Screen printing surface finishing technology

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 and ice we are more familiar with the effect, and in the beautification of packaging, improve product quality to obtain the full customer Accredited. At the same time, with the development of science and technology, new processes and technologies are still emerging. With the gradual maturation of the process, the grain pattern effect has become one of the more popular surface finishing methods.

Process characteristics

As we all know, imitation metal etching is to use the grainy feeling of the matte ink to print matte effect on gold and silver card paper. The thickness of the sand and the feel of the sand 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 features of the imitation embossing process. Starting from the plate-making film, the original background color is replaced by a uniform circular or square dot pattern, and the original color is printed. 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.

Platemaking

The high-quality screen printing plates are 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, the need for overprinting should be taken into account. There should be a certain gap between each link. 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 method using a single unitary move. 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 air bubbles on 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 accurate 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.

Oil ink

Screen printing effect of cloth pattern, you 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 residual residue during printing, and the printing points at the beginning of printing are clear. 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.

Sheets of paper

In order to improve the quality of products, 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 into the scope of quality inspection of raw materials, so as to avoid the paper due to the same batch of prints inconsistencies.

Problems to pay attention to in printing

In the process of screen printing, as the number of squeegees and plate was increased, the photosensitive layer of the screen gradually became thinner. When a certain amount of printing was reached, the printed product would have a greater effect on the new version. The difference. Therefore, it is necessary to check the proofs in production and replace the screens in time.

When the printing effect is printed, the screen's own cleaning ability is worse than that of the printed ones. 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.

Keywords: Screen Printing/Surface Finishing

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