PURPOSE:To further proceed the thermocrystallization of a mouth tube and further enhance the density of the mouth tube by a method wherein the mount tube of a preform which is a molding product is heated and, after that, the mouth tube is further heated from the inside by fitting a heating nozzle to the inside of the mouth tube. CONSTITUTION:A mouth tube 2 is heated from the outside with a far infrared heater 4 or the like up to a temperature, for example, of 180-200 deg.C. Next, a heating nozzle is fitted to the mouth tube after 20-30sec of said heating process for the thermocrystallization of the mouth tube. The temperature of the heating nozzle is not especially limited. However, higher density can be obtained in case of the setting temperature of 140 deg.C than in case of that of 90 deg.C and the optimum setting temperature of the heating nozzle is about 190 deg.C. Further, the heating nozzle 3 is formed in a convergent tapered shape so as to set to expand the upper end part, which shrinks most, of the mouth tube more than the remaining part. Natural cooling is preferable for the final slow cooling of the mouth tube.
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