To keep the color consistency is the basic requirement of packaging printing. Therefore, packaging and printing enterprises adopt different control methods. However, with the continuous innovation of detection technology, the comprehensive judgment mode of "human visual sensory comparison + color difference value judgment + spectral reflectance judgment" has become a consensus, that is, it is required that the printing sample and the standard sample "face color" are the same, and "same color and spectrum".
What is homochromatic spectrum?
The spectral reflectance curve refers to the ratio of the luminous flux reflected by the object to the luminous flux incident on the object, that is, the relation curve between the optical reflectance ratio and the wavelength. It is an inherent characteristic of the material itself, which can be called the "fingerprint" of the material. The spectral data of the curve can be regarded as the "DNA" of the material.
If two colors have the same tristimulus value and the same spectral reflectance curve, they are called homochromatic and homospectral colors, and the color phase of homochromatic and homospectral colors is consistent under different lighting sources and different field of view; if two colors have the same tristimulus value but different spectral reflectance, they are called homochromatic and heterospectral colors The same color and different spectrum may present different hues under different lighting sources and different field of view.
In actual production, there is no absolute homochromatic color. It is required that the spectral reflectance curve of the printed sample is consistent with the standard sample, and the color difference value Δ e * AB is controlled within the specified range of the standard.
Common problems in printing color control
1. Color difference in standard sample
There are many influencing factors in the process of comparison between customer's signature sample and sample sheet, in addition, there is color difference in the standard sample issued.
2. There is difference between test equipment
There are many brands and models of color detection equipment used by customers and printing enterprises, such as X-Rite ci7x00 / ci6x / sp6x in the United States, Datacolor dcx00 in Germany, and Minolta cm-2x00 / cm-3x00 in Japan. A few printing enterprises use domestic detection equipment, which has different degree of platform difference, and the standard establishment and color measurement method are different.
3. There are many factors of color change in the printing process
Before batch production, the color matching process, the change of ink viscosity, the color difference between paper and paper, the difference between printing batch and printing plate, and the difference between printing plate and printing plate will cause the hue change of printing process exceed the standard of color difference.
4. Limitations of routine sampling inspection
In the printing process, the number of sampling inspection is limited, which only represents the inspection individual and can not truly reflect the printing color of batch products. In addition, the feedback of quality information in the sampling process is not timely, and some unqualified products have been produced when the sampling inspection is unqualified, which can only be cleaned through selection, "postmortem examination after death".
5. No spectral reflectance control standard
According to the color standard set by the printing enterprise, the traditional testing equipment can judge the color difference value Δ e * AB, and display and derive the spectral reflectance curve and data in the testing software, but there is no judgment standard and basis.
Control measures of printing color phase "same color and same spectrum"
1. Construction process
According to the quality control requirements of the target customers of the printing enterprise, as well as the objective reality of the software and hardware configuration of the enterprise, establish a color control process suitable for the enterprise itself, and effectively monitor the whole process of raw material warehousing, pre production preparation, printing production process, finished product inspection, etc. see Table 1 for the key points of control.
2. Construction standard
No matter for new product proofing or production with samples, standard samples shall be printed with the same color, and the unified color measuring position and initial color standard shall be determined with customers. After confirming the standard sample with the customer, the warehousing inspection standard, production process technology standard and delivery inspection standard for the key raw materials such as paper and ink of the product shall be established.
According to the requirements of the same color and spectrum management and control, the requirements for the special color printing ink, paper color and spectral reflectance shall be specified in the warehousing inspection standard of raw materials, and the first pass shall be strictly controlled; the production process shall be in accordance with the requirements of the technological standards, the formula composition of the special color printing ink, the added model, proportion and limit value of the color mixing ink, and the requirements for the viscosity standard of the printing ink shall be specified, so as to avoid the problems caused by the color materials The difference of spectral composition or the value of chromatic difference is too large, which results in homochromatic difference. In particular, the internal control quality index of the enterprise should be properly stricter than the customer standard to ensure the quality of delivery and inspection.
3. Purchase of equipment
In order to accurately detect the color difference value and spectral reflectance data of printing products, meet the multiple control needs of portable use in the production process, regular spot check and real-time monitoring of the color phase of batch products, enterprises should reasonably configure the printing online and offline color phase detection equipment according to their own reality.
According to the management and control requirements, our company has purchased a number of sets of testing equipment, such as Ashley ci7800 desktop spectrophotometer, Ashley ci64 portable spectrophotometer, single sheet multi spectral color measurement system, online multi spectral color measurement system for map printing, etc.
In particular, the single multispectral color measurement system can output the spectral reflectance data of any area of the whole breadth of the product, and carry out spectral comparison and judgment; the printing online multispectral color measurement system can carry out real-time detection of the color phase of the printed product and feed back the data. When the color difference reaches the early warning value, it will send out an audible and visual alarm warning to ensure that the target printing color phase and the standard sample are the same spectrum.
4. Detail control
(1) Ink viscosity control. As the key link of color control, ink and solvent should be added in the printing process in accordance with the principle of "small amount, multiple times and measurement" to ensure the color content and color concentration in the ink. In addition, the frequency of viscosity measurement shall be strictly specified to achieve timing measurement (such as once every 20 minutes). If the printing equipment has been equipped with automatic viscosity control system, its detection accuracy shall be rechecked and corrected to ensure the accuracy and stability of viscosity measurement.
(2) According to the data of plate printing times, the dynamic color standard is established. Gravure printing follows the principle of "light ink deep printing, deep ink shallow printing". In order to restore the same color effect, different depth of gravure mesh should be matched with different ink color. In the actual production process, ink color data or samples should be formed according to different intaglio mesh depth states (normal intaglio wear process). Before batch production, the ink should be pre adjusted by comparing the dynamic color standard data or printing samples, so as to reduce the number of color matching before printing and the unqualified products caused by color matching process and improve the production efficiency.
(3) Determination of color sequence of special paper printing. Before special paper printing production, paper shall be sorted according to brightness value or color shade of base paper to confirm printing color sequence. Generally, printing is carried out according to the principle of "first dark and then bright, first deep and then light", and printing ink is first light and then deep, which is convenient for ink color matching in the printing process and reduces the amount and times of color matching.
(4) Test equipment management and data analysis. As the key quality index of printing process, the color difference detection equipment shall be periodically verified and corrected (every year / half a year); during daily use, self inspection and correction requirements shall be established, self calibration (standard plate correction) shall be carried out every 8 hours or 12 hours, and the detection equipment shall be kept in normal use by measuring standard samples and other forms.
In addition, the printing enterprise shall make a special comparison plan according to various quality indexes and characteristics of testing equipment, regularly carry out comparison tests on its own series of brand testing equipment and customer testing equipment, determine the difference between each testing equipment based on the testing standards established by the customer and the actual testing data, and effectively modify and process the testing equipment of the enterprise itself The spectral reflectance data of Δ L *, Δ a *, Δ b * and different wavebands in the chromatic aberration value can truly reflect the detection data and effectively guide the production.
To sum up, this paper is dedicated by Shandong printing packaging, printing color control is the key index of printing enterprise quality control, how to form a set of systematic control measures based on the actual situation of the enterprise is very important. With the continuous innovation and development of industry technology, advanced management and control technology, testing equipment, testing methods emerge in endlessly, let's wait and see,.