Twin-screw extruder, gear box and method for extrusion

A twin-screw extruder comprising:

• two screws that extend in parallel to each other, wherein each screw has a cylindrical upstream screw and a downstream screw, wherein the upstream screw has a shaft hole that extends in a longitudinal direction and a screw flight on an outer circumferential surface thereof, and the downstream screw includes a large diameter portion having a screw flight on an outer circumferential surface thereof and a small diameter shaft portion that has a smaller diameter than the large diameter portion, wherein the small diameter shaft portion of the downstream screw is inserted into the shaft hole of the upstream screw, and the upstream screw and the downstream screw can be independently rotated;
• an upstream rotating mechanism that rotates the upstream screws of the two screws;
• a downstream rotating mechanism that is provided independent of the upstream rotating mechanism and that rotates the downstream screws of the two screws;
• an upstream input shaft;
• a downstream input shaft;
• a first upstream output shaft that is connected to the upstream screw of one of the screws and that has the shaft hole;
• a second upstream output shaft that is connected to the upstream screw of the remaining screw and that has the shaft hole;
• a first downstream output shaft that penetrates through the shaft hole of the first upstream output shaft and that is connected to the downstream screw of the one of screws;
• a second downstream output shaft that penetrates through the shaft hole of the second upstream output shaft and that is connected to the downstream screw of the remaining screw;
• an upstream transmission element that transmits rotation of the upstream input shaft to the first upstream output shaft and to the second upstream output shaft;
• and
• a downstream transmission element that transmits rotation of the downstream input shaft to the first downstream output shaft and to the second downstream output shaft.