Process and apparatus for bending and tempering glass sheets

Abstract: Process for bending and tempering glass sheets wherein the sheet (16) is heated up to its plastic state, and it bends by effect of gravity resting on a skelet-type mold (14), and next it is tempered by sudden cooling, in which process during the bending stage the glass sheet (16) is tilted with respect to the vertical direction by an angle of less than 90°. Apparatus for carrying out said process, compris­ ing a heating chamber (1) and a cooling chamber arranged in a sequence and assembled on a rotatable cylindrical frame whose axis is horizontal, said chambers being tiltable through rotation of the frame.

Description: The present invention relates to a process for bending and tempering glass sheets and to the apparatus employed. More particularly, the invention relates to a bending and tempering process of the so-called "skelet" or gravity type, wherein the bending operation is carried out with the sheet at a slope with respect to the vertical direction, preferably employing a new orientable apparatus. Two different types of processes exist at present for bending and tempering glass sheets: the two-step processes, wherein both heating and bending are performed in one step only, which is immediately followed by cooling tempering, and the three-step processes, wherein the glass sheet is first heated, then drawn out of the heating chamber, shaped and finally cooled for tempering. Two different techniques are provided for the three-stop process. The first technique makes use of a press for molding the sheet, with a punch and die system, and it operates on a glass sheet supported in the vertical position by means of pliers or clamps. The second technique, which is more recent, provides the heating of the glass sheet arranged horizontally and movable on a bed of rolls, and it employs, for performing the bending operation, the pressure exerted by an air jet at high temperature from below. Such jet pushes the sheet upwards and presses it against a suitable die. Next, a sudden cooling is realized to perform the tempering. The first technique disclosed above has nowadays become absolete, since it does not allow to obtain an acceptably regular profile of the bent sheet. Indeed, strains are induced at the edges of the glass sheet, as an effect of the pincers to which it is hung. Moreover, there are some difficulties in the setting of the mold as well as a very high percentage of broken or defective products, which affect both yield and productivity. As regards the second procedure, which is technologically suitable for a high quality production, it can be considered as limited to large scale production, as plant and operation costs are extremely high. In the conventional two-step processes the operations are carried out on glass sheets arranged in the horizontal position and supported on suitable molds. Such molds, called "skelets", copy along their perimeters the profile of the glass sheet. The bending phase occurs inside the heating chamber wherein the glass takes on the shape determined by said skelet as an effect of heating and gravity. Once the glass sheet has attained the desired shape and temperature, it is drawn out of the heating chamber and suddenly cooled by means of air jets in order to obtain tempering. By means of such technique the defects resulting from the presence of the clamps are avoided and a good reproduction is obtained of the bending profile at the points corresponding to the perimeter. However, a sufficient uniformity in the central zone of the glass sheet is not attained, because of the excessive heating that must be necessarily applied. Indeed, in order to obtain the bending of the sheet against the mold by effect of its own weight it is necessary to heat the glass up to the plasticity (or softening) temperature, with the drawback of a higher possibility of deformation and hence the danger of surface defects in the finished product. Moreover, twisting of the sheet may also occur because of the same reason and, on the whole, the size tolerances are scarcely respected. A particular kind of two-step process is disclosed in US-A-2,766,555, wherein the glass sheets are first passed through a heating chamber and subsequently passed through an annealing chamber. In order to heat to the plastic state only those portions of the glass sheets that are to be bent, the apparatus disclosed is provided with heat sources concentrated near the top of said heating chamber, and the glass sheets, lying on the relevant molds, are passed through said chamber in a tilted position, with the portions of glass sheet to be bent oriented towards the top of the chamber. The technical teaching of the present invention relates to the so-called conventional two-step processes, its object being to avoid the above-mentioned drawbacks and, more particularly, the excessive sheet deformation. According to such teaching a working procedure is provided in which the sheet is kept at a slope during the bending operation. In that way the resultant of the gravity forces which are equal to the weight of the sheet) distributes over a smaller area (which is equal to the orthogonal projection of the sheet on a horizontal plane), and said sheet is pushed downwards by a differently distributed load, with respect to the case with horizontal arrangement. This results in the possibility of working at high temperatures without affecting the quality of the product. Accordingly, it is possible to produce glass sheets of optimal size tolerances and free from localized deformations of twist. Thus, the present invention consists in a process for bending and tempering glass sheets wherein the sheet is heated up to plasticity, and it is bent by effect of gravity so as to rest on a skelet-type mold, the sheet being next tempered by sudden cooling, in which process during the heating stage the glass sheet together with said skelet-type mold, is initially kept approximately in the vertical position, thus forming a first angle of more than 0° with the vertical direction, and then, as soon as the plastic state is attained, the said glass sheet and mold are further tilted for the bending stage, thus forming a second angle greater than said first angle and smaller than 90°, chosen in accordance with the desired bending profile and the dimensions of the glass sheet, and in that the said glass sheet and skelet-type mold are kept at the same slope during the cooling and tempering stage. The glass sheet is tilted for the bending operation, and kept at the same slope during the cooling and tempering step, because it is preferable not to change the slope of the sheet between the bending and the tempering step so that the passage be as rapid as possible and the sheet be not exposed to the risk of deformation. The choice of the optimal slope depends on various structural and operative parameters, such as for instance the thickness and the size of the glass sheet, the desired bending profile etc.. Such choice can be effected for instance on the basis of an experimental test on the quality of the final product. In order to carry into effect the process of the invention an apparatus is suggested, which is made up of a heating and bending chamber and of a cooling and tempering section which are assembled in such a way as to make possible their setting at various slopes with respect to the vertical direction according to the needs in question. More particularly, the apparatus for bending and tempering glass sheets according to the present invention comprises means for supporting and driving the glass sheets through the apparatus, including skelet-type molds, and, in a sequence along a horizontal line, a heating chamber wherein the glass sheet is heated up to the plastic state and it bends so as to rest on the skelet-type mold, and a cooling section, in which said cooling section is a tempering section, both said heating chamber and said tempering section are assembled on a single cylindrical frame rotatable about a horizontal axis, and it further comprises means for tilting the heating chamber and the tempering section through rotation of the said frame. Said means for supporting and moving the glass sheets further comprise two guide rails, one being placed in an upper position and the other in a lower position, which rails longitudinally cross the apparatus, one or more carriages movable between said rails, the said skelet-type molds being supported by said carriages and supporting one or more glass sheets, and means for driving said carriages along said rails. Said last means are made up of, for instance, a chain driving device running above the upper rail, from one end to the other of the apparatus, said device being driven by an electric motor, to which device said carriages are connected when they enter the apparatus. Said skelet-type molds are made up of a number of articulated segments hinged to one another along horizontal axes, and movable between an open configuration of the mold and the configuration corresponding to the desired bending profile. During the working cycle, the skelet-type mold passes from the open configuration, when the glass sheet resting on the same is initially plane, to the final bent configuration, when the glass sheet in the plastic state adheres completely to its inner surface. Preferably the heating chamber is of the rapid ignition type and is made up of a parallelepipedal structure divided into two symmetrical longitudinal halves facing one another and spaced from one another, such halves being provided with closure means along their perimeters. Such chamber contains electrical resistances as the heating means, and it is completely coated with layers of a suitable insulating material. According to a preferred embodiment of the invention, the cooling section is of the so-called "organ pipes" type, and it comprises two arrays of horizontal pipes facing one another, having longitudinal lines of nozzles turned towards the zone therebetween, where the glass sheet passes, said pipes being connected by means of hoses to a source of cooling air, which is generally provided by a pair of fans. Preferably adjustable supporting means are provided for said pipes, so that their arrangement can be changed so as to closely follow the bending profile of the glass sheet placed therebetween. The two arrays of pipes can be provided with devices capable of keeping them moving during operation, so as to make the cooling action of the sheet much more uniform. The supporting frame of the apparatus preferably comprises three or more vertical rings made up of steel and connected to each other by longitudinal bars which also are made up of steel. The above-mentioned means for tilting the apparatus are preferably made up of pairs of supporting wheels mounted on horizontal bearings integral with the bedplate of the apparatus. On said wheels the rings of said frame rest in a slidable way. The means for tilting the apparatus also comprise two or more chain driving devices which are driven by and electric motor, each one of them acting on one of the rings. More particularly, each one of the chain driving devices comprises a chain encircling perimetrically the upper part of the corresponding ring of the frame, which chain is coupled in a fixed way to said ring at a point on its top, and two sprocket wheels with horizontal axes arranged symmetrically on the bedplate of the apparatus, on both sides with respect to said ring. Said chain runs around said sprocket wheels, one of them being connected to said electric motor. Preferably the driving sprockets of each one of said chain driving devices are assembled on a single horizontal shaft connected to one electric motor. According to a preferred embodiment of the present invention, said apparatus also comprises a return section of the carriages which is arranged parallel to the main body of said apparatus and spaced apart from the same, and two separate terminal sections. The loading section is close to the inlet of the heating chamber, while the unloading section is arranged near the outlet of the cooling section, each one being assembled on a rotatable cylindrical frame with horizontal axis. Such sections are movable along two straight paths at right angles to the axis of the apparatus, from a position coaxial with said main body to a position aligned with said return section. The whole assembly of the above-mentioned elements makes up an automatic load-unload and return system of carriages wherein the latter, constantly supported by rails, move along an ideally closed path. It is evident that the apparatus according to the present invention can be employed in an optimal way for carrying out the bending and tempering process of the invention in which the glass sheet enters the heating chamber in a vertical or almost vertical position and next, when the desired temperature has been reached, the whole apparatus tilts by the previously determined angle, rotating about the horizontal axis of the cylindrical frame, and keeps such slope while the sheet goes to the cooling section and is tempered therein. Anyway, it is to be remarked that the process of the present invention colud also be carried out employing an apparatus different from that suggested above, also not orientable, but constructed in such a way as to make it possible that the glass sheet be maintained within the furnace and the cooling section at a slope of less than 90° with respect to the vertical direction. The present invention will be now disclosed, for illustrative and not for limitative purposes, according to a preferred embodiment as illustrated in the enclosed drawings, wherein: Figure 1 represents a schematic side elevation view, partially in cross section, of an apparatus according to the present invention in the position taken on during the first heating stage; Figure 2 shows a schematic side view, partially in cross section, of the main body of the apparatus of Figure 1 at the beginning of the bending stage, with the glass sheet tilted; Figure 3 is a schematic cross sectional view of the cooling section of the apparatus shown in Figure 1; Figure 4 is a schematic horizontal longitudinal sectional view of the apparatus shown in Figure 1; and Figure 5 is a schematic vertical longitudinal sectional view of the same apparatus. Figure 1 shows schematically a side view of the complete apparatus wherein the main body 1, the return section 2 and the unloading section 3 (in dotted lines) of the carriages are shown. The furnace or heating chamber is designated by the reference numeral 4, such furnace being shown as a partial cross section; the figure shows the longitudinal walls 5 facing one another and the closure member 6, which consists of a two-section door encircling perimetrically the furnace 4 (the upper and the rear parts not shown). The whole assembly is coated with a suitable insulating material. The figure also shown the electric heating resistances 7, the guiding rails 8 and 9, which are part of the system for supporting and driving the glass sheets, and the carriage 10, which is slidable on the rails 8 and 9 through the rolls 11 and 12. Said carriage 10 is connected to the chain driving system 13, which causes it to move horizontally across the whole apparatus. The articulated mold or skelet 14 is supported by the carriage 10; on said skelet the glass sheet 16 is arranged. Figure 1 shows the apparatus at the beginning of the heating step when the glass sheet 16 just put into the furnace 4 has a plane surface and the slope of the whole assembly is almost vertical. As can be remarked, such initial shope has been chosen, in the preferred embodiment illustrated, so as to be of 15° with respect to the vertical direction, in order to increase the stability of the skelet-sheet assembly during the shift of the same. The apparatus is assembled on a cylindrical steel frame comprising essentially the vertical rings 17 in number of three, and the horizontal bars 18, in addition to all other structures allowing the heating chamber 4, the cooling section (not shown), the rails 8 and 9 and all other fittings to be positioned and supported. The rings 17 are slidably resting on two wheels 19 supported on the fixed bedplate 20 of the whole apparatus; the whole cylindrical frame can rotate about its horizontal axis running on the wheels 19, driven by the two chain drives 21, the first one being connected to the first ring 17 while the other one is connected to the third ring (not shown), at the outlet end of the cooling section. It is well evident that the transverse rings and the chain driving devices can vary in number, for instance according to the size of the apparatus. The chain device 21 is made up of a chain 22 encircling the upper part of the ring 17 and fastened to the same at the point 23, and of two sprocket wheels 24 and 24 arranged on the bedplate 20, said chain 22 running around said sprockets 24 and 25; one of them, which is designated by numeral 24, is connected to the electric motor 26. By driving the device 21 when the temperature of the glass sheet has reached the desired level, the heating chamber 4 is tilted integrally with the whole main body of the apparatus by the predetermined angle as shown in Figure 2, and the bending step is started. In Figure 2, where the elements shown in Figure 1 are pointed out by corresponding numerals, the slope of the furnace 4 and of the glass sheet 16 with respect to the vertical direction is 50°. As the sheet 16 becomes softened, its profile bends, followed by that of the articulated skelet 14, that passes from the spread or open configuration shown in Figures 1 and 2 to the configuration corresponding to the final bending profile shown in dotted lines with numeral 27 in Figure 1 and with numeral 28 in Figure 3. This latter Figure represents a schematic cross section of the cooling section 29 arranged on the cylindrical frame immediately after the heating chamber 4. Only the traces of the two arrays 30 and 31 of the "organ pipes" 32 are shown in said figure, the pipes bearing the nozzles 33, whereas there are not shown the supporting structure system, the supporting frames of the arrays 30 and 31, the means for adjusting the arrangement of the pipes 32. Such means allow the pipes 32 to be shifted as shown in the figure, so as to follow the profile of the whole assembly 28, consisting of the glass sheet 16, of the skelet 14 and of the carriage 10. Also not shown are the means for keeping the two arrays 30 and 31 oscillating during working, such means being possibly made up, for instance of a mechanical device arranged on top of the cylindrical frame, causing the two frames and the two arrays 30 and 31 to move alternately upwards and downwards. Figure 3 also shows the guiding rails 8 and 9 to which the carriage 10 is connected, and the chain driving system 13 for causin the sheets to move. The ring 17 at the outlet end of the apparatus is provided with a second chain driving device 21 for causing the apparatus to rotate, said device consisting of elements whose reference numerals correspond to those of the similar elements shown in Figure 1. The driving sprocket wheel 24 of the second chain device 21 is assembled on the same shaft (34 of Figure 4) of the sprocket wheel 24 of the first device, which shaft is connected in turn to the motor 26. The glass sheet 16, just after passing from the furnace 4 to the cooling section 29, undergoes a sudden cooling owing to some air jets which are conveyed by the pipes 32 and by the nozzles 33, so that the sheet is tempered. Figure 4 shows a horizontal sectional view of the whole assembly of the apparatus according to the present invention, wherein, in addition to the main body 1 of the apparatus comprising the furnace 4 and the heating section 29, there are also shown the carriage return section 2, the loading section 35 and the unloading section 3. As shown also in Figure 1, said return section is merely made up of a pair of rails 8a and 9a similar to the rails 8 and 9 and supported by the upright members 36 and 37, which are fastened to the bedplate 20 of the main body 1, and of a chain driving system 13a similar to 13, which pushes the carriages in the direction opposite to the advancement direction within the apparatus. The slope (15°) in the same way that of the inlet into the furnace. As can be observed in Figures 1 and 4 as well as in Figure 5, which represents a vertical sectional view of the apparatus, the two sections 35 and 3, respectively for loading and unloading, are made up of two cylindrical frames with the rails 8b and 9b, similar to the rails 8, 8a and 9, 9a. Such rails support the carriages during translation from the line of the main apparatus 1 to the line of the return section 2, and vice versa. The two section 35 and 3 can move along the directions shown by the arrows in Figure 4 between a position aligned with the return section 2 and a position coaxial with the main body 1 of the apparatus. Figure 1 clearly shows the bedplate 38 of the unloading section 3, with the wheels 39 for the motion of the same. The two loading and unloading sections 35 and 3 are orientable as they are endowed with means for causing them to tilt through the rotation of the frame, such means being similar to those of the main body 1 of the apparatus. For the unloading section 3 shown in Figure 1 such means comprise the wheels 19a for supporting the frame and the chain driving system 21a comprising the chain 22a, fastened at the point 23a, the two sprocket wheels 24a and 25a, and the electric motor 26a. Thus it is possible to tilt the loading and unloading sections according to what is necessary to obtain a perfect alignment of the rails for the passage to or from the main body 1, and to or from the return section 2. The corresponding rail pairs are endowed with clutch locking systems for making such pairs integral. Figure 5, wherein the various members are always designated by the same reference numerals, also shows the glass sheet 16 resting on the skelet-type mold 15 within the furnace 4, the organ pipes 32 of one array 30 of the cooling section 29 and the air feeding system which is made up of fans 40 and of hoses 41. The apparatus disclosed above allows the process of the present invention to carried out optimally, and such apparatus can also be fitted with a set of control units and timing units and/or thermostats allowing the whole working cycle to be automatically controlled, by driving in addition the carriages, controlling the opening and closing of the furnace doors, the rotation of the main apparatus at a predetermined moment and by a predetermined sloping angle, the activation of the cooling system as well as the rotation and traverse of the two loading and unloading sections.

Claims: A process for bending and tempering glass sheets (16,28) wherein the glass sheet (16) is heated up to the plastic state, and it bends by effect of gravity so as to rest on a skelet-type mold (14), and then it is tempered by sudden cooling, in which process during the heating stage the glass sheet (16), together with said skelet-type mold (14), is initially kept approximately in the vertical position, thus forming a first angle of more than 0° with the vertical direction, and then, as soon as the plastic state is attained, the said glass sheet (16) and mold (14) are further tilted for the bending stage, thus forming a second angle greater than said first angle and smaller than 90°, chosen in accordance with the desired bending profile and the dimensions of the glass sheet (16), and in which the said glass sheet (28) and skelet-type mold (14) are kept at the same slope during the cooling and tempering stage. An apparatus for bending and tempering glass sheets (16,28) according to the process claimed in claim 1, said apparatus comprising means (8-13) for supporting and driving the glass sheets through the apparatus, including skelet-type molds (14), and, in a sequence along a horizontal line, a heating chamber (4) wherein the glass sheet (16) is heated up to the plastic state and it bends so as to rest on the skelet-type mold (14), and a cooling section, in which said cooling section is a tempering section (29), both said heating chamber (4) and said tempering section (29) are assembled on a single cylindrical frame (17,18) rotatable about a horizontal axis, and it further comprises means (19, 21-26) for tilting the heating chamber (4) and the tempering section (29) through rotation of the said frame (17,18). An apparatus according to claim 2, wherein said means for supporting and driving the glass sheets (16,28) further comprise upper and lower guide rails (9,8), both rails crossing longitudinally said apparatus, one or more carriages (10-12) which can move between said rails (9,8), the said skelet-type molds (14) being supported by said carriages (10-12), and supporting one or more glass sheets (16), and means (13) for causing said carriages (10-12) to move along said rails (9,8). An apparatus according to claim 3, wherein said skelet-type molds (14) are made up of a number of articulated segments which are hinged to one another according to horizontal axes and which can move from an open or spread configuration of the mold (14) to the configuration which corresponds to the desired bending profile. An apparatus according to claim 2, wherein said heating chamber (4) is of the rapid ignition type and is made up of a parallelepipedal structure divided into two symmetrical longitudinal halves (5) facing one another and spaced from one another, which halves (5) are also provided with perimetrical closure means (6), said structure containing electrical resistances (7) as heating means and being fully coated with layers of a suitable insulating material. An apparatus according to claim 2, wherein said tempering section (29) comprises two arrays (30,31) of horizontal nozzle-bearing pipes (32) facing one another, having longitudinal lines of nozzles (33) turned towards the zone therebetween, where the glass sheet passes, said pipes (32) being connected by means of hoses to an air source for cooling. An apparatus according to claim 6, wherein said nozzle-bearing pipes (32) are fastened to adjustable supporting means for changing their arrangement so as to closely follow the bending profile of the glass sheet (28) placed therebetween. An apparatus according to claims 6 or 7, wherein said pipe arrays (30,31) are provided with devices which are capable of keeping them moving during operation. An apparatus according to claim 2, wherein said cylindrical frame (17,18) comprises two or more vertical steel rings (17) connected to each other by longitudinal steel bars (18). An apparatus according to claim 9, wherein said means for tilting the apparatus consist of pairs of supporting wheels (19) mounted on horizontal bearings integral with the apparatus bedplate (20), said frame rings (17) being slidably resting on said wheels (19), and of two or more chain driving devices (21-25) actuated by an electric motor (26), each one driving one of said rings (17). An apparatus according to claim 10, wherein each one of said chain driving devices (21-25) comprises a chain (21) encircling perimetrically the upper part of the corresponding ring (17) of the frame (17,18) and fixedly coupled to said ring (17) at a point (23) on its top, each one of said driving devices also comprising two sprocket wheels (24,25) having horizontal axes and arranged symmetrically on the bedplate (20) of the apparatus, on both sides with respect to said ring (17), said chain (21) running around said two sprocket wheels (24,25), one of them being connected to said electric motor (26). An apparatus according to claim 2, which also comprises a return section (2) for said carriages (10-12), parallel to the axis of the main body of said apparatus and spaced apart from the same, and two separate terminal sections (3,35), namely a loading section (35) arranged at the inlet of said heating chamber (4), and an unloading section (3) arranged at the outlet of said tempering section (29), each section being mounted on a rotatable cylindrical frame with horizontal axis and movable along a straight path at right angles to the axis of the apparatus, from a position coaxial with said main body to a position aligned with said return section (2).

Inventors: MAZZAROPPI GIUSEPPE, MAZZAROPPI, GIUSEPPE
Applicants: MAZZAROPPI SRL, MAZZAROPPI S.R.L.
Application year: 1987
Main classification: C03B23
IPC codes: C03B27, C03B29, C03B23