[0001] The invention relates to a method of drape drawing, by means of a drawing process,
a shadow mask sheet for a colour display tube consisting of an iron-nickel alloy of
the invar type, in which a skirt is formed on the shadow mask sheet.
[0002] The invention furthermore relates to a shadow mask manufactured by the method according
to the invention.
[0003] The invention also relates to a colour display tube comprising a shadow mask according
to the invention.
[0004] Such a method has been proposed in the previously filed EP-A 179 506 in which a process
is described for drape drawing a shadow mask sheet consisting of an iron-nickel alloy.
Prior to the actual drawing process, the shadow mask sheet is subjected to an annealing
treatment to produce a complete recrystallisation of the material of the shadow mask
sheet without an essential grain growth occurring and to reduce the tensile stress
at the 0.2 % proof stress of the material, which tensile stress is too high at room
temperature to draw the shadow mask sheet to the desired shape. A further reduction
of the tensile stress, however, is necessary to obtain a reproducible process for
drape drawing the shadow mask sheet. In order to realise this, the shadow mask sheet
during the drawing process is not drape drawn at room temperature but at a higher
temperature. During this drape drawing process of the shadow mask sheet, the shadow
mask sheet is generally first given the desired shape, in particular an arcuate shape,
and the shadow mask sheet is then provided with a skirt. This skirt is formed by bending
over the edge of the shadow mask sheet and it may be used for the connection of the
shadow mask in a colour display tube. After the skirt has been formed, the drawing
process is terminated and the shadow mask is cooled to ambient temperature and the
material substantially regains its original, comparatively high 0.2 % proof stress.
A similar method is also known from EP-A 124 354.
[0005] It has been found that as a result of this comparatively high 0.2 % proof stress
the skirt resiles outwards slightly and as a result of this can adversely influence
the shape of the shadow mask sheet. Moreover, it is usual to weld the shadow mask
formed to a supporting frame by providing a number of spot welds between said supporting
frame and the skirt. An outwardly resiled skirt, however, does not fully engage the
supporting frame and thus presents a problem in welding the shadow mask to the supporting
frame.
[0006] It is the object of the invention to provide a method of drape drawing a shadow mask
consisting of an iron-nickel alloy of the invar type, in which a good reproducibility
of the shape of the shadow mask sheet is obtained.
[0007] For that purpose a method of the kind mentioned in the opening paragraph is characterized
according to the invention in that in addition to the formation of the skirt, said
skirt is plastically deformed permanently. It has been found that as a result of the
permanent plastic deformation of the skirt, the shape and the position of the skirt
are very much controlled. The extent of outward resiling is reduced as a result of
which a good reproducibility of the shape of the shadow mask sheet is obtained. An
additional advantage of the invention is that the skirt can fully engage a supporting
frame and can more easily be welded to said supporting frame.
[0008] An embodiment of a method in accordance with the invention is characterized in that
the permanent plastic deformation of the skirt takes place at a temperature between
150
°C and 250
°C. When the plastic deformation is carried out at room temperature, the comparatively
high 0.2 % proof stress leads to the material being difficult to work mechanically.
The tensile stress at which the shadow mask material reaches the 0.2 % proof stress
is reduced when the plastic deformation takes place at a temperature between 150°C
and 250°C. As a result of this the mechanical workability is increased and the level
of the reproducibility of the drawing process is satisfactory.
[0009] A further embodiment of a method in accordance with the invention is characterized
in that the plastic deformation of the skirt takes place at least substantially simultaneously
with the formation of the skirt, and that a flange is formed on said skirt. The skirt
is provided on the shadow mask sheet by means of a drawing operation. The plastic
deformation of said skirt involves a complementary operation which takes some time.
By keeping the periphery of the shadow mask clamped in a slip-free manner during the
drawing process, the formation of the skirt and the plastic deformation thereof can
be done simultaneously. This provides a saving in time since according to the invention
no complementary operation is necessary any longer for the deformation. Moreover,
a complementary heating for the plastic deformation of the skirt of the shadow mask
sheet is no longer necessary since the drawing process of the shadow mask sheet can
take place already at a temperature between 150°C and 250
° C.
[0010] Still a further embodiment of a method in accordance with the invention is characterized
in that prior to the formation of the flange recesses are provided in the shadow mask
sheet, said recesses being formed at least for the greater part in that part of the
shadow mask sheet which afterwards constitutes the flange. If the shadow mask is to
be connected to a supporting frame, for example by means of spot-welding, it must
be possible for the welding apparatus to contact the skirt. The outwardly directed
flange could consitute an obstruction therefor. By providing recesses in the flange
at the areas where the spot welds have to be provided, the welding apparatus can reach
the skirt at these places. Prior to the drawing process, said recesses can be provided
in the shadow mask sheet by means of an etching process, for example the same etching
process with which mask holes are provided in the shadow mask sheet. However, it is
also possible to provide the recesses after the flange has been formed, for example,
by cutting away flange material, although this is time-consuming.
[0011] An embodiment of the invention will now be described in greater detail with reference
to the drawings, in which
Figure 1 shows the tensile stress of an annealed iron-nickel alloy of the invar type
as a function of the temperature during the drawing process,
Figures 2 to 8 show the method step by step,
Figure 9 shows a shadow mask manufactured by said method, and
Figure 10 is a sectional view of a shadow mask connected to a supporting frame.
[0012] A shadow mask sheet consisting of an iron-nickel alloy of the invar type which comprises
35-37 % by weight of nickel, can be subjected to an annealing treatment preceding
the actual drawing process, for example, at a temperature between 700°C and 820
°C for a period of time which is sufficient to produce complete recrystallisation of
the material of the shadow mask sheet without essential grain growth occurring. As
a result of this the tensile stress at which the 0.2 % proof stress is reached is
reduced. However, the 0.2 % proof stress reached is still too high to obtain a reproducible
process for drape drawing the shadow mask sheet. For that purpose a further reduction
of the 0.2 % proof stress has proved necessary. In order to realise this the shadow
mask sheet is not drape drawn at room temperature but at a temperature between 150
°C and 250
°C. Figure 1 shows the variation of the tensile stress at the 0.2 % proof stress as
a function of the temperature. In the temperature range from 150
°C to 250
°C the temperature dependance on the 0.2 % proof stress decreases considerably with
increasing temperature. At temperatures above 250
°C a comparatively small reduction of the 0.2 % proof stress is still obtained. However,
at such high temperatures practical problems start playing a role with regard to the
drawing tools which no longer outweigh the advantage of a lower 0.2 % proof stress.
[0013] Figure 2 is a diagrammatic sectional view of the device for drape drawing a shadow
mask sheet 1. The device for drape drawing the shadow mask sheet 1 comprises a draw
die 2 (sometimes termed "mandril"), a pressure ring 3 (sometimes termed "pleat holder")
and a draw ring 4. A shadow mask sheet 1 is laid on the draw die 2. The draw ring
4 is then lowered towards the pressure ring 3 so that the shadow mask sheet 1 becomes
clamped on opposite sides at its periphery between the draw ring 4 and the pressure
ring 3 in a slip-free manner (see Figure 3). The actual drape drawing of the shadow
mask sheet 1 takes place by simultaneously lowering the draw ring 4 and the pressure
ring 3 as is shown in Figure 4. The shadow mask sheet 1 is then drawn over the draw
die 2. During this process the temperature of the shadow mask sheet 1 is adjusted
to between 1500C and 250
°C. For this purpose the draw die 2 comprises a copper block 5 in which electrical
heating elements 6 are embedded. Similarly, the pressure ring 3 comprises copper blocks
7 with heating elemens 8 and the draw ring 4 comprises copper blocks 9 with heating
elements 10. The shadow mask sheet 1 can be heated by heating the drawing tools to
between 150°C and 250°C. However, the shadow mask sheet 1 may alternatively be heated
previously in a furnace at a temperature between 150°C and 250°C. In order to distribute
the temperature uniformly over the shadow mask sheet during the drawing process the
draw die 2 comprises a number of heat pipes 11 which ensure temperature equalisation
at the surface of the draw die 2. After the shadow mask sheet 1 has been drape drawn,
it is provided at its periphery with a skirt by bending over peripheral portions of
the shadow mask sheet 1. For this purpose the pressure ring 3 is lowered until it
bears freely on springs 12 (see Figure 5). The shadow mask sheet 2 is then no longer
clamped at its periphery between the pressure ring 3 and the draw ring 4, but is clamped
between an ejector 13 and the draw die 2. The ejector 13 also comprises a copper block
14 with heating elements 15 so that the shadow mask sheet 1 contacts the ejector 13
which is also heated to between 150°C and 250
°C. The draw ring 4 is then lowered further so that a rim 16 of the skirt 22 becomes
clamped between the draw ring 4 and the pressure ring 3 (Figure 6). The pressure ring
3 and the draw ring 4 then compress the springs 12 over a readily defined distance
while the rim 16 of the skirt 22 remains clamped. The skirt 22 is thereby stretched
and consequently plastically deformed. The springs 12 are located directly below the
faces of the draw ring 4 and pressure ring 3 between which the shadow mask sheet 1
is clamped. The number of springs 12 is chosen so that the pressure force of the draw
ring 4 is distributed uniformly between said springs 12. The back pressure provided
by the springs 12 when the skirt 22 is formed is equal to the pressure with which
the pressure ring 3 and the draw ring 4 are lowered. The rim 16 must be sufficiently
wide to ensure permanent good clamping. The springs 12 are then relaxed by raising
the ejector 13 (Figure 7). After the forming of the skirt 22 the draw ring 4 is raised
and lifts the shadow mask 1 with it. Finally, the shadow mask 1 is ejected from the
draw ring 4 by the ejector 13 (Figure 8) and discharged. If the shadow mask 1 is removed
rapidly from the draw die 2 the shadow mask 1 may be deformed as a result of this
rapid raising because the shadow mask 1 is heated. In order to prevent this the shadow
mask 1, while still clamped between the pressure ring 3 and the draw ring 4, can be
raised slowly by means of a lever system 17 to a position some distance above the
draw die 2. As a result of this the possibility of deformation of the shadow mask
1 is reduced. The lever system 17 may be operated by means of compressed air, for
example. After the shadow mask 1 has been raised slowly to said position above the
draw die 2, the shadow mask 1 can be rapidly raised further.
[0014] It is to be noted that the operating members for the draw ring 4, the pressure ring
3 and the ejector 13 are not shown in the drawings since these do not directly relate
to the invention.
[0015] In addition to the said possibility of plastically deforming the skirt 22 substantially
simultaneously with the formation of the skirt 22, by which plastic deformation a
flange 18 is formed, it is also possible to deform the skirt 22 plastically after
completion of the drawing process. This may be done at room temperature although in
that case the mechanical workability is impeded by the comparatively high 0.2 % proof
stress. When said plastic deformation is carried out at a temperature between 150°C
and 250
°C, the tensile stress at which the 0.2 % proof stress is reached is reduced and the
mechanical workability is increased.
[0016] The shape given to the skirt 22 by the plastic deformation need not be restricted
to an outwardly directed flange 18 as described. When after termination of the drawing
process the skirt 22 is plastically deformed, this may be done in various manners.
If, however, it is desired for the plastic deformation of the skirt 22 to take place
substantially simultaneously with the formation of the skirt 22, according to the
embodiment of the invention the form of said plastic deformation is restricted to
the outwardly directed flange 18.
[0017] A shadow mask 1 manufactured by means of the method according to the invention is
shown in Figure 9. As a result of the permanent plastic deformation during the formation
of the skirt 22 of the shadow mask 1, the shape of the shadow mask 1 has become readily
reproducible. When the preferred form of a method in accordance with the invention
is used an outwardly directed flange 18 has moreover been formed on the skirt 22.
The width of the flange 18 corresponds to the width of the rim 16 which is still clamped
between the draw ring 4 and the pressure ring 3 at the end of the drawing process.
As already said, the rim 16 must be sufficiently wide to ensure a good clamping and
consequently a readily reproducible shape of the shadow mask 1. On the other hand
it is not advisable to make the flange 18 too wide because in that case an amount
of excessive material is obtained. The width of the flange 18 should preferably be
between 1 and 5 mm.
[0018] If it is desired to secure the shadow mask 1 to a supporting frame 19 as is shown
in Figure 10, for example by means of spot welding the outwardly directed flange 18
constitutes an obstruction. It is of advantage to make a few recesses 20 in the rim
of the shadow mask sheet 1, for example, by etching the recesses during the etching
of the mask holes 21. Due to these recesses 20, which are present in the flange 18
after the drawing process, the welding apparatus during welding can reach the skirt
22 of the shadow mask 1 without any obstruction at the area of the recesses 20. The
width of the recesses 20 should be adapted to the width of the spot welding apparatus.
Excessively wide recesses result during the formation of the skirt 22, in variations
in tensile stress in the shadow mask 1 between those places of the skirt 22 which
are in the elongation of the recesses 20 and that part of the skirt 22 which is present
between the recesses 20. This would detrimentally influence the shape of the shadow
mask sheet 1. The width of the recesses 20 should preferably be between 16 and 16
mm. When selecting the depth of the recesses 20 the same two arguments should be considered
which played a role in the selection of the width of the recesses 20. A depth of the
recess 20 which is at least substantially equal to the flange width is to be preferred.
1. A method of drape drawing, by means of a drawing process, a shadow mask sheet for
a colour display tube consisting of an iron-nickel alloy of the invar type, in which
a skirt is formed on the shadow mask sheet, characterized in that in addition to the
formation of the skirt, said skirt is plastically deformed permanently.
2. A method as claimed in Claim 1, characterized in that the permanent plastic deformation
of the skirt takes place at a temperature between 150°C and 250°C.
3. A method as claimed in Claim 1 or 2, characterized in that the plastic deformation
of the skirt takes place at least substantially simultaneously with the formation
of the skirt, and that a flange is formed on said skirt.
4. A method as claimed in Claim 3, characterized in that prior to the formation of
the flange recesses are provided in the shadow mask sheet, said recesses being formed
at least for the greater part in that part of the shadow mask sheet which afterwards
constitutes the flange.
5. A method as claimed in any of the preceding Claims, characterized in that an alloy
which contains 35 to 37 % by weight of nickel is chosen for the iron-nickel alloy
of the invar type.
6. A shadow mask manufactured by means of the method as claimed in any of the preceding
Claims.
7. A colour display tube comprising a shadow mask as claimed in Claim 6.
1. Verfahren zum Tiefziehen eines aus einer Eisen-Nickel-Legierung vom Invar-Typ bestehenden
Lochmaskenblechs für eine Farbbildröhre mittels eines Ziehvorgangs, bei dem am Lochmaskenblech
ein Kragen geformt wird, dadurch gekennzeichnet, daß ergänzend zur Formung des Kragens
dieser Kragen bleibend plastisch verformt wird.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die bleibende plastische
Verformung des Kragens bei einer Temperatur zwischen 150 und 250°C erfolgt.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die plastische Verformung
des Kragens wenigstens nahezu gleichzeitig mit der Formung des Kragens erfolgt und
ein Flansch an diesem Kragen geformt wird.
4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß vor der Formung des Flansches
Ausnehmungen im Lochmaskenblech angebracht werden, die wenigstens zum größten Teil
in jenem Teil des Lochmaskenblechs geformt werden, der später den Flansch darstellt.
5. Verfahren nach einem oder mehreren der vorangehenden Ansprüche, dadurch gekennzeichnet,
daß als Eisen-Nickel-Legierung vom Invar-Typ eine Legierung mit 35 bis 37 Gew.% Nickel
gewählt wird.
6. Lochmaske in der Herstellung mit dem Verfahren nach einem oder mehreren der vorangehenden
Ansprüche.
7. Farbbildröhre mit einer Lochmaske nach Anspruch 6.
1. Procédé d'emboutissage, au moyen d'un processus d'étirage, d'une feuille de masqué
d'ombre pour un tube-image en couleur, constituée d'un alliage de nickel-fer du type
Invar, dans lequel la feuille de masque d'ombre est munie d'une jupe, caractérisé
en ce qu'additionnellement à la formation de la jupe, ladite jupe est soumise à une
déformation plastique permanente.
2. Procédé selon la revendication 1, caractérisé en ce que la déformation plastique
permanente de la jupe a lieu à une température située entre 150°C et 250°C.
3. Procédé selon la revendication 1 ou 2, caractérisé en ce que la déformation plastique
de la jupe a lieu au moins pratiquement simultanément avec la formation de la jupe
et en ce que ladite jupe est munie d'un rebord.
4. Procédé selon la revendication 3, caractérisé en ce qu'avant la formation du rebord,
des évidements sont réalisés dans la feuille de masque d'ombre, lesdits évidements
étant formés au moins en majeure partie dans la partie de la feuille de masque d'ombre
qui, par la suite, constituera le rebord.
5. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce
qu'un alliage contenant 35 à 37 pour cent en poids de nickel est choisi pour l'alliage
de nickel-fer du type Invar.
6. Masque d'ombre fabriqué au moyen du procédé selon l'une quelconque des revendications
précédentes.
7. Tube-image en couleur comportant un masque d'ombre selon la revendication 6.