a melt distribution block for a co - extrusion die has a generally annular body with opposed front and rear faces and opposed inner and outer faces . inlets at the outer face have channel ports for accepting flow of melted polymer into the distribution block . flow controlling valves independently control the melted polymer flow through each of the channel ports . melt flow channels on the front and rear faces between the inner and outer faces are in fluid communication with the channel ports . a thermal insulation layer within the distribution block between the front and rear faces separate all or part of the melt flow channels on the front face from all or part of the melt flow channels on the rear face . the blocks , together with separator blocks , are axially stackable front to rear around a central mandrel to form an extrusion die for making blown films into tubular form .