Microcellular injection molding is a technology to create very small cells in the wall of plastic parts. Microcellular injection molding needs specially designed equipment to create single phase solution from molten polymer and supercritical fluid and to create micro cells in molten polymer by quick pressure drop. Microcellular parts are used in automotive industries and electric/electronics industries. Recently the combination of microcellular injection molding and Heat&Cool technologies are studied actively to avoid swirl mark on the surface. To get high expansion ratio, core back technology is studied.