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One type of network-forming thermoset, polyurethane, is molded into parts such as automobile bumpers and inside panels through a process known as reaction PEEK Injection Molding, or RIM. The two liquid precursors of polyurethane are a multifunctional isocyanate and a prepolymer, a low-molecular-weight polyether or polyester bearing a multiplicity of reactive end-groups such as hydroxyl, amine, or amide. In the presence of a catalyst such as a tin soap, the two reactants rapidly form a network joined mainly by urethane groups. The reaction takes place so rapidly that the two precursors have to be combined in a special mixing head and immediately introduced into the mold. However, once in the mold, the product requires very little pressure to fill and conform to the mold—especially since a small amount of gas is evolved in the injection process, expanding the polymer volume and reducing resistance to flow. The low molding pressures allow relatively lightweight and inexpensive molds to be used, even when large items such as bumper assemblies or refrigerator doors are formed.