EP0914966B1 - Improved lithographic layer for printing blanket and blanket incorporating the same - Google Patents

Description

The main subject of the present invention is a improved lithographic layer for blanket printing.

It also targets a blanket fitted with this layer.

We know that blankets for offset printing generally include a coated support one or more successive layers, themselves coated with an outer or printing layer commonly known as the lithographic layer.

In known printing blankets, the layer lithographic was in an incompressible material, such than nitrile rubber, so as to obtain a good offset printing quality.

However, such a lithographic layer of material incompressible hard still presents a number disadvantages.

It is obviously not able to absorb irregularities in the underlying layer which can lead to printing defects.

Also, when duplex printing a sheet of paper passing between two cylinders or blankets printing occurs due to the incompressibility of the lithographic layer, a repression of the material of this layer, which translated by beads on one side and on the other of the contact area of the two blankets, i.e. at the entry of paper into said contact area and at the exit the paper from this area.

These beads are all the more marked as the layer lithographic is thick, as is the case for the most of the lithographic layers used with blankets of the prior art.

Such beads produce, as we understand, printing defects on paper.

It should also be added that with a lithographic layer incompressible, i.e. a deformable layer but nevertheless rigid in compression so that it retains its volume, we are facing printing defects constituted by a slight color shift on the paper, which gives the printed image a certain blurring or color separation effect. One more times, this is due to the deformation of the layer lithographic generating beads which deform somewhat the paper to be printed.

It should also be noted that when manufacturing a incompressible lithographic layer, the finish of this layer, that is to say the polishing operation, leaves despite all the precautions taken, wavelets on the outer face of said layer, which, of course, can only cause defects to printing on paper.

WO 96/40528 describes a foam sheet and a printing blanket comprising said sheet, where the foam-like compositions are formed by mechanical skimming.

EP-A-222680 describes a foam layer comprising voids, usable for making printing blankets, said layer being sandwiched between two layers of fabric substrate.

The object of the present invention is to solve in particular the above problems and drawbacks by providing the lithographic layer a certain degree of compressibility which, contrary to all expectations, and whereas the incompressible lithographic layers of prior art were deemed to have the maximum printing advantages, gives said layer very important or even exceptional advantages.

For this purpose, the lithographic or printing layer according to this invention is made slightly compressible by including a multiplicity of voids in the elastomeric material constituting said layer.

This layer is further characterized in that the voids consist of pre-expanded microspheres or expandable in the elastomeric material.

According to another characteristic, the content of microspheres in the elastomeric material is included between about 0.5 and 10% by weight of the layer and preferably between 2 and 4% by weight of said layer.

The lithographic layer of this invention is still characterized in that the diameter of the microspheres in the elastomeric material of the layer is between about 10 and 30 microns.

The lithographic layer according to this invention has a thickness of less than 0.7 mm, and preferably less than or equal to 0.3 mm.

This invention further relates to a printing blanket comprising a lithographic layer corresponding to one and / or the other of the above characteristics.

This printing blanket can advantageously coat the shape of a sleeve.

But other features and advantages of the invention will appear better in the description detailed which follows and refers to the attached drawings, given only by way of example, and in which:

Figure 1 is a schematic sectional view cross-section of a blanket embodiment printing equipped with an outer layer or lithographic sleeve, according to the invention.

Figure 2 is a schematic sectional view, more large scale, lithographic layer.

Figure 3 is a schematic sectional view of the contact area of two lithographic layers incompressible according to the prior art.

Figure 4 is a schematic sectional view of the external face of an incompressible lithographic layer according to the prior art, after finishing by polishing this layer.

Figure 5 illustrates the displacement D (microns) in blanket surface, i.e. displacement of the lithographic layer, as a function of position P (millimeters) in the contact area between two blankets.

We see in Figure 1 a printing blanket 1 1 which, following the example shown, has the form of a cylinder or sleeve made up, from the inside to the outside, of a support 2 made of metal or other material rigid, of a compressible layer 3 into an elastomer cellular or not, and a lithographic layer 4 according to this invention.

The support 2 can have a diameter of about 200 mm and a thickness between 0.1 and 0.3 mm.

We could perfectly, without going outside the framework of the invention, use with the lithographic layer 4 other layers (not shown) with the layer compressible 3, that is to say, in general, any combination of layers between support 2 and lithographic layer 4. Alternatively, support 2 may be omitted or replaced by means equivalents.

In accordance with the invention, and as can be seen clearly on Figure 2, the lithographic layer 4, which is in one elastomeric material is made compressible by the inclusion of a multiplicity of voids in said elastomeric material.

This elastomeric material can be any type of rubber known in the art such as rubber vulcanizable nitrile or even combinations nitrile rubber / polysulfide. Also, the material elastomer could be a thermoplastic elastomer crosslinked or not.

The voids consist of microspheres 5 incorporated into the elastomeric material of the layer lithographic 4.

These microspheres 5 can be pre-expanded and incorporated for example in an elastomeric material thermosetting to form the layer lithographic 4.

One can also use microspheres which are expandable in the elastomeric material, i.e. which contain a solvent which, under the effect of heat at about 100-150 °, causes the expansion of microspheres. Such expandable microspheres can be for example incorporated into an elastomer thermoplastic which can be crosslinked or not.

Other means can be used to create voids in the lithographic layer 4. Thus, we can use a chemical layer expansion method elastomer during the curing cycle.

The content of microspheres 5 in the lithographic layer 4 made of elastomeric material constitutes approximately 0.5 to 10% by weight of said layer.

Preferably, a microsphere content will be used between 2 and 4% by weight of layer 4.

Furthermore, the diameter of the microspheres 5 in the layer 4 elastomer material will be between about 10 and 30 microns.

In general, the thickness E of the layer lithographic 4 with incorporated microspheres 5 sera less than 0.7 mm.

Preferably, the thickness E of the layer 4 will be less than or equal to 0.3 mm.

We will also observe that the Young's modulus of the layer lithographic 4 will preferably be located in the interval 1 to 8 megapascals.

The exceptional and / or unpredictable advantages of the lithographic layer 4 made compressible according to the invention will be explained in detail below, in referring more particularly to FIG. 5 which illustrates the results of simulations and tests carried out on the lithographic layer 4, compared to a layer classic lithography of the prior art, that is to say an incompressible layer with no voids.

First, it will be observed that the lithographic layer 4 slightly compressible according to this invention will absorb advantageously possible irregularities of any underlying layer, so that the printing surface will be improved.

The lithographic layer 4, due to its compressibility, will avoid the presence of beads in the contact area between two printing cylinders, i.e. the area where the paper is pinched between these cylinders. In other words, it will only occur significantly no deformation at the surface of the lithographic layer 4, contrary to what was the case, as seen in FIG. 3, with lithographic layers 10 of the prior art which, at the level of the contact zone Z, necessarily had beads B at the entrance 11 of the paper along the path X-X ', and at the outlet of paper in contact zone Z. Faults printing due to these beads B during the rolling of the blankets are therefore removed with the layer lithographic 4 of the invention.

In addition, the absence of beads and therefore of deformations of the lithographic layer 4 will avoid any deformation paper and will avoid any offset or cropping of the colors on both sides of the paper. all distortion of the printed image will therefore be advantageously avoided.

Also the lithographic layer according to this invention is more tolerant of variations in thickness without compromise print quality.

In addition, with the lithographic layers incompressible from the prior art, there were manufacturing problems of these layers during polishing finishing which, as we can see on figure 4, caused wavelets marked V on the surface of the lithographic layer. These wavelets were due to a bearing of the grinder on the outside of the layer, which caused the removal of small chips of material generating the V wavelets in question.

Contrary to that, with the lithographic layer 4 more or less compressible according to the invention, one obtains polishing, on the surface of the diaper, wavelets much smaller, if not nonexistent, due to the flexibility of the lithographic layer 4 on the one hand, and on the other hand, due to the presence of numerous microspheres 5 which constitute as many sites of fracture on the surface of layer 4, so that finally the V wavelets are practically eliminated.

It should also be noted that when finishing with polishing of the lithographic layer 4 according to the invention, due to the flexibility and compressibility of this layer, heat production will be reduced and machining will be easier.

Returning to FIG. 5, we see three marked curves J, K and L respectively. The curve J relates to a layer incompressible lithographic according to the prior art, the curve K relates to a compressible lithographic layer according to the invention and containing a void content or 4% microspheres, and curve L concerns a layer lithographic produced according to the principles of the invention with a microsphere content of 10%.

It can be seen in FIG. 5 that the contact zone has a total length of about 8 mm, since it stops respectively on the abscissa -4 and +4 mm.

The beads B (Figure 3) created on the surface of the layer lithographic according to the prior art result in displacement peaks marked J1 in FIG. 4 in entering and leaving the contact zone, these beads affecting in particular the flow of paper and creating printing faults as previously explained.

With a printing blanket comprising a layer lithographic 4 according to the invention having a content in 4% microspheres (curve K), it can be seen that the peak displacement marked K1 in FIG. 5 at the input and at exit from the contact area is very amplitude scaled down. In other words, the surface deformation of the blanket or more precisely of the lithographic layer at the entrance and exit of the contact zone is almost zero since the amplitude of peak K1 is reduced by at least minus a factor of 3 compared to J1 peaks.

It can also be seen in FIG. 5 that the deformation in compression at the center of the contact area is increased, but this has no influence on the paper since it is totally confined in the pressure zone and cannot be deformed in compression.

As for the curve. L visible in Figure 5 and corresponding to a lithographic layer 4 according to the invention and which is very compressible since it has a void or microsphere content of 10%, sees that the peak of displacement in entry and exit of the contact area that we see on curves J and K a totally gone. This means that the deformation in surface of the lithographic layer 4 at the entrance to the contact area is weak and negative.

We therefore see that with a lithographic layer 4 slightly compressible (curve K) we minimize considerably the amplitude of the bead at the entrance and at the exit of the contact zone. This bead disappears completely for a sleeve with lithographic layer very compressible (curve L). However, given that such a very compressible sleeve could present certain drawbacks in terms of mechanical strength of the lithographic layer and in terms of the holding chemical layer that could degrade, we will adopt a compressibility range of said layer lithographic to optimize the paper throughput and keeping the characteristics and advantages expressed previously. In other words, we will adopt for the lithographic layer according to the invention a content of voids constituting between about 2 and 4% by weight of said layer.

Of course, the invention is in no way limited to the mode illustrated described embodiment which has only been given as an example.

This is how the elastomeric material constituting the lithographic layer and its manufacturing process could be any.

The invention therefore includes all technical equivalents means described as well as their combinations if these are carried out according to the subject of the claims.

Claims (6)

1. Lithographic or printing layer of an elastomeric material for printing blanket, which layer is made slightly compressible by inclusion of a multiplicity of voids in the elastomeric material which constitutes said layer (4), characterized in that the voids are constituted by microspheres (5) that are pre-expanded or expansible in the elastomeric material.
2. Lithographic layer according to claim 1, characterized in that the amount of microspheres (5) in the elastomeric material is comprised between about 0.5 and 10% by weight of the layer (4) and, preferably, between 2 and 4% by weight of said layer.
3. Lithographic layer according to claim 1 or 2, characterized in that the diameter of the microspheres (5) in the elastomeric material of the layer (4) is comprised between 10 and 30 microns.
4. Lithographic layer according to one of the preceding claims, characterized in that it has a thickness (E) smaller than 0.7 mm and, preferably, smaller than or equal to 0.3 mm.
5. Printing blanket comprising an outer lithographic layer (4) according to anyone of claims 1 to 4.
6. Printing blanket according to claim 5, characterized in that it has the shape of a sleeve.

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