1. Principle of overprint error detection

Overprinting is a printing technology that uses multiple colors of ink to print a superimposed pattern based on the same reference point. The printed product needs to be inked multiple times in the printing process to reproduce the original color, which requires accurate alignment for each coloring. If several colors are not aligned, the printed product will be unrecognizable, that is, there is an overprint error.

In the printing process, the overprint error is detected by detecting a mark on a specific position of the printed product. This mark is commonly referred to as the Mark line (printing mark line of each color). It is also referred to as color for short without confusing. Mark. A commonly used form of color mark marking. In the case of accurate overprinting, the color marks overlap and the picture and text are clear; the other is the case of large overprint errors. Due to inaccurate overprinting, the overall picture is very fuzzy. Therefore, the horizontal and vertical overprinting errors can be obtained by observing the degree of coincidence of the color marking lines.

The efficiency of traditional visual inspection is low, and it is easy to introduce human errors. With the improvement of social production efficiency, the disadvantages of visual inspection are becoming more and more obvious, and the intelligent detection technology based on machine vision has gradually been put into practical use. Using machine vision for online inspection not only has the advantages of high accuracy and high efficiency, but also overcomes many shortcomings of traditional visual inspection.

2. Overview of Machine Vision

Machine vision is to use the machine to replace the human eye for measurement and judgment, that is, to use a computer to simulate the human visual function, extract information from the image of objective things, process and understand it, and finally use it for actual detection, measurement and control. The biggest features of machine vision technology are fast speed, large amount of information and many functions. The machine vision system refers to the machine vision products (that is, the image pickup device, divided into two types of CMOS and CCD) to convert the captured object into an image signal, and send it to a dedicated image processing system, according to the pixel distribution and brightness, color and other information, change Into a digital signal; the image system performs various operations on these signals to extract the characteristics of the target, and then controls the operation of the equipment on the spot according to the result of the discrimination.
The machine vision system is mainly composed of four parts: industrial-grade cameras, lenses, light sources, image analysis and processing software, and these four parts themselves are a professional field with many performances and specifications. There are many things to consider to match the application. . However, each part cannot individually determine whether it is applicable to a certain application. Simply put, the composition of machine vision is different due to the different professions involved in each part, and it is inseparable for the achievement of application problems, plus the application itself has other A professional knowledge, which makes the difficulty of the machine vision system produced.

The advantages of machine vision systems are:

â‘  Non-contact measurement will not cause any damage to the observer and the observed person, thereby improving the reliability of the system.
â‘¡ has a wider spectral response range, for example, using infrared measurement invisible to the human eye, expanding the human visual range.
â‘¢ Stable work for a long time, it is difficult for humans to observe the same object for a long time, and machine vision can do measurement, analysis and recognition tasks for a long time.

With the continuous application of machine vision products and technologies in industrial technology, the concept of machine vision is no longer as unfamiliar as in previous years, and people have gradually begun to accept machine vision detection technology and have been widely used.

3. Factors affecting the accuracy of screen printing overprint

According to the actual printing situation, there are many factors that affect the printing quality, such as: the design and processing of pre-press graphic information, the impact of plate production, the impact of printing registration, the impact of plate quality and the impact of ink color matching, etc. This article mainly discusses the effect of printing registration accuracy on printing quality.

As the color printing plate used for color scraping should distinguish the ink color level, generally speaking, the light color should be printed first and then the dark color, and the secondary printing should be printed first, and the color sequence should be arranged when the printing plate is made. Considering the factors that affect the quality of screen printing from the perspective of printing overprinting, there are four main points: â‘  the influence of the operator; â‘¡ the influence of the substrate and the ink; â‘¢ the influence of the squeegee; â‘£ the influence of the screen printing machine.

According to the structure and movement mode of the screen printing machine, the machine's influence on the accuracy of overprinting is also very significant. Especially the composition and movement of the machine. The screen printing machine is based on machine vision technology.

4. Working principle and main working process

According to the working characteristics of the screen printing machine, that is, using machine vision technology to complete a series of actions such as image taking, signal conversion, and movement of the worktable, to achieve accurate registration.

1. Working principle

According to the motion matrix, the image can be moved from one position to another. When the image is deviated, it must first complete its rotation and then realize its translation.

2. The main working process based on machine vision technology

The main working process of this multi-color screen printing overprint precision detection and control based on machine vision technology is as follows:
â‘ Complete the printing of the first color, in this step, there is no need to detect and control the registration accuracy.
â‘¡ The image acquisition part sends start pulses to the camera and the lighting system respectively according to the preset procedure and delay.
â‘¢Under the control of computer signals, complete the scanning of the image.
â‘£The collected image information is digitized by A / D or directly receives the digital video data after the camera digitization, and the image acquisition part stores the digital image in the memory of the processor or computer.
⑤The processor processes, analyzes and recognizes the image to obtain the measurement result or logical control value.
â‘¥ The processing result controls the movement of the workbench, realizes the registration of the image and corrects the error of the movement, etc.

Compared with traditional methods, it not only has the advantages of high precision, fast speed, and high degree of automation, but also overcomes the shortcomings of traditional visual inspection that is easy to introduce human errors, and can improve the flexibility and automation of production.

Based on the above analysis, if there is an overprint error, the CMYK four-color component does not exactly overlap in the printing process, and the RGB three-component of the pattern does not exactly overlap on the 24-bit true color image collected by the CCD. Therefore, our intelligent detection based on machine vision is to divide the image of the product to be detected and the standard template image into three sub-pictures of R, G, and B, and apply the template matching method to detect R, G, and B on the image to be detected. For the coordinates of the template sub-picture, if the coordinate error between the three template sub-pictures of R, G, and B is less than a certain threshold, it can be considered that there is no overprint error, otherwise, it is considered that there is an overprint error. Once the processing unit determines that there is an overprint error, it issues a corresponding control command and the execution device corrects it, so as to achieve accurate overprint.

All in all, machine vision is a relatively complicated system. However, due to the superiority of machine vision technology, more and more people apply this technology. Not only can it save manpower, but it can also improve accuracy and make it meet people's needs. Therefore, the application fields of machine vision systems are becoming more and more extensive, not only in industry, agriculture, defense, transportation, medical treatment, finance, even sports, entertainment, etc., but also in screen printing technology. Application, and can better control the printing process, thereby improving the quality of printed products.

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