A, screen printing overprint error detection principle

       Screen printing overprint is to use a variety of color ink is based on the same reference point printing folding color pattern printing technology, printing quality need to pass through many times in the process of printing ink forme to reproduce the original color, this requires every color accurate alignment.If no alignment of a few kinds of color, printed printing quality will be unrecognisable, namely the overprint error.

       In the printing process, through the test print of the Mark in the specific location to implement the overprint error detection, the tag known as Mark line, all kinds of printing Mark line), in the case of not confused, also known as color standard.

       Traditional visual detection of low efficiency, and easy for the introduction of human error, and with the improvement of social productivity, the disadvantages of visual testing will become more and more obvious, and the intelligent detection based on machine vision technology has been gradually practical application.On-line detection using machine vision do not only has the advantages of high precision and high efficiency, but also to overcome the disadvantages of the traditional visual inspection.

Second, the overview of machine vision

       Machine vision is to use machines to replace human eyes do measurement and judgment, which USES computer to simulate human's visual function, and extract information from images of the objective things, and try to understand, finally used in the actual detection, measurement and control.The most obvious feature of the machine vision technology is fast, informative, functional.Machine vision system is to point to by machine vision products (namely image extraction device, CMOS and CCD two) will be taking the target into image signal, and send it to the dedicated image processing system, according to the pixel distribution and brightness, color and other information, converted to digital signal;Image system for various operation these signals to extract the characteristics of the target, and according to the results of determination to control the scene of the equipment.

        Machine vision system is mainly composed of industrial camera, camera, light source, image analysis and processing software of four parts, which itself has become a profession oneself four parts, quite a few performance and specifications, matching and application need to consider things very much.However, each part can't decide whether alone can be applied to a particular application, in a nutshell, the composition of machine vision because of the different parts involved professional identity, and to achieve integral application problems, combined with the application itself has its another kind of specialized knowledge, which makes the production increased the difficulty of the machine vision system.

The advantages of machine vision system are:

     1) non-contact measurement for the observer and to be the observer will not cause any damage, so as to improve the reliability of the system.

     (2) has a wide range of spectral response, such as the users see invisible infrared measurement, expands the scope of human eye vision.

     (3) work stably for a long time, the human long hard time to look at the same object, while machine vision can be a long time for measurement, analysis and recognition task.

     With machine vision products and technology application in industrial technology, the concept of machine vision is no longer as strange as in previous years, people have gradually started to accept machine vision inspection technology, and has been widely used.

Third, the factors influencing screen printing overprint precision

      According to the actual situation of printing, there are many factors affecting the quality of printing, such as: before printing graphic information design process, the influence of the influence of plate making, printing alignment, the influence of the quality of plate and ink color matching, and so on.This article mainly discuss the alignment precision of printing on printing quality.

     Due to the color to distinguish when a chromatograph scrapping printing ink level, generally should be printed after the light dark, after the first impression to the main, color sequence should be arranged in the production of plate.
From the aspects of printing overprint to considering the influencing factors on the quality of the silk screen printing, mainly summarized the following four points: 1) the influence of the operator;(2) substrates and the influence of ink;(3) the influence of the scraping blade;(4) the influence of screen printing machine.

      According to the structure and movement mode screen printing machine, the machine overprint precision is also very significant.Especially the machine way of composition and movement.The screen printing machine based on machine vision technology is done.

Fourth, work principle and the working process of the main

According to the working characteristics of the silk screen printing machine, it is using machine vision technology to accomplish image taken, signal conversion, the movement of the workbench and so on a series of actions, to implement the overprint precision.

Working principle of the 1.

     According to the motion matrix to realize the image position of a move to another location.As the image deviation occurs, to finish the first rotary motion, to achieve its translation.Specific as follows:

     Suppose images by p1q1 starting position to the position at the end of the pq, known from the beginning to the end of the location and the rigid body turning Angle of theta.Set the starting position around the P1 point turned theta Angle, arrived in p1q 'position, then by p1q' position at the end of the translation to the position of pq.

2. The main working process based on machine vision technology

       The multiple evanescent hues network based on machine vision technology printing overprint precision detection and control of main working process is as follows:

      (1) to complete the first color printing, in this step, don't need alignment precision of detection and control.

      (2) the image acquisition part in accordance with the procedures set in advance and delay, respectively to cameras and lighting system start pulse.

      (3) under the control of the computer signals, complete the image scanning.

      (4) the collected image information through A/D digital or is it directly after receiving digital camera digital video data, and image acquisition part will digital images stored in A processor or the computer's memory.

      (5) processor of image processing, analysis and identification, measurement results or logic control values.

      6 processing results control the movement of the workbench, the realization of image alignment as well as the correct motion error, etc.

      It compared with the traditional method, not only has the advantages of high accuracy, high speed and high degree of automation, can overcome the traditional visual inspection easy introduction of human error, and can improve the flexibility and automation of production.

      Overprint error based on the above analysis, if appear, on the printing process is shown as CMYK four-color component overlap, no precise performance in a 24-bit true color images by CCD acquisition on three component pattern of RGB are not accurate.So our intelligent detection based on machine vision is the product image and the standard template image to be detected respectively divided into R, G, B three son figure, the use of the method of template matching, the image to be detected on the detection of R, G, B three template subgraph coordinates, if R, G, B three template subgraph between the coordinates of the error is less than a certain threshold, can see no overprint error, otherwise, they think overprint error occurs.Once processing unit determine have overprint error, a corresponding control command by performing the correct equipment, thus to achieve the accurate overprint.

      In a nutshell, machine vision is a complex system.But due to the superiority of machine vision technology, people more and more application of the technology.Not only can save manpower, but also can improve the accuracy and make it meet the needs of people.Application field of machine vision systems, therefore, more and more widely, not only in industry, agriculture, national defense, transportation, medical, financial, and even sports, entertainment and so on has been widely applied, also in screen printing technology also obtained the very good application, and can better control the printing process, so as to improve the quality of the printing quality.