Today, the function of corrugated boxes has changed from transport packaging to sales packaging. Exquisitely printed packaging can not only increase the added value of goods, enhance the competitiveness of the goods, but also play an important role in opening up the market. Therefore, how can we produce high output? Quality and high-grade printing products have become the common goal and effort of all the technical personnel in the packaging and printing industry. Corrugated box printing is gradually changed from monochromatic or polychromatic printing to color tone printing. Flexographic printing has been widely used in the packaging and printing of corrugated cartons, and high-grade color-graded flexographic printing will become the protagonist of corrugated box printing. Corrugated carton flexo can be divided into high-end color tone printing and generally more simple lines and pattern printing. Flexographic printing quality is inseparable from the quality and performance of screen negative originals, printing plates, inks, and anilox rollers. This article will discuss the quality control issues of direct colour tuning of corrugated boxes.
First, the production of color network line negative original point of the original color line color line than the original monochrome line, pattern production is much more complex, more variable factors. In order to properly perform the flexographic plate separation, several data must be obtained, otherwise it is difficult to produce a high quality plate. These kinds of data are: the number of ink repelling roller lines, the screen angle of each color version, the amount of dot expansion, and the deformation compensation rate of printing plates.
1. Strictly control the maximum and minimum density values ​​of the original (negative film). When making a photosensitive resin plate, firstly, prepare a negative plate for printing. With high-quality matte film, the original maximum density D ≥ 4.0, the minimum density D ≤ 0.05.
2. The relationship between the number of screen lines and the number of lines of the screen roller The flex screen printing screen is generally thicker than the screen cable used in lithography. The number of screen lines should be controlled within the scope of flexo printing. The number of screens for the flexo version also depends on the dot gain and the media reproduction performance.
At present, the number of screens commonly used for printing corrugated boxes using flexographic printing is 40-80 lines/inch.
The number of screens for flexographic printing and the number of lines of the anilox roller have a matching relationship. The general rule is:
Number of screens = 1/4 screen roller number. That is, the number of anilox roller lines is four times the number of screens of printed products, and it can also be higher.
3, the determination of the screen angle of various colors (as shown in Table 1)
Group 1: Should meet the formula Offset - 7.5° = Flexo Printing Group 2: Should meet the formula Group 1 value + 15° = Group 2 value 4 Dot gain control Flex plate due to soft, flexible, plus Pressure is applied again during the printing process. Although light pressure printing is used in flexographic printing, it still leads to enlargement of printed image dots and elongation of images. When printing with low-viscosity water-based inks, as long as the pressure between the surface of the printing plate and the surface of the substrate is slightly higher, the ink on the printing plate will spread to the surrounding, making the dots form an intermediate ink layer with a thin ink layer. , Similar to hollow ideas, this is different from offset printing dot change.
In the high-light part, small dots (1%) have been lost in the high-light part and become a net-free part, 2% of the outlets have expanded to about 12%, and the 50% or less high-school adjustment area has been particularly serious. Close to midtones outlets. When the middle tone is about 40%, it will be deformed to the angle, resulting in a sudden change in the dot, which is extremely unstable. In dark tone, the flexo dot will become ground state at about 90%. In the process of flexo printing, dot gain is severe, and the tone reproduction range is 10% to 90%. Special treatment is required for highlights and dark notes. For the intermediate adjustment area, the network points in the compensation printing process have been severely expanded by narrowing the area ratio of the film Internet sites. For high-light areas, flat screens should be used as much as possible. For dark-tone areas, spot-color printing should be used.
5. Calculation of printing plate deformation compensation There are 2.84mm, 3.94mm and 7.00mm in flexographic printing for corrugated box printing. The commonly used is 3.94mm. The deformation rate and printing effect of plates of different thicknesses are different.
There are two parameters affecting the size of the flexible photosensitive resin plate layout. One is the thickness of the plate (especially the thickness of the photopolymer above the interface), and the other is the circumference of the plate cylinder. This perimeter is fixed with the plate. It is related to the outer diameter of the material layer up to the plate cylinder.
In order to compensate for the distortion of the image, the size of the corresponding graphic text in the circumferential direction must be reduced. The amount of decrease is calculated by the following formula:
Decreased percentage = K/R × 100%
Where R is the final copy length of the print.
Usually, the R value is given with the product. If no R value is given, the following formula can be used:
R=2Ï€(r+b+c)
In the formula, r—the radius of the plate cylinder b—the thickness of the adhesive liner c—the thickness of the printing plate The K value in the preceding formula is a constant and can be obtained by the following experiment.
Make a precise scale of the Yin picture, use it to expose a photosensitive flexo plate, apply the plate to the roller with standard loading process, ensure that the plate is installed correctly, and adjust the tension accurately before starting printing. Finally measure as accurately as possible a standard distance between the negative picture and the print, the same measurement is performed several times, and the average value is respectively calculated, and the average value obtained by measuring the negative picture is divided by the average value obtained by measuring the print product. This is the K value.
Second, the choice of plate material Corrugated carton flexo, both solid flexible plate material can also be used liquid flexo. However, the solid version is widely used because it has the advantages of good ink transmission performance, large latitude, high reproduction accuracy, good stability, no wear deformation, long service life, and convenient processing. Corrugated box printing solid flexible version 2.84mm, 3.94mm and 7.00mm several kinds, hardness HS25-50, commonly used 3.94mm. The thin version should be used as much as possible, because the thin version can not only perform flat solid printing, but also has a small dot enlargement, high overlay accuracy, stable print quality, and finer image layers compared with the thicker version. DuPont Selva corrugated carton printing thin version has the following: 3.94mm TDR (for B, C, BC and A type below 150g/m2); 2.84 - 3.18mm TDR (for above 170g / B2 type B corrugated or E type, EB type corrugated below 140g/m2); TDR 1.70-2.84mm (used for E corrugated). Thin plates must be used together with the pads to achieve good results.
When the quality of corrugated paper is poor, a thicker plate with a lower hardness should be used. When the quality of corrugated paper is good and small prints and screen printing are required, a thin plate with a hardness of HS35-45 should be selected.
Third, the use of water-based ink Water-based ink in the corrugated carton flexo printing has been widely used. The ink should be alkaline and have a pH of 8.5-9. However, if the alkalinity is too high, the paste will tend to occur.
The choice of water-based ink should pay attention to ink viscosity, concentration, fineness, dryness, abrasion resistance, water resistance and stability.
1. Viscosity The ink viscosity should be selected or adjusted according to the number of screen lines of the anilox roller, the fineness of the printing plate graphic, and the ink-absorbing performance of the paper product. When the number of reticulation lines is high, the printed text is small, or the ink absorption performance of the paper is poor, the ink with the lower viscosity should be used, and the ink with the higher viscosity should be used instead. Because the ink is water-soluble, it is more convenient to adjust the viscosity in use. Just add a little water and stir it evenly to make the required viscosity.
2, the density and fineness of the ink concentration that is the color concentration, only the color concentration is sufficient, can print a colorful pattern, and the fineness refers to the delicate degree of ink particles, for the high number of anilox roller should be Choose fine ink with good density and high concentration, otherwise it will block the mesh due to thick pigment particles, which will affect the effect of ink transfer and make the color of the printed product lighter. The printing effect is not good.
3. Adhesion, abrasion resistance, water resistance and dryness The choice of these properties should be based on the use of the corrugated box. When used in the packaging of frozen products, aquatic products or canned beverages, inks with strong adhesion, quick-drying, and water-resistant deinking should be selected.
In short, the variety, performance, and price of ink vary greatly. Although low-grade inks are inexpensive, they are often susceptible to blistering, stickiness, and affect the quality of printed products. They also contain more impurities, which can easily clog the mesh, not only affecting the amount of ink transferred but also accelerating. Anilox roller wear. Therefore, when selecting the ink, care should be taken not to lose it.
Fourth, white coated corrugated paper White coated corrugated paper, also known as white surface coated kraft paper, domestic carton factory has used this corrugated paper printing carton, obtained a good print quality and clear printing results. Compared with the non-coated paper, the coated paper has a higher printing ink density than the non-coated paper by 63% and a gloss of 350%. At present, the packaging boxes of branded beer, beverages, food, household appliances, and computers are all made of white-leaf kraft paperboard. For example, the boxes of HP printers were previously printed with offset printing paper and then covered with bottom paper. Now it is large. Most have adopted the method of printing directly on white-faced kraft paper with a flexo press. However, when printing with ordinary flexo ink on coated white kraft paper, ink inconsistency, slow drying speed, and low production efficiency often occur. Therefore, it is recommended to use a new generation of ink or a flexo printing machine with a drying device to print. It should be printed on a small area of ​​graphics, and then printed a large area of ​​graphics, printing color sequence can be black, blue, magenta, yellow color sequence for multi-color printing.
Fifth, the correct choice of lining plate Usually corrugated base material thickness is not uniform, the pressure is too light, may make the ink transfer uneven, easy to cause the printed Parts of the site white, or corrugated plate; and the pressure is too heavy, it will cause the substrate deformation, The expansion of outlets has affected fine lines and dot overprinting, and has also caused severe damage to the strength of corrugated cartons.
In traditional flexo printing, a 7mm thick printing plate is used. When the printing plate is pressed, the pressure of the raised graphic part is strong, and it is first compressed. The dot and text lines are easily deformed, the dot enlargement is intensified, and the plate making cost is also high.
If a 4mm thin plate with 3mm R/bak air cushion lining is used instead of a 7mm plate, the air cushion lining will have a hardness of 20-25 Shore, which is lower than the plate hardness (the current commonly used plate hardness is Shore 35). When the printing pressure is compressed, the air cushion lining is compressed first, so the printing plate is deformed little, the printing quality is greatly improved, and the plate making cost can also be reduced. The combination of this thin process and R/bak air-cushion lining technology is more suitable for printing fine images and is gradually replacing traditional thick ones.
First, the production of color network line negative original point of the original color line color line than the original monochrome line, pattern production is much more complex, more variable factors. In order to properly perform the flexographic plate separation, several data must be obtained, otherwise it is difficult to produce a high quality plate. These kinds of data are: the number of ink repelling roller lines, the screen angle of each color version, the amount of dot expansion, and the deformation compensation rate of printing plates.
1. Strictly control the maximum and minimum density values ​​of the original (negative film). When making a photosensitive resin plate, firstly, prepare a negative plate for printing. With high-quality matte film, the original maximum density D ≥ 4.0, the minimum density D ≤ 0.05.
2. The relationship between the number of screen lines and the number of lines of the screen roller The flex screen printing screen is generally thicker than the screen cable used in lithography. The number of screen lines should be controlled within the scope of flexo printing. The number of screens for the flexo version also depends on the dot gain and the media reproduction performance.
At present, the number of screens commonly used for printing corrugated boxes using flexographic printing is 40-80 lines/inch.
The number of screens for flexographic printing and the number of lines of the anilox roller have a matching relationship. The general rule is:
Number of screens = 1/4 screen roller number. That is, the number of anilox roller lines is four times the number of screens of printed products, and it can also be higher.
3, the determination of the screen angle of various colors (as shown in Table 1)
Table four commonly used four-color version of cable angle flexo printing
Offset printing commonly used four-color cable angle Flexo printing commonly used 232 four-color version of the cable angle of the first group: Offset -7.5 ° Group 2: The first group of values ​​+15 ° C version (C) 15 ° 7.5 ° 22.5 ° black version (K ) 45° 37.5° 52.5° Magenta Edition (M) 75° 67.5° 82.5° Yellow Edition (Y) 90° 82.5° 7.5°
Because the flexographic printing adopts the ink supply system of the ink roller short ink path ink supply system, in order to avoid the generation of moiré with the angle of the anilox roller network, the angles of the colored screen lines are adjusted. The following two sets of screen angles can be used. Group 1: Should meet the formula Offset - 7.5° = Flexo Printing Group 2: Should meet the formula Group 1 value + 15° = Group 2 value 4 Dot gain control Flex plate due to soft, flexible, plus Pressure is applied again during the printing process. Although light pressure printing is used in flexographic printing, it still leads to enlargement of printed image dots and elongation of images. When printing with low-viscosity water-based inks, as long as the pressure between the surface of the printing plate and the surface of the substrate is slightly higher, the ink on the printing plate will spread to the surrounding, making the dots form an intermediate ink layer with a thin ink layer. , Similar to hollow ideas, this is different from offset printing dot change.
In the high-light part, small dots (1%) have been lost in the high-light part and become a net-free part, 2% of the outlets have expanded to about 12%, and the 50% or less high-school adjustment area has been particularly serious. Close to midtones outlets. When the middle tone is about 40%, it will be deformed to the angle, resulting in a sudden change in the dot, which is extremely unstable. In dark tone, the flexo dot will become ground state at about 90%. In the process of flexo printing, dot gain is severe, and the tone reproduction range is 10% to 90%. Special treatment is required for highlights and dark notes. For the intermediate adjustment area, the network points in the compensation printing process have been severely expanded by narrowing the area ratio of the film Internet sites. For high-light areas, flat screens should be used as much as possible. For dark-tone areas, spot-color printing should be used.
5. Calculation of printing plate deformation compensation There are 2.84mm, 3.94mm and 7.00mm in flexographic printing for corrugated box printing. The commonly used is 3.94mm. The deformation rate and printing effect of plates of different thicknesses are different.
There are two parameters affecting the size of the flexible photosensitive resin plate layout. One is the thickness of the plate (especially the thickness of the photopolymer above the interface), and the other is the circumference of the plate cylinder. This perimeter is fixed with the plate. It is related to the outer diameter of the material layer up to the plate cylinder.
In order to compensate for the distortion of the image, the size of the corresponding graphic text in the circumferential direction must be reduced. The amount of decrease is calculated by the following formula:
Decreased percentage = K/R × 100%
Where R is the final copy length of the print.
Usually, the R value is given with the product. If no R value is given, the following formula can be used:
R=2Ï€(r+b+c)
In the formula, r—the radius of the plate cylinder b—the thickness of the adhesive liner c—the thickness of the printing plate The K value in the preceding formula is a constant and can be obtained by the following experiment.
Make a precise scale of the Yin picture, use it to expose a photosensitive flexo plate, apply the plate to the roller with standard loading process, ensure that the plate is installed correctly, and adjust the tension accurately before starting printing. Finally measure as accurately as possible a standard distance between the negative picture and the print, the same measurement is performed several times, and the average value is respectively calculated, and the average value obtained by measuring the negative picture is divided by the average value obtained by measuring the print product. This is the K value.
Second, the choice of plate material Corrugated carton flexo, both solid flexible plate material can also be used liquid flexo. However, the solid version is widely used because it has the advantages of good ink transmission performance, large latitude, high reproduction accuracy, good stability, no wear deformation, long service life, and convenient processing. Corrugated box printing solid flexible version 2.84mm, 3.94mm and 7.00mm several kinds, hardness HS25-50, commonly used 3.94mm. The thin version should be used as much as possible, because the thin version can not only perform flat solid printing, but also has a small dot enlargement, high overlay accuracy, stable print quality, and finer image layers compared with the thicker version. DuPont Selva corrugated carton printing thin version has the following: 3.94mm TDR (for B, C, BC and A type below 150g/m2); 2.84 - 3.18mm TDR (for above 170g / B2 type B corrugated or E type, EB type corrugated below 140g/m2); TDR 1.70-2.84mm (used for E corrugated). Thin plates must be used together with the pads to achieve good results.
When the quality of corrugated paper is poor, a thicker plate with a lower hardness should be used. When the quality of corrugated paper is good and small prints and screen printing are required, a thin plate with a hardness of HS35-45 should be selected.
Third, the use of water-based ink Water-based ink in the corrugated carton flexo printing has been widely used. The ink should be alkaline and have a pH of 8.5-9. However, if the alkalinity is too high, the paste will tend to occur.
The choice of water-based ink should pay attention to ink viscosity, concentration, fineness, dryness, abrasion resistance, water resistance and stability.
1. Viscosity The ink viscosity should be selected or adjusted according to the number of screen lines of the anilox roller, the fineness of the printing plate graphic, and the ink-absorbing performance of the paper product. When the number of reticulation lines is high, the printed text is small, or the ink absorption performance of the paper is poor, the ink with the lower viscosity should be used, and the ink with the higher viscosity should be used instead. Because the ink is water-soluble, it is more convenient to adjust the viscosity in use. Just add a little water and stir it evenly to make the required viscosity.
2, the density and fineness of the ink concentration that is the color concentration, only the color concentration is sufficient, can print a colorful pattern, and the fineness refers to the delicate degree of ink particles, for the high number of anilox roller should be Choose fine ink with good density and high concentration, otherwise it will block the mesh due to thick pigment particles, which will affect the effect of ink transfer and make the color of the printed product lighter. The printing effect is not good.
3. Adhesion, abrasion resistance, water resistance and dryness The choice of these properties should be based on the use of the corrugated box. When used in the packaging of frozen products, aquatic products or canned beverages, inks with strong adhesion, quick-drying, and water-resistant deinking should be selected.
In short, the variety, performance, and price of ink vary greatly. Although low-grade inks are inexpensive, they are often susceptible to blistering, stickiness, and affect the quality of printed products. They also contain more impurities, which can easily clog the mesh, not only affecting the amount of ink transferred but also accelerating. Anilox roller wear. Therefore, when selecting the ink, care should be taken not to lose it.
Fourth, white coated corrugated paper White coated corrugated paper, also known as white surface coated kraft paper, domestic carton factory has used this corrugated paper printing carton, obtained a good print quality and clear printing results. Compared with the non-coated paper, the coated paper has a higher printing ink density than the non-coated paper by 63% and a gloss of 350%. At present, the packaging boxes of branded beer, beverages, food, household appliances, and computers are all made of white-leaf kraft paperboard. For example, the boxes of HP printers were previously printed with offset printing paper and then covered with bottom paper. Now it is large. Most have adopted the method of printing directly on white-faced kraft paper with a flexo press. However, when printing with ordinary flexo ink on coated white kraft paper, ink inconsistency, slow drying speed, and low production efficiency often occur. Therefore, it is recommended to use a new generation of ink or a flexo printing machine with a drying device to print. It should be printed on a small area of ​​graphics, and then printed a large area of ​​graphics, printing color sequence can be black, blue, magenta, yellow color sequence for multi-color printing.
Fifth, the correct choice of lining plate Usually corrugated base material thickness is not uniform, the pressure is too light, may make the ink transfer uneven, easy to cause the printed Parts of the site white, or corrugated plate; and the pressure is too heavy, it will cause the substrate deformation, The expansion of outlets has affected fine lines and dot overprinting, and has also caused severe damage to the strength of corrugated cartons.
In traditional flexo printing, a 7mm thick printing plate is used. When the printing plate is pressed, the pressure of the raised graphic part is strong, and it is first compressed. The dot and text lines are easily deformed, the dot enlargement is intensified, and the plate making cost is also high.
If a 4mm thin plate with 3mm R/bak air cushion lining is used instead of a 7mm plate, the air cushion lining will have a hardness of 20-25 Shore, which is lower than the plate hardness (the current commonly used plate hardness is Shore 35). When the printing pressure is compressed, the air cushion lining is compressed first, so the printing plate is deformed little, the printing quality is greatly improved, and the plate making cost can also be reduced. The combination of this thin process and R/bak air-cushion lining technology is more suitable for printing fine images and is gradually replacing traditional thick ones.
Yong Kang Jun Ran Sports Factory , http://www.junran-sports.com