Kneading disc having band portion with bent part, screw element and extruder

A kneading disc that is fitted onto a rotatable shaft and that is arranged in a cylinder, the kneading disc comprising an outer circumferential surface that faces the cylinder, the outer circumferential surface comprising:

• a first portion that is adjacent to an inner surface of the cylinder as seen in an axial direction of the cylinder;
• and
• a second portion that is farther apart from the inner surface of the cylinder than the first portion, as seen in the axial direction,
• wherein the outer circumferential surface has a band portion that extends between both ends of the kneading disc with regard to an axial direction of the shaft and that forms a minimum gap with the cylinder,
• wherein the band portion is only provided on the first portion and is a single band portion, and the band portion includes at least one bent part,
• wherein the band portion includes a first part that extends diagonally relative to the axial direction and a second part that is connected to the first part at the bent part and that extends diagonally with regard to the axial direction in a direction opposite to the first part,
• wherein the first and second parts extend straight, and the first and second parts form acute angles with the axial direction, wherein the acute angles are equal to or more than 30 degrees.

An extruder comprising:

• a screw that is partitioned into a plurality of screw elements;
• a rotatable shaft that is fitted into the plurality of screw elements;
• and
• a cylinder in which the screw is arranged,
• wherein the plurality of screw elements includes a first screw element, a second screw element, and a third screw element, each comprising a plurality of kneading discs that are integrated in an axial direction of the cylinder,
• wherein each of the plurality of kneading discs is fitted onto the shaft and is arranged in the cylinder, each kneading disc comprising an outer circumferential surface that faces the cylinder, the outer circumferential surface has a band portion that extends between both ends of the kneading disc with regard to an axial direction of the shaft and that forms a minimum gap with the cylinder, wherein the band portion includes at least one bent part,
• wherein the first screw element is arranged upstream in a conveying direction of resin material, the second screw element is arranged downstream of the first screw element, and the third screw element is arranged downstream of the second screw element,
• wherein, as viewed from upstream to downstream in the conveying direction of the resin material,
• in the first screw element, an angular position of the bent part of each kneading disc is shifted relative to an angular position of the bent part of an adjacent kneading disc on a downstream side by an angle that is more than 0 degrees and equal to or less than 60 degrees in a direction opposite to a rotational direction of the shaft,
• in the second screw element, an angular position of the bent part of each kneading disc is shifted relative to an angular position of the bent part of an adjacent kneading disc on an upstream side by 90 degrees in a direction opposite to the rotational direction of the shaft, and
• in the third screw element, an angular position of the bent part of each kneading disc is shifted relative to an angular position of the bent part of an adjacent kneading disc on an upstream side by an angle that is more than 0 degrees and equal to or less than 60 degrees in a rotational direction of the shaft.