Mesh screen printing and elimination of moiré

【Abstract】This paper analyzes the moiré problem in fine color tone screenprinting, and discusses the method of eliminating moiré from five aspects according to one's own practical experience, and provides the necessary technological parameters.

[Keywords] screen number, screen angle, stretch screen, light-sensitive material, commonly used mesh tension value is:

S type 13(N/cm)

230～305 T type 14(N/cm)

HD Type 15(N/cm)

S type 12(N/cm)

330～420 T type 13(N/cm)

HD Type 14(N/cm)

The net is stretched and flattened to reach the index requirement. Bonding is performed after a period of time. After thorough drying, it is taken out for backup. It is best to store it for 24 hours before use to ensure stable performance of the net cloth and to help prevent moire or local moire. .

According to the specific screening angle of the film, it is necessary to perform angle-selecting stretch netting, and its purpose is to prevent the mesh from cutting the mesh points and destroy the original structure. Can also be in the film density of 60 ~ 70% between the best area to eliminate the moire under the stretching operation.

5 Requirements and Applications for Photosensitive Materials

Photosensitive materials are important basic materials for making screen printing plates, and their quality requirements are:

(1) High photospeed, which can achieve the purpose of energy saving, rapid and efficient.

(2) It has a suitable spectral sensitivity range, which is generally ideal in the 300 nm to 450 nm light region of the spectrum.

(3) The resolution is high and a line width of 100 μm to 50 μm can be produced.

(4) The film forming property is good, and the cured film has high strength and elasticity, and has strong anti-acid, alkaline and anti-organic solvents.

(5) The deep light transmission performance is good so that a thick film screen printing plate can be made.

(6) The binding force of photo-resist and screen is good, and the film is smooth.

(7) Good storage stability and long service life.

Diazo photosensitive adhesive is an ideal photosensitive material. It has a high resolution and a clear plate-making image. The photosensitizer is incorporated into the latex and preserved at room temperature for 1 to 2 months. When used, the photosensitive adhesive is applied directly on the screen plate. After the screen plate is coated once inside and outside, the printed surface is coated. After it is completely dried, it is repeatedly coated on the printing surface three or four times (but each time it is dried). , reaching a thickness of 6μm. In this way, the edges of the sun-dried halftones are neat, and there will be no expansion. If the screen is too thin, the dark part of the print (low-lying area) has already been stuck on printing.

Use the pre-coated photosensitive film (film film), first attached to the stretched screen, dried, then remove the transparent film base and then carry out the printing. It has extremely high resolution, chemical resistance and physical properties. The film surface is smooth and flat, and the film thickness is uniform on the four sides and in the middle to achieve the desired thickness. The sharpness of the dried image dots is good, and the level loss is small, which can make the copying effect more realistic and also conducive to the control of moiré.

6 Drying high quality printing plates

The film surface of the positive image should be tightly adhered to the surface of the screen printing film, and the vacuum exposure machine should be used for exposure when vacuum is applied. When printing, use a larger screen, and place the Yang picture on the center of the screen. Due to the squareness of the mesh in the middle area, there is also good stability, which also prevents the occurrence of moiré or local moiré.

The most important part of the printing process is the exposure conditions. If the conditions are not good, the printing will fail or the quality of the processed plate will be poor. Therefore, the exposure conditions should be based on the type of photosensitive material, the thickness of the coated photosensitive film, the type of light source, and the distance from the light source to the printing plate to determine the exposure time. Note that it is better to choose a blue-violet light source, the light zone is between 300nm and 450nm, its short wavelength, and strong irradiation power.

Point light source: iodine lamp, cold light source: ultraviolet lamp.

If the exposure time is too long, the high-profile part of the positive version of the positive version will be eaten by the light, causing even shadow difficulties. Insufficient exposure time will cause the printing plates to have more sand and sand, and there will be degumming phenomenon, and the low-staining sites will stick and die, which will increase the depth of the printing tone level. So the exposure time must be accurate, the development can be smooth, so as to produce a high-quality printing plate.

7 Preparation of screen printing tools and materials

Screen printing is the use of special rubber squeegee, a certain pressure on the ink in the printing plate, forcing the ink through the mesh on the plate to form a print on the substrate.

The squeegee is made of urethane rubber and has no bubble blisters. It has suitable hardness, abrasion resistance and solvent resistance. No matter manual printing or mechanical printing is an extremely important tool, it directly affects the quality of the printed matter.

Quality requirements for squeegee in sizing screen printing:

(1) The edge of the squeegee must be kept flat, because the unevenness of the squeegee will cause the ink to squeegee unevenly during printing.

(2) Since the hardness of the squeegee is different, the amount of ink to be ejected is also different, so the reproducibility of the image is also different. The hardness of the commonly used squeegee is mostly in the range of 70-85° (Shore hardness).

(3) In the printing process, due to the continuous friction of the scraper, the change of the shape of the edge of the scraper will bring about changes in the amount of ink to be transferred, and the wear resistance of the scraper is required to be stronger.

(4) The solvent in the ink causes the rubber of the squeegee to swell and cause the ghost of the printed matter. Therefore, the squeegee must have solvent resistance.

The use of ink in gradation screen printing is an extremely fine choice. The role of thixotropy ink in printing is extremely important. Thixotropy is a physical phenomenon of certain chemical components. When it is not in motion, it is half or half. Solid, and it becomes increasingly dilute once it is agitated. During operation, the ink can be fluidized by the continuous movement of the squeegee and the inked sheet, remaining in the liquid state in the screen plate and passing through the open area of ​​the printing plate. The thixotropy is converted by looser internal molecular exchange. The ink immediately passes through the screen and becomes “stable” on the substrate, stabilizing or gelatinizing, which limits the diffusion of the dots on the substrate. Variant, parallel, but there is a certain leveling phenomenon, which can well maintain the integrity of the printed image.

With the continuous updating and development of screen printing equipment, materials, and process technology, due to the increasing demand for high-end products, people are also required to compete with foreign products and technologies after entering the WTO, and further narrow the gap with foreign advanced levels in all aspects. Therefore, to increase the product's own high-tech content, we must continue to develop new processes, new technologies, and new products. In particular, the problem of eliminating moiré in fine-graded screen printing and eliminating it is the first problem to be solved. Based on the working practice, some technical parameters are available for reference.