KR20060103478A - Method of fabricating low-warp flat glass - Google Patents

Abstract

A method of fabricating a glass sheet comprises modifying the thermal stress in the glass such that it is a tensile stress or substantially zero stress in a particular temperature zone of the glass, with that zone selected such that the glass sheet is formed with reduced levels of curtain warping. In an example embodiment, the modifying of the thermal stress is effected by non-uniform cooling of the glass as it passes through its glass transition temperature range. This non-uniform cooling may, for example, be applied in cooling segments that are linear with at least two of the segments have differing slopes.

Description

Method for manufacturing low warp flat glass {Method of fabricating low-warp flat glass}

The present invention relates to the manufacture of glass sheets, such as glass sheets, used as substrates in display devices such as liquid crystal displays (LCDs). More specifically, the present invention relates to a method for reducing the problem known as " curtain warping " occurring in the manufacturing of such glass sheets, for example in the fusion-down draw process.

A. Display Device

Display devices are used in a variety of applications. Thin film transistor liquid crystal displays (TFT-LCDs), for example, are used in notebook computers, flat panel desktop monitors, LCD televisions, and the Internet and communications devices to name a few. Some displays, such as TFT-LCD panels and organic light emitting diode (OLED) panels, are made directly from flat glass sheets. With many displays, the glass used in the panel should be flat within approximately 150 to 250 micrometers across the surface of the glass. Any distortion or ripples on the glass will adversely affect the quality of the display.

For the purposes of explanation, in many displays, such as those mentioned above, it is useful to couple electrical components onto glass sheets (glass substrates) used in displays. Sometimes, electrical components are complementary metal oxide semiconductor (CMOS) devices that include TFTs. In this application, it is advantageous to form the semiconductor structure directly on the glass material of the display device.

Thus, many LCD displays often include a liquid crystal (LC) layer that is combined with a glass substrate over which a transistor is formed. The transistor is placed on an engraved arrangement and operated by peripheral circuitry to provide the desired voltage for the LC material to orient the molecule in the desired manner. The transistor is an essential element of the pixel of the display device.

As can easily be appreciated, any variation in the flatness of the glass panel can lead to deformation of the space of the transistor and the pixel.

This may cause distortion of the display panel. In that respect, in LCD and other glass display device applications, it would be of great advantage to provide a glass substrate that is within acceptable tolerances for flatness to at least avoid the problems of warped glass mentioned above.

B. Warping

Warping is a defect in the glass plate specified by bending from the plane. It has become one of the most demanding and persistent problems in the manufacture of LCD glass substrates. Various aspects of warping are known and the present invention relates to curtain warping. As shown in Figure 1, the curtain warping is characterized by curvature, such as a sinusoidal curve across the width of the plate. In particular, in glass plates made from drawn glass ribbons, for example glass plates made from glass ribbons produced by a downdraw fusion process (see FIG. 2), the curtain warp may cause the ribbon to be rolled. It is a curvature like sinusoid in the axial direction with respect to the direction. As demonstrated from FIG. 1, the term "curtain warp" is appropriate due to its ripples in glass resembles and in the direction of the hanging curtain in the vertical draw.

To date, there has been no fundamental understanding of the causes of curtain warping, and therefore no systematic approach to reducing / controlling it. What is needed, therefore, is a method of substantially manufacturing flat glass which overcomes at least such obstacles in the art.

Summary of the Invention

According to a first aspect, the present invention provides a method for manufacturing a glass sheet comprising a step of mitigating a tensile stress in a glass or a thermal stress that is substantially zero over a particular temperature region of a glass forming process, wherein the user By preselecting the area (hereinafter referred to as "TZ"), the curtain warping of the final, fully solidified, glass sheet is below a specified (specific) level. Preferably, the final, fully solidified, glass plate is oriented substantially free from curtain warping.

As will be appreciated by those skilled in the art, the level of curtain warp that can be recognized in the final glass sheet will depend on the intended application for that sheet. As a general guideline, the level of difference between the highest and lowest points of curtain warping across the width of the glass sheet is preferably less than 1000 microns, more preferably less than 600 microns, most preferably less than about 200 microns.

In a preferred embodiment in this aspect of the invention, TZ is preselected according to the nonlinear temperature coefficient of expansion (CTE) of the glass in the glass transition temperature range (GTTR). As will be explained fully below, according to the present invention, (1) a glass of the type used as the display substrate has a nonlinear CTE in its GTTR, and (2) a compression and tension band in a thermally induced glass. The reaction of this nonlinearity with the cooling pattern of the glass was found in the manufacturing process to provide.

By taking into account this nonlinearity, a cooling pattern, in particular a nonlinear cooling pattern, can be preselected, which means that the cooling pattern / nonlinear CTE response is substantially at the tension band or part of the cooling process at the point where the final shape of the glass is determined. Since providing a band with zero compression results in less curtain warping.

According to another aspect, the invention provides (selecting) one or more substantially non-linear cooling sequences (cooling patterns) over a glass transition temperature range (GTTR) and a glass representing at least a portion of the glass sheet. A method of making a glass sheet is provided that includes using one or more sequences to obtain sampled stress data for a sample. The method may also include selecting one of the one or more substantially nonlinear cooling sequences that results in a substantially non-compressive or tensile stress in the glass sample in the desired region, ie, TZ.

This substantially non-linear sequence can in turn be employed to produce glass sheets with curtained warps below a specified (specific) level.

According to another aspect, the present invention provides a method for manufacturing a glass sheet using a fusion forming apparatus (eg, isopipe) having a route, wherein the glass has a glass transition temperature region (GTTR), The method includes applying a cooling pattern to the glass as a distance action from the route and at least in the GTTR of the glass sufficient to be a glass sheet where the curtain warp may be at a specified level, i.e. substantially no curtain warp. Ensuring a cooling pattern comprising one non-linearity. In certain preferred embodiments, the at least one nonlinearity at the lower temperature end of the GTTR includes a nonlinearity that provides a stress band or a substantially zero compression band.

According to each of the above aspects of the present invention, the non-linearity in the cooling pattern of the glass may result in a decrease or increase in the cooling rate of the glass, for example, as a function of the distance (or equivalently, time) from the root of the isopipe. It may include. The decrease in cooling rate can be represented as an upward "defect" in the temperature versus distance (or time) plot for the glass, while an increase in cooling rate can be represented as a "defect" downward in that figure. have. By taking into account the nonlinear CTE such display defects and display glass intentionally introduced in that GTTR, the curtain warp can be controlled and substantially eliminated.

Additional features and advantages of the invention will be set forth in the description which follows, and from such description will be readily apparent to those skilled in the art, or may be recognized by practice of the invention described herein.

It is to be understood that the foregoing general description and the following detailed description are merely examples of the invention, and are intended to provide a glean for understanding the nature and properties of the invention as hereinafter claimed. In addition, the above-listed features of the present invention may be used individually or in part or in combination, as with other adopted specific descriptions of the present invention discussed and claimed below.

The accompanying drawings are included to provide a further understanding of the present invention, and form a part thereof. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the concept and operation of the invention. It should be noted that the various features shown in the figures need not be drawn to scale. In fact, the size may be arbitrarily large or small for clarity of discussion.

1 is a schematic diagram illustrating curtain type warping.

2 is a schematic diagram of a fusion glass maker according to an actual embodiment.

3 shows the occurrence of stress bands derived as a result of linear cooling of a glass sheet having a nonlinear thermal expansion coefficient. The tref for the secant plot was 25.

4 is a plot of CTE versus temperature for Corning's Code 1737 glass.

5 is a schematic representation of the stress of glass for various cooling rates as a schematic representation used to optimize glass thermal stress in accordance with an actual embodiment.

6 is a schematic diagram of the stress of the glass for various cooling rates in the range of 300-650

7 is a flow chart of a manufacturing process of a substantially planar fusion glass according to a practical embodiment.

FIG. 8 is a diagram illustrating various cooling patterns tested in Example 1. FIG.

9 is a diagram of three cooling patterns discussed in Example 2. FIG.

FIG. 10 is a comparison of measured expansion curves for Corning Code 1737 and Eagle 2000 glass.

In the following detailed description, for purposes of explanation without limitation, the specific embodiments in which a clear description is disclosed are presented to provide a thorough understanding of the present invention. However, it will be apparent to those of ordinary skill in the art having the benefit of this description, and that the invention will be practiced in other embodiments that depart from the detailed description disclosed herein. In addition, descriptions of well-known devices, methods, or materials may be omitted to clarify the description of the invention.

As described above, the present invention relates to warping in display glass, in particular curtain warping. Such glasses can be manufactured in various drawing processes, including down draw fusion processes.

2 is a schematic diagram of a glass manufacturing apparatus 100 of the type typically used in a fusion process. The apparatus includes a forming apparatus (isopipe), which receives molten glass (not shown) in the cavity 102.

The root of the isopipe is shown at 103 and after leaving the root, the sheet 104 of glass passes through the edge roller 105. The root 103 of the isopipe 101 is related to the position of the point where the molten glass meets each other from both outside of the isopipe 101. Finally, the position of the set of tubes (doctari) 106 is shown.

The tube regulates the partial temperature of the glass sheet (glass ribbon) and thus its thickness. Fusion (melting) devices are known in the art, and details are omitted for clarity of description of actual embodiments. However, it should be noted that other kinds of glass making apparatus can be used in combination with the present invention. Such devices are within the understanding of those skilled in the art of glass making.

In fusion or other types of glass making apparatus, when the glass sheet (glass ribbon) moves downward to the drawing portion of the apparatus, the sheet undergoes complex structural deformations at the molecular level as well as at the physical level. For example, the transition from a flexible 50 millimeter thick liquid phase at the root of the isopipe to a rigid glass sheet approximately 0.5 millimeter thick is precisely balanced with the mechanical and chemical requirements for achieving a liquid-to-solid transformation. Achieved by carefully selected temperature ranges. Temperature gradients below the perfect temperature cause curvature in the plane, especially curtain distortion.

Although trial and error changes in the cooling rate of the glass can be adopted as one approach to reducing the level of curtain warping or non-smoothness of the glass sheet, such trial and error changes are purely empirical and so generally for the final product. This results in reduced yields and higher costs. In addition, trial and error approaches should be repeated every time the composition of the glass and / or the design of the glass manufacturing equipment changes.

In order to understand the cause of the curtain warping, according to the present invention, a model has been developed that can find out the effect of temperature dispersion on the potential thermal stress in the sheet. In particular, according to a practical embodiment of the present invention, a model using ANSYS finite element software has been established. The sheet included in the model starts at some distance from the root (upper boundary) and extends to the point where the glass ribbon breaks during manufacture. The SHELL63 component is used to describe the glass sheet and the position of the upper boundary is specified according to the model inflow.

The temperature distribution in the sheet was also specified in this model along with the temperature dependent elastic module and the temperature dependent coefficient of thermal expansion (CTE). The ANSYS software then selected the elasticized sheet and calculated the thermal stress on that sheet. The special equations used are described in Appendix A. Out-of-plane buckling and membrane stress were considered, but viscoelasticity was not taken into account. The calculated stress is therefore used as the stress measurement in the sheet, not the actual stress.

The upper boundary is set at a location far enough from the route so that the glass has its final thickness before reaching the upper boundary. In addition, the position of the upper boundary is selected to extrude an area where the stress is immediately resolved because the glass is too soft, and does not include a glass area in which out-of-plane deformation is immobilized in the glass. .

Although it has been found that the aforementioned ANSYS elastic model can successfully perform the practice of the present invention, it should be understood that a wide variety of other models, modeling techniques, and modeling software may be used. For example, models that include the effect of viscoelasticity can also be used if desired.

To investigate curtain warping, the thermal stress model was first operated for a linear downward temperature profile of the sheet. The results are shown in FIG. As can be seen in the figure, at a temperature of about 650 to 830 ° C. (particularly 650 to 800 ° C. in FIG. 3), the linear temperature state is the tensile peak following the compression peak following the tensile peak, ie as shown in FIG. 3. The stress pattern in the glass is characterized by a positive-negative-positive stress pattern. In this pattern, the compressive stress band may occur in the glass transition region that is sufficiently rigid for the strain to stay when the sheet moves below the draw, but flexible enough to become a strain that results in a permanently distorted glass sheet. Especially important. Above the glass transition region, the glass is essentially a Newtonian fluid, in which the stress is quickly resolved, and below that region, the glass is hard enough to resist buckling.

According to the present invention, the cause of the curtain type warping in the glass transition region for the above type of compressive stress shown in FIG. 3 has been found. In other words, it was found that if the glass sheet had been allowed to cool linearly through its glass transition temperature region (GTTR), it would buckle and develop into curtain-type warping.

The stress pattern in FIG. 3 is a result of the nonlinear thermal expansion coefficient (CTE) of the glass in GTTR. FIG. 4 shows a plot of typical CTE versus temperature for Corning's Code 1737 glass. The nonlinear dependence of CTE on temperature in the GTTR shown in this figure results in uneven shrinkage of the glass sheet upon cooling through this region.

This produces the pattern of tensile-compression-tensile stress of FIG. 3. The compression portion in this pattern, in turn, forms a buckling in the treated glass and thus a curtain warp. Although the nonlinear CTE of FIG. 4 is not special for 1737 glass, it should be noted that it corresponds to a property generally seen in display glass.

According to the present invention, it has been found that the problem of curtain warping can be controlled by controlling the cooling rate of the glass sheet in the glass forming process. In particular, as discussed above, the present invention involves adjusting the thermal stress such that the thermal stress in a particular region becomes tensile stress or substantially zero, resulting in the glass sheet controlling the level of curtain warping, Provided is a method for producing a glass material that is substantially curtain free of warping.

According to the actual embodiment described herein, the cooling from the glass transition temperature zone GTTR versus the distance from the route substantially reduces the internal compressive stress in the undesired region where curtain-warped glass occurs. It is substantially nonlinear to avoid.

Nonlinear cooling over the GTTR may consist of various cooling segments. Each segment may be substantially linear, but need not be completely linear. That is, one or more of these segments may themselves be nonlinear. Ultimately, the slope and duration of these cooling segments are chosen to reduce or substantially eliminate the compressive stress at undesirable locations.

For example, the glass of an actual embodiment is a flat glass having a thickness of about 0.1 to 2.0 mm. The glass advantageously has a flatness across the substrate from about 150 microns to about 250 microns, depending on the size of the substrate. The glass can be used in glass displays such as those mentioned above, or in other applications where a flat or substantially ripple free glass surface is advantageous. As a representative example, the glass may be Corning's Code1737, Code Eagle 2000 glass, or glass of display applications produced by other manufacturers.

5 shows the effect of a representative nonlinear cooling pattern on the stress pattern calculated using the ANSYS computer model above. As shown in FIG. 5, non-linear cooling deviates the temperature profile from the linear dependence of the distance under the draw, thereby creating a stress pattern in the glass that is altered as shown in FIG. 3.

In particular, as shown in FIG. 5, a tension band surrounded by a compression band is joined to a stress pattern at all upward bend points of the temperature curve, and a compression band surrounded by a tension band has a respective downward bend. Are joined to a point. The upward bend points correspond to faster and slower cooling rate changes as the seat moves down the draw, while the downward bend points correspond to cooling rate changes that are slower and faster. By selecting the point of change in the cooling rate, the change is faster to slower or slower to faster, and the tensile stress band (or band with substantially zero compression) It may be located in one or more desired zones along the cooling curve of the glass.

In particular, a band with tensile stress or substantially zero compression may be located, for example, in TZ, such that the final glass may be less than a certain level of curtain warp, ie the final glass may experience substantially curtain warp. You can't.

Graph 300 of FIG. 5 shows, in particular, stress (left vertical axis) and temperature (right vertical axis) versus distance from the root of the isopipe. Optionally, the horizontal axis of the graph can be time from the route instead of distance. Graph 300 was mathematically modeled using a computer model, such as the model discussed above. The glass sample used for modeling is for example a glass material made of a glass sheet used as a glass display of the type mentioned above.

It should be noted that graph 300 may be used to perform the selection of the optimal cooling method described in connection with the actual embodiment of FIG. 7 described below. That is, out of three modeled cooling sequences (cooling curves 301, 302, 303), one will be selected to optimize the flatness (flatness) in the glass sheet made of the selected glass material.

Graph 300 includes three individual modeled cooling sequences for a particular glass material formed, for example, by known fusion (melting) techniques. The cooling sequence of FIG. 5 is in the glass transition region of the particular glass being manufactured, for example from about 850 ° C to about 650 ° C. The first sequence shown in the first curve 301 is substantially linear. The second sequence, which is a non-linear curve, is seen as the second curve 302, and the initial cooling rate is reduced compared to the first curve. The third sequence is indicated by curve 303, with an increased initial cooling rate compared to the first curve 301.

The first curve 301 yields the stress curve 304 of FIG. 5. Glass made using this cooling sequence will have a slight positive (tensile) stress, but a noticeable amount of compressive stress. In this example the glass sheet is subject to an unacceptable degree of compressive stress. Thus, the modeling method of the actual embodiment allows removal of this cooling sequence without substantially performing cooling of the sample. With the benefit of this embodiment of the invention, this reduces waste and increases manufacturing output.

The second curve 302 has three cooling segments. First segment 305, second segment 306, and third segment 307. In this embodiment, the first segment 305 and the second segment 306 have substantially the same gradient, while the cooling curve of the third segment has a different gradient than the segments 305 and 306. The cooling segment results in the stress of the glass described by curve 309. The last (fourth) segment 308 is also linear, as in the gradient for the last (fourth) segment of curves 301 and 303.

The cooling segment 305, which is the initial cooling rate of the second curve 302, is reduced compared to that of the gradient of the curve 301. This initially results in some tension, as seen in stress curve 309. However, the third segment 307, which has a steeper gradient relative to the second segment 306, results in significant compression in the glass. This is obvious as in stress peak 301. As such, the second sequence represented by the second curve is also not useful because the compression peak will have curtain-like distortion that is unacceptable when the compression peak corresponds to the position where the shape of the glass is fixed. Again, the modeling method of the present invention allows removal of this cooling sequence without actually performing the cooling of the sample. While retaining the benefit of this embodiment of the invention, this reduces waste and increases manufacturing output.

The third cooling sequence, shown by curve 303, also has three cooling segments, ie, first segment 311, second segment 312, and third segment 313. This third cooling sequence results in stress in the glass shown in stress curve 314. In this practical embodiment, the first segment 311 has a significantly increased gradient like other segments 312 and 313 compared to the curve 301. The first cooling segment 311 results in some compression as shown. The second and third cooling segments 312 and 313 have a significantly reduced gradient compared to the gradient of the first segment 311 and curve 301, respectively. This second cooling segment 312 produces a more pronounced upper, positive stress peak 315. This cooling profile provides compensation for the effect of the nonlinear thermal expansion coefficient in the glass transition temperature region, thus resulting in the desired tensile thermal stress as the glass passes through the glass transition region.

As can be estimated from graph 300, various stress curves can be calculated by selective changes to the gradient of the cooling segment. In addition, the length of the segment can also be changed. Each of these groups of cooling segments results in a stress curve that is generally unique to the particular glass material produced. In that way, various residual stress values can be realized. For example, the curve population is determined and characterized by faster to slow cooling of the glass in the range of approximately 780 to approximately 720 ° C., which is the lower end portion of the glass transition region for the sample of glass of this embodiment. From this group of curves, special curves can be selected to meet the desired special purpose results in the glass sheet. Similarly, it may be determined that the population of curves is specified by slow to faster cooling for the glass in or around the glass transition temperature region in which the special glass is made. From this alternative population, unique curves can be selected to meet the limited curtain warping needs. In the same sense, a combination of faster-to-slower and slower-to-faster patterns can be used if desired to achieve particular flatness criteria.

Quantitatively, in the cooling sequence of the actual embodiment described with respect to the third curve 303, the cooling segment (not shown) before the first segment 311 is relatively at the temperature above the glass transition region to substantially eliminate the curtain distortion. It has a high cooling rate. By way of example, this cooling rate is in the range of approximately 6 ° C./in to approximately 15 ° C./in. This occurs in an area about 25 inches to about 40 inches from the root.

Next, the cooling rate in the upper portion of the glass transition region and along the first segment 311 is reduced to a range of approximately 4 ° C./in to 10 ° C./in. This later cooling rate continues over the next approximately 10 inches to approximately 15 inches.

In an embodiment, during the second and third cooling segments 312 and 313, the cooling rate is the lower end of the glass transition region for the cooling rate in the range of approximately 2 ° C./in to approximately 5 ° C./in. in the glass transition region. This cooling rate affects the next approximately 15 inches to approximately 25 inches. After the glass passes through the glass transition region, the cooling rate is maintained at a substantially continuous level, such as for example in segment 308. Accordingly, embodiments include non-linear temperature profiles during the cooling process that substantially eliminate or certainly eliminate curtain-like warpage caused by compressive stress in the glass by providing the desired tensile stress in the glass.

FIG. 6 shows a more specific finding based on this modeling, ie, modeling that is substantially independent of the stress pattern in the critical glass transition region from the cooling rate at the lower point of the draw. As shown in the figure, the three gradients in the temperature range of 650 ° C to 300 ° C show the same stress pattern in the higher temperature range from 650 ° C to 850 ° C. This substantial independence has the advantage that it is easier to control curtain warping because it focuses on obtaining an effective cooling pattern for the GTTR as opposed to the entire temperature range of the draw.

In summary, it can be seen from the above that the correlation between the glass cooling rate and the sheet stress in the glass transition temperature region (GTTR) is the cornerstone for the control of the curtain distortion. By this correlation, tensile stress can be generated to keep the sheet flat while the sheet goes from the relatively soft glass of the transition region to the hard solid state below that region. In particular, the tension band at the lower end of the GTTR corresponding to the beginning of the solid sheet provides a final product with reduced curtain warping compared to products produced without such a tension band. .

7 is a flow chart for an exemplary method 200 for performing control of curtain warping in a glass making process. The practical embodiment for FIG. 7 can be applied, for example, to the manufacture of substantially flat glass panels using a manufacturing apparatus as described in connection with FIG. 2 above.

In step 201 of FIG. 7, uniform or linear cooling data for a special glass sample formed of sheet glass is obtained by computer-based mathematical modeling. As described above, computer modeling may be affected by the use of one or more of various well known mathematical modeling techniques.

These are as known, and details are omitted for clarity of description. This cooling data includes the stress (usually in p.s.i units) for the glass sample upon cooling of the manufacturing process. For example, such data may be the stress versus temperature and time (or distance from the root) when the glass cools from its molten state to its final state at room temperature when the glass leaves the isopipe.

Optionally, the data may only be for the portion of the cooling process from above the glass transition temperature range at the point where the individual glass sheets are separated from the continuous glass ribbon.

As described above, uniform or linear cooling of the glass results in curtain warping due to the compression of the glass in the glass transition temperature range where the glass is transformed from liquid to glass-like material. This glass transition temperature range occurs in many glasses in the temperature range between about 850 ° C and about 650 ° C. For example, such uniform cooling may be cooling of the glass over the inclusive or to some degree over the involvement throughout the manufacturing stage.

The compressive stress (negative stress) in the glass transition region is due to the nonlinear thermal expansion of the glass material as it is uniformly cooled through the glass transition temperature. In the glass transition temperature range, the coefficient of thermal expansion varies nonlinearly with respect to the linear change in temperature. Thus, the expansion of the glass with respect to temperature is also nonlinear, so it is not uniform. If not suppressed, this will result in compression in the glass and will cause curtain-like warping in the glass.

 As soon as the linear or uniform cooling stress data of the glass is obtained in step 201, the method includes in particular the modification of the cooling sequence through the glass transition temperature zone in step 202. One purpose of step 202 of this embodiment is to optimize the glass stress so that the glass has substantially zero stress, positive response or tension over the glass transition temperature range. At the end of the glass transition temperature region, the glass is substantially flat because it reduces harmful curtain-like distortions by positive stress induced by the selective change in cooling rate in that region. It should be noted that the control of the cooling rate may be achieved by heating / cooling with an external heating / cooling device that is cooler at a faster / slower rate than would be realized by using independent heat and convective radiation. Such heating / cooling devices within the understanding of those of ordinary skill in the glass sheet manufacturing art can be used to realize this controlled cooling rate.

It should be noted that the cooling sequence adjustment from the linear cooling sequence of step 201 can effectively optimize the cooling of the glass in the glass transition temperature region which substantially eliminates the compression and the resulting curtain warping. However, as discussed above, the removal of compression in the glass can be accomplished by various cooling sequences. Part of this sequence can cause the glass to have a net positive stress at the end of the glass transition region, while other sequences can cause the glass to be substantially stress free at the end of the glass transition region.

In practical embodiments, the glass is cooled at a relatively high rate at temperatures above the glass transition region, for example in the range of approximately 6 ° C./in to 40 ° C./in, to substantially eliminate curtain warping. This area can be approximately 25 inches to 40 inches from the root. It should be noted that the cooling rate in this and other areas after the route is controlled by adjusting the heating / cooling power on the glass surface.

Next, in the upper portion of the glass transition region, the cooling rate can be reduced at a ratio in the range of approximately 4 ° C./in to approximately 10 ° C./in. This later cooling rate may last for about 10 inches to about 15 inches.

For the next 15-25 inches, the cooling rate can be varied at the lower end of the GTTR for the cooling rate ranging from approximately 2 ° C./in to approximately 5 ° C./in. . After the glass passes through the glass transition region, the cooling rate can be maintained at a constant level, creating a nearly linear temperature profile.

It should be noted that the delay of the cooling rate at the beginning of the glass transition region causes the glass molecules to rearrange structurally, leading to reduced energy states and lower density or compression. Further reduction in cooling rate at the lower end of the glass transition region forms a tensile stress band at the width of the glass sheet. For example, tensile stress bands occur within a point where there is a decrease in the gradient of the cooling curve. Tensile in such glass is useful for promoting unfolding of the glass following glass deformation from the glassy / semi-liquid state to the solid state. This is very beneficial in preventing curtain warping of the glass due to compression.

Finally, controlling the cooling rate below the glass transition region is less stringent because it does not strongly affect the stress in the glass transition region where curtain warpage is likely to occur. In other words, the stress in the glass does not have a major effect during cooling from the end of the glass transition region to room temperature. However, the cooling rate below the glass transition region may still be transmitted to the glass within the glass transition region, and a gradient (i.e., a flexible temperature curve) to minimize the development of transient bends in the glass that causes additional morphology in the glass sheet. It should be noted that a smooth temperature curve can be adjusted to produce a curve that is substantially free from sudden changes.

It is possible that each cooling zone according to an actual embodiment has a substantially constant gradient with respect to the temperature versus distance curve. The rate of change of cooling (according to distance) can be determined based on the stress curve determined from the linear cooling (temperature) simulation. For example, when the temperature reaches a point corresponding to a peak (relative peak) internal compressive stress, a change in cooling rate may be employed. In practice, this can be done at a fixed location from the route or when the glass reaches this temperature in the cooling process.

More specifically, in the embodiments described above, the cooling rate across the glass transition region is such that each segment provides a certain ratio of linear cooling and each linear cooling segment begins with a nearly simultaneous change in cooling rate. Or a series of more controlled cooling segments. However, it is noted that this is merely an exemplary embodiment and that other embodiments can be used to realize the removal of the compression of the glass in the glass transition region. For example, it should be noted that linear cooling rates other than those mentioned above may be used to perform the desired removal of compression. In addition, depending on the glass material used, different linear cooling rates will be required to meet this desired purpose. Moreover, it should be noted that additional cooling segments can be used to meet this desired purpose for the same or different glass materials. Finally, note that the cooling sequence across the glass transition is not necessarily required to be the various linear cooling segments. Rather, one or more nonlinear cooling segments will be selected by a gradient of cooling rates that varies to achieve the desired reduction in compression or the generation of tension during cooling, or both.

Step 203 of the method illustrated in FIG. 7 is an optional step, which includes repeating the adjustment of the cooling sequence by selecting another cooling rate across the glass transition region. This will be done in a preferred form to optimize the cooling sequence to achieve a specific synthetic stress level or lower curtain warping.

Finally, step 204 of FIG. 7 is the application of the selected cooling sequence to the production process. The example of the glass making method described above promotes a major reduction in the complexity of the glass produced by reduced curtain warping.

As can be appreciated, these methods are performed by using modeling techniques to determine the appropriate cooling sequence, not empirical trial and error techniques. As such, a significant reduction in downtime and waste is realized by practical embodiments. To this end, in practice, new glass materials, or curtain warping requirements, or both, may affect manufacturing faster by the selection of a desired cooling sequence than through inefficient and inefficient trial and error techniques. Can be. These and other benefits of practical embodiments will be apparent to those skilled in the art of glass making.

Without intending to be bound by any means, the invention will be fully illustrated by the following examples.

Example  One

This embodiment of the invention relates to the selection of a cooling pattern for a new installation of a glass forming apparatus, in particular a glass forming apparatus which is designed to handle more glass pounds per hour than existing ones for producing larger sized glass sheets. Start application.

Initially, the cooling pattern that was successfully performed with the previous unit was met with the new unit. The cooling pattern showed unacceptable curtain warping in the 1000-2000 micron range. A trial and error approach was then performed to see if the cooling pattern exhibited curtain warping levels within the 200 micron range. The unnumbered curves in FIG. 8 show a representative profile that was tested without success.

The modeling process shown above was then adopted and represented by the 402 curve of FIG. 8. The cooling curve reduced the glass's curtain warping to 250-300 microns, down from the 1000-2000 micron range. As shown in FIG. 8, the cooling curve 402 changes from fast-to-slow cooling at 720-780 ° C., which is the lower end of the glass transition temperature range during the manufacture of the glass (ie, Corning's code 1735 glass). Have

Curve 402 requires a high level of cooling capacity at a distance in the range of 40-60 inches from the root of the isopipe. In order to reduce the need for cooling capacity, subsequent modeling was performed, and it was also found that smaller upward inflows at about 760-780 ° C. could reduce the level of curtain warping in the final glass. Curve 400 shows the cooling curve derived from this subsequent modeling. The 760-780 ° C. temperature range corresponds to the upper end of the 720-780 ° C. region, but is still included within the lower end of the glass transition temperature region for code 1737 glass. The cooling curve 400 was found to consistently achieve the level of curtain warping in the 200 micron range without the need to increase the cooling capacity of the new unit.

Example 2

This example illustrates the application of the invention to the selection of a cooling pattern for a change in the form in which a glass is processed, in particular a change from Corning's Code 1737 glass to Corning's Code Eagle 2000 glass.

Curve 408 of FIG. 9 shows the cooling pattern found to produce glass sheets with low curtain warping levels when 1737 glasses were processed in a particular fusion glass forming machine. The same device was used to process the Eagle 2000 glass using a cooling pattern that had substantially the same shape as the successful 1737 pattern at the time. Curve 404 shows the pattern of the Eagle 2000. As can be seen, parallel to the 1737 pattern but slightly higher temperature since the Eagle 2000 glass was treated at slightly higher temperatures.

Surprisingly, the 404 pattern resulted in unacceptable curtain warping levels for the Eagle 2000 glasses. The modeling technique was then used to explain and address this difference in behavior in the two glasses.

FIG. 10 is a plot of the expansion curve evaluated for 1737 (curve 410) and Eagle 2000 (curve 412). As shown in this figure, the Eagle 2000 shows an expansion less than 1737. The lower end of the GTTR for this glass can also be seen in this figure, ie it is generally the region between the peak of the expansion curve at about 780 ° C. and the beginning of the linear portion of the curve at about 720 ° C.

The average person skilled in the art may think that the Eagle 2000 will be less affected by curtain warping problems because of its lower expansion, but in fact using the modeling approach of the present invention, the lower expansion of the Eagle 2000 was It was found that they required a more aggressive cooling pattern at the lower end of the GTTR to control curtain warping than was needed.

As described above, using the modeling technique of the present invention, it has been found that curtain warping is related to thermal stress caused by the temperature gradient as the glass cools through the GTTR. The level of stress and the label are, in turn, related to the nonlinear CTE of the glass in the GTTR. The comparison of the CTE curve of 1737 and Eagle 2000 in FIG. 10 is generally similar, but shows that the Eagle 2000 CTE (curve 412) has a smaller temperature dependency than that of 1737 (curve 412). The lower CTE gradient of Eagle 2000 means that its stress is less sensitive to temperature changes in the GTTR. Therefore, in order to create a similar stress pattern in this critical region, a more urgent temperature profile is needed to compensate for the Eagle 2000's lower CTE gradient.

Curve 406 of FIG. 9 shows the cooling patterns tested for Eagle 2000 glasses based on the above analysis. As can be seen, this curve has a steeper temperature profile than curve 404 with inferior curtain warping performance for the Eagle 2000 glass, as described above. As expected by modeling, it was found that curve 406 with a steeper temperature profile produced glass with lower curtain warping levels.

Thus, as explained in this example, changing the glass composition, the CTE curve should be irradiated and the cooling profile should be adjusted based on such curve to achieve a low curtain warping level.

From the foregoing description, it can be seen that the most sensitive area for curtain type warping formation / control is at a temperature of approximately 650-850 ° C. in the display glass. The reason why this region is so important is: First, the nonlinear dependence of CTE at this region's temperature passes through this temperature region, causing the glass sheet to cool, causing uneven shrinkage. Secondly, the sheet in this area is flexible enough to cause deformation and rigid enough for sustained deformation, thereby causing permanent sheet deformation, ie curtain warping. By modeling the effects of nonlinear CTE on stress and adjusting the cooling rate associated with these effects, glass sheets with lower levels of curtain warping can be made.

Although the detailed description of the invention has been disclosed and described, it should be understood that modifications may be made without departing from the spirit and scope of the inventors. For example, as described in the above embodiment, it is merely an example of the invention that the non-linear cooling profile comprises a linear cooling segment. The method can include three or more cooling segments or fewer than three cooling segments. In addition, one or more of the cooling segments may be non-linear. Finally, a combination of linear and nonlinear cooling segments can be used to realize a nonlinear cooling sequence across the glass transition temperature region.

Although described in detail in the Examples, variations of the present invention will become apparent to those of ordinary skill in the art having the benefit of this specification. Such modifications and variations are included within the scope of the appended claims.

Appendix A

Thermal stress  For equation

The stresses and strains in the sheet must satisfy the following set of field equations.

compatibility( Compatibility)

(1)

(One)

ε is the total strain, subscripts represent components in traditional methods, and x, y, z are Cartesian coordinates. See, for example, Sokolnigof, I.S., 1956, Mathematical Viscosity Formula, Robert Krieger Publishing, Malabar, Florida. The fitness equations show that the plane of displacement is continuous. It indicates that they do not form holes in the body and that the same space is not filled with more than one part of the body.

은In the elastic model, the total strain is the sum of the elastic and thermal strain stresses. Elastic deformation

silver

(2)

(2)

α is the coefficient of thermal expansion, here adopted as isotropic, T is It is the temperature difference from the base temperature when the thermal stress is zero. Note that T can be a function of spatial location.

Structural Law Constitutive  Law)

(3)

(3)

σ _ij Is stress, E is Young's modulus, and υ is Poisson's ratio. E and υ can be temperature functions. This set of equations represents the stress-strain behavior of a material having linear elasticity, although the material may be viscoelastic if desired.

equilibrium( Equilibrium)

(4)

(4)

To find the thermally induced stresses, the set of connected equations described above was solved according to the given temperature dispersion and boundary conditions. In the present curtain warp model, the boundary condition is that the sheet is not affected by external forces.

If the thermal strain satisfies compatibility by itself, the total strain may simply be thermal strain and there will be no stress. For example, if the thermal strain is uniform or has a uniform slope, then it satisfies its own suitability, so there is no stress generation. If the thermal strain does not satisfy suitability, elastic (or viscoelastic) strain enters the situation and the total strain meets suitability.

Warping models are performed using ANSYS restrictor software. The set of equations described above and the order for solving them are established in ANSYS. Geometry, physical properties (E, υ, α) and temperature dispersion as a function of temperature are specified as conditions in ANSYS software.

Claims (30)

Providing a tensile stress or a substantially zero thermal stress in at least a portion of a particular temperature region subjected to the manufacture of the glass sheet, wherein after manufacture, the glass sheet is The glass of the glass sheet has a coefficient of temperature expansion that varies nonlinearly with temperature (i) without exceeding a predetermined curtain warping level at at least some temperature of (ii) substantially on the glass in the fluid. Method for producing a glass sheet, characterized in that the deformation into a material. The method of claim 1, The specific temperature range is all or part of the glass transition temperature region of the glass. The method of claim 2, Wherein said particular temperature range is about 60 ° C. The method of claim 2, Wherein said particular temperature region is about 20 ° C. The method of claim 2, Wherein the glass transition temperature region of the glass is between about 650 ° C. and about 850 ° C. 6. The method of claim 2, Wherein the glass transition temperature range of the glass is about 700 ° C to about 850 ° C. The method of claim 6, And wherein said particular temperature range is from about 720 ° C to about 780 ° C. The method of claim 6, Wherein said particular temperature range is from about 760 ° C to about 780 ° C. The method of claim 1, Providing the thermal stress is performed by cooling the glass sheet in an inhomogeneous manner in at least a portion of the particular temperature region. The method of claim 9, Wherein the non-homogeneous manner of cooling the glass sheet is determined using a computer model for the glass sheet reflecting a nonlinear change with temperature of the coefficient of thermal expansion of the glass. The method of claim 9, The cooling process includes cooling the glass sheet over at least a first cooling portion having a first gradient and a second continuous cooling portion having a second gradient, wherein the first gradient and the second gradient are different from each other. How to feature. The method of claim 11, Wherein said first cooling segment precedes said second cooling segment and said first gradient is much greater than said second gradient. The method of claim 11, The first cooling segment precedes the second cooling segment, and wherein the first gradient is less than the second gradient. The method of claim 11, At least one gradient of said cooling segments is non-linear. The method of claim 1, And wherein said preselected curtain distortion level is about 250 microns. Providing at least one substantially non-linear cooling sequence over the glass transition temperature for the glass of the glass sheet, and using at least one sequence to obtain standardized stress data for the glass sample provided on at least a portion of the sheet Method of producing a glass sheet comprising a. The method of claim 16, The method includes selecting one of one or more substantially nonlinear cooling sequences that are substantially free of compressive stress in the glass sample or that result in tensile stress in the glass sample in a desired region. . The method of claim 17, The selected substantially non-linear cooling sequence comprises a first cooling segment having a first gradient and a second continuous cooling segment having a second gradient, wherein the first gradient is different from the second gradient. The method of claim 18, The first cooling segment precedes the second cooling segment, and wherein the first gradient is much greater than the second gradient. The method of claim 18, Wherein the first cooling segment precedes the second cooling segment, and wherein the first gradient is smaller than the second gradient. The method of claim 18, At least one gradient of said cooling segments is non-linear. The method of claim 17, The method comprises using a substantially nonlinear cooling sequence selected in the manufacture of the glass sheet. The method of claim 22, Wherein said manufactured glass sheet exhibits no more than a preselected curtain warping level. The method of claim 16, And wherein the plurality of substantially nonlinear cooling sequences are provided repeatedly. A method of making a glass sheet using a fusion forming apparatus having a route, wherein the glass has a glass transition temperature range (GTTR), and the method is applied to the glass under the action of a distance from the route. Applying a cooling pattern for the glass and ensuring the cooling pattern comprising at least one nonlinearity in the GTTR of the glass sufficient to appear as a glass sheet with a curtain warp below a specified level. Manufacturing method. The method of claim 25, Wherein said at least one nonlinearity comprises nonlinearity producing a stress band or a substantially zero condensation band in the region of the lower temperature end of said GTTR. The method of claim 25, The gradient of cooling can vary within the cooling pattern. The method of claim 27, And the cooling pattern comprises a plurality of cooling segments. The method of claim 28, Wherein each of the plurality of cooling segments has a linear gradient and at least one gradient is different from at least one other gradient. The method of claim 28, At least one of said plurality of cooling segments has a non-linear gradient.

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