Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Backqround of the invention
A press blanket of this kind is known from US-A-4,978,428.
Press blankets of this kind are used especially in pressing
devices of papermaking machines in order to extract water from
a continuous web, i.e. a water-containing pulp sheet. Such a
pressing device comprises two press elements (for example two
press rolls or a press roll and a press shoe), that form together
a press nip. The pulp sheet, from which the water is to be
extracted, passes through this press nip, together with the
press blanket and, in some cases, together with one or two felts.
As a rule, the press blanket is endless and may have different
lenghts, in the traveling direction of the blanket. In the case
of long press blankets, as disclosed for example by GB-A-2 106
555, the press blanket runs about guide rolls outside the press
nip. If, however, the press blanket is relatively short in the
direction of movement, then it forms a hose-shape moving along
a substantially circular path outside the press nip.
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Press blankets of this kind are exposed to high stresses in
operation. In the press nip they are subjected to high pressures
and tensile stresses that may cause premature wear of the press
blanket. Especially on that surface of the press blanket, that
faces the web, considerable abrasion occurs. In the case of
conventional rolls this has the effect that the compression
of the press blanket in the press nip and, thus, the water
extraction efficiency will vary. For press blankets intended
for shoe presses, such circumstance will result in changes in
the distribution of pressure and also in variations in the water
extraction behavior.
In addition, grooved press blankets have become known (US-A-
4,559,106, W0-A-92/02678, US-A-4,478,428, US-A-5,062,924) which
are intended to improve the absorptive capacity in the area
of the press nip. Such grooved press blankets are likewise
subjected to the same stresses that have been described before,
so that as a result of the reduced strength brought about by
the grooves, and due to the higher tensile strains and shearing
stresses encountered in the area of the grooves, even a higher
degree of wear occurs in the area of the grooves, which is
connected with the before-mentioned disadvantages.
It was for this reason that the before-mentioned US-A-4,978,428
proposed an outer wear-resisting layer facing the pulp sheet,
whose hardness exceeds that of the next lower layer.
It has been found that in the case of such a design, where a
layer of greater hardness is arranged on a layer of lesser
hardness, there is a risk that the two layers may get detached
one from the other and, thus, a risk of destruction of the press
blanket.
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Summary of the Invention
It is an object of the present invention to provide an improved
press blanket which avoids the disadvantages of the prior art
and which, in particular, offers long service life and little
abrasion. A further object of the invention is to provide a
suitable method for producing such a press blanket.
With respect to the press blanket, this object is achieved by
the fact that the wear-resisting layer of a press blanket of
the afore-mentioned type is made from the same or a similar
matrix material as the inner layer and is reinforced by embedded
wear-resisting particles.
The fact that the wear-resisting layer consists of the same
or a chemically similar matrix material as the inner layer
ensures perfect bonding between the two layers so that any
detachment of the outer layer from the inner layer is avoided,
even in the presence of high stresses under continuous operation
conditions. By embedding wear-resisting particles in this matrix
material the wear-resistance of the compound so formed is greatly
increased, while the othermechanical properties, in particular
the elasticity and strength, are substantially determined by
the property of the matrix material. This requires of course
that the quantity of embedded wear-resisting particles must
be small enough so that altogether an embedding structure is
obtained whose matrix consists of the matrix material. If the
proportion of wear-resisting particles is below 50 ~ by volume,
that requirement will always be fulfilled. As a rule, an even
smaller proportion of wear-resisting particles will be added,
preferably a proportion in the range of between approximately
1 % by volume and 30 % by volume; the optimum mixing ratio can
be determined empirically and may be influenced by the conditions
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of use and the properties of the matrix material and the wear-
resisting particles embedded therein.
The matrix material used may, for example, be polyurethane,
while the wear-resisting particles may consist, for example,
of rock dust, especially granite dust, ceramic powder, especially
aluminium oxide power, or of glass frit.
According to an advantageous further development of the inven-
tion, the reinforcing threads comprise longitudinal threads
that extend transversely to the direction of movement of the
mat and on which cross-threads are provided that extend in spiral
shape substantially along the direction of movement of the mat
and up to the marginal area of the blanket.
While conventional press blankets, having their edges mounted
on carrier disks, should have the greatest possible flexibility
exactly in the marginal area, due to the necessity to mount
them on the carrying disks (compare DE-A-39 09 936), the
arrangement according to the invention provides for high strength
of the press blanket also in the marginal areas, in the direction
of movement of the mat, so that the press blanket according
to the invention can be driven advantageously by the carrying
disks in order to ensure improved running properties of the
mat in the press nip.
According to an advantageous further development of this
embodiment, the wear-resisting layer extends only over the
pressing area of the blanket, comprised between its two marginal
areas.
It is thus ensured that the blanket offers high strength in
the marginal areas in order to enable the blanket to be driven
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via the carrying disks, while on the other hand the necessary
reinforcement is provided only in the central pressing area
of the blanket in order to increase its wear resistance, so
that the elasticity in the marginal areas remains now as before
sufficient to secure the blanket on the carrying disks.
With respect to the method, the object of the present invention
is achieved by a method comprising the steps of providing a
casting mold having an elongate form and being symmetrical to
its longitudinal axis; rotating said casting mold about its
longitudinal axis and simultaneously pouring a first layer of
a first elastomeric matrix material, onto the casting mold,
during which process reinforcing threads are wound into the
material, and pouring a second layer on the first layer, before
the latter has cured, the second layer consisting of a mixture
of a second matrix material and wear-resisting particles.
The fact that the second layer is applied by pouring before
the first layer has cured guarantees a durable bond at the
interfaces between the two layers so that any detachment of
one layer from the other is excluded even under high stresses
in continuous operation.
According to an advantageous further development of this method,
the second matrix material and the first matrix material are
identical or chemically similar.
This feature further improves the bond between the two layers
so that i~ identical matrix materials are used for the two
layers, one practically obtains a consistent continuous layer,
except that the outer layer contains the embedded wear-resisting
particles which are, preferably, homogeneously dispersed
throughout the matrix.
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, .
It is understood that the features mentioned above and those
yet to be explained below can be used not only in the respective
combinations indicated, but also in other combinations or in
isolation, without leaving the scope of the present invention.
Brief DescriPtion of the Drawings
Further features and advantages will become apparent from the
following description of certain preferred embodiments of the
invention. In the drawings:
Fig. 1 shows a cross-section through a press blanket accor-
ding to the invention;
Fig. 2 shows a detail, in enlarged scale, of the press
blanket according to the invention, as shown in Fig.
l;
Fig. 3 shows a simplified representation of the press blanket
in the area of the press nip; and
Fig. 4 shows a simplified diagrammatic sectio~al represen-
tation of a device for producing a press blanket.
Detailed DescriPtion
In Figs. 1 to 3, a press blanket according to the invention
is generally indicated by reference numeral 10.
Fig. 3 is a diagrammatic representation of the conditions
existing at a press nip of a shoe press of conventional design,
where the press blanket 10 revolves between a lower press shoe
28 and a back-up roll 26, a web 30, being constituted by a pulp
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sheet containing water, being guided between the back-up roll
26 and a felt 32 that runs above the press blanket 10. Due to
a high pressure prevailing in the press nip 34, the water is
extracted from the pulp web 30 in the press nip 34 and is
absorbed by the felt 32.
According to Figs. 1 and 2, the press blanket 10 has the shape
of an endless hose 11, being reinforced in its central area,
the pressing area 12. The hose 11 consists of a first inner
layer 20 and a radially outer second layer 22 that is applied
only in the pressing area 12, while the inner layer 20 extends
over the entire cross-section of the blanket 10 up to and
including the two marginal areas 14.
The blanket 10 is reinforced by reinforcing threads 16, 18 in
a way that will be described in more detail further below.
The reinforcing threads 16, 18 of the blanket consist of radially
inner, mutually parallel longitudinal threads 16, facing away
from the pulp web 30 and extending at equal spacings in crosswise
direction between the edges of the blanket 10, and of cross-
threads 18 extending transversely thereto, i.e. substantially
in the direction of movement 36 of the pulp web, which cross-
threads are spirally wound above the longitudinal threads 16,
as will be described in more detail below, with reference to
Fig. 4.
Both the first layer 20 and the second layer 22 consist of an
elastomeric matrix material, preferably polyurethane, with
additional approximately 10 % by volume of particles 24,
consisting of granite dust, being embedded in the second layer
Z2.
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Before the matrix material of the second layer 22 is applied
by pouring, a liquid, homogeneous mixture of the matrix material
and approximately lo % by volume of granite dust 24 is prepared.
The production of the press blanket is then carried through
in a manner known as such, as described in detail by US patent
no. 5,134,010 which is incorporated by reference.
A device 50 for the production of the press blanket 10 comprises
a cylindrical casting mold 38 with two end faces between which
the longitudinal threads 16 are stretched in mutually parallel
arrangement and at a predetermined distance from the surface.
The polyurethane is fed from above through a first line 46
through a first casting nozzle 40, while the casting mold 44
is rotated about its longitudinal or central axis 52, in the
direction indicated by the arrow, and the casting nozzle 40
is advanced by means of a slide in a direction parallel to the
central axis 52 so that gradually a cylindrical press blanket
is produced by the continuous casting process. During the
process, the compound flows through the longitudinal threads
16 and down onto the casting mold 38, thereby forming the inner
layer 20 of the press blanket.
At the same time, a cross-thread 18 is spirally would into the
compound, resting on the outside of the longitudinal threads
16.
As a result of this process, the first inner layer 20 of the
press blanket 10 is produced.
According to the invention, the second outer layer 22 is
simultaneously applied by means of a second casting nozzle 42
arranged at a relatively small distance from the first casting
nozzle 40. This second casting nozzle 42 serves for applying,
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via a line 48, a pouring compound consisting of the same matrix
material as that applied through the first line 46, but mixed
homogeneously with 10 % by volume of granite dust.
Due to the fact that the second outer layer 22 consists of the
same matrix material as the first layer 20 and is applied on
the first layer 20 by casting before the latter has cured,
optimum bonding is obtained between the two layers 20, 22. At
the same time, the wear-resisting particles 24 embedded in the
second layer 22 guarantee high wear-resistance of the press
blanket lo.
