With the continuous development of packaging and printing, folding color cartons are increasingly appearing in the market. Many product packaging boxes were changed from original plastic, wood, tinplate, etc. to folding colored paper packaging boxes. The packaging box products printed with paper materials can break through the limitations of the previous packaging and give full play to the advantages of good physical properties and printing applicability of paper. Through the physical properties of paper and the adaptability of printing, we can design exquisite patterns and shapes, which can be realized through various processing methods of printing factories. For color carton products, customers have high quality requirements, and there are also prescribed standards in our industry.
The quality of packaging offset printing products has qualitative and quantitative standards. One of the most important requirements is the color requirements of the printed product (such as density, color difference, dot expansion, small dot reproduction, etc.). The color of the printed matter should be bright and the hue is correct, so that the color reproduction of the printed matter is correctly reproduced, and finally meets the requirements of the customer. This requires that we must have a full understanding of the color reproduction conditions.
One. First we analyze from the material
1. Ink
Ink is the most direct material for color development, and its quality plays an important role in color reproduction. The characteristics of the three primary colors ink directly affect the vividness of the color of the printed matter. Different models and different manufacturers of three primary color inks have different color deviation, gray scale, and color efficiency. So the vividness of the ink is also different. According to the calculation formula of LTF Institute in the United States:
Grayscale = (L / H) * 100%
Color deviation = (ML) / (HL) * 100%
Color efficiency = (1- (M + L) / (2 * H)) * 100%
Example: ×× Ink
Color filter
Color
×× Yellow
×× Red
×× 兰
B
H 1.12
M 0.71
M 0.69
G
M 0.14
H 1.37
L 0.26
R
L 0.10
L 0.20
H 1.86
×× Yellow
×× Red
×× Orchid
Color cast
4%
44%
27%
Grayscale
9 %
15%
14%
Color efficiency
89%
67%
74%
Therefore, we generally check the color of the ink. For some prints with high color requirements, select the color of the ink, that is, use an ink with a high color.
When the printing plant prepares its own ink, try not to add too much auxiliary material to the original ink, which can easily cause the saturation of the ink to decrease, and the ink is also easily emulsified.
The thickness of the ink layer affects the color reproduction in actual printing. The thickness of the ink layer is thin, and the picture and text displayed are gray and dull, and the thickness of the ink layer is thick. Therefore, we should control the amount of ink and the density value of the ink according to the proofs.
Print density range (CY / T5-1999)
Color
Fine print solid density
General print solid density
Yellow (Y)
0.85 ～ 1.10
0.80 ～ 1.05
Magenta (M)
1.25 ～ 1.50
1.15 ～ 1.40
Green (C)
1.30 ～ 1.55
1.25 ～ 1.50
Black (BK)
1.40 ～ 1.70
1.20 ～ 1.50
Field density index value GB7705-87 (National Standard for Packaging Lithography)
Indicator name
unit
symbol
Index value
Fine products
General products
Homochromatic density deviation
Ds
≤0.050
≤0.070
Same color difference in the same batch
CIEL * a * b *
ΔE
L *> 50.00
L * ≤50.00
L *> 50.00
L * ≤50.00
≤5.00
≤4.00
≤6.00
≤5.00
Ink layer gloss *
%
Gs (60)
≥30.00
-
In addition to four colors, spot color is often used in packaging offset printing products. These spot colors are generally allocated by the printing house itself. Therefore, in the preparation of spot color inks, we must first understand the commonly used ink hue and color deviation. To control the color of the color swatch, it is necessary to select the primary color hue ink. In addition, according to the principle of the subtractive color method of the color material, the variety of the three primary colors ink is reduced as much as possible, and the spot color is produced with the least three primary colors to make the ink bright. In order to make the spot color color correct, the scraping paper used to prepare spot color ink must be the same as the printing paper. In addition, in order to maintain the consistency of the ink color, the density value, â–³ E and L * a * b * value measurement records .
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2. Substrate
A. Whiteness of paper
The substrate is the basis of the color development of the printed matter. If the whiteness of the substrate is high, the brightness of the ink color can be improved. On the contrary, the whiteness of the substrate is low, just like adding a small amount of black ink to the ink, thus showing a dark color. Especially printing light color ink is more obvious.
Therefore, when we use substrates, we must use the same brand, the same weight, the same production date batch number. Otherwise, even if the ink, printing conditions, etc. remain unchanged, the color of the printed matter will still change.
B. Absorbency of paper
The absorbency of the substrate is also important. The absorbency of the printing material will affect the saturation of the color. For the printing material with large absorption, the imprint cannot form a sufficiently plump ink layer on its surface. The excessive penetration of the binder into the substrate makes the blot dry, and the substrate paper with large pores will cause the pigment and binder to penetrate into the paper together, thus reducing the color saturation and affecting the vividness of the color.
C. Paper gloss and smoothness
The formation of color is reflected by light, and the color is only seen through human eyes. The gloss and smoothness of the paper will affect the reflection of light, thereby changing the color of the human eye.
3. Plate
The printing plate is one of the foundations of the quality of image and text copying. The good reproducibility of the printing plate can ensure the stable ink transfer of the printing plate. If the dots or lines on the printing plate are lost, enlarged, pasted, etc., the ink color of the printed matter will inevitably be biased. Therefore, in the usual work, we must first ensure the quality of printing.
The plate material used for water offset printing is basically the PS plate, which is based on the polymer photosensitive layer as the graphic basis. When printing, we must control the time of exposure, the temperature of development, the concentration of developer, etc. Because changes in these conditions will cause a profound change in the graphics of the plate. We can use the test strip to check the quality of the printing plate. [next]
two. Craftsmanship
1. Overprint rate
In modern packaging offset printing, four-color or four-color special printing is generally adopted. When the color sequence of the printing changes, it will cause a color change. This is mainly a question of overprint rate, which is the ratio of the amount of ink printed by the latter color to the previous color and the amount of ink printed by the latter color directly on the paper. The size of the overprint rate will affect the color saturation of the secondary color. The higher the overprint rate, the greater the saturation of the color, and the brighter the color.
Therefore, the arrangement of color sequence is very important for packaging offset printing. For general color sequence arrangement, the following should be considered: the viscosity of the ink, the transparency of the ink, the size of the printing area, the main color of the graphic, the accurate overprint, the nature of the substrate, and the equipment.
Dv （1 + 2) ï¼DV (1)
Overprint rate = × 100%
DV (2)
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2. outlets
The graphic color of offset printing, the gradation of layers, are reflected by the dots. However, printing is stressful and will inevitably lead to the expansion of the network. The excessive expansion of the dots causes a great change in color. Therefore, we must control the expansion rate of the outlets as much as possible. For high-definition products, we need to measure the expansion value of the printing equipment, and take into account the expansion of printing when pre-separating the plates. This is conducive to the stability of the graphic color of the printed matter.
Screen printing requirements GB7705-87 (National Standard for Packaging and Lithography): The 50% dot expansion value requires that fine products should not exceed 12%; general products should not exceed 18%.
CY / T5-1999 dot printing requirements: clear dots, accurate angles, and no ghosting. The increase range of 50% dots of fine prints is 10% -20%; the increase range of 50% dots of general prints is 10% -25%.
3. Bad effect of ink roller
Whether the printed matter can get a uniform ink color, the quality of the rubber roller has a very important role. There are three types of rubber rollers: inking roller, ink roller, and inking roller.
The elasticity, viscosity, center of circle, hardness and surface finish of the ink roller all determine the quality of the ink color of the printed product. Therefore, the ink roller should be maintained and cleaned, and the pressure between the ink roller and the pressure between the ink roller and the printing plate should be adjusted.
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4. Printing process operation
A. Effect of dry-out density value
Just after printing offset printing, the ink is in a wet state, its density value is high, and the density value of the ink will be reduced after drying. This is called the phenomenon of dry density.
B. The effect of the size of the wetting fluid
The wetting fluid is mainly water, its size will affect the color of the ink, water is large and the ink is light, and water is small and the ink is deep. In printing, we must control the water-ink balance, minimize the amount of water, reduce the emulsification of the ink, and control the PH value and conductivity of the wetting fluid.
C. The impact of printing speed
The speed of the vehicle speed will affect the balance of ink and water. The water content of the express printing plate evaporates quickly, and the water content of the slow printing plate evaporates slowly. As the change of moisture leads to the imbalance of ink, it will inevitably cause the change of ink. Therefore, the normal speed must be stable. You can't drive fast or slow at one time. At the same time, you must adjust the most stable speed according to the equipment.
D. Influence of printing pressure
Printing pressure is one of the conditions for ink transfer. If the printing pressure is insufficient or uneven, the printed ink layer will show unevenness. Therefore, it is necessary to do three levels and two levels in operation.
E. Influence of light source characteristics
There must be a light source to observe the color. Without light, there is no color. The light source we are talking about refers to the standard light source. The distance between the paper and the light source under the same light source will reflect the intensity of the light. The closer is stronger, the farther is weak. In addition, the wavelength of light is different, and color chromatic aberration will also occur. For prints with high color requirements, special emphasis is placed on the standard of the light source and the viewing distance. Therefore, the difference between the characteristics of the light source and the viewing angle will directly affect the accurate discrimination of color.
F. Bad effects of equipment
If a worker wants to do his best, he must first sharpen his weapon. The quality and accuracy of the machine is a reliable guarantee for the color of the printed product. The unstable machine will inevitably lead to the instability of the color, so the maintenance of the equipment is very important. Offset workers should do three good, four meetings, and equipment calibration work, maintenance work, pay special attention to the lubrication of key parts of the machine, to prevent increased wear.
5. The effect of paper surface treatment on the color after printing
In addition to printing, there are many post-press processing techniques for packaging prints. A considerable part of them is the surface treatment of paper. Including coating (light film, sub film), glazing, calendering, oiling, printing and so on. After these surface treatments, the printed matter will have different degrees of hue changes. Some of these changes are physical changes and some are chemical changes. The physical change is mainly reflected in the increased specular reflection on the product surface, which has a certain effect on the color density. For example, light coating, UV varnish, calendering, etc., the color density will increase. The color density of the printed product decreases after the sub-film coating and the matte oil coating. The chemical changes mainly come from laminating glue, varnish, UV oil and so on. These materials contain a variety of solvents, which will cause the color of the printing ink layer to undergo a chemical reaction and cause a color change.
Therefore, as the printed matter of packaging offset printing, if there is a post-press processing process during printing, the physical and chemical changes of the post-press processing must be taken into consideration to determine the density value and L * a * b * of the ink layer during printing.
In short, the color of packaging offset printing products should be correctly restored, which involves a wide range of areas. We must pay attention to materials, equipment, and technology. Special attention should be paid to the process operation. At the same time, the control of color should not rely on experience alone, but must use data and instruments for measurement and analysis. Realize data, standardization and standardization, so that the quality of our packaging offset printing products can be improved.
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