NO136835B - PROCEDURES FOR HARDENING GLASS PLATES. - Google Patents

Description

The present invention relates to methods for hardening glass, and more specifically hardening of glass plates that are quenched in a boiling medium, e.g. a dressing liquid. Such plates are used e.g. when manufacturing windows for vehicles, e.g. windscreens for motor vehicles or aircraft.

A sheet of glass hardened by the method of the invention can be used alone as a windscreen for a car, or used as one of the laminates of a composite, laminated windscreen.

It is common to join two thin sheets of glass for formation

of a laminated windscreen using an intermediate layer of transparent plastic material, e.g. polyvinyl butyral. In such a windscreen, both sheets can be of annealed glass or both of tempered glass, but currently laminated windscreens are most often manufactured with a double breaking characteristic by using an annealed glass sheet as the outer layer and a sheet of tempered glass as the inner layer.

With such a windscreen, visibility is maintained through the pane itself

if the outer annealed plate is splintered by a sharp flying stone, while the inner hardened plate is quickly shattered into smaller pieces when it is hit by the head of a person in the vehicle.

When manufacturing a tempered glass sheet for use in a windscreen, a sheet of flat glass is usually cut to a shape corresponding to the window opening in the vehicle into which the windscreen is to be fitted, after which the cutting edges of the sheet are sanded to remove irregularities resulting from the cutting process. The glass is then heated to a suitable temperature for boyning by feeding it through a heater while the glass sheets are suspended in tongs from an overhead conveyor, which then feeds the glass between vertical boy forms that are closed around the glass sheet and bends it to the desired chrome finish.

The buoy molds are then opened and the glass is fed between blowing frames for hardening the glass plate, or the glass is brought from the buoy molds through an annealing furnace when an annealed plate is desired. During this process, the glass is suspended from pliers that grip the upper edge of the glass plate.

In a process for bending two sheets of glass which must have a similar curvature to each other for subsequent joining into a laminate, it has been common to bend the glass using a sieving process, during which the two sheets are placed on top of each other horizontally on a sieving mold and then fed through an oven in which the glass plates are heated and then assembled to the desired, adapted curvature.

In a later developed process for hardening glass, a flat sheet of glass that has been cut to the desired shape is suspended in tongs in a heating oven with an opening in the floor, and when the sheet has been heated to buoy temperature, it is lowered to a position between buoy forms which are closed again the suspended plate and bends it to the desired curvature, for the plate is further lowered through a pre-cooling step in which cooling air is blown against the side surfaces of the glass plate, immediately followed by a sudden cooling in a cooling liquid, such as e.g. may be a mineral oil with a small amount of a low-boiling liquid added. Toluene or carbon tetrachloride are suitable additives. This process has proven to be particularly effective in the production of high-strength glass for airplane windscreens, as well as boyd and tempered glass of thickness 1.5 - 3 mm for use in the production of laminated windscreens for motor vehicle toys.

In accordance with the invention, however, it has been found that in order to maintain the desired optical quality standard for the tempered glass sheets, as well as to reduce glass breakage during brazing, it is advantageous to give the part of the glass which first comes into contact with the curing medium, a higher temperature than the opposite end of the glass, and it is a main purpose of the present invention to indicate an improved method for utilizing this condition.

The invention thus relates to a method for hardening glass plates which are first fed in a vertical position through a heating zone and then quenched in a quenching medium, during which different parts of the plates are heated to different temperatures for quenching, the distinctive feature of the method according to the invention being that the leading edge of each plate , which is first brought into contact with the cooling medium, is heated to a higher temperature than the rear edge of the plate.

According to the invention, the speed of the movement of the glass through the heating zone can be suitably varied in such a way that the front edge of the plate is in the heating zone for a longer time than its rear edge.

The invention will now be explained in more detail with the help of a manufacturing example, and with reference to the attached drawings, on which: Figure 1 shows an elevation, partly in section, of an apparatus for manufacturing the invention, and which includes a loading station for flat glass plates , a heater, heating equipment and equipment for brazing in liquid; and Figure 2 shows a vertical section through panels of rapid heaters, a tempered glass plate being immersed between said heating elements for heating for curing;

Figure 1 shows the principle design of an apparatus for carrying out the invention, namely the heating, bending and hardening of glass plates which are advanced through the apparatus in an upright position during the entire relevant work operation, which means in an almost vertical position during heating, as well as in a completely vertical position after the boyning process and while the plate is brazed in a boiling liquid.

The oven chamber, which is described in Norwegian patent application no. 74.2548 is generally denoted by the reference number 1 and has a rectangular cross-sectional shape with a specially designed floor which will be described later, the oven chamber being carried at an angle of approximately 5° with the vertical plane in a supporting beam frame, which comprises support beams 2 which are connected at their ends to cross beams 3. From the ends of the beam frame, upright posts 4 are arranged which form an angle of e.g. 5° with the vertical direction,.

The upper ends of the upright posts 4 are interconnected by means of cross beams 5 in an inclined position of about 5° below the horizontal plane.

The furnace floor is supported by cross beams 6 which extend under the lower ends of the upright posts 4 and are designed to support the floor of the heating furnace.

The heaters 1 are a metal structure lined with heat-resistant material and with side walls that extend upwards from the floor, as well as a one-piece roof structure that is suspended from the upper beams. 5. Beams 7 in the longitudinal direction and fixed along the upper side of the heater, form supports for gear-boxes which enclose the upper ends of a number of mutually spaced, upright, nearly vertical rollers 8, which form inclined supports for glass plates 9 arranged to be advanced through the heating oxmen 1 for boyning and subsequent hardening by brazing in liquid, or annealing.

The rollers 8 are covered with asbestos or lined in heat-resistant stainless steel, and are mounted at an angle of 2 to 10° in relation to the vertical direction, e.g. at an angle of 5°, and forms part of a conveyor for the plates 9, the conveyor extending across the heater 1 from the loading station 10 to a boye station 11.

The conveyor comprises a movable carrier in the form of a carriage 12, with a V-shaped cross-section on which the lower edge of the glass plate 9

are placed, as well as protruding bottom rollers 13 which protrude through the spaces between the upright rollers 8, near the lower end of these, both in the loading station 10 and in the heater 1, as well as drive devices for advancing the carriage 12 through the heater with the glass plate 9 inclined towards the uprights rolls 8.

The bottom rollers 13, which are also made of heat-resistant, stainless steel or are covered with asbestos, are in the embodiment shown mounted at an acute angle of 50° in relation to the upright rollers 8.

The buoy station, which is described in Norwegian patent application no. 74.2549, is equipped with horizontally arranged buoy forms.

The forms are placed in a tilting chamber which is a heat-resistant lined metal construction in the form of a heat-resistant lined metal construction in the form of a heat chamber around the buoy forms. Through this chamber extends a conveyor comprising upright rollers 8 and bottom rollers 13 of the same type as the rollers in the heater, and which form an extension of the conveyor. The upright rollers 18 in the tilting chamber have short support surfaces in the area occupied by the buoy forms, so that the ring frame of the matrix form can move through and beyond the rollers.

Beyond the outlet from the tilting chamber, further upright rollers 8 and bottom rollers 13 are arranged, which form a further extension of the conveyor for receiving each carriage 12 after the plate which the carriage supports has been lifted away from the carriage for boyning between the buoy forms.

The tilting chamber is heated by gas burners to the same temperature as the glass reaches during its passage through the heating furnace, so that the boy forms will be at the same temperature as the glass when it is delivered to the forms for boyning.

The tilting chamber 17 is mounted on a massive pivotable beam frame

which comprises bottom beams 20 mounted on central pivots 21.

A hydraulic piston rod connected centrally to one end beam for the swiveling frame is arranged to tilt the frame from an angle of about 5° in relation to the horizontal plane, the rollers 18 in this position being in the same angular position in relation to the vertical direction as the rolls 18

in the heater, to a horizontal frame position in which the rollers 18 are vertical.

Initially, the chamber is in its tilted position and the patrice mold is moved into position as the carriage with the glass plate enters the tilting chamber, and as soon as the hot glass is placed between the molds, the matrix mold is displaced through the rollers 18 to press the plate against the patrice mold, while the swing frame tilted to a horizontal position as the bending of the plate progresses. During the movement of the matrix form, the glass sheet is lifted from the carriage by fingers on the matrix form, these fingers being lined under the lower edge of the glass sheet and lifting the sheet. When the swinging frame is in a horizontal position, a tongs beam 23 which carries grippers for the glass is lowered by means of a lifting mechanism denoted by 25, and which itself is arranged to be raised and lowered.

When the glass plate has been lifted from the carriage 12, this is lined

with increasing speed out of the tilting chamber to the outlet conveyor for the swing frame is tilted to its horizontal position, where the molds are opened and the glass, which is now suspended vertically in the tongs, is lowered through an opening at the bottom of the chamber for further processing.

During its feeding through the heater 1, the glass is heated to buoy temperature, e.g. 610°C, as the glass at this temperature can be bent satisfactorily and grasped by the tongs without it having become so soft that the smoothness of the glass surfaces is jeopardized during the bending process.

When the tempered glass is to be hardened, especially when a glass with high strength is required, it will be desirable to quench the glass from a higher glass temperature, e.g. 680°C, and in the embodiment shown in Figure 1, the glass is reheated, as described in Norwegian patent application no. 74.2551, because it is broken into a cooling liquid in a cooling tank 26, which is placed in a pit below the tilting chamber

Just below the outlet opening at the bottom of the tilting chamber, the glass is lined between two rows of electric heating elements 27, which are mounted in the monster shown and facing both sides of the glass. During passage of the glass downwards between these heating elements, the glass will from its buoy temperature, e.g. 610°C, be heated through its entire thickness to a suitable exit temperature for quenching, closer to the softening point of the glass, e.g. 680°C. The buoyed glass can be immersed at a uniform speed in order to maintain, as high as practically possible, a uniform temperature throughout the glass plate. Alternatively, the glass can be accelerated while it is lowered between the heating elements, thereby producing a uniform temperature gradient in the glass from a high temperature at the lower edge of the plate to a lower temperature at the upper edge of the plant.

Such a temperature gradient can be imposed on the glass for the building by making the lower row of heating elements on the walls of the heater hotter than the upper rows, or by placing the lower rows of heating elements closer to the glass. The lower part of the heater can e.g. have a temperature of 800°C, while the middle area of the heater's walls is at 750°C and the upper part of the heater at

700°C. The buoy form's patrice seat is then heated by means of internal heating elements to a temperature distribution in accordance with that given to the glass plate by means of a heater.

Below the heating elements 27, there are arranged two blowing boxes 28, both of which are supplied with air of ambient temperature, e.g. around 30°C, and which is emitted through nozzles 29 in the boxes uniformly towards both side surfaces of the glass plate. This connection of the glass surfaces after the further heating immediately produces temperature gradients from the central area of the glass to the glass surface floats. The central area of the glass remains at the temperature achieved between the additional heating elements, and the pre-cooling of the glass surfaces will be such that the glass plate can immediately be quenched in a cooling liquid before these temperature gradients are evened out and while the glass temperature is still above the glass's stress point.

When the glass is lowered from the buoy forms, the tank 26 with dressing liquid is raised on a scissor platform 30, which is located at the bottom of the pit. The tank 26 is raised until the top edge of the tank is located just below the bottom of the blow boxes 28 with the surface of the liquid in the tank located at a predetermined small distance from the bottom pieces 29 of the blow boxes. The boyed glass plate, in which there are temperature gradients from the core to the surfaces, is then brazed immediately in the cooling liquid, the plate being fed from the surrounding cooling air to the surface of the cooling liquid.

Two rows of additional heating elements 27 are shown in more detail in Figure 2. The upper edge of the additional heating elements 27 is located approximately 60 cm below the lower edge of the buoy forms, and the entire panel of additional heating elements is 90 cm high.

The rows of additional heating elements each comprise a heat-resistant panel 790 carrying a monster of heating elements 27. The heat-resistant panels 790 form walls in an elongated chamber with an open inlet opening 791 located below the outlet opening 269 from the tilting chamber. Sealing asbestos curtains 792 hang down from the sides of the elongate mouth 269 against pieces that form the inlet opening 791 into the heat resistant assembly for additional heating. The heat-resistant panels are arranged vertically, and the flexible asbestos curtains 792 form a flexible seal between the outlet opening 269 for the tilting chamber and the fixed inlet opening 791.

Each of the heat-resistant panels is equipped with a metal support structure 793. The heating elements 27 are arranged in the form of wire-wound elements on ceramic tubes, which are mounted on connecting pliers 794 made of steel, and which are fixed through the heat-resistant panels 790 and mounted in insulators which are held by the support structure 793.

The supply of electric current to the additional heating elements 27 is such that the elements are maintained at a temperature in the range of 750 to 1600°C, e.g. 1000 to 1200 C. During the downward movement of the bent glass sheet between the heating elements, the glass can be heated to a temperature of 60°C or more above the temperature at which it has left the bent forms, as the glass e.g. with a buoy temperature of 620°C can be heated to an output temperature of 680°C for the brackish cold.

In order to heat the lower edge area of the glass sheet to a higher temperature than its edge area, the glass can be accelerated from the moment the lower edge of the glass sheet is level with the lower end of the heating panels, e.g. from a lowering: of 15 cm/sec. to a lowering of 30 cm/sec. The heating time for the upper part of the glass between the heating panels will therefore be shorter, while each part of the glass plate is nevertheless heated to such an extent that there will be no significant temperature gradient across the thickness of the glass plate, or at most a temperature difference of 12°C which described. A linear temperature gradient will, however, be produced from a hdye temperature of e.g. 700°C at the lower edge of the glass plate to a lower temperature of e.g. 680°C at the upper edge of the glass plate, at which point the plate is passed past the lower end of the additional heating elements 27 to a pre-cooling stage for the sudden cooling in the oil tank 26.

A maximum lowering speed of 100 cm/sec. can be achieved.

Some exemplary embodiments are given in Tables I, II and III, which show the production of a linear temperature gradient of 10 to 30°C from a higher temperature at the lower edge of the plate to a lower temperature at its upper edge. These tables are based on a 25 cm glass plate which is immersed through an area of additional heating elements 2 7 of a height of 90 cm. Acceleration of the glass plate takes place from the initial to the final velocity when the lower edge of the glass plate has reached the lower end of the heating area. The lowering speed gives the approximate passage times for the upper and lower edge of the glass plate, respectively, between the additional heating elements. An essentially linear temperature gradient can be produced over the height of the glass plate, with the front edge of the plate hotter than the rear edge of the plate, by lowering the plate past additional heating elements 27 with constant acceleration. The area between the lower limit of the additional heating elements 27 and the upper edge of the blowing boxes 28 is in extent at least equal to the height of the glass plates to be treated, thereby allowing each plate to be brought to a constant speed as it passes between the blowing boxes. Any heat loss in this area can be compensated for by arranging a secondary heating zone in this area, which is maintained at the average temperature of the glass plate that leaves the area msd the additional heating elements.

Some exemplary embodiments for treatments of this nature are indicated

in tables IV, V, VI and VII

In each of these embodiments, a glass plate with a height of 6JD cm is lowered through rows of additional heating elements 27 over an area of height 90 cm.

To achieve the specified outlet velocity of 30.5 cm/sec.,

the values for the constant acceleration from the specified inlet speed must be as follows:

Similar results have been obtained with the glass plate of thickness 4 mm and height of 61 cm, as indicated in Table VII. The values / for the constant acceleration of the glass plate are also here as indicated in the table above in connection with table VI.

A linear temperature gradient can be produced in the glass plate by lowering the hot plate with constant acceleration between panels of extra hot elements, and whose height is less than the height of the glass plate. Design examples with different glass thicknesses are indicated in the following table VIII *

The values for the glass plate's constant acceleration are also in this case as indicated in connection with table VI.

Claims (8)

1. Method for hardening glass plates which are first passed in a vertical position through a heating zone and then flash-jolled in a dressing medium, during which different parts of the plates are heated to different temperatures for the flash-jolling, characterized by the leading edge of each plate, which is first brought into contact with the dressing medium, is heated to a higher temperature than the rear edge of the plate. 2. Method as stated in claim 1, characterized in that the speed of the glass through the heating zone is varied in such a way that the front edge of the plate is in the heating zone for a longer time than the rear edge of the plate. 3. Method as stated in claim 2, characterized in that the glass plate is first immersed in a. heating zone of greater extent than the height of the plate^ and then, a predetermined time after the introduction of the upper edge of the plate into the heating zone, and while the plate is still completely within this zone, is accelerated for immersion in a zone where the plate comes into contact with the dressing medium, whereby the plate's lower edge is heated to a higher temperature than its upper edge for insertion into the dressing medium. 4. Method as stated in claim 3, characterized in that the plate is accelerated to a higher speed at the time the lower edge of the plate reaches the lower end of the heating zone. 5. Method as stated in claim 1, characterized in that the glass plate is lowered into a heating zone of at least the same extent as the plate's height and kept stationary for a predetermined time in this zone, the temperature distribution of which is set so that the lower edge of its stationary glass plate is heated to a higher temperature than the plate's upper edge, after which the plate is lowered into the dressing medium. 6. Method as stated in claim 2, characterized in that the glass plate is lowered through the heating zone with constant acceleration., 7. Method as stated in claims 1 - 6, characterized in that the glass plate is heated in such a way in the heating zone that the temperature at the front edge of the plate is 5 - 40°C higher than the temperature at the back edge of the plate, and a mainly linear temperature gradient is achieved from the front edge to the back edge of the disc. 8. Method as stated in claim 7, characterized in that the glass plate is heated in such a way in the heating zone that the temperature at the front edge of the plate is 20°C higher than the temperature at the rear edge of the plate.