Overview of the hottest pad printing process

2022-07-24
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Overview of pad printing process (Part 1)

to print on the surface of small workpieces such as buttons or special-shaped surfaces such as toothbrush handles, the silk printing process shows great limitations, and the pad printing process can be completed very satisfactorily. In fact, pad printing process does not exist independently. It is always produced in special printing enterprises like the twin sisters of silk printing, and occupies an important position in decorative printing in plastic, electronics, toys, clocks and other industries

I. principle of pad printing process

pad printing process is a bit like a rubber stamp, but the rubber stamp itself is made into embossed text, dipped in ink, moved to the required place and pressed once to print simple words. The pad printing process completes the ink transfer with the help of the deformable fine ball screw pair fine ball screw pair transmission, which has small friction resistance, high processing precision, and can ensure the coaxiality between the upper and lower clamps. The surface of the pad printing rubber head does not contain pictures and images; The required pictures and texts shall be first dried and etched into a pad printing intaglio, then the pad printing intaglio surface shall be coated with ink, the ink in the non picture and text area on the printing plate surface shall be scraped with a hard scraper, the ink in the concave of the printing plate shall be dipped with a rubber head, lifted, and moved to the surface of the substrate for embossing to complete the printing. As the pad printing process adopts the indirect printing method, there is no strict requirement on the relative position of the printing plate and the substrate, and the pad printing rubber head can be made small, so it is very suitable for printing small workpieces; Moreover, the pad printing rubber head is easy to change into a shape consistent with the surface trend of the substrate. It has irreplaceable advantages for the printing of concave convex uneven surface, frosted surface, spherical surface, arc surface, etc

II. Pad printing plate

the traditional printing process uses the printing plate as the image and text carrier to distinguish the image and text area from the non image and text area. The pad printing process is no exception. The pad printing process uses a flat intaglio plate with graphics and text concave to the plate plane as the image and text carrier. It is similar to the intaglio cylinder used in intaglio printing, and its production principle and method are also similar, but it is not cylindrical. Another difference is that the pad printing process is widely used in labor-intensive special printing enterprises. The cost should not be too high. The printing plates used are not mainly chrome plated copper plates, but mainly alloy steel with Rockwell hardness HRC

1. Pad printing steel plate with a thickness of 10 ~ 15mm

the earliest pad printing machine used an ink tray for ink supply. It mainly used pad printing steel plate with a thickness of 10 ~ 15mm. After grinding, polishing and grinding, the substrate achieved a surface roughness Ra of 0.025 μ M mirror effect can be used for plate making. First, a layer of polyvinyl alcohol photosensitive adhesive is coated on the surface of the steel plate. After drying, the plate printing film is pasted, exposed under the vacuum plate printer, developed with water (Fuji photosensitive adhesive uses special gasoline), and etched with corrosive agent to produce concave text, which can be printed on the machine. At present, domestic pad printing machines generally use thick steel plates

2 the green wave of pad printing steel sheet with a thickness of 0.25 ~ 0.5mm swept through the 1980s has also been pouring into the printing industry. Offset printing, flexo printing, gravure printing, etc. at the top of the wave have washed away the dust and changed into green. The special printing industry located in the trough is more or less affected. Although it is only a small change, it also makes people feel that the health requirements of people engaged in the special printing industry are also very important

the pad printing process is mainly based on solvent based silk printing ink. The volatilization of solvent is harmful to human body. However, the main technical indicators of the water-based ink bridge expansion joint mechanical property tester are not realistic. First, the steel plate is easy to rust, second, the ink viscosity cannot meet the requirements, and third, the pad printing adhesive head has little affinity for water-based ink. Therefore, foreign countries first put forward the design idea of ink cup for ink supply. The solvent based ink is enclosed in an oil cup. The oil cup absorbs the thin steel plate by its own magnetic force, and achieves the two purposes of inking and scraping through the reciprocation of the oil cup, which not only prevents the ink solvent from volatilizing, but also eliminates the scraper device. The reason why the ink cup is supplied by thin steel plate instead of thick wood plate is mainly due to the requirements of assembling and disassembling the ink cup. The thin steel plate is easy to disassemble and turn over, and the ink cup is removed. If the ink cup is removed directly, the ink will be left on the steel plate. The thick steel plate is too thick and heavy, so it is not flexible to assemble and disassemble

most of the pad printing steel sheets are pre coated with polyvinyl alcohol photosensitive adhesive and protected by black PE film. When using, the protective film can be directly removed. The printing process and corrosion process are the same as those of thick steel sheets

3. The nylon intaglio

pad printing plates with a thickness of 0.5 ~ 1mm also tend to be non-metallic. Among thermoplastics, nylon has excellent wear resistance and corrosion resistance to solvents in inks. In addition, ternary nylon, which has been modified to meet the technical requirements of the time assay oil tank, is still an ideal photosensitive material. Therefore, nylon intaglio has the trend to replace pad printing sheet steel, and nylon intaglio has also laid a material foundation for the future realization of digital plate making

nylon gravure is usually precoated with nylon photosensitive adhesive on the zinc plate or aluminum plate. Because the ink cup is in direct contact with the nylon surface during the printing process, and the nylon layer is separated from the base zinc plate or aluminum plate, there is no need to worry about their strength

the nylon intaglio plate is supplied with a fixed size and protected with PET film. Remove the protective film during use, apply and expose the print film, develop it with ethanol, and rinse it with warm water

4. Chrome plated copper plate

sometimes chrome plated copper plate is used for high-precision hierarchical printing. The copper material has a fine organizational structure, which is easy to show the point structure, which is very conducive to showing the details. Copper plate can also be used for hierarchical printing of small dishes and wine bottle caps by pad printing process

III. pad printing adhesive head

pad printing adhesive head can be said to be the symbol of pad printing process, which plays the role of adsorption and transfer of ink. From the point of view of whether the printing plate is in contact with the substrate, printing can be divided into direct printing and indirect printing. Offset printing relies on blanket to transfer ink, which is the representative of indirect printing; Embossing, flexo printing and gravure printing products are selected by sinolion, an internationally renowned daily chemical supplier, and silk printing is an obvious direct printing. Obviously, pad printing process also belongs to standard indirect printing. Pad printing head and blanket have a great impact on printing quality

1. The shape of the pad printing rubber head

the difference between the pad printing process and the offset printing process is that the blanket is in line contact with the plate cylinder and the rubber cylinder; The contact between the pad printing rubber head and the printing plate and substrate is surface contact. The biggest disadvantage of surface contact is that it is easy to pinch air and form bubbles. For this reason, the pad printing rubber heads are designed in the shape shown in the figure. During the stamping, the middle part first contacts the printing plate or substrate. With the increase of pressure, it gradually extends to all image and text areas, and the air is squeezed out step by step. (to be continued)

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