A screw extruder, for use with or without a calender, has a divided or split extruder head with upper and lower parts or halves hingedly mounted on a collar secured to a flange at the discharge end of the extruder cylinder. Each of the upper and lower extruder halves is movable between an open innoperative position and a closed operative position by a piston cylinder unit acting between a fixture on the respective extruder head part and a fixture on the extruder cylinder. When in closed position, the upper and lower extruder head parts or halves are pressed toward one another by a pair of vertical piston-cylinder units, each having a cylinder with lower end engaging the upper extruder head part and a projection on a piston rod engaging the lower extruder head part. The vertical piston-cylinder units are mounted on pivoted carriers mounted on the collar on the extruder cylinder. The two carriers are interconnected by a horizontal piston-cylinder and are also movable vertically. Operation of the apparatus is effected by an electro-hydraulic control circuit having a processor by which sequential operation of the piston-cylinder units is controlled.