Theoretical Analysis and Research of Wire Mesh Parameters and Minimum Dot Size

Screen printing dots are directly blocked and restricted by the screen mesh, which can easily cause serious damage to the dots. In order to solve this problem, the foreign screen printing industry has done a lot of work, strictly limiting the selection of screens and major technical parameters, and providing an important guarantee for improving the reproducibility of the halftone screen printing. Nowadays, screen printing adopts the traditional plate-making technology. Its tone reproduction range reaches 20% in the bright spot, 80% in the dark spot. It adopts advanced plate making technology and fine mesh, and its tone is adjusted. The reproduction range is up to 10% in the bright spot and 90% in the dark spot. At present, in China, although screen-printing screen printing technology has made some progress, it is still at a low level on the whole, mainly as follows:
(1) The scope of printing tone adjustment is relatively narrow, generally between 20% and 80%, and needs further improvement.
(2) There is no basis for the determination of the number of screens in the Yang picture. The selection of the number of screens depends largely on experience, and it is difficult to obtain the desired reproduction of the tone.
(3) There is a lack of systematic research on the main process parameters of screen printing screen printing prepress systems. For example, what is the relationship between the minimum dot diameter to be printed and the silk screen diameter? Although several formulae were introduced in the data, the results of these formulas calculated using the same parameters are quite different, so that the relationship between the minimum dot diameter and the screen wire diameter has been in an uncertain state, and it is impossible to guide production at all. Therefore, it is absolutely necessary to theoretically analyze and study the screen selection and the control of the minimum dot size for screen printing.
This article will focus on the interrelationships between the main parameters related to screen selection. Provide theoretical basis for the choice of screen.
Determining the Number of Screen Lines In screen screen printing, determining the number of screen lines for positive pictures is the key to achieving original tone and color reproduction. The lower the number of screen lines, the poorer the reproducibility of printed images at a fine level; conversely, the higher the number of screen lines, the higher the image clarity, but this will increase the requirements for screens, inks and other printed materials, resulting in increased printing costs. . Therefore, a reasonable determination of the number of screens should comprehensively consider various factors.
The screen-printing printing products have a wide viewing distance ranging from 50mm to tens of meters. This requires that the number of screens of the positive image should be compatible with the viewing distance of the printed product to achieve continuous adjustment. Color reproduction effect. From this point of view, the quality of the printing effect is reflected by the quality of the visual effects obtained within the actual viewing distance. Therefore, the number of screens for screen printing for Yang screen printing should mainly be based on the quality of the printed products. Look at the distance to decide. At different viewing distances, different screening lines should be selected.
In order to reasonably determine the number of screens for positive images, the minimum viewing distance and the maximum viewing distance corresponding to the number of different screen lines can be obtained through experimental testing. This range is called the visible distance, as shown in Table 1. . When determining the number of screens, note the following two points:
(1) The viewing distance of printed products is the main basis for determining the number of screens. When determining the number of screens for positive images, the viewing distance of printed products must be clearly defined so that the required number of screens can be easily determined.
(2) The number of screens should be low, not high. If the number of screen lines is too high, the number of network sites per unit area will increase, which will increase the difficulty of controlling the density of network sites, which will affect the stability of the quality of printed products.
Finding the minimum dot diameter In order to reasonably select the number of screen meshes, the relationship between the minimum dot diameter of the printing and the selected screen must be analyzed in depth.
In the halftone printing, after determining the number of screen lines and the percentage of bright and dark dots in the positive picture, the minimum dot diameter required for printing can be obtained.
The minimum dot diameter to be printed is proportional to the square root of the percentage of dots in the highlights, which is inversely proportional to the number of screen lines, ie:
D=1.1284×/L x 10000 (1)
In herbs, D—the minimum dot diameter (μm) to be printed
L——The number of screens for Yang pictures (line/cm)
H—Percentage of dots on the bright spot (%)
According to the formula (1), the number of different screen lines of the positive picture can be obtained, and the minimum dot diameter that can be printed by the percentage of different dots can be obtained.
The relationship between the screen and the minimum dot diameter In the screen printing, due to the limitation of the screen mesh, it will be difficult to print out the dots on the ultra-light or dark areas of the Yang picture. Therefore, the analysis of the relationship between the minimum dot diameter of the printing and the diameter and aperture of the silk screen silk and the search for its internal connection is an important prerequisite for reasonably determining the mesh number.
1. Basic basis (1) Reticle of mesh reproducibility: For a mesh, once the wire diameter is determined, the blocking effect of mesh on the mesh is a fixed value. The larger the wire diameter, the greater the barrier effect of the wire on the screen, and the larger the diameter of the smallest screen that can be reproduced. On the other hand, the smaller the wire diameter, the smaller the blocking effect of the wire on the screen, and the smallest that can be reproduced. The dot diameter also decreases accordingly.
(2) Influence of screen aperture on dot reproducibility When the wire diameter of the screen is determined, the larger the aperture of the screen, the larger the aperture ratio of the screen, the better the ink's permeation performance, and the more reproducibility of the dot. The smallest dot diameter that can be printed is also reduced.
(3) Using coarse mesh, it is impossible to weave high mesh screens. To weave high-mesh screens, fine mesh must be used, which is why the general use of high-mesh screen printing high-precision products is the root cause.
2. The relationship between the minimum dot diameter and the screen is discussed in three different situations.
(1) The wire diameter of the screen is larger than the aperture: For this type of screen, since the aperture ratio is the smallest, the ink transmission performance is the worst, and the dot loss is the most serious, so in general, it is not suitable for screen printing. To further illustrate this issue, a statistical analysis was conducted on the data provided by monofilament nylon and polyester mesh produced in Japan and Switzerland. It was found that in the specification of all wire meshes listed, polyester meshes were From 5 meshes to 495 meshes, nylon meshes range from 5 meshes to 510 meshes. Except for a few high mesh mesh meshes, the wire diameter is slightly larger than the pore size, and the ratio of the other wire diameters to the pore diameter is less than one.
(2) The wire diameter of the wire mesh is equal to the hole diameter: Compared with the wire diameter larger than the hole diameter, the wire dot reproduction performance is enhanced when the wire diameter is equal to the hole diameter. In order to seek the relationship between the diameter of the smallest dot of the printing and the diameter and aperture of the screen wire, it is used as a graphic method for discussion.
When the printed dot diameter is D=2d (d is the screen wire diameter), the dot loss is the most serious when the dot is exactly at the intersection point of the filament. At this time, the area of ​​the dots is 3.14d2, and the area occupied by the dots by the dots is approximately 3,14×d2-(2×2d×d-d2)=0.14d2. The proportion of mesh through-holes occupied by the outlets to the total area of ​​the outlets was 0.14d2/3.14d2=4.5%. Such a small ink transmission rate will result in the loss of small outlets. When the minimum dot diameter for printing needs D≥3d, the dot can cover at least one mesh even if the dot falls on the most unfavorable position on the screen. That is, as long as the minimum dot diameter of printing is D ≥ 3d, the selected mesh can reproduce this dot. Of course, the position of the dots on the screen is different, and the loss of the dots will be a little different, but the pressure of the squeegee when printing. The effect of the ink flow, the influence of the net distance, etc., will increase the number of outlets to a certain degree, and can compensate for the losses incurred by the outlets due to the action of the screen. In addition, at the time of development, the mesh that does not cover the photosensitive film also absorbs a certain amount of ink during printing, so that the through-hole portion covered by the screen mesh can also transmit a certain amount of ink, which is also when the screen is printed in the field. The reason is very good.
(3) The wire diameter is smaller than the pore diameter: Compared with the above two cases, the wire diameter is smaller than the pore diameter, and the dot reproducibility is the best. The dot with the diameter D≥3d of the dot to be printed can be better reproduced.
Through the above analysis, the following conclusions can be drawn: In the screen-mesh screen printing, regardless of the relationship between the diameter of the wire mesh and the aperture, the minimum diameter of the printed dot and the wire diameter of the wire mesh have the following relationship, namely:
D≥3d (2)
That is, as long as the minimum dot diameter to be printed is greater than or equal to 3 times the screen silk diameter, this dot can be reproduced.
Calculation of screen wire diameter After the above discussion, the relationship between the minimum wire diameter D that the print product needs to reproduce should be equal to or greater than 3 times the screen wire diameter d, then it can provide a convenient choice for the screen mesh number selection. Calculation method.
The technical parameter that we are most concerned about is the wire diameter of the wire mesh. If the wire diameter of the wire mesh can be calculated directly from the dot diameter that needs to be printed, the selection of the mesh mesh number will be greatly simplified. From the formula (1) and the above-discussed conclusions, the calculation formula of the screen wire diameter can be derived, namely:
D≤0.376 × /L×10000 (3)
Where d is the diameter of the selected wire mesh (μm)
L——The number of screens for Yang pictures (line/cm)
H—Percentage of dots on the bright spot (%)
It can be seen from the above equation that the wire diameter of the screen can be easily calculated as long as the number of screen lines of the positive picture and the percentage of the screen points of the highlight position are determined.
The relationship between the screen wire diameter and the screen mesh number After the wire diameter of the screen wire is determined, the corresponding screen mesh number can be found out from the technical information provided by the screen manufacturer. The corresponding relationship between the screen wire diameter and the mesh number of the monofilament polyester mesh produced in Japan (at present, the screen mesh diameter of the highest mesh size is not less than 30 μm).
For example, it is known that the number of screens for a positive picture is 28 lines/cm, and the percentage of screen points for a bright spot is 20%. Proceed as follows:
(1) From the known conditions from the table 3 to determine the wire diameter of the wire is 60μm;
(2) The mesh size corresponding to the 60 μm wire diameter should be equal to or less than 1 75 mesh/inch.
This article only analyzes the problem of how to determine the screen mesh number reasonably for reproducing the minimum dot print. In fact, in addition to the main factors that determine the number of screens, there are problems with the correct "thread-to-head ratio" in order to prevent printing moirés and the required thickness of the ink layer to match the number of screen meshes. Only by considering the above three factors can we finally determine the number of screens.