A method for heating a preform - for manufacturing a polygonal bottle from the preform, - the preform comprising an open neck portion at a first end, a closed base portion at a second end, and a body portion extending along a longitudinal axis between the neck portion and the base portion, said method comprising the following steps: - introducing the preform into a heating apparatus comprising an array of infrared emitters arranged in multiple columns (Cj) and multiple rows (Ri), the preform and said array being disposed such that the longitudinal axis and the columns (Cj) of the array of infrared emitters are parallel or directed obliquely with respect to each other; - orienting angularly the preform at an input angular position by rotating the preform around the longitudinal axis; - setting power levels of the infrared emitters so as to divide the array of infrared emitters into subsets of columns (SCn), each subset of columns (SCn) generating heat at a different power level from an adjacent subset of columns (SCn); and - heating the preform with the array of infrared emitters while translating the preform in a direction parallel to the rows (Ri) of the array at a translation speed, and simultaneously rotating said preform around its longitudinal axis in front of said infrared emitters at a rotation speed, the rotation speed, the translation speed, the input angular position and the power levels of the infrared emitters being set so that the power levels of the subsets of columns (SCn) facing at least three zones of the body portion extending longitudinally are different from the power levels of the subsets of columns (SCn) facing the rest of the body portion, said zones of the body portion extending relative to one another in a polygonal array, said method being characterized in that - said zones of the body portion have a greater thickness than the thickness of the rest of the body portion; - the power levels of the subsets of columns (SCn) facing said zones of the body portion being set higher than the power levels of the subsets of columns (SCn) facing the rest of the body portion if and only if the said zones of the body portion have a greater thickness than the thickness of the rest of the body portion; and - the said zones of the body portion corresponding to the corners of the polygonal bottle. - for manufacturing a polygonal bottle from the preform, - the preform comprising an open neck portion at a first end, a closed base portion at a second end, and a body portion extending along a longitudinal axis between the neck portion and the base portion, - introducing the preform into a heating apparatus comprising an array of infrared emitters arranged in multiple columns (Cj) and multiple rows (Ri), the preform and said array being disposed such that the longitudinal axis and the columns (Cj) of the array of infrared emitters are parallel or directed obliquely with respect to each other; - orienting angularly the preform at an input angular position by rotating the preform around the longitudinal axis; - setting power levels of the infrared emitters so as to divide the array of infrared emitters into subsets of columns (SCn), each subset of columns (SCn) generating heat at a different power level from an adjacent subset of columns (SCn); and - heating the preform with the array of infrared emitters while translating the preform in a direction parallel to the rows (Ri) of the array at a translation speed, and simultaneously rotating said preform around its longitudinal axis in front of said infrared emitters at a rotation speed, the rotation speed, the translation speed, the input angular position and the power levels of the infrared emitters being set so that the power levels of the subsets of columns (SCn) facing at least three zones of the body portion extending longitudinally are different from the power levels of the subsets of columns (SCn) facing the rest of the body portion, said zones of the body portion extending relative to one another in a polygonal array, - said zones of the body portion have a greater thickness than the thickness of the rest of the body portion; - the power levels of the subsets of columns (SCn) facing said zones of the body portion being set higher than the power levels of the subsets of columns (SCn) facing the rest of the body portion if and only if the said zones of the body portion have a greater thickness than the thickness of the rest of the body portion; and - the said zones of the body portion corresponding to the corners of the polygonal bottle.