Abstract:
Optical quality of float glass is improved by attenuating longitudinally prior to attenuating laterally and by carrying out each attenuation step within specified temperature ranges.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of U.S. Patent Application Ser. No. 137,329 filed Apr. 4, 1980, now U.S. Pat. No. 4,305,745, the disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for manufacturing flat glass wherein the glass is formed into a flat sheet while supported on a surface of a pool of molten metal, commonly referred to as the float process. More particularly, this invention relates to a process for attenuating the glass while supported on the molten metal to a thickness below the equilibrium thickness of the glass in such a manner so as to minimize distortion in the product glass. 
     In a float forming process, molten glass is delivered onto a pool of molten metal and thereafter formed into a continuous ribbon or sheet of glass as disclosed, for example, in U.S. Pat. No. 710,357 of Heal; U.S. Pat. No. 789,911 of Hitchcock; U.S. Pat. Nos. 3,083,551 and 3,220,816 of Pilkington; and U.S. Pat. No. 3,843,346 of Edge et al. Under the competing forces of gravity and surface tension, the molten glass on the molten metal spreads outwardly to an equilibrium thickness of about 0.27 inches. In order to produce glass of thicknesses less than the equilibrium thickness, the prior art has resorted to various arrangements for stretching the glass while still in a viscous state on the pool of molten metal. The simplest stretching technique is that shown in U.S. Pat. No. 3,215,516 of Pilkington wherein stretching is done in the longitudinal direction (the direction of glass travel) only, wherein the stretching force is provided by the tractive means withdrawing the glass from the float chamber. In such an arrangement, the ribbon loses width as it becomes thinner. A common refinement of this arrangement is to employ lateral stretching means in order to reduce the loss of ribbon width as it is being stretched longitudinally. Typical of this latter approach is the process shown in U.S. Pat. No. 3,695,859 to Dickinson et al. Another approach is to maintain the ribbon of glass at essentially constant width by applying lateral tractive forces to edge portions of the ribbon as the ribbon is being attenuated in the longitudinal direction as exemplified in U.S. Pat. No. 3,843,346 of Edge et al. 
     Process perturbations originating with the attenuating process affect the topography of the glass ribbon in ways that degrade the optical quality of the product glass. The topography of float glass is characterized by two types of elongated features, thickness variations and corrugations, which extend generally parallel to the direction of glass travel, i.e., the longitudinal direction. These deviations from perfect flatness are, in effect, cylindrical lenses which distort light reflected from and/or transmitted through the product glass sheet. Analysis of the distortion patterns using optical scanners in a direction transverse to the direction that the glass traveled in the forming process reveals that the distortion patterns can be considered as consisting of randomly superimposed sinusoidal waves whose wavelengths vary over a wide range. It has also been found that the dominant component of the instrumentally measured signal corresponding to transmitted light occurs at rather well defined wavelengths that may range from about 1.2 to 1.4 inches (3.0 to 3.6 centimeters) for a &#34;constant width&#34; float forming process as in the Edge et al. patent cited above, to about 0.25 to 1.0 inches (0.6 to 2.5 centimeters) in the freefall type of float forming as in the Pilkington patents cited above. Furthermore, these dominant wavelengths have been found to lie within a range to which the human eye is particularly sensitive for most applications. 
     Surface distortion in float glass is believed to arise from several categories of perturbations. First, inhomogeneities in the glass composition (&#34;ream&#34;) not only cause nonuniformity of the refractive index of the glass but also can contribute to surface distortion. Second, thermal nonuniformity either in the molten glass entering the float forming chamber or within the chamber itself can contribute to surface distortion. Third, variations in the flow of molten glass from the melter to the forming chamber, either volume flow rate fluctuations or inequalities in the thickness of entering molten glass across the width of the ribbon of glass. Fourth, mechanical perturbations from contact of various members of the forming apparatus with the deformable glass ribbon. These include, for example, the stretching machines and side barriers as well as fluctuations in the speed with which the dimensionally stable ribbon is withdrawn from the forming chamber. These perturbations in the glass/tin system generate thickness variations or corrugations in the glass through a variety of mechanisms such as differential stretching, viscous folding, wrinkling, embossing, and membrane stress. While minimizing the causes of these perturbations is desirable, such an approach is limited because the perturbations cannot be completely eliminated, particularly in the case where less than equilibrium thickness glass is being produced. Therefore, this invention relates to diminishing the effects on distortion of these perturbations rather than eliminating the perturbations themselves. 
     As a newly formed ribbon of glass still in a softened condition progresses along the molten tin bath its topography is continually changing as perturbations introduce new defects into the ribbon and previously introduced defects are changed in shape. A defect may decrease in amplitude by means of viscous decay, or the wavelength of a distortion pattern may be altered by extensive or compressive stresses. It would be desirable if these distortion decaying mechanisms could be coordinated with the attenuating process so as to minimize the amount of distortion imparted to the glass when attenuated to below equilibrium thicknesses. 
     Some attempts have been made in the prior art to correlate the manner of attenuation to minimizing surface defects such as in U.S. Pat. Nos. 3,440,030 (Thompson et al.); 3,533,772 (Itakura et al.); and 3,520,672 (Greenler et al.), but it is now believed that none of these approaches fully meets the problem. 
     SUMMARY OF THE INVENTION 
     In the aforementioned copending application Ser. No. 137,329, it is disclosed that applying attenuating forces to a ribbon of glass in a float chamber in a specific sequence can substantially reduce the amount of apparent optical distortion in below equilibrium thickness glass. In one embodiment, the sequence may be summarized as passing the glass first through a relaxation zone, then a longitudinal and lateral stretching zone before the glass has cooled sufficiently to become dimensionally stable. Another embodiment is characterized by passing a glass ribbon through a longitudinal stretching zone and subsequently through a lateral stretching zone. In the longitudinal stretching zone, the glass is attenuated in the longitudinal direction as it is mechanically restrained from shrinking substantially in width so that the attenuation occurs largely by virtue of thickness reduction. A substantial portion, e.g., about 50 percent, of the overall thickness reduction is effected in the longitudinal stretching zone. It is believed that most of the surface defects are imparted to the glass during this longitudinal stretching. Immediately following the longitudinal stretching zone is a lateral stretching zone in which mechanical forces are applied to the ribbon to increase its width while reducing the ribbon to its final thickness. Stretching in the lateral stretching zone is primarily in the lateral direction, but tractive force applied to convey the ribbon in the longitudinal direction is usually sufficient to at least prevent longitudinal shrinking. Following the lateral stretching zone is a quiescent zone in which the glass ribbon is permitted to cool to a condition at which it may be withdrawn from the float chamber without damaging its surfaces. 
     This particular sequence of attenuating a glass ribbon is designed to minimize the creation of observed surface distortion in the glass produced. The improvements are based on the recognition that the optical power of glass surface distortion is a strong factor of the spatial frequency of the distortion features in accordance with the following relationship which relates optical power of a defect to its geometry: 
     
         P=khf.sup.2 
    
     where P is optical power, k is a constant, h is the height or amplitude of the surface defect, and f is the spatial frequency of the distortion pattern. It can be seen from this relationship that while amplitude and frequency both affect the optical power, the frequency is a second power factor whereas the amplitude is merely a linear factor. Therefore, the primary objective is to reduce the frequency of the distortion features. However, additional improvement due to amplitude reduction is also attained. 
     The improved attenuation technique is also based on the finding that longitudinal stretching is not only a major source of mechanical perturbations, but even more significantly, serves to increase the frequency of surface defects introduced into or pre-existing in the glass ribbon. Accordingly, the longitudinal stretching zone of the present method preferably is preceded by the relaxation zone so as to minimize the effects of any perturbations on the glass entering the longitudinal stretch zone. Furtheremore, by carrying out most of the longitudinal stretching, which is accompanied by some of the most harmful perturbations, at a point where the ribbon is relatively narrow in width permits subsequent operations to be performed on the ribbon which reduce the amplitude and frequency of the surface distortion produced by the longitudinal stretching. More specifically, by widening the ribbon in the subsequent lateral stretching zone, the distortion patterns produced by the longitudinal stretching also become stretched in the lateral direction, thereby reducing their frequencies as well as their amplitudes. 
     The present invention is an improvement on the method of the previous application wherein preferred temperature ranges for the longitudinal stretching and lateral stretching steps have been determined. Longitudinal stretching is carried out at glass temperatures below 1700° F. (925° C.), preferably from 1550° F. to 1650° F. (840° C. to 900° C.). Lateral stretching is carried out in the range of 1450° F. to 1600° F. (790° C. to 870° C.), preferably 1450° F. to 1550° F. (790° C. to 840° C.). Qualitatively, this represents carrying out the lateral stretching step at relatively low temperatures, which is advantageous for minimizing the &#34;differential stretching&#34; effect. In other words, lateral stretching at relatively low temperature results in attenuation of the glass ribbon more by thickness reduction and less by reduction of the longitudinal dimension. Barriers within the molten metal may be used to aid in establishing the desired distinct temperature zones. 
     A variety of commercial configurations for forming flat glass on baths of molten metal may be adapted to practice the present invention, and several embodiments of such adaptations will be described herein. 
    
    
     THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional side view of a preferred embodiment of the present invention employed in conjunction with a freefall molten glass delivery system. 
     FIG. 2 is a schematic plan view of the glass forming chamber of FIG. 1. 
     FIG. 3 is a schematic cross-sectional side view of an alternate glass forming chamber embodiment of the non-freefall type incorporating the features of the present invention. 
     FIG. 4 is a schematic plan view of the glass forming chamber of FIG. 3. 
     FIG. 5 is an enlarged plan view of edge cooling means that may be employed with the present invention. 
     FIG. 6 is a side view of the edge cooling means of FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments depicted in FIGS. 1 and 2 relate to the type of float glass forming embodiments disclosed in U.S. Pat. Nos. 3,083,551 and 3,220,816 (Pilkington) which are in wide commercial use. Details of its construction and operation will be familiar to those of skill in the art. Generally, a mass of molten glass 10 from a melting furnace (not shown) is delivered by way of a canal 11 to a forming chamber 20. A tweel 14 extending through the roof 12 of the canal control the rate of delivery of the molten glass to the forming chamber. The chamber may comprise refractory floor 21, roof 22, and walls 23. A pool of molten metal 25 consists essentially of tin or an alloy thereof. The molten glass enters the forming chamber over a lip member 15 where it falls freely onto the molten metal to form a meniscus 26 which is permitted to spread laterally to the extent permitted by surface tension forces of the molten glass. The glass need not fall freely from the lip 15 but may be supported between the lip and the molten metal surface by a refractory member such as that shown in U.S. Pat. No. 4,055,407 (Heithoff et al.). This laterally spreading portion of the molten glass is designated zone A in FIG. 2 and constitutes the relaxation zone of the present invention. In zone A the glass is either at or above equilibrium thickness and is maintained at or above about 1600° F. (870° C.) up to a typical delivery temperature of about 2000° F. (1090° C.). 
     The principal function of zone A in the present invention is to maintain a relatively long residence time for the glass at this relatively high temperature range at which the glass will have a relatively low viscosity, which in turn encourages equilibrium of flow perturbations arising from delivery of the molten glass onto the pool of molten metal. This relatively long residence time is achieved by providing a relatively large volume of molten glass in zone A, such as by permitting the glass to spread laterally as shown in FIG. 2. Alternatively, the increased volume may be attained by enhancing the depth of the glass in zone A by means of side barriers or other means to urge the glass inwardly. 
     In FIG. 2, zone B represents a longitudinal stretching zone. The glass ribbon is at approximately the equilibrium thickness at the point where it is drawn into zone B. The longitudinal stretching of zone B is initiated at a location where the glass temperature is below 1700° F. (925° C.). Preferably, the longitudinal stretching is carried out at 1550° F. to 1650° F. (840° C. to 900° C.), optimally at about 1600° F. (870° C.). The temperature of the glass ribbon is permitted to fall as it passes through zone B but the temperature is controlled so that the temperature is not below 1500° F. (815° C.) when it enters the subsequent zone, zone C. Glass is drawn from zone A into zone B in the longitudinal direction whereby forces are applied to the glass which tend to cause the glass ribbon to be reduced in width and thickness. However, the reduction of width would be much more pronounced than the reduction in thickness if the longitudinal attenuation were permitted to proceed without restriction. This is disadvantageous since the ultimate object of attenuation is to reduce the glass thickness, and since a narrow glass ribbon is less useful commercially. Therefore, means are provided in zone B to restrict the narrowing of the ribbon and to force the attenuation in zone B to take place primarily at the expense of the thickness of the ribbon. The width-controlling means is preferably a set of rotating rolls 28 as shown in the art such as gas jets, blades or electromagnetic means. Preferably, the rolls 28 may be of the particular design shown in U.S. Pat. No. 3,929,444 (May et al.). A plurality of sets of rolls are provided in zone B so as to maintain the width of the ribbon substantially constant, each set consisting of a pair of rolls on opposite sides of the ribbon. The rolls engage the top surface of the edges of the ribbon, and their speeds of rotation are controlled so as to accelerate the longitudinal velocity of the ribbon as it passes through zone B. It is preferred that the rolls in zone B be angled outwardly slightly (about 5° to 10° from the direction of glass travel). In zone B the thickness of the glass is reduced from approximately the equilibrium thickness to a substantially reduced thickness typically on the order of about halfway or more toward the desired final thickness. This longitudinal attenuation is believed to induce a substantial amount of surface distortion in the glass, but that this distortion constitutes the majority of the distortion produced by the overall attenuation process. 
     Subsequently, the glass enters a lateral stretching zone, designated as zone C in FIG. 2, where the glass is brought to its final thickness. The glass in zone C may range in temperature from about 1600° F. (870° C.) to about 1450° F. (790° C.). Preferably, lateral stretching is carried out at glass temperature between 1450° F. (790° C.) and 1550° F. (840° C.), optimally at about 1500° F. In this final attenuation step the thickness reduction is achieved primarily by increasing the width of the ribbon. Lateral stretching forces are provided by means engaging the edges of the ribbon, such as sets of rolls 29 which may be the same design as rolls 28, or other known attenuating devices. The rolls 29 are angled so as to impart a lateral component of force to the glass ribbon. Longitudinal force is also applied to the glass in zone C by means of the rolls 29 as well as by the conveying means acting upon the formed ribbon beyond the exit of the forming chamber. The application of longitudinal force in zone C is desirable to assure that the final attenuation is accomplished through thickness reduction rather than by shortening of the longitudinal dimension. Some acceleration in the longitudinal direction may be imparted to the ribbon in zone C so as to stretch the ribbon in both the longitudinal and lateral directions, but the longitudinal stretching in zone C should be minor relative to that imparted to the glass in zone B. 
     The ratio of the final ribbon width to the ribbon width in zone B is directly proportional to the frequency of the optical power of the distortion. Therefore, it is desirable to maximize lateral attenuation in zone C. It has been found that a dominant distortion pattern due to thickness variation having a frequency ranging from about 0.70 to about 0.80 cycles per inch (0.28 to 0.32 cycles per centimeter) is created by longitudinal attenuation as in zone B. This frequency of optical distortion unfortunately happens to be in a region of frequencies which are highly sensitive to the human eye. The lateral attenuation in zone C advantageously reduced this frequency in accordance with the following relationship: 
     
         f.sub.2 =f.sub.1 ×W.sub.B /W.sub.D 
    
     where f 1  is the optical distortion frequency entering zone C, f 2  is the optical distortion frequency of the final glass product, W B  is the width of the glass ribbon in zone B, and W D  is the width of the glass ribbon in zone D. Accordingly, it is desirable to increase the ribbon width in the lateral attenuation zone C to at least 1.05 times the width of the ribbon in zone B, preferably by a factor of 1.1, and most preferably by a factor of 1.5 or higher. When feasible, it is desirable for the final ribbon width to exceed the maximum width of the glass in the relaxation zone. 
     After lateral attenuation, the glass ribbon enters zone D in FIG. 2 where it is permitted to cool without further attenuation to a temperature, typically about 1100° F. (595° C.), at which it is dimensionally stable and sufficiently hardened to be lifted from the pool of molten metal by means of lift-out rolls 31 at the exit lip 30 of the float chamber. Subsequently, the glass ribbon is typically conveyed on a roller conveyor through an annealing lehr. 
     In order to assist in establishing the thermal conditions disclosed above for zones B and C, it is preferred to employ barriers 35 and 36 submerged in the tin at the approximate boundaries between zones B and C and between zones C and D as shown in FIG. 2. The barriers may be of a known construction, such as the movable barriers disclosed in U.S. Pat. Nos. 3,930,829 (Sensi) and 4,099,952 (Schwenninger), and may be a two-piece construction as shown to facilitate insertion into the tin bath. The height of the barriers is less than the depth of the tin to avoid contacting the glass, but sufficient to retard the flow of tin in the longitudinal direction. By retarding the flow of tin from one zone to another, the desired thermal conditions can be established more readily in the respective zones. In order to help estabish the desired thermal conditions in a portion of the glass ribbon, the temperature of the adjacent molten metal is usually maintained slightly lower, typically about 30° F. (17° C.) lower. 
     FIGS. 3 and 4 depict an adaptation of the present invention to a &#34;constant width&#34; type forming process as disclosed in U.S. Pat. No. 3,843,346 (Edge et al.). This embodiment differs from the embodiment of FIGS. 1 and 2 in that molten glass is delivered onto the molten metal in the forming chamber by means of a wide threshold and without free fall or unhindered lateral spread. Molten glass 40 is contained in a melting furnace 41 provided with a metering tweel 43 at the junction between the melting furnace and the forming chamber 50. A wide threshold 44 underlies the metering tweel 43 and supports the glass during its delivery into the forming chamber until it is supported by the molten metal 55. The forming chamber 50 may consist of a bottom 51, roof 52, and sidewalls 53 of conventional construction in the art. In accordance with the present invention, the glass ribbon 57 passes in sequence through four zones designated Q, R, S, and T, in FIG. 4 and which respectively correspond in function to zones A, B, C, and D described above in connection with FIG. 2. Zone Q is the relaxation zone where, as previously described, the glass is maintained relatively undisturbed at a relatively high temperature in order to reduce the volumetric nonuniformities in the newly delivered layer of molten glass. Lateral spread in zone Q is restricted by means of side barriers 56 and 57. 
     Referring again to FIG. 4, the glass, after leaving relaxation zone Q, enters longitudinal stretching zone R wherein the ribbon is subjected to longitudinal attenuation to substantially reduce its thickness while maintaining substantially constant width by means of edge roll members 58 in the same manner described above in connection with zone B in FIG. 2. Likewise, subsequent lateral attenuation in a lateral stretching zone S, which includes outwardly angled edge roll means 59, is carried out in the same manner as in zone C described in connection with the FIG. 2 embodiment above. The temperature of the glass in zones Q, R and S is the same as that described above for zones A, B, and C respectively. Finally, the glass is permitted to cool, typically to a temperature of about 1100° F. (595° C.), in a cooling zone T after which the dimensionally stable ribbon of glass is lifted over exit lip 60 by means of liftout rolls 61. 
     A variation of the invention entails passing the glass from a relaxation zone such as A, or Q, as in the previously described embodiments into a combined longitudinal and lateral attenuation zone. In such a zone, the lateral and longitudinal attenuation may be carried out substantially simultaneously so that the ribbon of glass is increased in width to essentially its final width and is decreased in thickness to essentially its final thickness during passage therethrough. In such an embodiment, a substantial amount of longitudinal stretching would not be performed subsequent to the final lateral stretching. 
     Edge coolers 39 are shown in the lateral stretching zones of the embodiments of FIGS. 1 and 2 and FIGS. 3 and 4. The edge coolers serve to increase the viscosity of the edge portions of the glass ribbon, thereby permitting greater traction by the edge stretching rolls 29 or 59 on the glass ribbon, which is advantageous for imparting lateral stretching forces on the glass ribbon. Normally, edge stretching rolls pull a portion of the glass ribbon outwardly, and between the rolls the ribbon shrinks back partially to its original width, thereby producing a scalloped edge effect in a stretching zone. Cooling the edge portions of the glass ribbon reduces this scalloping, and as a result more uniform forces are applied to the ribbon. Another advantage of cooling the edges in the transverse stretching zone is that the relatively stiff edges transmit to the longitudinal stretching zone more of the longitudinal forces produced by the conveying means downstream from the exit of the forming chamber. Thus, the longitudinal tractive forces may serve to said stretching in the longitudinal zone with a diminished effect on the ribbon in the transverse stretching zone. Preferably, the marginal edge portions of the ribbon are cooled to about 50° F. (28° C.) below the temperature of the central portion of the glass ribbon. Thus the edge portions typically may range from about 1400° F. to 1500° F. (760° C. to 820° C.) after being cooled in the transverse stretching zone. 
     Suitable edge cooling means are known in the art, an example of which is disclosed in U.S. Pat. No. 3,692,508. A preferred arrangement comprises a plurality of water-cooled members 39 shown in FIGS. 1-4, details of which are illustrated in FIGS. 5 and 6. Each cooler 39 includes a hairpin bent water conduit 65, to the end of which is welded a metal plate 66. Beneath the plate 66 is affixed a graphite block 67 which serves to prevent sticking of the glass ribbon 27 to the cooler in the event of accidental contact. The conduit 65 may be mounted in a side seal unit 68 which may be inserted into the customary access openings in the side walls 23. 
     The use of edge coolers in combination with a bidirectional attenuation process is the subject of a copending U.S. Patent Application Ser. No. 307,815 entitled &#34;Method of Bidirectionally Attenuating Glass in a Float Process with Edge Cooling &#34; filed by J. R. Mouly on Oct. 2, 1981. 
     Other variations and modifications employing features known in the art will be apparent to those of skill in the art and are within the scope and spirit of the invention as defined by the following claims.