What To Do If The Ink Color Of Offset Printing Is Uniform And Inconsistent

Offset packaging products often have a large area of solid background color, requiring thick, uniform ink color. The consistency of the ink color includes two aspects: one refers to the consistency of the ink color of different printed sheets successively; the second refers to the consistency of the ink color of different parts of the same printed sheet. Among them, the ink color consistency requirements for different parts of the same printed sheet are particularly strict, which is a key factor for evaluating product quality. Offset printing ink layer is thin, prone to troubles such as streaks, smeared back, dark front and back light, and it is not easy to achieve thick and uniform ink color when printing large areas.

If you want to use the offset printing process to print a large area of solid background color, on the one hand, you need a high-precision printing machine, on the other hand, you need to adopt an appropriate process method.

 

1. Try to avoid thin lines in the spot color background

The thickness of the ink layer of color screen printing is not suitable for printing large-area background colors, especially when the surface of the substrate is not smooth enough, it will give people the feeling that the ink is not thick enough and uneven.

Spot color solid background printing generally needs to increase the amount of ink supply. The thicker ink layer on the layout will inevitably increase the probability of ink extending to the blank part. Therefore, when designing before printing, try to avoid extremely fine colors in the solid background color. If there are small characters, try to use boldface, because the strokes of boldface are relatively thick and not easy to paste.

Second, The background color effect of the second overprint is better

If there are both black background and color halftone images on a layout, then there must be both a large area of solid and halftone images on the black plate, which determines that the black ink cannot be printed too thick. In the case of not specially making a black background color plate, in order to achieve a thicker visual effect of the black background color, about 40% of the flat screen can be added to the corresponding part of the black background color of the cyan plate, because the black ink is overprinted on the cyan color. The flat screen is still black, and the result is that the ink layer thickens. With this method, a satisfactory thick effect can be achieved when the black ink is not printed very thickly.

In addition to the color image on the layout of some products, some deep spot color backgrounds, such as bright red backgrounds, dark blue backgrounds, etc., need to be printed. At this time, flat screen printing can be added to the corresponding parts of the background color on a certain color plate. Make a primer, and then overlay the spot color on it.

The advantage of overprinting a large area of background color by two printings is: on the one hand, under the condition of controlling the thickness of the ink layer of the background color plate, the overprinted ink layer is thicker; on the other hand, it can greatly reduce the ink skin and paper powder. , Streaks and other adverse effects on the printed screen.

3. The typesetting is as good as possible to even ink

Try to avoid slender blank skylights along the direction of drum rotation in the middle of the solid part. Since the ink string capability of the printing press is limited, the elongated skylight along the rotation direction of the printing cylinder will darken the ink at the upper and lower ends of the skylight during printing, which will seriously affect the appearance of the picture. If the skylight is not in the shape of the finished product, consider adding it to the same solid ground as the surrounding, or if possible, rotate the entire pattern 90 degrees. When making printing plates for packaged products, because multiple patterns are often crossed together, there are often slender blank parts where they meet. Such blank parts should be filled with the same solid ground as the surroundings. Although some ink is wasted, it is conducive to uniform ink color.

Fourth, printing is the most critical

The high precision of the printing machine is a prerequisite for printing a large area of background color. If a uniform solid background color can be printed with a lower printing pressure, it indicates that the offset press has a higher precision. The processing accuracy of roller bearings, transmission gears, etc. and the reliability of the clutch pressure mechanism and ink distribution mechanism are particularly important.

 

1. Printing pressure

Larger printing pressure when printing on the ground, on the one hand, is conducive to the full contact of the printing surface to obtain a thick and thick print; on the other hand, it can promote the pressure penetration of the ink on the paper, and accelerate the fixing and drying of the ink.

 

2. The stability of printing pressure

During the rolling process of the printing plate cylinder and blanket cylinder, blanket cylinder and impression cylinder, if the printing pressure changes, it will inevitably cause the change of the ink transfer rate, and eventually lead to uneven ink color on the ground.

 

3. The elasticity of the blanket

After long-term use, the blanket will gradually age, and the ink on the surface of the blanket will not be in full contact with the surface of the paper, resulting in poor ink transfer and actual unevenness and thickness.

In addition, the tighter the blanket is, the more the rubber layer becomes thinner and the more serious the loss of elasticity. For air cushion blankets, the air cushion layer may even fail completely, which is not conducive to obtaining uniform ink color.

 

4. Roller lining

Improper lining of the printing plate cylinder and blanket cylinder will cause large sliding on the imprinting surface, and the sliding amount of the printing plate surface is inconsistent, resulting in ink streaks and affecting the uniformity of the solid ink layer.

 

5. Printing plate

For full-page printing, a solid printing plate should be printed as usual, but the old printing plate can also be used for full-page printing, which can save production costs and improve efficiency. However, the photosensitive resin in the graphic part of the PS plate and the alumina in the blank part have slightly different ink absorption and transfer capabilities. The ink layer on the printed product corresponding to the graphic part of the old printing plate will be blank than the old printing plate. Part of the corresponding ink layer is slightly darker. For printing strong colors, such as dark blue, it is best to use a completely blank printing plate, or to install the old printing plate in reverse (front face in, reverse face out) for printing; printing weak colors, such as yellow and bright oil, because It is difficult for the human eye to distinguish such small differences, and the old printing plates left after the last printing can be used directly.

 

6. The blending of ink

The fluidity of the ink used in the printing field is slightly larger than that of the halftone image. If the ink fluidity is too small, it is not easy to spread evenly between the ink rollers, which may cause the ink color to be dark in the front and light in the back. The fluidity is greater. After the ink is printed on the paper, it can be leveled quickly before drying, which is beneficial to increase the solid coverage and achieve the effect of uniform and thick ink. The worse the paper, the more it is necessary to improve the fluidity of the ink when printing on the ground.

The fluidity adjustment of the ink is mainly to adjust the viscosity of the ink. Generally add low-viscosity No. 6 varnish to dilute the ink, but the amount must be strictly controlled, because the viscosity is too low and the ink is easy to emulsify.

When printing coated paper, in order to prevent smearing on the back side, a small amount of desiccant or detackifier needs to be added to the ink.

 

7. Ink transferability of the ink roller

The ink roller will become smooth due to aging or improper cleaning, and its ability to absorb and transfer ink will be weakened. Especially the inking roller, if it cannot absorb enough ink, when it first transfers the ink after it comes into contact with the graphics and text of the printing plate, due to the small amount of ink storage, the dark tone and solid ground of the image on the printing plate have absorbed the ink on its surface Most of the ink rollers cannot get enough ink immediately to replenish them. Therefore, when the ink is transferred in the second transfer, the ink obtained by the dark part of the image or the solid area is much less than that in the first transfer. Although there are 4 inking rollers to transfer the ink, the ink color still appears dark in the front and light in the back. Therefore, the inking roller, especially the first two inking rollers, must use the rubber roller with good surface ink transfer performance.

 

8. Pressure adjustment of ink roller

The pressure between the ink roller and the ink roller, between the ink roller and the printing plate should be adjusted appropriately. If the pressure is too small, the ink transfer will be poor, especially if the contact pressure between the ink form roller and the ink string roller is too light, so that the ink form roller cannot get enough ink, the ink color of the layout will be light and heavy. Accelerate the aging of the ink roller, especially if the pressure between the ink form roller and the printing plate is too large, it will produce shock jumps at the gripper, and ink streaks will appear. The pressure between the same inking roller and the inking roller is slightly less than its contact pressure with the printing plate. This is to ensure that the surface linear speed of the inking roller is inconsistent with the surface linear speed of the printing plate to ensure that the inking roller can follow the pressure. The side of the printing plate, that is, the side of the printing plate, runs synchronously, so that the ink form roller and the surface of the printing plate are as free as possible.

 

9. Pressure adjustment of water roller

If the pressure between the water roller and the water roller, between the water roller and the printing plate is too small or too large, it will cause poor water transfer and cause the plate surface to get dirty. Too much pressure between the dampening roller and the printing plate, and too much pressure between the dampening roller and the stringing roller, may cause white bars.

 

10. Adjustment of ink roller

In order to achieve the best ink distribution effect, the axial string momentum of the stringing roller is generally at the position of the maximum string momentum. Being in this position is also helpful for the operator to quickly adjust the axial ink volume of the layout evenly. In some printing machines, the inking roller can also be moved in series after adjustment. In order to achieve a better ink distribution effect, the inking roller can be moved in an appropriate amount. By adjusting the starting point of the string movement of the string roller, the circumferential distribution of the ink can be changed. It is difficult to determine which step position must be the best starting position. Generally, one can only determine a position that is more conducive to uniform ink color by gradually adjusting the string starting position.

 

11. Control the influence of emulsification on ink color

When printing solid background colors, the emulsification consumes a lot of water and the water supply is large. The axial ends of the solid background color part, especially when the background color part approaches or reaches the edge of the paper, there is no ink and water emulsification in the area outside the paper, and no blank part of the paper absorbs water. As a result, excess water will follow The string movement of the water roller enters the background color parts at both ends of the paper axis, causing the background color ink layer at both ends of the paper to be overemulsified, the ink color becomes lighter, and the back side smear phenomenon may occur. A reasonable approach is to use water blowing devices as much as possible, or apply stickers on the water bucket roller to reduce the water supply at both ends of the printing plate in the axial direction, and control the emulsification degree of the entire page.