Abstract:
A flat elliptic thin glass tube for a discharge tube is produced by the following steps: (a) a cylindrical glass tube is hermetically sealed; (b) the cylindrical glass tube is heated and deformed in a mold by an increased internal pressure of the glass tube caused by the heating of the glass tube to form a flat elliptic glass tube, the mold having means for defining at least the minor axis of the flat elliptic glass tube; and (c) the flat elliptic glass tube is heated and drawn to form the flat elliptic thin glass tube.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to methods for making glass discharge tubes having a flat elliptic cross-section. In particular, the present invention relates to a method for making an elliptic thin glass tube at high accuracy and low cost.  
           [0003]    2. Description of the Related Art  
           [0004]    Glass tubes having a flat elliptic cross-section have been generally formed by tube drawing called a Danner process. In the method for making glass tubes by the Danner process shown in FIG. 7, a molten glass material that is melt in a melting furnace (not shown) at 1,300° C. to 1,500° C. in introduced into a platinum cylinder called a sleeve  74  to form a cylindrical glass tube, and the cylindrical glass tube passes through a shaping unit  72  at a temperature above the softening point of the glass in a production line. The shaping unit  72  has at least a pair of upper and lower rollers  73 . The glass tube is pressed by the upper and lower rollers  73  to be deformed into a flat elliptic cross-section.  
           [0005]    Unfortunately, the flat elliptic glass tubes directly produced by the Danner process from the molten glass exhibits poor shaping stability. Furthermore, it is difficult to produce thin glass tubes with an inner diameter of about 0.5 mm to 5 mm with high accuracy by the Danner process.  
           [0006]    General glass tubes produced under predetermined processes have high productivity, for example, several tens of tons every day; hence, yearly required amounts of glass tubes could be produced within several days. However, control of the shape of flat elliptic glass tubes requires many hours. Thus, production dedicated to fine discharge tubes inevitably consumes much expense.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the present invention is to provide a method for readily making a flat elliptic thin glass tube at high accuracy and low cost prefaerably the tube having an inner diameter of about 0.5 mm to 5 mm.  
           [0008]    The present inventors made flat elliptic glass tubes by sealing their two ends of inexpensive glass tubes with a circular cross-section formed by a Danner process and then shaping the glass tubes in a shaping unit that determines their outer shape. The cross-section and the thickness of each flat elliptic glass tube were proportionally contracted by a redrawing process to produce a flat elliptic thin glass tube for a discharge tube. The present invention has been accomplished based on these experiments.  
           [0009]    According to the present invention, a method for making a flat elliptic thin glass tube for a discharge tube includes the following steps of (a) hermetically sealing a cylindrical glass tube; (b) heating and deforming the cylindrical glass tube in a mold by an increased internal pressure of the glass tube caused by the heating of the glass tube to form a flat elliptic glass tube, the mold having means for defining at least the minor axis of the flat elliptic glass tube; and (c) drawing while heating the flat elliptic glass tube to form the flat elliptic thin glass tube.  
           [0010]    Preferably, in the step (b), the cylindrical glass tube is maintained at a temperature which is 70% to 90% of the softening point of the glass tube.  
           [0011]    Preferably, in the step (c), the length of a region at a maximum temperature of a heating path for heating the flat elliptic glass tube is 10% or less of the total length of the heating path.  
           [0012]    Preferably, the maximum temperature of the heating path is 1.07 times to 1.1 times the softening point of the flat elliptic glass tube.  
           [0013]    Preferably, the maximum temperature of the heating path is 1.08 times to 1.09 times the softening point of the flat elliptic glass tube.  
           [0014]    Preferably, the heating rate is in the range of 10° C./min to 300° C./min in a heating portion of the heating path.  
           [0015]    Preferably, in the step (c), the feeding rate of the flat elliptic thin glass tube is 20 times to 400 times the feeding rate of the flat elliptic glass tube.  
           [0016]    According to the present invention, a flat elliptic thin glass tube with a predetermined size and shape is readily produced at high accuracy and low cost by using a commercially available inexpensive cylindrical glass tube. Discharge tubes formed of this flat elliptic thin glass tube have a stable size and shape and thus exhibit uniform discharge characteristics. The discharge tubes are preferably used in a display apparatus. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows a schemetivally perspective view of a display device including flat elliptic thin glass tubes produced by a method according to the present invention;  
         [0018]    [0018]FIG. 2 shows a schemetivally perspective view of an apparatus for making a flat elliptic glass tube;  
         [0019]    [0019]FIGS. 3A to  3 C show schemetivally cross-sectional views of the apparatus shown in FIG. 2;  
         [0020]    [0020]FIG. 4 shows a schemetivally schematic illustration of an apparatus for making a flat elliptic thin glass tube;  
         [0021]    [0021]FIG. 5 is a graph showing a temperature profile in a heating furnace used in an experiment for making a thin glass tube according to the present invention;  
         [0022]    [0022]FIGS. 6A and 6B show a schemetivally front view and a schemetivally side view, respectively, of a redrawing apparatus for making a thin glass tube according to the present invention; and  
         [0023]    [0023]FIG. 7 shows a schemetivally schematic illustration of a conventional Danner process for making an elliptic glass tube. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 is a perspective view of a display device including flat elliptic thin glass tubes produced by the method according to the present invention. A rear support  1  composed of a resin or glass substrate is provided with a plurality of data electrodes  13  (three electrodes for displaying red, green, and blue colors, respectively are drawn in the drawing) thereon. Hereinafter, red, green and blue colors are referenced as R, G, and B, respectively. R tube, for example, means a tube for red color. R, G, and B flat elliptic thin glass tubes produced by a method described below are in contact with the respective data electrodes  13 . Plural pairs of display electrodes  11  perpendicular to electrodes  13  are arranged on a transparent sheet  3 ; the outer face of each flat elliptic thin glass tube  2  is in contact with the corresponding data electrode  13  at the bottom and with the display electrodes  11  at the top. The display electrodes  11  are covered with the transparent sheet  3  that functions as a front support. The transparent sheet  3  is bonded to the thin glass tubes with an adhesive layer (not shown). Although is not shown in the drawing, each display electrode  11  has a composite structure including a transparent electrode and a metal bus electrode to reduce its line resistance and the shading area so that visual light can be effectively emitted through the thin glass tubes.  
         [0025]    Each thin glass tube is filled with discharge gas and has an electron-emitting layer  14  and three primary color fluorescents layers  16 R,  16 G, and  16 B on the inner wall. These fluorescents layers  16 R,  16 G, and  16 B are preliminarily formed on a fluorescent support  15  and the flourescent support  15  is placed at a predetermined position in the thin glass tube.  
         [0026]    For performing display, selective discharge is generated between a data electrode  13  in contact with a selected thin glass tube and a pair of display electrodes  11  and then continuous discharge is generated between the pair of display electrodes  11 .  
         [0027]    A method for making the above flat elliptic thin glass tube will now be described according to the steps.  
         [0028]    Steps for Making Flat Elliptic Glass Tube  
         [0029]    [0029]FIG. 2 is a perspective view of an apparatus for making the flat elliptic glass tube, and FIGS. 3A to  3 C are cross-sectional views of the apparatus.  
         [0030]    Referring to the left in FIG. 2, both sides of a glass tube  21  (Pyrex # 7740  made by Corning, diameter: 10 mm, thickness: 1.0 mm, length: 500 mm, softening point: 821° C.) are sealed by melting. The sealed glass tube  21  is placed into a 500 mm long shaping unit  22  composed of carbon, quartz, or silicon carbide and having a rectangular cross-section of 8.6 mm by 11.8 mm. The two ends of the sealed glass tube  21  may put into the shaping unit  22  or may lie outside the shaping unit  22 , as shown in the drawing. FIG. 3A shows a state of the glass tube  21  in the shaping unit  22 .  
         [0031]    The glass tube  21  in the shaping unit  22  is placed in a heating furnace (not shown in the drawing) and is heated to 640° C to cause deformation of the glass tube  21  into a shape (flat elliptic cross-section) all along the inner shape of the shaping unit  22  due to an increased inner pressure and the softening of the glass tube  21 , as shown in the right in FIG. 2 and FIG. 3A. After the deformation of the glass tube  21 , the glass tube  21  with the shaping unit  22  is cooled. A flat elliptic glass tube  23  is thereby formed. Since the glass tube is more rapidly cooled than air in the tube in the cooling process, the glass tube  23  maintains its flat elliptic cross-sectional shape. Preferably, the maximum temperature of the heating furnace is in the range of 600° C. to 720° C. for Pyrex glass or is in the range of 70% to 90% of the softening point for other glass materials.  
         [0032]    Referring to FIG. 3B, the glass tube  21  that is placed into the shaping unit  22  may have an outer diameter larger than the short side of the shaping unit  22 . In such a case, the glass tube  21  in the shaping unit  22  is placed in the heating furnace in a state that one side plate  22   a  of the shaping unit  22  is separated from other portions. A predetermined pressure  25  applied from the side  22   a  causes deformation of the softened glass tube  21  into a flat elliptic cross-sectional shape along the cross-sectional shape of the shaping unit  22 .  
         [0033]    Referring to the left in FIG. 3C, alternatively, a flat elliptic glass tube  26  may be used for forming the flat elliptic glass tube  23  having a desired cross-sectional shape.  
         [0034]    In this embodiment, the both sides of the glass tube placed into the shaping unit are preliminarily sealed. Alternatively, an open glass tube may be used. In such a case, the open glass tube is placed into the shaping unit and is sealed in the shaping unit.  
         [0035]    Steps for Making Flat Elliptic Thin Glass Tube  
         [0036]    [0036]FIG. 4 is a schematic illustration of an apparatus for making a flat elliptic thin glass tube. The flat elliptic thin glass tube is formed of a flat elliptic glass tube  43  produced in the above steps. The flat elliptic glass tube  43  is heated in a heater  41  provided around a furnace wall  42  and redrawn while its shape being maintained to form a flat elliptic thin glass tube  44  having a predetermined size and shape. In an actual production apparatus, the heater  41  is divided into a plurality of segments (not shown in the drawing), each provided with a thermosensor  45  of a thermocouple. The temperature detected by the thermosensor  45  is fed back to control the current in the heater  41  for maintaining the furnace temperature within a predetermined range.  
         [0037]    The flat elliptic glass tube  43  is fed in the direction shown by arrow A at a feed rate v, while the flat elliptic thin glass tube  44  is being drawn in the direction shown by arrow B at a drawing rate cxv where c is a drawing factor. Feeding of the flat elliptic glass tube  43  and the drawing of the flat elliptic thin glass tube  44  are performed by a plurality of rollers (not shown) disposed on both sides of the tubes. The drawing factor c depends on the material and the size of the flat elliptic glass tube  43  and is preferably in the range of 20 to 400. At a drawing factor c of less than  20 , the cross-sectional homothetic ratio is about 4.5; hence, the major axis of the flat elliptic glass tube  43  must be 4.5 mm in order to form a flat elliptic thin glass tube  44  with a major axis of 1 mm. This figure is not practical. At a drawing factor c exceeding  400 , the heating of the glass tube cannot follow the temperature of the heating furnace. As a result, the glass tube will break because of insufficient softening during drawing. Accordingly, the drawing factor c is preferably in the range of 20 to 400.  
         [0038]    [0038]FIG. 5 is a graph showing a temperature profile in the heating furnace used in the experiment. The vertical axis represents the temperature in the heating furnace, whereas the horizontal axis is a distance from the entrance of the heating furnace. The temperature profile must have three regions, i.e., a heating region for raising the temperature of the glass tube, a holding region for holding a predetermined maximum temperature, and a cooling region for decreasing the temperature of the glass tube. In the heating region, the heating rate is in the range of 10° C./min to 300° C./min. A heating rate exceeding 300° C./min causes insufficient softening of the glass tube because of insufficient heating of the glass tube in the heating furnace. Thus, the glass would be broked by tensile force in the direction of B shown in FIG. 4 in the drawing process. A heating rate of less than 10° C./min requires an impractical longer heating furnace for sufficiently heating the glass tube.  
         [0039]    The holding region is preferably short. At a long holding region, the softened glass tube tends to deform from the flat elliptical cross-section to a circular cross-section by surface tension of the glass. Thus, the length of the holding region is preferably 10% or less of the length of the heater of the heating furnace to maintain the flat elliptical cross-section. The temperature of the holding region is preferably in the range of 891° C.±10° C. for Pyrex and more preferably 891° C.±3° C. for Pyrex. For any other glass, the temperature is preferably in the range of 1.07 times to 1.1 times and more preferably 1.08 times to 1.09 times the softening point of the glass. If the holding region has an uneven temperature profile, the high temperature portion of the glass is drawn while the low temperature portion is not readily drawn, resulting in an uneven cross-sectional shape of the thin glass tube.  
         [0040]    In the cooling region, the glass tube is slowly cooled until the temperature reaches the strain point (510° C. for Pyrex in this embodiment) to remove permanent strain in the glass tube.  
         [0041]    [0041]FIGS. 6A and 6B are a front view and a side view, respectively, of a redrawing apparatus  61 . The redrawing apparatus  61  may be placed vertically or horizontally. The redrawing apparatus  61  has a slider  62  and a pair of drawing rollers  63 . As described above, the slider  62  feeds a glass tube  43  at a feed rate v while the drawing rollers  63  draw the thin glass tube  44  at a drawing rate cxv.  
         [0042]    In this embodiment, the apparatus is used for making a thin glass tube made of Pyrex glass. Any other glass may be used in the present invention. Examples of usable glasses include soda lime glass, borosilicate glass, and quartz glass. The temperature profile of the heating furnace is preferably determined according to the softening point of the glass used.