Reaction impingement molding

FIGURE 19.5 Stack and ampoule assembly of a lead/fluoboric acid/Iead dioxide reserve battery supported by potting in epoxy in a molded case (left) and by in situ molding using a reaction impingement molded polyurethane foam (right). (Courtesy of U.S. Department of the Army.)... 

Morganstein, M., and A. B. Goldberg Reaction Impingement Molding (RIM) Encapsulation of a Fuze Power Supply, Proc. of the 4th International SAMPE Electronics Conference, Society for the Advancement of Material and Process Engineering, Covina, CA, pp. 753-764, 1990. 

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. 

Fig. 13.40 Schematic representation of a typical RIM machine. The machine can be divided into three basic parts (1) low-pressure recirculation or conditioning (bounded by the dotted lines) (2) high-pressure metering and (3) the impingement mixhead. The mold is usually considered separately. The figure shows the machine in low-pressure recycle mode. [Reprinted by permission from C. W. Macosko, RIM Fundamentals of Reaction Injection Molding, Hanser, Munich, 1989.]...

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. 

The rapid blending of two steams that is achieved by impinging jet technology, as developed for reaction injection molding by Edwards (1984) was adapted for crystaUization by Midler et al. (1994) and further developed by others [examples Mahajan and Kirwan (1996),... 

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]. 

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