling rates of finished products more consistent. Optimization of residual stresses, modification of mold design, optimization of the production process, and annealing treatment are common responses. With the development of computer-aided simulation and analysis techniques, mold flow analysis programs have been employed for high-accuracy prediction of residual stress, ,. Nontransparent parts are often inspected after partial material removal (delamination and drilling) and chemical coating have been performed. The residual stress of transparent parts, which are often examined using a polarizing strain gauge, is measured using a photoelasticimeter. This inconsistency causes compression and tension stresses between the skin and core layers inside a product. Heat stress is mainly caused by an inconsistency in the cooling rates of the molten plastic skin and core layer during the cooling process. If overfilling and overpressing occur during the packing stage of the injection molding process, molecules are easily compressed, which results in the production of high stresses and large volume shrinkage differences in different areas of the finished product thus, the resulting product’s quality is poor. The molecular chains in the skin layer of a finished product are subjected to compressive stress in the direction of the product’s thickness however, the inner core layer is subjected to tensile stress. This phenomenon is affected by the shearing effect when the molten plastic is flowing. During the filling stage of injection molding, the long chains of plastic molecules are stretched and aligned. The residual stress occurring during the injection molding process is mainly divided into two categories: flow stress and heat stress. This force is called the residual stress. While a product gradually solidifies during the molding process, its molecules attempt to return to their original state, which results in the generation of a force inside the product. Residual stress is mainly caused by the long chain-like molecules of plastic materials, which become deformed because of shear stress, pressure, and shrinkage during the injection process. Residual stresses generally tend to be concentrated at the corners of finished products, areas with drastic thickness variation, and areas near the gates of molds. The internal polarization characteristics of optical products change with the level of residual stress, and certain polarization can result in birefringence caused by different rates of light passing through products, the cracking of finished products because of material and environmental factors, and warpage deformation of finished products because of the stress distribution in different areas of the products. In the injection molding process, product quality is negatively affected if the residual stress is excessively high. This monitoring method can provide a new concept of online monitoring technology for the injection molding of optical products. Therefore, the online monitoring of residual stress near the gate is feasible. When a single sensor was used to measure process variables, correlation coefficients of between 0.48 and 0.59 were obtained, and when multiple sensors were used, correlation coefficients of between 0.80 and 0.92 were obtained, which indicated strong correlation. The study found that residual stress near the gate could not be accurately measured with only a single sensor because measurements were susceptible to interference with process parameters. This study proposes a method for the online monitoring of residual stress near the gate of a mold using multiple pressure sensors to measure process variables and verifies the feasibility of online monitoring. Therefore, the online monitoring of critical process variables is necessary for stabilizing product quality and reducing manufacturing cost. Regulating process parameters is a critical element in quality control. Residual stress negatively affects the quality of optical products and is a difficult problem to solve.
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