Dot gain and control technique inventory in web offset printing
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1. Printing pressure causes mechanical dot gain
The transfer of ink and the transfer of the print during the printing process require a suitable printing pressure, and the deformation of the dots and the deformation of the blanket by the roller tend to cause deformation and increase of the dots. Therefore, when printing the PS version, it is necessary to combine the different models (the different printing machines have different dot gain values) to select the appropriate exposure time, so that the dots on the PS plate are slightly smaller than the proof, and the printed matter can be consistent with the proof.
2. Poor printability of poor printing plates
Some of the PS versions have thicker meshes, and some have poorer photosensitivity. It is easy to form a faint around the dots after exposure. After the PS version is inked, the faint part is also inked. The actual effect is to increase the dot area, causing the dot to increase, and it is necessary to replace the PS plate with better quality.
3. Ink transfer device is not well adjusted
The relative position between the ink rollers changes, and the shear force between the rollers and the rollers also changes, causing damage to the ink transfer relationship. The pressure of the plate roller is heavy, the deformation of the plate is large, the diameter of the rubber roller is thinner, the line speed of the plate roller and the plate roller are inconsistent, and the mesh point is elongated in the circumferential direction of the roller to cause tensile deformation.
4. The blanket is too thick or too soft
The blanket 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 roller cylinder is both the transfer roller and the impression cylinder), and the compression deformation of the blanket 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 is relatively high.
5. The amount of 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 is increased, and the separation force is also rapidly increased. The excess ink cannot be transferred instantaneously by the substrate, and the ink is accumulated on the surface of the printing plate and the blanket, and the ink starts to spread, causing the dot to increase and even A paste appeared.
6. Paper tension control is not good or blanket wrapping is inconsistent
If the paper tension is not well controlled or the blanket is inconsistent, the print of the previous color will be misaligned on the blanket of the next color set, and the image will appear, and the image transferred to the surface of the paper will also appear to be superimposed and blurred, and the dots will be enlarged. .
7. Ink viscosity is low and fluidity is too high
The hydrophilicity of the ink is too sensitive, the thixotropic and yield values are too small, the viscosity is too low, the ink is too long, and the fluidity is too strong, and more dampening solution is required to compete with it during printing. The emulsified ink is easy to spread on the PS plate, which tends to cause the dot to increase. Transfer to the substrate is not conducive to the solidification of the ink, and a clear mesh image cannot be obtained.
An effective way to control dot gain
Although the dot gain 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 printing is controlled within 35% (generally referred to as 50% dot). In the production process, it can be effectively controlled from the following aspects.
1. Choose high quality media, reasonable control of exposure time and developer concentration
The printed plates are clear and the dots are strong, so as to ensure the transfer effect of the dots, and to ensure that the plates have a high printing durability. Usually, 5% of the outlets are not lost, and 95% of the outlets are 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 to 22 to 28 filaments, and the two ink rollers are parallel to the plate cylinder, and the diameter is less than 100 mm. The printing ink roller and the plate cylinder have an embossing width of 5 to 6 mm, and the printing roller and the plate cylinder having a diameter of 100 mm or more have an embossing width of 6 to 7 mm.
3. Strictly control the ink and water balance in printing
In the printing process, if the water supply is too large, the amount of ink supplied to the layout will increase, the ink layer will become thicker, and the ink will be emulsified seriously, resulting in a dull and dull impression. 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. When the printing is normal, the water quantity should be gradually reduced to the normal value to ensure that the graphic is clear, saturated and shiny.
4. Correctly control the concentration of dampening solution
The control of the concentration of the fountain solution plays a crucial role in the balance of the ink and the quality of the printed product. A suitable concentration of dampening solution can quickly wash away the “dirt” around the dots. If the concentration of the dampening solution is low, the ink around the dot can not be removed, which will cause the dot to increase, and the paste will cause the paste; if the concentration of the dampening solution is too high, the printing plate of the PS plate will be reduced and the dampening solution will be A lot of waste. Normally, the pH of the dampening solution is controlled at 4.8 to 5.3, and the conductivity is controlled at 800 to 1200 μs.
5. Control the paper tension
There are two main reasons for the paper tension control. One is the primary tension formed by the paper holder with the tension control system, and the other 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 larger than the tension of the paper in the printing, the dot can be better reproduced and the last color (usually black) is not blooming.