Two key points of automatic color control for commercial web offset presses
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Commercial web offset presses are often referred to simply as "commercial reels" or "commercial wheels." Different from sheetfed offset presses and other forms of rotary presses, the merchant wheel takes into account production speed, print quality and functionality, and ensures printing accuracy at high speeds. At the same time, it can complete a series of work such as double-sided printing, drying, folding, etc., and can expand additional functions such as connecting glue, connecting ridges, connecting cutting, etc., which can be widely used in large batches. Print production with short lead times, such as full-color magazines, promotional materials, direct mail, color newspapers, and yellow pages.
Market profile of the merchant ship
From the end of the 20th century to the beginning of the 21st century, China's commercial shipping market flourished. Many well-funded and strong book printing companies have purchased new importers. At that time, China's annual installed capacity of commercial vehicles was close to 10. After 2010, affected by the impact of electronic publishing and the overall shrinking of the book printing market, the new installed capacity of commercial ships fell sharply, but the domestic equipment holdings remained high, which also made the market increasingly fierce. In order for a printing company to gain a competitive advantage, it must start by reducing production losses, improving the stability of printing quality, and reducing the number of printing jobs for commercial wheels.
In addition to management methods, the application of automated auxiliary devices is an important way to help printing companies achieve the above requirements. The use of automatic color control system can not only greatly reduce the loss of the initial start-up of the equipment, but also maintain the stability of color and quality during the printing process. Loss, maintain print quality and reduce the number of prints. The systems associated with the automatic color control of the merchant wheel are mainly two parts of the CIP4 and the color control system of the printing press.
CIP4
If the color control system of the printing press is the "commander" of the printing color control, then CIP4 is the "pioneer" before the start of printing. Through CIP4, various information related to printing can be smoothly transmitted to various processes of the entire printing production process through JDF files, and information can be interconnected. In order to realize the color control of the merchant wheel through CIP4, the following requirements must be met.
First, the press and the prepress system must be connected to the same network. When the original is processed before printing, a JDF file is generated to store various types of data. Specifically, it includes: basic information, such as job name, customer name, number of jobs and delivery deadline, etc.; preset data of the printing machine, such as color separation preview image, ink preset data, ink fountain roller speed, water fountain roller speed, etc.; Binding and processing data, such as cutting size, polyline position, number of posts, etc.
Second, CTP platemaking must be used. The CTP process is the key to CIP4's integration of prepress and printing, enabling the instruction set of JDF files to be sent directly to the press. At the same time as the CTP prints the plates, the ink amount information of each ink zone of each color group will be written into the JDF file.
Finally, before printing, the merchant operator needs to enter CIP4 integrated on the merchant wheel to call the JDF file of the corresponding job. After the JDF file is called, the ink preset information, folder data, etc. contained in the file will be downloaded to the central control system of the merchant wheel, and then the central control system issues an instruction to pre-adjust the opening and closing angle and folding of the ink button of the ink fountain. The pre-adjustment position of each machine such as the page machine ensures the smooth production of the work piece.
When the dealership is up and running, the equipment operator can focus more on monitoring the status of the equipment and the quality of the prints.
Printing machine color control system
1. From open loop to closed loop
Most of the early commercial ships used the open-loop color control system, that is, the operator repeatedly took the printed sheets during the printing process, and adjusted the ink supply of the printing machine by measuring or visually comparing with the standard proofs to achieve stable printing color. the goal of. The main disadvantages are: slow response, long adjustment time, large printing loss, and mainly rely on the skill level of the operator.
In order to solve the above problems, the closed-loop color control system came into being, and its working process was mainly based on automatic measurement and control of the computer system, which greatly reduced the influence of human factors. The main principle is: during the printing process, the measuring device continuously scans the color control strip on the substrate, and after the background calculation and comparison with the ink preset standard, the instructions for the ink amount adjustment of each color group and each ink area are given, and the printing control is performed. The table sends an adjustment signal to the printing ink fountain to adjust or correct the thickness of the ink layer to achieve the purpose of automatically adjusting the printing color without interruption. This process theoretically does not require manual participation, and the adjustment speed is fast and accurate.
2. Two color control modes of closed-loop color control system
At present, there are not many manufacturers that can provide closed-loop color control systems for commercial ships in the world. The structure and operation principle of each manufacturer's closed-loop color control system are similar, mainly composed of speed measuring device, operating terminal, control PC and spectrophotometer scanning head. (As shown in the red box area in Figure 1). Wherein, the speed measuring device is mounted on the drum shaft of the printing unit for detecting the printing speed of the printing machine, so that the moving speed of the scanning head can match the printing speed; the spectrophotometer scanner is installed in the drying device and the folding mechanism. Between the time when the commercial wheel operates at 40,000 rpm, the spectrophotometer scans the head approximately every 20 seconds.
The closed-loop color control system used by our company provides the following two color control modes, which can be used for different printing standards.
(1) Field density control mode
The field density control mode is mainly suitable for jobs with clear printing parameters, such as four-color density or L*a*b*. In printing, the spectrophotometer scan head continuously scans the color control strip on the printed sheet, and measures and calculates the density of the solid and 75%, 50%, 25% dots, L*a*b*, ΔE, TVI and other parameters. And compared with the preset parameter requirements, automatically calculate and convert to the ink amount difference, and then send the adjustment information to the ink fountain control system of the printing machine, adjust the ink supply amount, and complete a cycle. Then the next measurement is made, and the adjustment is continuously corrected so that the difference between the measurement result and the printing parameter requirement is kept within a stable range.
The characteristics of the field density control mode are intuitive and clear. It provides various common printing parameters, which can help the operator to quickly judge the state of the equipment. When the printing machine itself has problems, the reference data can be given in the case that the automatic control system cannot be adjusted. Manually participate and solve directly.
(2) Gray balance control mode
The gray balance control mode is mainly applied to print jobs based on color drafts. Generally, there is no clear printing parameter requirement, but each printing standard is used as a control target. The system automatically calculates and adjusts the amount of ink in each unit by measuring the gray balance of the printed color control bar and the dot gain value of the middle adjustment, and controls the printing color by controlling the gray balance. Before use, the operator selects different printing standards built into the system. After the system is started, it will evaluate from two aspects of printing effect and hardware condition, and automatically adjust it continuously, so that the evaluation level of printing effect is continuously improved and stabilized, and the evaluation of hardware conditions (equipment status, printing materials, etc.) can be operated by the operator. Provide hardware tuning recommendations.
When the printing effect reaches the preset state or the customer approves the effect, you can choose to enter the “sample” mode, after which the operator's intervention is no longer needed. The system controls the spectrophotometer to scan the signal bar continuously, according to the “sample” standard. The data of each ink zone captured is subjected to color balance analysis, then fed back to the press control system, and the press is automatically adjusted accordingly.
The gray balance control mode is characterized by high hardware requirements and stable printing conditions. For example, paper and ink should conform to international standards, and parameters such as increased printing dot must also be within the standard range.
Generally speaking, the closed-loop color control system is used on the merchant wheel. After printing, 1200 sheets of paper can be printed to achieve the general printing color requirement. Under normal circumstances, the preset color effect can be achieved within 5000 sheets. Compared with the commercial wheel without the closed-loop color control system, the startup loss can be reduced by more than 30%, and the printing process is more stable without large fluctuations due to human negligence.
The printing speed of the commercial wheel is fast, the color control is difficult, and the technical requirements of the operator are very high. The closed-loop color control system combined with the use of CIP4 shortens the preparation time before printing, greatly reduces the workload of the operator, reduces the influence of subjective factors such as the operator's technical experience, and ensures the stability of the printed product quality. Sexuality reduces paper loss and the demand for printing, which enables the printing industry to help printing companies maintain strong competitiveness.