Forced ventilation system for motor vehicles

A method of making a forced ventilation apparatus for equalizing a pressure difference between a passenger compartment and surroundings of a motor vehicle, the apparatus including a frame being made of hard plastics and delimiting an aperture, said method comprising the steps of:

• injection-molding a ventilation flap of an elastomer material to the frame in an opened position of the ventilation flap, the ventilation flap having a common contact area with the frame and a free end, and the elastomer material being pressed from the free end of the ventilation flap underneath and beyond a barrier being located in front of the contact area and opposing the frame into the common contact area with the frame;
• and
• cooling down the elastomer material of the ventilation flap to attain a resilient restoring force counteracting opening movement of the ventilation flap from a closed position in which the ventilation flap rests on the frame and it closes the aperture being arranged in the frame.