Reaction injection molding is a simple concept. As its name suggests, the process is based on a chemical reaction. A reactive liquid mixture (usually polyol and isocyanate) is injected or poured into a mold where a chemical reaction takes place. After an exothermic (heat generating) reaction occurs, the finished part is removed from the mold. Depending on the chemical formulation, the end product can take on a wide range of physical characteristics: foam or solid, highly rigid or very flexible. Nitrogen is used in the process to keep the chemicals dry and oxygen free.