Print quality refers to the comprehensive effect of various appearance characteristics of prints. From the perspective of reproduction technology, the quality of printing should be based on the "faithful reproduction of the original". Whether in the traditional printing process or in the digital process, the printed matter must be faithfully reproduced in the original.
Print Quality Control Elements
There are 4 control elements for printed matter: color, gradation, sharpness, consistency.
Color is the basis of product quality and directly determines the quality of the product. Color control or management has always been a key link in the research and analysis of printing professionals.
That is, the tone refers to the step change in the shades of colors that can be recognized by the image. It is the basis for accurate color reproduction.
Sharpness refers to the sharpness of image details, including three aspects, the sharpness of the image's subtle levels, the sharpness of the edges of the image outline, and the sharpness of image details.
Consistency is uniformity, which includes 2 aspects. On the one hand, it refers to the consistency of the ink quantity in different parts of the same batch of printed matter, that is, different ink areas. Generally, the consistency of the vertical and horizontal solid density of the printed matter is used to measure, which reflects the stability of different parts of the printed matter printed at the same time. On the other hand, it refers to the consistency of the density of different batches of printed matter in the same part, which reflects the stability of the printing machine.
As for the printed matter, as long as the four aspects are controlled well, that is, the color, gradation, sharpness, and consistency of the printed matter can be well controlled, a high-quality printed matter can be obtained.
Print quality control methods
One of the methods of print quality control is to use scientific signal strips, test strips, control strips, ladders, and test equipment and charts to perform scientific quantitative control on print quality (including print quality and printing engineering quality). At present, there are many types of signal strips and test strips used in various countries, such as the GATF system in the United States, the Brunal system in Switzerland, the Vogel system in Germany, and the Gelinda system. In China, the signals and test strips of Meiwei GATF and Swiss Brunal are mostly used.
Testing tools commonly used in printing include signal strips, test strips, control strips, and ladder scales. The signal bar is mainly used for visual evaluation, and the function is relatively single, which can only express the quality information of the appearance of the printed matter. Such as the sun ladder, GATF code signal bar, color signal bar, etc. The test strip is a multi-functional marking element mainly based on the density meter detection and evaluation. The test strip is a combination of visual identification and density meter test and numerical calculation with the help of charts and curves. The control bar is a multifunctional control tool that combines the visual evaluation and test evaluation of the signal bar and the test bar. Such as Brunal's third-generation control bar. Ladders with equal density or dots are used to control the quality of printing and printing.
The printing process mentioned above requires the use of signal bars and control bars to record data in the operation of its main links to lay the foundation for standardized production. Quality inspection and control generally detect problems by proofing. In the traditional printing process, traditional proofing is used, that is, proofing by means of round flattening. In the digital process, digital proofing has gradually replaced the traditional proofing. Digital files to be printed can be directly output to the proofing machine to see if there are any problems with text, specifications, and images. As film is no longer used in the digital process, and even printing plates are no longer used, the evaluation and control of quality depends to a large extent on the digital proofing process. So, in today's digital and networked environment, what factors restrict its print quality in the digital process?
What are the difficulties and advantages of quality inspection and control?
Method for detecting and controlling printing quality in digital environment
1. Transfer of data flow in digital processes
The digital production process makes the data flow and control flow in the actual production less intuitive than the traditional process. What we see is only the input layout elements, the layout information expressed by the display equipment, and the final output color digital proofing proofs and CTP printing plate. The job information is in the form of data. Any small mistake or error will cause the failure of data transmission or abnormal output results. It will also exceed the control ability of the operator. It is difficult or impossible to check the problem. Therefore, these variables become The uncontrollable factors in the prepress operation. In order to ensure that data can flow correctly and smoothly, we must find a set of practical data flow control methods.
In the printing industry, there are two types of information flow in printing jobs, namely "graphic information flow" and "production control information flow". The graphic information flow solves the problem of "what to do", while the control information flow solves the problems of "how to do" and "what to do". In the digitization process, both types of information are digitized, and both can be recorded, processed, and transmitted by computer storage. The control nodes of the data flow in the digital process are file pre-check, print output, PDF normalized digital layout, and RIP output.
2. Color management in an open system
Color reproduction is an important measure of image reproduction quality. The color control in the traditional process is based on the definition and description of color attributes with density control as the core. The color recognition and correction are performed through a closed closed-loop system. Specific control methods include gray balance, color correction, optimal printing density control, halftone dot transfer and its control.
The digital printing process is an open system. Input, processing, and output equipment may come from different manufacturers. Various devices have different descriptions and expressions of colors. As the number of uses increases, the same device will also lose, and the expressive power of colors will also vary, thereby increasing the difficulty of color reproduction. And because of the emergence of the Internet and the need for information exchange, designed color images or color image files are no longer limited to local printing and printing; they often need to be viewed or copied off-site. The color information of a color image must not only be displayed on different devices, such as computer monitors (even the same device, the devices of different manufacturers have different abilities to display color), but also need to be transmitted between different media.
This requires a system whose function is to make color transfer results basically consistent across all media without aliasing. Also known as "what you see is what you get", this system is called a color management system. The International Color Organization ICC has developed a standard for describing the color performance of devices-ICCProfile. Using the ICCProfile profile, you can achieve cross-platform color communication, so that when other computers add color management functions, these files can be converted at will. Color management is done through a profile. A color management system is an application system that includes computer hardware, computer software, and color measurement equipment. The goal is to form an environment that allows various equipment and materials that support this environment to match each other in color information transmission and achieve undistorted transmission.
A new color control method with color space transformation as its core, color management, has enabled the technical process in the field of printing and communication to enter a new digital production stage, and has solved many problems of color reference and conversion in the production of print media. New technologies and equipment such as Digital proofing, CTP, digital printing, and digital production processes continue to emerge, and are moving towards the goal of “what you see is what you get”.
3. Data transmission and management
With the deepening of digitization in the digitization process, the amount of data increases geometrically.
Although there is a fast network, there is still a need to optimize the transmission and management of data on the network. To this end, the prepress field has formulated two related specifications: the OPI (Open Prepress Interface) specification and the DCS (Desktop Color Separation) specification. The OPI specification allows a low-resolution replacement image to be used for imposition, and is automatically replaced by the corresponding high-resolution image by the OPI server when color separation is output, thereby reducing the amount of file transmission on the network. The DCS specification is an extension of the EPS file format, which can manage the entire color separation process of the desktop publishing system, which is beneficial to shortening production time and reducing equipment requirements. During the printing process, data flows in all links. Therefore, it is extremely important to ensure that the data file is not missing during the transmission process and that the interpretation of the file is consistent across different platforms. Otherwise, files transmitted to the back end will get incorrect output results due to data loss. In order to carry out unified file control in printing integrated production, the CIP3 standard has been developed to CIP4, and its members have also developed a new file format, JDF.
4. Digital proofing
Proofing is divided into traditional proofing and digital proofing. In the traditional production process, the traditional proofing is basically used, and the round flattening proofing method is generally used. Of course, digital proofing is also used within a certain range, but there are problems in the quality and accuracy of proofing.
With the development of digital proofing, digital proofing is very useful in digital workflow. Digital proofing is a "no-film" and digital proofing technology produced with the development of network technology, CTP technology and pre-press digitalization. It is an important part and development direction of the digital process of modern printing technology. Digital proofing refers to the output of pages (printed layouts) produced by the color desktop publishing system without any analog processing, and is directly output by color printing equipment (inkjet, laser, electrostatic, etc.) digitally, which can meet the design, The quality and error checking of the prepress process, such as production, can also provide P and standards for subsequent printing, and can also be used as the user's signature proofing technology based on proofs. Digital proofing system generally consists of a color inkjet printer or a color laser printer. The color printer simulates the color of printing proofs, replacing the long process flow of traditional output film, printing, mechanical proofing, etc., using the original page (printing page). Data to get color proofs.
The development of a new generation of RIP has strongly promoted digital proofing. "RIP once, output many times" ensures that the files used for proofing and printing are consistent. In this process, the file is only interpreted once by RIP, and it can be output to printers and platesetters, eliminating many repetitive tasks, saving time, and ensuring the consistency of proofs. In the digital printing production process, digital proofing is essential. The digital proofing uses ink and paper, which is different from the printing ink and substrate, which determines that the color reproduction and printing effect of digital proofing will be different. This distinction is currently narrowing. Some new printers have 7 or even 8 colors, so that the proofing effect is closer to the actual printing effect.
Digital proofing is not only different from the traditional flat press round printing method, but also different from the round press round printing method of the press. It is based on the color range of the printed color and the same RIP data as the printed content, and uses the color gamut. Larger space color printing matches the printing mode with smaller color gamut space to reproduce printing colors, which can meet the requirements of various printing methods such as flat, concave, convex, flexible, and screen, and can produce proofs according to the actual printing conditions of the user. , Solve the problem that proofing cannot be matched with the subsequent actual printing process, which brings difficulties to printing.
Compared with traditional proofing, digital proofing has the following advantages: ① less investment in equipment, small footprint, and low environmental requirements. ② Save human resources, reduce costs, and rely less on operator experience. ③ Fast speed, stable quality, strong repeatability and low cost. ④ Wide adaptability, especially suitable for direct plate making, gravure and flexo printing, which can not be proofed or difficult to proof. It can not only simulate the effects of various printing methods, but also can be combined with digital devices of CTP and digital printing presses to truly realize an automated workflow.
5. Online detection technology
At present, many countries in the world have begun to use advanced online measurement and control printing technology . Conventional tests usually use printed measurement and control strips. The test results cannot be fed back online to the printing process to timely control the printing process and improve the printing quality. One-time printing process is controlled; while the advanced online measurement and control printing technology directly tests and controls at the same time of the test, the test can not only test and control the strip, but also directly test the specific printed matter. The basis of this development is the development of computer technology, calculation Speed has allowed full-picture analysis and testing in a very short time, such as Heidelberg's online quality control scanning system and online ink control system for printing ink.
On-line inspection usually uses CCD camera lenses to continuously print printed matter on the printing production line, and send the captured images to the computer for analysis and processing in time. The detection content can include: color detection, layout detection, handwriting detection, position registration detection Missing prints, offset printing, dirty printing, etc.