A heating unit for heating preforms made of plastics material and having a flange separating a body and neck. The heating unit comprising a plurality of supports, each bearing a gripping member for gripping the preforms. The supports are movable in the heating unit such that each gripping member moves along a conveying path of the preforms. The heating unit also comprises a plurality of shells secured to the supports, where each shell is connected to one or more supports by an actuating member. The actuating member is mounted on said support to pivot about an axis situated at a distance from the conveying path of the preforms. The heating unit further comprising means for pivoting the actuating member to drive the shell from an open position to a protection position when the support to which said shell is connected is passing from an admission/releasing zone to a holding zone.