An implosion-protected cathode-ray tube has a bulb and a metallic band tightly binding the front outer peripheral portion of the bulb to suppress the occurrence of accidental implosion of the bulb during the manufacturing processes including a heating and evacuation process and a rapid cooling process after the heating and evacuation process as well as after the manufacture of the cathode-ray tube. The material forming the metallic band and the size of the metallic band meet the following conditions: i) the thermal expansion coefficient .alpha..sub.s of the material is not less than the thermal expansion coefficient .alpha..sub.g of the glass forming the bulb; ii) a value expressed by: L(1+.alpha..sub.s..delta.t), where L is the inner perimeter of the metallic band at the ambient temperature when the tightened metal band is removed from the bulb and where .alpha.t is the temperature differential between the ambient temperature and a temperature for a heating and evacuation process, is smaller than the perimeter of the front outer peripheral portion of the bulb on which the metallic band is shrunk, during the heating and evacuation process, and iii) a stress corresponding to the yield point of the material of the metallic band remains in the metallic band at the ambient temperature.

BACKGROUND OF THE INVENTION 
The present invention relates to an implosion-protected cathode-ray tube 
comprising a bulb, and a metallic band tight binding a front outer 
peripheral portion of the bulb near the front face of the bulb, and 
designed for efficient mass production. 
Because it is evacuated to a high vacuum and continuously subjected to a 
pressure corresponding to the atmospheric pressure, the cathode-ray tube 
can implode suddenly if the surface of the glass bulb is damaged or an 
impact is applied thereto resulting in a dangerous scattering of pieces of 
broken glass. Various means have been proposed and applied to the 
cathode-ray tube to prevent the implosion of the cathode-ray tube. A 
currently most prevalent means for avoiding the implosion of the 
cathode-ray tube and the resulting scattering of pieces of broken glass 
even when the airtightness of the bulb is broken employs a metallic band 
tightened around a largest outer peripheral portion of the bulb near the 
front face to bind the largest front portion tight so that stress induced 
in the bulb by the atmospheric pressure is relaxed and the propagation of 
cracks may be suppressed. 
In most cases, the metallic band is tightened around the largest outer 
peripheral portion of the bulb after the heating and evacuation of the 
cathode-ray tube to prevent implosion. A stress is induced in the 
evacuated glass bulb by the pressure difference between the outside and 
inside of the glass bulb and by the sudden drop of the temperature of the 
glass bulb when the glass bulb is taken out after the heating and 
evacuation from a heating furnace heated at a temperature on the order of 
400.degree. C. Accordingly, there is a danger that the bulb can implode 
suddenly and pieces of broken glass can scatter in the heating and 
evacuation process including the process of taking out the bulb from the 
heating furnace, when the bulb has cracks or an impact is applied to the 
bulb. Recently, such a danger has progressively increased with the 
increasing use of large cathode-ray tubes having a nearly flat face. As 
stated in Japanese Patent Laid-Open No. 62-5533, even if a metallic band 
is tightened around the largest outer peripheral portion of a bulb prior 
to the heating and evacuation process, the metallic band provides a 
thermal expansion greater than that of the glass bulb in the heating 
furnace during the heating and evacuation process when any special 
consideration is not given, so that the metallic band is unable to exert 
its effect properly. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
implosion-protected cathode-ray tube solving the foregoing problems in the 
conventional cathode-ray tube, provided with a metallic band for 
protecting the glass bulb of the cathode-ray tube from implosion, and 
capable of effectively providing implosion prevention during the heating 
and evacuation process. 
To achieve this object, the present invention provides an 
implosion-protected cathode-ray tube manufactured through the heating and 
evacuation of a bulb after tightening a metallic band around the largest 
outer peripheral portion of the bulb to protect the bulb from implosion, 
in which the thermal expansion coefficient .alpha..sub.s of the material 
of the metallic band is determined selectively so that the thermal 
expansion coefficient .alpha..sub.s of the material of the metallic band 
is not less than the thermal expansion coefficient .alpha..sub.g of the 
glass forming the bulb, and a value expressed by: 
L(1+.alpha..sub.s..delta.t), where L is the inner perimeter of the 
metallic band at the ambient temperature when the tightened metal band is 
removed from the bulb, and .delta.t is a temperature differential for the 
heating and evacuation process, namely, the difference between the ambient 
temperature and a temperature for the heating and evacuation, is smaller 
than the perimeter of the largest outer peripheral portion of the bulb in 
the heating and evacuation process. 
Before subjecting the bulb to the heating and evacuation process, a 
non-inflammable buffer member is wound round the largest outer peripheral 
portion of the bulb at the ambient temperature, and then the heated 
metallic band is shrunk on the largest outer peripheral portion of the 
bulb in a shrinkage fit so that a residual tensile stress in the metallic 
band at the ambient temperature corresponds to the yield point of the 
material of the metallic band.

DETAILED DESCRIPTION OF THE INVENTION 
When the thermal expansion coefficient .alpha..sub.s of the material of the 
metallic band is smaller than the thermal expansion coefficient 
.alpha..sub.g of the glass forming the bulb, it is possible that the 
metallic band is subject to elongation beyond the yield point during the 
heating and evacuation process, so that the metallic band is unable to 
stay tight at a proper tensile stress around the largest outer peripheral 
portion of the bulb during a cooling process after the heating and 
evacuation process. On the contrary, when the thermal expansion 
coefficient of the material of the metallic band is not less than that of 
the glass forming the bulb, the metallic band is not subject to elongation 
beyond the yield point during the heating and evacuation process. 
Furthermore, when the material of the metallic band is determined so that a 
value expressed by: L(1+.alpha..sub.s. .delta.t), where L is the inner 
perimeter of the metallic band at the ambient temperature when the 
tightened metal band is removed from the bulb, and .delta.t is a 
temperature differential for the heating and evacuation process, is 
smaller than the perimeter of the largest outer peripheral portion of the 
bulb during the heating and evacuation process, the metallic band is kept 
tight round the largest front outer peripheral portion of the bulb during 
the heating and well evacuation process and never fall off the bulb. 
The present invention will be described more quantitatively hereinafter 
using the following symbols. 
DEFINITION OF SYMBOLS 
L.sub.B : The perimeter of a largest front outer peripheral portion of a 
bulb, L.sub.i : The inner perimeter of a metallic band before shrinkage 
fit process, L: The inner perimeter of the metallic band at the ambient 
temperature when the tightened metal band is removed from a bulb, 
.epsilon..sub.y : Strain at the yield point of the metallic band before 
being put on the bulb, .epsilon..sub.ya : Strain at the yield point of the 
metallic band after being put on the bulb, .alpha..sub.g : The thermal 
expansion coefficient of the glass forming the bulb, .alpha..sub.s : The 
thermal expansion coefficient of the material of the metallic band, 
.delta.t: Temperature differential for heating and evacuation (the 
difference between the ambient temperature and a temperature for heating 
and evacuation process), .delta.t': Temperature differential for shrinkage 
fit (the difference between the ambient temperature and a temperature for 
the shrinkage fit process). 
As stated above, since a residual stress in the metallic band at the 
ambient temperature after the metallic band has been shrunk on the bulb in 
the shrinkage fit corresponds to the yield point, the difference between 
L.sub.B and L corresponds to a strain at the yield point, that is, 
EQU L.sub.B =L(1+.epsilon..sub.ya).apprxeq.L(1+.epsilon..sub.y) 
EQU L.apprxeq.L.sub.b /(1+.epsilon..sub.y) . . . (1) 
The inner perimeter L(1+.alpha..sub.s..delta.t) of the metallic band at a 
temperature for heating and evacuation must be smaller than the perimeter 
L.sub.B (1+.alpha..sub.g..delta.t) of the largest front outer peripheral 
portion of the bulb at the same temperature so that the metallic band 
cannot fall from the bulb, that is: 
EQU L(1+.alpha..sub.s..delta.t)&lt;L.sub.B (1+.alpha..sub.g..delta.t) 
By substituting expression (1) into this expression, we obtain, 
EQU {L.sub.B /(1+.epsilon..sub.y)}(1+.alpha..sub.s..delta.t)&lt;L.sub.B 
(1+.alpha..sub.g..delta.t) .alpha..sub.s 
&lt;{(1+.alpha..sub.g..delta.t)(1+.epsilon..sub.y)-1)/.delta.t 
On the other hand, the thermal expansion coefficient .alpha..sub.s of the 
material of the metallic band must be not less than the thermal expansion 
coefficient .alpha..sub.g of the glass forming the bulb so that the 
metallic band does not elongate beyond the yield point during the heating 
and evacuation process, that is, 
EQU .alpha..sub.g .ltoreq..alpha..sub.s 
Therefore, the range of the thermal expansion coefficient .alpha..sub.s of 
the material of the metallic band must meet an inequality: 
EQU .alpha..sub.g .ltoreq..alpha..sub.s 
&lt;{1+.alpha..sub.g..delta.t)(1+.epsilon..sub.y)-1})/.delta.t (2) 
Therefore, the material of the metallic band may be selected such that the 
thermal expansion coefficient .alpha..sub.s thereof and the strain 
.epsilon..sub.y at the yield point thereof satisfy the expression (2). 
Incidentally, since the metallic band is shrunk on the largest front outer 
peripheral portion of the bulb in a shrinkage fit, L.sub.B is greater than 
L.sub.i and, since the metallic band is subject to elongation beyond the 
yield point in shrinking the metallic band on the bulb, the difference 
L.sub.B -L.sub.i is greater than its strain L.sub.i..epsilon..sub.y at the 
yield point, that is, 
EQU L.sub.B -L.sub.i &gt;L.sub.i..epsilon..sub.y hence 
EQU L.sub.i &lt;L.sub.B /(1+.epsilon..sub.y) (3) 
To tighten the metallic band around the largest front outer peripheral 
portion of the bulb in a shrinkage fit, the inner perimeter L.sub.i 
(1+.alpha..sub.s. .delta.t') of the metallic band at the temperature for 
shrinkage fit process must be greater than the perimeter L.sub.B of the 
largest front outer peripheral portion of the bulb at the ambient 
temperature, that is, 
EQU L.sub.B &lt;L.sub.i (1+.alpha..sub.s..delta.t') (4) 
Therefore, the inner perimeter L.sub.i of the metallic band before the 
shrinkage fit process and the difference between the ambient temperature 
and the temperature for shrinkage fit process are determined so as to meet 
expressions (3) and (4) respectively. 
The thermal expansion coefficient .alpha..sub.s and inner perimeter L.sub.i 
before shrinkage fit process of the metallic band employed in the present 
invention and the temperature differential .delta.t', i.e., the difference 
between a temperature for shrinkage fit process and the ambient 
temperature, are determined by the following procedure. 
i) The thermal expansion coefficient .alpha..sub.g of the glass forming the 
bulb, and the temperature differential .delta.t, i.e., the difference 
between a temperature for heating and evacuation process and the ambient 
temperature, are given. The values may be determined by a conventional 
technique. 
ii) A material for forming the metallic band, having .alpha..sub.s and 
.epsilon..sub.y meeting expression (2) is selected. 
iii) A value for the inner perimeter L.sub.i of the metallic band before 
the shrinkage fit process which meets expression (3) is determined. 
iv) A value for the temperature differential .delta.t' between the 
temperature of the metallic band for shrinkage fit and the ambient 
temperature, i.e., a temperature increment for the shrinkage fit proess 
which meets expression (4) is determined. 
v) If the values of L.sub.i and .delta.t' determined through the foregoing 
steps are inappropriate, the same steps are repeated for another material 
satisfying the expression (2) to determine an appropriate material for the 
metallic band. 
vi) The metallic band is tested on the actual bulb to confirm the 
appropriateness of the values of L.sub.i and .delta.t' determined through 
the foregoing steps or to make the values of L.sub.i and .delta.t' 
appropriate by a cut-and-try method. 
Since the non-inflammable buffer member has a very small thickness in the 
range of 0.01 to 0.1 mm, the thickness of the non-inflammable buffer 
member is neglected in the foregoing quantitative description. 
When the non-inflammable buffer member is wound around the largest front 
outer peripheral portion of the bulb, and then the metallic band is shrunk 
on the largest front outer peripheral portion of the bulb in a shrinkage 
fit before the heating and evacuation process so that a stress 
corresponding to the yield point of the material of the metallic band 
remains within the metallic band when the metallic band is cooled to the 
ambient temperature after shrinkage fit, a variation in the perimeter of 
the largest front outer peripheral portion of the bulb or in the perimeter 
of the same with the noninflammable buffer member, if any, is absorbed by 
the elongation of the metallic band, and a fixed stress corresponding to 
the yield point of the material of the metallic band remains in the 
metallic band. 
The non-inflammable buffer member is, for example, a glass cloth tape, a 
strip of aluminum foil or a strip of copper foil. 
The thickness of the metallic band is, usually, in the range of 0.5 to 2.0 
mm, which is a range for the thickness in the prior art. A thickness 
exceeding 2 mm is undesirable, because such a large thickness affects the 
workability of the metallic band adversely and increases the weight of the 
metallic band excessively. 
Although the thermal expansion coefficient .alpha..sub.s of the material of 
the metallic band may be of any value in a range meeting expression (2), a 
preferable value in general is in the range of 10.4.times.10.sup.-6 to 
12.3.times.10.sup.-6 /.degree. C. in view of conditions for the heating 
and evacuation process and the glass material of the bulb employed usually 
at present. 
The temperature differential .delta.t for the heating and evacuation 
process can be in the range of 400.degree. to 700.degree. C., which is 
also a range for the conventional heating and evacuation. 
Mild steels, which are materials forming the conventional metallic bands, 
are unsuitable. Materials having values thermal expansion coefficient in 
the aforesaid appropriate range are Fe-Cr alloys, Fe-Ni alloys and 
Fe-Ni-Cr alloys. However, from the viewpoint of economical effect, chrome 
steels containing 10 to 30% chromium, typically, 10 and several % 
chromium, are suitable. 
The thermal expansion coefficients .alpha..sub.s of such chrome steels are 
in the range of 10.5.times.10.sup.-6 to 11.5.times.10.sup.-6 /.degree. C., 
and the strains .epsilon..sub.y at the yield points of the same are 0.001. 
Ni(52%)-Fe alloy has .alpha..sub.s in the range of 10.4.times.10.sup.-6 to 
10.8.times.10.sup.-6 /.degree. C., and .epsilon..sub.y of 0.001. 
Ni(42%)-Cr(6%)-Fe alloy has .alpha..sub.s in the range of 
10.5.times.10.sup.-6 to 11.1.times.10.sup.-6 /.degree. C. and 
.epsilon..sub.y of 0.001. The unit of content expressed by y "%" is 
percent by weight. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An implosion-protected cathode-ray tube in an embodiment according to the 
present invention will be described with reference to FIG. 1. 
A bulb 3 is formed by sealing a glass panel portion 1 provided with a 
phosphor screen on the inner surface thereof to a funnel portion 2 with a 
low-melting point glass. A neck portion 4 is formed integrally with the 
funnel portion 2. A glass stem 6 holding an electron gun structure 5 is 
inserted into the neck portion 4, and then the glass stem 6 is welded and 
sealed to the extremity of the neck portion 4. Then, a non-inflammable 
buffer member 7, such as a glass cloth tape, is wound round the skirt of 
the panel portion 1 or all round the junction of the panel portion 1 and 
the funnel portion 2, sealed with the low-melting point glass. Since it is 
difficult to maintain the state of the wound non-inflammable buffer member 
without an auxiliary member, the non-inflammble buffer member 7 is held in 
place with an auxiliary member coated with an adhesive. Since the adhesive 
contaminates the interior of the evacuating furnace heated at a high 
temperature and the bulb 3 when the adhesive burnt in the evacuating 
furnace, the auxiliary member is removed before the heating and evacuating 
process. In this embodiment, the auxiliary member coated with an adhesive 
is an adhesive tape. The adhesive tape is wound on and stuck to the front 
periphery portion of the bulb and the edge (front edge in the present 
embodiment) of the non-inflammable buffer member 7 so as to overlap the 
non-inflammable buffer member 7 by a width of about 5 mm. A metallic band 
8 is then put on the bulb 3 over the non-inflammable buffer member 7 so 
that the metallic band 8 may not overlap the adhesive tape. After putting 
the metallic band 8 on the bulb 3 in shrinkage fit, only the adhesive tape 
is removed. The metallic band 8 is formed beforehand by welding the 
opposite ends of a metal strip in a frame having a shape and size 
respectively corresponding to those of the bulb portion on which the 
metallic band 8 is mounted. The metallic band 8 having the shape of such a 
frame is heated for expansion, the expanded metallic band 8 is put on the 
non-inflammable buffer member 7, and then the metallic band 8 is shrunk by 
cooling. Then, the bulb 3 is subjected to the heating and evacuation 
process. 
The size of the metallic band 8 is based on the size of the outer 
peripheral portion of the bulb on which the thickness of the 
non-inflammable buffer member 7 is added. However, as stated above, the 
thickness of the non-inflammable buffer member 7 may be neglected in 
determining the inner perimeter of the metallic band 8. 
The composition of the glass forming the bulb 3 is dependent on the type of 
the corresponding cathode-ray tube, and the portions of the bulb 3. The 
panel portion 1 of the bulb 3 in an exemplary embodiment, as intended for 
use for forming a cathode-ray tube for a display, was formed principally 
of a conventional glass 8% BaO-10% SrO-1% ZrO.sub.2 -SiO.sub.2. The 
thermal expansion coefficient .alpha..sub.g of this glass was 
10.4.times.10.sup.-6 /.degree. C. The value of the perimeter L.sub.B of 
the front outer peripheral portion of the bulb 3 is dependent on the type 
and size of the corresponding cathode-ray tube. The perimeter L.sub.B Of 
the front outer peripheral portion of the bulb 3 for a cathode-ray tube of 
20 in. nominal size was 1456.2.+-.1.8 mm. 
The metallic band 8 was formed of a Cr(18%)-Fe alloy, and had a thermal 
expansion coefficient us of 11.0.times.10.sup.-6 /.degree. C., a strain 
.epsilon..sub.y at the yield point of 0.001, an inner perimeter L.sub.i of 
1450.8.sub.-1.sup.+0 mm (in the case of a cathode-ray tube of 20 in. 
nominal size for a display), and a thickness of 1.0 mm. 
The thickness of the non-inflammable buffer member 7, i.e., the glass cloth 
tape, was 0.08 mm. 
The temperature for shrinkage fit was, 550.degree. C., and hence the 
temperature differential .delta.t' for shrinkage fit 525.degree. C., when 
the ambient temperature was 25.degree. C. 
The evacuation is conducted employing a discharge tube 9 inserted through 
the glass stem 6 into the bulb 3. The discharge tube 9 is chipped off to 
seal the bulb 3 hermetically after the completion of heating and 
evacuation. The bulb 3 is heated at 400.degree. C. (the temperature 
differential .delta.t is 375.degree. C. when the ambient temperature is 
25.degree. C.) in a heating furnace for evacuation and it is evacuated. 
The bulb 3 is taken out from the heating furnace after the completion of 
heating and evacuation. 
The metallic band 8 is formed so that the metallic band 8 is strained 
beyond the strain at the yield point and a tensile stress corresponding to 
the yield point remains therein when the metallic band 8 is cooled to the 
ambient temperature after shrinkage fit. Although the metallic band is 
heated at a temperature of about 400.degree. C. for heating and 
evacuation, the metallic band 8 is not subject to a tensile stress 
exceeding the yield point and hence the metallic band 8 maintains its 
tightening effect after the heating and evacuation process, because the 
thermal expansion coefficient of the metallic band 8 is equal to or 
greater than that of the glass forming the bulb 3. Although the metallic 
band 8 expands during the heating and evacuation process, the metallic 
band 8 maintains its tightening effect to prevent the implosion of the 
bulb 3 during the heating and evacuation process, because the value of 
.alpha..sub.s is determined selectively so that the perimeter 
L(1+.alpha..sub.s ..delta.t) of the metallic band 8 during the heating 
and evacuation process is smaller than that of the bulb 3. 
In this embodiment, the metallic band 8 was shrunk on the outer peripheral 
portion of the bulb after sealing the electron gun structure 5 to the bulb 
3, however, the metallic band 8 may be shrunk thereon before sealing the 
electron gun structure 5 to the bulb 3. In this embodiment, the metallic 
band 8 is shrunk on the outer peripheral portion of the bulb after sealing 
the panel portion 1 to the funnel portion 2 with the low-melting point 
glass, however, when necessary, the the metallic band 8 may be shrunk on 
the outer peripheral portion of the bulb before sealing the panel portion 
1 to the funnel portion 2 with the low-melting point glass. 
As described above, according to the present invention, an 
implosion-preventive metallic band formed of a material having an 
appropriate thermal expansion coefficient is shrunk on the front outer 
peripheral portion of a bulb for a cathode-ray tube to suppress the 
occurrence of accidental implosion of the bulb during the heating and 
evacuation process and the subsequent rapid cooling process, regardless of 
some dimensional variations in the associated members.