Patent Application
20050084781
The present invention relates to a method and an apparatus for manufacturing the phosphor screen of a display tube, comprising the steps of:
First a phosphor slurry applying step is carried out in such manner that, while rotating the panel at a proper speed, a proper amount of the phosphor slurry is injected into the inner face of the panel so as for the phosphor slurry to be spread on the whole inner face of the panel, after which the phosphor layer is subjected to a drying stage. Often the drying stage is preceded or interrupted by an excess phosphor slurry recovering step which is carried out in such a manner that the panel is made to spin at a high speed so as for the excess slurry to be recovered.
Thereafter, an exposing step is carried out in such a manner that the phosphor dots of the required pattern are formed on the inner face of the panel through the use of an exposing apparatus and, upon completion of the exposure, the phosphor layer is washed and developed.
The above-described processes are repeated three times in the case that three R, G, B phosphors have to be separately applied.
In such a conventional phosphor screen manufacturing process, a flowcoat mill is used for applying the phosphors. In a current flowcoat process, ⅔ of the flowcoat mill is used to dry the picture area and the panel corners. Especially for larger panel sizes and/or for larger length-to-width ratios, drying of the corners is a bottleneck. For introduction of color filters, the size of the equipment is too large because of the long drying step.
In the above-mentioned process the flowcoat suspension is dosed and the slightly tilted panel is rotated until the layer is settled homogeneously. After spinning to get an even phosphor layer, the screen is rotated to a more or less vertical position to the drying positions. The centrifugal forces, and the inflow from the picture area, force the suspension into the corners. Drying by IR heaters of the picture area is finished fairly quickly, but especially the corners take a very long time to get dry. Moreover, in case of breakdown, the extra longer heat input to the panels causes extra rejects.
In the current process, the heat necessary for the evaporation of the solvent (usually water) is coming from the IR-heaters. The positions of the IR-heaters are fixed.
The panel rotates in a plane in front of the IR-heaters, so that the evaporation solvent is removed by the air movement resulting from the panel rotation.
Therefore, it is an object of the present invention to provide a manufacturing method for the phosphor screen of a color cathode ray tube, which allows faster drying of the corners, preferably in a time which is comparable to the drying time of the picture area of the phosphor screen.
In achieving the above object, the manufacturing method for the phosphor screen of a color cathode ray tube according to the present invention is characterized in that during the drying step heated air is forced to flow toward, and through, each inner corner of the panel by means of an air conveying means which is arranged so as to be stationary relative to the panel.
More specifically, the air conveying means (ACM) is arranged to be co-rotatable with the panel.
According to the present invention, the removal of evaporating liquids is forced by a directed flow of air, and the special characteristic is that the flow is forced towards the corners, so that the corners are dried much faster. The air can advantageously be conditioned on temperature (higher temperature gives faster evaporation) and on relative humidity (lowering the relative humidity increases evaporation).
According to a first embodiment the air conveying means comprises a cover plate which is capable of covering at least the central portion of the open side of the panel.
The cover plate can be designed such that it can bear on the skirt of the panel so that it substantially “closes” the panel.
In a practical embodiment the cover plate has a central air inlet opening for enabling (heated) air to be blown in, and air outlet openings arranged adjacent each inner corner of the panel.
To enable controlled dosing of the air flow, the cover plate is preferably provided with a perforated dosing plate which in operation is located between the cover plate and the phosphor layer.
By arranging the perforated dosing plate at a small distance above the phosphor layer the blown-in air can flow in a controlled manner in the space between the perforated plate and the phosphor layer.
In a further practical embodiment the air conveying means is put into position while the panel is in a substantial horizontal position.
After the ACM has been put into position in the flowcoat equipment, blowing air in and the resulting evaporation of the solvent heed not to start immediately.
Therefore, it is possible to carry out spinning and drying as two separate, sequential steps. Certain artifacts such as the so-called north/south stripes and the 60° cross appear to be reduced by the decoupling of spinning and drying, which is not possible when using the current flowcoating equipment.
Using the above features, the process steps can be as follows:
The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which:
First (see
The phosphors are suspended in water, to which the light-sensitive PVA-ADC system is added. The suspension, first with green phosphor, later with blue phosphor and finally with red phosphor, is poured into the slowly rotating panel and spread evenly.
A next step of the manufacturing process is carried out in such a manner that, with the inner face of the panel directed upwardly in a slightly inclined position, the panel 1, as shown in
In a drying step the water of the suspension is made to evaporate; this produces a level, dry coating. The above process is known by the name flow coating.
The great heat of evaporation of water is a problem in the application of an even coating of water-based suspension. And the temperature is also of great importance for the photosensitive system. During drying the photosensitive coating may not exceed a temperature of 45° C. because of the occurrence of the so-called ‘dark reaction’.
During the drying of the coating the rate of evaporation of the water must as far as possible be the same all over. The rate of evaporation should not be too high because otherwise the thickening PVA solution cannot flow sufficiently into the porous phosphor coating. The result would be that the residual PVA remains tacky and be the cause of color contamination during the application of subsequent coatings. A compromise has been adopted. The suspension coating is evenly spread by rotating the screen. The applied coating is then dried by controlled heating.
A shadow mask is then placed in the screen which is exposed to the right quantity of light to obtain the required line width (or dot size) and developed.
Developing involves spraying water at the screen, the spraying pressure, the spraying time and the water temperature being the usual variables. After drying, the screen is ready for the application of the next color.
One of the possible solutions in the framework of the invention for faster drying of the wet corners during flow coating is the use of a co-rotating air conveying means or air box (CRAB).
Panel 1, which has a peripheral skirt 6, is supported by a table 4. Panel 1 has a phosphor layer 20 (
As can be seen in
A ventilator 14 (
The ventilator 14 is controlled by a (frequency) controller. The air velocity flowing through the air conveying means can thus be varied.
A hot air blower 18 is positioned adjacent the ventilator inlet 16 to control the temperature of the air. The air temperature can be measured at the outlet side of ventilator 14 by means of a thermocouple 17.
Several experiments were done using the CRAB device. After suspension dosing (step 1), the screen rotation was stopped to insert the CRAB (step 2). The ventilator and hot air blower are then working; but the air supply is not connected to the CRAB. After arrangement (step 2) the screen rotation resumed. During the subsequent settling phase (step 3) the suspension is distributed over the screen and the phosphor settles. At the end of the settling phase the screen rotational speed is increased to spin off the excess of suspension (step 4). After the spin-off phase the air supply is connected and the drying phase starts (step 5).
In the drying experiments with the CRAB, the air velocity during drying was varied to levels of 0.12, 0.25 and 0.42 m/s.
Satisfactory results were obtained. The drying time was reduced by about 50 to 75% as compared to infra-red drying. However, when drying panels having mask suspension pins arranged in the corners, in some cases suspension drops from the mask pins flow back into the picture area. For increasing air velocities, this effect decreases.
The problem with faster drying of the wet corners is how to prevent the formation of a droplet or a thick suspension layer below the mask pin. Possible solutions to solve this problem could be the following:
Separation of Drying and Spinning
In the current flowcoat program drying and spinning occur at the same time during approximately the first minute of the total drying time. This means that there is still new suspension flowing through the screen corners during drying. By separating spinning and drying, drying is started when the required wet layer thickness has been reached, so no additional flow of suspension occurs during the drying process. If the spinning and drying process can be separated effectively, also all kinds of defects due to combined spinning and drying will disappear.
After spinning off the excess of suspension (step 4), the rotational speed is reduced to, for example, 30 rpm during the drying phase (step 5).
A further improvement can be obtained as follows:
Still making use of separation of dying and spinning, the layer is first dried at a low rotation speed of 30 rpm until the picture area is almost dry and then the rotation is increased to 200 rpm with a high air velocity for the drying of edge and corners. The idea is that the small thick edge is smeared out to a large thin layer due to the high rotation and that drying should then be easier.
Another possibility for solving the wet corner problem is the use of an additional air jet in the corner of the screen beneath the mask pin. This air-jet leads to faster drying rates in the corner and/or the droplet is blown away from beneath the pin.
The application of an air jet directed towards the corner is shown in
In
In
Summarizing, the invention relates to a method for drying the inside of a CRT-panel after flow coating of, for example, the phosphors, using convection drying with air. The method of the invention is characterized in that the airflow is forced to the corners of the CRT panel, so that the corners are dried much faster. The crux of the invention is the use of a so-called co-rotating barrier plate which covers the CRT-panel and forces the airflow into the corners of the panel.
Furthermore, the air used for the drying process can be conditioned on temperature (higher temperature yields a faster evaporation) and on relative humidity (lowering the relative humidity increases the evaporation).
Presently, the drying of CRT panels is carried out using IR heating elements. In this process especially the corners of the CRT panel take a very long time to dry.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0120493.6 | Dec 2001 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB02/05106 | 12/2/2002 | WO |
