JPH0544902B

PURPOSE:To make it possible to smoothly lower molten resin, between an inner wall surface and which friction is reduced with friction relaxing liquid, without developing deformation by a method wherein excess lubricant is discharged outside from its stagnant portion on the basis of the temperature near a diametrically enlarging guide tube. CONSTITUTION:Molten resin 15 is thermally crosslinked while being extruded in hollow cylindrical form along the inner wall surface of a hollow cylindrical crosslinking tube 7 having a vertically extending center axis line. Next, the diameter of the crosslinked molten resin 15 is enlarged by being guided along the inner wall surface of a diametrically enlarging guide tube 10. Next, the diametrically enlarged hollow cylindrical molten resin 15 is cooled down and stabilized. And, at the beginning of said thermal crosslinking, lubricant 16 is continuously charged between the crosslinking tube 7 and the molten resin 15. At the same time, the inside temperature of the tube wall at the inlet point P1 and the outlet point P2 of the diametrically enlarging guide tube 10 are measured so as to properly discharge the friction relaxing liquid 16 outside at the positions of the inlet point P1 and the outlet point P2 in accordance with the values of the measured temperatures. Further, the numeral 17 is a discharge pipe provided at the inlet point P1 and that at the outlet point P2. The numerals 41 and 42 represent solenoid valves provided on the discharge pipe 17. The numerals 21 and 22 indicate temperature measuring terminals.