Method of coating the inner surface of a display window of a television display tube to form a display screen

A method for coating with a liquid the window panel of a display tube by means of at least one brush mounted for rotation in a reservoir containing the liquid. As the partially submerged brush is rotated about its axis, drops of liquid picked up by the brush hairs are flung against the window panel. This method results in continuous mixing of the coating material and the return of the excess coating liquid back into the reservoir so that losses of the coating material are minimized.

The invention relates to a method of coating the inner surface of a display 
window of a television display tube to form a display screen, in which the 
material necessary for the coating is provided in a liquid form on the 
display window. 
Various methods are known for the manufacture of a display screen of a 
colour television display tube. 
In one such method, phosphor particles suspended in a liquid are caused to 
flow from above into the display window mounted at an angle on a 
correspondingly constructed support device. As the liquid flows over the 
display window, the window is rotated about its axis and shortly before 
the end of the flow, the speed of rotation is increased producing a 
coating which is sufficiently uniform on the inner surface of the display 
window. Excess liquid and suspension with sediments is received at the 
corners of the display window by special receivers and from there returned 
to receiving reservoirs. The very expensive phosphors are recovered from 
this liquid, while the carrier emulsions which cannot be reused are 
drained. 
In addition to wet-chemical coating methods, electrophotographic methods 
are also known in which the inner surface of the display window is coated 
with a photoconductive layer. It is also known, for example from French 
Pat. Nos. 1,588,531 and 1,602,204, to temporarily spray, during the 
coating process, the inner surface of the display window from below 
through a nozzle which is suitably moved towards the screen. 
In the first-mentioned method, the flow rate and the speed of rotation of 
the display window must be very accurately matched to each other in order 
to obtain uniform layer thicknesses, because the phosphors suspended in 
the liquid start to settle as soon as the liquid lands on the inner 
surface of the display window, so that different layer thicknesses tend to 
be obtained which, give rise to different light intensities at various 
places on the display screen. 
One drawback of the coating methods disclosed in the above cited French 
Pat. No. is that the spraying has to be interrupted when the display 
screen is shifted. Since the coating agents, for example, when the 
electrophotographic method is used, the developer suspension, dry readily, 
the nozzles very rapidly clogged during the above-mentioned shifting time. 
It is the object of the invention to avoid these important drawbacks and to 
provide a method which can produce a sufficiently uniform coating on the 
inner surface of the display windows, and in which, also, excess liquid is 
returned simultaneously and directly to the useful liquid and is readily 
mixed again with the starting liquid. 
U.S. Pat. No. 1,520,446 relates to a laundry sprayer and discloses a 
rotatable brush co-operable with an interned edge of a housing. The 
relevant hairs of the brush which are directed downwards are dipped in the 
liquid and when encountering the interned edge the brush hairs are first 
restrained and then allowed to spring forward so as to spray the laundry. 
In a method of the kind mentioned in the first paragraph, the object 
according to the invention is achieved in that, by means of a brush which 
is provided in a reservoir containing the liquid and being covered on the 
upper side by the display window to be coated, and which brush is provided 
on a shaft, so as to be rotatable and adjustable in such manner that its 
relevant downwardly projecting brush hairs are dipped in the liquid, drops 
of liquid are taken along by the brush hairs by rotating the brush and are 
flung away. 
In a preferred embodiment of the invention an edge member is provided on 
one side of the reservoir approximately at the level of the upper side of 
the shaft of the brush, so that brush hairs strike the edge member and are 
flicked past it and drops are flung against the display window. 
Alternatively, two brushes driven in opposite directions may be arranged 
in the reservoir. Of the two brushes, the one which is driven more slowly 
has its hairs dipping in the liquid and flings liquid onto the other brush 
which is driven at a higher speed and which flings drops of liquid against 
the display window. The brushes may be constructed as rollers. 
When using the invention the depth to which the brush hairs are immersed in 
the liquid can be adjusted by adjusting the height of the shaft. When, 
however, only one brush and one brush roller, respectively, is used, it is 
found that the liquid at the required number of revolutions is taken along 
in the direction of rotation of the brush and is consequently thrown up 
periodically more or less at the edge of the window panel. An improvement 
in this respect is obtained by providing an edge member co-operating with 
the brush. This ensures that the brush hairs are obstructed and then leave 
the rake at a higher speed so that the drops are flung away at a much 
higher speed. As a result, the size of the drops is also reduced so that a 
better or finer coating is obtained. A further improvement is obtained, 
finally, when two brushes driven in opposite directions are provided in 
the reservoir and only the brush driven at lower speed is dipped in the 
liquid. This latter brush then transports the drops to the brush driven at 
higher speed and the latter throws and flings, respectively, the drops 
towards the inner surface of the display screen. 
The resulting layers are of improved uniformity. The excess liquid may 
drain and flow back into the reservoir itself. Since the hairs of the 
brush are always dipped in and take along fresh drops from the liquid, 
good mixing of the liquid is obtained obviating the need for the 
recovering devices required in the prior art. By means of a simple device 
it can be achieved that the level of the liquid in the reservoir is always 
at the same level. 
The liquid which is processed in the reservoir depends on the method. Thus 
it is possible to fling developer suspensions which are necessary for 
electrophotographic methods, as well as liquids in which phosphor 
suspensions, also for electrophotographic methods, are present. However, 
liquids may also be used which are used for the so-called conventional 
wet-chemical method. The speeds of rotation of the brush or brushes is 
adjusted in accordance with the viscosity of the liquid and/or the 
sedimentation in the liquid.

FIG. 1 shows a display window 1 of a colour television display tube whose 
inner surface 2 is to be coated. For that purpose, the display window 1 is 
laid on the open top of a reservoir 3 of liquid 4 of which only the inner 
wall is shown diagrammatically. A rotary brush 6 having radially extending 
hairs 7 is mounted on a shaft 5 not shown. When the brush roller 6 is 
rotated in the direction of the arrow 8, the liquid 4, as indicated at 9, 
is driven upwardly along the wall of the reservoir and finally drops again 
as a result of gravity, so that at that area the liquid undulates. At the 
same time, individual drops 10, which are withdrawn from the liquid, are 
thrown against the inner surface 2 of the display window 1 and the excess 
liquid flows outwardly and downwardly over the inner surface. 
FIG. 2 shows an improvement in which an edge member 11 is added on one side 
of the wall of the reservoir 3. The brush hairs strike the edge member and 
flick past it, so that the drops 10 reach a higher speed and also reach 
the inner surface 2 of the display window in a more finely divided manner. 
FIG. 3 shows a further improvement with respect to the uniform distribution 
of the drops over the whole inner surface 2 of the display window 1. In 
this embodiment two cylindrical rotary brushes are provided in the 
reservoir 3 in a manner such that one brush 12 rotate to the right, as 
denoted by the arrow 13, and the other brush 14 rotates to the left, as 
denoted by the arrow 15. When the brush 12 rotates, it takes along drops 
from the liquid 4 and, as denoted at 16, throws them on the other brush 
14. Brush 14 rotates at a considerably higher speed and then throws the 
drops 10 against the inner surface 2 of the display window 1. The catch 
brush 12 rotates at approximately 60 to 250 rpm and the other roller 14 
rotates at 1000 rpm. 
In all cases the excess liquid is immediately recovered and can be used 
again directly. This is also possible in the embodiment of FIG. 3 if, for 
example, the reservoir for the liquid 4 is made slightly deeper than that 
below the brush roller 14.