Glass sheet forming and annealing system providing edge stress control

A system for glass sheet forming and annealing comprising:

• a furnace for heating a glass sheet to its forming temperature;
• the furnace including a preforming end section having inclined bending rolls for preforming the heated glass sheet with an upwardly concave shape;
• a forming station located adjacent the furnace;
• a conveyor for conveying the preformed glass sheet from the furnace to the forming station;
• the forming station having an upper forming mold which is heated and has a downwardly facing forming face;
• the forming station also having a lower forming mold that is movable upwardly to lift the preformed glass sheet from the conveyor to the upper forming mold for pressing that provides the glass sheet with a formed shape, the lower forming mold having four sides defining a ring shape and each side having a T-shaped cross section including a vertical stem and a crossbar supported on the vertical stem and having an upwardly facing forming surface, each side of the lower forming mold having a pair of electrical heating elements at lower locations below its crossbar where it is supported on its vertical stem, the forming face of the crossbar of each side having three grooves and three electrical heating elements respectively received within the three grooves to limit heat loss from the glass sheet, and the ring shape of the forming mold having insulation within its interior and around its exterior to also limit heat loss from the glass sheet;
• a source of vacuum for drawing a vacuum at the upper forming mold to maintain the formed glass sheet on the upper forming mold as the lower mold is moved downwardly after the press forming;
• an annealing ring that is movable under the heated upper forming mold and the formed glass sheet supported thereon to receive the formed glass sheet upon termination of the vacuum drawn at the upper forming mold, the annealing ring including heaters and also including upwardly and downwardly extending baffles at upstream and downstream locations relative to the direction of subsequent movement of the annealing ring from the forming station to limit cooling from air flow across the formed glass sheet during that movement, and insulation within the annealing ring and around its exterior to limit heat flow from the formed glass sheet to slow its cooling;
• and
• a controller that controls operation of the forming station, the conveyor, and the annealing ring, and that after the formed glass sheet is received by the annealing ring the controller is configured to maintain the annealing ring and the formed glass sheet thereon in the forming station below the heated upper forming mold for at least five seconds, and for some glass sheets at least 6.5 seconds, prior to movement from the forming station so as to provide the slow cooling of the formed glass sheet to the strain point temperature and thereby control edge stresses upon final cooling to ambient temperature.