Analysis and Research of Screen Printing Register Accuracy Based on Machine Vision

First, the principle of overprint error detection

Overprinting is a printing technique that prints a superimposed pattern based on the same reference point using multiple colors of ink. In the printing process, multiple inks are required to reproduce the original color. This requires that each coloring is accurately aligned. If several colors are not aligned, the printed products will be completely different, that is, overprint errors occur.

In the printing process, the overprinting error is detected by detecting the mark on the specific position of the print product. This mark is the so-called Mark line (printing mark line of each color), and is also simply referred to as color in the case of not being confused. Standard. A commonly used form of color mark, in the case of accurate overprinting, the color mark coincides, the picture is clear; the other is the case of overprint error, two color lines are not coincident, staggered with each other, indicating that there is a larger Overprinting errors, due to inaccurate overprinting, the graphic as a whole is very vague. Therefore, lateral and longitudinal overprint errors can be obtained by observing the degree of coincidence of the color marking lines.

The traditional visual inspection is inefficient, and it is easy to introduce human error. 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 been gradually applied. Adopting machine vision to do online detection not only has the advantages of high precision, high efficiency, but also overcomes many shortcomings of the traditional visual inspection.

Second, the overview of machine vision

Machine vision is the use of machines instead of the human eye to make measurements and judgments. It uses computers to simulate human visual functions, extract information from images of objective things, process them and understand them, and finally use them 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 conversion of the captured target into an image signal through a machine vision product (ie, an image pickup device, which is divided into CMOS and CCD), and is transmitted to a dedicated image processing system, which changes according to pixel distribution, brightness, color, and other information. Digital signals are generated; the image system performs various operations on these signals to extract the characteristics of the target, and then controls the on-site equipment actions according to the results of the discrimination.
The machine vision system is mainly composed of industrial cameras, lenses, light sources, image analysis and processing software. These four parts are themselves a professional field, and have quite a lot of performance and specifications. There are a lot of things to consider when matching with applications. . However, each part can not be individually determined whether it can be applied to an application. In short, the composition of machine vision is different due to the different disciplines involved in its own parts, and it is inseparable to achieve the application problems, plus the application itself has its own A professional knowledge, which makes the machine vision system produced more difficult.

The advantages of the machine vision system are:

1 Non-contact measurement will not cause any damage to the observer and the observed person, thus improving the reliability of the system.

2 has a wider spectral response range, such as using infrared measurements invisible to the human eye, extending the visual range of the human eye.

3 The long-term stable work makes it difficult for humans to observe the same object for a long time, while machine vision can measure, analyze, and identify 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 strange as in previous years. People have gradually begun to accept machine vision detection technology and have been widely used.

Third, the 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: pre-press graphic information design and processing, the influence of printing plate production, the influence of printing registration, the influence of the quality of the plate, and the influence of ink color matching. This article mainly discusses the impact of print register accuracy on print quality.

Since the ink color layer is to be distinguished when it is used for color registration of color printing, in general, it should be printed with light color and then printed with dark colors. The printing should be performed after the printing is mainly performed. The color sequence should be arranged when the printing plate is produced. Considering the factors affecting the quality of screen printing from the aspect of overprinting, the following are mainly summarized in the following four points: 1 The influence of the operator; 2 The influence of the substrate and the ink; 3 The influence of the doctor blade; 4 The influence of the screen printer.

According to the structure and movement of the screen printing machine, the impact of the machine overprint accuracy is also very significant. Especially the composition and movement of the machine. The screen printer is based on machine vision technology.

Fourth, the working principle and the main working process

According to the operating characteristics of the screen printing machine, machine vision technology is used to complete image acquisition, signal conversion, and a series of movements of the table to achieve overprint accuracy.

1. How it works

According to the motion matrix, the movement of one position of the image to another position is realized. When the image is deviated, it must first complete its rotational motion and then achieve its translation.

2. The main working process based on machine vision technology

The main working process of this multi-color screen printing overprint accuracy detection and control based on machine vision technology is as follows:

1 Finish the printing of the first color. In this step, the detection and control of register accuracy is not required.

2 The image acquisition part sends start pulses to the camera and the lighting system, respectively, according to the previously set program and delay.

3 Under the control of the computer signal, complete the scan of the image.

4 The collected image information is digitized by A/D or directly received digital video data digitized by the camera, and the image capturing part stores the digital image in the memory of the processor or the computer.

5 The processor processes, analyzes, and identifies the image to obtain measurement results or logic control values.

6 Processing results Control the movement of the table to achieve image registration and correct movement errors.

Compared with traditional methods, it not only has the advantages of high precision, high speed, high degree of automation, but also overcomes the shortcomings of the traditional visual inspection that can easily introduce human error, and can increase the flexibility and automation of production.

Based on the above analysis, if overprinting errors occur, the four-color component of CMYK does not overlap exactly in the printing process. This is reflected in the 24-bit true-color image collected by the CCD, that is, the RGB three-component components of the pattern do not overlap precisely. 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 subgraphs R, G and B, and apply the template matching method to detect R, G and B on the image to be detected. The coordinates of the template submap, if R, G, B three template sub-image coordinates of the error is less than a certain threshold, it can be considered that there is no overprint error, otherwise it is considered overprint error. Once the processing unit determines that there is an overprint error, the corresponding control command is issued to correct it by the executing device, so as to achieve overprinting accuracy.

In short, machine vision is a more complex system. However, due to the superiority of machine vision technology, people are increasingly applying this technology. Not only can you save manpower, but you can also increase accuracy to meet people's needs. Therefore, the application of machine vision systems has become more and more extensive, not only widely used in industry, agriculture, national defense, transportation, medical care, 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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