US3493356A - Process for producing a color cathode ray tube having a prestressed faceplate panel - Google Patents

Description

Feb. 3, 1970 J. A. TORRENCE PROCESS FOR PRODUCING A COLOR CATHODE RAY TUBE HAVING A PRESTRESSED FACEPLATE PANEL Original Filed Nov.' 25, 1964 2 Sheets-Sheet 1 Affys.

Feb. 3, 1970 J. A. ToRRENcE PROCESS FOR PRODUCING A COLOR CATHODE RAY TUBE HAVING A PRESTRESSED FACEPLATE PANEL 2 Sheets-Sheet 2 Original Filed Nov. 25, 1964 64 FIG. 3

lnvenior James A. Terrence By United States Patent O 3,493,356 PROCESS FOR PRODUCING A COLOR CATHODE RAY TUBE HAVING A PRESTRESSED FACE- PLATE PANEL James A. Torrence, Franklin Park, Ill., assignor to Motorola, Inc., Franklin Park, Ill., a corporation of Illinois Continuation of application Ser. No. 413,778, Nov. 25, 1964. This application Oct. 28, 1968, Ser. N0. 771,696 Int. Cl. C03b 23/24 U.S. Cl. 65-58 7 Claims ABSTRACT OF THE DISCLOSURE A process for assembling a cathode ray tube which includes the step of pre-stressing the glass faceplate panel by heating the same to the annealing range temperature in an environment that provides similar heating, cooling and temperature differentials in the glass panel that exist when the panel is frit sealed to the funnel. Subsequently, when the pre-stressed faceplate panel is frit sealed to the funnel by heating the joined faceplate and funnel to the annealing range temperature, panel distortion will be limited.

This is a continuation of application Ser. No. 413,778, led Nov. 25, 1964, which was assigned to the assignee of this application, now abandoned.

This invention relates to color cathode ray tubes and more particularly to the preparation of faceplate panels for such tubes which utilize a shadow mask and multiple electron beams for producing a composite image in color.

In the widely used tri-beam shadow mask color picture tube phosphor dots are arranged in triad groupings on the screen or faceplate panel of the tube so that when these dots are impinged by an electron beam individual dots in each triad will emit red, green or blue light. A shadow mask screen having apertures precisely located with respect to each of the triad groups of phosphor dots is positioned behind the screen of the tube such that each of three electron beams will impinge only an associated phosphor dot in each triad group.

The enclosing envelope of such a cathode ray tube is generally formed of glass and consists of three main sections: the faceplate panel which forms the viewing area and retains the shadow mask assembly; a neck portion in which the electron gun assembly is mounted; and a funnel portion joining the faceplate panel with the neck portion and providing a drift space for the tube so that deflection of the beam by a given angle will produce an image of the desired size.

In order to deposit the phosphor dots on the faceplate panel and to secure the shadow mask so that it is properly aligned with respect to the dots, the faceplate panel is normally completely manufactured as a subassembly and then frit sealed, or glass soldered, to the funnel portion. Subsequently, the electron gun assembly is installed in the neck of the tube in precise alignment with the faceplate panel subassembly so that the angle at which each electron beam passes through the shadow mask will insure that each beam strikes only its associated phosphor dots. In this way, three slightly displaced images of primary colors are produced to appear as a single image of color corresponding to the original scene. To display the separate parts of the composite television image correctly, it is necessary that the shadow mask be accurately formed and positioned with respect to the accurately designed contour of the viewing area of the faceplate panel.

In such a shadow mask type color picture tube the means for retaining the mask within the tube, for example, metal stud members, must be exactly positioned to locate the mask, and the studs must be mechanically secured in 3,493,356 Patented Feb. 3, 1970 the glass panel without unduly weakening it. Generally prior art production of face-plate panels has been difcult and expensive due to the attempts to satisfactorily solve these problems.

Upon consideration of the above it is seen that there are stringent requirements on the dimensional accuracy and structural rigidity of the various component parts of a color television tube so that beam landing errors are minimized. The viewing area of the faceplate panel is required to be accurately contoured and matched to the design contour of the apertured shadow mask. The panel must also include exactly dimensioned side flanges for sealing to the funnel portion of the tube so that the alignment of the panel and the electron gun assembly can be accurately established.

In the past it has been common to use glass of special hardness and thickness (as distinguished from the standard commercially available glass used in the manufacture of black and white type cathode ray tubes) in order to maintain the dimensional tolerances during manufacture, and to insure necessary alignment of the various tube parts for correct color reproduction. The annealing temperatures of this special glass are far above the temperatures u'sed in sealing the faceplate panel to the funnel. For example, the range of annealing temperature used with a particular composition of hard glass ranges from 450 C. to 490 C. The frit sealing temperatures used range from 420 C. to 440 C. Thus the temperature of the sealing process is lower than the annealing temperature and the sealing operation may not produce strains in the faceplate panel which will distort the faceplate panel and produce misalignment between the phosphor dots and the shadow mask. However, when faceplate panels manufactured from the sof glass normally used in the manufacture of black and White type cathode ray tubes are sealed to the funnels, the sealing temperatures used are of the same order of magnitude as the annealing temperatures of the glass and strain patterns are introduced into the faceplate. For example, the range of annealing temperatures used for a soft glass suitable for use as a faceplate panel of a cathode ray tube is from 406 C. to 444 C. The presence of the funnel during the sealing process introduces strain patterns in the glass different from those which existed before, thus causing distortion and misalignment of the shadow mask with the phosphor dot pattern. For this reason prior attempts to use commercial soft glass normally used in the production of black and white cathode ray tubes were abandoned because of the distortions which take place during the sealing process between the faceplate panel and the funnel.

It is, therefore, an object of this invention to provide an improved color cathode ray tube of the shadow mask type With the tube having constructional features which may enjoy increased consumer acceptance while at the same time being capable of production through a process which results in significant manufacturing advantages.

Another object of this invention is to process faceplate panels of the commercial soft glass now used for black and white type cathode ray tubes so that such panels can be used in color cathode ray tubes.

Another object of this invention is to process a faceplate panel of the commercial soft glass now used for black and white type cathode ray tubes so that such panels can be sealed to cathode ray tube funnels without introducing distortion.

A feature of this invention is the provision of an annealing process in which the faceplate panel manufactured from soft glass and ready for phosphor coating is annealed with the same heat cycle used subsequently to seal the faceplate panel to the funnel while the faceplate panel edges are properly supported or weighted.

In the drawings:

FIG. 1 is a process chart descriptive of the processing steps in producing a faceplate panel;

FIG. 2 is a drawing of a cathode ray picture tube made according to the invention;

FIG. 3 is a perspective view of the mechanism used to anneal the faceplate panel; and

FIG. 4 is a View showing an alternate procedure in practicing the invention.

In practicing this invention la faceplate panel manufactured from soft glass normally used in the manufacture of black and white cathode lray tubes is provided. The faceplate panel is placed on a moldto properly dimension the viewing screen and side walls and studs are inserted in the side walls for properly positioning the shadow mask. The faceplate panel is then annealed and when cooled the edges of the faceplate panel are ground to the proper dimensions and cleaned by acid fortifying. The faceplate panel is then placed upon a funnel of the type to which it will be later sealed, or it is in some manner weighted as it will be during sealing. The faceplate panel so disposed is passed through a furnace which subjects it to the same temperature cycle which will be later used for frit sealing the faceplace panel to the funnel. If the panel is not placed on the funnel it may be annealed while in contact with a second faceplate panel, for example. This annealing process, with the faceplate panel in contact with a funnel or another faceplate panel, pre-establishes strain patterns in the panel which would otherwise undesirably develop during the later process of sealing the panel to the finished funnel. The faceplate panel so treated is then coated with phosphor to form the screen, the shadow mask is installed and the panel is sealed to a funnel with sealing frit.

FIG. 1 illustrates the process in which a faceplate panel of the type normally used for black and white tubes and manufactured from soft glass is processed for use in a color tube of the shadow mask type By way of example, although not limiting, commercial sof glass may include those lead-barium type glasses commercially available for use in forming the envelope of black and white cathode ray tubes. Typically they have annealing points in the ranges of 406 C. to 444 C., softening pointsin the order of 655 C. or less, and have a thermal coefficient of expansion in the range of 85-100X 10-7/ C.

Referring to FIG. 2 the tube 50 generally consistsof three main sections: a rectangular faceplate panel 52 having a shadow mask 54 positioned therein; a neck 56 which contains and positions the electron gun assembly; and a funnel section 58 joining the neck and the faceplate panels to constitute the drift space of the tube. The faceplate panel 52 is accurately contoured and dimensioned and has a precisely positioned shadow mask structure 54 mounted on studs -61 positioned on the side walls of the faceplate panel. The faceplate panel 52 has a rectangular viewing area 63 with tri-color light emitting phosphor dots disposed thereon and side anges 64 having their ground bottom edges 65 joined to the ground edges of the wide end of the funnel 58 by a glass-to-glass frit seal.

In the process depicted in FIG. l at step a faceplate panel 52 manufactured of soft glass of the type normally used for black and white tubes is provided. This panel may have relatively loose dimensional tolerances, for example, varying as much as 1% inch in the case of a 23 inch (diagonal) size panel. At step 11 the faceplate panel is placed on a mold and passed through an oven for recontouring In this step the surface 63 of panel 52 is accurately contoured and the side wall flanges 64 are accurately shaped for later precise matching with the funnel 58 of the finished tube.

At step 13 the metallic stud members 61 are effectively melted into the side wall flanges 64 of faceplate panel 52 with the panel on its mold so the studs are accurately positioned with respect to the face of the panel. At step 15 the contoured and studded faceplate panel, sufficiently cooled for handling without distortion, is removed from the mold and placed in an annealing oven where it is .reheated above the strain point and allowed to slowly cool down below the strain point of the glass that any stresses in faceplate panel 52 from the contouring or studding operations are removed.

After properly annealing the panel the edges 65 of the flange are ground in step 16 to an accurate dimension with respect to the studs 61. Subsequent to the grinding of the flanges, the panel 52 is acid fortified at step 18 with a solution of hydrofluoric and sulphuric acid to remove all sharp peaks and edges that might tend to promote a cracking or rupture of the glass when it is finally formed into a completed tube under vacuum.

The accurately dimensioned and contoured faceplate panel 52 can now have its inside surface 63 coated with the phosphor comprising the triad dot pattern as shown in step 24. The shadow mask S4 would also be installed permanently during this step. In step 28 the neck 56 and funnnel 58 from step 26 are positioned in frit xture 75 of FIG. 3. A bead of glass solder or frit is applied to the edges 76 of the wide portion of the funnel 58 and faceplate panel 52 is positioned thereon. Faceplate panel 52 is oriented with respect to the vertical axis of frit fixture by three indexing pins 78. Frit fixture 7S is then conveyed through an oven or lehr 80 and gradually brought to a temperature of approximately 425 C., maintained at that temperature for a period of an hour or more and then gradually cooled to ambient, During this process the frit is vitrified and fuses the faceplate panel 52 to the funnel 58 to provide a vacuum type glass-to-glass seal.

During the sealing process the temperature used to solder the faceplate panel 52 to the funnel 58 is susbtantially the same as the annealing temperature of the faceplate panel. Therefore, faceplate panel 52 is again annealed and because of the close contact between the fiat edges of the faceplate side walls and the fiat edge of funnel 58, strains are set up in the faceplate panel which cause distortion of the panel and the shadow mask structure 54, This distortion is sufficient to cause misalignment of the shadow mask and the phosphor dot triads so that the tube would not be satisfactory in a color television set. To prevent this distortion which may be developed in the sealing process, steps 20' and 22 of FIG. l have been incorporated in the tube manufacturing process.

After the faceplate panel 52 has been acid fortified in step 18 the faceplate panel 52 is again annealed. In step 20 a funnel 58 of the same type as will be used in the completed tube is provided and positioned on a fixture 75 which may be the same as, or similar to, the fixture used in the sealing process. Faceplate panel 52 is placed on funnel 58 in the same relative position that it will occupy in the finished tube with the ground edges of panel and funnel meeting and the weight of the panel (eg. 15 pounds) pressing the edges together. In step 22 the faceplate panel and funnel are passed through the same temperature and time cycle used for frit sealing in step 30, eg. slow heating to 425 C. for an hour or more. Annealing the faceplate panel in the presence of the funnel 58 produces, or pre-establishes, a strain pattern in faceplate panel 52 which will be substantially the same as the final strain pattern developed during the frit sealing step. Thus, there will be no change in strain pattern during the sealing step and distortion of the faceplate panel 52 and the shadow mask 54 after the phosphor screen is laid will not occur. The annealed faceplate panel from step 22 is then processed through steps 24, 28, 30 as previously described and manufactured into a complete tube.

In another embodiment of this process two faceplate panels are placed with the hat edges of their side walls in contact with each other and passed through the oven at a temperature used for frit sealing. This operation is shown in FIG. 4. A faceplate panel 52a is positioned on a three-point mounting structure '74. A second faceplate panel 52h is positioned on top of panel 52a with the flat edges of the faceplate panels pressed into contact by the weight of the upper panel. This assembly is passed through a frit sealing oven, such as oven 80, to anneal the faceplate panels at at least the frit sealing temperature.

It should be noted that in the disclosed heat treating process which corresponds to a subsequent frit sealing time and temperature, the operation should preferably be carried out with the edges of the faceplate panel constrained as described. Prior attempts to anneal the panel in some way without so constraining the edges thereof have been found to be unsatisfactory. During the frit sealing of the glass faceplate panel to the funnel there is a heating of the glass from the outside of the structure as it passes through the oven and a nonuniformity of heat distribution across the tube due to the size of the assembly (for example, the diagonal of the faceplate may be 23 inches or more) and due to the presence of air which is essentially trapped within the assembly. Of course there would be a similar nonuniformity in the cooling as the frit sealed funnel and panel passed out of the sealing oven. It is thought that this unevenness in heating and cooling, particularly in the case of soft black and white glass, causes distortion of the faceplate and a change in the relative positions of the mask and faceplate. For good color purity in the picture tube it is necessary for the shadow mask to be in precisely the same position relative to the faceplate as it was during the prcess of exposing the phosphor dots of the faceplate. By utilizing the herein described process it is possible to overcome such distortion and maintain necessarily close dimensional tolerances.

What is claimed is:

1. A process for manufacturing a faceplate panel which is frit sealed to a funnel portion to complete the envelope of a color cathode ray tube, and on which are deposited a plurality of phosphor dot triads which are accurately aligned with the apertures in a shadow mask assembly with the same being mounted within the faceplate panel, the process including the steps of providing a faceplate panel having an accurately contoured and shaped surface and side wall flanges, melting stud members for supporting the shadow mask assembly into the side walls accurately positioned with respect to the face panel, grinding the edges of the side wall anges to an accurate dimension with respect to the studs, and heat treating the faceplate panel by heating the same to the annealing range temperature under conditions providing similar heating, cooling and temperature differentials in the panel that exist when the panel is frit sealed to the funnel thereby producing a desired stress pattern in the same.

2. The process of claim 1 further including engaging the side wall flanges of the faceplate panel with a cover member to trap air therewithin, and wherein the heat treating step is accomplished by heating the panel to the annealing range temperature with the cover member in place.

3. The process of claim 2 wherein the cover member is a second faceplate panel having the side wall flanges thereof pressed into contact with the sidewall flanges of the rst faceplate panel.

4. The process of claim 2 wherein the cover member is a tube funnel, and the faceplate panel engages the upper are of the funnel with the corresponding edges of each member pressed into contact.

5. A process for manufacturing cathode ray tubes, including the steps of, providing a faceplate panel dimensioned for frit sealing to a cathode ray tube funnel, engaging the edges of the faceplate panel with a cover member to trap air therewith, producing a stress pattern in the faceplate panel by heating the same to the annealing range temperature with the cover member in place thereby providing similar heating, cooling and temperature differentials in the panel that exist when the panel is frit sealed to the funnel, mounting and aligning a shadow mask and phosphor dots in said faceplate panel, and frit sealing said faceplate panel to the funnel by heating the joined faceplate and funnel to the same annealing range temperature.

6. The process of claim 5 wherein the faceplate panel is composed of commercial soft glass and has a face area bonded by depending side walls, and wherein said cover member is a funnel, and the process further inclodes the steps of supporting the panel on the upper flare of the funnel with the corresponding edges of each member pressed into contact, and passing the funnel and panel so supported through an oven to heat the same to the annealing range temperature thereby producing a stress pattern in the faceplate panel.

7. The process of claim 5 wherein the faceplate panel is composed of commercial soft glass and has a face area bonded by depending side walls, and wherein said cover member is a second faceplate panel, and the process further includes the steps of supporting the rst panel on the second Ipanel with the corresponding edges of each member pressed into contact, and passing the panels so supported through an oven to heat the same to the annealing range temperature thereby producing a stress pattern in the faceplate panel.

References Cited UNITED STATES PATENTS 2,642,633 6/1953 Dalton 161--45 X 2,889,952 6/1959 Claypool 65-43 X 2,894,359 7/1959 Pawlicki 65-43 X 2,936,923 5/1960 Veres 65-43 X 3,002,645 10/1961 Kegg 65-43 X S. LEON BASHORE, Primary Examiner ROBERT LINDSAY, J R., Assistant Examiner U.S. Cl. X.R. 29-25.l3; 65-117

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