1. Strong adhesion (to printing surface).
2. Good liquidity. Some inks are pasty and cannot flow in oil cups or basins.
3. When the diluent evaporates as the ink is absorbed, it can exert its viscosity.
4. the pigment should be fine, so that the luster is more obvious and the transparency is reduced. The thickness of ordinary ink is 7 microns in general printing.
5. The gloss of the ink will not change due to the printed surface.
6. Quick drying without changing its viscosity.
In order to obtain ideal printing effect, transferable ink must be used. Because pad printing transfers less ink each time, it requires high color content of ink. The characteristics of ink are as follows: easy to operate; Long storage period; Do not endanger health and protect the environment; Easy to clean; Good adhesion, and the material does not need pretreatment and post-treatment; High color density; Good covering and fine ink; Short thread, no wire drawing; Quick drying; Lower than the substrate surface tension and ideal viscosity;
1, what is pad printing?
Definition of 1. 1 pad printing
Pad printing is a special printing method. It is to make the pattern to be printed into an intaglio by means of photo-plate-making, and then transfer it to the printed object through a special silicone printing head, so that characters, figures and images can be printed on the surface of irregular objects. Now it is becoming an important special printing. For example, the characters and patterns on the surface of mobile phones are printed in this way, and the surface printing of many electronic products such as computer keyboards, instruments and meters is completed by pad printing.
The pad printing process is very simple. With a steel (or copper, thermoplastic) intaglio and an arc pad printing head made of silicone rubber, the ink on the intaglio is dipped on the surface of the pad printing head, and then pressed on the surface of the desired object, so that characters and patterns can be printed.
1.2 pad printing principle
Principle of transfer printing: first, the designed pattern is etched on the printing plate, and the etched printing plate is coated with ink. Then, most of the ink is transferred to the printed object through the silica gel head.
The specific transfer process is as follows:
1. When the pad head moves forward, the sucker rod etches the pattern and fills it with ink.
2. When the pad head moves backward, the scraper scrapes off the excess ink on the steel plate, leaving only the ink on the pattern.
3. The pad presses on the etching and pulls up the pattern.
4. The pad head moves forward to the printing position, and the printing plate is filled with ink again.
5. The pad is pressed to the surface of the product, and the ink is transferred to the product with other shapes.
6. Due to the characteristics of silica gel, when the transfer is completed, the glue head will return to its original state.
Figure 1. 1. 1 transfer process
Methods of coating ink on etched plate (steel plate): There are many methods of coating ink on etched plate. First, spray the ink on the etching plate, and then scrape off the excess ink with a retractable scraper. At this time, the solvent in the ink left in the etching area volatilizes to form a gelatinous surface, and then the glue head descends to the etching plate to suck the ink away.
Ink absorption and printing: the rubber head absorbs most of the ink on the etching plate and rises. At this time, this part of the ink volatilizes, and the remaining wet ink surface is more conducive to the close combination of printed matter and rubber head. The shape of the glue head should be able to produce rolling action and discharge the excess air on the surface of the etching plate and ink.
In the production process, the best cooperation between ink and glue head is to transfer all the ink on the etching plate to the printed matter. In the production process (nearly 10 micron or 0.0 1 mm thick ink is transferred to the substrate), offset printing is easily affected by air, temperature and static electricity. If the evaporation speed and dissolution speed are just controlled in a balanced state in the whole process from the etching plate to the transfer glue head and then to the substrate, then the printing can be considered successful. If it evaporates too quickly, the ink will dry before it is absorbed. If the evaporation is too slow and the ink surface has not formed gel, it is not easy to stick the glue head on the substrate.
1.3 present situation and development direction of pad printing technology
Pad printing is a special printing technology, which has obvious advantages in products with small printing area and uneven surface. Plate-making is a prelude to printing, and pad printing is a combination of gravure plate-making technology and sign corrosion technology. The operation process of acid corrosion is easy to master and the cost is low. Although the environmental pollution is serious, it is still adopted by most enterprises. Steel plate must have fine structure and high surface finish. The former ensures the neatness of the edges of corroded pictures and texts, and then ensures the integrity of dot structure, so as to maximize the sharpness of cutting and the reduction of color of printed pictures and texts; The latter ensures the cleanliness and printing resistance of scraping ink.
The application of resin plate in pad printing process is not long, but it has been paid attention to. The main resin plate is nylon photosensitive glue, which is cast on the surface of zinc plate, similar to the resin plate used in self-adhesive trademark. Nylon has very good wear resistance and can be used for repeated friction of scraper after photosensitive curing. The advantage of using resin plate is that it is easy to obtain extremely fine dots. Resin plate is the first choice when printing delicate small objects.
At present, the production of pad printing steel plate is mainly corroded by nitric acid solution and ferric chloride solution with protective agent. This corrosive acid smells bad, and the nitric oxide and nitrogen dioxide produced are very harmful to human body. After the steel plate is corroded, most chemical solvents with hydrogen peroxide as the main body are used, which is also harmful to human skin. So it has long been banned in European countries. They began to use resin plate or laser carving to make plates.
At present, pad printing ink is still mainly solvent ink, which has faster and more stable drying speed and better thixotropy. The ink layer of pad printing process is relatively thin. In order to obtain more eye-catching handwriting, the pigment in ink has higher purity and finer fineness, which are the difficulties in manufacturing pad printing ink. In recent years, domestic ink production has developed very rapidly, among which Zhongshan Zhong Yi Ink Co., Ltd., Hangzhou Kewang Ink Co., Ltd. and Shenzhen Meihua Ink Co., Ltd. are the most prominent.
UV pad printing ink needs pad printing rubber to transfer acrylic monomer in the ink well. Generally speaking, pad printing adhesive is made of silicone rubber, and the poor affinity between silicone rubber and acrylic monomer will affect the transfer efficiency. On the other hand, solvent-based ink must add necessary fillers to improve the finishing uniformity of the imprint. However, UV pad printing ink can't add too much filler, which will not only affect the curing efficiency, but also make the ink transparent.
Development of pad printing machine: from traditional open pad printing machine to oil cup pad printing machine. The solvent in solvent-based ink will be captured by the human respiratory system when it volatilizes into the air, most of which are harmful to human health. In order to protect human safety, Germany first produced a pad printing machine to control solvent volatilization by cutting off solvent volatilization, that is, an oil cup pad printing machine. The main difference between it and the traditional open pad printing machine lies in the different ink supply methods. In the ink supply part of the ink cup pad printing machine, the ink cup filled with ink and solvent is used to continuously supply ink to the pad printing plate. Because the volatilization of solvent is controlled, the viscosity and drying speed of ink tend to be stable during use, which is more conducive to obtaining uniform printing quality.
Development from oil cup pad printing machine to UV pad printing technology. The oil cup pad printing machine has not completely eliminated the damage to the environment. Part of the solvent sealed in the oil cup reaches a dynamic balance in the oil cup, and the other part will be transferred to the surface of the product together with the pigment and resin, and the solvent will eventually evaporate into the air. The main function of oil cup is to change the evaporation of solvent in ink into effective evaporation. Neither the debugging process of ink loading nor the cleaning process can completely eliminate the volatilization of solvent. Therefore, the oil cup pad printing machine will face the challenge of UV pad printing technology sooner or later. UV technology has long used photosensitive ink with acrylic resin as the main component instead of solvent-based ink. The related technology can be put into production with a little modification. For example, pad printing involves relatively small products, which can be easily realized by setting a miniaturized curing device. The pad printing machine integrating prepress treatment, printing system and curing device should be put on the agenda. It is believed that in recent years, UV pad printing technology will be a critical period for further improvement and maturity.
The new highlight of pad printing technology: the development of pad printing machine to automation should be recognized by the industry, which is the fundamental way to improve production efficiency and reduce costs, and it is also the inevitable requirement for pad printing technology to change from labor-intensive industry to technology-intensive industry.
At present, the pad printing machine with high degree of automation is the pad printing machine with turntable conveying, which is completely automatic in the printing process, but the process of placing and taking out the workpiece still depends on manual work. The degree of automation loves to influence. There are still some imperfections in the consideration of quality problems in the printing process of most production enterprises, and even such pad printing machines will face many challenges if they want to complete real automatic printing.
Web pad printing machine: Any printing process is faced with two paper feeding methods: one is sheet-fed paper and the other is web paper. The only machine that does not print web during pad printing. As mentioned above, pad printing technology is the only technology that is not integrated with paper printing. If pad printing technology has no advantage on paper, it should be mainly attributed to the large paper format. It may not be an idiotic dream to carry out the pad printing process like conventional self-adhesive paper. There is no technical problem in printing pictures and texts on paper, and its printing area is very small. Compared with resin plate self-adhesive printing machine, pad printing web machine is less difficult to manufacture, more convenient to operate and lower in machine price. From a technical point of view, because screen printing can be used for web printing, there is no problem in pad printing for web printing. Some manufacturers are developing monochrome web pad printing machines, and it is estimated that there will be a prototype in 1 year.
Pad printing machine with cleaning device: Japan first installed rubber head cleaning device on pad printing machine, which improved printing quality. In the printing process, because the ink on the surface of the rubber head can not be completely transferred to the printed matter, the residual ink of the resin will gradually change the edge of the graphic trace on the surface of the rubber head, resulting in a decline in clarity. There are two ways to solve this problem: one is to improve the deinking ability of rubber head; The other is to install a rubber head cleaning device. The characteristics of the rubber head cleaning device are: in the process of pad printing rubber head returning, there is one more printing action on the tape than ordinary pad printing machine, and the residual ink on its surface will be removed.
Multi-color pad printing machine with accurate overprinter: The traditional rotary pad printing machine has high production efficiency, but the overprinter accuracy is poor. It's hard to say that fine dot printing can't be completed, and it's hard to say that the color block overprint is slightly higher. This is related to its own structural defects. In 2003, Japan took the lead in introducing 1 new four-color pad printing machine, and its movement form was completely different from the traditional multi-color pad printing machine. It consists of four groups of rubber head systems rotating around the central axis, which rotate 90 degrees each time; The ink supply device consists of four groups of printing plate systems rotating around another rotation center, and each group of rubber heads only contact with the fixed printing plate, so the overprint is very accurate. This pad printing machine uses two precision rotating rollers to control the rotation of the rubber head system and the printing plate system respectively. Compact structure, coordinated action, almost no parts processed by the mechanism, and good overprinter effect.
2, pad printing ink
2. 1 ink composition
Ink contains binders, pigments, solvents and additives. Adhesives contain one or more resins, which are powders in most cases and must be dissolved in a suitable solvent or mixed solvent. The selection and synthesis of resin determines the printing characteristics of ink, such as transfer and adhesion to substrate, gloss and chemical resistance.
Solvent is the decisive factor of the drying speed of pad printing ink, and it also affects the printing speed and adhesion. The influence of slow-drying agent on pad printing is not too great, because the speed of pad printing is very fast, and it is only needed when using quick-drying ink. Slow drying agent volatilizes slowly, which will not have much influence on printing.
Solvent contains diluent, which can be mixed with ink to obtain ideal viscosity, which is convenient for ink transfer. Pigments reflect the hue of ink and determine its background color, which can be divided into organic pigments and inorganic pigments, and require that pigments do not contain heavy metals.
Additives are usually used in small amounts to adjust the characteristics of ink. Additives include leveling agent, thickener, etc. No matter what kind of ink it is, you should adjust as much as you use at a time. In order to ensure the printable viscosity, the ink should be properly adjusted during pad printing to ensure continuous pad printing.
Because this aspect has been introduced in detail in the painting part, I won't go into details here.
2.2 Basic characteristics of pad printing ink
Because the construction method of pad printing ink is different from baking paint, its ink still has different characteristics of paint to match its unique construction method. The main features are as follows:
1. Strong adhesion (to printing surface).
2. Good liquidity. Some inks are pasty and cannot flow in oil cups or basins.
3. When the diluent evaporates as the ink is absorbed, it can exert its viscosity.
4. the pigment should be fine, so that the luster is more obvious and the transparency is reduced. The thickness of ordinary ink is 7 microns in general printing.
5. The gloss of the ink will not change due to the printed surface.
6. Quick drying without changing its viscosity.
In order to obtain ideal printing effect, transferable ink must be used. Because pad printing transfers less ink each time, it requires high color content of ink. The characteristics of ink are as follows: easy to operate; Long storage period; Do not endanger health and protect the environment; Easy to clean; Good adhesion, and the material does not need pretreatment and post-treatment; High color density; Good covering and fine ink; Short thread, no wire drawing; Quick drying; Lower than the substrate surface tension and ideal viscosity; For various substrates)