Screen printing process

In screen printing, special effects that cannot be achieved by some other printing methods have been widely used in high-end packaging, such as imitation metal etching, ice flower, and other well-known effects, and have been fully recognized by customers in beautifying packaging and improving product quality. . At the same time, with the development of science and technology, new processes and technologies are still emerging, and with the gradual maturation of the process, the texture effect has become one of the more popular surface finishing methods.

Process characteristics As we all know, imitation metal etching is the use of matte ink's own graininess, printed on the gold and silver card paper matte effect. The thickness of the sand, the feel, etc., are all determined by the ink used, and the limitations of adjustment in operation are large. On the basis of the imitation metal etching process, the embossing process absorbs some characteristics of the imitation embossing process. Starting from the plate-making film, the original background color is replaced by a uniform circular or square dot matrix pattern. Has a coarse cloth texture similar to matte. At the same time, the size, shape, etc. of the texture can be adjusted by the plate-making film, and the autonomy is stronger. The inks used in printing are no longer limited to matte inks. Some other types of inks, such as dimming inks and pearlescent inks, can also be used to provide different colorful effects and have higher aesthetic value.

Plate Making The quality of screen printing plates is an important prerequisite for obtaining fine grain printing effects. Therefore, it is very important to control the platemaking process.

First of all, when making a large-scale imprinted film, it is necessary to take into account the requirements for over-printing during printing, and certain gaps should be maintained between the links. Because the texture film is different from the real fill pattern. In the use of film pre-shrinking, the use of the past "digging" method to improve the overprint will undermine the integrity of the graphic and thus can only improve the overprinting using a single-unit overall movement method. The remaining gap will leave room for the later die-cutting process.

Second, the tension of the screen should meet the requirements. If the tension of the screen is insufficient, there will be a series of problems such as imprint stretching, overprinting, and hue instability during printing. In general, for a mesh of 300 mesh/inch, the tension should be controlled between 20 and 23 N/cm. In addition, the thickness of the coating photoresist should be moderate, to avoid the occurrence of uneven thickness and bubbles in a screen plate, so as to avoid a greater impact on the printing quality. Finally, for different screens with different number of screens, use a dedicated test strip to measure the exact exposure time. After each print, make a detailed record to achieve data management, so that the future work is well documented.

For the cloth pattern products mainly based on bitmaps, if the matching of screen mesh number and screen number is unreasonable, due to the limitations of the screen's own structure and the existing plate-making system, it is easy to have a moiré phenomenon that interferes with the text. Moreover, the higher the number of screen lines, the finer the graphics and the higher the probability of moiré. Through multiple proofing printing, the moire can be overcome by adjusting the angle of the printing pattern on a 300 mesh/inch screen to overcome the 60 line/inch dot. Based on this, the number of screen lines increased, and the number of screen meshes selected must be increased accordingly to improve accuracy and reduce moire. On the contrary, the screen mesh can be properly reduced to improve the feel.

If the angle is not set properly, the silk screen wire will interfere with the integrity of the dot, affecting the generation of the texture effect. There are two specific methods of operation: First, the “slanting and direct drying method” means that when the net is stretched, the screen and the frame are tilted at a certain angle, and when the printing is done, the angle between the film and the frame is kept unchanged; The “straight diagonal stretch drying method” means that the right angle is maintained when the net is stretched, and the film is tilted by 4 to 8 degrees in the net frame during the printing. The “straight diagonal drying method” can be adjusted on the printing press because the tilt angle is not large. The “slanting and direct drying” method is not recommended due to the need to purchase a special widening screen, and it is difficult to use when stretching the net and the screen is wasted. After the exposure, the screen should be developed with a high-pressure, low-flow spray gun to prevent the dots from being washed out due to insufficient water pressure, or the flow rate being too large to flush the dots. Before printing on the machine, check whether the screen is complete and whether the screen is full. For some suspicion of ink can be modified under the magnifying glass with an embroidery needle piercing method.

Ink printing screen printing effect, can choose more types of ink, generally according to different design requirements choose different types of ink. For some products that are mainly based on the actual background color and supplemented by the cloth pattern effect, pearlescent inks and color-changing inks of colored type can be used to cover the background color, so that they can produce pearlescent, discolored, and other colorful effects when changing angles. . If you use cloth texture as the main color directly on gold and silver card paper, you can use some colorless matte ink.

When selecting the ink, "over-net rate" is a parameter to be considered. The poor ink rate of over-netting rate has more residues remaining in the printing process. The printing sites that have just started printing are clear, and after a certain number of printing, there will be incomplete and incomplete dots. Should try to use some non-networking uV ink to avoid frequent washing in the printing, reducing labor intensity, at the same time, UV ink is also more secure than solvent-based ink, and environmental protection.

Paper In order to improve the quality of the product, more and more packaging options are coated with gold and silver cardboard. When using a colorless ink to print a cloth pattern effect, since the ink particle itself is colorless and transparent, the hue reflected by the product largely depends on the color of the gold and silver cardboard itself. If the same paper used in the same batch of paper has different shades of color and the difference is greater, then the hue of the printed product will certainly be inconsistent. Therefore, when purchasing paper, the supply department should not only consider the paper size, thickness, stretch rate and other indicators, but also should be included in the paper's color quality inspection of raw materials, so as to avoid paper due to the same batch of prints inconsistencies.

The problem that should be paid attention to in the printing process is that during the screen printing process, as the number of squeegees and the number of washing plates increases, the photosensitive layer of the screen plate gradually becomes thinner, and when a certain printing quantity is reached, the printed product is printed on the effect. There will be a big difference with the new version. Therefore, it is necessary to check the proofs in production and replace the screens in time.

When the printing effect is printed, the cleaning ability of the screen itself is worse than that of the printed document. When printing the actual graphics, the paper powder may drop by itself after printing several sheets, and there is no need for frequent washing. However, in the case of the printing effect, after printing thousands of sheets, the paper powder may still adhere to the screen, which has a great influence on the quality of the product. If the surface of the paper has a lot of paper and paper powder, the operators will be struggling to cope with it when printing. Therefore, when paper is cut, the generation of paper hair and paper powder should be avoided as much as possible. The offset printing process used in the screen printing process should be a UV offset printing process that does not use powder.

For the printed products using matte ink printing, hot stamping, polishing and other processes should be arranged after the offset printing process and before the screen printing process, to avoid hot stamping and inequality after screen printing.

Reprinted from: Printing Industry