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. Quantification of residual stresses through experimental measurement, and finite element modeling is of interest in this research.Residual stress negatively affects the quality of optical products and is a difficult problem to solve. A schematic pattern of distribution of residual stress due to collision of a single particle with the substrate is shown. Due to the self-equilibrated nature of residual stress (i.e., zero net internal forces), associated with the compressive residual stress at and near the surface, there is tensile residual stress through the depth to balance off the residual force. Such stresses are beneficial for fatigue life of the coated part. The dent resulted from particle impact causes tension at the surface which upon unloading (adhesion of particle to surface) creates compressive residual stresses at and near the surface. Due to the localized effect of the collision, the area went under plastic deformation is surrounded by a large elastic domain resulting in formation of local residual stresses. Figure depicts such plastic zone formation schematically. The high particle impact velocity causes high local stresses which lead to plastic deformation in the substrate in the proximity of the particle-substrate interface. A by-product of cold spray coating is the formation of residual stress due to the peening effect of the particles collision with the substrate. Cold spray coating uses high particle velocity and impact energy to enable material coating on a substrate at a relatively low temperature.
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