What is the effect of water in offset printing (2)

3, waterless offset printing principle and process

Waterless offset printing is a lithographic offset printing technology that uses a special silicone rubber coated plate, special inks, and a strict temperature control system on the press to complete the lithographic printing process. The unwatermarked version is a flat intaglio with the graphic part below the surface of the printing plate. This special silicone rubber-coated printing plate has an ink-repelling silicone rubber layer as a blank part of the printing plate, while a slight depression forming an ink-acceptable polymer surface layer is a graphic part. Figure 1.

Now CD offset printing mainly uses Toray without watermark version. Here we mainly introduce the Toray waterless printing system.

3.1 Toray System

Toray waterless printing system is mainly composed of three parts: no watermark version, waterless ink and temperature control system on the printing press.

(1) Toray without watermark version

The Toray waterless version has two types, negative and positive. At present, the most used type is the negative image type without watermark.

Toray has no watermark version with three main layers. As shown in Figure 2.

a) Aluminium substrate, base layer (approximately 0.5 to 1 mm).

b) Photosensitive resin layer (approximately 3 to 3.5 μm): A diazo photosensitive material. The positive-type photosensitive layer is a photosensitive combination type, and the negative-type photosensitive layer is a photosensitive decomposition-type functional structure.

c) Silica gel layer (approximately 2 μm): Strong ink repellency, as a blank on the plate.

d) Cover film (protective layer, about 6 μm): In order to increase the adhesion of the film and the plate at the time of printing, the surface of the cover layer was processed into a 2 μm concave-convex shape.

e) In order to bond the aluminum plate and the photosensitive layer (diazo photosensitive material), the photosensitive layer (diazo photosensitive material) and the silica gel layer well, they are also coated with a photosensitive diazonium compound as a raw material. Gluing layer.

According to the type of Toray waterless version, the print volume varies from 100000 to 600000 prints. The amount of print depends on other factors.

Like the traditional PS version, the Toray waterless version can be regenerated.

(2) Plate-making step: Waterless offset printing The key to plate-making development is to cut off the silica gel layer corresponding to the graphic part. The shear force is stronger than the adhesion between the silicone layer and the photo-sensitive layer in the graphic part. Applying it to the interface between the two layers can remove the graphic part of the silicone layer.

a) Imposition: Films for waterless printing can be either negative or positive. The so-called imposition is the process of pasting and fixing the film on the printing plate according to certain requirements and operating specifications. The imposition should be based on the film and lithographic type to determine whether to use light blocking film.

b) Exposure: The Toray waterless version is exposed using a conventional UV light source exposure device and a light source. The exposure time is also about 60 seconds. The negative-pattern waterless version passes through the UV light of the film during exposure, penetrates the silicone rubber layer, and reaches the photosensitive resin layer. The photosensitive resin in the graphic part absorbs UV light and reacts and falls off at the junction of the photosensitive resin and the silicone rubber layer. This kind of reaction is very fine, and the printing plate is also very easy to get high resolution. When the number of screen lines is 175 lines/inch, it can reproduce 0.5% to 99.5% of the dots.

c) In addition to the protective film: Toray waterless version after exposure, but also need to be processed. Prior to processing the plate, the protective film on the silicone layer must be removed.

d) Swelling: The development processing equipment is unique to the system and uses special chemical and mechanical methods to process the plates. The photosensitive layer of the graphic part is swollen with the appropriate PPG (polypropylene glycol) and PEG (polyethylene glycol), ie TORAY PA-1 pretreatment solution, and the silicone layer is floated, so that the adhesive force between the two layers is greatly increased. Weakened. The blank part cannot afford chemical changes.

e) Mechanical peeling: If a negative image is used, the photosensitive portion is peeled off and the unexposed portion (ie, the blank portion) retains the silicone layer; if a positive image is used, the photosensitive portion hardens, and the unexposed portion falls off to form the graphic portion. The puffed waterless version is uniformly applied with shear force to the interface of the swollen silicone layer by the nylon brush, and the silicone layer of the printed part is peeled off. Thus, the watermark printing process has not been completed, and the waterless version is manufactured as such. .

f) Staining: The developed waterless version can be printed without dyeing. The dyeing function is mainly used to identify and check the quality of the printing plate.

One point to be emphasized here is that waterless offset printing produces a flat gravure plate. The non-graphic part of the plate is an ink-repellant silicone rubber layer. In the graphic part, the silicone rubber layer is removed, leaving the ink absorbing. The resin layer forms an "ink-absorbing hole" structure. These "ink-absorbing holes" provide a supporting net wall as the ink is transferred from the printing plate to the blanket cylinder. In this way, in the process of ink transfer, the dot enlargement formed under the effect of the printing pressure is very small, and the faithful reproduction of the graphic and picture is basically achieved.

Figure 3, no horizontal plate exposure development process
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(3) waterless offset ink

Waterless offset uses a specific ink. It requires a higher viscosity to ensure good repellency of non-textured silicone rubber layers, that is, high viscosity inks can ensure that the silicone rubber layer does not stick ink, no dirty plate appears (blank part without ink), at the same time The graphic part of the printing plate has a good affinity, and can match the type and material of the blanket to ensure the smooth transfer and transfer of the ink. It also requires that the ink does not contain coarse particles to prevent scratches on the protective film on the surface of the plate, while avoiding the friction of the particles to generate heat and reduce the viscosity of the ink.

(4) Temperature Control System

For high-viscosity waterless offset inks, the important factor affecting the viscosity of the ink is temperature. Small changes in temperature can cause the ink viscosity to change greatly. For example: The viscosity of the ink at 25°C is 100-150 poise, but at 40°C the viscosity drops sharply to 30-40 poise. In the continuous printing process, due to mechanical friction (between the rollers, between the roller and the plate, and between the plate and the blanket) factors such as the plate surface temperature will increase, because of the viscosity of water-free ink Large, friction between the waterless ink and the ink roller will also increase the temperature, resulting in a drop in the viscosity of the printing ink.

It is because of the high requirements for the viscosity of the ink that a temperature control system has to be used on the printing press to precisely control the temperature of the ink. Air-cooled and water-cooled methods are commonly used in CD offset printing machines. Whichever method is used, the temperature is controlled at a constant value to maintain the viscosity of the ink without water.

Air-cooling method: the constant temperature cold air is blown to the surface of the ink roller and plate cylinder to reduce the temperature of the plate surface, maintain the ink temperature, and maintain the viscosity of the ink. Such as the German KAMMANN offset press.

Water-cooling method: mainly in the lithography of the offset printing press, it is provided with enough cooling water to control the temperature. Such as Taiwan lithographic offset press.

Although different brands of waterless offset inks have different suitable temperatures, it is necessary to control the temperature difference between the platen and the roller to maintain it within the ±15°C range. Some inks have a temperature tolerance of ±0.8°C, while others Has a temperature tolerance of ±1.5°C. If the temperature is too high, the non-graphical part of the silicone resin will be stained with ink and a dirty plate will appear. If the temperature is too low, the ink will be transferred poorly, and the full version will not occur with white spots. Therefore, the balance control technology for the temperature-free offset printing ink is a key technology for the horizontal offset printing press.

In the actual printing production manipulation, the operator should adjust the temperature accordingly according to the selected ink and printing speed.

3.2 Presstek System

The Presstek PEARL dry waterless version consists of two parts: a waterless version and a waterless ink similar to Toray.

Presstek claims that the temperature control system on the press is an option. PEARL dry no watermark version with a maximum capacity of 50,000 impressions. If the print volume is less than 10,000 prints, there is no effect on the temperature on the press. Only the long-run version requires a temperature control system. PEARL dry is not watermarked, but it can also be recycled. The maximum size is four open.

The PEARL dry waterless version consists of four layers: an ink-repellent silicone rubber layer, a light-absorbing imaging layer, an ink-receiving layer, an aluminum base or a polyester base layer. PEARL dry without watermark imaging, no need for film, no exposure, no processing. Its imaging uses mainly an ablation technique. Use an infrared laser diode to ablate the silicone rubber coating on the plate to expose the underlying ink-receptive layer. During imaging, the laser rapidly heats the imaging layer to record images. The imaging layer is evaporated and the upper silicone rubber coating is stripped from the plate to form a graphic. After the plates are imaged, they must be cleaned and printed on the plate cylinder.

The system can be imaged using Presstek's direct platemaker.