Thermoplastic method of reducing the diameter of a metal tube

A thermoplastic method of reducing the diameter of a metal tube, which comprises holding around the metal tube a plurality of annular heaters spaced from each other in the longitudinal direction of the metal tube, each annular heater being capable of applying locatized heat to annular part of the metal tube, also holding around the metal tube a plurality of annular coolers disposed alternately with the annular heaters, simultaneously moving the annular heater and the annular coolers relative to the metal tube in the longitudinal direction of the latter for successively heating annular parts of the metal tube throughout the length thereof, those parts of the metal tube which are adjacent to the annular parts being heated being cooled by the annular coolers to a temperature sufficiently low to limit the free thermal expansion of the annular parts being heated, whereby the diameter of the metal tube on cooling is made less than that before the heating.

A thermoplastic method of reducing the diameter of a metal tube, which comprises:

• holding around the metal tube an annular heater capable of applying localized heat to the annular part of the metal tube so as to cause softening thereof;
• moving the annular heater and the metal tube relative to each other in the axial direction of the metal tube for producing a travelling annular heated part of the metal tube throughout the length thereof;
• and cooling the metal tube by submerging the same entirely in a coolant bath, while the metal tube is being heated by said annular heater, the cooling by the coolant bath being such as to hold at least parts of the metal tube neighboring the annular heated part at a temperature sufficiently low to limit the free radially outward thermal expansion of the annular heated part and to cause yielding of the same, whereby the diameter of the metal tube is made less than that before the heating.