What is a pad printing machine? What is the difference between it and screen printing?

What is a pad printing machine? What is the difference between it and screen printing?

Pad printing technology is a special printing technology that was introduced to China in the 1980s. Because of its significant advantages in printing on small-area, uneven surfaces, it compensates for the shortcomings of screen printing technology. Therefore, it has developed rapidly in recent years. In the early 1990s, with the further opening of the Chinese market, a large number of foreign-funded enterprises mainly engaged in traditional industries such as electronics, plastics, gifts, and toys entered the Chinese market. Pad printing and screen printing technologies, as the main decoration methods, have experienced extraordinary development. According to incomplete statistics, the application of pad printing and screen printing technologies in the above industries reached 27%, 64%, 51%, and 66% respectively.

Pad printing and screen printing technologies have many similarities, such as wide adaptability to substrates; a wide variety of inks, with pad printing inks and screen printing inks being basically interchangeable; and the ability to easily print different products by changing fixtures and printing plates. Therefore, in many cases, pad printing and screen printing technologies are inseparable, playing an important role in special printing. Because pad printing and screen printing technologies are so closely related, most printing experts classify it under the screen printing field. To date, there are no specialized institutions researching pad printing technology, nor are there institutions conducting detailed investigations into the market distribution of pad printing technology. Much of the data from pad printing companies cannot independently reflect the actual situation of pad printing technology.

However, pad printing and screen printing have many differences, such as: significant differences in the ink transfer process; different printing machine structures; and vastly different substrate types. In fact, the differences between them are far greater than the similarities. The reason we habitually bundle them together is because they face a common market. In this common market, pad printing cannot exist without screen printing, and screen printing cannot exist without pad printing. Separating pad printing from the screen printing industry is difficult and unnecessary. However, to gain a deeper understanding of the current status of pad printing technology, one cannot view it from the perspective of the screen printing industry.

We classify all printing involving paper as mainstream printing. Mainstream printing has always received great attention from the theoretical community. In recent years, screen printing has received attention primarily because its advantages in paper printing have been discovered, rather than its advantages in the range of substrates. For example, using screen printing to achieve partial UV coating on paper, and printing special UV inks such as coral ink, ice flower ink, and wrinkle ink, yields better results compared to offset and flexographic printing.

Pad printing technology has not been so fortunate. Today, all printing technologies are exploring how to leverage their respective advantages and achieve integration. Pad printing technology, however, maintains its independent identity. The connection between paper and pad printing technology remains blank. However, pad printing technology has not stopped its rapid growth because it excels in areas that no other printing technology can achieve. The prosperous industries in the Pearl River Delta strongly support the 18% annual growth rate of the pad printing industry. The job opportunities created annually by the pad printing technology-related industry have also been a focus of government departments.

Overall, pad printing technology has a low technical content. In developed countries, where production costs are high, continued development is difficult. Therefore, the transfer of the pad printing industry to developing countries like China with low labor costs is an inevitable trend. Currently, apart from a few foreign companies that still manufacture highly automated pad printing production lines, most companies have moved their production bases to China. This presents a historical opportunity for China. What is the current development level of pad printing technology in China? What are the latest advancements? This article offers some preliminary insights that may be helpful to readers.

I. Plate Making

Plate making is the prelude to printing. Traditional printing invariably involves first transferring the images and text from the original to the surface of the printing plate. In terms of pad printing plate making, it should be a combination of intaglio plate making technology and etching nameplate technology. Intaglio plate making technology has now developed into electronic engraving and laser engraving, and nameplate etching has gradually eliminated acid etching technology. However, pad printing plate making remains at a very old-fashioned stage, which is related to the conditions of pad printing technology itself. First, the cost of pad printing processing is relatively low, and the cost of electronic engraving and laser engraving is too high for most companies to afford. Many companies have proposed electro-etching technology, but no technology from modern warfare companies can guarantee that the etched printing plates can be used for printing. The acid etching process is easy to master and has low costs. Despite serious environmental pollution, it is still used by the vast majority of companies.

II. Pad Printing Steel Plate

Pad printing steel plates dominate pad printing technology. 100% of oil-pan-type pad printing machines and 16% of oil-cup-type pad printing machines use thick steel plates with a thickness of around 15 mm; another 77% of oil-cup-type pad printing machines use thin steel plates with a thickness of 0.25 mm and 0.5 mm. Whether it is a thin steel plate or a thick steel plate, the plate making process is the same. The base materials used are also roughly the same. The steel plate must have a fine organizational structure and a sufficiently high surface finish. The former ensures the neatness of the edges of the etched images and text, thereby ensuring the integrity of the dot structure, maximizing the sharpness and color reproduction of the printed images and text; the latter ensures the cleanliness of the ink scraping and the printing durability. The surface flatness tolerance of the thick steel plates used in oil-cup pad printing machines is even higher. Excessive surface flatness tolerance accelerates the wear of the oil cup and affects the ink sealing. Steel plates used in oil-pan-type pad printing machines, if used in oil-cup machines, must ensure higher surface finish and lower flatness tolerance.

Thin steel plates have better flexibility, so the cleanliness of ink scraping is easier to ensure. However, this is related to the manufacturing precision of the substrate supporting the thin steel plate. If the surface flatness tolerance of the substrate does not meet the requirements, the ink will leak during the sliding process between the oil cup and the steel plate. Another factor is the manufacturing precision of the oil cup sliding guide shaft, such as surface roughness and mating tolerance, which is reflected in the indicators of the overall machine assembly level. How to control the parallelism tolerance between the guide shaft and the steel plate is very important. From this point of view, thick steel plate oil-cup pad printing machines are more difficult to manufacture.

III. Resin Plate

Resin plates have not been used in pad printing technology for a long time, but they have already received attention. Resin plates mainly use photosensitive nylon resin, cast on the surface of zinc plates, similar to the resin plates used on pressure-sensitive labels. Nylon has very good wear resistance and, after photosensitive curing, can withstand repeated friction from the oil cup or scraper. The advantages of using resin plates are that they can easily obtain extremely fine dots. When printing exquisite small items, resin plates are the preferred choice. If the manufacturing of oil-cup pad printing machines reflects a company's manufacturing level, then manufacturing pad printing machines that use resin plates will be even more difficult. After all, photosensitive nylon resin belongs to a polymer material. When the substrate supporting it has poor manufacturing precision, it is not easy for the pad printing machine to complete ordinary printing. The surface state of polymer materials is very fine, and slight defects will seriously affect the printing quality.

IV. Plate Making Process

Currently, the production of pad printing steel plates mainly involves etching with a nitric acid solution and ferric chloride solution containing a protective agent. The acid used in this etching process has a strong and unpleasant odor, and the production of nitrogen monoxide and nitrogen dioxide is very harmful to human health. The development of the etched steel plates mostly uses chemical solvents based on hydrogen peroxide, which can also damage human skin. Therefore, it has been banned in European countries. They have started using resin plates or laser engraving for plate making. Using a powder-free etching machine for zinc plates to etch steel plates is a relatively novel idea, and some manufacturers have successfully tested this method, which greatly reduces environmental pollution. Resin plate making is much more environmentally friendly and safer. Resin plates use nylon photosensitive glue and can be developed with alcohol. With the popularity of digitalization, it won't be long before digital methods are used to produce pad printing resin plates.

V. Ink

Pad printing inks are still mainly solvent-based. Screen printing inks have been used as substitutes for pad printing inks, and the effect is also very good. Currently, the vast majority of ink manufacturers produce pad printing inks and screen printing inks as a series. However, many problems can arise when screen printing inks are used in pad printing processes. MARABU was the first company to develop pad printing inks. Compared to screen printing inks, dedicated pad printing inks have faster and more stable drying speeds and better thixotropy. The ink layer in pad printing is relatively thin. To obtain more striking characters, the pigments in the ink must have higher purity and finer fineness. These are the difficulties in manufacturing pad printing inks. Currently, in addition to MARABU's dedicated series, DUJO's pad printing inks from Germany are also beginning to occupy a certain market share. Apart from these, there are few dedicated pad printing inks. Coates inks from the UK, known for their screen printing inks, also have a considerable market share as a substitute for mid-to-low-end products.

The pad printing ink market is vast and profitable, attracting many domestic enterprises to conduct research and development. Domestic ink production has seen very rapid development recently, with prominent companies including Zhongshan Zhongyi Ink Co., Ltd., Hangzhou Kewang Ink Co., Ltd., and Shenzhen Meilihua Ink Co., Ltd.

UV inks have been widely used in screen printing, and the last bastion is that UV pad printing inks cannot be replaced by solvent-based screen printing inks. The production process of UV screen printing inks is much simpler. As long as the ink has good permeability, the printing effect is easy to guarantee. However, UV pad printing inks require that the pad printing head has good transferability to the acrylic resin monomers in the ink. Generally speaking, the pad printing head is made of silicone rubber, and the affinity between silicone rubber and acrylic resin monomers is very poor, which will affect the transfer efficiency. On the other hand, solvent-based inks must add necessary fillers to improve the neatness of the print edges. However, UV pad printing inks cannot add too many such fillers, which will not only affect the curing efficiency but also make it difficult to guarantee the transparency of the ink. In the research and development of UV pad printing inks, ensuring high pigment color concentration while maintaining good transparency and transferability is a challenge. These problems have now been overcome. MARABU, a professional manufacturer of pad printing inks, has fully launched its TPC series of UV pad printing inks to the market.

The successful development of UV pad printing inks has solved some problems that have long plagued people. For a long time, the gloss and wear resistance of the characters on computer keyboards have not met customer requirements. Using UV inks has become the goal of many manufacturers. However, screen printing UV inks has many problems. Now, using the pad printing method to transfer UV inks easily meets customer requirements. The characteristic of pad printing is that pressure is applied from directly above the substrate to complete the printing, so the printing process easily prevents the product from shaking.

VI. Cup-type Pad Printing Machine

China has developed rapidly in recent years, and the advantages of Western countries in traditional industries have gradually been replaced by China. In order to protect the development of their domestic industries, some European and American countries have formulated very strict market access systems to restrict the entry of Chinese products into the EU market, such as requiring that export enterprises to the EU comply with the ISO14001 environmental protection standard. Objectively speaking, the technical barriers of European and American countries are very unfavorable to Chinese enterprises, but in the long run, these systems may force us to continuously adopt new technologies to improve the technology and safety level of our products. In the field of pad printing technology, the adoption of more environmentally friendly technologies must be given attention by Chinese enterprises.

The solvents in solvent-based inks volatilize into the air and are captured by the human respiratory system, and most of them are harmful to the human body. This has been widely resisted by Western societies, which mainly advocate for the protection of human safety. European and American countries have successively formulated strict regulations to control the use of solvent-based inks. Germany was the first to produce a pad printing machine that controls solvent volatilization by cutting off the volatilization pathway. This is the cup-type pad printing machine. Its main difference from the traditional open pad printing machine lies in the difference in ink supply methods.

The ink supply part of the cup-type pad printing machine uses a cup containing ink and solvent that is inverted on the pad printing plate to continuously supply ink to the plate. Because the solvent volatilization is controlled, the viscosity and drying speed of the ink during use tend to be stable, which is more conducive to obtaining uniform and consistent printing quality. The stable printability is the most commendable advantage of the cup-type pad printing machine. The effective reduction of solvent volatilization also improves the working environment for operators. It is worth pointing out that the manufacturing level of cup-type pad printing machines reflects the level of a company's manufacturing capabilities. Currently, there are very few domestic manufacturers of cup-type pad printing machines with high-quality products.

VII. Application of UV Pad Printing Technology

The advantages of cup-type pad printing machines have been favored by enterprises that are applying for or have obtained ISO14000 environmental management certification. However, we cannot deny that cup-type pad printing machines do not absolutely eliminate environmental damage. Part of the solvent sealed in the cup achieves a dynamic equilibrium inside the cup, and another part is transferred to the product surface along with the pigment and resin. The solvent will eventually volatilize into the air. The main function of the cup is to make the volatilization of the solvent in the ink effective volatilization. Whether it is the ink loading and adjustment process or the cleaning process, we cannot completely eliminate the volatilization of the solvent. Therefore, cup-type pad printing machines will eventually face the challenge of UV pad printing technology. UV technology is an environmentally friendly technology that became popular in the 1980s. It uses photosensitive inks based on acrylic resin as the main component to replace the long-standing solvent-based inks. In the fields of offset printing, gravure printing, flexographic printing, and screen printing, UV technology is becoming more widespread. In contrast, UV pad printing technology has been slow to develop.

The reason for the slow progress of UV technology is that the development of UV inks faces some difficulties, as mentioned earlier. Any new technology will face various difficulties, but no difficulty can stop the development of UV pad printing technology. With more enterprises paying attention to UV pad printing technology, the popularization of UV technology in the pad printing field is only a matter of time.

With the successful development of UV inks, the main obstacles of UV technology have been eliminated, and other related technologies only require minor modifications before they can be put into production. For example, pad printing involves relatively small products, so it is easy to set up miniaturized curing devices. An integrated pad printing machine combining pre-press processing, printing system, and curing device should be put on the agenda. It is believed that the next few years will be a crucial period for the further improvement and maturity of UV pad printing technology.

VIII. New Highlights of Pad Printing Machine Technology

The development of pad printing machines towards automation should be a common consensus in the industry. This is the fundamental way to improve production efficiency and reduce costs, and it is also a necessary requirement for pad printing technology to transform from a labor-intensive industry to a technology-intensive industry.

Currently, the most common highly automated pad printing machines are those with turntable-type conveying systems. The printing process is fully automatic, but the processes of placing and removing workpieces still rely on manual labor, affecting the degree of automation. Most manufacturing companies still have shortcomings in addressing quality issues that arise during the printing process. Even for such pad printing machines, achieving truly automated printing faces many challenges.

General-purpose fully automatic pad printing machines are relatively rare, but there are many surprises in fully automatic pad printing machines designed for specific products. For example, fully automatic pad printing machines for printing CDs and fully automatic pad printing machines for printing bottle caps. The automation of pad printing machines essentially means the automation of product conveying; the pad printing part itself changes very little. Various new technologies have broadened our horizons, the introduction of other printing technologies has given us new inspiration, and customer interviews have fueled our passion for the practical requirements of pad printing machine automation. The following text may not represent the direction of development of pad printing machines, but can you resist its fresh appeal?

IX. Pad Printing Machines for Roll Paper

Any printing process faces two paper feeding methods: single sheets and roll paper. Pad printing is the only printing process without machines for printing roll paper. As mentioned earlier, pad printing technology is the only technology that has not merged with paper printing. If pad printing technology does indeed lack advantages in terms of paper, I think this is mainly due to the large format of the paper. Pad printing for printing types like conventional self-adhesive paper may not be a pipe dream. There are no technical problems in printing graphics on paper; the printing area is small, and compared to resin plate self-adhesive printing machines, pad printing roll paper machines are easier to manufacture, operate, and are less expensive. From a technical perspective, since screen printing can be used for roll paper printing, there is even less of a problem using pad printing for roll paper. To our understanding, some manufacturers are developing monochrome roll paper pad printing machines, and sample machines are expected to be available within a year.

X. Pad Printing Machines with Cleaning Devices

Japan was the first to add a pad cleaning device to pad printing machines, improving print quality. During the printing process, since the ink on the pad surface cannot be completely transferred to the printed matter, the remaining resin ink will gradually change the edges of the graphics on the pad surface, causing a decrease in clarity. There are two ways to solve this problem: one is to improve the ink release ability of the pad; the other is to add a pad cleaning device. The pad cleaning device is characterized by an additional printing action on the pad during the retraction of the pad printing pad, during which the residual ink on its surface is removed.

For multicolor printing, adding a cleaning device significantly improves the accuracy of register. If multicolor printing involves halftone reproduction, the phenomenon of halftone expansion will be significantly suppressed. Currently, there is no trend in China to add pad cleaning devices to pad printing machines, which may be related to our understanding of pad printing technology. We always believe that pad printing is not a very precise printing technology and that the details of the printing process do not need to be overly emphasized.

Multicolor Pad Printing Machines with Accurate Register. Traditional rotary pad printing machines have higher production efficiency but lower register accuracy. Precise halftone printing is impossible, and even slightly higher requirements for color block register are difficult to achieve. This is related to the inherent structural defects. In 2003, a Japanese pad printing machine manufacturer was the first to launch a new four-color pad printing machine. The movement of this multicolor pad printing machine is completely different from that of traditional multicolor pad printing machines; it consists of a printing device composed of four sets of pad systems rotating around a central axis, rotating 90° with each print; and an inking device composed of four sets of plate systems rotating around another rotating center, also rotating 90° with each print. Each pad system only contacts a fixed plate, resulting in very accurate register. As shown in the figure. This pad printing machine uses two precision rotary cylinders to control the rotation of the pad system and the plate system respectively. The structure is compact, the movements are coordinated, and there are almost no parts requiring machining, so the register effect is very good.

Many craft gifts require fine printing. Inaccurate register will inevitably affect the value of the product itself, so developing pad printing machines with accurate register is not optional. If we do not yet have better design ideas, we can refer to the above examples.