How does RIM technology complete reaction injection molding?
The RIM reaction injection molding operation is mainly divided into three stages: raw material preparation, mold filling and molding, curing and shaping.
(1) Preparation of raw materials The raw materials in RIM are composed of two-component or multi-component raw material slurries. In addition to resin monomers, fillers and other additives can be added to each group of raw material slurries. Raw material slurries of different properties should be stored in different storage tanks. The storage tanks are equipped with agitators to prevent precipitation of the raw material slurry and heat exchangers to adjust the temperature to keep the raw material slurry in the storage tank in a constant temperature and uniform state. Then, the raw material slurry is metered and output by a metering pump. In order to strictly control the correct proportion of each reaction component, the metering accuracy should not be less than ±1.5%. The metered raw material paddles enter the mixing head through their respective channels. In the mixing head, the high-pressure material and liquid collide with each other at high speed to achieve uniform mixing. In this way, the material to be injected is ready.
(2) The filling point of the material liquid in the mold filling model RIM is that the speed of the material liquid is very high. For this reason, the viscosity of the mixed liquid should not be too high. Too high viscosity is difficult to flow at high speed, and too low viscosity will fill the mold. Troubles: First, the mixture is easy to leak, second, the material flow may entrain air into the mold cavity, which will cause unstable mold filling in severe cases; third, accelerate the chemical reaction process, and generate a large amount of reaction heat, and the temperature rise caused by the reaction heat , The lighter one will increase the molding shrinkage rate of the plastic part, and the heavy one will cause thermal degradation; fourth, it will cause the solid particles in the mixture to settle out during the flow, making the quality of the product unstable. When filling the mold, the mold is closed first, and the mixture is injected into the cavity from the nozzle under pressure. Since the mixture is liquid, the mold is filled from the lower layer and the mold is filled with the liquid level rising, and the gas is from the parting surface The exhaust gap escapes. It should be noted that the material flow must not form turbulence, because the formation of turbulence will cause undesirable air bubbles in the plastic parts and cause defects in the plastic parts.
(3) Curing and setting. After the mixed liquid fills the cavity in a very short time, under the promotion of the hot mold, the curing reaction is accelerated, and the curing and setting can be completed in a short time. Due to the poor thermal conductivity of plastics, a large amount of reaction heat makes the temperature inside the molded object much higher than the surface temperature, so that the curing of the molded object proceeds from the inside to the outside. At this time, the heat exchange function of the mold is mainly for heat dissipation, so that the maximum temperature in the mold cavity is controlled below the degradation temperature of the resin. The curing time of the molded object in the reaction injection mold is mainly determined by the formula of the molding material and the size of the plastic part. For plastic parts that need to be heated and cured, appropriately increasing the heating temperature of the mold can not only shorten the curing time, but also make the inside and outside of the plastic part have a more uniform curing degree. The proper time for ejecting the plastic part from the mold should be carried out after the plastic part has gained sufficient strength and rigidity.