NL1025549C2 - Screen printing process involves arranging halftone pattern on screen for each color of ink and forcing ink through screen which is arranged in specified diagonal orientation - Google Patents

Abstract

A halftone pattern is arranged on the screen (1) for each color of the ink and the ink is forced through the screen. The diagonal orientation of the screen and the halftone pattern is +30[deg] or -30[deg]. An independent claim is also included for group of screen for use in screen printing.

Description

Method for screen printing and group seven for use with this method

The invention relates to a method for screen printing one or more color ink on a carrier, wherein for each ink a screen is applied to a screen and the ink is printed through the screen with screen on the carrier.

Such a method is known, for example, from EP-A-0 761 434. In screen printing of a color image, the color image is usually split into four half-tone images with the colors cyan, black, magenta and yellow. These four halftone images are each recorded in a film screen, this film screen being transferred to a screen. The four sieves are then each used to apply the relevant halftone image of the color image to the carrier in the corresponding color. It is known that in screen printing, moire patterns can be caused by the interfering patterns of the halftone images that are printed over one another by the different screens and interference between the pattern of the screen and the pattern of the film screen of the halftone image.

It is known that the formation of disturbing moire patterns can be prevented by having the various half-tone grids enclose an angle of 30 ° with each other.

For example, it is usual to place the so-called raster angles below 15 °, 45 ° and 75 ° relative to each other. As long as the three contrasting primary colors of cyan, magenta and black are at an angle of 30 ° to each other, no disruptive form of moire occurs between the primary colors. The color yellow can be placed as a non-contrasting color at an angle of 0 ° between the other colors. In the method according to EP-A-0 761 434 an attempt is made to avoid disturbing moire patterns by using sieves of different geometry, for example using three different sieves, the second sieve enclosing an angle of 30 ° with the first and the third sieve

102S§49 «II

2 includes an angle of 60® with the first screen. In order to avoid interference between the halftone screen and the screen, this known method makes use of a so-called frequency-modulated halftone technique. This frequency-modulated halftone technique, however, is difficult to apply to screen printing and gives uncontrollable shifts in density, especially in the mid-tone region.

It is an object of the invention to provide a method of the type mentioned in the preamble, in which the occurrence of disturbing moire patterns can be avoided when using an analog or amplitude modulated halftone technique.

According to the invention, the method for this purpose is characterized in that the diagonal screen direction of the screen and the direction of the screen include an angle of + 30 ° or -30 °.

The invention is based on the insight that a correct choice of the position of the grating with respect to the diagonal screening direction can prevent disturbing moire patterns, an angle of + 30 ° or -30 ° being preferred. serves. With the expression "diagonal screen direction" the | 20 intended to be diagonal which includes an angle of 45 ° with the direction 1 of the screen wires.

According to the invention, the best results are achieved with single-color printing if the diagonal screen direction is 75 ° and the direction of the screen is 45 °.

According to the invention two sets of sieves are used in four-color printing, which have a diagonal sieving direction of 45 ° resp. of 75 °, with the direction of the grids at the first set of seven 75 ° and Is 15 ° and the direction of the grids in the second set of seven is 45 ° and Is 15 °. The one set of seven therefore comprises strained straps, while

In the other set of sieves, the sieve threads are tensioned at an angle of 30 °.

According to the invention it is furthermore of great importance that the correct ratio of the number of wires per centimeter of the screen to the number of lines per centimeter of the screen is chosen. According to the invention, it is preferable if the number of lines per centimeter of the grid is equal to or smaller than (Z / n) x cos 30 °, where Z is the 102 SS499 number of wires per centimeter of the screen and n a integer or equal to 2.33.

It is important here that the number of threads per centimeter of the screen is determined after tensioning the screen.

According to the invention, a group of sieves is also provided for use in the described screen printing method, wherein each sieve has a diagonal sieving direction, wherein according to the invention the group of seven comprises two sets of mutually equal sieves, the diagonal sieving directions of I

the seven of the two different sets of a group enclose an angle of 30 °.

The invention is explained in more detail below with reference to the drawing. FIG. 1 is a highly schematic view of a strained strainer used in an embodiment of the method according to the invention for four-color printing, and

FIG. 2 shows a screen tensioned at 30 ° and used in this embodiment of the method according to the invention.

In the FIG. 1 and 2 schematically two sieves 1,2 are shown on a greatly enlarged scale, wherein the sieve 1 is a straight-stretched sieve and the sieve 2 is a tensioned under 30 "

sieve is. This means that the screen wires 3 of the screen 1 lead

25, respectively, running parallel to the frame parts 4,

while at the screen 2 the screen threads 3 enclose an angle of 30 °

with the frame part 4 horizontal in the drawing. I

According to the method according to the invention, in screen printing of an image in four-color printing, use is made of two seven 1 and two seven 2. The seven 1 are

then used, for example, to print the half-tone I

images in cyan and black. Cyan I color film screen

for example, at an angle of 75 ° on the first screen, 1 l

as shown in FIG. 1 is indicated by the line C. The grid of the color black then becomes up at an angle of 15 °

the second screen 1, as shown in FIG. 1 with the line B is I

indicated. This means that the cyan and black grids have an angle I

of + 30 ° and -30 ° respectively with the diagonal sieve direction I

1025549 »!! Include 4 of 45 °. The diagonal screen direction is indicated in the drawing by the shaded screen openings.

Correspondingly, the grids of the colors yellow and magenta are applied to the first and second sieve 2 at an angle of 15 ° (the line Y in Fig. 2) and 45 ° (the line M in Fig. 2), respectively. These grids also enclose an angle of + 30 ° and -30 ° with the diagonal screening direction of 75 °. In addition, the three contrasting colors cyan, black and magenta also include an angle of 30 ° with each other. In this way the occurrence of disturbing moire patterns is completely avoided. In this case the color yellow is at the same angle as the color black, which presents no problems with a normal color separation. If a PCR or UCR color separation is used, the color yellow is set at an angle of 0 °.

According to the invention it is furthermore of great importance that the correct ratio between the number of wires per centimeter of the screen and the number of lines per centimeter of the screen is chosen. According to the invention, the number of lines per centimeter of the grid is equal to or less than (Z / n) x cos 30 °, where Z is the number of wires per centimeter of the screen and n is an integer. Tests have shown that a good ratio between the number of wires per centimeter of the screen and the number of lines per 25 centimeters of the screen is also obtained with n = 2.33. The factor (cos 30 °) takes into account the position of the screen with respect to the diagonal screen direction when determining the ratio of screen wires to screen. Experiments have shown that preferably the number of lines per centimeter of the grid 30 is greater than (Z / n) x cos 36 °. The number obtained from these formulas is of course rounded to a whole number of lines per centimeter.

According to the invention, when determining the number of wires per centimeter of the screen, the number of wires per centimeter after tensioning the screen is preferably used. Research has shown that the number of wires per centimeter of the screen after tensioning can be a few percent lower than the number of wires per centimeter specified for the screen material used.

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An example is a screen with 165 wires. Measurements have shown that with a conventional tensioning force the screen after stretching has an average of 152.5 threads per centimeter. If n = 2, the upper limit for the 5 number of lines per centimeter for the grid is 76.25 x cos 30 ° = 66.03, 66 lines per centimeter is rounded. The lower limit is 76.25 x cos 36 ° = 61.68, is rounded 62 lines per centimeter. When these calculations for n = 2.33, n = 3, n = 4, n = 5 and n = 6, suitable grid values follow: 21-22 10 (n = 6), 24-26 (n = 5) , 31-33 (n = 4), 41-44 (n = 3), 53-56 (n = 2.33) and 62-66 (n = 2).

It is noted that when an angle of 30 ° is mentioned in the foregoing, this angle is not limited to an angle that is exactly 30 °. A deviation of approximately + 3 ° or -3 ° 15 is possible without this leading to strongly disturbing moire patterns.

The invention is not limited to the exemplary embodiment described above, which can be varied in different ways within the scope of the claims.

1025549 "«

Claims (8)

Method for screen printing one or more color ink on a support, wherein for each ink a screen is applied to a screen and the ink is printed through the screen with screen on the support, characterized in that the diagonal total sieve direction of the sieve and the direction of the grid include an angle of + 30 ° or -30 °. A method according to claim 1, wherein at one-color printing the diagonal screen direction is 75 ° and the direction of the screen is 45 °. Method according to claim 1 or 2, wherein two sets of sieves are used in four-color printing, which have a diagonal sieving direction of 45 ° resp. of 15 °, wherein the direction of the grids in the first set of seven is 75 ° and Is 15 ° and the direction of the grids in the second set of seven is 45 ° resp. Is 15 ° or 0 °. A method according to any one of the preceding claims, wherein the number of lines per centimeter of the grid is equal to or smaller than (Z / n) x cos 30 °, wherein Z is the number of wires per centimeter of the screen and n is an integer or at least approximately equal to 2.33. The method of claim 4, wherein the number of lines per centimeter of the grid is greater than (z / n) x cos 36 °. 6. Method as claimed in claim 4 or 5, wherein the number of wires per centimeter of the screen is the number of wires after tensioning the screen. ί 7. Group seven for use in a screen printing method as claimed in any of the foregoing claims, wherein each screen has a diagonal screen direction, characterized in that the group seven comprises two sets of mutually equal seven, the diagonal screen directions of the seven of the seven include two different sets of a group at an angle of 30 °. 8. Group seven according to claim 7, wherein the diagonal screening direction of the first set of seven is 45 ° and of the second set of seven is 75 °. ί 1025549 »