lip die structures include thrust developing elements spaced along one lip at first points for flexing those first points toward and away from the other lip and controlled by a thickness monitor spaced downstream along the web being produced from the lip die . first heating elements are carried by the one lip aligned with the thrust developing elements and are controlled by the thickness monitor to selectively heat the first heating elements responsive to monitored decreases in web thickness . second heating elements are also carried by the first lip intermediate the first heating elements and are controlled by the thickness monitor to actuate , selectively , the second heating elements responsive to monitored decreases in thickness of said web .