FIELD OF THE INVENTION

The present invention relates to a rotogravure printing device for electrostatically assisted printing, said device comprising a surface arranged with numerous ink cells, each having a wall portion and a bottom portion.

BACKGROUND INFORMATION

The present invention relates to rotogravure printing having an electrostatic assist. Rotogravure printing is one of the conventional methods of printing on a sheet, web or other substrate. This printing is capable of reproducing both subtle shades of colour and black and white, and is particularly well suited for printing great numbers of copies precisely and rapidly.

Rotogravure printing is a commercial printing process which can control both the ink film thickness and the area of coverage. This is achieved by the engraving of recessed microscopic wells, frequently referred to as cells of varying depth and area in the printing medium or image carrier surface, usually a gravure cylinder. The amount of ink available for placement on the substrate is governed to generate an image composed of an arrangement of large and small dots and is controlled by the size and depth of the cells.

The gravure cylinder is mounted for rotation about a horizontal axis, and supplied with a liquid ink of high fluidity. A doctor blade removes ink from such portions of its peripheral surface as do not contain the cells, these cells contain ink until it is deposited upon the surface of the substrate. This is accomplished by passing the substrate between the gravure cylinder and an impression cylinder having a resilient covering which presses the lower surface of the substrate against the gravure cylinder so that ink in the cells may be deposited upon that surface. In ordinary gravure printing the ink is not always fully deposited from all of the cells and these incomplete or even missing dots caused by this result in faulty prints. The transfer of ink from gravure cells to the substrate is enhanced by the application of an electric field between the gravure and impression cylinder passing through the portion of the substrate that is located at the nip between the cylinders (Electrostatic Assist, ESA). ESA is used on gravure printing presses to overcome the incomplete transfer of ink from the cells to the substrate passing above it. The principle of ESA is to generate an electric field in the region of the nip between the impression- and gravure cylinder where ink transfer takes place. The ink is electrostatically pulled by the field onto the substrate. Due to dielectrophoresis (translational motion of neutral matter caused by polarization effects in a nonuniform electric field), the ink moves towards regions of denser field lines which are found at the edges of the ink cell. In consequence no lifting force acts on the ink in the centre of the ink cell and that results in a doughnut shaped ink lifting.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or at least minimize at least one of the drawbacks and disadvantages of the above described printing method. This can be obtained by a rotogravure printing device for electrostatically assisted printing in accordance with claim 1.

Thanks to this invention a lifting force acts on the ink even in the middle of the cell which yields a more even print out, a better tone value and a reduction of ink consumption.

According to one aspect of the invention the bottom of the ink cells has at least one protruding part that acts as an attractor for field lines and as a consequence a lifting force acts on the ink also in the middle and the ink is lifted more uniformly out of the cell and the resulting print dots are full dots without a hole in the middle.

Most preferably, the wall portion of the ink cell extends substantially vertically. In this way, the ink lift happens faster since the concentration of field lines is larger at the edges than in the case of the walls are tilted. According to yet another aspect of the invention the protruding part has a height that is equal to or smaller than the height of the wall portions. According to still another aspect of the invention the base width of the protruding part is at least one fourth the size of the width of the cell so no areas with sparse field lines occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to the enclosed figures, in which:

Fig. 1 A) and B) shows the sequential process of ink transfer of prior art rotogravure printing,

Fig. 2 is a cross sectional view of a laser etched U-shaped ink cell 1 of a prior art kind, as that used in fig. 1 B, Fig. 3 is a cross sectional view of a mechanically engraved V-shaped ink cell of a prior art kind, as that used in fig. 1 A,

Fig. 4 shows a view from above of a section of a gravure device arranged with ink cells according to the invention,

Fig. 5 shows a cross sectional view, along C - C in fig. 4, of an ink cell according to the invention,

Fig. 6 shows a view from above of a section of a gravure device arranged with alternative ink cells according to the invention,

Fig. 7 is a cross sectional view, along D - D in fig. 6, of an alternative ink cell according to the invention, Fig. 8 is a cross sectional view of still an alternative embodiment of an ink cell according to the invention, and Fig. 9 is a cross sectional view of yet an alternative embodiment of an ink cell according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description, and the examples contained therein, are provided for the purpose of describing and illustrating certain embodiments of the invention only and are not intended to limit the scope of the invention in any way. Fig. 1 A) and B) shows the sequential process of ink transfer of prior art electrostatically assisted rotogravure printing, obtained by means of a gravure cylinder 4 having numerous recessed microscopic wells 2 (fig A) and 1 (Fig B) , so called cells. The amount of ink available for placement on a substrate is controlled by the size and depth of the cells 1,2. The gravure cylinder 4 is mounted for rotation about a horizontal axis and supplied with ink of high fluidity. A doctor blade (not shown) removes ink from such portions of its peripheral surface as do not contain the cells, these cells contain ink 5 until it is deposited upon the surface of a substrate. This is accomplished by passing the substrate between the gravure cylinder 4 and an impression cylinder (not shown) having a resilient covering which presses the lower surface of the substrate against the gravure cylinder 4 so that ink 5 in the cells 1,2 may be deposited upon that surface. The transfer of ink 5 from gravure cells 1,2 to the substrate is enhanced by the application of an electric field between the gravure and impression cylinder passing through the ink 5, due to dielectrophoresis. In Fig 1 there is shown sequentially the process of transfer of the ink 5, by means of in the lower most part a) showing it before nip contact, in an intermediate part b) showing the phase of ink lifting and in uppermost part c) the nip contact area. As can be noted a doughnut shaped ink lifting is obtained, causing not all of the ink 5 to deposit from all of the cells 1,2, resulting in incomplete dots leading to reduced quality and sometimes therefore faulty prints.

Fig. 2 is a cross sectional view of a laser etched U-shaped ink cell 1 of a prior art kind, as that used in fig. 1 B. These cells 1 are in the form of circular engravings, having substantially vertical edges 11,12 and a substantially horizontal bottom 14. The concentration of field lines 10 is larger at the cell's 1 edges 11,12 than at the centre 13 of the cell 1. The consequence of this is that no lifting force acts on the ink in the centre 13 of the ink cell 1 and the result is a doughnut shaped ink lifting as can be seen in figure 1 B.

Fig. 3 is a cross sectional view of a mechanically engraved V-shaped ink cell 2 of a prior art kind, as that used in fig. 1 A. The volume of the cell 2 forms an upside-down pyramid with four faces; seen from above, the cell 2 look like a rhomb. The cell 2 has inclined edges 21,22 that meet at a centre 23 of the cell 2. The concentration of field lines 10 is larger at the cell's 2 edges 21,22 than at the centre 23 of the cell 2. The consequence of this is that no lifting force acts on the ink in the centre 23 of the ink cell 2 and the result is a doughnut shaped ink lifting as can be seen in figure 1 A.

It has been shown that the shape of the ink cell 1,2 influences the effectiveness of ink transfer. The ink lifting from an ink cell with U-shape 1 (laser etched) happens faster than from an ink cell with V-shape 2 (mechanically engraved). Looking to the field lines 10,20 as indicated in figures 2 and 3 the concentration of field lines 10 at the edges 11,12 of a U-shaped ink cell 1 is larger than in the case of a V-shaped ink cell 2. For both cell types 1,2 the field lines 10,20 are sparse at the centre 13,23 of the ink cell 1,2. As consequence no lifting force acts on the ink in the centre of the ink cell 1,2 and the result is a doughnut shaped ink lifting, as can be seen in figures 1 A) and B).

Fig. 4 shows a view from above of a section of a gravure device 4, e.g. a gravure cylinder, arranged with ink cells 3 according to a preferred embodiment of the invention. As can be noted each cell 3 is formed by a circular outer wall 31, and a protruding peak 34 in the centre 35 of the cell 3. The gravure device 4 according to the invention is further arranged with means (not shown) for applying an electric field between a gravure cylinder and an impression cylinder.

In fig 5 there is shown a cross sectional view of an ink cell 3, as shown in fig 4, along C - C in fig. 4. The ink cell 3 has substantially vertical walls 31 and a substantially horizontal bottom 33, having a height h of the walls 31 and a width w of the bottom 33, that depends on screen and tone. At the centre 35 of the bottom 33 an additional peak 34 is protruding vertically, i.e. preferably generally parallel with walls 31. The peak 34 is cone shaped, having its thickest part at the bottom 33. The width W of the thickest part of the peak 34 is in the interval 0.05w - 0.5w, i.e. 0.05w < W < 0.5w, preferably O.lw < W < 0.4w.The height H of the peak 34 is in between 0. lh - 0.9h, i.e. O. lh < H < 0.9h, preferably 0.5h < H < 0.8h. This additional peak 34 in the centre 35 of the ink cell 3 acts as an attractor for further field lines 10 and the consequence of this is that a lifting force acts on the ink also in the centre 35 of the ink cell 3. In this way the ink may be lifted more uniformly out of the cell 3 and the resulting print dots are full dots without a hole in the middle. The shape of the dots are the result of different interacting forces, e.g. electric forces lifting the ink out of the ink cell 3 at the edges 31/walls of the cell 3. These lifting forces are transmitted to the ink surface in the centre 35 of the ink cell 3 by surface tension. The bulk of the ink follows the ink surface due to cohesion forces. However, in these interactions the transmission of the ink lifting from the edges to the middle by surface tension is quite sensitive, which means that there is some unpredictability of the result (the exact dot size and shape), leading to unpredictability of the dot size and shape means on macroscopic scale about the tone value and evenness of the print result.

Thanks to the use of the protruding part 34, the lifting force now works in principle over the whole ink cell 3. Furthermore the novel arrangement may provide a more uniform pattern of field lines 10 and the ink lifting process is made more uniform, resulting in more predictable tone values, which in turn may result in less proofing. It may also provide more even print out and - in consequence of the resulting better ink coverage - a reduction of ink consumption. According to another aspect the protruding part 34 may have other shapes that creates possibilities to arrange different patterns of field lines 10 which render a possibility to control the print in the macroscopic scale.

Fig. 6 shows a view from above of a section of a gravure device 4, e.g. a gravure cylinder, arranged with an alternative embodiment of ink cells 6 according to the invention. As can be noted each cell 6 is formed by a hexagonal outer wall 61, and a protruding peak 64 in the centre 65 of the cell 6.

In fig 7 there is shown a cross sectional view of an ink cell 6, as shown in fig 6, along D - D. The ink cell 6 has substantially vertical walls 61 and a substantially horizontal bottom 63, having a height h of the walls 61 and a width w of the bottom 63, that depends on screen and tone. At the centre 65 of the bottom 63 an additional peak 64 is protruding vertically, i.e. preferably generally parallel with walls 61. This additional peak 64 is in the form of a hexagonal cut-off cone having a base width W in the interval 0.08w - 0.8w, i.e. 0.08w < W < 0.8w, preferably 0.15w < W < 0.6w. The height H of the peak 64 is in between 0. lh - 0.9h, i.e. 0. lh < H < 0.9h, preferably 0.5h < H < 0.8h. The hexagonal cut-off cone 64 in the centre 65 of the ink cell 6 acts as an attractor for further field lines 10 and the consequence of this is that a lifting force acts even in the middle of the ink cell 6. In this way the ink may be lifted more uniformly out of the cell 6 and the resulting print dots are full dots without a hole in the middle.

Fig. 8 is a cross sectional view of still an alternative embodiment of an ink cell 7 according to the invention. The ink cell 7 has substantially vertical walls 71 and a substantially horizontal bottom 73, having a height h of the walls 71 and a width w of the bottom 73, that depends on screen and tone. At the centre 75 of the bottom 73 an additional peak 74 is protruding vertically, i.e. preferably generally parallel with walls 71. The peak 74 has the shape of a pillar having a width W in the interval 0.05w - 0.5w, i.e. 0.05w < W < 0.5w, preferably O.lw < W < 0.4w. The height H of the pillar is in between O.lh - 0.9h, i.e. O.lh < H < 0.9h, preferably 0.5h < H < 0.8h. This additional peak 74 in the centre 75 of the ink cell 7 acts as an attractor for further field lines 10 and the consequence of this is that a lifting force acts over the middle of the ink cell 7. In this way the ink may be lifted more uniformly out of the cell 7 and the resulting print dots are full dots without a hole in the middle.

Fig. 9 is a cross sectional view of still an alternative embodiment of an ink cell 9 according to the invention. The ink cell 9 is V-shaped having inclined edges 91,92 with a height h and the whole cell 9 has a width w, the height h and the width w depends on screen and tone. At the centre 93 of the cell 9 an additional peak 94 is protruding vertically. The peak 94 is cone shaped, having its thickest part at the bottom of the cell 9. The width W of the thickest part of the peak 94 is in the interval 0.05w - 0.5w, i.e. 0.05w < W <0.5w, preferably 0. lw < W < 0.4w. The height H of the peak 94 is in between O.lh - 0.9h, i.e. O.lh < H < 0.9h, preferably 0.5h < H < 0.8h. This additional peak 94 in the centre 93 of the ink cell 9 acts as an attractor for further field lines 10 and the consequence of this is that a lifting force acts over the middle of the ink cell 9. In this way the ink may be lifted more uniformly out of the cell 9 and the resulting print dots are full dots without a hole in the middle.

As will be understood by those skilled in the present field of art, numerous changes and modifications may be made to the above described and other embodiments of the present invention, without departing from its scope as defined in the appending claims. For example, the ink cells may have other embodiments than those described above e.g. the ink cells may have an octagonal outer wall and the walls may be slightly inclined or sharply inclined. The protruding part at the bottom may have other embodiments than those described above, the main point is that the protruding part adds further non-horizontal surfaces as contact points for further field lines. Other possible embodiments may for example be a pyramid, an obelisk with e.g. three or four faces, a cut-off cone etc. The protruding part may also be a hollow structure like a ring which is hollow in the middle or a ridge extending over a part or the whole bottom of the ink cell or many shorter ridges. It is also possible to have more than one protruding part in the same ink cell. The different embodiments on the ink cells and the protruding parts described above may of course be combined in different ways than described here.

The skilled person also realize that the size, height and width of the ink cell may vary all depending on what is supposed to be printed.