Rapid tooling technology is a new technology developed with the emergence of rapid prototyping technology. Low pressure injection molding of rapid tooling refers to the process in which the bicomponent materials with high chemical activity and low relative molecular weight are mixed in a certain proportion and injected into the closed mold under low pressure (0.4 ~ 0.5MPa), which completes the chemical reactions such as polymerization, crosslinking and curing, and finally forms products after cooling and solidification. It is the process of low pressure perfusion molding (RIM) technology in rapid tooling products molding Field development. The product material is generally polyurethane resin, which has the properties of thermosetting material after molding - the product quality is closely related to the mold structure, filling process, mold temperature, initial temperature of raw materials, material mixing ratio, mixing uniformity, curing time, etc.
Technological process and forming principle of low pressure pouring molding
Low pressure injection molding of rapid tooling refers to the process of injecting polyurethane resin into rapid tooling and curing molding. Silicone mold, polyurethane mold, epoxy resin mold, ABS mold and aluminum alloy mold can be used. Low pressure injection molding equipment is mainly composed of control box, quantitative pump, mixing head and nozzle. The process flow is shown in Figure 1. Under the low pressure of 0.4 ~ 0.5MPa, the two-component polyurethane material is pumped into the mixing head by a quantitative pump, and then injected into the mold after impact mixing. After mixing, the two-component materials react rapidly in the mold and are polymerized and crosslinked. In order to ensure that the material is fully solidified and reduce the deformation of the product, the mold should be moved to the oven for post curing after the injection, and the product can be demoulded after about 1.5 ~ 2.0 hours. Generally, the production cycle of a pair of dies is 3-4 days, which can produce about 100-500 pieces of products. According to the process requirements, the mold and material should be heated before injection. Due to the curing reaction of the two-component material, the temperature of the whole reaction system will rise further and exothermic phenomenon will appear in the initial stage of the molding process.
Polyurethane is the main material used in low pressure injection molding. The physical properties of cured materials are similar to those of ordinary thermoplastic materials ABS, PP or PS. It is suitable for the production of larger wall thickness (> 4mm) and non-uniform wall thickness products
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