EP0448469A1 - Elastic and compressible blanket forming element - Google Patents

Abstract

The present invention concerns an elastic and compressible blanket- forming printing element. This element essentially comprises a lithographic or printing layer (1), a hard elastomer layer (2), a layer of cellular rubber (3), a layer of fabric (4) and another layer (5) of cellular rubber forming a base layer whose modulus of elasticity in compression is identical to or less than the modulus of elasticity of the layer (3). This printing element or blanket applies to all types of offset printing machines. <IMAGE>

Description

The present invention essentially relates to an elastic and compressible printing element, this element constituting what is called a blanket and can be fixed on the surface of cylinders that are found in printing machines.

Most printing blankets currently used in printing machines essentially comprise a lithographic layer allowing the transfer of information from the blanket cylinder onto a strip of paper, and a compressible structure consisting of a layer of cellular rubber taken up sandwich between two fabric layers.

However, the compressibility of such a blanket, that is to say its ability to deform under the effect of a force exerted by the machine on the blanket cylinder bearing on another blanket cylinder, by example, remains limited. Consequently, a relatively large effort will have to be exerted on the blanket and this effort will result, in the long run, in a sagging of the blanket which will no longer resume its initial thickness or geometry. In addition, it should be noted that, in modern and fast printing machines, there will be overheating favoring the sagging of the blanket, and, obviously, the quality of the printing of the strip of paper passing between the two cylinders carries -blanchet and the scrolling of this strip will be strongly affected.

In addition, with such blankets which are not very compressible and which generate high pressures for a given interval between the two cylinders blanket holder, there is a significant risk of shearing. More precisely, the material which constitutes the blanket tends to move laterally in a way to form protuberances, which is highly detrimental to the print quality of the paper strip and to the correct running of this strip.

Document FR-A-2 461 596 has also proposed a blanket essentially comprising a lithographic layer, a layer of cellular rubber and a stabilization or base layer composed of several layers of tissue adhered with neoprene.

But, here again, such a blanket is likely to generate high pressures in the interval between two cylinders carrying the blankets, while this should be avoided so as not to alter the structure of the blanket, and such a blanket can also present risks of shearing, that is to say of lateral displacement of the blanket material, which leads to the drawbacks mentioned above.

Also, the present invention aims to remedy all these drawbacks by proposing a blanket which will have no tendency, when compressed, to cause lateral displacements of the material which constitutes it so that the printing quality and the scrolling of the paper strip will remain satisfactory, and which will always exhibit a neutral dynamic behavior under the effect of a compression force.

To this end, the subject of the invention is an elastic and compressible printing element of the type comprising a lithographic or printing layer with which is associated a structure with compressible layers, characterized in that said structure consists of at least one fabric layer or the like sandwiched between two layers of cellular rubber having identical or different compressibility properties.

According to a preferred embodiment, the layer of cellular rubber situated on the side of the lithographic layer has a higher modulus of elasticity in compression than the other layer of cellular rubber.

According to another characteristic of the invention, the layer of lower modulus of elasticity is associated with a base layer of fabric or other reinforcing material.

According to another embodiment, the layer of lower modulus of elasticity itself constitutes the base layer.

The elastic and compressible printing element according to this invention also comprises a layer of hard and possibly reinforced elastomer, which is interposed between the lithographic layer and the layer of cellular rubber closest to this lithographic layer.

According to yet another characteristic of the invention, the thickness of the fabric layer or the like is between approximately 0.1 and 1 mm, and the thicknesses of the two cellular rubber layers are each between 0.1 and 0, 8 mm.

According to a preferred embodiment, the thickness of the tissue layer or the like is 0.35 mm, the thickness of the cellular layer with a lower modulus of elasticity is 0.5 mm, and the cellular rubber layer has higher modulus of elasticity is 0.45 mm.

It will also be specified here that the thickness of the abovementioned layer of hard elastomer is between approximately 0.5 and 0.05 mm, and is preferably equal to 0.15 mm.

According to yet another characteristic of the printing element according to this invention, the two above-mentioned cellular rubber layers have a modulus of elasticity of between approximately 0.2 and 50 megapascals (MPa) for the layer of modulus of elasticity plus high, and between about 0.1 and 25 MPa for the lower elasticity layer.

According to a preferred embodiment, the elastic modulus of the layer of cellular rubber located on the side of the lithographic layer is 10 MPa, while the elastic modulus of the other layer of cellular rubber is 5 MPa.

It will also be specified here that the percentage by volume of the gas enclosed in the two layers of cellular rubber, preferably in closed cells, is between approximately 10 and 80%, and is preferably equal to 30% for the layer of higher modulus of elasticity and 35% for the layer of lower modulus of elasticity.

The invention also relates to the cylinders equipped with a printing element forming a blanket meeting the above characteristics.

However, other characteristics and advantages of the invention will appear better in the detailed description which follows and refers to the appended drawings, given solely by way of example, and in which.

Figure 1 is an enlarged sectional view of a printing blanket according to the invention.

Figure 2 is an enlarged sectional view of another embodiment of the blanket according to this invention.

FIG. 3 schematically and partially illustrates two blankets according to this invention, in the compressed state, and carried respectively by two cylinders between which a strip of paper passes, the assembly being related to a reference system with axes of orthogonal coordinates where l he abscissa axis coincides with the direction of the paper strip and where the ordinate axis intersects the abscissa axis in the center of the compressed area of the two blankets.

FIG. 4 represents three curves illustrating, in millimeters, the lateral displacement of the material constituting the blanket on either side of the center O of the compression zone - visible deformation in FIG. 3, as a function of the distance, from this point O and along the abscissa axis OX in FIG. 3, these three curves corresponding respectively to a blanket as commonly used today and which will be called hereinafter blanket of the prior art, a blanket according to document FR-A-2 461 596, and a blanket according to the invention.

FIG. 5 also represents three curves illustrating the relative pressure in the interval between blankets, for the three blankets above, as a function of the position, on the axis of the abscissae OX, relative to the center O of the pressure zone .

According to the embodiment shown in FIG. 1, an elastic and compressible element or printing blanket according to this invention successively comprises an external lithographic or printing layer 1, a layer of hard elastomer 2, a layer of cellular rubber 3, a layer of fabric or the like 4 and another layer 5 of cellular rubber.

In the variant shown in Figure 2, there are in the same order, the layers 1 to 5 mentioned above, but here, the blanket also has a base layer 6 of fabric or other suitable reinforcing material.

When fixing the blanket to a cylinder (not shown), the base layer 6 will be applied to the peripheral surface of this cylinder, while, in the case of the blanket of FIG. 1, the base layer applied to the cylinder consists of the layer of cellular rubber 5.

In the two embodiments shown in FIGS. 1 and 2, the layers of cellular rubber 3 and 5 can be identical or different in terms of their elastic modulus in particular.

Preferably, the layer 3 located on the side of the lithographic layer 1 will have a higher modulus of elasticity in compression and between approximately 0.2 and 50 MPa, while the layer 5 forming the base layer (FIG. 1 ) or associated with the base layer 6 (FIG. 2) will have a lower modulus of elasticity between approximately 0.1 and 25 MPa. Thus the modulus of elasticity of the layer 3 can be equal to 10 MPa, and the modulus of elasticity in compression of the other layer 5 of cellular rubber can be equal to 5 MPa.

It will be observed here that the difference in elastic modulus between the two layers of cellular rubber 3 and 5 can be obtained by varying the percentage by volume of the gas enclosed in the two layers, preferably in closed cells, as shown diagrammatically on the figures. Thus, the percentage in question may be equal to 30% for layer 3 of higher elastic modulus, and 35% for layer 5 of lower elastic modulus.

As for the thicknesses of the two cellular rubber layers 3 and 5, they can each be between 0.1 and 0.8 mm. We can thus provide a thickness of 0.45 mm for layer 3 and a thickness of 0.5 mm for layer 5.

The thickness of the fabric layer or the like 4 sandwiched between the two cellular rubber layers 3 and 5 may be between approximately 0.1 and 1 mm and for example be equal to 0.35 mm.

As for the thickness of the layer 2 of hard elastomer, it will be between approximately 0.5 and 0.05 mm and will preferably be equal to 0.15 mm.

But, for a better understanding of the invention, the interest and the exceptional advantages of the blanket of the invention will be explained below compared to a blanket of the prior art, that is to say a blanket of the type commonly used today, and a blanket according to document FR-A-2 461 596.

Reference is made to the table which follows and which states the various layers of the three blankets above and for which the same thickness has been adopted so that the results are comparable.

More precisely, we see on this table, from the left, that the first column indicates the thicknesses in millimeters of the layers. In the second column appear the successive layers of a blanket according to the invention and as shown in FIG. 2. In the third column appear the successive layers of a blanket according to the prior art, which comprises, as mentioned at the beginning from this description, a layer of cellular rubber interposed between two layers of tissue, while the blanket of the invention comprises, conversely, a layer of tissue interposed between two layers of cellular rubber. Finally, in the fourth column of the table, there are the blanket layers according to document FR-A-2 461 596 which, it should be remembered, comprises only a single layer of cellular rubber and a base layer formed by several layers of fabric bonded with neoprene. In the three cases, as seen on the table, a hard elastomer layer is associated with the lithographic layer.

The compression behavior of the three blankets above has been studied with regard to the blanket material which is driven out or displaced, during compression, to the right and / or left of the center O of the pressure zone, as materialized by the arrows F in Figure 3.

Reference will be made to FIG. 4 which represents the displacement of material or the shear C as a function of the distance D relative to the center O of the pressure zone, and this for the three blankets of the table above. It can be seen immediately that the curve C1 which corresponds to the blanket of the invention is substantially flat, that is to say that practically no lateral movement of the blanket material according to the invention does not occurs, under compression, unlike the blanket of the prior art (curve C2) and the blanket according to document FR-A-2 461 596 (curve C3), which however has a less marked tendency to shear than the blanket according to the prior art (curve C2).

Thus, it is understood that with the blanket of the invention, the print quality and the movement of the strip of paper P passing between two blanket cylinders (FIG. 3) will be very greatly improved due to the absence of lateral displacement or surface shear. This means that the blankets of the invention will have a neutral dynamic behavior on rolling on the strip of paper P.

Furthermore, it should be observed that for a distance AA ′ given by the machine (see FIG. 3), it is advantageous to have a lower generated pressure. In other words, it is advantageous to obtain a gain in compressibility so as in particular, as explained at the beginning of this description, to avoid collapse in the long run of the blanket.

This gain in compressibility is clearly illustrated by the curve C4 in FIG. 5 corresponding to the blanket according to this invention. This curve shows that the gain in compressibility compared to the blankets of the prior art (curve C5) and the blankets according to document FR-A-2 461 596 (curve C6) is significantly improved, the latter however having a certain gain in compressibility, compared to the blankets of the prior art (curve C5).

It follows from all of the above that the blanket according to the invention is remarkable in particular from the point of view of the gain in compressibility and the absence of shear, which has an advantageous effect on the print quality of the paper, on the throughput or the flow of the paper, and on the longevity of the blanket, this essentially thanks to the provision in said blanket of a layer of sandwiched fabric between two layers of cellular rubber having identical or different modulus of elasticity.

Of course, the invention is in no way limited to the embodiments described and illustrated which have been given only by way of example.

On the contrary, the invention includes all the technical equivalents of the means described as well as their combinations, if these are carried out according to the spirit.

Claims (11)

Elastic and compressible printing element of the type comprising a lithographic or printing layer (1) with which is associated a structure with compressible layers consisting of at least one fabric layer or the like (4) associated with a layer (3) of cellular rubber, characterized in that said fabric layer (4) sandwiched between two layers (3,5) of cellular rubber having identical or different modulus of elasticity. Element according to claim 1, characterized in that the layer (3) of cellular rubber situated on the side of the lithographic layer (1) has a higher modulus of elasticity in compression than the other layer (5) of cellular rubber. Element according to claim 2, characterized in that said other layer (5) of lower modulus of elasticity is associated with a base layer (6) of fabric or other reinforcing material. Element according to claim 2, characterized in that the layer of lower modulus of elasticity (5) itself constitutes the base layer. Element according to one of claims 1 to 4, characterized in that it comprises a layer of hard, and possibly reinforced, elastomer (2), interposed between the lithographic layer (1) and the layer (3) of cellular rubber. closer to the lithographic layer (1). Element according to one of claims 1 to 5, characterized in that the thickness of the fabric layer or the like (4) is between approximately 0.1 and 1 mm, and the thicknesses of the two layers of cellular rubber (3 , 5) are each between 0.1 and 0.8 mm. Element according to claim 6, characterized in that the thickness of the fabric layer or the like (4) is 0.35 mm, the thickness of the layer of cellular rubber (5) with a lower modulus of elasticity is 0.5 mm, and the layer of cellular rubber (3) with a higher modulus of elasticity is 0.45 mm. Element according to claim 5, characterized in that the thickness of the said layer of hard elastomer (2) is between approximately 0.5 and 0.05 mm, and preferably equal to 0.15 mm. Element according to one of the preceding claims, characterized in that the aforementioned two layers of cellular rubber (3,5) have a modulus of elasticity of between approximately 0.2 and 50 megapascals (MPa) for the layer (3) of higher modulus of elasticity and between about 0.1 and 25 MPa for the layer (5) of lower modulus of elasticity. Element according to claim 9, characterized in that the elastic modulus of the layer (3) of cellular rubber located on the side of the lithographic layer (1) is 10 MPa, and the elastic modulus of the other layer (5) of cellular rubber is 5 MPa. Element according to one of claims 1 to 10, characterized in that the percentage by volume of the gas enclosed in the two layers (3,5) of cellular rubber, preferably in closed cells, is between approximately 10 and 80% , and is preferably 30% for the layer (3) of higher elastic modulus and 35% for the layer (5) of lower elastic modulus.

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