Originally published April 1996
Fred T. Wickis, Jr.
Evergreen Molding, Greenville, SC
Reaction injection molding (RIM) creates parts by using impingement mixing to combine reactive liquid intermediates as they enter a mold. RIM differs from traditional injection molding because it forms solid parts by cross-linking or polymerization in the mold rather than by cooling. The process does not use hot mold cavities to activate the reaction; in fact, RIM molds must often be cooled because an exotherm forms when the intermediates are mixed in the presence of heat. Completed parts can often be demolded in less than 20 seconds.
RIM developed from polyurethane foam technology. The needs of the automotive market spurred major growth of the process in the United States, and this market remains the primary application for RIM. Most of the technology's advances have resulted from efforts to improve the materials and processes for use in automobile production. However, there are some significant applications for RIM in the medical device industry, including structural foam cabinet parts, wheelchair seating and structural parts, and reusable foam patient positioners.
Equipment. Equipment for RIM was first developed in Germany and is now sold by several different manufacturers. The basic elements of a RIM molding system include a conditioning system that prepares the liquid intermediates for use, a metered pumping system that ensures delivery of the intermediates in appropriate quantity and pressure, one or more high-pressure mixing heads where the liquid intermediates are combined through impingement, and a mold carrier that orients the mold as required and opens and closes it for cleaning and demolding.
Unlike thermoplastic molding, RIM uses liquids that have a low viscosity during mold filling, and fills out the part using only internally generated pressure. Consequently, molding pressures in RIM can be as little as 50 psi (compared with 5000 psi or more for thermoplastic molding), making it possible for small machines with very limited clamping force to produce even relatively large parts in large numbers. For the same reasons, RIM molds are typically much less expensive than those used in thermoplastic processing. However, RIM molds made using the criteria for traditional injection molding are often unsuccessful. Molds for RIM have unique requirements related to the low-viscosity liquids they are filled with, and molds from other processes are rarely adaptable.
Low viscosity, low mold pressure, and inexpensive mold costs make RIM attractive for short production runs and prototyping. Selection of equipment appropriate to the anticipated application is critical for successful use of RIM. Key parameters for equipment selection include the type of material to be used (e.g., foam, elastomer), suitability for the size of parts being manufactured, and the desired throughput. As development of the technology has accelerated, corresponding equipment improvements have been made. Options now available include systems that incorporate multiple mixing heads and equipment for a range of processing limitations. Equipment options include various types and sizes of mixing heads, temperature controls for both materials and molds, programmable shot time controls, and process control alarms.
Materials. The earliest use of RIM was with polyurethane, but advances with the technique now offer opportunities for use with many other materials. Depending on the intermediates used, RIM can be used to produce soft foams, rigid foams, and solid elastomers. For example, RIM technology has produced a reusable foam that would have been impossible to produce with any other technology. The flexibility of the RIM process can enable companies to resolve problems unsolved by attempts using other materials such as plastic, rubber, or even steel.
Processing Parameters. Although RIM's use of low-viscosity intermediates is an advantage for productivity, it also has some disadvantages. Handling of such reactive or hazardous raw materials requires special equipment and procedures, including spill-cleanup materials. Gowning for operators should include protective coverings, eye protection, and sometimes air filtration masks. Since some components freeze at room temperature, a temperature-controlled environment is required for their shipping and storage.
Gas bubbles can be trapped during filling, and molds can be difficult to seal, which increases flash. Such problems can normally be overcome by careful attention to materials selection, mold design and orientation, shot time, and venting. Because the low-viscosity materials generally penetrate molds, the development of mold release agents for use in RIM has been difficult. Also, the recent exclusion of certain blowing agents (such as chlorofluorocarbons and hydrochlorofluorocarbons) has created the need for extensive research to find suitable replacements.
Successful molders must use RIM long enough to learn all the peculiarities of chemical manipulation, mold manufacture, and processing parameters. These typically differ for each project, resulting in a long learning curve for the few companies that choose to offer RIM-produced products.
Design Considerations. Inserts and reinforcement materials can be readily used. Reinforcement materials may include fiberglass, scrap plastics, metal, or wood. Fillers may also be added to improve the flexural modulus of the finished product or reduce its shrink rate in processing.