Patent Application
20130263749
This invention relates to a litho strip for electrochemical roughening consisting of a rolled aluminium alloy. The invention further relates to a method for the manufacture of a litho strip of this type, in which a litho strip consisting of an aluminium alloy is cold rolled and in which after the final cold rolling action the litho strip is subject to a degreasing process with a simultaneous pickling process in an aqueous pickling medium, wherein the aqueous pickling medium contains at least 1.5% to 3% by weight of a mixture of 5% to 40% sodium tripolyphosphate, 3% to 10% sodium gluconate, 3% to 8% non-ionic and anionic surfactants and optionally 0.5% to 70% soda and the sodium hydroxide concentration in the aqueous pickling medium is between 0.1% and 5% by weight. Finally, the invention also relates to a method for the manufacture of a printing plate carrier and its advantageous use.
Particularly high requirements are placed on the surface conditions of litho strips, in other words of aluminium strips for the manufacture of lithographic printing plates. Litho strips are generally subject to a process of electrochemical roughening, the aim of which is to ensure a complete roughening and an appearance of no structure. The roughened structure is important for placing a photosensitive layer on the printing plate carriers manufactured from the litho strips. In order to manufacture an equally roughened surface, a particularly flat surface is required on the litho strips. The topography of the litho strip surface is essentially an imprint of the rolling topography of the final cold rolling step. Elevations and recesses in the rolling surface lead to grooves and webs in the litho strip surfaces which can in part remain in the further processing steps in the manufacture of the printing plate carriers. The quality of the litho strip surfaces and therefore of the printing plate carries is therefore determined by the quality of the rolling surfaces and thus on the one hand by the grinding action when treating the surface of the rollers and on the other hand by ongoing wear to the rollers.
A measurement to determine the quality of the surface of the litho strip is the average roughness Ra in accordance with DIN EN ISO 4287 and DIN EN ISO 4288. In the current method for the manufacture of litho strips, surfaces with a usual average roughness value Ra of around 0.15 μm to 0.25 μm are generate in the final cold rolling step. These roughness values are sufficient for many areas of application.
However, in the past few years there has been an increasing demand for printing plates with very flat roughening structures and/or a relatively photosensitive coating. These are used, for example, in the field of CtP technology, which is increasing in size and in which the printing plates can be illuminated directly using a computer. Furthermore, the thickness of the coating is decreasing and its complexity is increasing. The number of printing errors is increasing when the currently available printing plate carriers are used in this way. A flat topography of the litho strip after rolling is therefore a quality criterion for litho strips which is becoming ever more important.
An attempt has been made to optimise the grinding of the rollers in order to obtain flat roller structures. However, the grinding actions have already been extensively optimised, so further increases in quality are very difficult to achieve in this way. Furthermore, the quality of the surfaces of the rollers decreases after grinding as a result of wear to the rollers, so it is frequently necessary to re-grind the rollers. Ultimately, very flat roller surfaces only exert a small amount of friction on the litho strip surfaces, potentially resulting in a slip between the roller and the litho strip which causes a disruption to the rolling process or damage to the litho strip.
Printing plate carriers for lithographic printing are known from EP 1 172 228 A2, EP 0 778 158 A1 and EP 1 232 878 A2.
In other known methods from the prior art from WO 2006/122852 A1 and WO 2007/141300 A1, the litho strips are pickled after rolling in order to remove damaging pockets of oxides on the surface of the strips and therefore to improve the subsequent electrochemical roughening. In this way, the surface quality of the printing plate carriers can in principle be improved, but the problem of printing errors mentioned above remains.
Starting from the prior art, the object of the present invention is to provide a litho strip and a method for its manufacture with which the above mentioned disadvantages from the prior art can be avoided or at least reduced.
This object is achieved according to the invention in a suitable litho strip by the strip surface having a topography with a maximum peak height Rp and/or Sp of a maximum of 1.4 μm, preferably a maximum of 1.2 μm, more preferably a maximum of 1.0 μm.
The topography of a strip surface means its deviation from an ideal plane. It can be described using a function Z(x, y), which indicates the local deviation from the average height of the surface at each point along the surface (x, y). In accordance with this, the average value of the function Z(x, y), in other words the position of the average surface, is set at 0, as shown by the following formula:
F is the size of the integration surface. Local elevations correspond to positive values and local dips correspond to negative values of Z (x, y).
The features of a topography of this type can be determined using various parameters. A usual parameter is the average roughness Ra or the average quadratic roughness Rq in accordance with DIN EN ISO 4287 and DIN EN ISO 4288. These parameters can be defined using the following equation:
Z(x) is a profile of the surface, in other words a one-dimensional section through the function Z(x, y). L is the length of the integration interval. In order to determine the surface quality of a surface, one-dimensional profiles Z(x) are generally measured by means of linear scanning at various locations on the surface and the corresponding Ra and Rq values are determined.
The values for Sa and Sq are determined on the basis of a two-dimensional measurement of the surface, in other words the topography Z(x, y). The calculation of the values Sa and Sq is made on the basis of the following equation, wherein A is the size of the integration surface:
In the context of the present invention, it was recognised that the printing errors which occur in the prior art are often caused by individual, particularly high roller webs, which in part remain in the manufacture of pressure plate carriers. When coating the pressure plate carriers, this can lead to interruptions in the photosensitive layer in the region of these roller webs, which in turn causes printing errors in the use of the completed printing plates. The high roller webs have proven to be particularly problematic in printing plates with a flat roughening structure and/or a relatively thin photosensitive coating.
The existence of individual high roller webs is, however, only insufficiently included in the parameters Ra and Sa used to date to characterise the litho strip surfaces. In contrast to this, the likelihood of high roller webs and, therefore, the occurrence of the above mentioned printing errors can be reduced by the litho strip and the method for its manufacture being optimised in terms of another, as yet unused roughness value. By limiting the maximum peak height Rp and/or Sp to a maximum of 1.4 μm, preferably a maximum of 1.2 μm, more preferably a maximum of 1.0 μm, litho strips can be provided which are sufficient for the current high requirements in terms of surface quality, for example when using CtP technology.
In order to determine the maximum peak height Rp of a litho strip, three positions on the litho strip can generally be measured transverse to the roller direction profile Z(x) over a length of, for example, 4.8 mm in each case, in order to determine a value for Rp. For each of these profiles, the following applies:
R
pmax(Z(x)), (4)
wherein the function max(Z) provides the maximum value of Z(x). Sp is determined using a surface measurement with the equation
S
p=max(Z(x,v)) (5)
wherein the function max(Z) provides the maximum value of Z(x, y). The surface to be measured can in practice be, for example, quadratic and have an edge length of 800 μm.
Preferably, a profile Z(x) is measured at each of the centre and the sides of the litho strip to determine the maximum peak height Rp.
It is understood that for the measurement of the profile Z(x) and the topography Z(x, y) only those regions of the litho strip which will later be further processed into printing plate carriers are taken into consideration. Damaged regions or regions with roller defects are, for example, not taken into account.
In a first embodiment of the litho strip, the surface of the strip has a topography with a reduced peak height Rpk and/or Spk of a maximum of 0.4 μm, preferably a maximum of 0.37 μm. It has been demonstrated that the quality of the strip surfaces in terms of a lack of printing errors can be further improved by an additional check of the reduced peak height Rpk and/or Spk.
The reduced peak height Rpk is determined in accordance with DIN EN ISO 13 5 65. The reduced peak height Spk is determined in accordance with DIN EN ISO 13 565 using a surface measurement. In practice, the profiles Z(x) and the topography Z(x, y) are measured as described above for Rp and Sp.
In a further embodiment, the thickness of the litho strip is 0.5 mm to 0.1 mm. It has been shown that conventional litho strips of a low thickness can have high roller webs. The surface quality of thin litho strips can, therefore, be improved in particular by limiting the maximum peak height Rp and/or Sp or the reduced peak height Rpk and/or Spk.
In a further embodiment of the litho strip, good material features of the litho strips are achieved by the litho strip consisting of an AA1050, AA1100, AA3103 or AlMg0.5 alloy.
In a further preferred embodiment the litho strip has the following alloy compositions in percent by weight:
plus residual Al and unavoidable impurities, to an individual maximum of 0.05% and totaling a maximum of 0.15%.
In this way, the litho strip can be improved in a targeted manner for use in terms of its strength and heat resistance characteristics.
High resistance to bending and simultaneously very good thermal stability of the litho strip can be achieved in a further embodiment by the litho strip having the following alloy content in percent by weight:
In a further preferred embodiment the litho strip has the following alloy contents in percent by weight:
In this way, the roughening characteristics and the heat resistance of the litho strip can be improved.
In accordance with a further embodiment, the impurities in the alloy of the litho strip have the following threshold values in percent by weight:
Titanium can also be intentionally added for grain refinement up to a concentration of 0.04% by weight.
In a further teaching of the invention in a suitable method for manufacturing a litho strip in accordance with the invention, the above mentioned object is achieved by the surface erosion caused by degreasing treatment with simultaneous pickling being at least 0.25 g/m2, preferably at least 0.4 g/m2.
It is recognised that the high roller webs on the litho strip surfaces which cause disruptions can be reduced after the final cold rolling step by a specific degreasing treatment. Pickling treatments to remove pockets of oxides are known, the targeted removal of roller webs was not previously known. By the special selection of the pickling and degreasing medium and the process parameters, it is now possible, however, to achieve a topography of the litho strip surface either instead or additionally which has a much lower susceptibility to errors than the litho strips previously known due to high roller webs. Since the degreasing treatment with a pickling step is a very critical process for litho strips, the method requires a very careful selection of process parameters. In particular, the composition of the pickling medium and the pickling temperature and duration should be set such that during the degreasing treatment with pickling, surface erosion of at least 0.25 g/m2 is achieved on the litho strip surfaces. In this way, a topography of the litho strip surfaces can be achieved with a maximum peak height Rp and/or Sp of a maximum of 1.4 μm, preferably a maximum of 1.2 μm, more preferably a maximum of 1.0 μm.
Surface erosion means the weight of the litho strip removed during the degreasing treatment and pickling per surface. In order to determine the surface erosion, the litho strip is weighed before and after the degreasing treatment with pickling. The loss in weight calculated as a result divided by the size of the surface treated gives the surface erosion. If both sides of the litho strip are subject to degreasing treatment with pickling, the surface on the front side and the rear side should subsequently be added.
A surface erosion of between 0.25 g/m2 and 0.6 g/m2, preferably between 0.4 g/m2 and 0.6 g/m2 has been shown to be particularly advantageous. In this way, the erosion is on the one hand high enough to reduce the high webs, and on the other hand does not reduce the thickness of the litho strip too much. In principle, however, the erosion should be kept as low as possible so the material loss during the degreasing treatment with pickling is as low as possible.
In a preferred embodiment of the method, the topography of the litho strip surface can be improved by the sodium hydroxide concentration in the aqueous pickling medium being between 2% and 3.5% by weight and optionally by the degreasing treatment with pickling taking place at temperatures between 70 and 85° C. for a duration of between 1 and 3.5 seconds. At these concentrations, temperatures and treatment durations, the topography according to the invention can be achieved in a particularly reliable manner.
A further improvement is achieved by the sodium hydroxide concentration in the aqueous pickling medium being between 2.6% and 3.5% by weight and/or the pickling temperature being between 76 and 84° C. This enables a shorter treatment duration with a removal of the high roller webs which is nevertheless homogeneous. A further improvement in the speed of the degreasing treatment with pickling of the litho strip can be achieved by the pickling duration being between 1 and 2 seconds, preferably between 1.1 and 1.9 seconds.
In accordance with a further embodiment of the method, the litho strip is rolled in the final cold rolling step to a final thickness of 0.5 mm to 0.1 mm. In this roller thickness which is preferably used, roller webs occur particularly frequently which can be reduced considerably through the degreasing treatment with pickling.
AA1050, AA1100, AA3103 or AlMg0.5 is used as an aluminium alloy in accordance with a further embodiment. These aluminium alloys have proven to be particularly advantageous for the features of the litho strips.
In a further embodiment of the method, the aluminium alloy has the following composition in percent by weight:
plus residual Al and unavoidable impurities, to an individual maximum of 0.05% and totaling a maximum of 0.15%.
The effect of the degreasing treatment with pickling is influenced by the alloy of the litho strip. It has been determined that with this alloy composition very good results in terms of the surface topography and simultaneously good material characteristics of the litho strips can be achieved with the selected process parameters for the degreasing treatment with pickling.
In further embodiments of the method, the aluminium alloy has the following alloy contents in percent by weight:
The impurities in the alloy of the litho strip have the following threshold values in accordance with a further embodiment:
wherein titanium can also be intentionally added for grain refinement up to a concentration of 0.04% by weight.
Reference is made to the relevant embodiment of the litho strip for the advantages of the preferred alloy compositions.
In a further embodiment of the method, the structural features of the litho strip can be improved by the litho strip being warm rolled before the cold rolling and optionally by a homogenisation treatment being carried out before the heat rolling and/or intermediate annealing being carried out during the cold rolling.
The above mentioned object is achieved in accordance with a further teaching of the present invention by a printing plate carrier having a topography with a maximum peak height Rp and/or Sp up to a maximum of 1.4 μm, preferably 1.2 μm, more preferably 1.0 μm. The printing plate carrier is preferably manufactured from a litho strip in accordance with the invention.
In a preferred embodiment of the printing plate carrier, said printing plate carrier has a photosensitive coating with a thickness of less than 2 μm, preferably less than 1 μm.
The high roller web in previous litho sheets led to printing errors, particularly with thin photosensitive coatings, so that in this case a particular improvement in printing plate quality is achieved. The printing plate carrier preferably has a transparent photosensitive layer which offers advantages on exposure. In these layers, the complete covering of the printing plate carrier can only be determined after printing, so faulty printing plate carriers are very costly. By improving the topography and as a result reducing the printing errors, the costs caused by printing errors can be reduced considerably.
The printing plate carrier can preferably have a width of 200 mm to 2,800 mm, more preferably 800 mm to 1,900 mm, most preferably 1,700 mm to 1,900 mm, and a length of 300 mm to 1,200 mm, more preferably 800 mm to 1,200 mm.
The printing plate carrier according to the invention can preferably be used in CtP technology, in other words for a CtP printing plate. In CtP technology, the surface structure of the printing plate carrier is particularly critical since the flat roughening structures and the relatively thin photosensitive coating can lead to increased numbers of printing errors with high roller webs. In addition to this, transparent photosensitive layers are frequently used in CtP technology, leading to the above mentioned problems. Due to the flat topography of the printing plate carrier in accordance with the invention as compared with printing plate carriers in the prior art, the printing quality can be improved and the costs reduced.
Further features and advantages of the present invention can be derived from the following description of embodiments of the litho strip in accordance with the invention and the method in accordance with the invention, in which the attached diagrams are taken into account, in which:
The left-hand region 2 of
The right-hand region 4 of
L is the length of the profile Z(x) measured, in other words the size of the definition region of Z(x). The integration region is the part of the total length to which the inequality Z(x)≧ZAF(Q) applies.
By placing a tangent 8 through the inflection point of the Abbott-Firestone curve 6, the points of intersection of this tangent 8 with the 0%-line 10 and the 100%-line 12 define a core region of the surface, the expansion of which is designated as the core roughness depth Rk. The averaged height determined of the peaks which extend out of the core region is designated as the reduced peak height Rpk and the averaged depth determined of the grooves which extend out of the core region is designated as the reduced groove depth Rvk. Furthermore, the maximum peak height Rp is also plotted in
The maximum peak height Rp and the reduced peak height Rpk can be determined in practice, for example, from profiles Z(x) measured at various positions of the litho strip transverse to the direction of rolling.
The reduced peak height Spk can in practice be determined accordingly from a known surface measurement. The calculation is made analogue to the reduced peak height Rpk, wherein the Abbott-Firestone curve ZAF (Q) for Spk can be defined implicitly using the following equation:
A is the size of the surface measured, in other words the size of the definition region of Z(x, y). The integration region is the part of the total length to which the inequality Z(x, y) ZAF(Q) applies.
plus an inflow of residual Al and unavoidable impurities, to an individual maximum of 0.05% and totaling a maximum of 0.15%.
The casting can generally be continuous or discontinuous, in particular it can be part of a continuous, semi-continuous or discontinuous casting process. In an optional step 24, the casting product, in other words in particular the cast ingots or the cast strip is subject to a further processing through a homogenisation treatment, for example in the temperature range between 480° C. and 620° C. for at least two hours. In the subsequent step 26 the casting product is optionally warm rolled, preferably to a thickness between 7 mm and 2 mm. Warm rolling can for example be foregone in a litho strip manufactured in a double strip casting process. The warm strip is then cold rolled in the step 28, in particular to a thickness between 0.5 mm and 0.1 mm. An intermediate annealing can take place optionally during the cold rolling. After the final cold rolling step, the litho strip is subject to a degreasing treatment with pickling with an aqueous pickling medium in a step 30, wherein the aqueous pickling medium contains at least 1.5% to 3% by weight a mixture of 5% to 40% sodium tripolyphosphate, 3% to 10% sodium gluconate, 3% to 8% non-ionic and anionic surfactants and optionally 0.5% to 70% soda, wherein the sodium hydroxide concentration in the aqueous pickling medium is between 0.1% and 5% by weight, in particular between 2% and 3.5% by weight, the degreasing treatment with pickling takes place at temperatures between 70 and 85° C. for a duration of between 1 and 3.5 seconds and a surface erosion of at least 0.25 g/m2 is set by the degreasing treatment with pickling.
The selected surface erosion can reduce high roller webs in the surface of the strip such that after the degreasing treatment with pickling the litho strip has a topography with a maximum peak height Rp and/or Sp of a maximum of 1.4 μm, preferably a maximum of 1.2 μm, more preferably a maximum of 1.0 μm and is particularly suitable for CtP printing plate carriers.
It is therefore possible to use the method in accordance with the invention to manufacture a strip surface with a maximum peak height Rp and/or Sp of a maximum of 1.4 μm, preferably a maximum of 1.2 μm, more preferably a maximum of 1.0 μm.
In order to ensure in practice that the maximum peak heights Rp are maintained in the production of the litho strips, three profile measurements can for example be taken transverse to the direction of rolling, on the outside and in the centre of the strip, wherein the length of the profile can for example be 4.8 mm. The value for Sp can be determined on the basis of a quadratic surface measurement with a side length of 800 μm.
As a comparison of
A litho strip with a surface topography as shown in
Additional features and characteristics of the invention can be derived from the roughness measurements taken from embodiments of the litho strip in accordance with the invention shown below.
Litho strips with an aluminium content which in addition to impurities caused by manufacture have the following alloy contents in percent by weight:
plus residual Al, are cold rolled to a final thickness of 0.14 mm, 0.28 mm or 0.38 mm. In the degreasing treatment with simultaneous pickling, identical parameters are set as for the embodiment in
Before and after the degreasing treatment, roughness measurements are taken on the upper sides of the litho strips, both in the edge regions and in the centre of the litho strips. The roughness measurements determine the average roughness Sa, the reduced groove depth Svk, the reduced peak height Spk and the maximum peak height Sp. The results for the litho strip with a thickness of 0.14 mm are shown in table 1.
In the prior art, the average surface roughness Sa has been used to characterise the litho strips to date. Table 1 shows that this roughness parameter is not suitable to demonstrate the effect of the degreasing treatment with pickling in accordance with the invention or the surface quality of the litho strips in terms of individual high roller webs. Its value remains essentially unchanged after the degreasing treatment with pickling. The reduced groove depth Svk is also evidently not suitable as an indicator of high roller webs. In contrast to this, the values for the maximum peak height Sp are considerably reduced and therefore show the improvement of the litho strip surfaces in terms of the damaging high roller webs. An optimisation of the litho strips and the method for their manufacture using the roughness parameter Sp therefore leads to a particularly infrequent occurrence of the above mentioned printing errors. The reduced peak height Spk is also decreased through the degreasing treatment with pickling and can be used as an additional roughness parameter.
Table 2 shows the results for the maximum peak height Sp from the roughness measurements on litho strips of different thicknesses. In particular, litho strips with a thickness of 0.3 mm to 0.1 mm benefit greatly from the method in accordance with the invention, as these have a relatively high Sp value of more than 1.5 μm immediately after the final cold rolling step and are therefore susceptible to the above mentioned printing errors. The maximum peak height Sp for all strip thicknesses measured can essentially be reduced to the same value through the degreasing treatment with pickling. As a consequence, the surface quality of thin litho strips can be improved particularly with the method in accordance with the present invention.
The results in tables 1 and 2 further show that high roller webs occur in particular on the edges of the strips. The degreasing treatment with pickling can therefore take place for example selectively in the edge region of the litho strips.
Table 3 shows the roughness parameters Sa, Svk, Spk and Sp determined in average on litho strips of different thicknesses. The results clearly show that the average roughness Sa which has been used to date to characterise litho strips is not suitable to improve the quality of a litho strip surface in terms of the damaging high roller webs. In contrast to this, the values of the maximum peak height Rp and/or Sp and the reduced peak height Rpk and/or Spk after the degreasing treatment with pickling show a considerable reduction, such that the litho strip and the method for its manufacture can be improved considerably by an optimisation of the parameters Rp and/or Sp, where necessary in combination with Rpk and/or Spk.
In order to manufacture the litho strip in accordance with the invention, the method in accordance with the invention can for example be used. However, the litho strip in accordance with the invention is not limited to this method of manufacture. On the basis of the present invention, the person skilled in the art can develop further methods to achieve a litho strip in accordance with the invention by optimising the roughness parameter Rp and/or Sp.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10188553.1 | Oct 2010 | EP | regional |
This patent application is a continuation of PCT/EP2011/067896, filed Oct. 13, 2011, which claims priority to European Application No. 10188553.1, filed Oct. 22, 2010, the entire teachings and disclosure of which are incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2011/067896 | Oct 2011 | US |
| Child | 13866639 | US |
