EP0014360B1 - Screen application method for colour picture tubes - Google Patents

Description

The invention is based on a shielding method for color picture tubes according to the preamble of patent claim 1.

The problems of such shielding methods are described in detail in the article by W. Möller, "Influence of shielding technology on the luminance of color picture tubes", in Techn. Mitt. AEG-Telefunken 64 (1974).

Particular attention should be paid to the shielding temperatures. This is especially for controlling the weight of the screen, as mentioned on page 4, column 2 of the article cited. The weight of the screen significantly influences the brightness of the image and other optically recognizable sizes.

The temperature of the glass tubs to be screened is typically approximately 313 K, while the screen suspension is applied at a considerably lower temperature, namely approximately 293 K. If the excess, heated shield suspension is used in a cyclic process, e.g. re-used as described in DE-PS 1614372, the return or the shielding suspension must of course be cooled to a predetermined temperature of at most 293 K. This fact is also described in DE-AS 25 13 907.

This necessary cooling process is completely sufficient to be able to maintain a specified weight of the screen permanently. In the case of problems caused by new techniques, which are described in the following, the previously known shielding method only permits production with high rejects.

In the case of the color picture tubes produced hitherto, the width of the slits in the center of the shadow mask was approximately 160 μm and the width of the phosphor strips obtained on the screen by exposure through the shadow mask was approximately 250 μm. In recent times, the trend has been towards picture tubes with greater image brightness, which is achieved by increasing the transparency of the shadow mask. For this purpose, the slit width in the shadow mask is increased to approximately 190 μm. Since the width of the phosphor strips on the screen is still approximately 250 pm, either the exposure time must be shortened or the amount of the photosensitizer reduced.

The reduction in the addition of photosensitizer leads to non-smooth-edged boundary lines of the phosphor strips. Experience has shown that the shorter exposure time results in fluorescent strips that adhere poorly to the glass. It was found that the exposure time and thus the adhesiveness can be increased if the shielding suspension is cooled too much. However, since the freezing point of the suspension is a few degrees below the freezing point of the water, cooling is only possible to a limited extent. However, the possible lower cooling temperature and the associated increase in exposure time were not sufficient to obtain firmly adhering strips when exposed through the wide mask slits with high reproducibility. With this method, too, the committee for shielding is still too high.

The invention is therefore based on the object of specifying a method which allows firmly adhering fluorescent strips of a predetermined width, e.g. 250 µm, even when exposed through very wide mask slits, e.g. Get 190um and more.

The solution to the problem can be found in the main claim. The temperature of the starting components is preferably 278 K, but at least 288 K. The lower possible temperature is given by the freezing points of the suspensions.

With the method according to the invention, it is readily possible to obtain fluorescent strips adhering to the wide mask slits described practically without rejects in production. The fluorescent strips show the further advantage of a very sharply cut edge.

The invention will now be explained in more detail using an exemplary embodiment.

A suspension of binder, e.g. PVA, fluorescent, water and suspension auxiliaries manufactured. Now, viscosity and density are not simply set, and with the sensitizer, e.g. ADC, activated, e.g. in the method described in DE-PS 16 14 372. Rather, the invention is based on the fact that binder and photosensitizer may only be mixed in the cooled state. The suspension produced in the ball mill is cooled to approx. 278 K. The solutions or suspensions with which the viscosity or density are adjusted and the ADC activation solution, all cooled to 278 K, are then added. The shielding suspension is obtained from the supplementary suspension prepared in this way by dilution.

All suspensions used or occurring in a shielding process, e.g. those described in DE-PS 1614372 are continuously cooled to approx. 278 K.

The cooling temperature can vary between the freezing point of the individual suspensions and approx. 288 K. It is essential to the invention that the cooling takes place before the suspensions containing the binder and the suspensions containing the photosensitizer are mixed, and that the cooling is then maintained continuously.

Claims (1)

1. Method for screening of glass shells for colour picture tubes in which a screening suspension of particles of luminescent material in an aqueous solution of binding agent, photosensitiser and suspension adjuvants is poured onto the glass shell, in which then the surplus screening suspension is centrifuged off and the reflux together with a replenishing suspension containing the same constituents and restoring the reflux to the concentration of the screening suspension stock is returned to the stock, measures being taken in the suspension circulation always to pour on the screening suspension at a temperature not exceeding 293 K, caracterised in that in the production of the screening suspension and the replenishing suspension all the constituents of the said suspensions are precooled before being mixed with the photosensitiser solution to preferably 278 K, but at least 288 K.