1. Field of the Invention
This invention relates to a metal tube of irregular shape which has at least two inner diameters, and its production method. To be more specific, this invention relates to a small diameter, irregular shaped metal tube which can be used for a pin, injection needle, connector, electron gun for TV liquid crystal, and the like, and its production method.
2. Prior Art
Metal tubes of small diameter such as those having, for example, an outer diameter of up to 2 mm and used for a medical pin, injection needle, connector, electron gun for TV, or the like are typically produced by curling a metal thin plate having a thickness of up to 0.2 mm simultaneously with the drawing, welding the abutting edges of the thin plate just before its entrance into a drawing die, drawing the welded member through the drawing die to form a tube having an outer diameter of about 4 to 6 mm, and repeating the drawing process to thereby produce a final tube product having a tapered or stepped side profile with at least two inner diameters. FIG. 12 shows typical process of drawing. In FIG. 12, a metal tube 1 which has been formed to an outer diameter of about 4 to 6 mm is drawn through a die 2 having a die bore of smaller cross section to thereby reduce the outer diameter and produce a tube having a cross section identical with the bore of the die. In the drawing, a plug 3 is inserted in the tube 1 to thereby prevent the occurrence of creases on the inner surface of the tube 1 during the drawing process.
However, when the diameter of the tube 1 becomes reduced after repeated drawings, insertion of the plug 3 in the tube 1 becomes impossible, and such drawing with no plug 3 is inevitable. When the tube 1 is drawn with no plug 3 inserted in the tube 1, the inner surface of the tube 1 becomes creased and the inner surface will suffer from an increased surface roughness which results in the increase in the resistance of a fluid passing through the tube 1. Increase in the surface roughness also results in the increase in the surface area which invites adhesion of dirt and foreign matters. This is a serious problem when the tube is used for a medical application where hygiene is of serious interest. In spite of such situation, there has so far been no small diameter tube which is free from the problem of the rough inner surface.
As described above, there is a strong demand for a small diameter metal tube with a smooth inner surface having at least two inner diameters and which can be used for an injection needle, as well as its production method.