Abstract: A method of forming a sole assembly for footwear construction comprises providing a first polymeric composition having a first density and a second polymeric composition having a second density in a mold to form a pre-expanded midsole. The pre-expanded midsole is expanded using a supercritical fluid to form a midsole. The midsole includes an exterior portion having a third density and an interior portion having a fourth density, which is different than the third density. The midsole defines a first surface and a second surface opposite the first surface. An outsole is joined with the second surface of the midsole.