Waterless offset printing is an ideal process with low overall cost and high printing quality, but the industry lacks a unified understanding of the current value and major development potential of waterless offset printing, resulting in a total of over 30 years of development history of technology. Is not given the attention it deserves.
The most obvious difference between waterless offset printing and water offset printing lies in the different printing plates used. According to the printing plate method, waterless offset printing can be divided into two categories: the traditional photosensitive waterless offset printing technology led by Japan Toray Corporation and the digital waterless offset printing system represented by the United States Presstek. The traditional photosensitive offset printing was first developed by the 3M company in the United States in the late 1960s. In 1972, Toray purchased the patented technology obtained by the 3M company in waterless offset printing, and began to work in this area. Continue to develop. The first practical waterless offset printing technology was introduced by Toray Industries of Japan at Drupa International Printing Exhibition in 1977. The development of the waterless offset printing technology of the United States Presstek Company experienced several stages such as direct printing (GTO-DI of Heidelberg), a computer-to-plate system, and rapid control of direct imaging technology. Many of the world's printing press manufacturers now use the direct imaging system of the United States Presstek as a key component of their waterless offset presses, such as Heidelberg Speedmaster GTO-DI46-4, Karat 74, Ryobi 3404-DI, and Adast 705-. C and so on. Presstek's image separation system from the United States Presstek has extended the computer-to-plate waterless offset printing technology to a wider range of applications and has become a pioneer in digital waterless offset printing. In recent years, the waterless offset printing market has developed rapidly and its market share has continuously expanded. With the joint efforts of various printing plates, equipment, inks and other manufacturers, the achievements made by waterless offset printing have made it a place. Today, there are more than 2,000 waterless offset presses in the world.
Traditional waterless offset printing technology:
Traditional waterless offset printing technology is represented by Toray of Japan. Compared with water offset printing, Toray Photosensitive Waterless Offset Printing System has three parts: photosensitive waterless offset printing plate, special ink specially designed for waterless offset printing, and special temperature control system. .
1. Structure and production of waterless offset printing plate
Currently the most widely used Toray waterless offset printing plate is a roller compacted plate, which is a printing plate with a special layer of ink repellent silicone rubber. This silicone rubber layer can resist ink because it has a very low surface energy, and the viscosity of the ink is particularly large, which makes the cohesion of the ink greater than the adhesion between the ink and the silicone rubber layer, so the silicone rubber layer Macroscopically, it is expressed as ink repellent. Unexposed plate is divided into 5 layers.
(1) Protective layer. The protective layer is seamlessly and tightly connected to the underlying silicone rubber layer, which can protect the printing plate from being scratched or subject to human damage during manual operation.
(2) Silicone rubber layer. The silicone rubber layer constitutes a non-exposed portion, that is, a non-graphic portion.
(3) Photosensitive layer. The cross-linking reaction occurs between the photosensitive layer and the silicone rubber layer, so that the photosensitive layer and the silicone rubber layer are tightly connected to form a unitary body.
(4) Glue layer. The glue layer tightly adheres the aluminum plate and the photosensitive layer to prevent the two from falling off. It can also absorb the light reflected from the aluminum plate, reducing the dot gain during plate making.
(5) Aluminum plate base. Aluminum base is the main support for printing plates.
There is no obvious difference between the process of making Toray offset printing plates and water offset printing, and it can be exposed by using the same plate-making machine and printing plate light source with water offset printing. The exposure time is similar to that of a water offset printing plate, but its internal principle is different. The development device and the developer are specially designed. This type of plate can be stored for a long time and can be recycled like an ordinary plate.
Toray's waterless offset printing plates can be exposed using positive and negative images. Their exposure reaction mechanism is the same except that the developer used during development is different. If using a negative image exposure, develop a special developer to make the photosensitive part fall off, leaving the silicon rubber layer of the unexposed part (non-graphic part); if you are using a positive image exposure, use another special type of development. The chemicals caused the unexposed parts to come off, leaving the silicone rubber layer of the photosensitive part (non-graphic part), but the final result was the same. The following is an example of exposure and development of a positive picture, and its process of plate making is introduced. Figure 2 shows the entire process of exposure and development of a positive image. There are 5 main stages.
1 before exposure: the positive image and the printing plate closely attached together, the vacuum in the middle, so that the positive picture on the printing plate of the first layer of protective film on the exposure frame for exposure. The light source and time used for exposure are basically the same as those of ordinary offset printing plates.
2 Exposure stage: Exposure through the positive image, light is transmitted through the non-graphical portion of the positive image onto the plate, the photosensitive layer is partially exposed, photopolymerization takes place between the photosensitive portion and the silicone rubber layer thereon, so that two layers Tightly cross-linked together to form a whole.
(3) Developing stage: Firstly, the printing plate is soaked with specially formulated special chemicals during development, and the silicon rubber layer of the exposed part (non-graphic part) of the plate after the soaking protrudes because it is not cross-linked with the photosensitive layer.
(4) Fixing stage: Mechanical methods, such as brushing with a special brush, remove the part of the protruding uncrosslinked silicone rubber layer to expose the photosensitive layer part below (image part), and use the oleophilicity of the photosensitive layer to create the ink layer.
5 Inking stage: Since the silicone rubber layer has oleophobicity, the ink can only be carried in the lipophilic area of ​​the graphic part, ie, the whole process of plate making and inking is completed.
2. Waterless offset ink
There is no difference in the choice of pigments for waterless and offset inks. The main difference between them is the resin used in the ink, that is, the binder. Due to the nature of waterless offset printing, the inks used have the following special properties compared to water-based offset inks.
(1) There is a need for higher viscosity and viscosity than a water-based offset ink, so that it is possible to provide a larger ink cohesion to be larger than the force between the ink and the silicone rubber layer, so that the silicone rubber layer becomes sparse. Oily, so that it plays the role of water in the water offset printing.
(2) must meet the special rheological properties of waterless offset printing. Because the waterless offset ink has a high viscosity, its flow between the ink roller and the plate becomes difficult, which requires a good rheological property of the ink.
(3) Due to the special influence of the temperature in waterless offset printing, the waterless offset ink preferably has a relatively wide printing temperature range.
3. Waterless offset temperature control system
Another important component of the waterless offset system is its temperature control system, which is one of the most obvious differences between it and water offset on the mechanical structure. In water offset printing, in addition to maintaining the balance between water and ink, another important role of fountain solution is to take away the heat generated by the high-speed rotation of the plate, so that the plate can be maintained in a relatively stable, suitable for ink transfer Within the temperature range. Since no water is used in waterless offset printing, a temperature control system is needed to maintain a temperature range suitable for ink transfer. The heat of offset printing mainly comes from two aspects: one is the friction between the plate cylinder, the blanket cylinder and the impression cylinder; the second is the characteristics of the high viscosity of the waterless offset ink, which makes the ink roller and ink roller between Heat is generated due to friction during the ink transfer process. If these heats are not eliminated in time, the temperature of the printing plate and the ink roller will rise sharply, and the rheological properties and viscosity of the ink will be greatly changed, resulting in extremely unstable conditions, and the normal transfer of the ink cannot be completed.
One of the more commonly used temperature control systems is through the hollow part of the tandem roller. The circulation of the coolant takes away heat to lower the temperature so that it can be maintained in a relatively stable temperature range suitable for ink transfer. This cooling method has been used in high-speed web printing for many years, so it is technically mature. Nearly all printer manufacturers offer hollow ink fountains, and this is an economical solution. Figure 3 shows a waterless offset temperature control system. It should be noted here that this device is only used to maintain a stable temperature range, and not to freeze the ink, because if the temperature is too low, it will cause the viscosity of the ink to rise. As a result, the cohesion between the inks is increased. Inhibit the separation of ink, and even the phenomenon that the ink cannot be transferred to the printing plate. With this cooling method to control the temperature, in order to achieve good results, the location of the ink roller must be optimized. With this temperature control system, it is possible to ensure that the ink is transferred within a suitable temperature range.
Digital waterless printing technology:
In recent years, with the development of computers and information technology, the digital workflow has become the mainstream development direction of various industries. Digital waterless offset printing technology is developed under this background. It has become one of the most rapidly developing and promising technologies in the printing industry. At present, the world's digital waterless offset printing is dominated by the solution of the United States Presstek. Its direct imaging system (DI) and Pearl Dry printing plates are widely used in digital waterless printing systems of major printing press manufacturers, such as Heidelberg Speedmaster GTO-DI and KBA's new Karat 74.
Compared to offset printing, the Presstek system consists of two special parts: the printing plate and the special ink. Among them, the ink used has the same ink characteristics as the Toray company's printing plate, and its temperature control device is optional. This is because the maximum print number of the Pearl Dry plate is 50,000 sheets. Below 10,000 sheets, the company's technical department believes that printing temperature changes will not be a particularly important factor. Its impact on the Pearl Dry printing plate is not very prominent, so it can be selected according to the requirements. And printing plates can also be recycled.
Presstek's digital waterless offset printing plate is divided into four layers. The bottom is an aluminum plate or polymer plate base. The top is a lipophilic layer, and the top is an image layer. The top layer is an oil-repellent layer. Plate exposure imaging does not require film and development. Instead, it uses an imaging technique called ablation that uses infrared-based high-powered lasers to illuminate the plate, vaporizing the image layer on the plate, and then The outermost silicone rubber layer was removed and the last exposed oleophilic layer was used for ink. In the preprinting stage of digital waterless offset printing, the computer is used for graphic processing. The processed image is converted by RIP. The converted data and the infrared laser head drive are used to control the infrared laser head array, and then The plate is exposed. The entire production process of the printing plate is very much like the CTP technology that is currently developing very rapidly. After the irradiated plate, the image forming part of the substance rapidly warms up to become gas, the gas expands to release the silicone rubber layer on the plate from the plate, and after dust removal, the oleophilic layer of the plate is exposed. Printing plates can be inked and printed
Waterless offset and disadvantage analysis
(1) From the effect of print copying,