CA2145965A1

CA2145965A1 - Press blanket

Info

Publication number: CA2145965A1
Application number: CA002145965A
Authority: CA
Inventors: Udo Grossmann; Uwe Matuschczyk; Harald Aufrecht
Original assignee: Individual
Current assignee: Voith Sulzer Papiermaschinen GmbH
Priority date: 1994-04-02
Filing date: 1995-03-30
Publication date: 1995-10-03
Also published as: FI109816B; JP4072791B2; DE4411620C2; JPH07305291A; FI951540A; DE4411620A1; FI951540A0

Abstract

A press blanket for a pressing device intended for extracting liquid from a pulp web in the extended press nip of a papermaking machine, comprises an inner layer that faces away from the pulp web and consists of an elastomeric matrix material, in which reinforcing threads are embedded and on which an outer wear-resisting layer is provided on the surface facing the web. The wear-resisting layer consists preferably of the same matrix material as the inner layer and is reinforced by embedded wear-resisting particles consisting of rock dust, ceramic powder or glass frit. During production, the elastomeric material is poured onto a casting mold while the reinforcing threads are wound into the material, and the second layer is poured onto the first layer before the latter has cured so that an especially good bond is obtained between the two layers.

Description

21~5965 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.

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 21~ ~ 9 6 5 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 21 4~6~

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 21459~i~

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, ~ 21~96~

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.

Claims

1. Press blanket for use in a nip press for extracting liquid from a web, said press blanket comprising:
- an inner layer facing away from the web and made from an elastomeric material, said inner layer comprising reinforcing threads; and - an outer layer provided on said inner layer and facing the web, said outer layer being configured as a wear-resisting layer made from a matrix material substan-tially similiar to the matrix material of the inner layer and being reinforced by wear-resisting particles embedded therein.

2. Press blanket according to claim 1, wherein said matrix materials of said outer and said inner layers are the same.

3. Press blanket according to claim 1, wherein said wear-resisting particles are selected from a group comprising rock dust, ceramic powder and glass frit.

4. Press blanket according to claim 1, wherein said reinforcing threads comprise longitudinal threads extending substan-tially transversely to a direction of movement of said web, and cross-threads extending substantially transversely on said longitudinal threads and also extending into a marginal area of said press blanket.

5. Press blanket according to claim 4, wherein said wear-resisting layer extends only throughout a pressing area of the press blanket passing directly through an extended nip of said press and being enclosed by two lateral marginal areas.

6. Method for producing a press blanket for use in a nip press for extracting liquid from a web, said method comprising the steps of:
(a) providing a casting mold having an elongate form and being symmetrical to its longitudinal axis;
(b) rotating said casting mold about its longitudinal axis and simultaneously pouring a first layer of a first elastomeric material onto said casting mold;
(c) simultaneously to step (b) winding reinforcing threads into said first layer;
(d) pouring a second layer consisting of a mixture of a second elastomeric matrix material and wear-resist-ing particles onto said first layer before said first layer is cured.

7. Method according to claim 6, wherein said first and said second matrix materials are substantially similiar.

8. Method according to claim 6, wherein said first and second elastomeric materials are the same.

9. Method according to claim 6, wherein said wear-resisting particles are homogeneously mixed with said second matrix material.

10. Method according to claim 6, wherein said wear-resisting particles are selected from a group comprising rock dust, ceramic powder, and glass frit.