Reaction Injection Moldings

The RIM process predominandy uses TS polyurethane (PUR) plastics. Others include nylon, TS polyester, and epoxy. PUR offers a large range of product performance properties. As an example PUR has a modulus of elasticity in bending of 200 to 1400 MPa (29,000 to 203,000 psi) and heat resistance in the range of 90 to 200°C (122 to 392°F). The higher values are obtained when glass-fiber reinforces the PUR (also with nylon, etc.). The reinforced RIM process is called RRIM or structural RIM (SRIM). Large and very thick RIM products can be molded with or without reinforcements using fast cycles.

When compared to injection molding (IM) that processes a plastic compound (polymer plus additives, etc.), RIM uses two liquid PUR chemical monomer components (polyol and isocyanate) that are mixed to produce the polymer (plastic). Additives such as catalysts, surfactants, fillers, reinforcements, and/or blowing agents are also incorporated. Their purpose is to propagate the reaction and form a finished product possessing the desired properties

Mixing is by a rapid impingement in a chamber (under high pressure in a specially designed mixing head) at relatively low temperatures before being injected into a closed mold cavity at low pressure. An exothermic chemical reaction occurs during mixing and in the cavity requiring less energy than the conventional IM system. Polymerization of the monomer mixture in the mold allows for the custom formulation of material properties and kinetics to suit a particular product application.

RIM is the logical process to consider at least for molding large and/or thick products. With RIM technology, cycle times of 2 min and less have been achieved in production for molding large and thick [10 cm (3.9 in.)] products. It is less competitive for small products. Capital requirements for RIM processing equipment are rather low when compared with injection molding equipment (includes mold) that would be necessary to mold products of similar large size.

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