Dot gain and control techniques in the most popula

2022-08-06
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Point increase and control skills of rotary offset press

1 The printing pressure causes the increase of mechanical points

appropriate printing pressure is required for the transfer of ink and imprint during the printing process. The extrusion of cutting off the power supply through the roller and the deformation of the blanket will inevitably cause the deformation and increase of points. Therefore, when drying the PS plate, it is necessary to select the appropriate exposure time in combination with different models (different printing machines have different point increase values), so that the points on the PS plate are slightly smaller than the proofs, so that the printed matter can be consistent with the samples

2. Poor photosensitivity of poor printing plate

ps version has some coarse eyes and some have poor photosensitivity, which is easy to form virtual halo around the point after exposure. After the PS version is inked, the faint part is also inked. The actual effect is to increase the point area and the starting point. It is necessary to replace the PS version with a better quality

3. Poor adjustment of inking device

the relative position between inking rollers will change, and the shear force between rollers will also change, which will damage the inking relationship. The pressure of the plate roller is heavy, the amount of extrusion deformation is large, the diameter of the rubber roller becomes thinner, the linear speed of the plate roller and the plate roller is inconsistent, and the point is elongated in the circumferential direction of the roller, resulting in tensile deformation

4. The rubber cloth lining is too thick or too soft

the rubber cloth lining is too thick or too soft, which increases the pressure between the printing plate cylinder and the rubber cylinder, the rubber cylinder and the impression cylinder (the rubber cylinder of the rotary machine is both the transfer cylinder and the impression cylinder). The refrigerator manufacturers also improve the pressure between many working forces. The rubber cloth compression deformation and the width of the contact area are relatively large, the point quality is not ideal, the point reproducibility is poor, and the point increase value is relatively high

5. Excessive ink volume

under ideal conditions, the thickness of the transferred ink layer should be controlled at 25 μ Within M. With the increase of ink supply, the thickness of the ink film increases, and the separation force also increases rapidly. The excess ink cannot be transferred by the substrate immediately. There is more ink on the printing plate surface and the blanket, and the ink begins to spread, causing the dot to increase, or even the plate paste

6. Poor control of paper tension or inconsistent rubber cloth lining

poor control of paper tension or inconsistent rubber cloth lining, the impression of the previous color will be misaligned on the rubber cloth of the next color group, resulting in overlapping shadow. The pictures and texts transferred to the paper surface also appear overlapping shadow and blur, and the dots will be increased

7. Ink viscosity is small and fluidity is too high

ink is too sensitive to hydrophilicity, thixotropy and yield value are too small, viscosity is too low, ink filament is too long and fluidity is too strong, so more fountain solution is required to compete with it during printing. The emulsified ink is easy to spread on the PS plate, which is easy to cause dot increase. Transfer to the substrate is not conducive to the solidification of the ink, and clear eye tone images can not be obtained

effective method for increasing control point

point increase is inevitable, but it can be controlled. Under normal circumstances, the increased value of tissue and organ repair points of fine prints of coated paper in modern medicine should be less than 12%, that of general prints should be less than 18%, and that of newspaper printing should be controlled within 35% (generally refers to 50%). In the production process, the following aspects can be effectively controlled

1. Select high-quality plates, reasonably control the exposure time and developer concentration

the printing plate with clear graphics and strong dots can not only ensure the dot transfer effect, but also ensure the high printing resistance of the automatic shutdown of the printing plate machine. Generally, 5% of the points shall not be lost and 95% of the points shall not be blurred

2. Control various printing pressures within a reasonable range

control the pressure between the printing plate cylinder and the rubber cylinder at 15 ~ 20 wires, and the pressure between the two rubber cylinders at 22 ~ 28 wires, and ensure that the two plate inking rollers are parallel to the printing plate cylinder. The printing width between the plate inking roller and the printing plate cylinder with a diameter of less than 100mm is 5 ~ 6mm, and the printing width between the plate inking roller and the printing plate cylinder with a diameter of more than 100mm is 6 ~ 7mm

3. Strictly control the balance of water and ink in printing

in the printing process, if the water supply is too large, the ink supply to the layout will increase, the ink layer will become thicker, and the ink emulsification will be serious, resulting in dim and lusterless impressions. In the production process, in order to reduce the waste of startup, the water level is usually increased to 150% of the normal value at startup. After normal printing, the water volume should be gradually reduced to the normal value to ensure that the graphics and texts are clear, saturated and glossy

4. Properly control the concentration of fountain solution

the control of fountain solution concentration plays an important role in ink balance and print quality. The appropriate concentration of fountain solution can quickly wash away the "dirt" around the point. If the fountain solution concentration is low, the ink around the dot cannot be removed, which will cause the dot to increase if it is light, or cause a paste if it is heavy; If the fountain solution concentration is too high, it will lead to a decline in the printing resistance of PS version and a large amount of waste of fountain solution. Generally, the pH value of the fountain solution is controlled at 4.8 ~ 5.3, and the conductivity value is controlled at 800 ~ 1200 μ s。

5. Control the paper tension well

there are two main points in the paper tension control: one is the primary tension formed by the paper rack with tension control system, and the other is the secondary tension formed by the stepless transmission behind the printing unit. Under normal circumstances, it is necessary to ensure that the secondary tension is slightly greater than the paper tension in printing, so as to achieve better reproduction and the last color (usually black) will not be scratched

reprinted from: printed in Shenzhen

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