Discussion on the Process of Low Pressure PerfusionLow-pressure perfusion process: monomer or prepolymer in a liquid state by a metering pump to a certain ratio into the mixing head for mixing. After the mixture is injected into the mold, it is rapidly reacted in the mold and crosslinked and cured, and the mold is RIM. This process can be simplified as: storage → metering → mixing → filling → curing → stripping → post-processing.
The two-component stock solution used in the RIM process is typically stored in two reservoirs at a constant temperature, and the reservoir is typically a pressure vessel. In the absence of molding, the stock solution is usually in the 0.2 ~ 0.3 MPa low pressure, in the reservoir, heat exchanger and mixing head in the non-stop cycle. For polyurethane, the temperature of the stock solution is generally 20 ~ 40 ℃, the temperature control accuracy of ± 1 ℃.
The measurement of the two components is usually carried out by the hydraulic system, the hydraulic system consists of pumps, valves and accessories (control of liquid materials, piping systems and control the distribution of the cylinder system). Injection is also required through the high and low voltage conversion device to convert the pressure required for injection pressure. The raw fluid is measured with a hydraulic quantitative pump, requiring a measurement accuracy of at least ± 1.5%, preferably at ± 1%.
In the low-pressure infusion products molding, the quality of the product depends largely on the quality of mixing the mixing head, the production capacity is entirely dependent on the mixing quality of the mixing head. Generally used in the pressure of about 10MPa, in order to obtain a better mixing effect.
The reaction of the injection material is characterized by a high flow rate. For this reason, the viscosity of the stock solution is not too high. For example, the viscosity of the polyurethane mixture is about 0.1 Pa.s. Flow control: 600g / s.
Polyurethane two-component mixture has a 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 plastic, a large number of reaction heat can not be distributed in time, so the internal temperature of the molding is much higher than the surface temperature, resulting in the molding of the curing from the inside out. In order to prevent the cavity temperature is too high (not higher than the thermal decomposition temperature of the resin), should give full play to the mold heat transfer function to distribute heat.
The curing time in the reaction injection mold is mainly determined by the formulation of the molding material and the size of the product. In addition, secondary heat curing is required after the reaction injection product is released from the mold.
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