Machine Vision in Printing Industry Applications
| By: Nello Zuech, Contributing Editor
The print industry has been a user of electro-optical sensors for registration control and print-to-print control for many years. Over the years, machine vision-based sensors were substituted for the original electro-optical sensors. As machine vision technology became more robust, it was substituted for people sitting watching the print using a strobe to essentially stop action on a single print and observe any changes from the original print pattern. These camera-based systems were often mounted on a traversing mechanism permitting inspection, on a sample basis, or sections of the pattern across the web.
These original systems were more of an aid to the operator, alarming him whenever a change in the repeat pattern was observed. These changes could stem from registration errors, color errors or actual defects – missing or added print, streaks, etc. Today, with the advances in all of the underlying machine vision infrastructure components (optics, cameras, lighting, microprocessors) and the ability to handle ever more sophisticated algorithms at higher speeds, a number of companies are now offering systems that will perform 100% inspection of the entire web or printed material. In addition, most of these systems now offer the ability to handle web wandering and web stretch. Some offer defect classification, which can lead to feedback control of the printing process.
Printing line speeds and web widths present challenges for machine vision. Hence, many of these systems use line scan cameras to acquire the image data, often in color and take advantage of cheap memory to assemble the image data on which to operate. The high speeds do present challenges for the line scan cameras and high levels of illumination are critical to provide a sufficient number of photons to overcome signal-to-noise issues.
Furthermore, systems are offered with more user-friendly interfaces, making it easier for operators to take ownership of systems. Data analysis and reporting is offered with most of these systems integrating directly into plant-wide management information systems that provide many layers of management to handle quality issues virtually instantaneously.
For the most part, print production can be characterized as high mix/high volume – many different production runs on the same press. With advances in user interfaces, these systems now respond to the requirement for quick and easy changeovers that line operators perform.
Not all segments of the print industry, however, are embracing this new technology. For the most part, it is being used at printers that print currency, bank notes, stamps, lottery tickets, labels and packaging materials. Labels and cartons printed for the pharmaceutical industry must be 100% inspected to assure compliance of every detail with the original artwork. In the case of packaging, the more quality conscious customers of the printers are often demanding that their printed materials also be 100% inspected.
One of the latest print industry applications for which automatic inspection by machine vision systems is suitable is the printing of electronic patterns reflecting RFID circuitry, as well as patterns on flex circuits. It is still puzzling why some high-valued segments of the print industry, such as wall paper and counter laminate, are not widely adopting machine vision.
To gain some insights into what is currently available in the way of machine vision-based systems for print applications; input for this article was canvassed from all known suppliers of such systems. The following companies graciously provided input:
- Gal Shamri – Vice President Marketing – Advanced Vision Technology (AVT)
- Dr. Stephan Krebs - Head of Business Unit Print Inspection - Erhardt+Leimer GmbH
- Juergen Kreis - Business Unit Manager – ISRA VISION
- Roberto Salgari – President - PARVIS systems and services
- John Woolley - Vice President - PC Industries
- Christiane Blasius – Sales and Marketing Assistant – TEMA GmbH
1. What specific generic print industry applications do your machine vision products address? In each case, is the specific application addressed as a 100%-based inspection or sample-based inspection? In each case, is the scene interpretation fully automatic or is human intervention required?
[Gal Shamri – Advanced Vision Technology] AVT has evaluated the different print market segments and the requirements for machine vision based solutions. The company started addressing the packaging segment, and then entered the label segment and recently the sheet-fed folding carton market. Specializing in providing solutions for the printing industry, we have developed a dedicated solution for each market segment.
Packaging market - This segment typically runs wide web presses and requires both process control and 100% quality assurance. AVT has a wide variety of machine-vision based solutions to address the growing needs of this segment, including:
- Process control: Based on area-scan cameras, this on-press solution provides automatic inspection and early warning for print defects. The system is designed to detect process defects to allow an operator to fix a problem before they produce waste material. Process control solutions include the ability to host many add-on modules, such as barcode verification, in-line color measurement and press control solutions.
- Press control solutions: AVT has developed closed loop solutions that can fully automate the printing press, dramatically reducing setup time. These solutions include automatic color register control and pressure control for CI Flexo presses.
- 100% Quality Assurance solution: Using high-quality line scan CCD cameras, a quality assurance solution detects all printing defects 100% of the time. AVT quality assurance solutions can be installed on press, as a separate system or as a combination with area scan camera-based process control systems. In many cases, the quality assurance solution is installed at post-press stages, such as doctor machines, laminators or slitter rewinders.
Label market - Driven by the pharmaceutical, cosmetic and other high-quality segments, the need for 100% automatic inspection in the label market has grown significantly in the last few years. To answer the challenges of this market segment, AVT has developed a dedicated line scan CCD-based 100% automatic inspection solution that can be installed either on-press or on-rewinder. While installed on the rewinder, the AVT solution detects all converting process defects and automatically stops the rewinder to allow efficient removal of defective material.
Sheet-fed folding carton market - AVT is developing dedicated solutions for this market based on line scan cameras. The solutions will include on-line and off-line systems, providing automatic inspection for complete sheet and die-cut parts (‘‘signal package’‘), including fully automatic sorting of defective parts.
[Dr. Stephan Krebs – Erhardt+Leimer GmbH] Our NYSCAN systems are used for 100% print inspection. This means inspection of 100% of the web at 100% of the time. We provide single camera solutions as well as multiple camera solutions, depending on the web width and the desired resolution. Our target markets are labels, film, foil printing and general packaging industries. Our systems are used in critical applications, such as pharmaceutical or security industries, as well as standard printing. Depending on the application we use either high-speed high-resolution grayscale line scan cameras or high-quality 3 CCD color linescan cameras. Our company offers additional product lines for the print industry: ELSIS surface inspection solutions based on line scan camera technology and ELSCAN web viewing systems based on area scan cameras.
In the case of an installation on a rewinder, our NYSCAN inspection system is a fully integrated solution and will automatically stop the rewinder if a print or process defect is detected. The information is displayed on the screen for the operator, who will proceed with the corrective action.
More human interaction is required in the case of an installation on a printing press. The NYSCAN inspection system provides important information that allows the press operator to make an intelligent decision whether to stop the press and avoid further waste production or continue production at a commercially acceptable quality level.
[Juergen Kreis – ISRA VISION] For surface vision, we provide a comprehensive portfolio of fully scalable systems, providing 100% inspection of virtually any substrate, from paper, plastics and films, to non-woven and metal. The business unit PRINT focuses on systems for the inspection of printed surfaces, where we address web-fed and sheet-fed applications alike, as well as dedicated solutions for the inspection of blanks/singles. Additionally, we offer dedicated solutions for printed electronics with a specific focus on flexible electronics, both in sheet and reel-to-reel processes.
Respectively, we offer a unique portfolio of solutions for both the printing and converting industries, covering all relevant production steps including pre-press (inspection of substrates and raw materials), in-press, and post-press inspection, where we offer systems for offline print inspection on blanks, sheets and, in the case of web processes, for inspection on rewinder/doctor machines and slitters.
All of our systems provide a fully automated 100% inspection of goods. This approach ensures monitoring of the complete production volume at full production speed over the full web/sheet width on every repeat. Our print inspection approach is based on a reference comparison (golden template match), including, where necessary, real-time compensation of web wandering.
Right now, we see the need for print inspection in all segments, where print adds substantial value to the product or is an essential safety feature. This applies especially to printing of high quality packaging materials as well as pharmaceutical packaging. Most common packaging materials hereunder are either made of flexible substrates, produced in a reel-to-reel process, or made as folding cartons, mostly produced by sheet printing, dye-cutting, and gluing/folding, sometimes accompanied by value added applications such as hot-foil lamination or cold-seal application and stamping/embossing. In the case of sheet printers, there is a certain tendency to procure paper in rolls and cut them to sheets in order to reduce costs. In this case an additional substrate inspection of the paper roll prior to cutting is recommended.
In the following, I would like to outline in two examples of how and where our inspection system solutions can be used in the process of package printing:
1. Flexible packaging- Flexible packaging is printed on flexible substrates, made of various types of plastic or metal films. Here, ISRA VISION provides inline inspection systems from our SMASH series for a 100% inspection of the processed substrate (either metal film such as aluminum, or plastic film at the extrusion line). The material is processed in a reel-to-reel (web-fed) process. Respectively, we integrate our print inspection systems (SMASH Print series) into the web-fed printing presses to perform a 100% inspection of the printed patterns, on each repeat, for the full length of printed material.
Though possible for any print technology, most common processes employ either flexographic or gravure print. As today, typical web speeds range between 150m/min up to 600m/min at web widths of 500 to 1500mm. The free scalability of ISRA's technology allows us to provide adequate layouts (suitable resolution and coverage in cross web and machine direction), covering the full range of processes found in the field. After printing on typically wider web widths, the printed rolls are then slit, allowing for additional post-press inspection on the slitting machine or the rewinder.
2. Folding cartons - Folding cartons are found in the packaging industry (for cereals, consumer electronics, tobacco and many other products) and are also widely used for pharmaceutical packaging, where the flawless quality (of the information printed on the boxes) is mandatory. The cardboard is mostly processed in sheets and printed on offset printing presses. During paper production, ISRA VISION’s surface inspection solutions for paper (SMASH series) can help the paper manufacturers to monitor their production quality.
When processing paper in rolls (e.g., on high-speed rotogravure presses used in the newspaper business) another application of pre-press inspection is the deployment of ISRA's PrePress inspection system. This system monitors the paper fed into the press and if that paper is torn (possibly causing the printer to shutdown the press for several hours), to react to the tear-sensors and visualize the surface quality of that portion of the paper that caused the tear. This comprehensive documentation simplifies an objective discussion of fault reasons and enhances the constructive partnership between paper supplier and printer – in the worst case; it also helps to enforce reimbursement of related expenses from the paper supplier by having a proof of the faulty raw material (i.e., the paper quality).
In sheet-fed presses, in-press inspection systems from ISRA VISION, similar to their use on web-fed presses, again can monitor each printed sheet for the full repeat and will safely detect flaws in print quality as well as surface defects.
After printing, customers in the folding carton business may consider an additional offline sorting of the printed sheets, thus allowing them to sort printed sheets coming from several presses on one single station. This approach provides more flexibility when deciding to perform a 100% inspection on only those products that require the highest quality standards. Additionally, it allows separation of a-grade from b-grade sheets, and then having blanks coming from b-graded stacks be inspected piece-by-piece (again, either manually or fully automated with ISRA’s blank inspection system).
For high-quality folding cartons, it becomes more and more common to have additional applications applied to the product after print, such as hot-foil lamination of metallized films, embossing of structures, and other applications. Here, an offline inspection solution provides the means to have these features inspected as well.
For the above described offline inspection solutions, ISRA VISION together with one of our partner companies, provides dedicated inspection and sorting systems, including material handling for sheets and blanks alike, and for different widths and resolutions. After dye-cut, inspection of the blanks provides the benefit of sorting out the defects instead of disposing of a whole sheet. Given the price of cardboard used for high-quality print, this allows for additional savings.
Blank inspection systems from ISRA VISION are available as standalone inspection stations (again, including a sorting unit); solutions designed as additions to common folding-gluing systems are under consideration, thus helping to mark defective blanks and to use the sorter integrated in the folder-gluer.
3. Metal print - Metal sheets are printed and used for making boxes (e.g., for expensive cosmetics packaging) and cans. Leaving aside the manufacturing of the sheet metal itself (ISRA VISION also offers surface inspection systems for metal, used in hot mills and cold mills alike), inspection of the print quality (similar to cardboard printing), could be done in-press or offline. In handling metal sheets, special attention must be paid to avoid scratches on the substrate when handling these sheets. After print, metal sheets often are treated with additional lacquer or embossing. ISRA’s multi-camera/multi-bank systems allow for the inspection before and after the respective application.
[Roberto Salgari – PARVIS] Our applications are mostly in the area of security printing, which includes tax stamps, stamps, ID cards, lottery tickets, etc. But the most important and specific market that we address is that of banknote production. We do both on-line 100% inspection and off-line sampling.
The first is used mainly on web presses or at the end of a production line (final inspection and sorting). The second is used on sheet-fed presses when there is a need for high resolution and precision controls in order to detect process drifts.
All of our systems analyze image content in a fully automatic way (they are not just image grabbing and display systems). The operator can then make the decision about how to intervene on the press in order to correct the detected problem.
[John Wooley – PC Industries] We have 3 primary applications:
- a. 100% inspection for critical or high-value printing, such as pharmaceutical labeling
- b. 100% inspection of variable printing, typically for security printing
- c. Sampling inspection for non-critical, low-value printing
[Christiane Blasius – TEMA GmbH] Our systems are mainly used in:
- Can, cup, and tube industry (PI-Cylinder): 100% inspection of decoration, fully automatic
- Metal sheet industry (PI-Sheets): 100% inspection of decoration, fully automatic
- Security printing industry (PI-Sheets, NASC): 100% print inspection of sheets (offset and intaglio), additional control of security features and serial numbers, fully automatic
- Web printing industry (PI-Web): 100% inspection of decorated web material, fully automatic
Our vision systems are either integrated into our customers’ printing machines or provided as stand-alone units with complete sheet handling, from in-feed, to accept and reject sorting.
2. What are the benefits the print industry derives from the use of your machine vision-based products?
[Stephan] 100% inspection enables the printer to make an intelligent decision during the press run. This helps reduce the waste created by the press significantly. A sample-based system cannot provide this feedback in most cases, as it only covers a fraction of the web. With today's computer technology, all defects can be stored in a database called roll map giving the user full transparency about each produced roll. The data can be reviewed after the press run as a ‘‘virtual roll’‘ and a decision about further processing can be made. Today, having a 100% inspection system provides a competitive advantage to the printer. It is often a selling tool when bidding for a large quantity print job.
[Juergen] The major benefit of inspection systems providing a real 100% inspection of the production is the complete monitoring of the whole production batch and providing comprehensive documentation and history of production quality and line performance. The information obtained can be used internally to optimize processes as well as externally, in communication with customers, as proof of dedication to excellent quality and, in the case of claims, as objective documentation.
Value-added functionality, inherently available with each of our systems, includes the provision of several simultaneous real-time visuals of the printed web (to be compared with having several web viewers mounted on the same machine, thus allowing monitoring several locations across the web), the possibility to monitor color deviations, stretch, or mis-registration, which provides the printers at all process steps with valuable information, resulting in shorter ramp-up times (‘‘fast-to-print’‘); and a comprehensive monitoring of the processes. Special algorithms allow for early trend indications.
As an example, I’d like to mention our Advanced Streak Detection (ASD) feature, which allows the proactive detection of streaks earlier than the human operator would be able to do. Streaks are a most common defect on gravure presses, originating from contaminated or damaged doctor blades, starting as tiny, low-contrast streaks, and are, with standard algorithms hard to identify for a vision system.
The non-monetary effect of demonstrating commitment to highest quality standards by employing a 100% inspection system in their print processes allows print shops to separate themselves from their competition. Early trend-indication and full process monitoring and documentation of the printed jobs results in increasing yield, reducing waste, and generating higher ROIs.
[Roberto] With on-line or ‘‘Near The Line’‘ process control systems a significant waste reduction is normally achieved. Ideally all systematic errors are prevented. Furthermore, the production outcome is more stable (repeatable), which makes the use of automatic inspection and sorting systems at the end of the production line much easier.
These last types of systems, automatic final sorters, are used in many banknote printing plants, replacing manual inspection. They inspect 100% of the finished products at very high speeds, providing faster, less expensive and more reliable control.
[John] Improved quality, reduced waste, minimized liability, higher production rates.
[Christiane] Simply said: saving time and money. 100% inspection covering all relevant criteria guarantees constant quality at high throughput rates. However, this is only one aspect. The other important benefit is cost reduction. The inspection results are permanently available and allow for quick interventions to prevent material waste. The whole inspection process is recorded and documented in detail – valuable information available to customers' data management systems. Individual inspection zones with different tolerances, accuracies, and algorithms enable precise defect detection and, at the same time, the acceptance of tolerable quality variations – unnecessary waste drops to a minimum.
Our vision systems can be integrated into different types of machines, even under space-critical conditions; upgrading of older machines is possible. Our print inspection systems save time – and time is money. They can be calibrated for a new product type quickly. Most parameters are automatically determined by the system and no hardware adjustment is required. Changeovers can be done in a matter of minutes. When developing our GUI we asked ourselves two questions again and again: What can we do to make using this system even easier and which information does the operator need at what time? The result: Operators with no reservations that need only short training periods.
[Gal] The advantages of machine vision-based products are divided into several categories:
Reducing production cost and the cost of quality: automatic inspection solutions enable printers to reduce make-ready time and material by more than 70% (compared to manual operation) and run-time material waste by more than 50%. These incredible savings are reflected directly in the bottom line, improving profit in this margin-pressured market.
Improving production utilization: by using machine vision, printers tend to run their production machines faster and with fewer stops, increasing production capability while using the same equipment.
Eliminating customer rejection and improving customer satisfaction: quality assurance solutions eliminate customer rejection and very often are used as a marketing tool to obtain high quality, high margin jobs.
Based on the experience we gained with more than 1600 automatic inspection platforms installed, at more than 700 customers, these benefits are well proven and are the main drivers for the growing demand for automatic inspection solutions and for repeat and multi-unit orders from existing customers.
3. Can you briefly describe your machine vision-based solutions for the print industry? Specify the type of machine vision hardware (frame grabbers, smart cameras, embedded vision processors, etc.), lighting arrangement and camera details. Why did you select the specific components in your designs?
[Juergen] Our goal is to provide standardized solutions rather than solutions that are specific to each customer. Hence, a wide selection of standardized components and an extremely flexible hardware concept allows us to offer standard solutions, optimized for each specific customer, yet extremely robust and easy to maintain.
All of our 100% inspection systems do employ line scan cameras. For print inspection (SMASH Print series), we typically use color line scan cameras. The open and versatile architecture of ISRA’s inspection systems allows us to use various suppliers’ cameras, and, where reasonable, to provide systems with black and white, color, or combinations thereof.
We opted for the use of high-performance line scan cameras from the leading brands, rather than developing proprietary solutions for these applications. We see the particular benefit of this decision in providing the latest available technology (why compete with companies that have specialized in optimizing camera technology?), plus the additional advantage of long-lasting spare part availability. Given that customers in the printing industry often use their printing presses for several decades, this, too, is an important criterion.
Particularly on wider webs, the amount of data produced by a suitable vision system performing a 100% inspection exceeds the performance capabilities of even the latest personal computers available in the market. In order to scale our systems according to the necessities of the application, we at ISRA VISION employ our own intelligent frame grabber cards, the SMASH WEB Processor card. Being optimized for surface vision applications, these cards themselves are scalable and perform the major part of image preprocessing, compensation of web wandering, reference comparison and several other tasks. These cards, integrated into a master-slave-PC environment common to all our surface vision systems guarantees the unprecedented performance of our solutions.
The flexibility of the approach not only allows employing and synchronizing multiple line scan cameras within one vision system, but also combining several setups of cameras (called camera banks) within one system. As an example, imagine printing flexible packaging, where one camera bank monitors the printed results on the topside, while a secondary bank monitors the backside of the web, where a cold-seal has been applied.
For print inspection, our systems have to deal with a variety of substrates, ranging from opaque diffuse materials to (partially) transparent, glossy, or highly specular products. With a wide range of standardized illumination modules we can provide ideal lighting conditions for any process. In print inspection applications, we mostly employ CCFL light sources, which in comparison to LEDs, have an economical advantage. However, if space or lifetime of the light source is of paramount concern, adequate LED sources are readily available as well.
The software structure and the user interface (GUI) are made with the intention of assisting the printer by providing useful information rather than creating another burden or obstacle to overcome. Hence, our software engineers paid special attention to a most versatile framework, which, together with a user administration system, can be easily configured to the individual tastes of our diverse customers. An underlying SQL database keeps track not only of the current inspection task, but of the comprehensive history of inspection jobs performed with the system.
On a corporate level, optional data mining software, available for all of our surface vision systems, allows for a consolidated view on the performance data from several production lines or even production sites, including the possibility of the creation of a wide range of reports to give a comprehensive overview of production efficiency, yield, and quality levels at the different lines and sites.
[Roberto] PROXIMA is a ‘‘Near The Line’‘ printing process control system. It is fully automatic and accurate. It is internally calibrated. Its main application is in the banknote industry to monitor all the steps of the process (press, plates, raw materials, operator). It has proven to be very effective in detecting deviations before they reach a critical level. PROXIMA is a Statistical Process Control system that prevents waste generation by fully analyzing a printed sheet in 100 seconds (600 Mb of data), performing automatic inspection based on geometric, colorimetric, security features and print integrity measures and controls.
A second system is REGOLO. This is an on-line narrow web print inspection system.
REGOLO is very flexible and adaptable to provide the most effective and cost convenient solution. It can range from 1 to 6 cameras to inspect print integrity, plate registration, color, fluorescence and variable data (OCR, bar code) on both sides of a web, at 300 meters/minute (full resolution, square pixel). Our systems can inspect different product sizes: single banknotes (4’‘), narrow web (20’‘) or sheets (32’‘).
For the time being, we do not use smart cameras. We use frame grabbers and the most powerful PCs (dual Xeon quad-core). Most of our applications are based on line scan cameras, grey-scale processing, using color-prism and color tri-line (line scanners). The number of pixels range from 512 to 10,200; 4K-8K is the most common. When needed (mainly for banknote inspection), we use lenses designed according to our specifications (MTF, aperture, vignetting and distortion are all parameters that we specify precisely).
For illumination, in the past we have made extensive use of fibre optics light lines, with halogen lamps, and aperture fluorescent tubes. After extensive testing, one year ago we switched to white LEDs, which are now available with sufficient power, stability and acceptable light spectrum (white).
[John] PCI typically uses digital non-frame grabber-based cameras and PCI-designed xenon strobes. PCI chose digital because it doesn't require a proprietary frame grabber and provides relatively high performance in terms of resolution and frame rates. Xenon strobes were selected because most of our applications are high speed.
[Christiane] Our systems are PC-based with frame grabbers. We use high-end standard products. Currently the operating system is Windows XP.