High definition carton printing generally refers to 80-line flexo printing and above carton printing. Most of the substrate materials are coated paper. Because of its ink absorption performance and surface roughness, it is very different from the boxboard and whiteboard of the paper used in ordinary carton printing. Therefore, the phenomena, causes and solutions of overprinting quality problems are different. For the printing of some new types of plastic corrugated board boxes, because its ink absorption performance is extremely small compared with coated paper, the drying of its ink and wash depends entirely on volatilization, so its overprinting problem is similar to that of plastic film printing. In the article "Causes and Solutions of Poor Overprinting of Ordinary Cartons", we expounded some basic knowledge and principles related to overprinting, which will not be fully repeated here.

 

1. Basic conditions for overprinting of high definition cartons

1. Plates

Flexo plate is the basic condition for printing to reproduce the original color. The prepress plate making of high definition carton flexo printing must pay attention to the following factors:

(1) Do a good job in the pre-press test for the printing equipment, and obtain the dot expansion curve of each color on the printing equipment, the field density of the ink, the hue value, the impact of the dots on the screen, the influence of the color sequence, the reduction of fine lines, overprinting errors and the pressure balance ability of the printing work. Based on this, the plate-making company can establish a set of color matching and management systems for this printing equipment, such as the establishment of dot compensation curves for dot expansion, etc., to correctly determine the plate-making process and color separation scheme.

◼ test the ink absorption properties of the substrates.

◼ testing the number of lines, ink carrying capacity, ink feeding capacity and control accuracy of the anilox roller is an important parameter for evaluating the ink supply capacity and accuracy.

◼ test the accuracy of flexographic printing equipment. For flexographic printing, the flexographic deformation of the flexographic plate during printing is inevitable, and the accuracy of the equipment will affect it. Different flexographic printing equipment has different precision, and the expansion degree of printing dots, small characters and lines on the printed product is also different. Therefore, when determining the number of screen lines added to the printing plate and making reasonable adjustments to the grey release of four-color overprinting, factors such as the expansion and deformation of printing dots, small characters and lines should be considered according to the tested data.

◼ test the printing performance of water-based inks and collect accurate data for the color management of printing plates, which can be used as the basis for the adjustment of plate-making parameters. Ink products from different manufacturers, and even different batches of products, due to differences in raw materials, formulations, ink modulation, and user application management of ink, will lead to changes in the hue of water-based inks and affect the hue deviation of printed products.

(2) The flexographic plate design should try to avoid stacking on the ground and stacking dots on the ground. When solving the problem of how to stack on the ground, the scheme of secondary printing of the same color and printing dots first and then printing on the ground can be used.

(3) To control the area of the dots of the overprinted printing plate *** points), the sum of the dots of the four-color overprinting (the total grey release of the four colors) should not exceed 220%. The overprinting of dots with too high fineness will lead to poor overprinting because the overprinting ink layer is too thick and not easy to dry.

(4) The trapping should be made light and not dark, the dark color in the superimposed color should not be moved, the original size should be maintained, and the light color should be expanded and shrunk. No, expand the dots, because the dots can be covered by the field.

(5) The material of the flexo plate meets the requirements of high definition carton flexo printing, and the hardness of the printing plate should be uniform. For high definition plates with more than 100 lines, choose a plate with a thickness of 2.84 mm. The small dots of the 2.84 mm plate are not easy to drop, and the small dots of the 3.94 mm plate (especially the highlight part) are easy to drop when erasing.

 

2. high definition ink

 

High definition ink is different from ordinary ink, it is designed for the characteristics of high definition flexo printing process, with the following characteristics:

(1) Fineness ≤ 5 microns (pre-printed ink standard), the dots after printing are clear, bright colors and delicate patterns.

(2) good transparency, high definition ink using transparent resin, when the light color superimposed dark color, the front of the dark printing will not change, the color, but will be more vivid.

(3) High gloss, the printed products are bright and shiny.

(4) High color content, full color and rich connotation of printed products.

(5) Low viscosity, it is not easy to block the screen, paste and ink bars during printing, the pictures and texts are clean, and the layout is clean.

(6) Appropriate drying speed, good color stacking effect, no drag flowers.

(7) Good leveling, uniform color and no bottom exposure.

(8) Excellent overprinting performance, good compatibility between various colors of ink and wash, natural and harmonious colors, and rich layers.

(9) Good adhesion and no fading.

3. Ink volume and control accuracy

Due to the poor ink absorption of coated paper, the high number of screen lines on the printing plate and the small number of printing dots, high definition carton overprinting is very sensitive to ink volume and control accuracy.

4. Equipment accuracy

High definition carton overprinting requires high precision of printing equipment, and requires good adjustment accuracy of three printing working pressures and transmission accuracy of substrates.

 

The premise of describing the overprinting problem of high definition carton in this paper is that it has been tested before printing and has been reasonably adjusted and processed during plate making. The use of high-definition special ink, the ink volume control accuracy and the equipment accuracy are all suitable for high definition carton printing.

 

2. Common problems in high definition carton overprinting

In high definition carton overprinting, the coated paper as the substrate material is tight and smooth, and the ink absorption is small. How is the ink printed on it dry? Part of the ink penetrates into the paper fibers and is absorbed by the paper, and the other part is volatilized in the air. The color sequence of overlay printing on coated paper is from dark to light (black, blue, red, yellow). Once the pre-printing color is caused by factors such as thick ink layer, fast printing speed or high ink viscosity, the color will dry poorly. The post-printing color is similar to the printing state of "wet pressing and wet" during the overprinting process, which makes flexo printing a taboo, and is prone to printing defects such as string color, ghost shadow, and color bias. If the drying speed of water-based ink is adjusted too quickly, it is also prone to paste, clogging screen, roll mesh and There will also be problems such as color channeling.

 

1. Overprint color channeling:

The ink color of the pre-printing color group enters the ink or varnish of the post-printing color group, resulting in contamination of the printing surface of the post-printing product.

 

2. Overprint ghosts:

Its blurry phantom appears in the printed area where a particular image should not appear.

There are two kinds of ghost phenomenon, one appears on the same color, and the other is that the graphic outline of the pre-printed color appears in the post-printed color, which is actually a phenomenon of color channeling.

 

 

3. Overprint color cast (hue difference):

During the printing process, due to the large or small amount of ink and printing pressure of different color groups, the area of the network points of each color overprinted changes, so that the hue and level after overprinting do not match the standard.

 

 

4. Overprint paste:

Unwanted ink appears on the dots or color blocks, lines and text edges of the printing plate, resulting in overlapping dots, blurred or dirty images, and darkening of local stacked color patterns.

 

 

5. The mesh hole of the anilox roller is blocked:

The drying speed of the ink is too fast or the anilox roller cannot be cleaned and cleaned in time, and the sudden mechanical and electrical failure causes the shutdown, etc., resulting in the ink on the anilox roller blocking the mesh due to drying up, affecting the ink transfer of the anilox roller.

 

 

3. Causes and solutions of common problems in high definition carton overprinting

1. Reasons and solutions for overprinting color channeling

(1) Reasons for overprinting color channeling

We all know that the ideal flexo overprinting should be wet and dry printing. When the ink to be printed first is fixed on the substrate and is close to drying, the post-printing ink will be overprinted on it. In this way, the post-printing ink can be effectively overprinted on the pre-printing ink. So how is the color channeling caused?

◼Insufficient drying of the ink before the

 

 

For flexo web pre-printing, the flexo printing machine has effective drying facilities, which generally ensures the wet press-drying of overprinting.

For high definition carton post-printing equipment, although some are also equipped with drying facilities, due to the insufficient drying time during high-speed printing, the drying degree of the pre-printing color often cannot meet the overprinting requirements during high-speed printing. During overprinting, if it is not dry, the pre-printing ink will be brought into the ink of the post-printing color printing plate, which will pollute the post-printing color. At the same time, it is brought into the anilox roller, which is then transferred to the printing plate and printed on the substrate through the printing plate, resulting in the phenomenon of color channeling. Under normal circumstances, the color of high definition carton printing is from deep to light, and the phenomenon of color channeling is the dark color of the pre-printing into the light color of the post-printing, so it will affect the quality of the printed product.

The amount of ink in the printing water before

◼ is too large

The excessive amount of ink in the pre-printing ink will cause the ink layer of the printing dot to thicken, which will also make the ink not sufficiently dry, resulting in color channeling.

 

 

The anilox roller of the color group of the flexographic preprinting machine will be composed of anilox rollers with different number of lines and ink carrying capacity according to the printing requirements of the product. If the anilox roller with too low number of lines and large ink carrying capacity is selected for high ** point overprinting, it will cause insufficient drying due to the excessive amount of ink, resulting in color channeling phenomenon.

Abnormal pressure of the scraper will reduce the ink delivery accuracy of the anilox roller, and the local printing area with too much ink will also be too thick due to the ink layer of the printing dots, resulting in too slow drying of the ink and color channeling.

Unreasonable configuration of anilox rollers in carton post-printing equipment will also lead to excessive ink volume in overprinting color groups.

◼ ink dries too quickly

 

 

If the ink dries too quickly, there will also be a phenomenon of color channeling. The quick-drying ink printed on the coated paper is easily glued down by the post-printed printing plate and brought to the anilox roll again. Due to the poor fluidity and re-solubility of the quick-drying ink on the surface of the anilox roll, it will cause color channeling and ghost shadows.

(2) Solution for overprinting color channeling

◼ adjust the performance of ink.

A. The viscosity of the printing color before overprinting ink is lower than that of the post-printing color. The high viscosity of the ink will make the printing ink layer thicker, resulting in the phenomenon of color channeling and ghost shadows that the ink cannot dry in time.

B. The drying speed of the overprint ink should gradually slow down from the front printing color to the back. Ensure that under certain printing speed conditions, when printing and color ink after printing, the front printing ink has reached the initial dryness, and there is no ink accumulation around the dots and text.

C. Appropriately improve the re-solubility of ink to improve the fluidity of ink on the surface of the anilox roller.

D. Make sure that the color content of the ink meets the requirements of high definition ink. If the ink with low color content needs to reduce the viscosity, the printing color will become lighter.

◼ for pre-printing, it is necessary to reasonably select the number of lines and ink carrying capacity of the anilox roller of the pre-printing color ink printing group to ensure that the ink ink output of the pre-printing color meets the requirements of stacking color printing.

For carton post-printing, the anilox roller is difficult to easily replace. When selecting the equipment, it is necessary to evaluate the anilox roller according to the requirements of the printed product, the number of lines, the amount of ink loaded, and the latitude of the amount of ink fed (including the scraping accuracy and adjustable range), must be able to meet the upper limit (large color fast printing) and lower limit (the number of dot lines and the minimum dot of the printing plate) of the printed product.

◼ check the ink scraping accuracy to prevent scraper wear or improper adjustment to increase the amount of ink, resulting in slowing down the drying speed of ink.

◼ adjust the spacing between the easy-to-channeling color groups, and take a distance between one or more groups to increase the drying time.

◼ the color channeling caused by varnish, choose varnish with low solubility to ink.

◼ conditions to improve the drying capacity of printing equipment.

◼ conditional, you can use the hot cardboard when the cardboard production line is offline for color stacking printing to help the ink dry.

◼ reduce the printing speed, so that the pre-printed color has enough drying time.

◼ choose the printing material reasonably according to the ink absorption performance. The printing material of high definition carton flexo printing is basically light coated paper, which is not suitable for recoating. At present, the light coated paper manufacturers suitable for high definition watermarking in China include: Century Sunshine, Ji'an Paper, Nine Dragons Paper, etc. (good ink absorption performance, no powder loss, smooth surface, uniform color, high temperature resistance, etc.).

2. Causes and solutions of ghosts

(1) The reason for the ghost

There are two kinds of ghost shadows in flexo printing, one is the ghost that appears on the same color group, and the other is the appearance of the pre-print on the post-print color graphic, the outline of the color graphic, which is actually the phenomenon of the pre-print color rushing into the post-print color.

◼ the first kind of ghost is due to the inconsistent local ink supply of the anilox roller during printing, which makes the local ink amount of the printing plate different, so the local color depth of the printed product appears, mainly in the flexographic preprinting.

During web pre-printing, when the repeating circumference of the printing plate is smaller than the circumference of the anilox roller, such ghosts are prone to occur, that is, on the printed product, the color saturation of the color block or the local area of the graphic and text changes, and the color appears Shades, especially the dark colors are more obvious. However, the graphic outline of the part that shows the light color is a part of the entire graphic outline. This kind of ghost appears on the same color group. After several graphic and text are printed to repeat the circumference, the ghosts begin to appear. Since the position of the ink on the anilox roller to the printing plate graphic and text is constantly displaced along the circumference, at this time, the area on the surface of the anilox roller that has not been in contact with the printing plate graphic and text at one time and the area where the printing plate graphic and text have been in contact with the ink transfer, although at the same time, ink is applied once through the scraping chamber, but the amount of ink in these two areas on the anilox roller is different, and the amount of ink in the area that does not contact the printing plate with the ink transfer will be greater than the area where the ink passed through the last time, and the color printed on the paper will be darker, and the light color outline is the outline of the graphic text That's right. Since the anilox roller corresponds to the printing plate graphic, the position of the anilox roller will continue to move due to the ratio of the repeated circumference of the printing plate graphic and the circumference of the anilox roller, and the position of the ghost will also correspond

Change, if it happens to be a common multiple or the interval is not large, the position of the ghost will not change much. When the repeated circumference of the printing plate graphic is larger than the circumference of the anilox roller, the ghost is not easy to appear.

◼ the appearance of the second kind of ghost is the printing color before the ink is dry, and the color of the outline of the pre-printing color appears on the picture and text of the post-printing color, which is a kind of color phenomenon of the pre-printing color rushing into the post-printing color.

 

 

The current printing color is not dry. When printing after printing, the post-printing color printing plate will bring the ink on the pre-printing color graphic text on the substrate into the post-printing, and then into the anilox roller. Since the diameter of the anilox roller is often smaller than the diameter of the printing roller, and the color sequence of the high-definition watermark is arranged from deep to light, the dark pre-printing color on the anilox roller is randomly transferred to the post-printing color flexo plate graphic text, and then transferred to the subsequent substrate together with the light-colored graphic text after printing, resulting in the post-printing Ghosts appear in the sequel. This kind of ghost is actually caused by the pre-print color rushing into the post-print color.

(2) The solution of ghosts

◼ for the first ghost, the ink drying speed can be adjusted to improve the re-solubility of the ink, and a slow drying agent can be appropriately added to make the ink easy to flow and level on the surface of the anilox roll, so as to eliminate the printing plate on the anilox roll. The outline traces of the graphic text can achieve the purpose of eliminating ghosts, and if possible, the circumference of the anilox roll is selected to be larger than the repeated circumference of the printing plate graphic text.

◼ the second ghost solution is mainly to solve the ink volume and drying problems of the pre-printing ink. (refer to the color channeling solution)

A. Reasonably select the number of lines and ink carrying capacity of the anilox roller in the front color ink printing color group.

B. Check the scraping accuracy.

C. Adjust the ink performance.

D. Adjust the color sequence between the easy-to-channeling color groups, and take a distance between one or several groups to increase the drying time.

E. Conditional, you can use the hot cardboard when the cardboard production line is offline for color stacking printing to help the ink dry.

F. If conditions permit, change the graphic and text colors that are easy to change into spot colors and then print them.

G. In winter, when the indoor temperature is lower than 5 degrees, if conditions permit, an ink and wash constant temperature system can be added to the equipment, which will be of great help to reduce color channeling and ghosts.

3. Causes and solutions of color cast

Color cast is that the color of the printed picture and text deviates from the color of the original. The quality of plate making is not discussed here. We discuss the color cast problem that occurs in the printing operation on the premise of confirming that the plate-making print and printing proofing meet the requirements of the original.

(1) Causes of color cast

◼ printing due to dot enlargement or deformation caused by color cast.

As we all know, the color of printed pictures and texts is composed of color blocks, lines and dots of different areas through C (cyan), M (magenta), Y (yellow), K (black) 3 primary colors plus black overprinting. 4 ** points of different areas are superprinted in various proportions, showing a variety of colorful colors on the printed product. Due to the material characteristics of the flexographic plate dots, during the printing process, it can be printed on the surface of the substrate with slightly less flatness, but also brings about the disadvantage that the printing dots are easily deformed and expanded, which leads to The proportion of ** points changes, and the color also changes.

In superimposed colors, light-colored (such as yellow) printing dot expansion and deformation have a relatively small impact on color cast than dark colors, and black has the greatest impact.

The expansion or deformation of dots of different colors has different effects on color casts. Black ones tend to darken and darken the overlay of the printed product; magenta ones will be reddish, while cyan will be bluish. When dots expand or deform in the multicolor group, magenta and cyan will be purple.

Magenta and yellow will be orange, and cyan and yellow will be green.

Causes of dot enlargement or deformation during printing:

A. The ink load of some or all of the anilox rollers in the overprint color group is too large or unreasonable and unbalanced.

B. Partial or total scratch pressure of the overprint color group is abnormal.

C. The printing pressure of part or all of the overprint color group is too large or uneven.

D. The ink performance does not match the printing conditions, or has changed during use due to a long cycle and being affected by ambient temperature and humidity.

E. The accuracy of the printing equipment is poor. The three printing pressure controls such as ink scraping, ink transfer and embossing are unstable.

F. There is ink slag on the printing pad, or the pad with aging, uneven elasticity and uneven thickness after using it for too long,

G. The deformation of the dots under pressure during printing is not considered before printing (generally, the deformation is 5%)

H. Serious blockage of the anilox roller will also cause color cast due to the reduction of the amount of ink.

I. Local burns on the surface of the substrate will cause uneven ink absorption. (The phenomenon of ribs in printing is partly caused by serious burns on the corrugated peaks of the cardboard).

J. There is ink slag on the surface of the embossing roller, and the foreign matter is not cleaned up, which increases the local printing pressure and causes the deformation of the dots to expand.

◼ color cast caused by poor overprinting

As mentioned earlier, when the overprint is wet and pressed, it will lead to poor overprinting. For high definition carton printing, the color sequence is from deep to light, the first color can always be printed solid, and then the printed color is difficult to adhere to, so the post-printed color is often overprinted, and the color density of the printed dots becomes smaller, and the degree of overprinting is different. If it also causes color channeling, the color cast phenomenon is more serious.

Generally, after obtaining the data through the prepress system test, the color management and printing color sequence in the bad state of overprinting will be considered when determining the plate making scheme, so the color sequence cannot be changed at will during printing. Since the first color of overprinting is always the most solid printing, once

Change the color order, the direction of the color cast will be cast with the color of the overprinted first color.

The causes of dampness and pressure are basically the same as the aforementioned causes of color channeling.

(2) Solution of color cast

The solution to exclude overprint color cast is to check and adjust from the following aspects:

◼ the number of lines and ink carrying capacity of the anilox roller.

◼ squeegee accuracy and squeegee selection.

◼ ink performance.

◼ check the gasket and replace the high quality gasket.

◼ check the surface of the substrate and replace it with a cardboard with no burns and better ink absorption.

◼ check and clean clogged anilox rollers.

◼ can only reduce the printing speed without conditions.

◼ check whether there is ink slag on the surface of the embossing roller and clean it in time.

4. Reasons and Solutions for Overprinting Paste

Paste plate phenomenon is the most common problem in high definition carton printing. Once the paste plate occurs, it is necessary to stop the machine to wipe the plate, which affects the normal production, which is very annoying. With magnifying glass, you can clearly see that the printing dots overlap, the color block lines and the edge of the text appear, and the ink brush is burnt.

 

 

(1) Causes of overprinting and pasting of high definition cartons

◼ the amount of ink in the ink is too large, causing the printing dots to expand and overlap.

 

 

◼ the ink transfer pressure between the anilox roll and the plate roll is too large, squeezing the ink to the edge of the flexographic dots, color blocks, lines and text outlines.

 

 

◼ due to the small ink absorption of the coated paper and the excessive printing pressure, the ink will also be squeezed to the edge of the flexographic dots, color blocks, lines and text outlines.

 

 

◼ the performance of the ink does not match the printing conditions or the performance has changed. If the viscosity of the ink is too high, it will cause the paste phenomenon; in order to solve the problem of drying the pre-printing color, the ink is often adjusted to dry too quickly, and the ink is easy to dry on the printing plate and block the paste.

 

 

◼ the hot air of the drying system between the color groups of some carton post-printing equipment will cause the ink on the surface of the printing plate to dry faster and cause paste.

(2) Solutions to the phenomenon of overprinting and pasting

◼ check and reasonably select the wire and ink load of the anilox roller

◼ reasonably adjust the three printing working pressures of scraping, inking and embossing.

◼ reasonable selection of scraper to ensure the precision of scraping ink.

◼ adjust the ink properties (drying speed, viscosity, re-solubility, etc.).

5. Causes and solutions of mesh clogging of anilox rollers

(See "Causes of clogging of ink transfer mesh and how to improve it" in the e-book)

(1) Reasons for blockage of mesh holes of anilox rollers

◼ because of high definition carton printing in order to solve the problem of pre-printing color drying, the ink is often adjusted too quickly, resulting in the anilox roller blocking the mesh due to the rapid drying of the ink.

 

 

◼ the hot air conduction of the drying system between the color groups of some carton post-printing equipment will cause the ink on the surface of the anilox roller to dry faster.

◼ excessive pressure between the scraper and the anilox roller causes the wear of the scraper to increase, resulting in the blockage of the anilox roller. Plastics in the dry ink in the mesh, plastic particles caused by the wear of the scraper, metal particles caused by the wear of the metal scraper, paper ash on the substrate, grinding debris from the slitting knife and dust particles in the environment are brought into the ink, making it easier for the ink to clog the mesh and difficult to clean.

 

 

◼ the ink is not cleaned thoroughly after each replacement and shutdown, so that the dry ink at the bottom of the mesh is gradually accumulating and blocked.

◼ mechanical and electrical faults caused the anilox roller to stop running, and the ink was not returned and cleaned in time, resulting in the dry blockage of the ink in the mesh.

 

 

(2) Solution for mesh clogging of anilox rollers

◼ choose the ink suitable for its own printing conditions according to the product structure of the factory, the performance of the printing machine, the printing speed, etc.;

◼ establish the technical standards for ink painting suitable for the actual printing conditions of the enterprise and the management specifications for maintenance and use;

◼ establish effective filtration facilities for import and export in the ink supply circulation system to remove impurities such as ink ballast, sand, paper wool and so on in the ink;

 

 

◼ every time the ink is replaced and after normal and abnormal shutdown, the anilox roller is thoroughly cleaned.

◼ check and eliminate the influence of hot air conduction on the surface of the anilox roller in the inter-color drying system of the post-printing equipment.

 

4. Summary

The problem of poor overprinting of high definition cartons is a system problem. Substrate materials, printing plates, water-based inks, ink volume and control accuracy and equipment accuracy must follow and meet the special requirements of high definition carton overprinting.

1. The printing material should be light coated paper with good ink absorption performance, high temperature resistance, no powder drop and other stable properties.

2. The printing plate should be reasonably adjusted on the basis of prepress test data.

3. Water-based ink should use high definition ink, and the printing performance adjustment of ink should be in line with the printing method of wet press-dry at the ideal printing speed.

4. The ink supply system should meet the ink quantity and control accuracy required to stably provide high definition carton overprinting. The use of ink is timely and reasonable, maintenance and adjustment, and the performance of ink is continuously stable.

5. The working accuracy of the printing equipment should meet the requirements of high definition carton flexo printing.

6. The most important thing is to establish a complete and sound high definition carton flexo printing process management specification, and can be effectively implemented.

In short, before the high definition carton flexo printing work to do a good job in the above aspects of work, when there is a problem of high definition carton overprinting, according to the phenomenon according to the causes analyzed in this article to investigate, identify the source of the right medicine, it is bound to reduce and eliminate the problem of high definition carton overprinting.