Electric lamp and discharge devices – Cathode ray tube – Shadow mask – support or shield
Reexamination Certificate
2003-10-10
2004-10-19
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Shadow mask, support or shield
C313S405000
Reexamination Certificate
active
06806631
ABSTRACT:
TECHNICAL FIELD
This invention belongs to the technical field of color CRT's (color cathode ray tubes) (also referred to as color picture tubes) used in television receivers, displays, and the like. More specifically, this invention relates to a mask frame, a steel sheet used in manufacturing the mask frame and a manufacturing method therefor. The mask frame is a member which supports under tension a tension-type shadow mask which is disposed within a color CRT (in this specification referred to as a color CRT mask frame).
BACKGROUND ART
A color CRT has in its interior three electron guns for red, blue, and green and a fluorescent screen (screen) impacted by electron beams discharged therefrom. The surface of the fluorescent screen has fluorescent dots formed thereon which generate the above-mentioned three colors and which are arranged in a regular sequence.
In the type of shadow mask type used in the majority of color CRT's, a rectangular shadow mask having a large number of aligned beam passage holes is disposed just in front of the fluorescent screen. The shadow mask is a member for performing alignment of the electron beams and the fluorescent dots so that each electron beam irradiates the fluorescent dots of the corresponding color.
A conventional ordinary shadow mask is made of a cold rolled steel sheet having a thickness of 0.15-0.28 mm in which fine holes with a regular spacing are formed by etching. After the shadow mask is bent by press forming, its four sides are welded to a mask frame and secured. The curvature of the mask is necessary so that thermal expansion of the mask and vibrations transmitted from the outside are absorbed by the mask and positional deviation of the holes in the mask does not take place. Accordingly, this type of mask can not adequately cope with flattening of the mask surface.
A more recently developed type is a tension-type shadow mask. A typical tension-type shadow mask is made of a thin steel sheet having a thickness of 0.05-0.15 mm in which small holes for the passage of beams are formed. It is attached to a mask frame in a state in which a tensile force is applied to it in the vertical direction. Thermal expansion and vibration of the mask can be absorbed by the tension, and the mask can be made flat. A tension-type shadow mask in which bi-directional tension is applied in both the vertical direction and the horizontal direction is also possible.
A mask frame for supporting the above-described typical tension-type shadow mask is normally assembled by welding two long-side frame members extending in the horizontal direction which form upper and lower frame portions and two short-side frame members extending in the vertical direction which form left and right frame portions. The long-side frame members are made from a steel sheet shaped by press forming or roll forming of the steel sheet. The thickness of the steel sheet is in the range of 3-6 mm, and it is selected in accordance with the size of the CRT. Round or rectangular pipes or bars are normally used as the short-side frame members.
Before attaching the shadow mask to the mask frame, blackening treatment of the mask frame is carried out. The blackening treatment is treatment in which a black film made of Fe
3
O
4
is formed on the surface of the steel by heat treatment. The black film increases the thermal emissivity of the surface of the material, it increases the absorption and irradiation of electron beams, and it also has the effect of preventing the generation of secondary electrons and the formation of rust. The heating conditions for this blackening treatment are normally 450-680° C. for 10-30 minutes.
Attaching the above-described tension-type shadow mask to a mask frame is carried out by welding the upper and lower edges of the shadow mask to the upper frame portion and the lower frame portion of the frame while compressing from the outside towards the inside the upper frame portion and the lower frame portion of the mask frame formed from the long-side frame members, and if necessary simultaneously applying tension to the shadow mask in the vertical direction. Then, when the pressure applied to the upper frame portion and the lower frame portion of the frame is removed, due to the rebound force of the frame, the shadow mask is supported by the frame in a state in which it is pulled in the vertical direction. The left and right edges of the shadow mask are not secured to the left and right frame portions of the mask frame (made from the short-side frame members).
In a structure in which a shadow mask is attached to a mask frame (also referred to below as a shadow mask/frame structure) in this manner, the upper frame portion and the lower frame portion of the mask frame are in a state in which a bending stress is applied, and the shadow mask is in a state in which it receives a tensile force in the vertical direction. The left and right frame portions of the mask frame perform the function of supporting the upper and lower frame portions which are under a bending stress.
Finally, stress relief annealing is applied to the shadow mask/frame structure, and strains occurring at the time of mask installation are removed. The stress relief annealing is generally carried out by heating at a temperature of 400-680° C. for 10-30 minutes.
The order of steps of the above-described process (referred to below as Process A) is as follows:
Forming mask frame members-->assembly of mask frame-->blackening treatment of mask frame-->mounting of shadow mask on frame-->stress relief annealing
In the above order of steps, the step of blackening treatment and the step of stress relief annealing can be reversed. In this case, in the step of stress relief annealing, strains in the mask frame due to forming and welding are removed. In the step of blackening treatment, the mask frame and the shadow mask together undergo blackening treatment, and the strains resulting from installation of the shadow mask are also removed during the blackening treatment. The order of steps in this process (referred to below as Process B) is as follows:
Forming-->assembly-->stress relief annealing-->installation of shadow mask-->blackening treatment
In either of these processes, in the step of blackening treatment, it is desired to form a black film having good adhesion. If the adhesion of the black film is poor, there are cases in which the black film peels off during use of a color CRT, pieces of the black film fall down inside the CRT, the beam passage holes in the shadow mask are plugged and the like, and as a result, the image receiving properties of the CRT are greatly damaged.
The heat treatment step which is carried out after installation of the shadow mask (stress relief annealing in the case of Process A and blackening treatment in the case of Process B) is heat treatment carried out under the special circumstances in which the upper and lower frame portions of the mask frame are subjected to a bending stress and the shadow mask receives tension. When the bending stresses in the upper and lower frame portions of the mask frame are greatly alleviated by this heat treatment, there is the possibility of deformation of the mask frame taking place. A tensile force in the vertical direction is applied to the shadow mask by the mask frame, so deformation of the upper and lower frame portions of the mask frame causes a reduction in the tensile force acting on the shadow mask and causes surface strains. As a result, wrinkles and non-uniformity of the pitch of the holes develop, and are cases in which a deterioration of properties occurs such as impurity of color. Accordingly, it is important to suppress deformation of the mask frame during heat treatment.
In order to decrease the deformation of the mask frame during the above-described heat treatment step, which is a cause of a decrease in tensile force and surface strains of a tension-type shadow mask, the long-side frame members which make up the upper and lower frame portions of the mask frame which supports the shadow mask under
Nagao Noriaki
Ohmae Hideharu
Tsuge Shinji
Yoshii Tatsuo
Burns Doane Swecker & Mathis L.L.P.
Patel Vip
Sumitomo Metal Industries Ltd.
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