Discussion on the Difficulty and Technique of Detecting Area Area Ratio of Printing Plates

The process of plate-making is the process of production, transmission and change of dots. The dot size, shape and ink condition directly determine the tone level and color of the image. Controlling the change of outlets is the key to controlling the printing quality.

The printing plate is a bridge that transfers the dots on the original film to the substrate. Due to the influence of the original film and plate properties, exposure and development, the dot data on the film and the dot data on the plate are not the same. The change of outlets at different tone levels is also different. Due to different printing conditions and operating skills, the dot transfer data on the printing plate is also different from that on the proofing plate. During the printing process, dots will increase, printing pressure and other process parameters will be different, paper, ink and other materials will be different, and dot gain will be different. In order to ensure the printing quality, it is necessary to implement standardized and data management for each process. In the process of plate making and printing, not only the dot data on the film and the printed matter are to be inspected, but also the dot data on the printing plate need to be inspected in order to clarify the rules of the dot transfer during the whole process of plate making and printing, including proofing, and determine the quality of the printing plate. data range. In this sense, the detection of the dot area ratio of printing plates is very necessary.

1. Difficulty of printing plate dot area rate detection

The actual dot gain value at the time of printing should be the dot area ratio on the printed matter minus the dot area ratio on the printing plate. However, for a long time the dot gain has been defined as the difference between the dot area ratio on the printed matter and the dot area ratio on the original film, and the absolute increase at the 50% tone is usually used as the amplitude modulation net. The reason is that the measurement of printing plate dot data is very difficult and troublesome. The film is transparent, the fog is generally 0.03, and the contrast is above 3.5. The transmission density meter is used to measure the field and dot density values, and then the Marley-Davis formula can accurately calculate the dot area ratio on the film. Printing plates are different, and all tests that use the reflectance densitometer to measure the dot area ratio on the plate accurately are considered to be failures. There are two main reasons: First, on various printing plates, the contrast between the ink and blank areas is very small, or the photosensitive layer does not have a proper tone. Second, because of the diffusion effect of light, under certain conditions, Due to the roughness of the plate surface, the reflection densitometer cannot determine the zero value. Therefore, especially the dark and midtone measurements are very inaccurate. Therefore, for the quality control of the printing plate, it is usually changed to use grayscale ruler, Bruner test bar, GATF signal bar, etc.

2. Traditional printing plate dot area rate detection instrument

Although the dot area ratio of the printing plate is difficult to detect, people have never given up trying to use a test instrument to detect the dot area ratio of the printing plate. To sum up, the use of scale microscope, dot integrator, reflective dot density meter three kinds of instruments for testing.

The scale microscope can only measure the diameter of a single or a few dots. The dot percentage is calculated by calculation and there is a great chance. The dot integrator is accurate and can measure the dot of any shape, but it takes time and labor, and the cost is too high. Affects the production efficiency; the reflective dot density meter has large measurement error and low accuracy. According to foreign studies, the use of densitometers to detect FM networks has greater errors. Therefore, these three methods are basically not used in actual production except for basic research. In actual production, a magnifying glass is often used to detect the actuality of the dot, and a test strip is used to control the quality of the plate.

3. New printing plate dot area rate detection equipment

As a difficult point in the control of the printing process, the detection of the dot area ratio of printing plates has attracted the attention of many scholars. With the development of computer image processing technology, several new types of printing plate dot area rate detection instruments have appeared on the market in recent years. The structure and operation of these instruments have their own characteristics, but the basic functions are roughly the same. In addition to the fast and accurate detection of the dot area ratio of the printing plate, the dot diameter (net dot size) and the number of screen lines can be quickly and accurately detected. Web angle, and can display dot shape on LED display or computer monitor. The DMS910 tester is used as an example to illustrate the detection principle and detection process.

The instrument is mainly composed of three parts, namely high resolution digital camera module, optical system and image processing software. The detection principle is as follows: The printed plate under the illumination of the standard internal light source of the instrument is imaged by a complex optical system (lens, etc.) onto the CCD image sensor, and the image data is converted into a computer recognizable after the electronic components such as the image acquisition card. The image format, then the instrument supporting software uses digital image processing analysis technology to calculate the dot area rate, the number of screen lines and screen angle, and display the processing results.

Figure 1 is the original (untreated) plate halftone image collected with the DMS910. The dot shape, defects, and exposure errors on the plate are clearly visible. The operator can intuitively check the quality of the plate. The image after the software TECHKON DMS Pro preprocessing (denoising, sharpening, etc.) is shown in Figure 2. The software quickly calculates the dot area rate, the number of screen lines, and the screen angle according to this image. Intuitively displayed on the screen, and analysis of the dot gain situation, showing dot gain curves. The entire measurement process is completed in the Windows user interface. The instrument is suitable for both FM and AM images.

Original plate dot image

Preprocessed dot image

The detection operation of the instrument is quite simple. Simply place the calibrated instrument on the plate to be tested. Use the probe hole on the top of the instrument or the image displayed on the monitor to align the area to be measured. Press the only button on the instrument or the software interface. The "Start" in the middle can be measured. The instrument has a precision of ±0.5% when measuring 150 lines/inch of dots, and the measurement time is within 2 seconds. The instrument has a volume of 50mm (height) × 65mm (width) × 135mm (length), weighs only 280g, and is connected to a computer with a USB interface. It is actually a lightweight and fast measuring instrument.

The appearance of new printing plate dot data detection instruments makes it possible to quickly and accurately detect printing plate dot data, which is of great significance to the standardization, dataization and improvement of printing quality of the printing and copying process. At present, there are no similar domestic instruments, and we look forward to the efforts of scientific personnel in the domestic printing industry.