a method and an apparatus to thermocrystallize amorphous thermoplastic material in a bottom portion of a preform while producing a container . by means of at least one heating device the plastic material is heated to an elevated temperature at which the material crystallizes thermally . the supply of energy is discontinued once the material crystallizes at a speed entailing that the thus released effect corresponds to or exceeds that effect which departs from the material undergoing crystallization . the apparatus includes a mandrel having a forming surface which at an increased temperature is brought into abutment against a defining surface of the bottom potion . the forming surface has a curvature which deviates from the curvature of the defining surface so that during the initial phase of the abutment the forming surface is abutting against the defining surface only in a central area thereof . on continued displacement of material in the bottom portion in order to form the portion the size of the central area thermoplastic material intended for temperature - conditioning abuts against the forming surfaces .