Reactive liquid molding

Figure 16.1 Polyurethane composites of nano-HA (a) and (b), commercially available MasterGrafpM (MG) (c), and bioglass fabricated via electro spinning (a), foaming (b) and (d), and in situ reactive liquid molding (c).

Methods are used to produce the more costly rapid prototypes include those that produce models within a few hours. They include photopolymerization, laser tooling, and their modifications. The laser sintering process uses powdered TP rather than chemically reactive liquid photopolymer used in stereolithography. Models are usually made from certain types of plastics. Also used in the different processes are metals (steel, hard alloys, copper-based alloys, and powdered metals). With powder metal molds, they can be used as inserts in a mold ready to produce prototype products. These systems enable having precise control over the process and constructing products with complex geometries. 

The RIM process involves the high-pressure impingement mixing of two or more reactive liquid components and injection of the mixture into a closed mold at low pressures. Large and thick products can be molded using fast cycles with relatively low-cost materials. Its low energy requirements with relatively low investment costs make RIM attractive (9). 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

Reactive (chemical) molding technology and the equipment used depend on the geometrical shape of the article produced. In all cases, initial liquid products are delivered into a mold where the processes of polymerization (solidification), structure formation, and shaping proceed simultaneously. 

When the mixture of monomers is cast in an open mold, the air bubbles formed at the jet nozzle in the mold usually have enough time to leave the material due to the low viscosity of the reactive liquid. When more viscous oligomers and prepolymers are used, particularly in the case of low-lifetime reactive mixtures, it may be necessary to use some simple procedures, for example, filling through a pipe immersed into the mold, to prevent the formation of air bubbles in the product. 

Finally, in the reaction injection-molding (RIM) process, low-viscosity reacting monomers or prepolymers are intimately mixed just before being injected into a hot cavity, where they react further and solidify. The RIM process, then, is a variation of the casting process, where highly reactive liquid systems are injected quickly, rather than being allowed to flow by gravity, into complex shape cavities, where they quickly react and solidify. 

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. 

Flexible foam is made from long-chain diols with a small amount of triols for cross-linking to give strength and resilience. Foaming is produced by reaction between measured amounts of isocyanate and water to liberate carbon dioxide. Molded products are made by pouring the reactive liquid mixture into a mold cavity at 50°C. They foam and cure in 2-10 min, after which they are temporarily crushed or vacuum-shocked to open the cell walls and insure softness. Then they are allowed to condition for several hours to finish the cure. The major products are auto seating and headrests, and furniture cushions. 

Casting. The reactive liquid mixture is poured into a mold, and allowed to polymerize and cure. This is used to make solid tires for industrial vehicles, and to make decorative items such as simulated wood trim, furniture, picture frames, and lamp bases. 

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). 

LIM Reactive liquids are mixed batchwise in a small chamber and injected into a mold where they foam and cure rapidly. 

RIM Reactive liquids are mixed continuously and used to Teed a series of molds in which the mixture foams and cures. 

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