Optimization of compression molding for double-concave lenses

Most lenses on the market can be made by injection molding. However, defects such as weld lines and residual stress are often generated during the production of concave products. Compression molding eliminates these problems, and the cost of the required molding machinery is relatively low compared to other processes. The process can also be highly automated, which is essential for plastic manufacturing. This study proposes an innovative mold for compression molding for a double-concave lens. There is no requirement for angular alignment because the lens is axisymmetric. A compact compression mold without leader pins is created. Parameter optimization for the powder and granular PMMA is optimized using the Taguchi method. The significant compression molding parameters are the pre-pressing period, the pressing temperature, the pressing force, and the pressing period. The surface profiles of finished products are measured, and a factor response analysis is used to determine the effect of various parameters on the finished product. The pressing force was shown to be the most significant factor for powder. However, granules need a long enough pre-pressing time because the gap widens. A confirmation experiment was conducted with the optimized parameters. The profile of the compressed double-concave lens is measured and compared with the mold insert. Powder PMMA is more suitable for compression molding than granular PMMA. However, regardless of the material type, the compression-molded double-concave lens is 96% replicable or more.