JP2017007923A - Heating method for reinforcing glass plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heating method for reinforcing a glass plate (G).SOLUTION: The method of the invention includes: transporting the glass plate (G) on a roller (5) in a roller hearth furnace (3); and heating the glass plate (G) in the roller hearth furnace (3) to a transferring temperature on transferring the glass plate (G) into an air support furnace (3'). The glass plate (G) is transported on an air support table (10) while it is on an air cushion (3''), and the glass plate (G) is heated to a reinforcing temperature in the air support furnace (3'). The transferring temperature is not lower than 620°C and not higher than 675°C, and the reinforcement temperature is not lower than 650°C, and not higher than 720°C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method of heating to reinforce a glass plate, the method comprising conveying a glass plate on a roller in a roller hearth furnace, an air support table while the glass plate rests on an air cushion It includes heating the glass plate in a roller hearth furnace to a transition temperature when the glass plate is transferred into the air support furnace carried above, and heating the glass plate to a tempering temperature in the air support furnace.

  In a normal tempering line (conventional float glass or soda lime glass), heating from room temperature to tempering temperature takes place in a furnace where the glass plate is supported through a heating cycle by a rotating ceramic roller. The normal furnace temperature is 700 ° C. The strengthening temperature of a 3 mm thick glass is generally about 640 ° C. The tempering temperature can be lowered as the glass becomes thicker, and the tempering temperature is about 625 ° C. for a glass having a thickness of 10 mm. In the furnace, the glass rests on rollers that are spaced about 100 mm apart.

  The elastic plate returns to its original shape as soon as it is released from the forced tension. The plastic sheet will not return to its original shape when deformed. Creep refers to permanent or plastic deformation related to time or temperature that occurs in response to a constant load or stress.

  Glass begins to convert from an elastic plate to a plastic material at a temperature of about 500 ° C. The instantaneous deformation that occurs in the glass returns slowly and slightly as the temperature increases. Furthermore, the glass begins to creep due to its own weight. For example, in a roller hearth furnace, the glass creeps down over the length between the rollers. As a result, the glass changes its shape from a linear shape to a corrugated sheet, that is, a so-called roller wave is formed in the glass. The creep rate increases in conjunction with the glass temperature. The roller wave generated in the glass depends not only on the strengthening temperature, but also on at least the heating time, the distance between the rollers and the speed of movement of the glass in the furnace. Even a tempered glass having a thickness of 3 mm heated to a tempering temperature of just 640 ° C. in a roller hearth furnace may detect a slight waveform distortion generated in the glass in the roller hearth furnace. In the case of a 3 mm thick glass, simply raising the tempering temperature to 670 ° C. produces a clearly noticeable corrugated pattern, which makes the glass no longer acceptable in terms of quality. On the other hand, the above-mentioned increase in the tempering temperature (640 → 670 ° C.) gives the glass a clearly high strength, that is, increases the compressive stress on the surface of the glass. Moreover, even if the performance of the reinforced fan is significantly reduced, the same strength can be imparted.

  In general, the strength of heat-strengthened glass is about half that of normal tempered glass. In contrast to heat tempered glass, tempered safety glass breaks into pieces that are small enough not to tear. When the tempering temperature is 640 ° C., it is too low to satisfactorily strengthen the glass having a thickness of 2 mm. In the final 2 mm thick glass, when heated to a tempering temperature of 640 ° C. in a roller hearth furnace, the roller wave becomes clearer than the 3 mm thick glass. Therefore, the problem of roller waves becomes more pronounced as the glass becomes thinner.

  In addition to roller waves, there is what is said to be edge sagging that increases as the strengthening temperature rises from 640 ° C. to 670 ° C. in a roller hearth furnace. Edge sag refers to the deflection below the leading and trailing edges of the glass at a distance of 50-100 mm.

  The market for higher strength 1.7-2.6 mm thick tempered glass, which is also required to break without risk of tearing, is most common in the solar energy, furniture and vehicle industries. The glass used for solar panels must be robust, especially to withstand the mechanical impacts and cooling stimuli that occur when hail falls. On the other hand, the efficiency of solar panels improves as the glass becomes thinner. This is because more solar radiation can penetrate the glass and reach the actual semiconductor cell. Glass used in the furniture and vehicle industry is generally required to be safety glass, which is durable and breaks without risk of tearing. The advantage of using thin glass is that it is lightweight.

  In fireproof tempered glass (FRG glass = fireproof glass), its strength is at least about 1.5 times that of ordinary tempered glass, and exhibits a compressive stress of about 160 MPa on the glass surface. High surface compression provides greater resistance to temperature differences (thermal stress), thereby increasing heat resistance time. Thus, FRG glass can be used to send a fire spreading from one building, section or room to another. A normal FRG glass has a thickness of 6 mm. More stringent strength standards have been introduced into the FRG market, so there is a new market opportunity for slightly thinner glass than before.

  The compressive stress (strength) generated on the glass surface in the tempering process depends on the temperature difference between the glass surface and the inside of the glass when the glass cools in a transition region of about 600 → 500 ° C. Usually, this temperature difference is about 100 ° C., and the vertical temperature profile is parabolic. Thin glass requires a small cooling effect when obtaining the same temperature difference as above. For example, a 3 mm thick glass tempering requires about 5 times higher cooling fan engine performance per glass area than a 4 mm thick glass tempered. On the other hand, thin glass cools clearly faster as a result of high cooling and low thermal effects. Thus, the cooling zone may not be as long as that required for 4 mm thick glass, and the multiple 5 described above is reduced. In an intermediate width tempering line for 3 mm thick glass, a common requirement is that the cooling fan engine performance is about 800 kW. Therefore, the production capacity is increased. The problem of energy consumption occurs, and this high production capacity increases the price of electrical parts.

  Increasing the glass tempering temperature increases the strength achieved in the tempering process, because the temperature difference and profile described above is such that the temperature of the glass surface and surface layer is clearly lower than the upper limit of the transition region. This is because it progresses to a more finished state before falling down.

  In a roller hearth furnace, it is relatively easy to adjust the heat transfer to the glass so that the glass remains straight in the furnace. As long as its vertical temperature profile is reasonably symmetric, the glass remains straight in the furnace. Perfect symmetry is not required for the temperature profile as it attempts to bring the flat part of the glass into contact with the roller. Accordingly, the force generated by the thermal stress is required to exceed the resistance of gravity that causes the glass to bend. The symmetric temperature profile is the result of the same thermal effect being supplied to the glass from both above and below. Excessive non-uniform heating of the glass from above and below leads to glass deflection. Thus, the glass may be bent as a result of thermal stress. The risk of bending is reduced with increasing temperature, as the temperature difference between the glass and the furnace decreases, the thermal effect passing through the glass is essentially reduced. As the thermal effect decreases, the temperature difference that can occur between the upper and lower sides also decreases, making it more difficult to produce a sufficiently asymmetric temperature profile.

  Even at the end of heating, if the furnace undergoes a sudden change in its temperature or convection jets, the glass may bend in response to thermal stress. In general, the furnace is not subject to such a sudden change in heating effect because the furnace structure (roller hearth furnace) is usually the same over its entire length and is not adjusted for gradual changes. . As the glass advances from the roller hearth furnace into the air support furnace, the structure of the furnace, particularly the structure under the glass, changes significantly. In a roller hearth furnace, heat is transferred from the furnace floor to the bottom of the glass by means of rollers, lower resistance and radiation. Heat is also transferred to the glass by conduction from the point of contact between the roller and the glass and by convection from the air in the bottom section of the furnace (the furnace includes a device for convective injection). Bottom convection is forced convection from air injection into the glass when convection injection is active.

  On the other hand, in an air support furnace, heat is transferred from the air support table to the bottom surface of the glass by radiation and convection from a thin air cushion between the glass and the table. The main function of the air cushion is to support the glass, because the jet pressure of the air support table jet cannot be used to adjust the heat transfer. The heat transfer from the air cushion to the glass can be limitedly controlled by adjusting the temperature of the circulating air. On the other hand, the temperature of the circulating air also depends on the temperature of the upper section of the air support furnace and the temperature of at least part of the roller hearth furnace at the intersection. As a result of the furnace structure change consistent with the transition phase described above, the heat transfer under the glass will change stepwise. Furthermore, in an air support furnace, it is difficult to adjust the heat transfer so that the thermal effect passing through the glass is approximately equal on the top and bottom surfaces of the glass. This is particularly difficult in the transition phase because the temperature of at least part of the roller hearth furnace also affects the first part of the air support furnace.

  Instantaneous deflection of the glass in a roller hearth furnace is the biggest defect in glass quality. The air cushion has a thickness of less than 1 mm, that is, even if the air support furnace is slightly bent, the glass may be brought into contact with the ceramic table, which may cause the glass to slow down or stop. This can result in at least a serious quality defect and can be easy to stop production and cool the furnace because the glass left in the air support furnace is difficult to remove from the hot furnace. Therefore, the glass should not bend and contact the table in the air support furnace.

  In the actual tempering process for glass with a thickness of 2 mm, the tempering temperature needs to be much higher than 640 ° C., but because of the roller waves mentioned above, this cannot be done in a roller hearth furnace alone. Glass that can be used as FRG glass can impart sufficient strength with a smaller cooling effect as the tempering temperature is increased. Overheating is therefore advantageous, but the temperature is limited even for thick glass due to roller waves.

  The air support table of the air support furnace is composed of continuous ceramic table parts, and these are provided with a laterally extending path having no discharge port and injection port. The table parts are supported on ceramic vertical wall tiles. Since the table must be continuous and flat, the dimensional tolerances of these parts are small. Since the ceramic parts described above are expensive, the cost per meter of the air support furnace is significantly higher than the cost per meter of the roller hearth furnace. Therefore, the higher the transition temperature at which the glass is heated in the roller hearth furnace, the more advantageous. As the transition temperature increases and the strengthening temperature remains unchanged, the length of the expensive air support furnace is reduced and the overall furnace cost is also reduced.

  The object of the present invention is to eliminate or substantially reduce the above defects.

  The above objective is accomplished by the present invention such that the transition temperature is not lower than 620 ° C. and not higher than 675 ° C. and the strengthening temperature is not lower than 650 ° C. and not higher than 720 ° C.

  In addition to the above, preferred embodiments of the invention are described in the dependent claims.

  The combination of a roller hearth furnace followed by an air support furnace makes it possible to use a higher strengthening temperature than conventional, even a strengthening temperature as high as 700 ° C. A strengthening temperature above 720 ° C., in particular above 730 ° C., is no longer beneficial in view of the fact that strength is only slightly improved and the manufacturing problems are clearly increased.

  The purpose of the air cushion under the glass is to support the glass evenly throughout its front area. Thus, the glass does not generate roller waves or other such distortions at the transition and tempering temperatures of the present invention, and the method of the present invention is also applied to thin glass sheets, eg, 2 mm thick. In a glass strengthening line using only a roller hearth furnace, it is experimentally known that, for example, glass having a thickness of 3 mm generates roller waves when the strengthening temperature is 650 ° C. On the other hand, the 3 mm thick glass strengthened by the 650 ° C transition temperature and the tempering temperature of 650 ° C in the test operation performed in the combination of the roller hearth and the air support furnace does not generate a noticeable roller wave. It was. Therefore, the air support furnace reduces at least the roller wave generated in the roller hearth furnace, and removes the roller wave even if the strengthening temperature is higher than 650 ° C. The roller wave disappears slowly as the glass temperature decreases in the air support furnace.

  The method of the invention and some preferred embodiments thereof will be described more precisely with reference to the accompanying drawings.

1 is a schematic illustration of a furnace intended to strengthen a glass sheet to carry out the method of the present invention. It is a theoretical temperature graph of one glass sheet in a strengthening process.

  The method according to the invention can be applied in several ways within the scope of protection as defined in the claims.

  Thus, shown in FIG. 1 is a tempering furnace divided into two sections, which facilitates the work according to the method of the present invention. Reference numeral 1 is attached to the tempering furnace. In the tempering line of FIG. 1, the heating of the glass G from room temperature to the transition temperature occurs first in the roller hearth furnace 3 (first section of the tempering furnace), and the glass G moves while resting on the ceramic roller 5. The roller hearth furnace 3 for heating comprises an upper heating means 4 and a lower heating means 6 known per se. These include radiant heating means and / or convective jet means. All the final heating from the transition temperature to the tempering temperature takes place in the air support furnace 3 '(second section tempering furnace) and the glass G is between a ceramic table or so-called air support table 10 and the glass plate G. It floats at the top of the thin air cushion 3 ″. The air support table 10 is inclined at 1 to 20 ° C., and this inclination pushes the side edge portion of the glass G to a conveyance roller (not shown). The moving speed of the glass G in the air support furnace 3 'is equal to the peripheral speed of the transporting roller. The frictional force with which the glass G engages the transport roller, particularly at small tilt angles, is so small that local glass slack in contact with the table at least slows and stops the movement of the glass. The horizontal (0 ° C.) position of the air support table 10 is possible only when the air support table 10 is changed to a conveyor that holds the transport roller glass G. The air support table 10 includes an injection port and a discharge port (not shown). The airflow from the injection port under the glass generates an overpressure under the glass against the existing pressure in the furnace, which is the basis for the floating of the glass G on the air cushion 3 ″. Air exits from the bottom of the glass G and around the edge of the air cushion 3 ″ through the outlet.

  It is described in the literature that the transition temperature or conversion temperature of glass (soda lime glass) is about 570 ° C. A temperature range of about 500 ° C. to 600 ° C. is said to be a glass transition region or a conversion region. Glass can be classified as a solid material at temperatures below the transition region, and can be classified as liquid at temperatures above the transition region. Thus, in the glass heating process, the transition from solid to liquid does not occur suddenly at the transition or conversion temperature. Metastasis is also time dependent.

  In the tempering furnace 1, the glass temperature continues to rise from 500 ° C., while the elasticity of the glass continues to decrease. Further temperature increase shortens the time spent for disappearance of the glass stress, i.e. the relaxation time. Thus, the risk of deflection described above begins to be dramatically reduced. The temperature difference of the glass plate G can no longer generate the same high stress on the glass, and the resulting stress disappears faster and faster. Finally, the glass sheet G is not bent or sagged as a result of the temperature difference generated in the glass sheet G in the tempering furnace 1.

  The air support table 10 has a thickness and is made of a low heat transfer ceramic. When the glass plate G cooler than the table 10 arrives and is supported by air, the upper surface of the table 10 begins to cool slowly because heat is slowly transferred from the inside of the ceramic to the surface. The continuous glass sheet G continues to cool the table 10 further, so that the next glass sheet 10 always undergoes a different heat transfer from the preceding sheet. This becomes a problem in terms of stability of the heating process. The increase in transition temperature performed in the present invention is beneficially expensive in this respect because the upper surface of the table (10) slowly cools due to the decrease in temperature difference between the glass plate G and the air support furnace 3.

  In the conventional method, when the process proceeds at a transition temperature from the roller hearth furnace to the air support furnace, the transfer of heat applied to the glass plate changes stepwise and may be bent and contact the air support table. The transition temperature of US Pat. No. 3,409,422 does not exceed 980 ° F. (= 527 ° C.) and US Pat. No. 3,223,501 is about 950 ° F. (= 510 ° C.). These patents stipulate that the glass is an important factor regarding the risk of flexing and contacting. In practice, in the transition phase, the glass simply warps and contacts the ceramic table at temperatures of 560 ° C and 580 ° C. At a temperature of 600 ° C., contact due to deflection hardly occurs. At the transition temperature of 620 ° C., contact by deflection is no longer observed. On the other hand, at the transition temperature of 680 ° C., the glass is already too soft around its edge area because it moves freely in the roller hearth furnace and proceeds from the last roller onto the air support table. By this time, the glass has already generated roller waves and edge sag that were too strong to pass through the air support furnace for floating in the roller hearth furnace. Furthermore, thinner glass that stays at 680 ° C. in a roller hearth furnace is more susceptible to other quality problems, such as hot spots. The term hot spot is used to describe impurities that migrate from the indentations and rollers to the glass. On the other hand, glass heated to a transition temperature of 650 ° C. in a trial operation in a roller hearth furnace does not generate roller waves even when the tempering temperature is 670 ° C. and the thickness is 2 mm. The transition temperature at which roller waves begin to develop in the roller hearth furnace increases as the glass plate becomes thicker. On the other hand, this roller wave disappears when the glass is heated to a high tempering temperature which is a prerequisite for obtaining the strength and / or cooling energy efficiency achieved by the present invention in an air support furnace.

  Further, by increasing the transition temperature, the upper surface of the air support table is not cooled so much that the stability of the heating process is improved. Similarly, if the transition temperature is increased, the manufacturing cost of the furnace can be reduced for the reasons described above. On the other hand, higher transition temperatures do not affect glass with problems associated with roller waves because the roller waves generated in the roller hearth furnace disappear or almost disappear on the air support surface.

  It can be said that the object of the present invention is achieved by a transition temperature above 620 ° C. but not exceeding 675 ° C. and a strengthening temperature exceeding 650 ° C. but not exceeding 720 ° C. More preferably, the object of the invention is achieved by a transfer temperature above 630 ° C. (or even above 640 ° C.) but not exceeding 660 ° C. and a strengthening temperature above 660 ° C. but not above 700 ° C.

  In a continuous tempering line, the glass moves in only one direction. The moving speed is constant throughout almost the entire length of the furnace, not until the glass is accelerated to a suitable speed for cooling at the end of the furnace. In the vibration strengthening line, the glass load alternately passes through the furnace from the loading table, and the glass moves back and forth until the heating time elapses. At this point, the glass in the furnace is accelerated to a speed suitable for cooling as soon as new glass enters the furnace 1. A continuous furnace is several times longer than a vibrating furnace and can provide high production capacity. Usually, the length of a continuous furnace is at least three times that of a vibrating furnace. In some cases, the continuous furnace can be shortened. The advantage of a vibrating line over a continuous furnace is the price of that line. A short oscillating furnace is less expensive than a long one. Vibrating lines are more suitable for manufacturing where the type and thickness of the glass changes frequently. Vibrating lines account for most of the world's float glass tempering lines.

  In a line according to a preferred embodiment of the invention, the roller hearth furnace (for example 2 mm thick) has the heating function of heating the glass from room temperature (20 ° C.) to the high transition temperature of the invention (for example 640 ° C.). In the air support furnace 3 ′, the role of heating the glass plate G from the transition temperature to a strengthening temperature (for example, 680 ° C.) that is at least 650 ° C. remains.

  FIG. 2 is a graph of the temperature calculated theoretically when described in the parentheses. In this figure, the glass plate G stays in the roller hearth furnace for 100 seconds and in the air support furnace for 20 seconds. The air support surface 3 ′ can thus obviously be shorter than the roller hearth furnace 3. The high transition temperature of the present invention shortens the length of the air support furnace and lengthens the roller hearth furnace. The air support furnace 3 'can be longer than what is normal in the vibrating glass tempering line. Therefore, it is recommended to configure the air support furnace in a continuous structure. On the other hand, the length of the roller hearth furnace can be dramatically shortened by vibrating it or at least partially vibrating it. The combination in which the roller hearth furnace 3 vibrates and the air support furnace 3 'is a continuous structure is due to the high transition temperature, which is increasingly competitive in the industrial machinery market.

  The object of the present invention is to make it possible in particular to strengthen glass with a thickness of less than 2.7 mm to a strength of more than 10 pieces in the glass crushing test described in the EN 12150-1 standard. Furthermore, the glass is required to clear the limited value of the roller wave described in the above standard.

  In addition to what has been shown above, the object of the present invention is to provide the required strength of 6 mm fire resistant (FRG) glass in a more energy saving manner than is currently possible. Another object of the present invention is to increase the strength requirements set for thicker ones, such as 6 mm heat resistant glass, to obtain a thinner heat resistant glass. However, as a matter of course, the present invention is not limited to the glass having the above thickness. The glass must clear the limit value of the roller wave described in the standard cited above. Thus, in one preferred embodiment of the invention, the glass plate (G) has a thickness of 3.8 to 6.4 mm, the glass plate (G) being used for fire resistant (FRG) glass and at least 160 MPa. It is strengthened by the method of the present invention to have a surface compressive stress.

U.S. Pat. No. 3,409,422 U.S. Pat. No. 3,223,501

Claims (8)

  Conveying glass plate (G) on roller (5) in roller hearth furnace (3), air support table (10) while glass plate (G) rests on air cushion (3 '') Heating the glass plate (G) in the roller hearth furnace (3) to the transition temperature when the glass plate (G) is transferred into the air support furnace (3 ') carried above, the air support furnace (3' ) In which the transition temperature is not lower than 620 ° C. and not higher than 675 ° C. in a method of heating to strengthen the glass plate (G), comprising heating the glass plate (G) to a strengthening temperature in A method characterized in that the strengthening temperature is not lower than 650 ° C and not higher than 720 ° C.   The method of claim 1, wherein the transition temperature is not lower than 630 ° C and not higher than 660 ° C.   The method of claim 1, wherein the transition temperature is not lower than 640 ° C and not higher than 660 ° C.   The method of claim 1 or 2, wherein the strengthening temperature is not lower than 660 ° C and not higher than 700 ° C.   The method according to any one of claims 1 to 3, characterized in that the roller hearth furnace (3) vibrates and the air support furnace (3 ') has a continuous structure.   4. The continuous roller hearth furnace (3) has a length at least several times the length of a continuous air support furnace (3 '). Method.   The method according to any one of claims 1 to 5, wherein the thickness of the glass plate (G) is less than 2.7 mm.   The glass plate (G) has a thickness of 3.8 to 6.4 mm, and the glass plate (G) is reinforced to have a compressive surface stress of at least 160 MPa for refractory (FRG) glass. 6. A method according to any one of claims 1-5.

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