| Two-component molding is a novel process for manufacturing polymer products with a sandwich structure or a hollow structure. Typically, two different materials are injected or transferred into a mold sequentially or simultaneously. The skin is generally a prime polymer with required surface and bulk properties for intended use. The core can be solid, foam or gas. Obtaining a uniform encapsulated structure is difficult and there are no science-based rules for optimization of process setup. Thus, a physical model and process simulations have been developed based on the kinematics and dynamics of a moving interface, and Hele-Shaw approximation. The model has incorporated temperature and shear rate dependences of viscosity of both skin and core component into the transient interface evolution. Based on the developed model, simulations have been carried out to study flow rate controlled simultaneous co-injection molding of thermoplastics, pressure-controlled sequential transfer molding of rubber compounds, and gas-assisted injection molding (GAIM). The simulation results were compared with the experimental data, and in general, good agreement was found between the predicted and experimentally measured interface distribution in moldings.; For simultaneous co-injection molding, it is found that material pairs with a broad range of viscosities may be utilized. Breakthrough phenomena are mainly determined by the volume of melt of initial single phase injection and rheological properties of material combinations. When the core has a lower viscosity than the skin, or the volume of initial injection of skin melt is smaller, breakthrough is very likely. However, the breakthrough can be eliminated by controlling injection rate of the skin and core melts. For sequential transfer molding, it is found that the rubber distribution in moldings are dominated by the rheological properties of components and the volume fraction transferred, but independent of the gate pressure. When the core rubber has a lower viscosity than the skin rubber, the core front exhibits more block-shape penetration into the skin rubber. The penetration length of the core into the skin increases with an increase of the volume fraction transferred. For GAIM, it is found that the shot size, injection speed and gas injection delay time have the strongest effect on the gas penetration. A lower injection speed and a longer gas injection delay time lead to a smaller bubble diameter and longer gas penetration. A smaller shot size results in a longer gas penetration, eventually leading to a blow through of the gas. |
