Abstract: In accordance with the instant invention, biaxially oriented polymer sheet is produced by continuously forging polymer feedstock between a pair of converging belts in an isochoric deformation region to form the sheet from the feedstock. The resulting sheet is immediately annealed after being formed and subsequently cooled after being annealed by passage through an isobaric region defined by a pair of opposed parallel belts backed by a pair of opposed parallel platens which interface with the belts via an oil seal so as to transfer heat from the sheet through the belts to the cooled platens. The resulting sheet has substantially uniform strain distribution over the width and length thereof as well as through the thickness thereof while maintaining its flatness after emerging from the apparatus.