Dr. Stephan Krebs, founder of Nyquist Systems and head of the printing inspection business department of Erhardt & Leimer in Germany, has a complete set of insights on the current state-of-the-art 100% inspection system and its future development trends.
The paper roll observation system is the standard configuration of almost all printing machines today. These observation systems play a very important role in the non-fixed inspection of important parts of the image. Due to the camera resolution of area array devices, the displayed image size is quite small, but using the zoom function, the image can be enlarged.
This situation lasted for several years. On the other hand, the 100% print inspection system based on line array device cameras has been widely used in paper roll observation systems based on area array device cameras. Here, I would like to talk about the application potential of this new camera technology.
What does 100% print inspection system mean?
People often regard 100% printed matter inspection as 100% defect inspection. In fact, 100% printed product inspection does not mean that as a control system, it can only detect defects within a specified area. Simply put, 100% printed product detection means that the printing paper roll is continuously tested with a specific sensitivity to the entire width. The sensitivity depends mainly on the resolution of the camera.
What types of cameras are currently available on the market?
In recent years, camera technology has developed rapidly. Not long ago, the highest resolution that a camera could achieve was only 2000 pixels; now, the resolution of a monochrome linear array camera can reach 8000 pixels, and the development of a color linear array camera. Although most color line array device cameras have a resolution of 2000 pixels, the application of 3-chip technology has greatly improved their line scan frequency and image quality. In addition, the current novel interface technology greatly expands the industrial applicability of line array device cameras. Therefore, as long as the standard optical cable is used, the distance between the camera and the processor up to 10 meters can be solved.
Detectable defect size
The smallest defect size that the system can detect depends on the resolution of the camera and the paper web itself. In the case of controlling the coating surface, it can be assumed that the paper roll contrast is evenly distributed. When upper and lower thresholds are set, deviations from these thresholds as small as 1 pixel can be detected. However, this situation is somewhat different in print image control.
In order to reliably detect a defect with a pixel of 1 in a text, the detection of adjacent pixels must be considered. Practice has proved that the smallest defect size that can be detected is about 3 × 3 pixels. The letter resolution shown in Figure 1 is 6000 pixels and 2000 pixels. If the horizontal line of the letter "e" falls, it will be deformed like the letter "c". At this time, it is obvious that the control system operating at 2000 pixels cannot find this defect.
Figure 1: (Left) enlarged view of 6000 pixel letters; (right) enlarged view of 2000 pixel letters
Operation speed of modern printing image control system
The capabilities of modern printing image control systems are not only affected by the resolution of a given system, but also by the specified defect size that the printing image control system can detect at a certain paper roll operating speed. The three system sizes that determine the size of defects that can be detected include: paper roll width, paper roll speed, and the smallest defect size to be detected. These three factors combined with the resolution and speed of the camera determine the capabilities of the inspection system.
Figure 2 shows the speed that a modern camera with a resolution of 6000 pixels and a line scan frequency of 25 kHz can be achieved with different width paper rolls. The values ​​in the table are assuming that the entire paper roll has the same resolution-both the width and the paper roll running direction have the same resolution. The speed of the paper roll can be increased by increasing the resolution, so that the detection size in which the running direction of the paper roll is detected can be increased. Therefore, pixels no longer refer to "square" but "triangle".
Under such a hypothetical situation, the image processing computer can process a large amount of data. The above-mentioned 6000 pixel resolution camera can produce a resolution of up to 160 megapixels per second. Due to the dynamic development of PC devices, this bottleneck has been considerably improved in the past few years.
Figure 2 Assuming that the pixels are "triangular", the paper roll speed and minimum defect size for the entire width
New workflow concept
The first part of this article describes the capabilities of different control systems. The second part will focus on the application of 100% printed image detection control.
In theory, print image control can be used in the standard workflow of any printing press:
â–³ Before printing: prepare the manuscript required for printing in this process. Various proofing systems ensure that the final print contains all the elements in the original pdf file.
â–³ Printing: The paper roll is printed in this process. 100% print inspection will check for possible defects. This test directly affects the quantity of waste produced.
â–³ Post-press processing: The printing paper roll is processed on the rewinding machine in this process. In this step, a 100% print inspection system is usually used to ensure that only intact materials are sent to customers.
The rapid development of the storage capacity and processing speed of PC devices has greatly promoted the applicability of 100% printed matter detection system to the detection of all printed matter in the printing workflow.
What does it mean? In the past, the defective image was only displayed on the monitor. Nowadays, all the inspection process files can be provided continuously. At the same time, the printing personnel can get the inspection report called the "operation chart" during the production process. In this way, the operator can repeatedly check the same problem at a certain location and make a correct decision whether to stop.
Figure 3 shows a "running diagram" situation. Inspection is performed on the entire paper roll area, and defects are marked with a triangle symbol. Click on the triangle, and the defect image represented by it is displayed on the right for further analysis.
What about defects that have been detected in the production process but have not yet been dealt with? Since all the defects have been stored in the "run graph" memory, if you need to deal with the next rewinding step, these data can be called immediately. This is where the effectiveness of the detection system is most significant. Defects can first be analyzed on the “running diagramâ€, and it is determined whether the detected defects are relevant before starting post-press processing. Therefore, it is possible to edit the "running diagram", program the rewinding device, and only stop at the relevant position.
Figure 3 "Operation Diagram" section, the selected defect image is displayed on the right.
The use of this so-called "stroke arrangement" greatly improves production efficiency relative to the post-press processing process that uses a printed image control system on the rewinder. In the former case, the rewinder will be shut down wherever a defect is detected, requiring the operator to make a decision before the post-press processing continues. If the detected defects are irrelevant, the operator wastes 30 seconds; if the irrelevant defects appear from time to time, these downtimes will not only reduce production efficiency, but also make the operator have to reduce the printing image control system Sensitivity. In this case, it will significantly affect the quality of the control process.
If you use the "operation schedule", you must clarify the absolute position of the paper roll, so that the defects listed in the "operation diagram" memory are clear. For this reason, the position code is usually marked with a printed mark at the end of the paper roll. In order to ensure the best use of the rewinder, care must be taken to ensure that the position code can be detected even at the highest speed of the rewinder.
The following table summarizes the advantages and disadvantages of the image control system on the rewinder and printing machine:
Winder:
+ Issue the final inspection of the coil.
+ Usually there is only one rewinder for several printing machines.
-The long-term "stop-open" leads to the occurrence of false defect judgments. Therefore, the sensitivity of the printed image control system needs to be reduced frequently.
-In general, employees are not very familiar with PC operation.
-The printed image control system is usually slower than the maximum speed of the rewinder.
-Since the printing process has been completed, it only reduces the waste of quality costs.
Printing machine:
+ Continuous paper roll movement reduces the occurrence of false defects, so the printed image control system can be set to operate with high sensitivity.
+ People are already familiar with PC operation control.
+ The speed of the print image control system is about the same as the speed of the printing press.
+ Optimized the efficiency of the rewinding machine by using "operation schedule".
+ Since the detected defects are processed before rewinding, waste is greatly reduced.
+ The highest production efficiency.
-Each printing machine requires its own independent printing image control system. The cost of the control system itself is about the same as the reduced cost of waste.
-Use "Run Schedule" to request tag code.
-Since there is no final control during rewinding, process control must be very strict.
to sum up
100% printed image control is a relatively mature technology. It is expected that further development of cameras and computer systems can drive new developments in label quality management and printing.
Today, the static method is still the most advanced, and it must process a large number of pixels with the limited capacity of the computer. In the future, we will increase the use of "appropriate" methods. The integration of the network and data management functions in the workflow will also be considered in the future 100% printing image control.
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