Extruder, dehydration cylinder, dehydrating method and manufacturing method of resin pellet

An extruder comprising:

• a cylinder;
• a supplying unit supplying a resin material containing moisture into the cylinder;
• a screw built in the cylinder and conveying the resin material supplied to the cylinder;
• and
• a dehydration cylinder portion provided in middle of the cylinder and discharging the moisture separated from the resin material,
• wherein the dehydration cylinder portion has a structure in which a plurality of plate-shaped members, each having an opening, are arranged in a long-axis direction of the cylinder, the screw passing through the opening;
• an outer periphery of the plurality of plate-shaped members is covered by a cover member, the cover member including a plurality of openings for discharging the moisture from the dehydration cylinder portion;
• surface roughness of mutually opposing surfaces of the plurality of plate-shaped members is rougher than surface roughness of an inner wall of the cylinder;
• the surface roughness of the mutually opposing surfaces of the plurality of plate-shaped members is 1.6a to 25a in terms of arithmetic mean roughness Ra;
• a distance between the outer periphery of each of the plate-shaped members and corresponding openings in the plate-shaped members is 10 mm to 20 mm;
• and
• a distance between a front surface and a back surface of each plate-shaped member along the long-axis direction is 0.5 mm to 5 mm.