11.01.2007 | NYSCAN - 100% Print Inspection – chances and limits
100% Print Inspection – chances and limits
By Dr. Stephan Krebs
Web viewing systems are standard today on almost all printing presses. They also play an important role during make-ready for sporadic controls of critical parts. The size of the images on display is rather small given the resolution of the area cameras. The images may be blown-up, though, to considerable sizes with the zoom function.
These last few years on the other hand 100% print inspection systems based on line cameras have become widely used in parallel to the web viewing systems based on area cameras. The following paper deals with the application potential of this new camera technology.
What is meant by 100% print inspection?
Quite frequently 100% print inspection is though to mean that 100% of all defects will be detected. Naturally this is not the point as a control system can only detect defects of a certain physically specified size. 100% print inspection simply means that the print web is being inspected continuously over the entire width and at a specified sensitivity. The sensitivity itself depends substantially on the camera resolution.
What types of cameras are available today?
Camera technology has seen an astonishing development recently. Not so long ago the highest possible resolution was 2000 pixels. Actually monochrome line cameras are available with over 8000 pixels. Color line cameras too have evolved. Though actually most of the color line cameras have a resolution of 2000 pixels, the 3-chip technology has considerably increased their line frequency and image quality. In addition, their industrial suitability has been significantly broadened by the novel interface technologies now available. It is no longer a problem therefore to bridge distances of up to 10 meters between camera and processor by just using a standard cable.
Possible defect sizes
The minimum defect size which can possibly be detected depends on the camera resolution and on the web itself. In case of surface control it can be presumed that there is a homogeneous distribution of web contrasts. When upper and lower threshold values are set, variations from these values as small as one pixel can be detected. However, the situation is somewhat different in print image control. In order to reliably detect a one pixel defect within a text field the neighboring pixels must also be considered. Praxis has shown that the smallest detectable defect size is about 3 x 3 pixels. Figure 1 shows the resolution of a letter with 6000 pixels and with 2000 pixels. If, for example, the horizontal line in the letter “e” had been dropped for some reason or other making the “e” resemble a “c”, ”, it becomes quite obvious that a control system operating with 2000 pixels would probably not catch the defect.
Operating speed of a modern print image control system
The capabilities of a modern print image control system are not only determined by the given system resolution. Rather they are a matter of up to which web speed print image control is able to detect a specified defect size. Thus there are altogether three system dimensions which determine the size down to which defects will be detected, namely the given web width, the web speed and the minimum defect size to be detected. These three factors are put in relation to one another by the camera resolution and speed.
Figure 2 shows which web speed can be reached at a given web width if a modern camera with 6000 pixels and a line frequency of 25 kHz is used. The values of the table presume that there is an identical resolution all through the web, that is crosswise and in the direction of the web run. In fact it is possible to increase the web speed by increasing the resolution and thus the defect detection size in the direction of the web run. In this case the pixels are no longer referred to as “square” but rather as “rectangular”.
Such an application presumes, however, that the image processing computer can handle an immense data volume. The 6000 pixels resolution camera mentioned above provides up to 160 million pixels per second. This bottleneck has been eased considerably these past years by the dynamic development of the PC sector.
New workflow concepts thanks to 100% print image inspection
The first part of this paper described the capabilities of different control systems. The second part will have a detailed look at intelligent, appropriate applications for print image control.
On principle, print image control can be used within the standard workflow at any printer’s:
Prepress: This is where the artwork required for the print job is being prepared. Various proofing systems will make sure that the final print in fact comprises all the elements of the original pdf.-file.
Press: This is where the web will be printed. 100% Print Inspection will detect possible defects. This inspection has an immediate impact on the amount of waste created.
Finishing: This is where the printed rolls are processed on a rewinder. At this stage 100% Print Inspection is frequently applied in order to make sure that only perfect material is being shipped to the customer.
It is above all the ever fast development on the PC sector as far as memory space and processing speeds are concerned which has contributed enormously to making100% Print Inspection suitable for every print inspection job within the workflow.
What does this mean? Whereas in the past the actual defect image was just shown, it is possible today to document uninterruptedly the entire inspection process and to make this inspection protocol continuously available to the printer during production as a so-called “Roll Map”. The operator is thus in a position to identity heavy, repetitive defects and to make a well-considered decision as to whether the machine should be stopped or not.
Figure 3 shows a “Roll Map“. Areas will be defined across the web in which defects will be displayed with a rectangular symbol. By clicking on the rectangle the defect image for which the rectangle stands will be displayed to the right for further analysis.
What happens with those defects which have not been considered during the print process though they have been detected? As all defects are stored in the “Roll Map“ memory, this data is readily available if required for further print processing on the rewinder. This availability is the real benefit of the inspection system. Defects can first be analyzed on the “Roll Map“ and decisions can be taken before finishing as to whether a detected defect is of any relevance or not. The „Roll Map“ will be edited accordingly and the re-winder will programmed to stop only at those relevant positions.
This so-called „Scheduling“ quite obviously leads to increased productivity compared to the finishing process which uses print image control on the re-winder. With the later processing procedure the re-winder is stopped at every detected defect and the operator is called upon to make a decision before finishing continues. If the defect is considered irrelevant, the operator has wasted about 30 seconds time. If irrelevant defects occur more or less frequently, these stops will either reduce the productivity or the operator will have to reduce the sensitivity of the print image control system. This decision, however, considerably influences the quality of the control process.
If „Scheduling“ is used an absolute web position must be known so that the defects listed in the „Roll Map“ memory can be clearly attributed. For this purpose it is frequent practice to apply a position code at the rear of the web using a marking printer. To assure optimum use of the re-winder care must be taken that the position code can in fact be detected even if the re-winder runs at maximum speed.
The following table summarizes the benefits and drawbacks of print image control on a re-winder and on a printer:
Re-winder:
Printer:
+ Final check of outgoing roll
+ Usually there is one re-winder for several printing presses
+ Even movements reduce pseudo defects, print image control can operate at high sensitivity settings
+ The personnel is already familiar with operation control by PC
+ The speed of print image control more or less equals the speed of the printing press
+ With the use of “Scheduling” re-winder control is at its best
+ Significant waste reduction as defects are handled prior to winding
+ Highest productivity
- Permanent Stop and Go causes frequent pseudo defects. Therefore, it is usually necessary to reduce the sensitivity of the print image control system.
- Generally the personnel is not so familiar with PC operation.
- Usually the print image control system is slower than the max. speed of the re-winder.
- As the printing process is already concluded, waste can be reduced only at the expense of quality.
- A separate print image control system is required per printing press. The system pays for itself shortly as it contributes largely to waste elimination.
- The use of “Scheduling“ requires a marking code.
- As there is no final control on the re-winder process control must be strictly correct.
Summary:
100% print image control is a rather young technology. As various further developments of the actual camera and computer systems can reasonably be expected, considerable new developments in the quality management of label, film and flexo printing can also be anticipated. Whereas today rather static algorithms as still the state of the art given the extremely high number of pixels which must be handled by somewhat limited computer capabilities, we can reckon with the increased use of adaptive procedures in the future. Networking within the workflow and the integration of data management functions will also have to be considered for 100% Print Image Control in the future. Of course there are other systems which are of importance for 1005 Print Image Control but which have not been considered in this paper for reasons of space. These are, for example, web lighting, compensation of web or print distortions, web guiding etc. . Figure 4 shows a 100% Print Image Inspection System from Erhardt+Leimer used in label printing and mounted on a re-winder.
Short biographical notes:
Dr. Stephan Krebs (
s.krebs@erhardt-leimer.com) has worked for over 15 years as head of Development in electronics and drive engineering. He gained a sound and comprehensive expert knowledge particularly in the control and inspection of running webs. In Toronto/Canada he established in 2003 a company called Nyquist Systems which specialized in the development of print image control systems. Towards the end of 2005 the company was acquired by Erhardt+Leimer GmbH/Germany. Since then Dr. Krebs is responsible for the business unit “Print Inspection“ at Erhardt+Leimer. This company has a vast range of products available for print image inspection to cover the entire print workflow, from pres-press to press and finishing.
