To reduce the risk of breakage of a mandrel ring in a mandrel in which the mandrel ring is attachable to and detachable from a stem and to secure the high positional accuracy of the mandrel ring even when the difference between the coefficients of thermal expansion of the stem and the mandrel ring is large.
The mandrel (30) has the stem (32) and the mandrel ring (35), the mandrel ring (35) is composed of a material with a coefficient of thermal expansion smaller than that of the stem (32). The outer circumferential surface of the stem (32) and the inner circumferential surface of the mandrel ring (35) are set in such a manner that in a state in which the mandrel ring (35) is fitted externally to the stem (32): a clearance (S1) is present between both surfaces at ordinary temperature; and at the die temperature at extrusion, the clearance (S1) eliminates in at least a part in the axial direction of the mandrel (30) and both surfaces are brought into contact with each other as a result, and at least one hollow part (55) is provided along the axial direction on the stem (32).
Hisayoshi Shimizu
Ken Takada