Reaction Injection Molding (RIM) mixes the reactant liquid intermediates as they enter the mold to create a part. Unlike conventional injection molding, reactive injection molding forms solid components by cross-linking or polymerizing in a mold rather than by cooling. This process does not require the use of a hot mold cavity to activate the reaction. In fact, heat is generated when the intermediate is mixed under heat, so the RIM mold usually needs to be cooled. The demolding time of the molded part is generally not more than 20 seconds.
The development of the reaction injection molding process stems from the polyurethane foam technology. Due to the demand of the automotive market, this process has achieved tremendous growth in the United States, and the automotive market is still the main use for reactive injection molding. Advances in this technology have come from improvements in materials and processes used in automotive production. However, reactive injection molding is also used in the medical device industry, including structural foam cabinet components, wheelchair seats, structural components, and reusable foam patient positioning components.
Advantages of reaction injection molding
In general, reactive injection molding can make structural steel parts lighter, such as automotive bumpers and other types of strong resilience plastics used in vibration-proof products or vibration-damping products. With this process, maintenance costs, insurance costs, and fuel costs can be reduced, resulting in significant savings for the owner.
The social benefit of this process is to reduce air pollution. The National Science Foundation (NSF) has been supporting research on reactive injection molding. In the 1980s, software developed by researchers supported by NSF gained commercial success. Union Carbide, DOD and NaSa also provide support.
Reaction injection molding requirements
equipment
The reaction injection molding equipment was originally developed in Germany and is now sold by several manufacturers. The basic components of a reaction injection molding system include: a state conditioning system for preparing liquid intermediates; a metering pump system to ensure pumping of intermediates in a specified amount and pressure; and one or more mixing heads for mixing liquids by hop mixing Intermediate; the mold carrier, the mold is oriented according to the requirements, timely mold opening and closing, easy to clean and demoulding.
Unlike thermoplastic injection molding, reactive injection molding uses a low-viscosity liquid during the filling process and can be filled into parts only by internal pressure. As a result, injection molding can be used with injection molding pressures as low as 50 psi (thermoplastic injection molding pressures of not less than 5000 psi), allowing small machines with limited clamping forces to mass produce larger parts. For the same reason, RIM molds are much cheaper than thermoplastic processes. However, RIM molds made using traditional injection molding standards have a low success rate. RIM molds have unique requirements for filling low viscosity liquids, and molds using other processes are difficult to adapt.
The reaction injection molding has the characteristics of low viscosity, low mold pressure and low mold cost, and is especially suitable for short process production and prototype production. In order to respond to the successful application of injection molding, it is very important to select equipment according to local conditions. The main parameters on which the equipment is selected are: the type of material to be used (eg foam, elastomer, etc.), the dimensional suitability of the part to be produced and the required yield. With the development of technology, the equipment has been improved accordingly. There are many options available in the market, one of which is to use a system consisting of multiple mixing heads and equipment to solve a variety of processing limitations. In terms of equipment, there are also a variety of options, including various mixing head types and sizes, material and mold temperature control, programmable injection time control, and process control warnings.
material
The first material to be injection molded is polyurethane. However, as technology advances, many other materials are now available in reactive injection molding processes. Reaction injection molding can be used to produce flexible foams, rigid foams and solid elastomers, depending on the intermediate used. For example, it reflects that the injection molding process has been used to produce reusable foams that cannot be processed by other processes. The reaction injection molding process has a high degree of flexibility, and customers can solve problems that cannot be solved with other materials such as plastics, rubber and even steel.
Processing parameters
The use of low-viscosity intermediates in reaction injection molding is a major advantage in improving production efficiency, but this process also has its shortcomings. The handling of this reactive or hazardous raw material requires special equipment and procedures, including spill cleanup materials. Operators should wear protective clothing and eyewear, and sometimes even wear an air filter mask. Some materials are easy to freeze at room temperature, so shipping and storage should be done in a temperature-controlled environment.
The process is prone to bubble inclusion during filling, and the mold is difficult to seal, which makes the flash more serious. These problems can generally be solved by careful material selection, careful design of the mold and orientation, injection time and exhaust. Since the low-viscosity material easily penetrates the mold, it is difficult to develop a release agent suitable for reaction injection molding. Moreover, regulations prohibiting the use of certain blowing agents, including chlorofluorocarbons and hydrofluorocarbons, have recently been enacted and, therefore, extensive research is required to find suitable substitutes.
Injection molders must succeed in the long-term use of reactive injection molding processes to master chemical handling, mold manufacturing and processing parameters. These characteristics vary from project to project, resulting in companies that provide reactive injection molding products that experience a longer learning curve.
Factors to be considered in the design
Existing inserts and reinforcements can be used. Reinforcing materials include fiberglass, waste plastic, metal or wood. A filler may be added to improve the flexural modulus of the finished product or to reduce the processing shrinkage.
