A Study on the Resistance of PS Plate to India

The pre-coated photosensitive plate (also known as the PS plate) has been applied since the 1950s. It has gradually replaced the plate materials such as protein plates and multi-layered metal plates due to its high resolution, clean dots, and high resistance to printing. New type material. At present, under normal circumstances, the PS version of the print resistance is generally about 100,000 prints, after baking the PS version of the print force can be increased by 3-5 times to reach 30 million to 500,000 prints, but now there are many manufacturers reflect the PS The print resistance of the print is not high, generally only 210,000 prints, or even thousands of prints. Here, I only personal experience, in accordance with the PS version of the production process, on the PS version of the printing force plate production and plate-making process of rough discussion and analysis.

First, the choice of plates

The base of the PS plate is aluminum. The aluminum material used in the PS plate belongs to the general industrial grade pure aluminum, the thickness is generally 0.3mm, and the content is 99.5%. The main impurities are iron and silicon, followed by copper, magnesium, zinc, manganese and titanium. The presence of a small amount of impurities in the aluminum plate can increase the tensile strength and hardness of the pure aluminum, reduce the elongation, and reduce the conductive properties of the aluminum.

Second, the sensitizer composition and coating

The sensitizer for PS plate is composed of a photosensitive material, a film-forming polymer resin, a dye, and an organic solvent. For a photosensitive layer coated with a photosensitive agent formulated with a photosensitizer alone, its alkali resistance is strong, the film forming property is poor, and development is difficult; an appropriate amount of phenolic resin is added to the photosensitizer. Although the alkali resistance is poor, the film forming property and the adsorptivity are improved, and the ratio of both is 0.5:1 or 0.62:1. If there are few photosensitizers and there are many film-forming resins, the alkali resistance is reduced and the dots are easily lost. The low residual film rate directly reduces the printing plate's resistance to printing force.

The application of the PS version of the sensitizer requires the use of a special coating machine to ensure that the thickness of the photosensitive layer after coating is uniform. Coating machines are generally divided into two types, single-sheet and roll-type. In coating methods, there are paving, roller coating, extrusion and electrostatic coating. We do not recommend the use of centrifugal coating. China's commonly used roller coating type, in order to ensure the coating quality, the general negative picture type photosensitive layer solid content of about 1g/m2. The positive pattern is 2-2.5 g/m2, and the thickness is generally (20±1.0) mg/dm2. When the coating produces lateral stripes and other faults should be promptly removed.

Third, the PS version of the plate

The key to PS plate making is to maximize the reproduction of the dot and tone of the film, forming a solid lipophilic basis and a stable hydrophilic foundation on the plate surface. Otherwise, it not only wastes time, but also seriously reduces the printing plate's resistance to printing and wastes the plate. A series of factors that directly affect the quality of platemaking include plate making originals. PS version of dot reproducibility, plate making equipment and conditions. Platemaking process and operation, especially exposure, development conditions, and the like.

1. Original edition

PS version of the positive or negative original, can be made using traditional photolithography, electronic color separation and phototypes and other methods. In addition to the adjustment of the tone reproduction curve, it is also necessary to pay attention to the original density contrast and dot finish. Because the original Y value is too low, exposure, underdevelopment, or excessive thinning are the causes. It is easy to form soft dots with low density or active dots with high density and low edge density. The printing stability of these two types of outlets is poor, and the exposure and development are slightly changed, which will cause the printing plates to have different shades, which will directly affect the printing plate's resistance to printing forces. The original version of the hard dot, due to its high density and clean dots, has a large exposure and development latitude. So the quality is stable. Therefore, the original printing pattern used in the printing must be clear and have a high density and contrast. Take Huaguang brand YZ-600 11 film as an example. The original maximum density should be above 4.0. The minimum density is below 0.05, the gamma ratio is above 8.0, and the layout clean reduces haze and collage.

2. Shaiban light source

For printing light source. The sensitization of the sensitizer is a redox reaction. Only the sensitizer fully absorbs enough light energy of T to occur, and its degree of decomposition, cross-linking, and bonding are directly proportional to the required light intensity and the illumination time. Therefore, the emission spectrum of the printing plate light source and the photosensitizer photosensitive curve Sensitive areas should try to match. Experiments have shown that the photosensitivity of the positive photosensitive thorn is within 300nm-460nm, and the figure is within 200nm-500nm. Take into account the effects of the printing plate and the printing plate. Spectral design is suitable in the 350nm-460nm region. Therefore, the ideal light source for printing PS plates is iodine lamp.

3. Exposure

In the normal plate making process. Exposure is the most important factor affecting network changes and layout depth. We know that the exposure of the printing plate = illumination x printing time. As the commonly used positive version of the PS version belongs to the photolysis type. Therefore, when the exposure exceeds the critical light amount, that is, overexposure, the dots will be reduced, fine strokes and small dots will be difficult to reproduce, and when the amount of exposure is less than the critical light amount, that is, when the exposure is insufficient. The outlets will expand accordingly. Make the layout dirty and affect the depth of the layout. Therefore, during the printing process, the exposure amount is controlled. The continuous scale or Bruner test strip should be used for monitoring. The function is to check the exposure after development. The general proofing version is controlled at grade 3-4 (density is 0.4-0.55) and the printing plate is controlled at grade 4.5-5.5 (density 0.625-0.775). When printing a positive plate, if a pair of 0.5% dots and 99.5% hollow dots are sunned in pairs, such a printing plate can be considered qualified.

4. Development

The development of the PS plate consists in removing the photosensitive layer of the blank portion to maintain the hydrophilicity of the surface of the PS plate substrate. It is an important part of the PS plate making process. It is not only restricted by the nature of the photosensitizer and the original state, but also affected by the development conditions such as developer concentration, temperature, fatigue, and development time.

(1) Effect of concentration

Developer concentration. Generally refers to the content of the developer base. Insufficient developer concentration will not only increase the development time but also cause incomplete development. Causes the layout of the face to rise; the concentration is too large, the development time is accelerated, the entire process is difficult to control, easy to cause excessive development, network points are reduced or damaged, dissolve the unsensitized photosensitive layer, reduce the printing plate's sensory and resistance to India. Therefore, in order to accurately control the concentration, the plate can be developed after 30-60s, observe whether the hollow point of the Bruner test strip 98%-99% is clear and whether the 1%-2% small black point is firm, if The idea of ​​clear and sturdy indicates that the concentration is appropriate.

(2) Effect of temperature

In the case of other relatively stable development conditions. The higher the development temperature, the faster the development speed and vice versa. The general development temperature should be controlled at about 20 °C. The automatic developing machine should be controlled at 25°C, and when it exceeds 30°C, the dots will dissolve. In the photosensitive layer, the binding force between the photosensitive material and the film forming material is weakened, the alkali resistance is decreased, the area percentage of the screen area is reduced, and the reproduction of the printing plate is damaged.

(3) Effect of developer fatigue

The fatigue of the developer refers to a decrease in developing ability of the developer. Such as the use of hand-developed development, due to the exposure of the developer to the air, the absorption of water and CO2, accelerated developer degeneration, affecting the development performance. Another cause of the fatigue of the developing solution is that the photodegradable substance is dissolved therein, and the developing ability is significantly reduced. Therefore, the currently used alkaline developer should minimize contact with air in the process of storage and use, reduce the impact of moisture and CO2 on it, and regularly replace and quantitatively make up the medicine.

(4) Effect of development time

Because the development time is influenced by factors such as the exposure amount of the printing plate, the performance of the sensitizer, the development method, and the status of the developer, so, to control the degree of development, it is first necessary to grasp and stabilize the above-mentioned variable factors, and then perform the test with the gray scale. To determine the development conditions and the correct development time, the general development time is about 1 minute.

Fourth, baking version

After the printing plate is baked, the photosensitive base in the photosensitive layer disappears, and the phenolic resin undergoes thermal crosslinking reaction by heat, and the linear molecule turns into a large-scale network-like molecule. At this time, the solvent resistance, corrosion resistance, and abrasion resistance of the printing plate and the adsorption of the photosensitive layer to the plate surface are greatly improved. However, during baking, in addition to the change in the photosensitive portion of the graphic part, the micropores of the anodized aluminum substrate surface that was closed by the plugging treatment may cause the flow of water to be increased due to the loss of water, causing the plate to be dirty. Therefore, before baking, it is necessary to evenly apply a layer of protective agent to the plate with absorbent cotton to protect the blank from dirt. The general coating amount is one pair of open coating 20ml-30ml. Baking plate temperature control at 230 °C -250 °C. The time is 5-8 minutes. It must be noted that the special roasting machine used in the roasting plate must have a constant temperature control device, and the temperature difference should be within ±2°C. If the baking temperature is too high or the time is too long, the photosensitive layer will be carbonized and deformed, and the aluminum plate will deform and mechanical strength. Reducing affects the printing plate's resistance to printing force and the life of the plate base; the temperature is too low or the time is too short, and the thermal curing degree of the photosensitive layer is not enough, which will also affect the printing resistance. After baking, it must be cooled naturally, and it should not be cooled by water to avoid deformation of the plate base. After cooling, the protective agent is washed off with a waste developer, neutralized with 3% phosphoric acid, and washed with water, dried, and temporarily coated with gum arabic for protection.

The production and drying of the PS plate play a decisive role in improving the printing plate's resistance to printing. When the printing process is operated in accordance with data, standardization and standardization, the production and drying of printing plates are particularly important. therefore. We only carefully deal with the above issues, eliminate the external and human factors in the production and printing process, strongly promote data, standardization, and standardization of drying, ensure maximum plate quality, and improve the printing plate resistance to printing. Put it into practice.

Source: Graphic Arts