A process and device for the stable widthwise regulation of the bulb angle in connection with the extrusion of sheets of heat-sensitive plastic (2) in an extrusion device with a calender opening formed by two intake rollers (3, 2) and a traversing measuring device (7) provided according to the calender slot by altering positioning elements on the extrusion device. At least two traversing measurements of a parameter correlating with the bulb angle are started consecutively on the heat-sensitive plastic sheet synchronously with the positioning of one of the two calender rollers. Like measurement conditions therefore exist and the deviation of the measured values obtained on the basis of the measurements in relation to specified expected values is used to regulate the bulb angle over its width by altering a setting element on the extrusion device.