Reaction-injection molding impingement mixing

The largest segment of the CASE family of polyurethanes are elastomers. Cast polyurethane elastomers reached a new dimension when high pressure impingement mixing led to reaction injection molding (RIM). This technology is used widely in the automotive industry, and reinforced versions (RRIM) and stmctural molded parts (SRIM) have been added in more recent years. 

Reaction injection molding (RIM) is a fast, low-pressure, low-temperature, low-cost process for one-step conversion of reactive liquids into large finished solid plastic products. Liquid polyol and liquid diisocyanate are mixed by impingement, pumped instantly to fill a large mold cavity, and polymerize/ cure rapidly to form a thermoset polyurethane product. The cured polymer may be a stiffly flexible product such as automotive bumper covers, front ends, and trim or a rigid foamed product such as furniture and housings (cabinets) for computers, business machines, TY and radio. 

It is different to reaction injection molding (RIM) where it uses a mechanical mixing rather than a high-pressure impingement mixer (Figures. 11.1 and 11.2). The entire shot is mixed in a chamber before injection into the mold, rather than being continuously mixed and injected, as in the RIM process. LIM is used to mold smaller parts that are below the desired capacity of RIM. LIM also allows higher-viscosity materials to be processed. 

For our purposes, reaction injection molding is defined as a process in which two or more reactive streams are combined and mixed with an impingement mixing device and injected into a closed mold where they polymerize to rapidly form a plastic part. To compensate for polymerization shrinkage, RIM parts are foamed to some degree. 

In many instances, one wishes to produce solid parts so as to maximize the toughness of the product. In some cases, the economics of the busines will not permit the high cost and process sophistication of reaction injection molding equipment. Low-pressure casting is then the process of choice. Casting machine prices are usually 20 to 50% of those of RIM machines. They are typically smaller and simpler machines. They use static or dynamic mixers rather than impingement mix heads. 

Two-impinging-jets are also used in reaction-injection-molding processing equipment to provide good micromixing for viscous fluids [3-6]. It is one of the techniques used to reduce the timescale of micro-mixing in precipitators [6]. 

Reaction injection molding creates its own resin by mixing two highly reactive liquid components which react with each other by high-pressure impingement. The reaction is completed shortly after the resin is injected into the mold. This is accomplished at a pressure of 100 Ib/in, which permits the use of electro-formed and vapor-formed nickel lined epoxy, aluminum, and kirksite tools at a considerably reduced tooling cost. (Machined and plated cast steel molds can also be used.) As a closed-mold process, both sides of the part are finished and can have textures, raised numbers, and raised letters, but molded-in labels are not feasible. The finished part can be trimmed in the mold. 

Reaction-injection-molded PU offers a solution for the rapid production of complex plastic parts directly from low-viscosity monomers (single chemical unit) and oligomers (chain of multiple chemical units). These liquids are combined by impingement mixing as they enter the mold. 

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