In a packaging machine, typically a cellophaner for overwrapping packets of cigarettes, each of the packets is enveloped by a wrapping unit in a heat shrinkable wrapper presenting at least two opposite faces disposed parallel one with another. The machine also comprises a unit by which heat is applied to the wrappers, equipped with a pair of belt conveyors that present two branches, facing one another and set in motion in such a way as to transport the packets by gripping the two opposite faces of each wrapper, and at least one heating head associated with each transport branch; the heating heads are faced with a leaf of lamellar graphite bonded to the head on one side and offered in direct contact on the other to the respective branch of the corresponding belt conveyor, so as to minimize friction between the belt and the head.