Investigation of interfacial bonding behavior in multilayer injection molding of optical products

Multilayer injection molded transparent polymers are widely used in a variety of commercial and industrial applications, as key optical component. In multilayer injection molding, as the most typical feature of the process, the fusion interfaces and bonding behaviors between layers is a critical factor influencing the performance of the final product, while the characteristics has not been fully revealed in existing studies. Here, through theoretical and experimental perspectives, we thoroughly investigated the fusion interface characteristics of transparent polymers produced by multilayer injection molding. The fusion interface strength is used as the evaluation index of interface performance, and a theoretical strength model considering the characteristics of the multilayer injection molding process was established based on the molecular diffusion theory. Experimental studies on the fusion interface were conducted using a specially designed multilayer injection mold. Mechanical and microscopic characterizations show that increasing process temperatures, such as insert, melt, and mold temperatures, can significantly improve interface bonding strength. The proposed bonding strength theory can accurately predict experimental results, with the RMS error does not exceed 4 %. Moreover, the correlation between fusion interface properties and optical performance such as transmittance and haze were verified. Based on the above research, ultra-thick-walled optical products with a thickness of up to 30 mm were molded, achieving transmittance and haze values of 82.77 % and 0.05, respectively. The results reveal the properties of multilayer injection molding fusion interfaces of transparent polymers and will aid in the improvement of quality of thick-walled polymer optical products.