The generation and solution of large-scale solid ground color

Offset packaging products often have large areas of solid ground color, requiring thick, uniform ink colors. The consistency of the ink color includes two aspects: one refers to the consistency of the ink color of different printed sheets printed in succession; the second refers to the consistency of the ink color of different parts of the same printed sheet. Among them, the consistency of ink color in different parts of the same printed sheet is particularly strict, which is the key factor to evaluate the quality of the product. The offset ink layer is thin, prone to streaks, smudging on the back, and dark and light before and after the ink color. It is not easy to achieve thick and uniform ink color when printing on a large area. If you want to use the offset printing process to print a large area of ​​solid ground color. On the one hand, a high-precision printing machine is required, and on the other hand, an appropriate process method is required.

1. Try to avoid thin lines in spot colors

The thickness of the ink layer for color mesh printing is not suitable for printing large-area background colors, especially when the surface of the substrate is not smooth enough, it will give the impression that the ink color 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 the ink expanding to the blank part, so it is necessary to avoid extremely fine ground solid colors during pre-press design And small strokes; if there are small characters, try to use bold fonts because the strokes of bold fonts are relatively thick and not easy to paste.

2. The background color of the second overprint is better

If there is both a black background and a color mesh image on a layout, then the black version must have both a large area and a mesh image, which determines that the black ink cannot be printed too thick. In the case of not specially making a black background color version, in order to achieve a thicker visual effect of the black background color, you can add about 40% of the flat screen to the corresponding part of the black background color of the blue version, because the black ink is overprinted in the blue color It's still black on the flat net, and the result is that the ink layer becomes thicker. In this way, it is possible to obtain a more satisfactory thickening effect when the black ink is not very thick.

In addition to the color image, some products need to print some dark spot color background colors, such as scarlet background color, dark blue background color, etc. At this time, you can add flat screen printing to the corresponding part of the background color on a certain color version Make a primer and overprint spot colors on it. If printing a red background, you can add a suitable number of flat screens to the parts corresponding to the red background on the magenta version; if printing a dark blue background, you can add the appropriate number of parts to the dark blue background on the blue version Flat net.

The advantages of large-area background color overprinted by two printings are: on the one hand, under the control of 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 , Streak and other adverse effects on the printed screen. For example, when the black background color is printed only once on the white paper, due to the influence of paper scraps, ink skin, or other reasons affecting the ink transfer during the printing process, very obvious spots and trachoma will appear on the black background color. If the black background color is overprinted on the cyan flat screen, even if the black ink layer has trachoma, since the exposed is no longer the white paper base, but the cyan flat screen ink layer, compared with the black self-strong contrast, the black-cyan contrast It's less obvious.

3. Typesetting is as good as possible for ink distribution

Try to avoid the middle of the solid part with an elongated blank skylight along the rotation direction of the drum. Due to the limited ink-crossing capacity of the printing press, the elongated skylights along the direction of rotation of the printing cylinder will cause the darkening of the upper and lower ends of the skylights during printing, which seriously affects the appearance of the picture. If the skylight is not within the shape of the finished product, consider adding it to the same ground as the surrounding area, or if possible, rotate the entire pattern by 90 degrees. In the production of printing plates for packaging products, since multiple patterns are often stitched together, slender blanks often appear at the joints. Such blanks should be filled with the same solid areas as possible. Although some ink is wasted, it is conducive to uniform ink color.

Printing is the most important

The high precision of the printing press is a prerequisite for printing large-area background colors. If you can print a uniform solid ground color with a lower printing pressure, it means that the offset press has higher accuracy. The machining accuracy of roller bearings, transmission gears, etc. and the reliability of clutch pressure mechanism and ink leveling mechanism are particularly important.

1. Printing pressure

The use of larger printing pressures when printing on the field is beneficial 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 speed up the drying of the ink.

2. Stability of printing pressure

In the process of rolling the plate cylinder and rubber cylinder, rubber cylinder and impression cylinder for one week, if the printing pressure changes, it will inevitably cause the change of the ink transfer rate, which eventually leads to uneven ink color on the ground.

3. Elasticity of blanket

The blanket will gradually aging after long-term use, and the ink on the surface of the blanket and the surface of the paper cannot fully contact, resulting in poor ink transfer, which is actually uneven and not thick.

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

4. Roller lining

Inappropriate lining of the plate cylinder and rubber cylinder will cause large slippage on the platen surface, and the amount of slippage on the plate surface will be inconsistent, resulting in ink sticks, affecting the uniformity of the ink layer on the ground.

5. Printing plate

For full-field printing, a solid printing plate should be printed as usual, but old printing plates can also be used for full-page printing, which can save production costs and improve efficiency. However, the photosensitive resin of the PS version of the graphic part and the alumina of the blank part have a slightly different adsorption and transfer ability to the ink. The ink layer on the printed product corresponding to the graphic part of the old printing plate will be blanker than the old printing plate. The corresponding ink layer is slightly darker. For printing strong colors, such as dark blue, it is best to use completely blank printing plates, or install the old printing plates in reverse (front side inward, reverse side outward) for printing; printing weak colors, such as yellow and varnish, because It is difficult for the human eye to distinguish this slight difference, and the old printing plate left after the last printing can be used directly.

6. Ink preparation

The ink used for printing in the field is slightly more fluid than the screen-printed image. The fluidity of the ink is too small, and it is not easy to spread evenly between the ink rollers, which may cause the ink color to be darker and lighter. The fluidity is greater. After the ink is printed on the paper, it can be quickly leveled before drying, which is beneficial to increase the solid coverage and achieve the effect of uniform and thick ink color. The worse the paper, the more it is necessary to improve the fluidity of the ink when printing on the ground.

The adjustment of the fluidity of the ink is mainly to adjust the viscosity of the ink. Generally add low viscosity No. 6 ink to dilute the ink, but the dosage should be strictly controlled. Because the viscosity is too low, the ink is easily emulsified.

When printing coated paper, in order to prevent the back side from smudging, a small amount of desiccant or tackifier needs to be added to the ink.

7. Ink transfer of ink roller

The ink roller will become smooth due to aging or improper cleaning, and the ability to absorb and transfer ink will be weakened. Especially the inking roller, if it can not absorb enough ink, when it first transfers the ink after it contacts the plate and text, due to the small amount of ink storage, the dark tone of the image on the plate and the solid have absorbed the ink on its surface Most of them, and the ink roller cannot get enough ink to replenish immediately, so in the second transfer of ink, the ink obtained in the dark part of the image or on the spot is much less than the first transfer. Although there are 4 ink rollers to transfer ink, the ink color will still be dark before and then light. Therefore, the inking roller, especially the first two inking rollers, must choose the rubber roller with good surface ink transfer performance.

8. Ink roller pressure adjustment

The pressure between the ink roller and the ink roller, and between the ink roller and the printing plate should be adjusted appropriately. If the pressure is too low, the ink transfer is poor, especially if the contact pressure between the inking roller and the inking roller is too light, so that the inking roller can not get enough ink, the phenomenon that the ink color of the layout will be lighter and heavier will appear; Accelerating the aging of the ink roller, especially when the pressure between the ink roller and the printing plate is too large, will cause an impact jump at the mouth, and the ink bar will appear. The pressure between the same inking roller and the inking roller is slightly less than the contact pressure with the printing plate. This is to ensure that when the surface linear speed of the inking roller and the surface speed of the printing plate are inconsistent, ensure that the inking roller can increase with the pressure. The side of the printing plate is synchronized with the side of the printing plate, so that the ink roller and the surface of the printing plate do not slide as much as possible.

9. Water roller pressure adjustment

Too little or too much pressure between the water roller and the water roller, and between the water roller and the printing plate will cause poor water transmission, which will cause the plate surface to become dirty. The pressure between the water roller and the printing plate is too large, and the pressure between the water roller and the water roller is too large, which may cause white bars.

10. Adjustment of ink roller

In order to achieve the best ink leveling effect, the axial tandem of the squeegee roller is generally at the position of the maximum tandem amount. It is also helpful for the operator to quickly adjust the axial ink volume of the layout evenly. In some printing presses, the inking roller can also move in series after being adjusted. In order to achieve a better ink leveling effect, the inking roller can be allowed to make an appropriate amount of in series. By adjusting the starting point of the inking of the inking 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, it is only possible to determine a position most conducive to uniform ink color by gradually adjusting the serial starting position.

11. Control the effect of emulsification on ink color

When printing solid ground colors, emulsification consumes much water and the water supply is large. The axial ends of the solid ground color part, especially when the ground color part is close to or reaches the edge of the paper, there is no ink and water emulsification in the area outside the paper, and there is no blank part of the paper to absorb moisture. As a result, excess moisture will The tandem motion of the water roller enters the background color of the paper at both ends of the axial direction, which causes the background ink layer at both ends of the paper to be emulsified too heavy, the ink color becomes lighter, and may cause the back side to smear. The reasonable way is to use the waterway blowing device or the sticker on the water bucket roller as much as possible to reduce the water supply at both ends of the printing plate axis and control the emulsification degree of the entire plate.

The effect of post-press processing on solid ground color

After the undercoat ink layer is polished and coated, the surface is smoother and tends to be mirror reflection, which can make the undercoat color appear thicker.

If excessive drying oil is added during printing, or the ink layer is crystallized for too long, it will cause poor adhesion of the varnish on the imprint and pitting, which will affect the uniformity of the background color. At this time, 5% lactic acid can be added to the varnish to enhance the adhesion of the varnish.

When printing dark colors, if too much powder is sprayed, the film will make the powder particles appear more obvious, which seriously affects the appearance.

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