Causes of differences in Offset Printing color
Current packaging prints overwhelmingly still take the form of a Offset Printing.Because the packaging printed products that have been glued, the patterns are hierarchical, highly textured, and abundantly colored.Under the equipment conditions of modern printing machines, multicolor primary molding is highly efficient.So it is quite favored by customers.However printed prints also have lethal weaknesses.Color differences are easily produced by relatively other printing modalities.
There are many color difference factors that lead to a print during the printing process, and the specificities can be explored analytically from the following aspects.
1、 Materials for signments:
1. Effect of paper on color
A. The whiteness of the paper:
Papers with different degrees of whiteness have different effects on the color visualization of the printed ink layers.The effect is particularly pronounced for light inks with a light intensity above 70.This is because the different whiteness equals the addition of different degrees of black or gray matter to the ink, so in printing although the amount of ink, the color phase does not change, the effect of the actual color is to cause color difference with the different whiteness of the paper.So when we usually print, we must pay attention to the white matter of the printed paper, testing the white matter of the paper.When opening sheets, the same lot number date paper must be used.Because gram weights, specifications, sizes, although identical, the batch dates produced differ, there will be some variability in the whiteness of the paper and, finally, color differences in the prints.So we have to use the same whiteness of paper as printed paper for the same product.
B. Paper absorbability:
Inks of the same color phase, under the same conditions, printed onto absorbent paper will have different printing gloss and color phase.Because the structure of paper is composed of plant fibers to form bumps and pores, it needs to coat different thickness coatings on the paper surface, so as to obtain good uniformity and smoothness on the paper surface.Then, the nature, thickness, and uniformity of the coating materials all determine the absorption capacity of the paper for the ink.The difference in the absorption of ink on the paper will cause the difference in color of the printed grinding layer, so we usually have to choose the difference between non coating paper and coating paper according to the customer's requirements of the product, and the absorptive state of the paper surface, and the amount of ink is strictly controlled.For special papers in packaging Offset Printing, such as aluminum foil paper (commonly known as gold, silver card paper) and so on, tests for paper dynes must also be performed to detect the absorption attachment capacity of the paper to the ink.
C. Smoothness and smoothness of paper:
The smoothness of the print depends on the smoothness versus smoothness of the paper.Latex color printing is the reflection of light onto the retina of the human eye as light rays incident onto the surface of the paper, where color is seen through photosynthesis accepted by the photoreceptor cells.If the smoothness and smoothness of the paper are high, the color we observe is essentially that reflected through the ink layer, with high saturation of the primary color light.If the surface of the paper is rough and glossy, it will have a diffuse reflection, which will reduce the saturation of the primary chromatic light and make the perception of color in our eye observation prints lighter.The same amount of ink was measured densitometrically.Paper with high smoothness and high density value.Paper with low smoothness and low density value.
2. Effect of paper surface treatment on color
Packaging printing in addition to printing, there are many post printing processing processes.A considerable part of this is the surface treatment of the paper.Composite films (light film, sub film), light loading, piezoelectric, over oil, print, and so on are included.The prints undergo these surface treatments with varying degrees of color hue change.Some of these changes are physical, some chemical.The physical changes mainly reflect the increased specular reflection on the surface of the products, which has a certain effect on the color density.For example, there will be an increase in color density with a refractoriness film, oil on UV light, compact light, and so on.The color density of the print decreases after submembrane, upper sub oil.The chemical changes are mainly from the compound film glue, shiny oil, UV oil, and so on.These materials contain a variety of solvents, all of which chemically react to cause color changes in the color of the printed ink layer.Therefore, as a print for packaging of the ink, the physical changes and chemical changes of the post printing processing must be taken into account if there is a post printing processing process at the time of printing, so that the density values of the ink layers and the L * a * b * at the time of printing can be determined.
2、 Printing process operation
1. Effect of density value of dry regression
The ink is still wet just after printing the products, when the density values measured are different from those obtained after drying the prints.There are high values of density at wet and low values at dry, which is the phenomenon of receding density values.This is because the ink layer that is just printed out has some leveling off.The specular reflection that appears as a surface is dominated and appears brightly colored and glossy.When the ink layers are dried, there is a diffuse reflection of the surface and the natural gloss appears dull and dull than when just printed out.In order to guarantee the lowest possible reduction of colour difference between batches when printed in plain format, we have taken to control by testing at the same wet density.I.e., the wet density value and the L * a * b * value were measured immediately after the signed consent of the customer in the wet state.When the product is printed later (wet state), it is controlled for color by the first measured density value and the L * a * b * value.Of course if it is used with a densitometer with a polarizing mirror device, it is able to eliminate the light produced by the inked surface due to specular reflection. The relatively close wet to dry density it measures can also be helpful in the control of chromatic aberration reduction.
2. Effect of wetting liquid size
The gelation is printed using the principle of oil-water phase exclusion.This water refers to the wetting liquid.Its size directly affects the color of the ink layer.The water macro ink layer is pale and the water small ink layer is dark.The greater the water emulsification of the ink is more severe.The emulsified ink is lusterless and light in color.The printing of emulsifying ink not only has serious color differences, but also produces sticky, non dry, blotting fast, and other quality problems.So in printing, operation must be strictly controlled the wetting liquid size of the plate format, and the work of water ink balance should be done.
3. Effect of printing speed
Fast and slow speed in printing will affect the darker color of the print.This is because the fast and slow speed of the body speed will affect the equilibrium state of the ink.The water content on the express car plates and that on the slow car plates varied.Quick car print edition water parts volatilize little, slow car print parts volatilize large.Because the size change of water parts causes imbalance of water ink, color change of the ink must be caused.So usually you have to pay attention to a constant speed while printing and definitely can't drive a express car a quick car a slow car a little later so that printed prints will necessarily produce a colour difference.At the same time, one must also adjust to one of the most stable speeds depending on the machine's circumstances.For example, some instructions for importing the printer state the maximum speed of 16000 / hour, some instructions for domestic machines state the maximum speed of 8000 / hour, and none of these speeds are stable.Generally speaking, the stable speed of the importing machine is 10000-12000 Zhang / hour, and the domestic machine 5000-6000 Zhang / hour, above such a speed is more able to control the balance of water ink and reduce color difference.
4. Effect of printing pressure
Printing pressure is one of the conditions for the transfer of ink, because the plate surface, oak cloth and so on, cannot be absolutely flat, the surface of the paper is also inevitable to have subtle asperity or thick and thin inhomogeneities.If the printing pressure is insufficient or uneven, the ink layer is prone to the uneven concentration, so a "" triple flatting "" is required in the process, that is, it requires the plate surface, oak and sequalae, and the cladding surface all achieve a relatively flat condition, so that a thinner ink layer can achieve a more uniform ink on the print by the action of balanced printing pressure.When there is insufficient or uneven printing pressure and there is bad contact between the liner and the plate, the inking vision of the plate needs to be satisfied by increasing the amount of inking.But this will not only increase the consumption of ink, but also easily produce the printing chromatic aberration and the print stick phenomenon, so it is good to 'triple flat' (ink roller flat, water roller flat, roller flat) adopt uniform and constant printing pressure to prevent the chromatic aberration caused by inappropriate pressure.
3、 Bad shadowing of plates
Plates are one of the copyediting quality bases.During the printing process. 1 only the plate is reductive, does not deform, and does not wear and tear to better ensure the inking stability of the plate.If there is a loss, enlargement, or paste of outlets or lines on the plate, and so on, it will necessarily bias the print's ink.So in the usual work, first of all, the quality of sunburn should be guaranteed, and the quality of sunburn will be examined by measuring and measuring strips in sunburn.In printing, we should pay attention to the controlled pressure of the well printed, ink roller, and water roller and the balance of the ink, which is beneficial to prevent the generation of color difference.
4、 Effect of ink roller imperfection
Gelation printing process, whether the print can achieve uniform ink color, and the quality of the glue roll have a significant role.Generally, there are three kinds of roller, blotting roller and blotting roller, which have a great influence on the light of ink concentration.How elastic, viscous, roundness, hardness, and surface finish the ink roller largely determines the ink color quality.In case of poor elasticity, lack of tack and eccentric roll body, colloidal over stiffening and colloidal aging, the printing process can easily cause uneven or unstable inking, making the preparation of the impression printing ink inconsistent.So when printing, the quality of the glue roll should be guaranteed, so the ink coated onto the plate at every printing cycle can be basically consistent.So in the usual work, we should wash the glue roll, so that the paper dust and impurities retained on the glue roll are removed, and the uniform transmission of the ink is guaranteed.At the same time, we should also adjust the contact pressure between the fine ink roller and between the ink roller and the plate, otherwise it will also cause uneven ink transfer and produce severe color difference.
5、 Influence of light source characteristics
Imprinted prints must have a light source when viewing colors and cannot see colors without light, but if the light sources have different characteristics, the differences in colors can be significant.In general we require that color vision be carried out under conditions of natural light (that is, standard light sources).If color is visualized with a common electric bulb as the light source, the color will appear yellow, making accurate color discrimination difficult, and finally the print will produce severe skewing.The intensity of additional light and the angle of illumination can also have an effect on color discrimination.At the same light source, the intensity of the light is reflected on the irradiated specimen, mainly determined by the distance of the specimen from the light source.Strong for close, weak for far.So we usually pay attention to two points in our work, whether during the day or night: first, a standard light source (wavelength around 400-700) is adopted, and second, we pay attention to the fact that the light source has a weak intensity and the angle of illumination is equal.For products with high color requirements, the ink sample of the impression is placed on top of the standard sample for contrast, so that the discrimination of color is relatively quasi due to the small viewing angle difference.So the process practice tells us that the characteristics of the light source and the differences in the viewing angle will directly affect the accurate discrimination of colors.
6、 Impact of ink deployment
Packaging gel prints apart from four-color inks, some are specially colored.Inking is generally required to be deployed by the printing plant itself.Then we have to have a record of the proportion of the recipe that is inked to each print, with data control.Record the name, amount of ink used.If these are not done well, when the monograph is reprinted a second time, a colour difference results.Of course if the ink volume is very large, you can also ask the ink plant to help with the customization.For the ink in the factory of the ink plant, we must also do a good determination, or we will cause color difference and color deviation because of the quality problem of the ink itself.Generally, we wish to test the three indicators of good color color color for the ink from the factory, namely, the value of color deviation, gray scale value, and color efficiency value.If the requirement of the index value is not reached, the ink is not used.
7、 Adverse device effects
"" the jobs want to improve the things, bizine its organ "".Maintaining good usage performance of the machine is a reliable guarantee for improving the quality of printing ink.If the machine is unstable, for example, with inconsistent pressure, the infusion paper is unstable, and empty, multiple sheets are frequent in printing.Is bound to cause color difference on printing.So usually we want to do good maintenance maintenance of the equipment, pay more attention to lubrication work in the parts that the machine must do, prevent the aggravation of wear, and keep the equipment in a stable state.As a printer, we should do "" three good "", "" four society "", and the horizontal detection work of equipment.