A sole assembly is formed by preheating a mold assembly, placing a quantity of rubber in a first portion of a lower recess of a mold assembly; placing a middle plate in contact with a bottom plate, and an upper plate in contact with the middle plate; subjecting the top, middle, and bottom plates to heat to semi-cure the rubber in the lower recess to form an outsole member; separating the top plate, the middle plate, and the bottom plate; placing the top plate in contact with the bottom plate, with polyolefin forming a midsole above the semi-cured rubber; moving the movable insert upwardly with respect to the top plate; subjecting the top and bottom plates to heat such that the polyolefin expands within the sole recess and the polyolefin and rubber in the sole recess fully cure, cross-link, and bond to one another to form a sole assembly; cooling the top and bottom plates; and removing the sole assembly.