Screen printing is a printing technology that transfers ink through a screen plate to a substrate through a squeegee with a certain tilt angle and pressure to realize image reproduction.
1. The characteristics of screen printing
1) Many types and shapes of substrates
It is an indisputable fact that silk screen printing can be applied to the surface of various materials and the surface of special shaped substrates (such as curved or spherical surfaces). With the progress of screen printing equipment, it is also possible to print a variety of large-scale advertisements and curtains. , printing on ultra-small, ultra-high-precision surface (circuit board); because of its soft and flexible printing plate, small embossing force, it can not only print on soft substrates such as paper and textiles, but also add Pressure is easily printed on various glass and ceramic wares, so screen printing is a flexible printing method with wide applicability.
2) Applicable ink types
There are many kinds of inks that can be used for screen printing. Such as oil-based inks, water-based inks, resin-based inks, toners, various coatings, dyes, etc., so screen printing inks are commonly referred to as screen printing inks.
3) Thick ink layer and strong coverage
The ink thickness of the screen printing can reach 30 to 100 μm. Therefore, the coverage of the ink is strong, the hand feeling is good, the three-dimensional effect is strong, and the thickness of the ink layer can be controlled to a large extent.
2. Requirements for the screen
In screen printing, the printing plate is the main factor affecting the quality of printed products, mainly in the following aspects:
1) Reproducibility of printing plates
It contains the screen resolution, resolution, resolution and other indicators. The accuracy of the printing plate refers to the accuracy of the position and size of the plate's images, characters, symbols, and the like, and relates to the plate making materials and methods. The sharpness of the printing plate refers to the degree of neatness of the edge of the printing plate image in both horizontal and vertical directions, which is reflected by the characteristics and flatness of the mesh bridge. The resolution of a printing plate refers to the smallest printable image and space size, which is related to the parameters of the screen.
2) Thickness of printing plate film
It will affect the thickness and clarity of the ink layer, especially for small images. When the product requires a thick ink layer and thick lines, the corresponding film thickness is required.
3) Printing Force of Printing Plate
The resistance force of India refers to the ability of a printing plate to withstand the number of printings. There are many factors that affect the resistance to printing, and from the standpoint of platemaking, it depends mainly on the ability of the printed version to withstand mechanical and chemical effects and its fastness to the screen. The printing resistance of the screen should be compatible with the printing task, and the quality standard of the printing plate is not as high as possible, but instead, the economy is better as long as the printing needs are met.
3 plate making methods
Screen-printed plate production can be generally divided into hand-engraved filmmaking, direct-photolithography and indirect photosensitivity.
1. Direct Platemaking Direct platemaking, also known as direct photolithography, is the process of applying a photo-sensitive liquid directly on a screen to form a photosensitive film and processing it to form a printing plate.
The photosensitive screen made of liquid sensitized film is the most commonly used printing plate. The wide selection of performance parameters of the screen sensitizer makes it more versatile and suitable for various printing requirements (as shown in Figure 2a).
The screen-printing sensitizer is a semi-adhesive liquid. The coating amount of the sensitizer can be controlled by the edge of the applicator (sizing machine) during coating. The coating thickness can be measured with a thickness meter. Screens coated with sensitizers should be dried in warm air. Screen sensitizers are sensitive to UV light, but they are not sensitive to red and yellow light, making it possible to prepare and produce plates in yellow safe light.
The capillary electrophotographic film used in the direct plate making method is a prefabricated photosensitized film. The use of such a photosensitive film can select the plate thickness and parameters within a certain range to suit a variety of ink and printing applications.
The capillary photosensitive film is a precisely controlled photosensitive film that coats a photosensitive layer on a polyester film-based support. When affixed to the degreased wet screen printing surface, the surface of the sensitizer film is softened by the water in the screen, and the film and the screen are attached together by surface tension. After drying, the protective film is peeled off, ie, a flat and uniform thickness printing plate is formed (as shown in FIG. 2c). This screen printing plate has a small thickness and is flat and can achieve high printing quality.
The capillary photosensitive film can also be used in combination with the photosensitive adhesive, that is, the photosensitive adhesive is coated on the screen scraping surface, which can increase the printing plate resistance force (as shown in FIG. 2b). This method is suitable for printing on thick ink layer prints, which can produce three-dimensional three-dimensional effects on the prints.
2. Indirect plate making
Engraved filmmaking and indirect filming are indirect platemaking. This type of plate making method involves first making a plate, and then transferring the graphic on the plate to a screen to form a printing plate.
Engraved film is made by manually or computer-assisted cutting machine to remove the paint film on the image area, leaving behind the film in the non-image area, and then sticking it under the wire mesh and washing it with water or dissolving solution. net.
Indirect photolithography is the use of UV-sensitive photosensitive film, and the exposure of the positive image in ultraviolet light, using hydrogen peroxide dilute solution for development, and then water will be developed after the photosensitive film is not cured part of the graphics removed. The remaining diaphragms are glued to the screen with a special adhesive to form a printing plate.
The indirect plate making method can obtain better edge definition, but its film is thinner, the deposition amount of the ink on the screen is small, the printing ink is thin, and the abrasion resistance is weak, and it is generally suitable for short-run product printing, as shown in FIG. 2d. As shown.
The principles of direct plate making and indirect plate making are similar. The screens and positive pictures that have been given a photosensitive coating are exposed to ultraviolet light so that the exposed areas are not dissolved in water. The unexposed areas are soluble in water and are washed away with water mist. The exposed image area makes the graphic area a transparent mesh, which allows ink to pass through during printing. In the plate-making process is affected by some parameters, such as the number of screen mesh, photosensitive film thickness, ink chemistry and so on. Also need to consider ink performance parameters, such as water-based inks, solvent-based inks, plastic inks, UV-curable inks and other specialty inks.
(to be continued)