The invention relates to a nip press belt for a wet press or a
calender, in particular in a paper, cardboard or tissue
machine, according to the precharacterizing clause of Claim 1.
A nip press belt of this kind, in German also called Preßmantel
(translatable as "press jacket"), is used in paper, cardboard
or tissue machines to extract most of the water from the
product concerned (wet press) or to finish the surface
(calender). Such presses or calenders have an elongated press
gap ("nip") and are therefore also called "nip presses".
Within the nip the press belt makes sliding contact, by way of
its back (lower) surface, with the actual pressing element;
therefore this back surface must have excellent sliding
properties. On its front (upper) surface the press belt makes
contact either with a pulp felt (by way of which it indirectly
contacts the product concerned) or - in the case of a calender
- directly with the product, against which it is pressed by a
rotating roller.
In addition to the above-mentioned sliding properties of the
back surface, another important factor is the impermeability of
the belt to liquid, because water should not pass from the
product or pulp felt to the pressing element, nor may lubricant
from the latter enter the product or the felt. Furthermore,
certain elasticity and flexibility characteristics are
required.
Nip press belts of various designs are known in the state of
the art.
For example, the patent EP 0 420 372 B1 describes a nip press
belt of this generic kind with a basic web in the form of an
endless loop covered on its inner and outer surfaces with a
smooth polymer-resin coating, which makes the web impermeable
to liquids and endows it with a uniform thickness. The polymer
resin that forms the elastomer is here specified as
polyurethane.
The patent DE 50 20 005 C1 discloses another band for use in
paper machines, in particular wet presses with elongated nip
(nip presses). The band has on its back surface a smooth,
flexible band layer that is impermeable to liquids, and on the
front surface there is a carrier tread with a fiber layer
attached thereto.
The patent DE 42 02 731 A1 also discloses a belt, here termed
"jacket", of this generic kind for a nip press, which comprises
an elastomeric jacket material and two layers of reinforcing
threads. As specification of the jacket material, reference is
made here on one hand to material capable of swelling, and on
the other hand to polyurethane.
In WO 95/16820 a paper-machine web - specifically a nip press
belt - is described in which a carrier web with a polymer
coating is provided, which comprises a thixotropic material
(for example, aramid or silica glass). The base material
specified here, again, is polyurethane.
Furthermore, the patent DE 44 38 354 A1 discloses another press
jacket made of elastomeric material, in which is embedded
substantially parallel to the surface a woven layer of an
extremely stable material, for example aramid fibers.
In EP 0 258 759 a wet press for extracting the water from
strips of fibrous material is described, which likewise
comprises a nip press belt (here called "continuous belt"), and
this document also discusses appropriate parameters for an
elastomeric or rubber coating of this belt as well as the
possibility of vulcanizing or pouring the elastomeric material
onto the armoring of this belt.
The document EP 0 939 162 describes a nip press belt for paper
machines with a multilayered construction, which also comprises
specially oriented or wound textile layers and is said to be
extraordinarily stable both in the machine direction and in the
direction transverse thereto.
A nip press belt of this kind that has been further developed
with respect to its elasticity properties and an advantageous
multidimensional bending behavior is described in the
applicant's patent EP 1 251 203 A1. This more recent nip press
belt is distinguished by being made of soft rubber with
particular properties and, in advantageous embodiments, by the
special structure of a textile reinforement layer. This new
belt, based on soft rubber, is preferably used in the so-called
brown-paper market, but is also equally interesting for tissue
applications, because the soft-rubber strip is well adapted to
the movements of the so-called Yankee and promotes the
production of a voluminous tissue product.
In the context of these known solutions, it is the objective of
the present invention to disclose a nip press belt with
especially simple construction and hence low manufacturing
costs, which nevertheless fulfills all the demands made in
practice and achieves a long running time. In addition, at
least one appropriate manufacturing procedure for such a nip
press belt is disclosed.
This objective is accomplished with respect to the apparatus by
a nip press belt with the characteristics given in Claim 1, and
with respect to method by a manufacturing method with the
characteristics given in Claim 9 or 10.
The invention includes the fundamental idea that the nip press
belt is integrally designed so as to produce the necessary
profile of properties over its cross section, from the surface
that contacts the pressing element to the surface that contacts
the strip of fibrous material. It further includes the idea of
implementing the water-extraction function of the belt by means
of at least a superficial region of the carrier or
reinforcement layer, which also provides the required tensile
strength and is to a great extent embedded in the belt.
Thus the carrier web serves both to ensure the necessary
stability and resistance to deformation and also to extract
water from the strip of fibrous material, so that there is no
longer any need for a separate layer with special properties
for water extraction. Hence a special manufacturing step to
produce such a layer can be eliminated, which means that there
is a considerable potential for cost reduction. The integral
belt proposed here is suitable for all nip-press applications.
The integral basic web can be constructed as a stitched-felt or
continuous fabric, and can be single- or multi-layered,
depending on the intended use. Furthermore, the decision as to
whether the components of the basic fabric should be twisted or
untwisted monofilaments, or a longitudinal/transverse
combination of these variants, is made according to the
specific purpose of the belt.
In a preferred embodiment of the proposed nip press belt a
substantial proportion, in particular in the range between 1/3
and 2/3, of the thickness of the integral carrier and
functional layer serves as the water-extraction profile for the
object being pressed, while the remaining fraction of the
thickness is fixedly connected to the surface on which the belt
slides. Also preferred is an embodiment with ca. 30-50% exposed
area (textile surface) on the water-extraction side (as "water-extraction
profile"). This feature advantageously enables the
surface facing the object to be pressed to contain none of the
water-extraction structure supplementarily impressed onto the
textile structure.
As far as the design of the sliding surface to achieve optimal
sliding properties is concerned, the first aspect to emphasize
is that it should be absolutely impermeable to water and oil.
Specific elastomer materials to be especially recommended are
polyurethane or a soft-rubber material. Alternatively, or in
combination therewith, a thermoplastic hair, in particular
PEEK, or a thermoplastic film can be employed in forming the
sliding surface.
A preferred embodiment of the method for manufacturing the
proposed nip press belt is distinguished by the fact that the
sliding layer is attached to a cylindrical carrier so that it
covers the circumference thereof, after which a spliced (or
continuous) textile material with shrinkage properties is
pulled onto the carrier, over the sliding layer, and the
sliding layer and textile carrier are together subjected to a
thermal treatment during which the textile carrier shrinks onto
the sliding layer and becomes intimately connected thereto.
In another form of the manufacturing method the spliced textile
material is stretched onto a cylindrical carrier and closed by
a stitched seam, and thereafter a sliding-layer material is put
onto the textile carrier - in particular, a thermoplastic
material is pressed onto the textile carrier during the
application of heat. In preparation for the latter procedure,
in particular the region of the seam is covered by a strip of
plastic film, in particular a Teflon strip, before the sliding-layer
material is applied.
Other advantages and useful aspects of the invention will be
apparent from the subordinate claims, as well as from the
following description in outline of an exemplary embodiment
with reference to the figures, wherein
- Fig. 1
- is a schematic drawing of the wet-press section of a
paper machine in longitudinal section, and
- Fig. 2
- is a simplified cross-sectional drawing of an
embodiment of the nip press belt of such a wet press,
constructed in accordance with the invention.
Figure 1 shows part of the nip of a wet press 1 (nip press) of
a paper machine with elongated nip. Opposite a pressing element
3 is disposed a press-roller 5, and in the gap between these
two, namely the nip 7, water is removed from a paper web 13
enclosed between the surface of the press-roller 5 and a nip
press belt 9 that slides along the pressing element 3, with a
pulp felt 11.
The nip press belt 9 must on one hand have elasticity and
flexibility such that it conforms as well as possible to the
curved surface of the pressing element 3 and exerts an elastic
pressure uniformly upon the paper web 13 (by way of the pulp
felt 11). On the other hand, it must be sufficiently stable to
withstand the high, long-term stress (tension, pressure and
vibration) in the nip 7 for a service time that is economically
acceptable.
A nip press belt 9 suitable to meet these demands is shown
(schematically) in cross section in Fig. 2. This belt is so
designed that it will remain stable in the long term at
temperatures above 100°C, and can tolerate brief temperature
peaks of up to ca. 130°C; its overall thickness is in the range
between 3 and 6 mm, and it has a hardness in the range between
5 and 30 P+J.
In the illustrated embodiment the nip press belt 9 consists of
a thick elastomeric sliding layer 9a, made for instance of a
thermoplastic resin or polyurethane, into which, while it was
still in the fluid state, a multilayered carrier fabric 9b has
been shrunk. Figure 2 should be regarded as a synoptic
representation, inasmuch as in the upper region various kinds
of multifilament threads 9c, 9c' and monofilaments 9d are
symbolized as a possible implementation of the threads in the
integral carrier fabric. The reference code OV designates the
open volume in the belt 9, namely the space formed by the freestanding
loops in the upper layer of the carrier fabric, which
constitutes the water-extraction profile on the felt side.
The belt 9 is preferably constructed so that the liquid or
viscous sliding layer is applied to a mandrel (or drum), after
which an overdimensioned textile fabric is pulled over it, and
finally this fabric is shrunk down to size by the application
of heat. The particular design of the fabric to be shrunk
and/or the choice of shrinkage temperature determines the force
that induces the shrinking and hence the degree to which the
continuously woven or spliced and seamed fabric can penetrate
the sliding-layer mass.
The hardened (or at least cross-linked until it has reached a
state in which it is suitable for handling) nip press belt is
subsequently removed from the processing mandrel, for example
by the application of compressed air. The process of pulling
the belt away from the mandrel is also facilitated by the fact
that prior to the application of the sliding-layer material,
the mandrel has been provided with a non-stick coating, by
covering it with one of the customary substances or by wrapping
a film around it.
The implementation of the invention is not restricted to the
construction disclosed and described above and the above-mentioned
manufacturing method, but is also possible in many
modifications that are within the competence of a person
skilled in the art.