Compared with traditional injection molding, co-injection molding of two materials has the following advantages: 1. Low-viscosity materials can be used as the core material to reduce the injection pressure. 2. Considering environmental protection, recycled core materials can be used. 3. According to the application characteristics, soft materials are applied to the outer layer of thick finished products, hard materials or foam materials are applied to the core so as to reduce the weight. 4. Low-quality core materials can be used to reduce the cost. 5. Expensive and special property of materials can be applied to the outer layer or the core, such as materials which prevent electromagnetic interference and high-conductivity materials to increase the performance of the product. 6. Proper outer and core materials can reduce the residual stress of finished products and increase the mechanical strength or product surface properties. 7. Products with marble grain can be produced.

Double-color injection mold is popular on the market. With this kind of process, the appearance of products is more beautiful, the color is changed more easily without coating, while this process is expensive and needs high technical requirements.