Gravure printing plate

A gravure printing plate is formed by a plurality of concave portions called "cells", arranged in vertical and lateral directions, four sides of each generally square cell being surrounded by walls. A "cell group" is formed of one core cell and four sets of paired cells adjacent to each other arranged to surround the core cell. A notched portion is formed in either one of two intersections of a wall and another wall intersecting therewith on the wall separating the paired cells. The positions of the notched portion is selected to be in point symmetry about the core cell. The direction of flow of a quick-drying ink, during use with the gravure printing plate, becomes random in the vertical and lateral directions in the cell group due to the notched portions, hence mottling of the ink can be prevented.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a gravure printing plate and, more 
specifically, to a gravure printing plate capable of improving flow of ink 
at shadow portions thereof. 
2. Description of the Related Art 
Recently, sheet materials for flexible packaging formed of synthetic resin 
materials and the like are used for packing goods. Printing is carried out 
on the sheet materials for flexible packaging so as to provide good 
appearance and to indicate goods and contents thereof. 
Gravure printing using a web-fed rotary gravure press, such as shown in 
FIG. 1, has been used for printing on the sheet materials for flexible 
packaging which is suitable for printing on the sheet materials. 
Referring to FIG. 1, the gravure press comprises a gravure cylinder 7 on 
which a gravure printing plate 8 is wound, an impression cylinder 12 
provided opposed to the gravure cylinder 7 for conveying the sheet 
material 11 for flexible packaging sandwiched therebetween, a doctor blade 
10 for adjusting the amount of quick drying ink 5 supplied to the gravure 
printing plate 8, and a furnisher roll 6. 
The web-fed rotary gravure press on the sheet material 11 for flexible 
packaging is generally referred to as "gravure for flexible packaging 
material", which is distinguished from "publication gravure" which is the 
web-fed gravure printing press on sheets of paper. The "gravure for 
flexible packaging material" includes three types dependent on the method 
of plate making, that is, conventional gravure, inverted halftone gravure, 
and electronic engraving gravure. 
The gravure printing technique is briefly described in the following. An 
outline of gravure printing is disclosed in, for example, Japanese Patent 
Publication No. 63-28798. Referring to this document, in the gravure for 
flexible packaging material, a gravure printing plate is formed based on 
an original, and printing is carried out based thereon. The step of 
printing, i.e., the step in which ink is transferred from the plate to the 
material to be printed, is the most unstable step in which loss and 
deformation of characters become the largest, in the steps of transferring 
and reproducing image information from the original to the printed matter. 
The gravure printing plate is formed of minute concave portions called 
cells, having volumes corresponding to image density of the original. 
As described above, there are three types dependent on the structure of the 
plate, that is, a conventional plate (FIG. 2A), an inverted halftone 
gravure plate (FIG. 2B) and an electronic engraving gravure plate (FIG. 
2C). In any of the plates, the image density is divided into a light 
portion A, a middle portion B and a shadow portion C, in accordance with 
the volume of the cells. In printing, the ink 5 filled in the cells of the 
printing plate surface 8 is transferred to a material 11 to be printed. In 
the portion has relatively high image density, i.e., the shadow portion C 
where the cell volume is large, there is a flow of ink on the material 11 
to be printed even in a section called a wall which is definitely 
distinguished on the plate surface 8. In the shadow portion C, the flow of 
ink 5 serves to assist the wall portion. 
However, such a flow of ink causes a problem in accurately transferring 
image information which is formed in sections as cells on the plate 
surface 8. Especially in the inverted halftone gravure plate (FIG. 2B) or 
in the electronic engraving plate (FIG. 2C), the image information is 
reproduced based on the size of the opening of the cell, i.e., on the size 
of the dot on the material 11 to be printed, and accordingly, the quality 
of the printed matter is largely dependent on the change of the size of 
dots caused by the flow of ink. 
The reason for this will be described with reference to FIGS. 3 and 4. FIG. 
3 shows the relation between the density of an original and the density of 
a printed matter in gravure printing. FIG. 4 shows, in enlargement, the 
relation between the plate surface F and the print surface P in a light 
portion A', middle portions B' and B" and a shadow portion C' of FIG. 3. 
In principle, the ideal relation between the densities of the original and 
the printed matter is as shown by the line I of FIG. 3. However, in 
practice, the above-described flow of ink is generated at the region in 
the middle portion, for example between the portions B' and B", where the 
density of the original changes slightly. Consequently, in the portion B", 
adjacent dots are brought into contact with each other so as to generate 
an abrupt change of the density of the printed matter between the portions 
B' and B". Namely, the density of the printed matter becomes as shown by 
the line II, in which a jump of density is generated at a certain portion 
of the middle portion, which causes reproduction with uneven tone. The 
portion where such jump occurs changes dependent on various conditions of 
the press machine (speed of printing, angle of inclination of the doctor 
blade, pressure of the impression cylinder, hardness, and so on), the 
plate (depths of cells, forms of cells and so on) and of the ink 
(composition, viscosity and other characteristics). 
In gravure printing, the dots of the printed matter are unavoidably brought 
into contact with adjacent dots at the middle portion of a certain 
density, as shown in FIGS. 3 and 4. The reason for this is that portions 
formed of smaller cells are more susceptible to the influence of ink flow 
compared with the portions formed of larger cells or portions formed of 
rough point patterns formed of groups of smaller cells, even if the 
surface area per unit occupied by the cell portion on the plate surface is 
the same. 
Examples of the above described gravure printing plates 8 comprise those 
disclosed in Japanese Patent Laying-Open No. 59-232347 (hereinafter 
referred to as a conventional example 1) and those which are commercially 
available shown in FIG. 6 (hereinafter referred to as a conventional 
example 2). 
FIGS. 5 and 6 are both partial enlarged perspective views schematically 
showing plate surfaces of the gravure printing plates. 
In the conventional example 1 shown in FIG. 5, the cells C for filling ink 
are formed by walls 2, and in the shadow portion, intersecting portions of 
the walls 2 are removed to form unwalled portions 3, with the bottom 
surfaces of the cells C continued through the unwalled portion 3. 
Meanwhile, in the conventional example shown in FIG. 6, the unwalled 
portions 3 are formed near the intersecting portions of the walls 2 in the 
shadow portion and the unwalled portions 3 are formed to be arranged in a 
horizontally or vertically staggered manner. The bottom surfaces of the 
cells C are continued through the unwalled portions 3. 
FIG. 7 shows a gravure printing plate 8 having the form of the conventional 
example 2 shown in FIG. 6, viewed from above. As shown in the figure, the 
ink 5 flows in a prescribed direction through unwalled portions 3. The ink 
5 flows through a prescribed direction on the surface of the plate, since 
the ink flows to a direction opposite to the rotation of the rotary press, 
as the printing plate 8 is rotated in a prescribed direction on the rotary 
press, as shown in FIG. 1. In gravure printing, since the flow of ink is 
approximately in a prescribed direction as the process of printing 
proceeds, and the ink is of a quick-drying type, uneven flow of ink like 
an orange peel is generated. Those skilled in the art call this uneven 
flow of ink "mottling". Mottling is especially conspicuous in the shadow 
portions (e.g., C in FIG. 2A). 
Therefore, conventionally, generation of "mottling" is reduced by setting 
the above mentioned conditions of the printing press and of the printing 
plate 8 at prescribed values based on experience. However, it is very 
difficult to maintain and control operating conditions. It is difficult in 
both conventional examples 1 and 2 to eliminate "mottling", since the 
unwalled portions 3 continue with prescribed pitches. When "mottling" is 
generated, the surface of the printed matter will appear to have thin 
stripes. 
The details of the "mottling" will be described in the following. As shown 
in FIG. 1, after the ink 5 is transferred from the cells of the printing 
plate 8 to the object 11 to be printed, it is fed to a drying box 13 where 
the ink is dried. The state of the ink transferred from the cells of the 
printing plate 8 to the object 11 to be printed is shown in FIG. 8A. 
Referring to FIG. 8A, when the ink 5 is transferred onto the object 11 to 
be printed, the ink of each cell has a convex shape. Although there is a 
small flow of ink between the convexes of the ink after the ink 5 is 
transferred until the printed matter 11 is transferred to the drying box 
13, the ink 5 does not become completely flat as shown in FIG. 8B, since 
the ink is of a quick-drying type and the flow of ink is in only a 
prescribed direction. Consequently, the heights of the convexes of the ink 
become varied, causing uneven density of ink. The unevenness of the ink 
density depends on the viscosity of the ink (viscosity), color, and the 
depths of the cells. In addition, it depends on the speed of printing 
(fast, slow). The unevenness of ink is generated when the printing speed 
is low and when the speed is high. When the speed is high, the 
stripe-shaped unevenness becomes more conspicuous. 
In addition, the unevenness of ink depends on the screen ruling (175, 150l, 
100l, and so on). Namely, if the ruling is small, i.e., if the mesh is 
rough, the unevenness becomes more conspicuous. 
In the conventional example 1, all intersecting portions of the walls 2 are 
removed. Therefore, the wall portions 2 are cut into small pieces, the 
walls 2 wear much during printing, and the life of the walls 2 is not very 
long. 
Although the problem of short life of the conventional example 1 is solved 
and the above-mentioned "mottling" can be reduced at a specified speed of 
printing in the conventional example 2, "mottling" is still generated when 
the speed of printing is changed. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a gravure 
printing plate capable of reducing wear of walls, improving life, and 
preventing "mottling" even if the speed of printing is changed. 
An object of the present invention is to provide a gravure printing plate 
in which mottling is not generated. 
Another object of the present invention is to provide a gravure printing 
plate capable of reduced wear of walls and improving the life thereof. 
A further object of the present invention is to provide a gravure printing 
plate in which mottling is not generated, regardless of the speed of 
printing. 
The above-described objects of the present invention can be attained by 
providing a gravure printing plate, comprising: a base; and a plurality of 
cell groups formed on the base, each cell group including nine cells 
formed of three cells each in a first direction and a second direction 
intersecting the first direction formed adjacent each other; each of the 
cells being surrounded by opposing two sets of walls formed in a third 
direction intersecting the first and second directions; an end portion in 
the third direction of the wall of the plurality cell groups having a flat 
surface forming a printing surface of the gravure printing plate. Each 
cell group includes a core cell positioned at the center and four paired 
cells adjacent each other sharing one end of the walls of the core cell, 
with a notched portion formed on an end portion of a wall intersecting the 
wall dividing the paired cells. 
Since the gravure printing plate of the present invention comprises the 
above-described components, notched portions are formed in the walls such 
that the ink flow is in random directions. Since the ink flows not in a 
prescribed direction, the generation of uneven ink density can be 
prevented. Consequently, a gravure printing plate capable of preventing 
uneven ink flow can be provided. 
In accordance with another aspect of the present invention, the gravure 
printing plate comprises a base; nine first cells each having four sides 
of walls formed adjacent each other in a first direction and in a second 
direction intersecting the first direction, three cells in each direction; 
one second cell formed adjacent, in the first or second directions, one 
cell at a corner of the nine first cells; the nine first cells and the one 
second cell forming one cell group, a wall of each cell formed in a third 
direction intersecting the first and second directions on the base; an end 
portion of the wall of the cell group in the third direction forming a 
flat surface, the flat surface constituting a printing surface of the 
gravure printing plate. Eight cells around the central cell out of the 
nine first cells form four paired cells adjacent each other, a notch 
portion is formed in a portion of the adjacent wall of the paired cells 
along the wall surface of the wall intersecting the adjacent wall, and the 
positions of the notched portions are arranged in point symmetrically 
about the central one of the nine first cells. A corner portion of the 
first cell of each of the four cell groups is adjacent each of the four 
sides of the second cell, and accordingly, four cell groups share the 
second cell. 
In accordance with this aspect of the present invention, since the gravure 
printing plate comprises the above described components, the gravure 
printing plate is formed based on a of four cell groups unit, each group 
including ten cells sharing one central cell. There is no notch in the 
central cell, and notched portions are formed on the surrounding cells 
such that the ink flows in random directions. Consequently, a gravure 
printing plate capable of preventing uneven flow of ink even in the shadow 
portion can be provided. 
The foregoing and other related objects, features, aspects and advantages 
of the present invention will become more apparent from the following 
detailed description of the present invention when taken in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described in the following 
with reference to the figures. FIG. 9 is a perspective view schematically 
showing a plate surface of a shadow portion of a gravure printing plate in 
accordance with a first embodiment of the invention, and FIG. 10 is a 
schematic view of a dot pattern (gravure contact screen) thereof. 
Referring to FIG. 9, in the gravure printing plate 1 of the present 
invention, one cell group G1 is formed by 9 cells C in total, i.e., three 
cells in a vertical direction and three cells in a lateral direction, each 
cell C (corresponding to the dot D in FIG. 10) having square shape divided 
by walls 2 (2d in FIG. 10). Adjacent cell groups G1 are arranged separated 
from each other in the vertical and the lateral directions by one cell C2. 
Each cell group G1 comprises a central core cell Cl and 8 cells C 
surrounding the central cell, with the 8 cells forming four pairs of 
cells. Each pair of cells has a bottom portion coupled through a notched 
potion 3 (coupling portion 3d in FIG. 10). The notched portion 3 formed 
near the intersecting portion of the walls 2 is arranged to be in point 
symmetry about the center of the cell group C1. When one cell C2 is 
surrounded by four cell groups G1 . . . , one cell group G2 is formed by 
the 8 cells C and the central cell C2 which is the core. Namely, by the 
above described arrangement of the notched portions 3 in each cell group 
G1 and relative arrangements of the adjacent cell groups, continuation of 
the notch portioned 3 in the direction of printing A (see FIG. 10) by the 
prescribed pitches can be prevented. Consequently, the direction of ink 
flow between the paired cells C becomes random, which makes "mottling" 
less conspicuous. 
In the present embodiment, the "mottling" in the shadow portion (solid 
portion) is eliminated when the screen ruling (number of dots per unit 
length =number of cells per unit length) is set to be 175/inch, one side 
of the cell is set to be a=4.times.b, the width of the wall is set to be 
b=32 to 35 .mu.m, the width of notch of the notched portion 3 is set to be 
e.apprxeq.b, the depth of the cell is set to be h=20 to 27 .mu.ma and the 
screen angle .theta. is set to be 18.5.degree., and at that time, the 
reflection density reaches 2.85, which is higher by 0.1 than that of the 
conventional example 1, and the gradation is superior. reflection density 
is represented as 
EQU Dr=log.sub.10 I.sub.0 /I.sub.1 (1) 
where 
I.sub.0 : intensity of reflected light of a reference white board under 
specific conditions of measurement, 
I.sub.1 : intensity of reflected light from a sample surface under the same 
conditions. 
The screen angle applicable to the present invention is .theta.=15.degree. 
to 25.degree.. 
As shown in FIG. 10, the width e of the notched portion 3 in the shadow 
portion may be about 1/3 to about 1/6 of the length a of one side of the 
cell C. If the width of the notch becomes larger than that, two cells 
serve as one rectangular cell due to the influence of side etching, which 
is undesirable. 
Side etching means that the shape of the wall becomes as shown by the 
dotted line in FIG. 11 but not as shown by the solid line. 
The above mentioned screen ruling, the ratio of the length "a" of one side 
of the cell and the width "b" of the wall, the cell depth "h" and the like 
may be arbitrarily set to be dependent on various predetermined 
conditions. 
FIGS. 12 to 14 show main portions of dot pattern (gravure contact screen) 
when the gravure printing plate of the first embodiment is made as an 
inverted halftone gravure. All of the patterns are the dot patterns 
recorded on a photosensitive film by an image scanning and recording 
apparatus (scanner). 
Referring to FIG. 12, the size of the dots D . . . corresponding to the 
cells C is defined group by group G1. When the "dot percent", which is the 
ratio of the dot D in a unit area S proportional to the image density, is 
not less than a set value (for example 30%), dots are paired to be 
continuous, and as the dot percent increases, the width of the coupling 
portion e of the coupling 3d becomes larger. The width "e" of the coupling 
becomes the largest at the shadow portion. When the dot percent is not 
more than 30%, the dots D are not coupled to each other as shown by the 
phantom line. The dots D are set in this manner in consideration of the 
fact that when the dot percent is small, the width of the wall is 
sufficiently large to prevent generation of "mottling", and as the dot 
percent increases, the width of the wall becomes smaller and "mottling" 
tends to occur. Thus, as the density increases to that of the shadow 
portion, the width of notches is made larger to facilitate random flow of 
ink. 
In the gravure contact screen shown in FIG. 12, the gradation is 
represented group by group G1, so that there is a possibility of a 
decrease in the resolution. In the gravure contact screen shown in FIG. 
13, the size of the dots D is defined independently. If the dot percent is 
not less than 30%, for example, the dots D are paired and coupled. In the 
gravure contact screen shown in FIG. 13, the width of the notch is made 
different in every paired dots having different dot percents, which 
further improves the expression of gradation by that area. 
In the gravure contact screen shown in FIG. 14, the size of the dot is 
defined pair by pair, in order to facilitate formation of the dot pattern. 
The dots A are formed pair by pair in each character, regardless of the 
dot percent. However, as described above, the width of the notch (the 
width of coupling) is made different dependent on the dot percent. 
FIG. 15 is a schematic diagram of a dot pattern (gravure contact screen) 
showing a second embodiment. This embodiment is a modification of the 
first embodiment. Referring to FIG. 15, this embodiment is different from 
the first one in that the coupling portion 3d, corresponding to the 
notched portion in each group G1, indicated by the phantom line, is 
arranged distant from the central dot D1. In other words, in another group 
G2, indicated by the dotted line in FIG. 15, the coupling portion 3d is 
formed near the central core dot D2. 
FIG. 16 shows a third embodiment, corresponding to FIG. 9. This embodiment 
differs from the first embodiment shown in FIG. 9 in that the number of 
notched portions 3 is increased so that 9 cells C in each group G1 
indicated by the phantom line of FIG. 16 are continuous to each other 
through the central cell C1. The position of the cell C2 without the 
notched portion is as shown in FIG. 17. 
FIG. 18 shows a fourth embodiment, corresponding to FIG. 9. This embodiment 
differs from the third embodiment in that the notch portions 3 are formed 
such that cells C are continuous to each other through the central core 
cell C2 also in the cell group G2, which is indicated by the dotted line 
in FIG. 16 of the third embodiment. 
In the above-described embodiments 1 to 4, each of the cells C or the dots 
D has a square shape. However, the present invention is not limited 
thereto, and the form of the cell or the dot may be rectangular, rhomboid 
or of any arbitrary shape. 
Although the present invention has been described and illustrated in 
detail, it should be clearly understood that the same is by way of 
illustration and example only and is not to be taken by way of limitation, 
the spirit and scope of the present invention being limited only by the 
terms of the appended claims.