An apparatus for tempering thin planar glass sheets with a thickness of not more than 4 mm to a surface compression stress of at least 100 MPa, said apparatus comprising:
- a furnace for heating glass sheets to a tempering temperature, the furnace including a conveyor track for glass sheets, and
- a quenching unit for cooling glass sheets, the quenching unit including a conveyor track and cooling air enclosures set above and below the quenching unit conveyor track and having cooling air blast openings in such a disposition that a cooling effect of a blast through the blast openings is directed to the top and bottom surfaces of a glass sheet across the entire width of the glass sheet moving in the quenching unit,
- wherein the blast openings have a shortest blasting distance of not more than 30mm from the surface of the glass sheet moving on the quenching unit conveyor track and a blasting pressure of not less than 6 kPa,
- wherein, in order to eliminate or reduce the bi-stability of a tempered flat glass sheet, at least a first cooling air enclosure above the quenching unit conveyor track includes a subarea of weakened or blocked cooling effect and at least a first cooling air enclosure below the quenching unit conveyor track includes a subarea of weakened or blocked cooling effect,
- wherein said subareas having a weakened or blocked cooling effect as compared to the cooling effect of cooling air enclosures outside the subareas over a surface area of cooling air enclosures equivalent to the subareas, the subareas being located above and below an intermediate portion of the moving glass sheet,
- whereby quenching of a top surface and a bottom surface of opposing side portions of the glass sheet commences earlier or more effectively than the quenching of a top surface and a bottom surface of the intermediate portion of the glass sheet, as a result of which the opposing side portions are provided, prior to the intermediate portion, with a temperature profile in the glass thickness direction which generates a tempering degree having the surface compression stress of at least 100 MPa,
- wherein quenching of the intermediate portion is commenced on both the top and bottom surfaces of the glass sheet not more than 30 cm after the opposing side portions, and
- wherein a distance from the furnace to below the first cooling air enclosure is not more than a thickness of an end insulation of the furnace plus 300 mm and the glass sheet has a motion speed of at least 300 mm/s, whereby a transfer time for a leading edge of the glass sheet from the end insulation of the furnace to below the first cooling air enclosures is not more than 1 s.