Quality Control in Digital Printing Process

Major printing production process

The DTP technology that emerged in the 1970s and 1980s brought great changes to the printing platemaking process. DTP started with a word processing system, which solved a part of the process before printing. By the early 1990s, the printing industry entered a more mature stage of color desktop publishing. During this period, desktop publishing has gradually developed into a set of processes. Process. CTP is an expanded form of digital output at the output. With the continuous decline in the prices of CTP plate materials and the continuous maturity of related technologies, by the mid-1990s, the CTP system has entered the stage of practical application. "Digital printing" is essentially an extension of digital printing before printing and printing. For the definition of digital printing, there is no unified statement. In a broad sense, so-called digital printing is a printing method in which data such as characters and images stored in a computer are digitally recorded on a certain type of media, and then the information is transferred from the medium to the substrate. At present, digital printing can be divided into two ways. First, digital printing that completely eliminates printing plates, uses electronic imaging or inkjet technology to directly image the media, such as the digital presses of HP Indigo and Xeikon, and the semi-digital printing presses with printing plates. , such as Heidelberg's DI (Digital Direct Imaging System) presses. The latter can also be summarized as on-machine CTP.

Depending on the degree of digitization, existing printing techniques can be divided into the following four workflows.

1.CTF process: DTP → CTF → Traditional Printing Machine → Print

2.CTP process: DTP → CTP (offline computer direct plate-making) → traditional printing press → print

3. Semi-digital printing process: DTP → semi-digital printing machine (direct computer plate making) → print

4. All-digital printing process: DTP → digital press → printed matter We call the first CTF process the traditional printing process, and the latter three are called digital processes.

Print quality control elements and control tools

The quality of printed matter refers to the comprehensive effect of various appearance characteristics of the printed matter. From the point of view of copying technology, the quality of printing should be based on "faithful reproduction of manuscripts." Whether in the traditional printing process or in the digital process, faithful to the original copy of the printed matter must be realized. There are four control elements for print: color, level, clarity, and consistency.

1. Color is the basis of product quality and directly determines the pros and cons of product quality. Color control or management is always a research and printing professional. 5.

2. Hierarchy is tonality, which refers to the change in the shade of light that the image can recognize. It is the basis for achieving accurate color reproduction.

3. Sharpness refers to the degree of clarity of the image details, including three aspects, the level of clarity of the subtle level of the image, the clarity of the edges of the image outline, and the clarity of the image details.

4. Consistency is uniformity, which includes two aspects. On the one hand, it refers to the consistent degree of ink in different parts of the same batch of printed matter, that is, the consistent degree of the vertical and horizontal solid density of the printed matter. It reflects the stability of different parts of printed matter printed at the same time. On the other hand, it refers to the consistency of the density of different batches of prints at the same site, which reflects the stability of the printing press.

For printed matter, as long as the four aspects are well controlled, that is, the color, level, definition, and consistency of the printed matter can be controlled well, high-quality printed matter can be obtained.

One of the methods of printing quality control is the use of conventional signal strips, test strips, control strips, step scales, together with test instruments and diagrams, and scientific quantitative control of print quality (including print quality and print 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 Bruner system in Switzerland, the Flagra system in Germany, and the Gelingda system. Our country uses the signals and test strips of GAFT and Bruner.

Commonly used detection tools for printing include signal strips, test strips, control strips, and step scales. The signal bar is mainly used for visual evaluation, and its function is relatively single. It can only express the appearance quality information of the printed matter. Such as the sun scale ladder, GATF word signal bar, color signal bar and so on. The test strip is a multi-function marker element based on densitometer detection and evaluation, a combination of visual identification and densitometer test, and a test strip for numerical calculation using graphs and curves. The control strip is a versatile control tool that combines visual evaluation and test evaluation of the signal strips and test strips. Such as Bruner's third-generation control bar. The step scale has an equal difference density or a ruler for the dot, which is used for controlling the printing plate and printing quality.

The printing process referred to in the previous section requires the use of signal bars and control strips to record data in the operation of its major links to lay the foundation for standardized production. Quality testing and control generally identify issues through proofing. In the traditional printing process, the traditional proofing is used, that is, proofing is done by round flattening. In the digitalization process, digital proofing has gradually replaced the traditional proofing. The digital files to be printed can be output directly to the proofing machine, so as to check whether there are any problems with the text, specifications, and images. Since film is no longer used in the digital process and even the plates are no longer used, the evaluation and control of quality largely depends on the digital proofing loop. So, in today's digital, networked environment, in the digital process, the quality of its print is again constrained by what factors? Where is the difficulty and advantage of quality inspection and control?

Printing quality inspection and control method under digital environment

1. Transfer of data flow in digital flow

The digital production process makes the data flow and control flow in actual production not as intuitive as the traditional flow. All we see are the input layout elements, the layout information displayed by the display device and the final output of the color digital proofs. CTP plate. The job information is in the form of data. Any small error or small error will result in the failure of data transmission or abnormal output results. It will also exceed the control ability of the operator and it is difficult or impossible to check the problem. Therefore, these variables become Uncontrollable factors in prepress operations. In order to ensure the correct and smooth flow of data, we must find a practical method of data flow control.

In the printing industry, there are two kinds of information flow in the printing job, namely, "graphic information flow" and "production control information flow." Graphic information flow solves the problem of "doing"; and controlling information flow solves the problems of "how to do" and "what to do." In the digital process, both types of information are digital and can be stored, processed, and delivered by computer storage. The data flow control node in the digital process is file preflight, printout, PDF normalized digital layout, and RIP output.

2. Color Management in Open System

Color reproduction is an important measure of image reproduction quality. The color control in the traditional process is based on the theory of the definition and description of the color attributes with density control as the core, and the color is identified and corrected by a closed closed-loop system. Specific control methods include gray balance, color correction, optimal printing density control, dot transfer and control.

The digital printing process is an open system. The input, processing, and output devices may all come from different manufacturers. Various devices have different descriptions and expressions of colors. With the increase in the number of use of the same device, loss will occur, and the expression of colors will also be different, thus increasing the difficulty of color reproduction. And due to the emergence of the network and the need for information exchange, the design of color images or color image files is no longer limited to local publishing and printing; it is often necessary to view or copy in different places. The color information of color images not only needs to be on different devices, such as the display of a computer (even if the same device, the ability of devices produced by different manufacturers to display colors is not the same), but also need to pass between different media.

This requires a system whose function is to make the results of color transfer between all media basically consistent and non-aggressive. This is commonly referred to 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 that describes the device's color performance - ICC Profile. Using the ICCProfile feature file, cross-platform color communication can be achieved so that when other computers incorporate color management features, these files can be freely converted. Color management is done through the profile. The color management system is an application system that includes computer hardware, computer software, and color measurement equipment. Its goal is to create an environment in which various devices and materials that support the environment are matched to each other in color information transfer, enabling distortion-free transmission.

The new color control method with color space transformation as the core - color management has enabled the technological process in the field of printing and distribution to enter a brand new digital production stage, solving many problems in 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 are constantly emerging and are moving towards the goal of “What You See Is What You Get”.

3. Data transmission and management

In the digitization process, as the digitization level deepens, 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 Internet. To this end, the prepress field has established two related specifications: the OPI (OpenPrepressInterface) specification and the DCS (Desktop Color Separation) specification. The OPI specification allows low-resolution alternative images to be used for imposition, and the OHP server automatically replaces the corresponding high-resolution images when outputting color separations, thereby reducing the amount of file transfer in the network. The DCS specification is an extension of the EPS file format that can manage the entire color separation process of a desktop publishing system, helping to shorten production time and reduce equipment requirements. In the printing process, data flows in all aspects. Therefore, it is extremely important to ensure that the data files do not produce any missing information in the process of delivery, and that the interpretation of the files by different platforms is consistent. Otherwise, files sent to the back end may get incorrect output due to data loss. In order to implement unified document control in the integrated production of printing, 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 use is basically a traditional proofing process, generally using a flat press proofing method. Of course, digital