RIM reaction injection molding and low pressure perfusion

RIM process: Monomer or prepolymer in liquid state through the dosing pump at a certain ratio into the mixing head for mixing. After the mixture is injected into the mold, it quickly reacts in the mold and cross-links to cure, and the RIM product is released after the mold. This process can be simplified as: storage → metering → mixing → filling → curing → stripping → post-treatment. (1) Storage. The two components of the liquid used in the RIM process are usually stored at a certain temperature in two separate reservoirs, which are usually pressure vessels. When it is not formed, the stock liquid is usually circulated continuously in the reservoir, heat exchanger and mixing head at a low pressure of 0.2~ 0.3MPa. For polyurethane, the temperature of the stock solution is generally 20~40℃, and the temperature control accuracy is ±1℃. (2) Measurement. The measurement of the two-component stock liquid is generally completed by the hydraulic system, which is composed of pumps, valves and accessories (piping system for controlling liquid materials and oil circuit system for controlling the work of the distribution cylinder). During injection, the pressure needs to be converted into the pressure required for injection by a high and low pressure conversion device. The bulk liquid is measured with a hydraulic quantitative pump, and the measurement accuracy is required to be at least ± 1.5%, and it is best to control it at ± 1%. (3) Mixing. In RIM product molding, the quality of the product is largely determined by the mixing quality of the mixing head, and the production capacity is completely determined by the mixing quality of the mixing head. Generally, the pressure used is about 10MPa to obtain a better mixing effect. (4) Mold filling. Reaction injection material filling is characterized by a high rate of material flow. For this reason, the viscosity of the stock solution is not required to be too high, for example, the viscosity of the polyurethane mixture when filling the mold is about 0.1Pa.s. Flow control: 600g/s. (5) Curing. The polyurethane two-component mixture has high reactivity after injection into the mold cavity, and can be cured in a very short time. However, due to the poor thermal conductivity of the plastic, a large amount of reaction heat can not be distributed in time, so that the internal temperature of the molding is much higher than the surface temperature, resulting in the solidification of the molding from the inside out. In order to prevent the temperature in the cavity from being too high (not higher than the thermal decomposition temperature of the resin), the heat transfer function of the mold should be given full play to dissipate heat. The curing time in the reaction injection mold is mainly determined by the formula of the molding material and the size of the product. In addition, the reaction injection product needs to undergo secondary heat curing after it is removed from the mold.