Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A flat structure permeable to liquid, and a method for
manufacturing such a structure
The present invention relates to a flat structure permeable
to liquid, in particular to a press felt used in papermaking
machines, and to a method for manufacturing such a
structure.
The structure a~cording to the invention is used primarily
as a press felt, but it can also be used as a press wire,
for example.
In a papermalc.tng machlne, water .is extracted .tn three
different sections, i.e. .tn the w.tre section by f.tltering,
in the press section by wet pressing, and in the dryer
section by evaporation. For economy in papermaking it is
important that the web is dewatered as thoroughly as
possible before it arrives in the dryer section.
In the wire section, i.e. the forming unit, of the
papermaking machine as much water as possible is extracted
from the stock by using a wire. By a wire is mean-t an
endless fabric through which water is filtered from the
stock. The wires are in general made of phosphor-bronze
or a synthetic fiber.
The fiber~web which has been formed in the wire section
of the papermaki.ng machine still has a high moisture
content, which is reduced in the wet press of the
papermaking machine by pressing the web, together wi.th one
or several press~elts, between rolls. The main function
of the wet pressin~ is thus to dewater the web, but its
functions also include compacting the web and strengthening
it in order to achieve sufficient wet strength. Sufficient.
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wet strength of the web is a prerequisite for the web
being capable of being moved, without breaking, from the
press section to the dryer section and further from one
drying drum to another. In the wet press it is possible
to use rolls of different types, such as plain,
perforated, grooved or coveredrolls. It is also possible
to use a press wire between the press felt and the roll
surface in order to improve the dewatering ability and to
eliminate marking of the paper by the roll.
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The primary function of the press felt is to receive the
water extracted from the web. It is also a functi.on of
the felt to support the web in the press and to protect lt
from being damaged in the press. The structure o the press
elt plays a crucial role in the final quality of the
paper. An effective press felt is required to have a good
dewatering ability, mechanical strength, a long usage
life,and an ability to contribute to producing unmarked
paper of an even quality.
In general, needled press felts having a high synthetic
fiber content are used. Typically these synthetic fibers
are polyamide, polyester and polyacrylic nitrile flbers.
In most felts there is a wad layer attached to the base
fabric. The properties of a completed felt can be adjusted
by using fibers of different fineness and of different
raw materials in the wad needled to the felt. It is
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combining different types of wad layers. In felts of this
type, achieving sufficLent smoothness of the felt is a
problem; endeavors have been made to improve the
smoothness of felts by, for example, needling a thin layer
; of finer fibers on the web-side surface of the elt.
The press felt has a crucial efect on dewatering, and
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therefore its quality is a very important factor. In
general ~he aim is to use uncompressible, stiff felts wi~h
an open stru~ture. However, a felt which is too open may
produce detrimental patterns in the web and an uneven
distribution t~f pressure o~h the f iblers c:1f the web .
Present-day felts made totally or in part of syn~hetic
fibers have a relatively poor abilitv to resist compression
and, ~onsequently, the~ ~ose $heir elasticity ~ld ~mpact,
and this decreases their dewatering ability. Also cl~gging
substance~ such as fibers promote the compaction of ~e
~tructure and thereby shorten the economical useful life
of the felt. ~ecause of the disadvantages prese~ted above,
particular attention must be paid to the mal~tenance of
the felts, and they must be replaced relatively often. In
preqent-day papermaking machines, contlnuous-worklng
felt-c~eaning systems are used; these cystems include
various felt-maintenance devi~s ~uch as pres~ure ~prays
and suction slits extendlng acro~s the wire.
: From U.S. Patent no. 4,446,187 there is known a structure used as a wire or a felt, a structure which comprises
a film part having, passing through the film, ducts
produced by laser ~eams, and a reinforcement part combined
with the film part, consisting of a fabric comprising
staple fibers. In a structure of this type the clogging
of the ducts and fabrics may also constitute a problem.
~he ob jec t of the present invention is to provide a f lat
~ structure permeable to l~quid, for use especially as a
:: : press~felt,:a structure which has a good dewatering
ability and a~ high mechanical strength, and by means of
which an even distribution Qf pressure is achieved at the
nip and the above-menti-oned disadvantages are avoided, It
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is also an object of the invention to provide a simple
structure which is easy to service, xequires little
maintenance, and has a long useful l:Lfe. These objectives
can be achi~ved by means of the structure according to the
invention, which is characterized in that it is made up
of a body made of a castable polymerj the body having
holes and possibly cavities formed during the casting
process, the diameter of these holes increasing
substantially progressively from one surface to the other
in such a way that the surface facing the web is
microporous and the other surface has substantially more
open pores, and possibly of a reinforcement material.
The body can be made from castable polymers suitable for
this purpose, preerably elastomers, such as casting
urethane. Casting urethane is a rubber-like material which
is also called urethane rubber. In addition to urethane
rubber if is also possible to use other liquid rubberS,
such as nitrile rubbers and silicone rubbers.
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The castable polymers, such as casting urethane, can
easily be blended with polymeric additives such as epoxy~
to form a uniform castable basic material.
The properties of the structure can, when so desired, be
modlfied in various ways.
Thus, the mechanical properties, such as stiffness,
hardness and~resistance to wear and flex, can be
adjusted by selecting a suitable polymer.
The strength~properties can be improved by means of various
reinforcement~materials such as glass fiber, chopped
fiber, monofilaments, multifilaments, andjor metal netting.
It is also possLble to use both~woven and non-woven
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reinforcement materials. Synthetic fibers such as
polyamide, aramide ( Kevlar ) a~d polyester fibers are
typical raw materials. The reinforcement materials are
preferably fitted inside the body.
Casting urethane, being a rubber-lik~e material, has good
mechanical-dynamic properties as the body material of ~he
structure, and, in addition, at the casting stage it is
hlghly reactjve, so that it ls easy to achieve a chemical
bond between it and most reinforcement materials.
~he dewatering ability of the structure can be improved
and its elasticity can be adjusted by forming in the
structure cavlties and/or pores of different shapes and
izes; these cavities and pores may be open or closed.
The open porousne~s may al80 be such that the pores are
progressiYely more open through the entire s~ructure.
It is also possible to form in the structure cavities
which interconnect holes, whereby a possibility for
internal equalization of hydraulic pressure ls produced
tn the horizontal direction.
Also the surface properties of the structure, as regards
friction and release, can be adjusted to be those desired.
The properties of the structure can also be modified by
forming a sandwich structure in which the different layers
behave mechanically ln different ways.
The web-side surface of the structure according t~ the
invention is very smooth and microporous, and the
dewatering ability of the structure is good. By means of
the~structure according to the invention, a very even
pressure is also achieved at the nip, and this even
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pressure contributes to the formation of paper of even
and good quality. In addition, when the structure according
to the invention is used, elastic covering of the rolls
is not necessary.
The invention also relates to a method for the manufacture
of a structure permeable to water, especially a press felt
for use in papermaking machines, which method is
characterized in that the raw materials of a polymeric
material are applied to an endless base which has conical
members projecting substantially perpendicularly from the
surface of the base; thls procedure is, when necessary,
followed by curing, whereafter the structure is detached
from the base.
The applying of the raw materlals to the base is preferably
done by casting, spreading or spraying. The base must in
general be treated with a lubricant so that the completed
structure can be easily detached from the base.
According to one preferred embodiment the endless base is
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fitted between two rolls.
According to another embodiment the endless base is fitted
~round a large-diameter roll. The cicrumferential surface
of a large-dlameter roll can also constitute the endless
base.
The lnvention is described in greater detail below with
reference to the accompanying drawing, in which
Figure 1 depicts a longitudinal section of the structure
according to one embodi.ment~of the invention,
Figure 2 depicts a top view of the same structure as~Figure
Figure 3 depicts a longitudinal section of th structure
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according to another embodiment,
Figure 4 depicts a longitudinal section of the structure
according to one further embodiment,
Figure 5 depicts a longitudinal section of the structure
according to one further embodiment, and
Figure 6 depicts a longitudinal section of an apparatus
used for the manufacture of the structure according to
the invention.
In Figure 1 the flat body made of castable polymer is
indicated by reference numeral 1. The body 1 is preferably
made of urethane rubber. The body 1 has conical holes 2.
The web-side surface 3 (Figures 1 and 2) ls microporous
and very smooth. The other surace, for its paxt, has
pores which are clearly more open. Owing to thls structure,
~he structure according to the invention, when used as a
press felt or press wire, has a good dewatering ability
and, furthermore, the holes 2 do not get easily clogged.
Figure 3 depicts a structure similar to that in Figure 1,
except that it has been reinforced with a reinforcement
material 4. Various synthetic fibers, threads and fabrics,
glass fi.bers, and metal wires and nettings are among
suitable reinforcement materials.
Figure 4 depicts a structure similar to that in Figure 1,
except that the structure has pores 5. The pores may be
open or closed pores. Open pores make the structure more
"sponge-like",~ thereby improving its dewatering ability.
Closed pores, for their part, improve the elasticity
properties of the structure.
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Figure 5 depicts~a structure similar to that in Figure 4,
except ~hat~the~structure has cavities 6 which interconnect
the holes, and cavities which open into the holes.
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Only a few embodiments according to the invention are
described abo~e, but it is evident that these embodiments
can be combined and modi.fied within the scope of the
claims. Thus, for example, the structure according to the
invention may both be reinforced and contain pores.
With reference to Figure 6, two methods are described
below for the manufacture o~ a structure according to the
invention. A belt-like mold 7 is stretched between two
rolls 9, the surface of the mold having conical spikes 8
the shape of which corresponds to the shape of the holes
desired ln the structure. The raw materials of a polymeric
materlal are applied to this "mat of nails" to produce a
structure of desired thickness. The raw material can be
applied by casting, spreading,or spraying. For the final
curing of the material, a heat treatment is usually
necessary. The completed structure is detached from the
"mat of nalls".
If it is desired to reinforce the structure, the applying
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of the raw material is carrled out in layers, the
reinforcement material being placed between layers.
If it is desired to i.ncrease the porosity of the structure,
suitable chemicals, such as volatile substances or
water-soluble sub?tances, are added to the polymeric
material during the manufacturing process, these substances
forming, under the effect of heat or water, pores in the
structure. One example of such water-soluble substances
is common salt, the grain size of which determines the
size of the pores.
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