Note: Descriptions are shown in the official language in which they were submitted.
t
5702
SYLVEBTER ESCHMANN
PRE~B FELT
The invention concerns a press felt for the pressing region of
a papermaking machine with a belt of support material consisting of
one or more fabrics or knits and comprising a layer of fibers
deposited at least on the paper side and connected to the belt of
support material, the surface of this layer of fibers forming the
side in contact with the paper.
Press felts are known from many publications, illustratively
from US patents 4,283,454 and 4,564,985. Both documents disclose
a press felt which on the side away from making contact with the
paper comprises a belt of support material consisting of two
superposed fabrics. These two fabrics are connected together by a
stitched layer of fibers, these fibers being deeply stitched into
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the two fabrics. The upper side of the layer of fibers forms the
paper contact-side.
Moreover press felts are known of which the belts of support
material consist of crosswise superposed filaments (European patent
document A 0, 038, 276j , where called for a layer of fibers also
being provided between the individual crosswise superposed
filaments (US patent 4,781,567j. The particular uppermost filament
netting extends longitudinally. Contrary to the case of the fabric
or knit support materials, the individual filament layers are not
connected to each other.
The US patent 1,651,476 discloses a press felt wherein a layer
of fibers is bonded onto a belt of support material in the form of
a fabric, the bonding being implemented by a coating of adhesive
deposited on the belt of support material. The bonding coating is
porous and accordingly is permeable to the liquid pressed out of
the paper web.
The problem of remoistening the paper web after it leaves the
pressing gap arises with press felts. The previously compressed
press felt then increases in cross-section. The water moves along ,
the higher capillarity and spreads over the entire cross-section of
the press felt as far as the paper contact-side. In this manner
part of the water already removed from the paper web returns to it.
To avoid such problems, the US patent 4,772,504 proposes a
press felt of which the paper contact-side is formed by a surface
layer deposited on the felt layer. This surface layer consists of
a plastic, for instance a foam, of which the pores are nearly
entirely filled by a sealing material. Accordingly the stated
maximum values of surface-layer permeability are minute and
approach zero. The purpose of the surface layer is to more impede
the water in the felt toward the paper contact-side than to the
other side and thereby to prevent remoistening the paper web.
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It has been overlooked in this press felt design that
its main purpose is to move the squeezed liquid though it and
that it must be accordingly permeable. If there is virtually
complete sealing of the paper contact-side, a corresponding
drop in dehydration performance must be expected -- which
cannot be compensated by the claimed reduction in remoistening.
On the whole the dehydration performance is not improved by
such a press felt over conventional ones, whereas its
manufacturing cost however is higher.
The object of the invention is to so further develop
a press felt of the initially cited kind that its dehydration
shall be improved, in particular by reducing the paper web's
remoistening.
This problem is solved by the invention in that the
permeable layer is designed to be a blocking layer covering 45
to 85% of the surface and shall comprise transverse, elongated
passages.
According to one aspect of the present invention,
there is provided a press felt for a papermaking machine,
comprising: a. a lower fabric layer comprised of a plurality of
interlaced longitudinal and cross filaments; and b. an upper
fabric layer overlaid on said lower fabric layer and having an
upper paper contacting surface, said upper fabric layer
comprising a blocking layer intermediate said lower fabric
layer and said paper contacting surface and said blocking layer
comprising a plurality of mutually spaced blocking layer
filaments extending transverse to the direction of advance of
the felt and a substantially continuous passage is disposed
between adjacent filaments of said blocking layer for
permitting water to flow therethrough to said lower fabric
layer while preventing water from flowing therethrough to said
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3a
upper fabric layer and said blocking layer disposed parallel to
said paper contacting surface.
According to another aspect of the present invention,
there is provided a paper machine press felt assembly,
comprising: a. a lower fabric layer comprised of a plurality of
interlaced longitudinal and cross filaments; and b. an upper
fabric layer overlaid on and stitched to said lower fabric
layer and having an upper paper contacting surface, said upper
fabric layer includes a blocking layer disposed parallel to
said paper contacting surface and comprising a plurality of
mutually spaced plastic filaments extending transverse to the
direction of felt advance and a plurality of soluble threads
extending parallel to the direction of felt advance so that,
after said soluble threads have been dissolved, a substantially
continuous passage is disposed between adjacent ones of said
plastic filaments for permitting water to flow therethrough to
said lower fabric layer while preventing water from flowing
therethrough to said upper fabric layer.
According to still another aspect of the present
invention, there is provided a press felt as described herein
wherein the blocking layer filaments cover from about 45% to
about 85~ of the upper fabric layer.
In the invention, the species-forming press felt is
designed as a blocking layer reducing the flow cross-section to
about 15 to 55~ of the press-felt area. As a result, the water
flowing through the press felt will be accelerated in the
vicinity of the blocking layer by the nozzle-effect of the
passages. Consequently a pressure drop propagating as far as
the paper web arises in the region between the paper contact-
side and the blocking layer and uniformly sucks the water out
of the paper web. In especially important manner, the passages
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are longitudinally elongated and transverse. In this way they
act like foils used in the forming zone of papermaking machines
and support the above mentioned suction effect. In spite of
the surface cover entailed by the blocking layer, dehydration
is improved.
After it leaves the pressing gap, the blocking layer
practically acts like a check valve. The felt will expand
again, however on account of the blocking layer it can only
attract little air or moisture from the zone underneath the
blocking layer. The press felt therefore does not expand as
fast, and as a result, it will
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not prematurely detach from the paper web. Thereby the remoistening
is reduced to a minimum. On the whole therefore the press felt of
the invention offers substantially better dehydration.
The blocking layer can be deposited directly on the belt of
support material. Once the layer of fibers has been stitched into
the belt of support material, it still shall be inside the layer of
fibers. However it is possible also to mount the blocking layer a
distance from the belt of support material, the distance between
the blocking layer and the paper contact-side appropriately at most
being as large as the distance to the belt of support material, so
that preferably the blocking layer shall be located in the vicinity
of the paper contact-side but not at its surface.
Preferably the passages are transverse elongated slots and are
as continuous as possible. The slots are formed by mutually spaced
cross filaments representing the blocking layer. Especially
advantageously the cross filaments shall be flat, for instance as
disclosed in the European patent O,p98,612. The above cited foil
effect then shall be especially pronounced.
The press felt can be manufactured in such a way that first a
fabric is formed by said cross filaments and by means of dissolv
ing, especially water-soluble longitudinal filaments and in that
after the making of the press felt, the longitudinal filaments
shall be dissolved using a solvent. Thereupon the cross filaments
forming the blocking layer shall remain.
The drawing elucidates the invention by means of an illustra-
tive embodiment.
Fig. 1, is a cross-section of a press felt, and
Fig. 2 is a longitudinal section of the press felt of Fig. 1.
The press felt 1 shown in the Figures comprises a support
fabric 2 in its lower region which consists of two layers of
longitudinal filaments illustratively denoted by 3 and of cross
filaments illustratively denoted by 4 which lace together the
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longitudinal filaments of both layers. The longitudinal filaments
3 and the 'transverse filaments 4 are monofilaments. Suitable
materials are plastics, for instance polyamides or polyesters.
A layer of fibers 5 is present at the top side of the support
fabric 2. The free side of this layer forms the paper contact-side
6, A blocking layer 7 consisting of a plurality of flat filaments
illustratively denoted by 8 which are transverse to the direction
of advance of the press felt 1 is inserted into the upper half of
the layer of fibers 5. The blocking layer 7 is parallel to the
l0 paper contact-side 6. The flat filaments 8 also consist of plastic
and their cross-section is approximately rectangular, their
extension in the plane of the blocking layer 7 being significantly
larger than perpendicularly to it. The flat filaments 8 are spaced
in such manner that slots illustratively denoted by 9 are created
between them. The total cross-section of these slots 9 shall be
between 15 and 55 % of the total surface of the press felt 1 in the
design of this invention.
The press felt 1 of the invention can be manufactured as
follows: First the support fabric 2 is woven. Then a first layer
of non-woven fibers is deposited on the support fabric 2 and
stitched to it. At the same time the blocking layer 7 initially in
the form of a fabric is prepared by interlacing flat filaments 8
with water-soluble longitudinal threads illustratively consisting
of polyvinyl alcohol. This fabric blocking layer 7 is deposited on
the first part of the layer of fibers 5. Then a further layer of
non-woven material is deposited on the blocking layer 7 and is
stitched to the remaining part of the press felt 1. Lastly the
press felt 1 is subjected to water treatment ensuring the dissolu-
tion of the longitudinal filaments of the blocking layers 7. Only
the flat filaments 8 remain. Now the press felt 1 assumes the
design shown in Figs. 1 and 2.
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In the vicinity of the press gap the flat filaments 8 assure
the deflection of water at the flat filaments 8 and its being
accelerated near the slots 9. The effect that ensues is similar to
that from foils in the forming region of papermaking machines. The
water moves faster than the press felt 1, so that in practice
suction is generated in the vicinity of the paper contact-side 6.
A vacuum is generated above the blocking layer 7 when passing
the pressing gap because the previously compressed press felt 1
will expand again, but hardly any air or moisture can follow from
below. Above the blocking layer 7, the press felt 1 therefore no
longer Fills with water. Because the press felt 1 expands only
after a delay, it detaches earlier from the paper web 1 whereby the
residual moisture absent above the blocking layer 7 no longer can
be introduced into 'the paper web.
