Analyze the reasons for the increase

1. Printing pressure causes mechanical dots to increase

In the printing process, the transfer of ink and the transfer of imprints require appropriate printing pressure. The extrusion of the cylinder and the deformation of the blanket will inevitably cause the deformation and increase of the dot. Therefore, when printing the PS version, it is necessary to combine different models (different printing machines have different dot increase values) to select the appropriate exposure time, so that the dots on the PS version are slightly smaller than the proof, the printed matter can be consistent with the sample .

2. Poor sensitivity of poor printing plate

The PS version has some coarse meshes and some poor photosensitivity, and it is easy to form a halo around the dots after exposure. After the PS version is inked, the virtual halo part is also inked. The actual effect is to increase the dot area and cause the dot to increase. You need to replace the better quality PS version.

3. Poor adjustment of ink transfer device

The relative position between the ink rollers changes, and the shearing force between the rollers also changes, which destroys the ink transfer relationship. The platen roller is under heavy pressure, the squeeze deformation is large, the diameter of the rubber roller becomes thinner, the line speed of the platen roller and the plate cylinder are inconsistent, and the dots are elongated in the circumferential direction of the cylinder, resulting in 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 rubber cylinder, the rubber cylinder and the impression cylinder (the roller rubber cylinder is both the transfer cylinder and the impression cylinder), and the amount of compression deformation of the blanket And the width of the contact area are relatively large, the dot quality is not ideal, the dot reproducibility is poor, and the dot increase value is relatively high.

5. Too much ink

Under ideal conditions, the thickness of the transferred ink layer should be controlled within 25 μm. As the amount of ink supplied increases, the thickness of the ink film increases, and the separation force also increases rapidly. The excess ink cannot be transferred immediately by the substrate. There is more ink accumulation on the surface of the printing plate and the blanket, and the ink begins to spread, causing the dots to increase, even Confused appears.

6. Poor paper tension control or inconsistent blanket wrapping

If the paper tension is not controlled well or the blankets are inconsistent, the imprint of the previous color will be misaligned on the blanket of the next color group, overlapping images will appear, and the graphics transferred to the surface of the paper will also appear overlapping and blurry, 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 thixotropy and yield value are too small, the viscosity is too low, the ink filament is too long, the fluidity is too strong, and more fountain solution is needed to counteract it during printing. The emulsified ink is easy to spread on the PS plate, which is easy to cause the increase of the dots. Transferring to the substrate is not conducive to the solidification of the ink, and a clear mesh tone image cannot be obtained.

An effective method to control the increase of network points

Although the increase of outlets is inevitable, it can be controlled. Under normal circumstances, the increase value of fine printing dots of coated paper is less than 12%, the general printing is less than 18%, and newspaper printing is controlled within 35% (generally refers to 50% dots). In the production process, you can effectively control from the following aspects.

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

The printed printing plate with clear graphics and strong dots can ensure the transfer effect of the dots, and also ensure that the printing plate has a high printing resistance. Under normal circumstances, it is necessary to ensure that 5% of the outlets are not lost, and 95% of the outlets are not blurred.

2. Control various printing pressures within a reasonable range

Control the pressure between the plate cylinder and the rubber cylinder at 15 to 20 filaments, and the pressure between the two rubber cylinders at 22 to 28 filaments, and ensure that the two plate ink rollers are parallel to the plate cylinder and the diameter is less than 100mm The embossing width of the form ink roller and plate cylinder is 5-6mm, and the embossing width of the form ink roller and 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, it will increase the ink supply on the plate, the ink layer will become thicker, the ink emulsification will be serious, and the blot will be dull and dull. In the production process, in order to reduce the waste of starting up, the water level is usually increased to 150% of the normal value when starting up. After normal printing, the amount of water should be gradually reduced to the normal value to ensure that the graphics are clear, saturated and shiny.

4. Correctly control the concentration of fountain solution

The control of the fountain solution concentration plays a vital role in the balance of water and ink and the quality of the printing quality. The appropriate concentration of fountain solution can quickly wash away the "dirt" around the dots. If the concentration of the fountain solution is low, the ink around the dots cannot be removed, which will cause the dots to increase, and the paste will cause the heavy version; if the concentration of the fountain solution is too high, it will cause the PS printing resistance to decline and the fountain A lot of waste. Normally, the pH value of fountain solution is controlled at 4.8 ~ 5.3, and the conductivity value is controlled at 800 ~ 1200μs.

5. Control the paper tension

There are two main points of paper tension control, one is the primary tension formed by the paper holder with tension control system, and the second is the secondary tension formed by the stepless gearbox drive after the printing unit. Under normal circumstances, it is necessary to ensure that the secondary tension is slightly greater than the tension of the paper in printing, so that the dots can be reproduced better and the last color (usually black) does not bloom.

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