An extrusion die for extruding thermoplastic materials to produce a sheet of substantially uniform thickness, the extrusion die having a manifold width that increases from the centerline toward the die exit or lip. The increasing width manifold arms, with the preland portion, provide a constant shear rate at all areas of the die and a constant residence time, so that every fluid particle path through the die has the same residence time. By having the increasing width manifold arms, the preland area of the extrusion die is decreased, usually by about 30%, often about 40-60%, compared to a Winter die, thus decreasing clam-shelling effect and allowing design and extrusion of wider sheets.