Reaction injection molding process

The sandwich-type stmcture of polyurethanes with a smooth integral skin produced by the reaction injection molding process provides a high degree of stiffness as weU as exceUent thermal and acoustical properties necessary for its use in housing and load-bearing stmctural components for the automotive, business machine, electrical, furniture, and materials-handling industry. 

Siloxane containing polyester, poly(alkylene oxide) and polystyrene type copolymers have been used to improve the heat resistance, lubricity and flow properties of epoxy resin powder coatings 43). Thermally stable polyester-polysiloxane segmented copolymers have been shown to improve the flow, antifriction properties and scratch resistance of acrylic based auto repair lacquers 408). Organohydroxy-terminated siloxanes are also effective internal mold release agents in polyurethane reaction injection molding processes 409). 

The Reaction-Injection-Molding process is schematically shown in Figure 6. In this case, the two monomeric or low molecular weight reactants, a polyisocyanate and a polyol, are accurately metered through separate pumps and mixed in a very small mix-... 

Figure 6. Schematic of the "reaction-injection-molding process A and B Raw material tanks (1) and (2) Mold (3) Reinforcements (4)...

There are no generalized models that include all these variables for thermosetting polymers. However, extensive work has been done on the viscosity of polyurethanes [9, 10] used in the reaction injection molding process. An empirical relation which models the viscosity of these mixing-activated polymers, given as a function of temperature and degree of cure, is written as... 

For the flow of polymer melts, the Reynolds number, Re = pUch/r], is usually 10 5, with the exception of the reaction injection molding process, RIM, where Re —> 1 — 100 at the gate. The geometric and dynamic conditions of the lubrication approximation, applied... 

Different casting processes are used. They tend to overlap and could be identified by other processing methods. An example is liquid injection molding that can be identified as injection molding (Chapter 4) or reaction injection molding (Chapter 12). Many decades ago the reaction injection molding process was initially called liquid injection molding. 

Snyder, C. D., Materials for Reaction Injection Molding Processing, Composite Fabrication Assoc., Oct. 3-6, 2001. 

Palmosina, M. F., Gating for the Reaction Injection Molding Process, Bayer Corp., Pittsburgh, PA, 2002. 

FIGURE 14-54 Schematic diagram illustrating the reaction injection molding process. 

To date, low volumes of materials have been produced commercially from norbomene and cyclo-octene. Numerous products are expected to result from the materitd produced by the ROMP of dicyclopentadiene in a RIM (reaction injection molding) process. In a RIM process, two streams of a monomer are mixed in the mold where it is polymerized to the final part. In this case, one of the monomer streams contains a tungsten complex while the second contains an alkyl aluminum activator. When the two streams of dicyclopentadiene are mixed, the metathesis catalyst is formed and the monomer is ROMP polymerized (equation 12). 

The latest trend in foamed composites is automotive structural parts produced by the SRIM (structural reaction-injection-molding) process. The SRIM process is a method of the reactive liquid injection molding (i.e., LIM). LIM includes RIM (reaction injection molding), RRIM (reinforced RIM) and RTM (resin transfer molding). IX M (Liquid Composite Molding) is a relatively new technical term which includes SRIM, RRIM and RTM (Resin Transfer Molding). 

In summary, the future of the Reaction Injection Molding process in the USA definitely appears to be extremely positive. Fueled by growing market awareness and interest, especially in the Inudstrial Consumer area, new material and process developments should bring forth a variety of new applications. The automotive market for RIM continues to be strong, and for the near term will be the major driving force for new material developments. 

Lee, L.J. Polyurethane reaction injection molding Process, materials, and properties. Rubber Chem. Technol. 1980, 53, 542. 

Thermosets, on the other hand, are polymers formed by the mixing and chemical reaction of fluid precursors into a mold once the precursors react, a crosslinked network that cannot flow anymore under heating is created therefore, reaction and molding into the final shape usually take place at the same time (by the RIM or reaction injection molding process). Examples of common thermosets are some polyesters, phenol-formaldehyde resins, epoxy resins, and polyurethanes, among others. Chapter 28 of this handbook elaborates on this topic. 

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]. 

The rapid reaction of isocyanates with hydroxyl-bearing prepolymers to make polyurethanes is typical of the RIM (Reaction Injection Molding) process. These... 

IWo specialized injection-molding processes will be considered in some detail. These processes are structural foam and reaction injection molding. In the case of structural foam, a gas is distributed throughout the molded plastic to form a foam. When cooled, the product is a structural thermoplastic foam. The reaction injection-molding process involves mixing two reactive components and injecting them into a mold, where the product is formed. 

As mentioned previously, the reaction injection-molding process combines two precisely metered and well-mixed reactive streams. An example is the process that reacts catalyzed highly reactive streams of urethane components. One stream contains a polyether backbone, a catalyst, and a cross-linking agent. The other stream has an isocyanate. In addition, a blowing agent is included in one of the streams. 

Two-component, catalyzed, interfacial polymerization of polyurethane has been previously modeled by means of a set of partial differential equations for the diffusion-reaction process, with concentration-dependent difiusivities [105-108]. Their main assumptions were that reactions were fadlitated only toward the end of the mixing process, such as at the end of the reaction injection-molding process, wherein molecular diffusion was significant. It was also assumed that the diisocyanate and diols formed lamellar mixing structures, whereby alternative striations of diisocyanates and diols were present. 

The matrix polymer must quickly fill the cavity of the mold and wet all fibers in the structural reaction injection molding process. Macosko analyzed the flow in terms of flow of liquid throughout the entire cavity and in terms of... 

Reaction injection molding processes utilize rapid chemical reaction, allowing the production of solid parts from liquid monomers with reaction times on the order of seconds to minutes. 

Both networks are formed with high reaction rate. Here there is no time for phase separation, and the structures, corresponding to the liquid one-phase state, may be frozen (such a case should be typical of the reaction injection molding processes used for the production of IPNs). This case is a real chemical quenching. 

In the reaction injection molding process, the release force also depends on cure time. As cure progresses, less free isocyanate groups are available for reaction with the mold surface and the release stress decreases. Longer cure times also give zinc stearate more time to migrate to the mold surface. 

Bruins PF (ed) (1976). Unsaturated Polyester Technology, Gordon Breach Science, New York. Castro, J (1980). Mold Filling and Curing Studies for the Polyurethane Reaction Injection Molding Process, Doctoral Dissertation, University of Minnesota, Minneapolis, Minnesota. Castro JM, Macosko CW (1982). AIChE J. 28 250. 

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