Color Measurement and Control of Gold and Silver Cardboard Printing

1 Introduction

At present, the variety of cigarettes on the domestic market is dazzling, and the replacement of cigarette packs is even more daunting. From paper flexible packaging to ordinary white card hard packs to glass cards, aluminum foil gold and silver cardboard, which has been popular in recent years. The use of new technologies has also emerged in an endless stream, creating a new fashion called “paper packs to see cigarette packs”.

Gold and silver cardboard is made of ordinary white cardboard or gold and silver ink. Due to the metallic hue of the gold and silver inks, the general surface presents a metallic luster, and the fine patterns of various colors are printed on the top, giving a high-grade and elegant feel. Generally, the price of a set of gold and silver cardboard cigarettes is equivalent to 3 to 4 times the price of ordinary whiteboard paper, which has a higher profit margin. Therefore, we have seen in the market that most cigarette makers use branded cigarettes in gold and silver cardboard packaging; and cigarette pack printing manufacturers are also willing to recommend gold and silver cardboard printing cigarette packs to cigarette factories.

However, due to the strong metallic luster on the surface of the gold and silver cardboard, the color during the printing process is not easily controlled. How to ensure that the ink color of the same batch of printed products and even different batches of printed products is the same, which is a basic requirement for customers and a difficult problem for the printer. If the apparent characteristics such as the color and texture of the product packaging appear to be different, consumers may think that the quality of the product is not stable and even suspect the authenticity of the product. Therefore, the general cigarette factory attaches great importance to the color of its product packaging, and the printed bag must have the same ink color. In fact, for printing, the color consistency can only be relative, to ensure that the same batch of print color consistency has been more difficult, and most of the cigarette package printing is a multi-batch printing and a large number of printing, how to control and ensure different batches The uniform color of the prints is even more difficult.

2 The optical characteristics of the printed surface of gold and paper cigarette packets

The incident light is irradiated on the surface of the print, mainly forming a diffuse reflection. In the field of color measurement we ideally refer to such an object as a "completely reflective diffuser." On the surface of the fully-reflecting diffuser, the light reflected in all directions is uniform. Therefore, we see that the colors of the objects are the same in different directions.

The reflection of the printed surface of the gold and silver cardboard and the gold and silver cardboard is flat and has a strong metallic luster. When the incident light is irradiated, a strong specular reflection occurs. Therefore, the color appearance of the object surface changes with the observation angle.

3 Optical geometry of color measuring instruments

In the print color measurement, in order to improve the measurement accuracy and uniform test methods, the CIE (International Commission on Illumination, an international organization for color and color measurement) officially recommended the illumination and detection geometry used by the four color measuring instruments in 1971. They are: 0/45°, 45°/0, 0/d, d/0. (In practical applications, considering the inclusion or exclusion of specular reflection effects, 8°/d and d/8° are used more often?).

We see that the basic principle of the 0/45° measurement geometry is that light rays are normally incident and are reflected at the object surface and received at 45°. (Depending on the reversibility of the optical path, we know that the measurement geometry of 45°/0 is similar in principle to this) Single-angle incidence, single-angle reception, principle of light reflection from the surface of the object We know that this type of measuring geometry is used for color measurement instruments It is more suitable for measuring the surface color of general paper prints. X-Rite's 900 Series instruments use this type of measurement principle, while the X-Rite 500 Series Density Meter uses 45/0° measurement geometry. The above two series of instruments can be well applied to the measurement and control of the color of general paper prints, and can achieve consistent and accurate results.

The basic principle of the d/8° integrating sphere measuring instrument is that the light rays are perpendicularly irradiated on the measuring object, and the reflected light is uniformly reflected in the integrating sphere (the inner surface of the integrating sphere is completely reflective and diffused, and the light can be reflected back and forth). Single angle reception. Equivalent to a uniform sphere glow, illuminate the surface of the measuring object and measure its spectral reflectance. A gloss-absorbing trap is generally installed inside the sphere at a position opposite to the receiving angle. When this gloss-absorbing trap is opened, any specular reflection can be reduced to a minimum value. This measurement is called "exclusion of specular reflection" (SPEX). The results are similar to those of the 0/45° instrument described above. When we measure the color appearance of the highlight surface and need to consider the gloss effect of the specular reflection, turn off the gloss absorption trap and take the “Contain Specular Reflection” measurement (SPIN). There is no doubt that the measurement and control of gold and silver cardboard cigarette packs should be measured in this manner.
4 Measuring characteristics of gold and silver cardboard cigarette packs with X-Rite SP series instruments

X-Rite's SP Series instruments use the d/8° measuring optical geometry principle, which is especially suitable for the color control of gold and silver cardboard and cigarette packets printed with gold and silver cardboard.

The design of packaging products such as cigarette packs generally uses spot color printing. We know that using traditional density measurements to control spot colors often fails to achieve ideal results, and there is no way to establish correlations with human eye observations. Therefore, spot color printing generally adopts a colorimetric method to control color difference to achieve color control. X-Rite SP series instrument spectrophotometer adopts the principle of splitting light, divides visible light from 400 to 700 nm into 31 measurement intervals, measures the spectral reflectance of the color of the object, and then obtains the color CIE Lab value and calculates directly on the instrument. Chromatic aberration.

X-Rite SP Series instruments provide professional-grade color difference control accuracy. Its point-of-sight measurement error can reach 0.05DEab color difference unit, and the difference between the instrument desk is 0.1DEcmc color difference unit. It has a built-in pulsed, gas-filled tungsten lamp to avoid visual inconsistencies caused by the human eye under different viewing conditions.

X-Rite SP series spectrophotometer provides Job, Project, Standard, Sample four levels of measurement management, one measurement can provide 8 standard light sources and two fields of view (2 ° and 10 °) CIE color space XYZ, Lab, Lch values ​​and color difference comparisons, and "color fingerprints" - spectral plots. The internal memory of the instrument can store 1024 standards and more than 2000 sample data. When you finish testing the quality of the workshop, you can go back to the office and download the data on the instrument to the computer through X-Rite's dedicated color quality control software. Back up, wait until the next reprint as a reference standard.

X-Rite SP series spectrophotometer is a Chinese-displayed instrument and the operation interface is a Chinese menu. No matter measurement or instrument calibration, it can be done easily.

5 Several Factors to Be Considered When Using SP Instruments to Control the Color Difference of Gold and Silver Cards

1) X-Rite SP series spectrophotometer provides 8 kinds of observation light sources and two kinds of visual field conditions. When doing color comparison, it should consider using uniform light source and visual field observation conditions to ensure the consistency and comparability of measurement results. .
2) Due to the high-gloss reflectance of gold and silver cardboard, the “Contains specular reflection” method (SPIN) measurement should be used when measuring gold and silver cardboard and its printed products with SP series instruments.
3) For some special printing inks with high hiding rate printed on gold and silver card paper, consider using the "Excluding Specular Reflection" (SPEX) method.
4) For the white paper cigarette packs or general paper prints, when measuring with the SP series instruments, measurements should be made in the "excluded specular reflection" (SPEX) method. The color data is comparable to the data measured by other instruments.
5) It is of great significance to choose a uniform color difference formula. According to my own experience, the choice of DEcmc color difference formula (LC=1.41) for measurement of gold and silver cardboard and its printed matter is in good agreement with the observation results of the human eye.
6) The final problem of color measurement is to solve the problem of color difference between printed products and standard color samples and how to maintain the color consistency of large-volume prints. Due to the unavoidable fluctuations in the printing production process, printing color fluctuations are unavoidable. How to stabilize the production process and determine the color tolerance is a practical problem that every printing company must face. According to my own experience, some printing clients will directly assign a color tolerance standard, and even the same sheet will be handed over to the printing factory. The products delivered by the printing factory must meet the tolerance range. However, at present, most printing customers do not have clear inspection standards. The printing companies themselves have not established internal color control standards according to their actual conditions, and finally caused quality disputes at the time of delivery. If a printing plant usually uses color measuring instruments to measure and draw color tolerance maps to determine its own internal color difference control level according to its own actual conditions, on the one hand, it can optimize internal management and improve product quality and production efficiency; on the other hand, it can be used in advance with customers. Conduct color quality communication to reduce disputes.

Source: World of Printing Liao Lizhen

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