Note: Descriptions are shown in the official language in which they were submitted.
~Z835~
This invention relates to a supporting member for a
papermaking screen used in paper machines, and more particularly,
as a member for contiguously supporting the continuously moving
papermaking screen.
Both the prior art and the invention will be described
by way of example with reference to the accompanyïna drawings,
in which:-
Figures 1 (~) and (s) are schematic drawings of papermachines known i~ themselves to which members according to the
invention have been applied;
Figure 2 is a longitudinal sectional view showing
an embodiment wherein the members accordi.ng to the invention
are applied to a forming board;
Figure 3 is a schematic illustration or an abrasion
tester with the use of which the performance both of the member
according to the invention and of conventional members is
tested;
Figure 4 is a graphical representation showing the
correlation between both the abraded loss amount of the screen
obtained by the same tester as in Figure 3 and the qualities
of materials of supporting members of various sorts; and
Figure 5 is a graphical representation showing the
correlation between the surface roughness and the test time for
each of the screens tested.
The long screen type paper making machine shown in
Figure l(A) is so designed that the paper material;'namelv
slurry consisting of paper pulp and water, is supplied from a
head box 1 onto the screen (principally: made of metal~ 2
which is driven by rollers 8,9 in the direction of the arrow
shown in the figure. The watery paper pulp 3 is dehydrated
progressivelyon the screen 2 with the aid of the tractive force
of rollers 12,13 positioned just u.nder the screen 2. These are
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followed by treatment involving rolling, drying, and so on.
With a view partly to effect dehydration and partly to support
the screen 2, the paper machine includes a forming board 4,
a deflector 5, a foil 6, and a suction box 7, arranged
contiguously just under the screen 2 in succession, and furtner
tension rollers 10,11. Another kind of paper machine, known
as a twin wire machine is shown in Figure l(s). This kind is
so designed that the two screens 2,2' are driven by a driving
roller 8 and tension rollers 10,11, while paper pulp 3, after
having been supplied from the head box 1, is compressed from
both sides between the two screens 2,2' supported by ~he support-
ing members 14 and is dehydrated by the suction box 7. The
resulting dehydrate is treated by rolling and drying.
Now, in these paper machines, the supporting
structures such as the forming board 4 and the foil 6 are
provided for the purpose of contiguously supporting the screen 2.
.~s shown in Figure 2, the forming board 4 in fact comprises a
plurality of supportinJ members 4' extending transversely to
the running direction of the screen 2. These members have been
~0 previously made from ceramic obtained from a sintered body of
alumina or silicon carbide and have worked reasonably satisfactor-
ily. However, -these conventional members bring about a large
degree of abrasion on the screen 2 running incessantly over
them.
In the latest high speed paper machines the use
of conventional apparatus composed of a conventional~forming
board, deflector, table roll, and suction box does not allow
sufficient dehydration to take place which leads to a
deterioration in the quality of the paper. To overcome this
problem, the foil is used both for the purpose of supporting
the screen and effecting dehydration. However, the foil
deteriorates considerably due to abrasion and the efficiency
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of dehydration which has a bad influence on the quality of
paper. This is alsa true of the supporting member provided on
the suction dehydrator, such as a suction box. ~urthermore,
previousl~v known supporting members made of alumina ceramic,
silicon carbide or the like, also tenaed to cause substantial
abrasion of the screen itself.
Accordingly, the present invention provides a support-
ing member for a papermaking screen in a paper machine, said
supporting member being adapted to extend transversel~ to the
running direction of the papermaking screen and comprising a
sintered body consisting essentially of silicon nitride having
a porosity of approximately 0.8% or less.
The invention will now be described in more detail, by
way of example only, with reference to the accompanying drawings
introduced above.
A sintered body consisting essentially of silicon nitride
has a number of characteristics such as good high-temperature
strength, excellent heat shock resistance, abrasion resistance,
and so on, which has led to it being widely used in some special
fields as heat resistiVe material. However, silicon nitride it-
self, which is a sintering substance having strong covalent bond,
is hard to sinter in a dense manner using the simple substance
along, unlike alumina ceramic. To achieve this, it has been
found desirable to add a sintering aid. The composition of sili-
con nitride used ~n an example in accordance with the present
invention is as follows:
Si3N4 = 74.0%
FeSi ~ 6.0%
Fe c 1.5%
Fe2 3
To such a compound of silicon nitride is added some
metal oxide as a sintering aid, such as, for example, MgO.
- 3 -
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335;2
Further, after adding ~o this material an adequate quantity of
binders of various kinds and then moldiny it into a desired
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: - , ,', .
352
shape, it is sintered in the range of 1400 to 1700C in an
atmosphere of N2 gas. The resultant sintered body is in the
state where, except its main phase ~-Si3N4, unreactive
ferrosilicon and metal oxide produced in the sintering process
remain in crystal or non-crystalloid form. The properties of
both sintered body of silicon nitride thus obtained and alumina
ceramic heretofore used as the supporting member of the screen
are listed in contrast in the following Table 1.
Table 1
material aintered body alumina-
~~-___ of ceramic
item ~ silicon nitride
___ _
appearance dense dense
.
coloration dark graywhite
__
bulk specific gravity 3.3 3.8
porosity (%) 0.3 0
__ __ __ ,
bending strength kg/mm2 25 31
_ _ __.____
Rockwell hardness HRA 87 87
~oung's modulus (x106) kg/cm2 _ 3.5
linear expansion coefficient 3.5 7.7
~0 (X-6)/C _
A number of experiments conducted with the use of
such a sintered body of silicon nitride wi:Ll now be more
particularly described. Figure 3 is a schematic illustration
of an abrasion tester of the screen in order to measure the
performance of the screen supporting member according to the
invention. Figure 4 is a graphical representation showing
the correlation between the abraded auantity of the screen due
to the abrasion tester and the supporting members different
in the quality of material each time. Figure 5 is a graphical
representation showing the correlation between the surface
roughness and the test time relatively to each screen different
from one another in the quantity of material.
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The abrasion tester M in question is shown
schematically in Figure 3, wherein the paper-making screen 2
having one end fixed and the o-ther end attached to a constant
weight A, is suspended on the roller R made of the same quality
of material as the supporting member of the screen, and wherein
the nozzle N allowing pulp materials 3' to fall on the screen
is provided above the roller R. The roller R is made to
continuously rotate in the direction of the arrow while the pulp
materials 3' fall onto the screen. This arrangement allows
the abraded condi.tion of the screen 2 in contact with the surface
of the above roller R to be investi.gated. The conditions of
the abrasion test conducted simulating the running of the screen
2 over the supporting members in paper machines are as follows:
(a) rotational frequency of roller : 1500 rpm
(b) circumferential speed thereof : 283 m/min
(c) concentration of raw material : 2 tulk %
(d) delivery rate of material : 0.6 Q/min
(e) tension of screen 2.5 kg/cm
(f) width of screen : 40 mm
(g) sort of screen :
(g-l) bronze 60 - mesh
(g-2) chrome plated 65 - mesh
copper alloy
(g-3) plastic 60 - mesh
(h) material of roller :
(h-l) alumina ceramic
(h-~j sintered body essentially of silicon
nitride
(i) shape of roller : 60 ~ x 60 long (mm)
(j) surface roughness of roller : Rmax = 4
(both alumina ceramic and sintered
body essentially of Si3N4~
~;~ 1~
" ~283~2
(k) abraded amount of screen :
This is expressed by calculating the loss
in weight of the screen per unit time
before and after the test.
The test results obtained under the foregoing
conditions will be given in Figure 4. Incidentally, each lines
given in both Figures 4 and 5 demonstrate such relations to the
quantities of rollers and screens as are shown in the following
Table 2.
Table 2
.... , . ___
material of
~~ ~ roller sintered body of
~-___ alumina ceramic silicon nitride
material of
screen
_ . _ ._
bronze _ o x
___ _ _ _
plastic _ _ _ O _ _ _ _ - - - x - - -
~_~__ _ _
chrome plated copper - o - . - ~ - x - .
. alloy
. .. . , _~_~.~
In Figure 4, the bronze screen and the chrome plated
copper allov are screen arranged in order of the magnitude of
the abraded amount. In either case, when the material forming
the roller R is alumina ceramic, the abraded amount of the
screen is greatest, while on the contrary when the sintered
body is made of silicon nitride the abraded amount is much
smaller. This indicates that the degree of the abrasion of the
running screen is smaller when using a supporting member made
of silicon nitride than is the case with conventional supporting
members formed of alumina ceramic.
Table 3 presents the results obtained from the
investigation as to what kinds of effect the difference in
porosity of the sintered body of silicon nitride has on the
2`~ amount of abrasion.
:i.
.,.
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Table 3
nd of sa~ple Our Invention Prior
item ~ - - A A~ B C
~ul~cspecific gravity 3.4 3.3 2.9 3.8 2.7
' . _ ___
xnding stren~th (kg/mm ) 30 25 37 31 25
. ~
?~c ell hardness 87 87 87 87 82
~oung6s m~du~us
10(x10 kg/cm ) :2.5 2.5 2.5 3.5 2.3
. _
Linear expansion 6
~oefficient (x10 /C) 3. 7 3.5 2.6 7.7 3.0
... . _ _ .
~orosity (%) 0. 05 0.3 0.8 0 1.1
~ u~
o ~ bronze 0.5 0.75 0.851.2 1.46
~,rt, __
chrcqne copper alloy n .03 O.10 0.15 0.25 O.5
plastic 0.1 O. 2 0.4 1.0 2.48
Note: The composition of silicon nitride in examples A' and A"
is the same as the before-mentioned (referred to as A).
Only the porosity is different, as shown in the table.
Sample B is the case of the well-known alumina ceramic.
Sample C is a silicon nitride composition having greater
porosity of the samples A, A', A".
From this table, it is apparent that the amount of abra-
sion becomes less as the porosity decreases. Optimum results are
obtained when the porosity is less than 0.8~.
Separately from the above, using the same tester, the
affect of the surface roughness of the roller made of silicon ni-
tride as the abrasion of the screen has been studied. As the re-
sult, the following facts have been realized, as apparent fromFigure 5:
(I) With the lapse of time the surface roughness becomes
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progressively smaller, displaying the practically
stable rou~hness.
(II) The above (I) brings about a reduction of the
abraded amount of the screen,
(III) Although there is some difference more or less
dependlng on the variation in the quality of ma-
terial, it is desirable for the surface roughness
of the roller R able to achieve a value (I) that
it is under 4~.
Description will now be d~rected to an example wherein
the sintered body of silicon nitride according to the invention
has been applied to a twin wire-type paper machine shown in Figure
l(B)-
As seen from the Figure, paper pulp coming down from
the head box 1 first is caught between the two running screens 2,
2', a~d then is dehydrated by the action of the suppor~ing members
14 designed so as to compress the respective screens 2,2' alterna-
tely from both sides. The dehydrating operation is t~en furthered
by the suction hox 7. In this case, the paper machine is operated
under the following conditions: the running speed of the screen
is 700 m/min, the material of the screen 2 is bronze, that of the
screen 2~ is plastic, respectively, and that the tension applied
to each screen is 4 kg/cm. In this way paper o Einq quality
(70 g/m ~ was actually produced.
In this connection, a sin-tered body of silicon nitride
was used as the supporting member 14; which compressed the screen
2 made of bronze at its part nearest the headbox 1 (however, in
the other part was used ultramacro=molecular polyethylene). The
useful life of the b~onze screen 2 was seven (7) days in the case
of conventional suppor~in~ members made o alumina ceramic.
Contrary to this, by adopting the above~mentioned new member the
lie of the screen could be increased to nine (9) days.
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It is now e~ident from the above description that the
present invention has many excellent advantages. Tn particular,
the life of the papermaking screen is increased without being
scarcely abraded irrespecti~e of the quality of material, and
it is possible to supply paper of fine quality at ].ow cost and
stability for a long time.
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