Abstract:
A glass envelope for a cathode ray tube ( 10 ) and a method of production. The envelope comprises ( 12 ) a panel member having a thickness (t) that is defined by an equation which involves net tensile stress; wherein the panel having a net tensile stress not exceeding 1150 psi, having a surface compression induced during panel manufacture, and having an implosion protection band ( 30 ) shrink-fitted around a skirt ( 20 ) on the panel that augments surface compression in the panel ( 12 ), whereby the panel thickness is substantially less than that of a corresponding untreated tube envelope.

Description:
FIELD OF THE INVENTION 
     An envelope for a cathode ray tube, especially the panel or face plate thereof. 
     BACKGROUND OF THE INVENTION 
     A cathode ray tube customarily has a glass envelope which is evacuated and hermetically sealed for tube operation. The envelope comprises a viewing panel member, a funnel member and a tubular neck member. An electron gun is mounted in the neck member. 
     The panel member is often referred to as a face plate, or a display window, since it provides a screen on which a picture or display is formed. Normally, the panel member has a depending, peripheral skirt that is sealed to the funnel member with a fused sealing material. 
     Since a tube operates in an evacuated state, there is a pressure difference of one atmosphere across the panel. This creates a risk of implosion when flaws develop as the glass cools, or are caused by scuffs or scratches during handling. To counter this, the panel thickness is increased. Normally, glass thickness in the panel is greater than that in the funnel and neck member, since these members are less prone to damage and are not exposed during tube operation. 
     The ever increasing demand for larger picture tube size has led to larger and larger panels. Coincident therewith has been the increase in panel and tube weight. This has led to difficulty in handling the heavy ware without damage. It has also entailed longer times and more energy to process the panel and the tube, thus adding to the cost. This has led to a strenuous search for an alternative to increased panel thickness as a protection against implosion. 
     Numerous proposals have been made to use a so-called implosion protection band. Such a band tightly encircles the skirt portion of the panel, and may be adherently sealed thereto. The protection band may be composed of metal or other strong materials. 
     It is also known to strengthen glass by tempering. This practice is commonly employed for such diverse items as automotive glazing and glass cookware. Glass may be thermally tempered by blowing cooling air on the surface. This accelerates setting of the surface glass, thereby inducing compressive stresses therein. An alternative practice, chemical tempering, has been proposed. In this procedure, ion exchange is employed to create compression by crowding ions in the glass surface. 
     The present invention is directed to producing an improved envelope for a cathode ray tube. In particular, it is directed to the production of larger sized tubes in excess of a 19″ panel dimension. It is further directed at minimizing the glass thickness in a cathode ray tube (CRT) panel while maintaining full protection against implosion. It is also directed to a method of producing a panel for a cathode ray tube that has minimum thickness consistent with full protection against implosion. 
     SUMMARY OF THE INVENTION 
     The article of the invention is a glass envelope for a cathode ray tube comprising a panel member having a thickness (t) defined by the equation: 
     
       
         (σ) P =kP(a/t) 2   
       
     
     wherein (σ) P  is the net tensile stress in the panel, (a) is one half the length of the minor axis in inches, (t) is the panel thickness in inches, (P) is atmospheric pressure in psi and (k) is a structural constant based on tube geometry. The panel has a net tensile stress not exceeding 1150 psi, a surface compression induced during panel manufacture, and an implosion protection band shrink-fitted around a skirt on the panel that augments surface compression in the panel, whereby the panel thickness is substantially less than that of a corresponding untreated tube envelope. 
     The invention further resides in a method of reducing the thickness of a panel member in a cathode ray tube glass envelope wherein the thickness (t) is defined by the equation (σ) P =kP(a/t) 2  wherein (σ) P  is the maximum tensile stress in the panel, the method comprising imparting compression in the panel surface by treatment of the surface, and augmenting this compression by shrink-fitting an implosion protecting band about a skirt on the panel, while maintaining the value of (σ) P  at not over 1150 psi, thereby permitting a reduction in panel thickness from that in an untreated panel. 
     PRIOR ART 
     Literature of possible interest is listed on an attached document. 
    
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
     FIG. 1 in the accompanying drawing is a side view, partially cut away, illustrating a cathode ray tube in accordance with the invention. 
     FIG. 2 is a top plan view of the panel member of FIG.  1 . 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention is concerned with the glass envelope employed in producing a cathode ray tube. It is particularly concerned with the viewing panel member of such glass envelope. Both the tube and the envelope are commonly identified by the geometry of the panel member. The present invention is not limited to any particular tube geometry. However, it was developed in connection with the popular rectangular envelope, and is so described. 
     FIG. 1 is a side view, partially cut away, illustrating a cathode ray tube construction generally designated  10 . Tube  10  comprises the standard glass envelope members: viewing panel  12 , funnel  14  and tubular neck  16 . Electron gun  18  is mounted in neck member  16 . Other tube components, not relevant to the present invention, such as a phosphor screen and a shadow mask, are omitted in the interest of clarity. 
     Viewing panel  12  has a depending skirt portion  20 . Panel  12  is joined to funnel  14  by a fusion seal  22  between the rims of skirt  20  and funnel  14 . 
     When tube  10  is evacuated, there exists a pressure differential of approximately one atmosphere between the external ambient and the internal pressure of essentially zero. The differential occurs over the entire tube surface, but is of particular concern with respect to panel  12  because of its exposure in a mounted television set. This differential creates a tensile stress across the panel. This tensile stress must be limited to avoid the propagation of surface flaws, and consequent collapse of the tube envelope. 
     FIG. 2 is a top plan view of rectangular panel member  12  showing the major axis  24  and the minor axis  26  in dashed lines. It has been found that the gusset region  28  near the end of minor axis  26  experiences the highest stress and is the weakest region in the panel. This is, the expected source of breakage initiation if and when breakage occurs. 
     Extensive studies have shown that implosion can be averted by limiting the net tensile stress across panel  12  to 1150 psi. Tests on hundreds of test pieces maintained at or below this stress level showed no breakage even on badly abraded samples. However, at higher levels, e.g. 1400 psi tensile stress, at least some breakage was encountered. Tensile stress (σ) P , is determined in accordance with the formula: 
     
       
         (σ) P =kP(a/t) 2 . 
       
     
     It is evident that the permissible panel thickness (t) is dictated by the net tensile stress. The latter must not exceed 1150 psi. It is also evident that the panel weight, in turn, increases as the panel thickness increases. The present invention is aimed at reducing the thickness, and hence the weight, required to stay within the 1150 psi limit on tensile stress. 
     We have found that the normal thickness of a tube panel can be reduced by anywhere from 10-35% depending on the consistency of manufacturing conditions. To accomplish this, we combine two means of inducing surface compression in the exterior surface of panel  12 . 
     Rapid cooling of panel member  12  during production can introduce significant levels of compression in the panel surface. Depending on the prevailing conditions, surface compression levels ranging from 450-1650 psi can be introduced. To some extent, this surface compression may be lost during the thermal treatment inherent in an exhaust bake-out cycle. The tube is exposed to such cycle before being finally sealed. Allowing for a possible loss of up to  30 %, compression stress values of at least 315-1155 psi may be retained. This compression stress counters a tensile stress value of 1150 psi to such an extent that the net tensile stress is 835 psi. 
     We have also found that a further increase in the compression stress level in the panel can be augmented by shrink-fitting an advanced form of metal band  30  around the skirt  20  of panel  12 . In this manner, a compressive stress level of 170-290 psi can be introduced. This, in turn, decreases the net tensile stress to about 665 psi. 
     It will be appreciated that the normal desire is not to decrease the tensile stress. Rather, it is to decrease panel thickness, and thereby tube weight. Consequently, the practical effect of the present invention is to retain the maximum tensile stress of 1150 psi in a tube panel. Then, a permissible minimum panel thickness is calculated employing the formula stated earlier. This calculation reveals that the normal thickness of an untreated envelope panel can be reduced by as much as 25%. Thus, a panel cooled in normal manner, and having a thickness of ½″, can have its thickness reduced to ⅜″. 
     While various types of shrink-fit rim bands may be employed, we have found best results with a welded connector plate type rim band. This type of band is described in a publication entitled, “Evaluation of Fabrication and Reliability of Metal Band Joining Techniques for Long Term Safety of CRTs” by Keith Guenther et al. in the May 21, 1996 issue of &#39;96 Displayworks. 
     Air cooling is a preferred means of inducing the compression stress level in a glass panel. However, it will be appreciated that chemical tempering by ion exchange would be equally effective, albeit not as practical. Controlled release of forming stresses through heat treatment is also an effective way to introduce high surface compression in the CRT panel. 
     The invention is further described with reference to a test in which several groups of sealed and evacuated CRTs were compared. The tubes in each group had essentially identical physical characteristics, but each group had different characteristics. In particular, each group had a unique combination of Center Face Thickness (CFT) values and High Surface Compression (HSC) values. 
     Center Face Thickness is the thickness in inches at the center of the tube panel. High Surface Compression is the compression in psi induced in the panel to counter tension developed by the pressure differential. The test was carried out over a period of 100 days; all tubes were prepared in accordance with standard commercial practice; and all panels were lightly abraded to simulate expected use. 
     TABLE I below shows the characteristic CFT and HSC values for each group (incoming HSC value measured at seal edge); the number of tubes in each group; and the number that failed. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 CFT (in.) 
                 HSC (psi) 
                 # Tested 
                 # Failed 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 A 
                 0.491 ± 0.006 
                 1700 ± 100 
                 10 
                 0 
               
               
                 B 
                 0.491 ± 0.006 
                  600 ± 100 
                 8 
                 8 
               
               
                 C 
                 0.521 ± 0.009 
                 1700 ± 100 
                 10 
                 0 
               
               
                 D 
                 0.551 ± 0.009 
                  600 ± 100 
                 10 
                 0 
               
               
                 E 
                 0.491 ± 0.006 
                 1700 ± 100 
                 9 
                 0 
               
               
                 F 
                 0.491 ± 0.006 
                  600 ± 200 
                 10 
                 4 
               
               
                 G 
                 0.491 ± 0.006 
                 1200 ± 100 
                 7 
                 0 
               
               
                 H 
                 0.491 ± 0.006 
                 1700 ± 100 
                 10 
                 0 
               
               
                 I 
                 0.521 ± 0.009 
                  600 ± 100 
                 9 
                 0 
               
               
                   
               
             
          
         
       
     
     It is apparent that, within limits, the CFT required varies inversely with HSC. Based on this and other tests, it has been determined that the CFT may vary upward from about 0.480″ in a 27″ panel while the HSC is varied between 600 and 1700 psi. 
     In addition, CRTs must pass impact tests. These tests are described in UL 1418, “Standard for Safety of CRTs” from Underwriters Lab, and in CAN/CSA-C22.2, No. 228-92, “Canadian Standards for CRTs”. 
     The tests involve impact with a ball or missile of specified weight under specified conditions. They are designed to ensure the safety of viewers in case the CRT implodes due to heavy impact by a large object. The distance over which glass fragments are thrown out, in case of implosion, is limited by these tests to 0-5 ft. How far these fragments travel depends on two factors namely i) the size of the fragment and hence its mass, and ii) the energy to propel it. 
     The former is influenced by the compressive stress induced by the band since higher compression will keep several fragments locked together thereby increasing their mass. The latter is composed of three sources of energy namely i) energy due to vacuum, ii) energy due to bending induced by pressure differential, and iii) energy due to local deformation induced by compressive stress from the band. The first two components of total energy are dictated by the size of CRT and hence are relatively fixed, i.e., they cannot be varied. The third component, however, can be adjusted by controlling the compressive stress from the band by simply changing its width or thickness or both. 
     In addition to moderating the energy, the local deformation caused by the band alters the doming value of the panel at its center The doming value is the distance that the central point on a tube panel rises, that is, bows outwardly or upwardly due to applied compressive stress. It should be noted that the center of the panel moves inward upon evacuation while the band helps bring it back to its original position. Thus, doming is the recovery value. A certain amount of doming is a necessary result of imparting the desired stress. However, tests have shown that doming must be limited to pass the safety tests. 
     TABLE II sets forth the CFT and HSC values, as well as doming values measured on several sets of completed CRTs that were subjected to the UL and CSA tests. It also sets forth the width of the band employed to induce compression stress. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Tube 
                 CFT 
                 HSC 
                 Doming 
                 Bandwidth 
                 # 
                 # 
               
               
                 Set 
                 (in.) 
                 (psi) 
                 (mm) 
                 (in.) 
                 Tested 
                 Failed 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0.491 
                 1700 
                 0.121 
                 1.496 
                 15 
                 1 
               
               
                 2 
                 0.491 
                 600 
                 0.110 
                 1.496 
                 15 
                 0 
               
               
                 3 
                 0.521 
                 1700 
                 0.103 
                 1.496 
                 15 
                 0 
               
               
                  4* 
                 0.551 
                 600 
                 0.104 
                 1.496 
                 15 
                 0 
               
               
                 5 
                 0.491 
                 1700 
                 0.128 
                 1.496 
                 15 
                 1 
               
               
                 6 
                 0.541 
                 1700 
                 0.110 
                 1.496 
                 8 
                 0 
               
               
                 7 
                 0.607 
                 1700 
                 0.095 
                 1.496 
                 8 
                 0 
               
               
                 8 
                 0.541 
                 1700 
                 0.094 
                 1.378 
                 22 
                 0 
               
               
                 9 
                 0.607 
                 1700 
                 0.088 
                 1.378 
                 4 
                 0 
               
               
                   
               
             
          
         
       
     
     The implosion test data, summarized in TABLE II, clearly show that too high a doming value, i.e., greater than 0.110 mm, caused by the standard band of 1.496″ width, does not permit CRTs with center face thickness of 0.491″ to pass the implosion test. 
     It is also evident that the band plays a dual role. In addition to inducing compressive stress in the panel, control of the band width controls doming, that is, the strain energy due to bending. Reducing the band width to 1.378″ results in lower compression and less doming which help pass the implosion test as noted in TABLE II. Thus for thinner panels, within the tolerance band of thickness specs, a narrower band is required to pass the implosion test. 
     Based on the foregoing tests, we find that the band width of the metal band applied to a panel skirt of a 27 inch CRT panel should fall in the range of 1.378 to 1.496 inches. Also, to insure passing the UL and CSA safety tests, the domning value should be in the range of 0.088 to 0.110 mm.