a thermoforming device heats a plastic film to a substantially uniform temperature , in order to impart a curved shape to said film . the device includes a chamber , the internal pressure of which is variable and which is closed by the film , a system for measuring a sag in the film arranged outside the chamber and opposite said film , and a system for heating the film . the heating system includes a hot air blowing unit which is suitable for producing a hot airflow flowing parallel to the film between two opposite edges of said film .