The overprint is not real, which often causes great trouble in the flexo printing operation of the carton.

Ordinary cartons (box board, whiteboard paper) and coated paper cartons have completely different phenomena, causes and solutions for overprinting quality problems due to the great difference in the ink absorption properties of the substrates. .

The printing color sequence of ordinary boxboard paper and whiteboard paper is usually from light to dark color, which is similar to the color sequence of offset printing. In the process of overprinting, the phenomenon of overprinting and revealing background color often occurs. The material of the face paper is loose, the surface is rough, and the ink absorption is strong, and under normal circumstances, there will be no cross-color and ghosting phenomenon.

The color sequence of coated paper printing, usually from dark to light color, is opposite to the printing of offset printing and ordinary boxboard and whiteboard paper, because the material of coated paper is compact and smooth, and the ink absorption is small. , In the process of overprinting, the printing of coated paper is often prone to cross-color and ghosting. Generally, the phenomenon of overprinting is rarely seen in the printing of coated paper. The problems and solutions related to overprinting of coated paper will be analyzed in detail separately.

Let's focus on analyzing the reasons for the misprinting of ordinary cartons (box board, white board), and find some solutions.

 

I. Reasons for misprinting

First of all, it is necessary for us to briefly review some basic principles related to overprinting, so as to have a clearer understanding of the basis for various solutions, and to adopt more, more reasonable and effective measures according to the basic principles of overprinting. s solution.

1. Overprint

 

 

(1) Definition of Overprint

In color printing, printing one color on top of another, so that the ink of the latter color is attached to the ink film layer of the previous color, is called overprinting of ink (also called ink transfer).

(2) The difference between overprinting and overprinting and trapping

⚫ Overprint:

 

 

Print one color on top of another, with the ink layer superimposed between them. The effect of overprinting will be affected by the characteristics of the ink

Influences of factors such as properties, ink volume, ink drying speed, ink absorption performance of the substrate, printing speed and temperature and humidity of the printing environment.

⚫ Overprinting:

 

 

In order to avoid the printing quality problems caused by the overprinting of the upper and lower inks, the two overlapping objects are hollowed out during the plate making to achieve an accurate nesting during printing, so that there is no occurrence of ink between the top and bottom objects. Superimposition, which is also called overprinting.

⚫ 陷印 (Trapping) ：

 

 

Trapping is also called trapping, also known as expansion and contraction, a printing process that makes the edges of the upper and lower colors overlap.

Why use trapping? Because overprinting is difficult to achieve error-free, in order to make up for the gap between two adjacent different colors caused by inaccurate printing overprinting, the trapping process is produced.

Principles of implementing trapping on printing plates:

Under normal circumstances, it is to expand the lower color without expanding the color, expand the light color without expanding the dark color, and expand the flat screen without expanding the solid, because the color can cover the lower color, and the dark color can cover the light and solid Can cover dots. Sometimes mutual expansion can also be carried out, and in special cases, reverse trapping is required, and a gap is even added between two adjacent colors to compensate for overprinting errors, so as to make the printed matter beautiful.

In trapping, the part where the two-color edges overlap during printing due to scaling processing is also overprinting. Therefore, in the overprinting part of trapping, there will also be problems such as false overprinting.

 

 

Depending on whether the ink film of the first printed color is dry when printing the color after printing, the overprinting is divided into: dry overprinting and wet overprinting.

⚫ Definition of Dry Overprint

The printing method in which the ink of the next color is superimposed on the substrate when the first ink is fixed on the substrate and is nearly dry. The overprinting method is called: wet-press-dry, and the carton flexographic multi-color printing belongs to wet-press-dry, whether it is pre-printing or post-printing.

⚫ Definition of Wet Overprint

When printing on a multi-color printing machine, the first printed ink has not yet come and dried, and the latter color ink is overprinted, and the inks adhere to each other in a wet state. The overprinting method is: wet-on-wet (Note: multi-color offset printing belongs to wet-on-wet, and offset printing ink overprinting is not within the scope of this article).

⚫ The difference between ink transfer in dry overprinting and wet overprinting

 

 

When wet pressed to dry (dry overprint), the ink splits at 1/2 (middle of the ink layer) of the ink layer of the subsequent color (the plate ink layer). Therefore, during wet-press-dry overprinting, the amount of ink transfer is large, and the overprinting effect is good.

 

 

When wet pressing wet (wet overprinting), since the first color ink is not dry, during overprinting, the last color ink is in the total ink layer of the superimposed two-color ink (the ink layer on the printing plate plus the ink printed on the substrate first). layer) splits at 1/2 (the middle of the total ink layer when overprinting).

Since only a small amount of ink is transferred to the previous wet ink film when wet presses wet, and most of the ink remains on the surface of the printing plate, wet overprinting has less ink transfer (low overprinting rate) than dry overprinting.

(4) Significance of overprint rate (also called ink receiving force) and detection of overprint rate

How to judge the effect of overprinting? The printing industry uses overprinting rate to evaluate the effect of overprinting.

⚫ Definition and representation of overprint rate

Overprint rate is a measure of good and poor ink overprinting. It refers to the percentage of the density of the solid color block of the three primary colors transferred to the ink layer of the first printing color compared with the density of the ink directly printed on the surface of the substrate.

A. Weight notation (representing the overprint rate by the amount of ink):

 

 

fD = y2,1/y1 ×100%

fD : Overprint ratio of the ink volume of the second color ink on the ink volume of the first color ink

y1 : The ink volume of the first color ink

y2,1 : The amount of second color ink superimposed on the first color ink

B. Optical Density Representation:

 

 

fD(2/1) = D2,1/D2 ×100%

= (D1+2 - D1 ) / D2 ×100%

fD(2/1): Overprint ratio of the density of the second color ink on the density of the first color ink

D1: Density of the first color ink

D2 : Density of the second color ink

D1+2: The total density of the two-color ink after overprinting

D2,1 : Density of the second color ink superimposed on the first color ink

⚫ The significance of detecting and controlling the overprint rate

The purpose of measuring and controlling the overprint rate is to detect and avoid the drawbacks of low overprint rate (overprinting) in time, so as to meet the needs of tone restoration.

The level of overprinting rate reflects the quality of the color rendering of the overprinted part during multi-color printing. The value of overprinting rate is between 0 and 100%. When the overprinting rate is 100%, it indicates that the color of the later printed color is on the first printed color. The effect is the same as that printed directly on the substrate. An overprint ratio of 0 indicates that the last print color cannot overprint onto the first print color. For color reproduction, the higher the overprint rate, the better. If the overprint rate is too low, the color reproduction range will be reduced, and some required hues cannot be obtained.

The level of overprinting rate is a test item for judging the quality of printing. For this reason, many developed countries regard primary color, secondary color and complex color solid block as the basic segment block that the printing quality measurement and control strip must have, and also according to the size of the overprinting rate. , to classify the grades of printed products.

(Primary color: The basic color in the pigment and the color in the light can not be decomposed, also called the first color. Secondary color: The color mixed by two primary colors, also called the second color. Complex color: Also known as secondary color, A tertiary color obtained by adding a primary color and a secondary color or adding two secondary colors.)

 

 

(5) Mechanism of overprinting

⚫ Hole Theory

The void theory holds that: in the process of ink splitting, the ink will produce voids under the action of shear stress. The voids are the beginning of the ink splitting (Note: the beginning, the beginning), and the speed and quantity of voids can be used as the ink splitting difficulty. Degree and Ink

A measure of how fast it splits. The speed and number of voids are first related to the shear stress inside, and under certain other conditions, it is first related to the viscosity of the ink.

Ink split transfer process during overprinting:

 

 

During printing, the ink layer is gathered and squeezed at high speed between the roller and the roller or between the roller and the substrate. After the ink passes through the extrusion and shearing section, due to the sudden drop in pressure, the ink begins to split due to the tension of the ink layers on both sides, forming an "empty space" inside the ink layer. The "core" and "hollow core" gradually grow up to form a "void", while the "void" is formed and expanded, the ink is also drawn into filaments, and finally the ink is broken and divided into three parts. A part of the ink remains on the printing plate, another part of the ink is overprinted on the ink layer of the previous printing color and on the substrate, and another part of the ink forms "ink mist" in the space during the springback of the drawing breakage.

When the ink is extruded and sheared, the viscosity of the ink plays a decisive role; and after the extruded and sheared section, the viscosity and elasticity of the ink are caused by the appearance of voids, filament formation, breakage, and transfer.

Affecting ink break transfer is related to the following factors:

A. Ink viscosity

The viscosity of the ink is low, it is easy to produce voids, the production speed is fast, and it is easy to split. This is the reason why the viscosity of the second color ink is lower than that of the first color in order to ensure the overprinting effect.

B. Number and nature of pigment particles (particles) in the ink

The more pigment particles in the ink, the stronger the oleophobicity (that is, it is difficult to be wetted by the binder), the greater the probability of voids, and the easier the ink is to split.

Using wet-press-dry printing, when the second color ink is overprinted, the surface of the first color ink layer tends to be dry, and the connecting material in the second color ink cannot penetrate, and the concentration of pigment particles in the ink is still uniform. The chances of voids occurring in the ink layer are also equal everywhere, and the ink layer is split in the middle and then transferred.

This is why the second ink layer is equally divided into two in dry printing and transferred to the first ink layer and the plate cylinder respectively.

If it is wet-on-wet printing, when the second color ink is overprinted, since the first color ink layer is not dry, the connecting material in the second color ink will partially penetrate into the first color ink layer, resulting in the second color ink layer. In the ink layer near the first color ink layer, the concentration of pigment particles is higher than elsewhere, and there are more opportunities for voids to occur and the ink layer to split than elsewhere, so the ink splits here and then transfers.

This is why in wet printing, the second color ink layer is split close to the first color ink layer, so that most of the ink is taken away by the printing plate cylinder, and a small part is transferred to the first color ink layer.

⚫ Kinetic Theory

The dynamic theory holds that the splitting of the ink layer must absorb part of the energy (that is, the destruction energy), and the destruction energy comes from the impact energy caused by the printing pressure during the printing process, and the impact energy propagates in the ink layer in the form of shock waves. When two colors are overprinted, the impact energy is transmitted from the interface (interface) of the two inks to the two ink layers at the same time. Among the two-color ink layers, those with high adhesiveness can withstand the shock wave; those with low adhesiveness will be damaged, causing the splitting of the ink layer.

(Ink adhesion: In the process of ink transfer, the ability of the ink film to prevent the rupture of the ink film under the dynamic stretching force is the adhesion of the ink, which is essentially a kind of ink cohesion under the action of adhesion. Performance.)

If the adhesiveness of the post-print ink is less than that of the pre-print ink and the adhesiveness between the pre-print and post-print inks, the ink can adhere well to the post-print ink layer. When the front printing ink is dry, its adhesion is very large, so it is only related to the adhesion between the front printing and post printing inks. Therefore, when two-color inks are overprinted, one ink layer is always split, and the other ink layer remains intact. In order to ensure the smooth progress of overprinting, the adhesiveness of the ink of the latter color should be lower than that of the ink of the former color.

2. Why is the overprint not true?

(1) Wet-on-wet (print the second color before the first color is dry)

When printing on a flexo printing machine without a drying system, due to the short distance of each color group, when the printing speed is faster than the drying speed of the ink printed on the surface of the substrate, the former color with a larger amount of ink has not yet been dried, and the latter color has been pressed. After printing, it is difficult for the ink of the following color to adhere to the previous color, and even the ink of the latter printing causes the ink of the former printing to peel off (called reverse overprinting).

(2) After color viscosity is too high

 

 

When the viscosity of the ink after printing is too high, the leveling performance of the ink is inhibited, and the depressions of some substrates with rough and loose surfaces cannot be filled and covered by the ink and the former color is exposed.

(3) The amount of post-print ink is too small

 

 

When the amount of after-print color ink is too small, the after-color ink cannot cover the previous print color.

3. Other negative effects of overprinting

(1) Color bleed (mainly on coated paper)

 

 

The overprinting caused by the wet ink of the previous color is not real, and the reverse overprinting phenomenon will also cause the printing plate to stick the non-drying ink of the previous color back to the printing plate.

Causes ink bleed.

(2) Cross color affects the hue of the back color ink (coated paper printing)

 

 

When the color channeling is serious, the dark front color will be reversely brought into the light back color ink through the printing plate and anilox roller, which will affect the hue of the back color ink.

 

2. The solution to the false overprinting of ordinary cartons

1. Prepress measures

(1) When overprinting on a solid plate or a large color block, you can choose to reduce the thickness of the front color ink layer for each printing, and make two printing plates for the front printing color respectively, and the front color full version (10~15%) ) The left and right halftone versions are primed (the appropriate grayscale value of the halftone dots should be selected according to the actual test situation), and the back color is printed for the second time with the full version of the same color, which reduces the thickness of each ink and makes it easier to The drying of the ink is also beneficial to reduce the phenomenon of paper wool and whitening. It is also conducive to full-form solid and large color block printing to reduce paper fuzz and whitening.

(2) When the high-color dots are multi-color overprinted, the black version can be used to strengthen the tone level of the black version, reduce the dot area of ​​other colors, and reduce the total amount of overprinted ink. Therefore, when making color separation, consider using as few colors as possible and as pure colors as possible to complete the superposition of colors. Small ink volume color overprinting is easy for ink to dry, suitable for increasing printing speed.

2. Reasonable configuration of anilox rollers

The printing surface paper of ordinary carton is boxboard or whiteboard. The paper is loose and rough, so the amount of ink absorption is large. The anilox rolls of the first color group of traditional printing presses are often equipped with low line count anilox rolls with the largest ink load, which is very disadvantageous for overprinting, because too much ink volume is not conducive to overprinting. It is recommended that the configuration of anilox rollers for ordinary cartons can refer to the following scheme:

⚫ 09 series, 12 series anilox roller configuration

Number	Anilox roll configuration	First color group	Second Color Group	Third color group	Fourth color group	Notes
1	Metal anilox roller with rubber roller	250 (l/in)~280((l/in)		220(l/in)~250(l/in)
2	Ceramic anilox roller with rubber roller	300(l/in)~350(l/in)		250(l/in)~280(l/in)
3	Ceramic anilox roll with scraper	250(l/in)~300(l/in)		220(l/in)~250(l/in)

 

⚫ 14 series, 16 series printing press configuration of anilox roller

Serial number	Anilox roll configuration	First color group	Second Color Group	Third color group	Fourth color group	Notes
1	Metal anilox roller with rubber roller	250(l/in)~280(l/in)		200(l/in)~250(l/in)
2	Ceramic anilox roller with rubber roller	260(l/in)~300(l/in)		220(l/in)~250(l/in)
3	Ceramic anilox roll with scraper	250(l/in)~280(l/in)		200(l/in)~250(l/in)

 

Note: Since the watermarking machine after the carton is from 09 to 16 series, the outer diameter of the printing press roller increases in turn, while the printing speed decreases in turn, so the printing speed of the 09 and 12 series printing presses is higher than that of the 14 and 16 series printing presses. Faster, the rotation speed of the anilox roller is also fast, the centrifugal force during ink transfer is relatively large, and the ink transfer efficiency is relatively high. Therefore, the ink carrying capacity of the 14 and 16 series anilox rollers is selected to be larger to meet the ink amount required for printing.

3. Adjust ink performance index

(1) Select low-viscosity, high-color-concentration ink to reduce the thickness of the ink layer while ensuring the covering performance of the overprinting ink.

(2) Use finer ink to reduce the thickness of the ink layer.

(3) Increase the drying speed of the ink in use to ensure wet pressing during overprinting.

(4) Improve the adhesion of the front color or reduce the adhesion of the back color ink, so that the latter ink can easily overlap the ink of the previous color.

(5) Reduce the viscosity of the pre-print ink to make it easier to dry. Generally, the viscosity of the overprint ink can be 2~ lower than that of the overprint ink.

3 seconds.

4. Change the printing environmental conditions

(1) Increase the temperature of the printing material to help increase the drying speed of the ink after printing, such as the hot cardboard immediately after the offline printing operation.

(2) Add hot air to assist drying, multi-color nesting requires the former color ink to dry as soon as possible, and the latter color can be stacked smoothly. Therefore, advanced flexographic printing machines use hot air drying or increase the distance between the units so that the cardboard has enough time to dry (fixed units can be used).

(3) It is also possible to add cold air drying or infrared drying between color groups to help the ink dry.

(4) In the low temperature environment, the ink is heated and kept at a constant temperature to maintain the drying performance of the ink.

5. Change the printing process conditions

(1) Unfold the color sequence of the overlapping color groups, and do overprinting in separate groups or in multiple groups, so that the printed ink can increase the drying time before overprinting. For example, the third color group is superimposed on the first color group, and the fourth color group is superimposed on the first or second color group. Make sure the base coat is dry enough to allow the postprint to build up.

(2) Reduce the production speed and increase the drying time of the ink before overprinting.

(3) For the overprinting of multi-color crossed color blocks, the color is relatively dark, and it is required that the overprinting of each color cannot be exposed, and the product needs to be die-cut in a large area. It is recommended to adjust the process and do two printings to ensure the overprinting effect.

 

 

3. Summary

1. Reasons for false overprinting of ordinary cartons:

(1) Because of wet pressing, the current color ink is not dry, and the latter color ink is overprinted on the surface of the wet ink.

 

 

(2) The amount of post-print ink is too small.

 

 

(3) The viscosity of the post-print color is too high.

 

 

2. Key points for all solutions:

(1) To achieve wet pressing, reduce the ink volume of the fore-color ink, reduce the thickness of the fore-color ink layer, and speed up the drying speed of the fore-color ink or increase the drying time.

(2) Ensure that the ink loading of the ink and the anilox roller matches the printing material and printing conditions, and implement standardized management of parameters for the ink.