PS version of the matte layer technology
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The PS version of the mat layer is mainly used on the surface of the photosensitive layer and is composed of a myriad of discontinuous mat particles, which can shorten the vacuum bonding time required for contact exposure and prevent imaging. blurry. The vacuum bonding time refers to the time required for the surface of the photosensitive layer of the film and the PS plate to be in full close contact during the vacuuming process of the PS plate.
The PS version of the plate is printed by contact exposure, the process is as follows.
In the image exposure process, the film is first covered on the surface of the PS plate, and then the image on the film is projected onto the photosensitive layer of the PS plate by the printing light source. In order to obtain a clear image, the film and the PS plate photosensitive layer need to be adhered to each other without voids. Therefore, it is common to use a vacuum evacuation frame to place the film and the PS plate between the glass plate and the blanket of the vacuum frame of the printer, and then evacuate the film to adhere the film to the PS plate.
However, in this way, since the surface of the PS plate is smooth, the edges of the film and the photosensitive layer of the PS plate are easily attached, which is not conducive to the extraction of the air in the middle portion, resulting in a long time for vacuuming, greatly reducing the plate making. The efficiency of the operation; and sometimes after a long time, the air is still not evacuated. Since the domestic offset printing plate generally uses the positive-working PS plate, the photosensitive resin will decompose and precipitate a certain amount of nitrogen during the exposure, as shown in Fig. 1, if it cannot be eliminated in time, there will be a gap between the PS plate and the film. The result is that the dots in the middle part of the exposed version are faint, resulting in blurred images.
In order to improve the adhesion quality of the printing plate and the film, and shorten the vacuum bonding time, people began to apply a coating on the photosensitive layer of the PS plate. Due to the presence of the matte particles, the surface of the PS plate photosensitive layer becomes rough, and a small gap is formed between the film and the photosensitive layer, so that the air in the intermediate portion is easily extracted, reducing the time for vacuuming. Therefore, the matte layer is also called the gas guide layer.
Another function of the matte layer is that when a large number of PS plates are stored in a stacked manner, the PS plate photosensitive layer may be stuck to the back side of the above PS plate, which is difficult to peel off. This phenomenon is called tack. . The solution to this problem is usually to add paper to isolate the plate, but this will bring a new problem, that is, when using the PS plate, extra labor is required to remove the liner, a large amount of liner is industrial waste, and some automatic The upper plate making machine does not automatically remove the paper. The matte layer can solve the above problems well.
Performance requirements for the matte layer
For the performance of the matte layer, in addition to the impact on the sensitization and processing properties of the PS plate, there are the following requirements:
Vacuum adhesion
Compared with the ordinary PS plate, the vacuum bonding time of the PS plate of the sprayed matte layer during the image exposure should be significantly shortened. Vacuum bonding time is a basic indicator for evaluating the performance of the matte layer.
2. Developability
The matte layer should be soluble in water and an alkaline developer, and easily removed during the PS plate development step, leaving no residue after development. The plate retains good development performance in an old developer or under high temperature and high humidity conditions.
3. Storage stability
The matte layer should be able to resist the friction and pressure of the PS plate during storage and transportation, and will not break and peel off, that is, it has a certain degree of fastness. If the surface hardness of the matte is not enough, not only the vacuum adhesion is affected, but also the matte particles are easily peeled off into powder, which contaminates the glass surface of the printer, and the powder easily blocks the vacuum pump when vacuuming. At the same time, it is required that the surface layer cannot be stuck when stored under conditions of high temperature and humidity.
Raw material
In the early application of the matte layer, the method of directly coating the particles such as hot-melt solid powder on the surface of the photosensitive layer is often adopted. The matte layer has the disadvantages that the particles are easily broken, easy to fall off, and the mechanical strength is poor; The ultrafine polyethylene particles are directly dispersed in the photosensitive glue and coated with the photosensitive adhesive. The surface hardness and strength of the matte layer are better, but the coating liquid needs to be filtered when it is added to the coating tank because Polyethylene particles can easily block the filter element; from the 1980s, the method of coating with water-soluble resin aqueous solution, the main granular material is carboxyl resin and sulfonic acid-based resin, which can effectively improve the fastness of the matte layer. However, such resins are highly hygroscopic, sticky in high-temperature and high-humidity environments, and easy to adhere to in storage; in order to solve the problem of sticking, there is also an acrylic resin neutralized with alkali, but due to the addition of an alkaline substance, After storage for a long time, the photosensitive layer is decomposed and the shelf life is shortened.
Since the 1990s, techniques for producing a matte layer by dissolving or dispersing an aqueous solution of an acrylic copolymer have been developed, and various properties can be better achieved. The solvent of the matte coating liquid is generally water or an aqueous organic solvent, which is mainly for safety and environmental protection, and at the same time, the lowest cost and good effect are obtained.
Hair surface layer production process
The coating method of the matte layer can be sprayed and electrostatically coated, or screen printing or gravure printing. Nowadays, the rotary atomizing electrostatic spraying method is used more. The process is as shown in FIG. 2. In the rotary atomizing electrostatic spraying device 3, the aqueous solution of the dissolved or dispersed resin is atomized by the centrifugal force of the cup-shaped spray head and the applied high voltage, and the spray head and the PS plate 1 are used. Under the action of a high electric field, the atomized and electrostatically charged fine droplets fly toward the surface of the photosensitive layer to perform electrostatic adhesion, as shown in FIG. After a few seconds, the PS plate is dried by the drying chamber 4 in an indoor atmosphere of high temperature and low humidity or by blowing warm air.
The size and coating amount of the matte particles have a great influence on the performance of the PS version of the matte layer. If the particle size of the matte surface is too large, or the coating amount is too large, the contact surface between the two PS plates becomes small, and the slidability is insufficient, which may easily cause scratching of the photosensitive layer. The larger surface of the matte surface will cause the peak of the particle to be too high, and the gap between the film and the photosensitive layer of the PS plate is too large, so that the fine image is difficult to be restored, the density of the dot of the highlight portion is lowered, and the level of the image tone is lost. If the particle height is too small, the gap is too small, the vacuum adhesion is insufficient, and the image is blurred. This is also the case if the coating amount is too small.
Of course, adding a matte layer to the PS layout will inevitably lead to an increase in the production cost of the PS plate. However, from the various benefits brought by the matte layer, it is necessary to add a matte layer, and it is also a need for the development of a high-quality PS plate.