The present invention relates to a method for manufacturing a glass funnel for a cathode ray tube, the funnel comprising a cone portion and a substantially cylindrical neck portion.

Such a method is well-known in practice. In the known method, the cone portion and the neck portion are separately formed from separate pieces of glass. The cone portion is produced by means of pressing, whereas the neck portion is usually produced by means of blowing. A back end of the cone portion and a front end of the neck portion are joined by means of melting.

A front end of the cone portion is suitable to be joined to a glass panel having a rectangular shape and larger sectional dimensions than the neck portion. It will be clear that the back end of the cone portion has a circular sectional shape and is relatively small in size, whereas the front end of the cone portion has a rectangular sectional shape and is relatively large in size. Apart from the fact that the sectional dimensions of the cone portion decrease from front to back, the wall thickness of the cone portion also decreases from front to back, as the wall thickness of the panel is larger than the wall thickness of the neck portion.

As there is a tendency towards manufacturing shallower cathode ray tubes having larger panels, there is a need to manufacture wide and at the same time shallow cone portions having a larger overall variation in sectional dimensions. In order to be able to support the increasing stresses which result from the required shape of the cone portion, the wall thickness of the cone portion must be larger. The pressing forces which are required during a pressing process of a cone portion having a larger wall thickness are dramatically higher, since there is a progressive exponential relation between wall thickness and pressing forces.

Manufacturing larger and shallower cone portions can be performed by utilizing presses which are upgraded towards higher and higher pressing force capabilities.

However, the higher the pressing forces, the longer the processing time. It is therefore an object of the present invention to provide a method for manufacturing a larger funnel for a

cathode ray tube, wherein known production methods and devices can be applied, but wherein upgrading of the presses utilized is not necessary.

This object is achieved by a method wherein the cone portion of the funnel is not manufactured in one piece. Instead, the cone portion is manufactured by separately forming a front section and a back section from separate pieces of glass, and subsequently joining the front section and the back section together.

In the method according to the invention, the cone portion is manufactured in parts, which are joined in a suitable way, for example by means of fritting. This method of constructing the funnel has many advantages over the known method, a number of which shall be mentioned below.

A first advantage of the method according to the invention is related to the pressing process of the front section and the back section. Compared to the pressing of an undivided cone portion, separately pressing the front section and the back section requires less force.

A second advantage is related to the pressing technology for pressing of the front section. According to the state of the art, two types of pressing technology are needed to produce the cathode ray tube. The pressing of the panel requires panel pressing technology, whereas the pressing of the cone portion requires cone pressing technology. Both technologies have their own specific features, the cone pressing technology being more complex than the panel pressing technology. In producing a cathode ray tube having a funnel according to the invention, only the panel pressing technology needs to be applied. The front section can be produced as a panel comprising a substantially rectangular window provided with an upright edge a hole being made in the window subsequent to pressing.

A third advantage is that the accuracy of both the front section and the back section can be improved separately. On the one hand, it is possible to focus on the overall accuracy of the back section, which needs to be high, since it directly influences the performance of the cathode ray tube. On the other hand, it is possible to focus on the accuracy of the periphery at a front end of the front section, which directly influences the quality of the joint between the front section and the panel.

A fourth advantage is that the preparation of moulds for the pressing of the front section and the back section will take less time than the preparation of one large mould for a solid cone portion if the moulds for the pressing of the front section and the back section

are prepared in parallel. Also, repair or modification of the front section and/or the back section will take less time than repair or modification of a solid cone portion.

A fifth advantage becomes apparent in the situation of packaging for storing and transportation of the elements of the funnel. The volume required by a solid cone portion is larger than the total volume required by a front section and a back section together.

A sixth advantage is that by applying the method according to the invention, it is possible to utilize the back section as a standard product. The reason for this is that the cone portion can be adapted to panels having different sizes by only adapting the front section.

In a preferred way of carrying out the method according to the invention, the neck portion and the back section are formed as an integral part from one piece of glass. It is very important that the neck portion and the back section are aligned. By producing an integral part which comprises both the neck portion and the back section, the required alignment can be performed with more accuracy than by producing the neck portion and the back section as separate parts which need to be joined.

In another possible way of carrying out the method according to the invention, the front section and the back section are joined together through at least one intermediate part.

The present invention will now be explained in greater detail with reference to the Figures, in which similar parts are indicated by means of the same reference signs, and in which: Fig. 1 is a sectional view of a cathode ray tube having a funnel, which is manufactured in accordance with a first preferred way of carrying out the method according to the invention; Fig. 2 is a perspective back view of the cathode ray tube as shown in figure 1; Fig. 3 is a diagrammatical perspective front view of a quarter part of the funnel which is part of the cathode ray tube as shown in figure 1; Fig. 4 is a diagrammatical sectional view of a half of parts of the funnel as shown in figure 3; Fig. 5 is a diagrammatical sectional view of a half of parts of a funnel, which is manufactured in accordance with a second preferred way of carrying out the method according to the invention; and

Fig. 6 is a diagrammatical sectional view of a half of parts of a funnel, which is manufactured in accordance with a third possible way of carrying out the method according to the invention.

Figures 1 and 2 show a cathode ray tube 1 having a glass envelope 2, which comprises a substantially rectangular panel 3, a neck portion 4, and a cone portion 5 between said panel and said neck portion. The neck portion 4 is substantially cylindrical, a neck wall 6 being substantially parallel to a longitudinal axis 7 of the cathode ray tube 1.

The combination of neck portion 4 and cone portion 5 is referred to as funnel 8. A diagrammatical perspective front view of a quarter part of the funnel 8 is shown in figure 3.

The panel 3 comprises a substantially rectangular window 9 provided with an upright edge 10. In the case of a colour cathode ray tube, the inner surface of the window 9 is provided with a display screen 11 comprising a large number of red, green and blue luminescent phosphor elements. The neck portion 4 accommodates an electrode system 12 having three electron guns for generating electron beams. Deflection coils 13 for deflecting the electron beams on their way to the display screen 11 are coaxially arranged about the longitudinal axis 7 of the cathode ray tube 1.

The cone portion 5 comprises a front section 15 and a back section 16. A back end 17 of the back section 16 is joined to a front end 18 of the neck portion 4 (Figure 4), for example by means of melting. The periphery of the back end 17 of the back section 16 is adapted to the periphery of the front end 18 of the neck portion 4 by being substantially circular. A front end 19 of the front section 15 is joined to the upright edge 10 of the panel 3, for example by means of fritting. The periphery of the front end 19 of the front section 15 is adapted to the periphery of the upright edge 10 by being substantially rectangular.

According to the state of the art, the cone portion 5 is formed in one piece from one piece of glass, by means of pressing. The neck portion 4 is formed from another piece of glass, usually by means of blowing. The funnel 8 is assembled by joining the neck portion 4 and the cone portion 5 together.

According to the present invention, the front section 15 and the back section 16 of the cone portion 5 are formed as separate parts from separate pieces of glass, preferably by two pressing operations. In a subsequent assembling operation, the front section 15 and the back section 16 are joined together.

Preferably, the front section 15 is manufactured in two steps, wherein the first step comprises the process of forming a panel-like product from a piece of glass, said product comprising a substantially rectangular window provided with an upright edge, and the second step comprises the process of making a hole in the window.

Figure 4 diagrammatically shows a half of the parts of the funnel 8, being the front section 15, the back section 16 and the neck portion 4. It can be assumed that the other half of the parts is identical to the half shown.

In the shown example, an inner surface 20 of the cone portion 5 has a convex- concave shape. An interface 21 of the front section 15 and the back section 16 is located at a transition point between the convex-shaped part of the inner surface 20 and the concave- shaped part of the inner surface 20. Consequently, an inner surface 22 of the back section 16 is fully convex, and an inner surface 23 of the front section 15 is fully concave. In this context, convex and concave are defined as seen from the outside of the funnel 8.

Within the scope of the present invention, the inner surface 20 of the cone portion 5 can have a shape other than convex-concave, for example completely concave.

Moreover, the interface 21 can be located anywhere along the cone portion 5.

In an advantageous embodiment, the back section 16 and the neck portion 4 are formed as an integral part 24. Figure 5 diagrammatically shows a half of the integral part 24 and the front section 15. It can be assumed that the other half of the integral part 24 and the front section 15 is identical to the half shown.

In another advantageous embodiment, the cone portion 5 further comprises an intermediate part 25, which is located between the front section 15 and the back section 16.

Figure 6 diagrammatically shows a half of such an intermediate part 25 and also shows its position with respect to the neck portion 4, the back section 16 and the front section 15.

Within the scope of the present invention, the cone portion 5 can comprise more than one intermediate part, which are all located between the front section 15 and the back section 16.

In the example shown in Figure 6, the neck portion 4 and the back section 16 are separate parts. Nevertheless, it is also possible to have a funnel 8 comprising an integral part 24 as shown in Figure 5, a front section 15 and one or more intermediate parts.

Summarizing, according to the state of the art, the funnel 8 has exactly one joint which is located between the neck portion 4 and the cone portion 5, whereas according to the present invention, the funnel 8 has at least one joint in the cone portion 5.

According to the present invention, the components of a glass envelope 2 of a cathode ray tube 1, in particular a cone portion 5, are virtually broken up into pieces which are joined together at a later stage. In fact, the idea could be described as the cutting and pasting of components.

It will be clear to a person skilled in the art that the scope of the present invention is not limited to the examples discussed in the foregoing, but that several amendments and modifications thereof are possible without deviating from the scope of the invention as defined in the appended claims.

For example, the neck portion 4 does not necessarily have to be formed in one piece, but may alternatively comprise two or more separate parts that one joined together.