Shadow mask for cathode ray tube and method of manufacturing same

A shadow mask includes an aperture portion having a plurality of apertures formed therein, the aperture portion being formed over a predetermined area in a center of the shadow mask; and a non-aperture portion defining a periphery of the shadow mask and formed adjacent to the aperture portion. Only the aperture portion is selectively heat-treated to result in a tensile strength of the aperture portion being 1.2 to 3 times greater than that of the non-aperture portion, and the modulus of elasticity of the aperture portion being 1.5 to 3 times greater than that of the non-aperture portion. The method includes the steps of selectively performing a heat-treating process on only the aperture portion of the shadow mask by mounting a separating cover on the non-aperture portion, the separating cover preventing contact of the non-aperture portion with the gaseous atmosphere present during the heat treating process. The shadow mask is then press-formed.

CROSS-REFERENCE TO RELATED APPLICATION
 This application claims priority of Korean Patent Application No. 98-1852,
 filed Jan. 22, 1998.
 FIELD OF THE INVENTION
 The present invention relates to a shadow mask for a cathode ray tube (CRT)
 and a method of manufacturing the same. More particularly, the present
 invention relates to a shadow mask having improved tensile strength,
 thereby preventing what is called a spring back phenomenon.
 BACKGROUND OF THE INVENTION
 A conventional CRT comprises an evacuated envelope having a viewing screen
 coated with an array of phosphor elements of three different emission
 colors arranged in a cyclic order, means for producing three convergent
 electron beams directed towards the screen and a color selection structure
 or shadow mask comprising a thin multi-apertured metal sheet precisely
 disposed between the screen and the beam-producing means. The shadow mask
 shadows the screen and the differences in convergence angles permit the
 transmitted portions of each beam to selectively excite phosphor elements
 of the desired emission color.
 The conventional CRT shadow mask is typically manufactured by first coating
 a photoresist on a thin metal plate made of Invar or aluminum-killed
 steel. The plate is then exposed to light, and developed and etched to
 form a plurality of holes therein. Thereafter, the plate formed with the
 holes is annealed using a heat treating process in a hydrogen atmosphere
 at a high temperature, thereby removing residual stress and providing
 softness to the plate. The plate is then formed into a predetermined mask
 shape by the use of a press, after which the plate is cleaned to remove
 all contaminants from its surface such as fingerprints, dust and other
 foreign substances. Finally, a blackening process is performed on the
 shaped plate to prevent doming of the same, thereby completing the
 manufacture of the shadow mask.
 The shadow mask acts as a bridge between electron beams emitted from three
 electron guns (means for producing three convergent electron beams) and
 red, green and blue phosphor pixels formed on the screen because it
 ensures that the electron beams land on the correct phosphor pixels.
 Accordingly, any deviation of the shadow mask from its original position
 acts to mis-direct the electron beams to excite unintended phosphor
 pixels.
 The shadow mask can be moved away from its originally-set position in the
 CRT if it receives external shock or vibrations such as by the loud audio
 from speakers in the TV set. As the resut electron beams passing through
 moved shadow mask will land on the wrong phosphor pixel causing
 deteriorated color purity. This will be described in more detail
 hereinbelow.
 FIG. 1 shows a partial sectional view of a conventional CRT. It shows a
 shadow mask 6 mounted to a side wall of the panel 1. More specifically a
 mask frame 5 joined to a periphery of the shadow mask 6 is coupled to a
 spring 4, which is in turn connected to a stud pin 3 protruding from the
 side wall of the panel 1. When the CRT receives a substantial external
 shock or vibrations, the shadow mask 6 is shaken and moves away from its
 initial position to a deviated position 7. As the result the electron
 beams 10 emitted from the electron gun 11 will pass through an unintended
 aperture of the shadow mask 6 resulting in the excitation of the wrong
 phosphor pixel. This is perceived as shaking of a displayed picture and
 thus causes a reduction in color purity and other picture quality
 problems.
 Furthermore, in the case where the CRT receives an extreme shock, such as
 when it is dropped, it is possible for the shadow mask 6 to be deformed.
 An example of this is shown in FIG. 2 in which a deformed area 12 is
 illustrated. Needless to say, spurious colors would appear.
 In an attempt to solve the above problem, the mask was heat-treated to
 improve its tensile strength and softness. However, since the
 heat-treating of a shadow mask increases its modulus of elasticity in the
 skirt portion of the shadow mask, an angle of the bend in the skirt
 portion made while press-forming the metal plate used to make the shadow
 mask is not the intended .theta. degrees, but rather a
 .theta.+.DELTA..theta. degrees as a result of the spring back phenomenon
 of the skirt portion.
 SUMMARY OF THE INVENTION
 The present invention has been made in an effort to solve the above
 problems.
 It is an object of the present invention to provide a shadow mask for a
 cathode ray tube (CRT) and a method of manufacturing the same in which
 only an aperture portion of the shadow mask is heat-treated to increase
 its tensile strength and modulus of elasticity, thereby preventing the
 occurrence of the spring back phenomenon in a skirt portion of the shadow
 mask.
 To achieve the above object, the present invention provides a shadow mask
 for a CRT and a method of manufacturing the same. The shadow mask includes
 an aperture portion having a plurality of apertures formed therein, the
 aperture portion being formed over a predetermined area in the shadow
 mask; and a non-aperture portion, formed adjacent to the aperture portion,
 defining the periphery of the shadow mask. Only the aperture portion is
 selectively heat-treated so that the tensile strength of the aperture
 portion is 1.2 to 3 times greater than that of the non-aperture portion,
 and the modulus of elasticity of the aperture portion is 1.5 to 3 times
 greater than that of the non-aperture portion.
 According to a feature of the present invention, the selective
 heat-treating of the aperture portion is performed in a gaseous atmosphere
 including at least one gas selected from the group consisting of RX,
 propane, ammonia, B.sub.2 H.sub.6 and BCl.sub.3.
 According to another feature of the present invention, the shadow mask is
 made of a material having a low thermal expansion rate, preferably of
 aluminum-killed (AK) steel or Invar.
 The method includes the steps of selectively performing a heat-treating
 process on only the aperture portion of the shadow mask by mounting a
 separating cover on the non-aperture portion, the separating cover
 preventing contact of the non-aperture portion with the gaseous atmosphere
 present during the heat-treating process; and press forming the shadow
 mask.
 According to a feature of the present invention, the heat-treating process
 is performed in a gaseous atmosphere including at least one gas selected
 from the group consisting of RX, propane, ammonia, B.sub.2 H.sub.6 and
 BCl.sub.3.
 According to another feature of the present invention, the shadow mask is
 made of a material having a low thermal expansion rate, preferably AK
 steel or Invar.
 According to yet another feature of the present invention, the separating
 cover is mounted simultaneously on the top and bottom of the shadow mask.
 According to still yet another feature of the present invention, the
 separating cover is suitably sized and shaped to fully cover the
 non-aperture portions while exposing the aperture portion of the shadow
 mask.

DETAILED DESCRIPTION OF THE INVENTION
 A CRT shadow mask according to the present invention is made of a low
 thermal expansion material such as AK steel or Invar. As shown in FIG. 4,
 the shadow mask includes an aperture portion 13 and a periphery or
 non-aperture portion 14 surrounding the aperture portion. In the inventive
 shadow mask, only the aperture portion is selectively heat-treated such
 that the tensional strength of the aperture portion is 1.2 to 3 times
 greater than that of the non-aperture portion and the modulus of
 elasticity of the aperture portion is 1.5 to 3 times greater than that of
 the non-aperture portion.
 The selective heat-treating of the aperture portion is performed in a
 gaseous atmosphere in which at least one of the following is present: an
 RX gas, a propane gas, an ammonia gas or a gas containing boron. The
 aperture portion is made of a material containing at least one element
 selected from the group consisting of carbon, nitrogen and boron. For the
 above boron gas, it is possible to use B.sub.2 H.sub.6 or BCl.sub.3.
 A method of manufacturing the shadow mask of the present invention will now
 be described in detail hereinafter.
 A predetermined number of metallic plates, having a plurality of apertures
 formed in a predetermined area to form aperture portions, is stacked and
 loaded on a tray. A separating cover is placed over the top metallic
 plate. The separating cover is designed to expose aperture portions of the
 metallic plates while covering peripheries, or non-aperture portions, of
 the same. After a pre-heating furnace is set to a temperature between
 100.degree. C. and 200.degree. C., the tray having the stacked metallic
 plates thereon is placed in the pre-heating furnace.
 Next, the RX gas, propane gas, ammonia gas, and/or gas containing boron is
 fed into a reacting furnace, which is heated to a temperature over
 150.degree. C. When RX gas is used, it comprises of 40% H.sub.2, 40%
 N.sub.2 and 20% CO; and the gas containing boron is, as described above,
 B.sub.2 H.sub.6 or BCl.sub.3. Subsequently, the temperature in the
 reacting furnace is increased to between 400 and 1000.degree. C., and the
 gaseous atmosphere therein is suitably maintained, after which the
 metallic plates in the pre-heating furnace are transferred to the reacting
 furnace.
 The metallic plates are heat-treated in the reacting furnace for between
 0.1 and 5 hours. After a predetermined amount of time has elapsed, the
 temperature in the reacting furnace is reduced to 150.degree. C. while the
 atmosphere therein is maintained in the present state. When this
 temperature is reached, the injection of gas into the reacting furnace is
 stopped. Next, the metallic plates are removed from the reacting furnace,
 and the separator is decoupled from the metallic plates. The metallic
 plates are then press-formed into the desired shadow mask shape.
 The tensile strengths of the aperture and non-aperture portions of the
 resulting shadow masks manufactured as described above were measured to be
 300-500 Mpa for the aperture portion and 200-300 Mpa for the non-aperture
 portion. Further, the modulus of elasticity for the aperture portion was
 200-400.times.100 Mpa, and that for the non-aperture portion was
 100-300.times.100 Mpa, for only the tensile strength and modulus of
 elasticity of the aperture portion are increased as a result of the
 heat-treating process, while the non-aperture portion is left unaffected.
 Here, the values for the tensional strength and the modulus of elasticity
 are merely used to illustrate that the aperture portion has higher values
 after manufacture using the above method of the present invention, and do
 not refer to differences and limitations in the properties of the aperture
 and non-aperture portions.
 FIG. 3 shows a plan schematic view of a separating cover 21 used in the
 above heat-treatment process for manufacturing the shadow mask. The
 present invention is not limited to the separating cover 21 illustrated in
 FIG. 3, and other configurations can be used as long as they are able to
 selectively shield the non-aperture portions of the shadow masks from the
 contact with the gases in the reacting furnace during the heat-treatment
 process while leaving the aperture portions exposed.
 As shown in the drawing, the separating cover 21 comprises a pair of lower
 straps 21b, 21b' and a pair of upper straps 21a, 21a'. The lower straps
 21b and 21b' are substantially parallel and disposed a predetermined
 distance from each other. Likewise the upper straps 21a and 21a' are
 substantially parallel and disposed a predetermined distance from each
 other. The lower straps 21b and 21b' are perpendicular to the upper straps
 21a and 21a'. The distances between the lower straps 21b, 21b' and between
 the upper straps 21a, 21a' as well as the widths and lengths of the straps
 are such that the non-aperture portion, or the area to be formed into the
 skirt portion, is covered.
 In addition, formed on ends of each of the straps 21a, 21a', 21b and 21b'
 are connecting holes 23 which are used to couple the separating cover 21
 to the tray after being placed over the stacked metallic plates.
 Accordingly, with the separating cover 21 mounted over the non-aperture
 portions of the metallic plates, the non-aperture portions of the metallic
 plates do not come into contact with the gaseous atmosphere such that this
 area is not altered by the heat-treatment process. As the result the
 modulus of elasticity of the non-aperture or skirt portion of the metallic
 plates remains the same so that there is no springing-back of the skirt
 portion after it is bent during the press. The gaseous atmosphere can
 include an RX gas, a propane gas, an ammonia gas and/or a gas containing
 boron. Also, for the boron gas, it is possible to use B.sub.2 H.sub.6 or
 BCl.sub.3.
 With the use of the separating cover of the present invention shown in FIG.
 3, the atoms of the gases used in the heating process are prevented from
 physically contacting the non-aperture portion of the metallic plate used
 to manufacture the shadow mask. Hence, the non-aperture or skirt portion
 of the shadow mask retains its modulus of elasticity even after the
 heating process, thereby preventing the spring back phenomenon of this
 area.
 EXAMPLE 1
 A predetermined number of metallic plates, having a plurality of apertures
 formed over a predetermined area to form aperture portions, were stacked
 and loaded on a tray. An inventive separating cover was mounted on the
 uppermost metallic plate of the stack. Next, a pre-heating furnace was set
 and maintained at 150.degree. C., after which the tray having the stacked
 metallic plates thereon was placed in the pre-heating furnace.
 Next, RX gas, propane gas, ammonia gas, and/or gas containing boron was fed
 into a reacting furnace heated to a temperature over 150.degree. C. When
 RX gas was used, it comprised 40% H.sub.2, 40% N.sub.2 and 20% CO.
 Subsequently, the temperature in the reacting furnace was increased to
 600.degree. C., and the gaseous atmosphere therein was suitably
 maintained, after which the metallic plates in the preheating furnace were
 transferred to the reacting furnace.
 The metallic plates were left to stand in the heated reacting furnace for 1
 hour. After 1 hour, the temperature in the reacting furnace was reduced to
 150.degree. C. while the atmosphere in the same was maintained in the
 present state. When this temperature was reached, the injection of gas
 into the reacting furnace was discontinued. Next, the metallic plates were
 removed from the reacting furnace, and the separating cover was decoupled
 from the metallic plates. The metallic plates were then press formed into
 the desired shadow mask shape.
 In the shadow mask manufactured using the method of the present invention,
 since only the aperture portion is exposed to the gaseous atmosphere
 during the heat-treatment process, whereas the non-aperture or skirt
 portion is blocked from contact with the gaseous atmosphere, the tensional
 strength and modulus of elasticity of the aperture portion are increased
 while the skirt portion is left unaffected. As a result, springing back of
 the skirt portion during press-forming is prevented.
 Although the present invention has been described in detail hereinabove, it
 should be clearly understood that many variations and/or modifications of
 the basic inventive concepts herein taught which may appear to those
 skilled in the present art will still fall within the spirit and scope of
 the present invention, as defined in the appended claims.