Views:8 Author:Site Editor Publish Time: 2021-07-08 Origin:Site
Low pressure perfusion, also known as low pressure reaction injection molding (Reaction Injection Moulding, RIM). It is a new process used in the production of rapid molding products. After mixing two-component polyurethane materials, it is injected into the rapid mold under normal temperature and low pressure environment, and formed through chemical and physical processes such as polymerization, crosslinking, and curing of the material. Products. It has the advantages of high efficiency, short production cycle, simple process and low cost. It is suitable for small batch trial production in the product development process, as well as small batch production, simple structure covering parts and large thick wall and uneven wall thickness production.
Applicable molds: ABS molds (more than 100 pieces); resin molds (more than 300 pieces); aluminum alloy molds (more than 1,000 pieces).
Casting material: two-component polyurethane PU. Currently used materials: HD-PU4210, DCP-RIM, AXSON-RIM 875.
Physical properties: similar to PP/ABS. The product has the characteristics of aging resistance, strong impact resistance, high consistency, and easy loading and unloading.
Injection pressure: about 10MPa. Flow control: 600g/s.
General working time: 15-30 minutes/piece. The demolding temperature can be controlled below 40℃. After demolding, the parts will be cured at 80℃ for 2-4 hours. The performance is best.
Standard copy accuracy: ±0.20mm/100mm.
Pouring sample thickness: the best pouring thickness is 4~6mm, and the maximum pouring thickness is 10mm.
The largest casting workpiece: 2000mmX1200mmX1000mm, 10KG.
The RIM process is as follows: the monomer or prepolymer enters the mixing head in a liquid state through a metering pump at a certain ratio for mixing. After the mixture is injected into the mold, it reacts quickly in the mold, cross-links and solidifies, and becomes a RIM product after demolding. This process can be simplified as: storage → metering → mixing → filling → curing → demoulding → post-processing.
(1) Storage. The two-component stock solution used in the RIM process is usually stored in two reservoirs at a certain temperature, and the reservoirs are generally pressure vessels. When it is not forming, the stock solution usually circulates continuously in the reservoir, heat exchanger and mixing head under the low pressure of 0.2~0.3 MPa. For polyurethane, the temperature of the stock solution is generally 20-40°C, and the temperature control accuracy is ±1°C.
(2) Measurement. The metering of the two-component raw liquid is generally completed by the hydraulic system, which is composed of pumps, valves and accessories (the piping system that controls the liquid material and the oil circuit system that controls the work of the distribution cylinder). During injection, the pressure is converted into the pressure required for injection through a high-low pressure conversion device. The original liquid is measured and output by 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 the molding of RIM products, the quality of the product largely depends on the mixing quality of the mixing head, and the production capacity completely depends on the mixing quality of the mixing head. The generally used pressure is about 10MPa to obtain a better mixing effect.
(4) Filling the mold. The characteristic of reaction injection material filling is that the material flow velocity is very high. For this reason, it is required that the viscosity of the stock solution should not be too high, for example, the viscosity of the polyurethane mixture when filling the mold is about 0.1 Pa.s. Flow control: 600g/s.
(5) Curing. The polyurethane two-component mixture has high reactivity after being injected into the mold cavity and can be cured and set in a short time. However, due to the poor thermal conductivity of the plastic, a large amount of reaction heat cannot be dissipated in time, so the internal temperature of the molded object is much higher than the surface temperature, causing the curing of the molded object to proceed from the inside to the outside. 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 exchange function of the mold should be fully utilized 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, after the reaction injection product is released from the mold, a secondary thermal curing is required.