Research on the multilayer injection molding of thick-walled polymer optical products

Abstract

Injection-molded polymer optical products have been widely used for their excellent comprehensive properties. However, these products often suffer from deformation during the molding process due to the inherent shrinkage properties of polymer, which is typical defects for large-thickness products. The challenges are further compounded when considering the optical performance, as large-thickness products experience complex historical thermal field changes. Multilayer injection molding was proposed to meet the requirements of products with large thickness dimensions. In this study, the principles of multilayer injection molding were studied. A novel multilayer injection mold capable of sequentially molding products of any desired thickness was designed. The cooling time setting scheme associated with the layering scheme was proposed. Effects of layering scheme on shrinkage and birefringence of multilayer injection molding was investigated. Results show that bidirectional injection can reduce shrinkage deformation by 26.5 % compared to unidirectional injection. A layer thickness of 5 mm ensures low shrinkage and birefringence with the shortest molding cycle. Subsequently, the effects of mold temperature, melt temperature, cooling time and packing pressure were analyzed by orthogonal experiment. Results indicate that cooling time had the greatest impact on both shrinkage and birefringence. Through optimization, shrinkage and birefringence of the samples could be reduced by 20.0 % and 8.3 %, respectively. Finally, an ultra-thick-walled product with a thickness of 100 mm was successfully molded. This will help the development of injection molding of thick-walled polymer optical products.