In the CTP workflow, digital proofing is considered the weakest. One of the main reasons is that the digital proofing method uses a continuous adjustment process, which cannot produce samples of semi-continuous adjustment dots. But without adopting halftone dot proofing, it is difficult to detect the moire faults that may occur in screen printing. This requires that the printing operator must learn to use the tone matching method to check the printing quality during the printing process to reduce printing faults, rather than using The traditional method of matching dot structure. Because the CTP process cannot adopt the method of proofing by film, the digital proofing method is an inevitable choice. Replacement of traditional proofing equipment is high-end inkjet and laser color printers and so on.
The reason for restricting the application of digital proofing technology is that the coloring material used for proofing differs from the printing ink coloring material, resulting in a difference between the proofing color and the printing color. Therefore, it is necessary to apply color management methods to compensate for differences caused by different color materials. The ultimate success of a CTP system is an important point in deciding what kind of filmless proofing method to use to obtain customer-acceptable proofs.
The main purpose of digital proofing is to provide reference proofs for printing and to provide customers with proofing basis. The core of digital proofing system is color management. Only when color management is done well, can digital proofs be guaranteed. So, how can we manage color management well? I propose the following steps here:
1. Make sure the printing process is stable
The effect of color management is far from good software and hardware, and there should be a stable printing process. The conditions such as printing pressure, dot increase, paper and ink should be stabilized first to achieve standardized operation, because the dot increase in the printing process, the printing color sequence and overprint rate, solid density, water and ink balance, printing contrast, etc. determine the color The level of reduction level. In order to achieve optimization and stability of these conditions, we have to formulate a general standard for printing operations, through strict control, so that all printers meet this standard, so as to achieve the stability of the printing process.
2. Select representative equipment and raw materials for ICC characteristic documents
Printing presses, paper and ink are important factors in printing elements, and they directly affect the printing color, tone and clarity. Inks of different manufacturers have different color rendering effects, and even inks of the same manufacturer are divided into high, medium, and low grades; different paper, different printing models, different degrees of use, different liners, etc., the printing effect will also be different. But if every combination of paper, ink and printing press is printed as a standard document sample, the characteristic curve of each combination of digital proofing is not only costly, but also prone to confusion during use. Therefore, we generally have to use the most commonly used paper in the factory and the commonly used printing inks as the basic materials for printing. Ask to say more.
3. According to the printing results, establish the printing dot increase value and the ink solid density
CTP can achieve the basic reduction of the dot without adjusting the curve. Under this condition, the CTP version is released, which includes standard test color blocks and various printing quality control strips. At the same time, the printing conditions should be controlled within the normal range, such as the environmental temperature and humidity are the ranges required by the equipment, and the content of the fountain solution, pH value, alcohol content, and conductivity of the fountain solution should be controlled within the normal range, printing pressure And blankets should be kept normal. During printing, observe the stars and gray balance color patches on the sample sheet to control the deformation of the printed dots and the density of the four-color ink in an appropriate range, maintain the uniformity of the sample density in the entire ink area, and mark clearly. After that, choose samples with different density values, select several for each type, measure the field density value of each one, and calculate the density value for each sample. Analyzing the above data, it can be concluded that when the four-color ink is printed, the printing density increases, and the value of the black plate increases accordingly. However, after the density value increases to a certain degree, the value of the black plate does not increase; when the density increases again, The value of the black version shows a downward trend. Therefore, the density value at which the value of the black plate reaches the highest point is the optimal printing density of this printing machine. Combined with the national printing standards, determine the printing contrast value and four-color printing density value suitable for the enterprise, and obtain the dot compensation curve output by CTP.
4. Print standard proofs
After determining the output dot compensation curve of CTP, reprint the standard proofs. When outputting the CTP version, the dot compensation curve should be loaded, and the printing conditions of the printing machine should be controlled to be consistent with the front side, and then the experienced master can find the best sample combination through judgment.
5. Accurate measurement
Use a spectrophotometer to measure the proofs. Because it is a manual measurement, the weight of the hand during measurement, the position and speed of the instrument may affect the measurement result. The color difference comparison function in the software can check the color difference of each measurement. If it deviates too much from the average value, it must be re-measured. Ensure the accuracy of the measured value.
6. Generate ICC Profile
Use color management software to generate printed ICC Profile files. After testing, according to the specific conditions to determine the various options suitable for the factory in the software. At the same time, choose professional digital proofing paper and use the color management function in the software to make the printer's linearization curve; then make the paper's linearization curve. Establish digital proofing ICC Profile. Set the density of the four-color ink so that the basic density value of the four-color digital proofing is consistent with the density value of the four-color ink printed by our factory.
7. Color deviation correction
Compare the color and tone reduction of digital proofs and printed matter, measure the color difference between printed proofs and digital proofs, use the software to select the color calibration and cycle correction function, and after multiple corrections, the color of the two proofs will be consistent.
After completing the above steps, it should also be noted that in the actual use process, when it is found that the color and gradation of the printed matter and the digital proof are inconsistent, it is necessary to observe whether several other products have the same problem. After the problem is identified, the digital Proofing data to adjust or redo ICC Profile file.
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