Method and furnace for bending glass sheets

Abstract: Glass sheets are heated to a bending temperature by supporting them on a ring mould (11) and carrying them from one heating station to another. In a heating and bending station (4), the glass sheet is heated for effecting a desired bending partially or entirely through the action of gravity. During a bending operation, the temperature distribution of radiation heat is adjusted. In at least one of the heating stations (2,3,4) for preheating (2), prebending (3) or bending (4), the glass sheet is subjected to the action of convection blasting (forced convection) in addition to the radiant heat from electric resistance heaters (12) mounted above the glass sheet.

Description: The present invention relates to a method for bending glass sheets, comprising the steps of placing a glass sheet on a ring mold, which is supported on a mold wagon; transferring the mold wagon to at least one pre-heating section; pre-heating the glass section by radiation and forced convection in the pre-heating section; transferring the mold wagon with the pre-heated glass sheet to a bending section; heating the glass sheet by radiation heat during bending of the class sheet. The invention relates also to a bending furnace with glass sheets comprising one or more heating stations including at least one pre-heating section; a heating and bending section with optional pre-bending section, which includes a heating element field, including a plurality of other heating elements on a top of the heating bending section, the heating element field covering a glass sheet disposed in the bending section and the other heating elements being individually smaller than the glass sheet disposed in the heating and bending sections; one or more cooling sections below the heating sections; one or more ring molds for supporting a glass sheet during the course of heating, bending and cooling; wagons for carrying the ring molds from one station to another; a convection air blasting pipe extending into the pre-heating section through a thermally insulated sealing or wall, the convection air blasting station having a blast orifice which opens below the glass sheet plate in the pre-heating section; means for measuring a temperature of a glass sheet in the bending section. The preferred embodiment of the present invention relates to a method for bending a pair of laminated glass sheets, comprising the following steps: placing a pair of laminated glass sheets on a ring mold, which is supported on a mold wagon; transferring the mold wagon along a first horizontal track by increments substantially corresponding to the length of the wagon in the transferring direction; stopping the mold wagon for a stoppage period after each transfer increment; a first step of pre-heating the pair of glass sheets by convection heat during stoppage periods after each of the first set of transfer increments further heating of the pair of glass sheets by radiation during at least one stoppage period after at least one further transfer increment for heating so that the pair of glass sheets are heated to a temperature for initial bending; transferring said wagon to a heating and bending section; heating the glass sheets by radiation heat in the bending section so that the pair of glass sheets are bent; lowering the wagon to a second horizontal track located underneath said first horizontal track when said pair of laminated glass sheets has reached its desired bending depths; transferring said mold wagon along said second horizontal track by increments equal to the transfer increments along said first horizontal track, the transferring direction along said first and second horizontal track being opposite each other; allowing gradual cooling of the glass sheets on the second track. The above type of method and furnace apparatus for bending glass sheets are prior known from the Applicants' Patent publications US-4,497,645 and US-4,986,842. This type of furnace is especially intended for bending laminated windshields gravitationally by means of a ring mould. Naturally, press bending can also be used to assist gravitational bending. When bending typical windshield profiles, heating resistances are used to make sure that the glass sheet has a heat distribution which contributes to the formation of a desired bending shape. For example, due to the steeper bending curves formed therein, the end sections are subjected to more radiation heat than the mid-section which remains relatively flat. US-A-4,986,842 discloses a heat transfer method in a glass sheet bending furnace and a glass sheet bending furnace for carrying out the method. The method comprises the step of placing glass sheets on bending molds supported by wagons and advancing same on conveyor tracks through the heating and cooling sections. Furthermore, it is disclosed that heat is transferred from bent, hot glass in a cooling section to unbent, cold glass section in a heating section by air which is sucked from the cooling section and blasted into the heating section. The structure of this bending furnace does however not enable pre-heating by radiation and forced convection in the same section. There are pre-heating sections with efficient force convection only and heating sections with radiation heating only. The forced convection in the section is the only source of heating with powerful hot air jets uniformly on both surfaces of the glass sheet. The purpose is to utilise the heat energy recovered from glass sheets to be cooled. Such utilisation of moist heat is only possible in the first sections, where the glass to be heated is still rather cool. Furthermore, the structure of the sections with forced convection is such that heat radiators cannot be used. If heat radiators would be installed in the sections, the upper surface of the glass sheets would become excessively heated relative to the lower surface which results in distortion or breakage of the glass sheet. Furthermore, US-A-2,967,378 discloses a method and apparatus for bending glass sheets upon curved shaped molds. In particular a method for bending glass sheets is described by progressively heating the glass to different temperatures to cause the glass to bend progressively along the creasing radii without overheating or marring those portions of the glass already in contact with the mold. EP-A-0 486 952 discloses a heating apparatus for a glass sheet bending station, comprising an array of elongated heating resistance elements, positioned side by side to develop a resistance field covering the area of a glass sheet to be bent, as well as a control mechanism for switching the resistance elements on and off in order to produce at least one heating line extending in the direction other than that of the resistance elements. US-A-4,755,204 describes the manufacture of a curved glass by supporting glass on a sag bending mold which is passed through a furnace. During initial heating of the glass on the mold, hot air is directed around the mold beneath he glass to minimise the temperature difference between the mold and the glass. There is however no indication of intensified convection of the lower surface relative to the upper surface but only a description of intensified convection of the mold at the corner area of the glass. It is prior known from patent publications US-5,009,691 and US-4,441,907 to employ forced convection in a heating step preceding the bending. Neither of these publications says anything about forced convection at the time a glass sheet is already in the mould where it is heated and bent. It is prior known from patent publication US-2,967,378 to employ the local application of radiation heat to a glass sheet to be bent in a mould. From patent publication US-4,755,204 it is prior known to minimize the temperature difference between the mold and the glass by directing hot air around the mold beneath the glass. The increase of production capacity is one of the basic objectives in further development of the bending lines in vehicle windshields and backlights. In addition, there has developed a need to produce also such bending shapes, wherein the mid-section of a glass sheet bends to a progressive or uniformly extending curve without the mid-section remaining any flatter than the rest of a glass sheet. Such a shape is preferred e.g. for the reason that the windshield wipers operate more efficiently as their pressure on the glass surface remains more uniform. The aerodynamic reasons can also be a basis for preferring a progressive bending curve. This is not possible to achieve by means of resistance fields used for bending conventional windshield configurations. In principle, it would be possible to alter the resistance field configuration and the disposition of resistances in a manner that one and the same apparatus can be used for bending both conventional windshield shapes and shapes with the mid-section bent to a progressive curve. However, the re-arrangement of resistance fields for this purpose is an inconvenient and expensive operation. The shape control becomes more difficult as the pocket to which the entire glass sheet should be bent becomes deeper and the surface area of a glass sheet becomes larger. In general, the sag bending takes a long time in order that also the bending shape is under control. An object of the invention is to provide a method and a furnace assembly of the above type which, are also capable of substantially improving production capacity and effecting intensified bending of the mid-section of a glass sheet in view of producing various bending shapes without having to resort to the structural re-arrangement of resistance fields. This object is achieved in respect to the method by the subject matter of claims 1, 3 and 10 and in respect to the furnace assembly by the subject matter of claim 17. In a preferred embodiment the blasted air is colder than the surrounding air in the furnace. As a result of this, there is no excessive heating at the blasting point, but the convection effect is spread along the glass surface such that it may be evenly divided or even stronger at the areas surrounding the blasting point. The convection blast can be focused to any section at which the bending to form a pocket should be intensified. In a typical case, the convection blast is focused on the mid-section of a glass sheet for bending it to a progressive curve. Convection blast can be maintained throughout the period that the glass is stationary in a preheating station (or in a subsequent prebending or bending station), allowing for the use of a very weak blast which does not locally overheat the glass surface so as to form a "heat lens". Though convection blast can also be used in a prebending station preceding the bending station and/or in the bending station itself, it is most advantageous to employ convection blast just in preheating stations for bending laminated glass sheets (windshields). The reason for this appears from the following. The invention is particularly suitable for use in such a method, wherein the bending operation of laminated glass sheets is followed by transferring the mould wagon and the pair of bent glass sheets from an upper track onto a lower track and by cooling the pair of bent glass sheets on the lower track below said preheating sections. This prior known method involves the following problem. Inside preheating sections develops a temperature difference of about 100°C between the bottom and top glass. This temperature difference does not equalize even in the bending section. A result of this is that the bottom glass "resists" bending of the top glass and decelerates bending as it is necessary to wait for the bottom glass to heat up to a bending temperature. This effect is particularly pronounced in the process of creating a progressive sag in the middle of a glass piece or in complex bends in the corners. In view of the above, it is a further object of the invention to overcome this problem and to equalize said temperature difference in a manner such that heating and bending can be accelerated and thus the production capacity of a furnace can be increased while facilitating the attainment of desired bending shapes. This object is achieved by intensifying the transfer of heat by means of convection to the bottom glass of a pair of glass sheets lying in preheating section by blasting a small amount of air to the bottom surface of said bottom glass for thus reducing a temperature difference between the glass sheets included in a pair of glass sheets lying in preheating section. In this particular type of furnace the transfer of heat to the bottom glass can be intensified by intensifying the transfer of heat from a glass piece cooling down on the lower track to the bottom glass piece lying on the upper track. In a particular application of the invention, this is effected by blowing a small amount of air through the open floor of a mould wagon from pipes fitted between the upper and lower tracks. The best mode of the invention offers the following benefits: the top and bottom glass pieces are brought to an equal temperature prior to the commencement of actual bending by making effective use of a temperature difference (about 100°C) between the glass pieces lying on lower track and on upper track for heating the bottom glass piece on the upper track by applying convection without separate heating. The hemothermal glass facilitates rapid bending of the glass since there is no need in a bending process to wait for the bottom glass to heat up to a bending temperature. The cooling process does not develop spreading or reverse bends in the glasses as a result of temperature differences between top and bottom glasses. The glass finds its way better to a shape determined by a ring mould, especially in complex bends. The bending of a progressive sag can be successfully effected with a short bending time. A furnace or leer assembly of the invention is characterized in that the preheating station and/or prebending station and/or bending station is provided with a convection air blast pipe extended through the thermally insulated ceiling or wall and having its blasting orifice opening below and/or above a glass sheet placed in the preheating station and/or prebending station and/or bending station for blasting air colder than the surrounding furnace air towards the glass surface from said blasting orifice. The convection air blast pipe can be fitted with a pyrometer for measuring the glass temperature. Thus, the blasting action e.g. termination of the blasting, can be controlled on the basis of a temperature measurement effected by the pyrometer. In the first instance,the glass temperature measurement by the pyrometer is used for controlling and adjusting the distribution of radiation heat in the bending station, as disclosed in more detail in publication EP-0486952. One exemplary embodiment of the invention will now be described in more detail with reference made to the accompanying drawings, in which fig. 1 shows a vertical section through the furnace assembly of the invention at preheating stations; fig. 2 shows a vertical section through the outlet end (bending station) of the same furnace assembly; fig. 3 shows a vertical section through the inlet end of the same furnace assembly; fig. 4 shows test curves about the deflections of a glass sheet as a blast of the invention is applied to the bottom surface, top surface of a glass sheet and without a blast. In all cases, the constant bending time was 200 s. fig. 5 is a schematic cross-section through a furnace at a preheating section 2; fig. 6 shows a portion of a blast pipe 13 and its orifices in a plan view; fig. 7 is a schematic cross-section through the heating and bending section; fig. 8 is a top plan view of the heating resistances in the heating and bending section, Fig. 8a showing an alternative to middle section resistances 12a; figs 9 and 10 show sag profile curves with different relations between the glass height (H) and the length (h) of middle section resistances 12a in Fig. 8. A furnace for applying the method includes a number of successive heating stations or sections 1, 2, 3. A pair of laminated glass sheets is carried by a ring mould 11 into the first furnace section 1. The term "a pair of laminated glass sheets" can be replaced hereinafter by "a glass sheet". Mould 11, together with its glass sheet, is advanced from one section to another by means of a mould wagon 9 provided with an end wall 9a which separates the successive sections from each other. In first section 1, a glass sheet is primarily heated with forced convection receiving its thermal energy from a bent glass sheet in the process of cooling in lower cooling sections 7, as described in more detail in the Applicant's patent US-4,986,842. In sections 1, the share of forced convection from the heating effect of a glass sheet is typically 90 % and the share of radiation is 10 %. The number of heating sections 1 based on the recovery of heat can be 3-6 and a glass sheet reaches therein a temperature of 230°C - 300°C prior to advancing into the following preheating section 2. In preheating sections 2, the heating is partially effected by means of heating resistances 12 and partially by convection heat, coming through the floor and/or around the edges of carrier wagons 9 and delivered by glass sheets in the process of cooling in lower cooling sections 6. In sections 2, the share of radiation is e.g. 40 % and that of convection heating is 60 %. The number of preheating sections is typically 3-6. From preheating sections 2 a glass sheet advances into a prebending section 3 as the glass temperature is e.g. about 530°C - 550°C. In prebending sections 3, the temperature is raised by a few dozen degrees, which already results in slight bending of a glass sheet. Finally, a glass sheet is advanced into an actual bending section 4, wherein its temperature is raised e.g. to a temperature range of 590°C - 635°C. At the bending temperature, glass is highly sensitive to temperature variations and, thus, a change of even a few degrees in the temperature of a glass sheet has a major effect on its susceptibility to bending. In bending section 4, the transfer of heat to glass is typically such that the share of radiation is 90 % and that of convection is 10 %. Thus, the periodical progress of wagons 9 on an upper track 18 results in the formation of successive sections 1, 2, 3 and 4. After a bending operation, said wagon 9 and a pair of bent glass sheets are transferred from upper track 18 onto a lower track 19 for conveying said wagons 9 in the opposite direction also periodically, whereby the currently cooling pairs of glass sheets are located below a pair of glass sheets being heated on upper track 18 at a given time. It has been realized in the invention that, in addition to a temperature distribution achieved by means of ON/OFF switching of the resistances 12 in the bending section 4, convection blasting can be used to provide a more diversified range of shapes that can be bent in a controlled manner. Particularly, there has been a need for bending glasses having a rather large surface area in view of bending the mid-section thereof to a progressive curve. For this purpose, one embodiment of the invention includes a convection-air blasting pipe 13', which has been extended through the thermally insulated ceiling of the bending station 4 (and/or prebending station 3) and between heating resistances 12 to the proximity of the top surface of a glass sheet placed in the bending station. The distance between the bottom end of pipe 13 and the surface of a glass sheet can be within the range of 10 to 20 cm. The diameter of pipe 13 can be e.g. from 8 to 12 mm. In a typical case, pipe 13 is used to blow convection air very gently throughout the entire bending operation essentially over the mid-section of a glass sheet. The blowing rate is less than 0,1 l/s and typically within the range of 0,1 to 0,03 l/s. In order to accelerate the bending, a very gentle blowing (e.g. 0,01 l/s) can be used also for conventional bending shapes by making the necessary modifications to the program for controlling the temperature distribution of a resistance field. A pyrometer 14 is mounted on pipe 13 for measuring the temperature of bending glass. According to a predetermined program, a control unit 15 is provided to switch the resistances 12 off during the course of bending such that the radiation heat distribution pattern changes in a predetermined manner during the bending. The control unit 15 can also be used for controlling valves 16 and 16' on the basis of temperature measurement. Valve 16 is a control valve for the regulation of flow rate and valve 16' is an on/off- valve. Naturally, both valve types can be used separately or together for controlling the blowing time or rate. In a normal case, the blow control is not required at all for the duration of bending a single glass sheet but only when the type of glass sheet is changed. The on/off switching of a blast is required at the time a glass sheet is transferred from one station into another, so that the blasting or blowing would not be applied to the edge areas during a transfer. This is necessary since the focusing of a blast in any of the stations 2, 3 and/or 4 is used to make sure that a desired bending shape is attained. Thus, a typical case involves the use of a continuous and constant blow during the stay of glass at each station and the distribution of heat transfer is adjusted by adjusting the temperature distribution of a resistance field 12. The adjustment concerns primarily as to when and how many of the mid-section resistances are switched off at the final stage of heating in the bending section 4. A similar convection-air blasting system can also be arranged in preheating station 3. The creation of a necessary convection blasting requires a very low-power pump 17. Since the amount of air to be blown or blasted is very small, the blast air can be obtained directly from the room surrounding the furnace. Therefore, the air discharging from the pipe 13' is colder than the air in the furnace, this being the case also when the air in the pipe 13' is preheated (e.g. by the surrounding air in the furnace). A convection blasting of the invention serves to achieve, in addition to improved control over a bending area, also an increase in capacity since the convection blasting makes the bending operation substantially quicker. This is not actually a result of the increase of heat transfer effected by convection but it is due to the following facts: (1) the temperature difference between top and bottom glasses (or surfaces of a glass sheet) can be equalized, (2) the adjustment of resistances is not the only means for controlling the heat transfer distribution, and (3) in many cases, the total amount of radiation heat can be increased if desired. The location of a blasting spot as well as the blasting distance and the temperature of discharging air each have an effect on the shape a glass sheet is bending to. As already pointed out, in preheating sections 2 (Fig. 1) there develops a temperature difference of about 100°C between top and bottom glasses. This temperature difference causes the drawbacks mentioned in the introduction. In preheating section 2, the temperature of bottom glass can be e.g. 400°C. The temperature of a pair of glass sheets in the process of cooling on lower track 19 therebelow can be e.g. 500°C. Now, the transfer of heat possessed by the glass on lower track 19 can be intensified by means of convection to the bottom glass of upper track 18 by blowing or blasting a small amount of air through the open floor of wagon 9 from pipes 13 fitted between the upper and lower tracks. There may be one or a plurality of blast pipes 13. It is possible to use e.g. horizontal pipes 13, having a diameter of 20 mm and provided with 2,0 mm orifices 20 over the entire length at 50 mm intervals. The size of orifices 20 in the middle may also be different to those on the edges. The air to be delivered into pipes 13 can be compressed or blast air, which is preheated in a pipe system circulating inside a furnace or by means of a separate heater. Pipe 13 can also be branched. When using boosted convection according to the invention, the top and bottom glass can be brought to an equal temperature prior to commencing the actual bending. Equal temperature or hemothermal glass facilitates rapid bending, since there is no need in the bending operation to wait for the bottom glass to heat up to a bending temperature. The glass finds its way better to a shape defined by a ring mould, especially in complex bends. Also the bending of a progressive sag is successfully effected with a short bending time. This is demonstrated by the test run curves shown in fig. 4. The deflection illustrated by the top curve is produced by using convection intensified according to the invention for raising the temperature of bottom glass. However, if convection blasting is not applied, the deflection obtained by the same bending time of 200 seconds will be that demonstrated by the bottom curve. The middle curve shows that forced convection onto the top surface of the glass sheet also accelerates the bending, but substantially lesser than in the case of bottom blow. In a typical case, said pipes 13 are used for blasting convection air throughout the heating period, as a glass sheet is stationary in section 2. When a glass sheet is transferred from one section to another, the blasting can be switched off, if necessary, in order not to apply the blast onto the edge portions. Although the amount of air to be blasted is relatively small, nor does it require separate heating, the method is capable of achieving an increase of about 30 % - 40 % in production capacity as compared to a similar type of furnace without blasting. Furthermore, the disposition of blast pipes 13 as well as the size and disposition of orifices 20 included in pipes 13 can be used for such an effect that the temperature distribution of a glass sheet will be beneficial in view of a desired bending shape. In other words, the location of a blasting spot or the locations and distribution of the blasting spots have an effect on the shape a glass sheet is bending to. Along with or in addition to the mid-section, the blasting can be focused or distributed on the corner sections of a glass sheet, which require relatively sharp bends in multiple directions. If desired, the blowing spot of blast pipe 13 can be adapted to be maneuverable in vertical and/or horizontal direction. The manipulation of a blasting spot can be effected from outside the furnace mechanically by means of a suitable operating leverage. Naturally, it is possible to use both overhead and underhand blasts applied to the top and bottom surface of glass. In a typical case, however, blasting is used below the mid-section of a glass sheet in the preheating sections 2, and the shape is also provided by means of a program controlling the distribution of radiation heat. In a preferred embodiment, in the heating and bending section 4 there are additional resistances 12w on the side walls of the furnace below the level of the edge of the glass to be heated. The resistances 12w increase the general temperature of the lower portion of the heating and bending section 4. These additional resistances particularly aid the bending of the corners of the glass sheet. It has found to be particularly advantageous to use only relatively short top resistances 12a above the middle area of the glass sheet when the glass sheet is bending in the heating and bending section 4 (Fig. 8). Fig. 9 shows a sag profile in the height direction of the glass sheet with the relation between the glass height H and the length h of the heated resistances 12a being about 2. (The resistances 12b are switched off during the sag bending). The resistances 12a with said relation 2 are still too long, i.e. the heated area extends too close to the lower and upper edge of the glass sheet, with the result that the sag profile is not progressive. Fig. 10 shows two different sag profiles with said relation being 2,2 (open dots) and 2,6 (black dots). The sag profile with said relation 2,6 is almost completely progressive. Most preferably said relation is more than 2,6 but less than 3,2. For this purpose the resistances 12a above the middle section can be divided in three sections, which can be separately switched on and off (Fig. 8a). Alternatively, the resistances above the middle section can be made substantially shorter than the resistances (12, 12b) surrounding said middle section (Fig. 8). The resistances 12, 12b surrounding the middle section must extend substantially beyond the glass contour. During the sag bending, only the middle section resistances 12a and the outermost resistances 12 at the corners as well as at the ends of the middle section, close or beyond the glass edges, are switched on. This provides progressive sag bending, and the complex bending at the corners of the glass sheet is ensured by the wall resistances 12w (Fig. 7). Switching off of the resistances 12, 12b around the middle section resistances 12a takes place at the beginning and/or during the sag bending. This switching off of the resistances is controlled by the measured glass temperature.

Claims: Method for bending glass sheets, comprising the steps of: a) placing a glass sheet on a ring mold which is supported on a mold wagon; b) transferring the mold wagon to at least one pre-heating section; c) pre-heating the glass sheet by radiation and forced convection in the pre-heating sections; d) transferring the mold wagon with the pre-heated glass sheet to a bending section; e) heating the glass sheet by radiation heat during bending of the glass sheet; and characterized by pre-heating the glass sheet in step c) by radiation and forced convection in the same pre-heating section; and f) adjusting a heat distribution of the radiation heat in the bending section during said bending of step e). A method as claimed in claim 1, wherein the forced convection is induced by air jets colder than the surrounding air in the pre-heating section. Method for bending a pair of laminated glass sheets comprising the following steps: placing a pair of laminated glass sheets on a ring mold, which is supported on a mold wagon; transferring the mold wagon along a first horizontal track by increments substantially corresponding to the length of the wagon in the transferring direction stopping the mold wagon for a stoppage period after each transfer increment a first step of pre-heating the pair of glass sheets by convection heat during stoppage periods after each of the first set of transfer increments; further heating the pair of glass sheets by radiation during at least one stoppage period after at least one further transfer increment for heating so that the pair of glass sheets are heated to a temperature for initial bending; transferring said wagon to a heating and bending section; heating the glass sheets by radiation heat in the bending section so that the pair of glass sheets are bent; lowering the wagon to a second horizontal track located underneath said first horizontal track when said pair of laminated glass sheets has reached its desired bending depth; transferring said mold wagon along said second horizontal track by increments equal to the transfer increments along said first horizontal track, the transferring direction along said first and second horizontal track being opposite each other; allowing gradual cooling of the glass sheets on the second track characterized by after said first step of pre-heating, a second step of pre-heating the pair of glass sheets by radiation from above and convection from below in the same section during stoppage periods after each of a second set of transfer increments, thereby reducing the temperature difference between an upper and lower one of the pair of glass sheets, while pre-heating the pair of glass sheets by intensifying the convection from below with forced convection to increase the temperature of the lower glass sheet with respect to a temperature of the upper glass sheet. A method as claimed in claim 3, wherein the local distribution of the radiation heat in the bending section during bending of the pair of glass sheets is adjusted by turning off the selected heating elements, forming a heating element field in the bending section, the heating element field covering the pair of glass sheets and the heating elements forming the heating element field being individual, smaller than the pair of glass sheets. A method as claimed in claim 3 or 4, wherein the heating energy for the pre-heating by convection from below is obtained from a pair of glass sheets cooling on the second track by allowing the convection air to flow through the open bottom or around the bottom edges of the wagon of the first track. A method as claimed in claim 3 or 4, wherein the convection is intensified with forced convection in some of the forced jets of air which are directed through open portions of the bottom of the mold wagon on the first track. A method as claimed either in claim 1 or in claim 3, wherein the forced convection is maintained substantially constant during pre-heating. A method as claimed in either claim 1 or claim 3, wherein the forced convection is directed on at least a central area of the glass sheet. A method as claimed in claim 8, wherein air is blown through a convection air blasting pipe to create the forced convection, the air blown through the convection air blasting pipe being drawn from the outside of the furnace and being colder than the surrounding air in the pre-heating section. A method for bending glass sheets, comprising the steps of: placing a glass sheet on a ring mold which is supported on a mold wagon; transferring the mold wagon to at least one pre-heating section; pre-heating the glass sheet in the pre-heating section; transferring the mold wagon with the pre-heat glass sheet to a pre-bending section; initially bending the glass sheet in the pre-bending section by raising the temperature of the glass sheet to a first temperature for initial bending; transferring the pre-bent glass sheet to a heating and bending section; characterized by heating the glass sheet by a combination of radiation heat and forced convection during pre-bending and/or bending in the same bending station so that the glass sheet is bend, the bending station including a heating element field, including a plurality of heating elements, the heating element field covering the pair of glass sheets and heating elements being individually smaller than the pair of glass sheets; and adjusting a heat distribution on the radiation heat in the heating and bending section during pre-bending and/or bending by turning on and off selected ones of the heating elements. A method as claimed in claim 10, wherein the forced convection is maintained substantially constant during said pre-bending and/or bending. A method as claimed in claim 10, wherein the forced convection is directed against both an upper and lower surface of the glass sheet. A method as claimed in claim 10, wherein air is blown through a convection air blasting pipe to create the forced convection, the air blown through the convection air blasting pipe being drawn from outside of the furnace and being colder than the surrounding air in the heating and pre-bending sections and/or in the heating and bending sections. A method as claimed either in claim 1 or in claim 3 or in claim 10, wherein the forced convection is directed against at least a central area of the glass sheets. A method as claimed either in claim 1 or in claim 3 or in claim 10, wherein bending takes place by gravity. A method as claimed in claim 1, wherein during bending, the glass is subjected to radiation heat from side heating elements on side walls of the furnace below an edge of the glass sheet and also from upper heating elements on top of the middle section of the glass sheet, the length of the upper heating elements being limited in the height direction to the glass sheet, wherein the relationship between the glass sheet height and the length of the upper heating element is greater than 2.2, preferably greater than 2.4, and most advantageously about 2.6 to 3.2. A bending furnace for glass sheets comprising: one or more heating sections (1,2,3) including at least one pre-heating section (2); a heating and bending section (4) with optional pre-bending section, which includes a heating element field, including a plurality of other heating elements on a top of the heating and bending section, the heating element field covering a glass sheet disposed in the bending section and the other heating elements being individually smaller than the glass sheet disposed in the heating and bending sections; one or more cooling stations (5,6,7) below the heating stations; one or more ring molds (11) for supporting a glass sheet during the course of heating, bending and cooling; wagons (9) for carrying the ring molds (11) from one station to another; a convection air blasting pipe (13) extending into the pre-heating station through a thermally insulated sealing or wall, the convection air blasting station having a blast orifice (20) which opens below the glass sheet plate in the pre-heating station; means (14) for measuring a temperature of a glass sheet in the bending section; and characterized by means for a combined effect of heating by radiation and convection in the pre-heating section and/or a convection air blasting pipe (13') extending into the bending section 4 and/or into the optional pre-bending section 3 through a thermally insulated sealing or wall for a combined effect of heating by radiation and convection and by means (12,15) for adjusting a distribution of radiation heat from the heating element field by selectively turning on and off individual ones of the upper heating elements during bending of the glass sheet in response to a temperature measurement by the measuring means. A bending furnace as set forth in claim 17, wherein the air discharged from the blast orifice (20) is drawn from outside of the furnace through the convection air blasting pipe and is colder than the air in the pre-heating station (2). A bending furnace as set forth in claim 17, wherein the bending section includes sides and top walls and side heating elements on the walls of the heating and bending section below the level of edges of the glass sheet in the heating and bending section and a length of the upper heating elements above a middle section of a glass sheet in the heating and bending section is limited in a height direction of the glass sheet such that a relationship between the glass sheet height and the length of the upper heating elements is greater than 2.2, preferably greater than 2.4, and most advantageously about 2.6 to 3.2.

Inventors: KAONPAEAE ARTO, NIKKANEN JUKKA, SALONEN TAPIO, YLI-VAKKURI ERKKI, KAONPAEAE, ARTO, NIKKANEN, JUKKA, SALONEN, TAPIO, YLI-VAKKURI, ERKKI, Kaonpää, Arto
Applicants: TAMGLASS ENG OY, TAMGLASS ENGINEERING OY
Application year: 1993
Main classification: C03B29
IPC codes: C03B29, C03B25, C03B23