The present invention relates to an apparatus for curving and tempering or heat toughening thin glass sheets.
Such products are frequently used as windows for vehicles in general, and more especially for automobiles.
Such application requires that the windows be made accurately complying with imposed tolerances and sizes, as well as with the configuration associated with the lines of the particular vehicle.
Furthermore, the clarity of vision through these windows must not be impeded by optical distorsion or by defects in the glass.
The heat treatment confers increased resistance to impact and guarantees that glass fragments produced in the event of breakage are less damaging than those which would have been produced in nontoughened glass.
As is already known, tempered or toughened glass sheets, more especially those used in the automobile industry, can also be manufactured by equipment having a horizontal layout which permits a high production output.
In these types of equipment, each glass sheet is conveyed horizontally through a continuous furnace in which it is heated to the softening temperature. The glass sheet is next transferred to a forming station if it is to be curved, then to the quenching station, followed by cooling.
For efficient tempering of the glass sheet, it must still retain a certain degree of plasticity after forming.
On the other hand, such conditions of plasticity can inevitably give rise to modification in the shape assumed by the glass prior to the forming station, owing to its own weight, as well as to the inertia of the glass sheet, to the action of the first blasts of quenching air and to possible asymmetrical cooling between the top and bottom faces of the glass sheet.
As is already known, the thinner is the glass sheet, the greater are the deformations, since if it is true that its own weight and its inertia decrease in proportion to the thickness of the sheet, it is also true, however, that resistance to deformation decreases with the cube of the glass sheet thickness.
The deformation effect of the quenching blasts is more strongly noticable in thinner glass sheets.
During the last few years, considerable commercial importance has been attached to the manufacture of curved and tempered glass sheets of increasingly less thickness, ranging from 5-6.5 mm down to 2.5-4 mm.
As a result, the manufacturing process has been affected by this reduction in thickness since resistance to deformation decreases--as already stated--by the cube of the thickness.
The increase of quenching blasts, required for the higher thermal gradient associated with the thinner glass sheets, in turn contributes, with the lower structural strength, in making it more difficult to control the shape of the product which, after the forming phase, is still in the plastic state.
The apparatuses already known in the prior art have been gradually modified to improve the curving and tempering or heat toughening process.
However, only the apparatus in accordance with the present invention, is able to satisfy the requirements associated with ever closer tolerances and with products of different curvature from a cylindrical profile.
Also well known in the prior art concerning glass sheets conveyed on rollers in a heating furnace is, for example, the U.S. Pat. No. 3,338,695 (to Ritter) which describes a process as well as an apparatus for curving and tempering glass sheets, especially suitable for glass sheets from 4 to 6 mm thick.
In French Pat. No. 1.474.251 (to LOF) means are provided for the first time for curving the glass sheet utilizing the gravity and pressure of a mould, by providing a mould on which the glass sheet to be curved is placed, so that forming is performed as accurately as possible.
In U.S. Pat. No. 3,684,473 (to Ritter) a process and apparatus are described for tempering or heat toughening glass sheets 3.2 to 2.4 mm thick, by using a so-called curving for "inertia and gravity" process and a device which supports the glass sheet while it is submitted to the action of the quenching blasts.
Lastly, in Patent LU No. 65.146 (to Ritter) a process and apparatus are described for tempering or toughening thin glass sheets, that is, less than 4 mm thick, by using the curving by "inertia and gravity" process; furthermore, quenching of the glass sheet is already started in the curve forming station by providing special nozzles which cool the glass while it is still in the curving ring.
The systems of the patents of the prior art are not, however, able to provide means which ensure curving and tempering or toughening of parts with cambers over 60 mm and at the same time with thicknesses less than 4 mm, such as rear windows for motor vehicles.
In fact, U.S. Pat. No. 3,338,695 (to Ritter) is not able to provide curved and tempered glas sheets with cambers over 60 mm, because the configuration of the conveyor rolls (see FIGS. 3 and 7) shows that the forming ring cannot travel down lower than the axis of such rolls, as these rolls are secured at their ends.
Yet again, with French Pat. No. 1.474.251 (to LOF) it is not possible to curve, with reasonable accuracy, glass sheets less than 4 mm thick; this is because a thin sheet, resting on a mould (as illustrated in FIG. 4), tends to curve during heating, in a marked manner along its axis parallel to the longitudinal axis of the furnace, owing to the effect of decreased resistance to deformation due to its thinness.
This inevitably makes it difficult to control the curving process, thereby resulting in failure to hold the work tolerances.
U.S. Pat. No. 3,684,473 (to Ritter) cannot even be applied for manufacturing parts whose cambers exceeds 60 mm because the utilization of the curving for "inertia and gravity" system leads to curving times increased more than those obtained by pressing. Such increase of time, estimated to be about 2 seconds more, is in contrast with the pressing need associated with thin sheets, to minimize the total time required for transferring the glass sheet from the heating furnace to the quenching blasts.
Lastly, in Patent LU No. 65.146 (to Ritter) there is provision for starting the quenching phase in the curve forming station; therefore it involves a process which is applicable with decisively negative results on strongly curved parts because such arrangement of the blowers produces a very uneven "pre-quenching" on the parts, hence it is difficult to achieve the required standards of quality.
Furthermore, the use of mechanical means to stop the glass sheet can lead to the formation of marks on the glass sheet with consequent impairing of the optical quality of the finished product.