Web Dot Increase and Control Technique Inventory in Web Offset Printing

Factors that cause dot gain

1. Printing pressure causes mechanical dots to increase

In the printing process, the transfer of the ink and the transfer of the print require proper printing pressure, and the deformation of the web and the deformation of the blanket are bound to cause deformation and increase of the dots. Therefore, in the process of drying the PS plate, it is necessary to combine different models (different printing presses have different dot gain values) to select an appropriate exposure time so that the dots on the PS plate are slightly smaller than the sample drafts, and the printed matter can be kept consistent with the proofs. .

2. Poor photosensitivity of poor printing plates

PS version of some of the coarse mesh, some poor photosensitivity, easy to form a halo around the exposure after the outlet. After the PS version was inked, the virtual halo part was also inked. The actual effect was to increase the dot area and cause the dot gain to increase. It is necessary to replace the PS plate with better quality.

3. Insufficient ink transfer device

The relative position between the ink rollers changes, and the shear force between the roller and the roller also changes, so that the ink transmission relationship is damaged. Relying on the pressure of the printing roller, the amount of extrusion deformation is large, the diameter of the rubber roller becomes fine, and the line speed of the plate roller and the platen roller are inconsistent, and the dot is stretched in the circumferential direction of the roller, resulting in tensile deformation.

4. Rubber blanket lining too thick or too soft

The blanket liner is too thick or too soft to increase the pressure between the plate cylinder and the blanket cylinder, the blanket cylinder and the impression cylinder (the transfer blanket blanket cylinder is both the transfer cylinder and the impression cylinder), and the blanket compression deformation amount And the width of the contact area is relatively large, the quality of the dot is not ideal, the reproducibility of the dot is poor, and the dot gain value is relatively high.

5. Ink is too large

Under ideal conditions, the thickness of the transferred ink layer should be controlled within 25 μm. As the amount of ink is increased, the thickness of the ink film increases, and the separation force increases rapidly. The excess ink cannot be instantly transferred by the substrate. The ink on the surface of the printing plate and the blanket is accumulated more, the ink starts to spread, resulting in an increase in dots, even A paste version appeared.

6. The paper tension is not good or the blanket lining is inconsistent

Paper tension control is not good or the blanket lining is inconsistent, the previous color of the imprint will be misplaced on the blanket of the next color group, there will be a shadow, the image transferred to the surface of the paper also appears overlapping shadows and fuzzy, the dot is increased .

7. Small ink viscosity and high fluidity

The hydrophilicity of the ink is too sensitive, the thixotropy and yield values ​​are too small, the viscosity is too low, the ink filament is too long, and the fluidity is too strong, and more fountain solution is required to counteract it during printing. The emulsified ink is easy to spread on the PS plate, which easily causes dot enlargement. The transfer to the substrate is not conducive to the solidification of the ink, and no clear halftone image can be obtained.

Effective way to control network increase

Although the increase in outlets is inevitable, it can be controlled. Under normal circumstances, the fine print dot gain of coated paper is less than 12%, the general print is less than 18%, and the newspaper print is controlled within 35% (generally 50% dot). In the production process, effective control can be made from the following aspects.

1. Select high quality media, reasonably control exposure time and developer concentration

The clear printing plate of the dried printing plate is clear and the outlets are solid, in order to guarantee the transfer effect of the outlets, and it can also ensure that the printing plate has a higher resistance to printing forces. Under normal circumstances to ensure that 5% of the network is not lost, 95% of the network is not blurred.

2. Control various printing pressures within a reasonable range

The pressure between the plate cylinder and the blanket cylinder is controlled between 15 and 20 filaments, and the pressure between the two blanket cylinders is controlled between 22 and 28 filaments, and it is ensured that the two inking rollers are parallel to the plate cylinder and the diameter is below 100 mm. The printing roller width of the plate roller and the printing plate is 5-6mm, and the printing width of the plate roller with the diameter of 100mm or more and the plate roller is 6-7mm.

3. Strictly control the ink and water balance in printing

In the printing process, if the water supply is too large, the ink supply on the layout will increase, the ink layer will become thicker, and the ink emulsification will be severe, resulting in dull and dull prints. In the production process, in order to reduce the start-up waste, the water level is usually increased to 150% of the normal value when the machine is turned on. After normal printing, the amount of water should be gradually reduced to the normal value to ensure clear graphics and saturation.

4. Correctly control the concentration of fountain solution

The control of the fountain solution concentration plays a crucial role in the balance between ink and ink and the quality of the printed product. A suitable concentration of fountain solution can quickly flush away the "dirt" around the dot. If the concentration of the fountain solution is too low, the ink around the dots cannot be removed, and if the dots are light, the dots will increase, but if the weight is too high, the paste will be caused. If the concentration of the fountain solution is too high, the resistance of the PS plate will decrease and the dampening solution A lot of waste. In general, the pH of the fountain solution is controlled at 4.8-5.3 and the conductivity is controlled at 800-1200 μs.

5. Control paper tension

The paper tension control mainly has two points, one is the primary tension formed by the paper frame with the tension control system, and the second is the secondary tension formed by the continuously variable transmission after the printing unit. Under normal circumstances, to ensure that the secondary tension is slightly greater than the tension of the paper in the printing, can achieve a good reproduction of the dots and the last color (usually black) is not blooming.