Automatic web inspection technology has come of age as a valued quality control tool for producing labels and packaging print. Barry Hunt reports on some recent developments
The human eye is not very effective at detecting minuscule defects on a fast moving web. Assuming we blink once every six seconds and each blink lasts 0.2 seconds, our eye would skip 3% of the product if the web were moving at the relatively slow speed of 0.5m/sec.
Imagine what’s missed under normal conditions. That’s why automated inspection technology is so important when aiming for zero-defect performance during printing, as well for finishing operations too. There are certainly many things that can go wrong.
Production defects can include ink streaks, plate damage, spots of ink or dust (called hickies), colour variations, mis-register from colour to colour or colour to die-cutting, ghosted images and imperfect varnishing. Any of them could lead to an expensive waste of material or the rejection of an entire print run, if not detected in time by the press operator.
That’s why web inspection systems are found on most narrow-web label and carton presses and are practically mandatory on mid and wide-web flexo or gravure packaging presses. For some critical applications, especially pharmaceutical labelling, a separate web-fed inspection process provides a final line of defence.
Functions include the detection of missing labels and verification of bar codes, which may augment the final slitting and rewinding stages of roll products. As a market, the packaging sector is highly dynamic according to Ray Scraggs, a director of Futec UK, with users aiming to save on material costs and reduce production risks.
“Obviously, printers do not want to have work rejected, especially when using expensive film and foil packaging materials. It’s become an even more important issue given the greater accent on quality involving rationalised designs for today’s globalised brands.” At their simplest, web inspection systems can comprise strobe lights or rotating mirror systems with a drum of front surfaced mirrors mounted horizontally and viewed through a small panel to ‘stop the action’.
Most companies use camera-based web inspection systems, with varying degrees of sophistication. Cameras are positioned close to the web and can be fixed or moved manually or automatically. They allow operators to view and inspect the quality of fast moving webs of print in real-time just as if the web was stationary, using a suitable colour monitor.
To take an electronic snapshot of the printed image involves keying-in such details as the circumferential size of the print cylinder to obtain the repeat length of the printed image. A zoom lens facility allows image enlargement to help bring jobs quickly into registration from start-ups.
Most video web systems can create a master image for making comparisons with the fast-moving image using a split-screen monitor. Sometimes two monitors can work in tandem, such as doctor blade streaks, colour variations or hickies, to track print defects that are especially hard to detect.
So-called ‘active’ video web systems can additionally learn from these stored images or a proofed sample and generate prioritised levels of visual and/or aural signals. Press diagnostics and statistical printouts give a form of statistical process control.
These functions allow the operator to access stored data from a press run in order to track when and where a defect occurred and how much of the material was defective. With some software it is also possible to compile a roll report so that defective label or packaging material can be removed at the rewinding stage.
Other options may include programmable positioning to select and store random positions on the web at different zoom ratios. When the ‘run’ button is pressed the system automatically displays each chosen position. A back strobe option allows the checking of ‘front-to-back’ registration.
Cameras with a motorised traverse move in both a lateral or linear plane for automated and constant scanning. The operator programs the interval between each new image and the camera’s movement in both directions.
Some models have modules for inspecting both sides of the web simultaneously and handle reflective surfaces like metallised film or paper, as well as metallic inks.
Another clever module can inspect the relative position of transparent varnish and cold-seal liquid adhesives to printed matter as well as spot any defects. Modules that verify bar codes in terms of modulation, print quality and contrast are now widely used for packaging applications.
Most conventional video web systems still use either single or three-chip CCD (charge-couple device) cameras with red, green and blue (RGB) filters designed to process CMYK printed images. Three-chip high-resolution cameras have three sets of single-colour filters.
A prism splits the light entering the camera so all three chips see the same subject matter. These cameras can find small defects and still display them on larger monitors, making them ideal for exacting process colour work.
Firing a high-intensity xenon strobe light to illuminate the web and act as a shutter creates the video image or frame. Motorised focusing and an iris diaphragm regulate the light entering the camera’s optical system.
Zoom lenses have magnifying dioptres that govern the trade off between the size and clarity of an image area with the magnification. For example, a 6:1 lens with a standard +7 dioptre sees an area of not more than 62x82mm, giving a 20 times magnification.
Naturally, there is also a connection between the speed of the web and the quality of camera resolution. For example, using the highest 2000x2000dpi resolution on a 330mm wide label press would limit the inspection speed to around 80m/min. This could increase to 200m/minute and over with the more commonly used 500x500dpi resolution.
A new generation of digital cameras has changed these relationships because these systems allow the operator to see more of the web, including repeats that can be as wide as the web.
The operator selects a part of the image on the screen and the camera centres on this area, thereby creating a digital repeat.
An example is BST ProMark’s Genius Digital, which has replaced its widely-used Eagle video web system (originally developed by ProMark in the US before being absorbed by the German company). Options include closed-loop register control and colour monitoring, while the bar code verification module identifies print problems on the fly and monitors the scannability of standard bar codes.
Colour monitors are a critical component. Most systems use Sony, JVC or NEC non-interlaced SVGA models, which ensure flicker-free picture quality. System control comes from a software-driven interface, variously using keypads, toggle switches, colour-coded membrane switches or small touch screens.
Some high-end systems offer extra large fields of view with 41 times magnification and dual camera systems that capture front-to-back composite images on packaging papers, translucent and semi-opaque substrates. As with all computerised systems, the cost of video web inspection systems has tumbled but even the simplest models are far more powerful and versatile than five years ago.
This competitive market is served by scores of European and US suppliers but not all offer systems suitable for demanding packaging applications.
Even the most sophisticated models can only examine a part of the web at any one time. Therefore, it is possible to miss flaws that repeat at frequent intervals down one side of the web and similar random defects.
Depending on circumstances this limitation may not be a drawback but makers of so-called visual systems state that their products allow a full 100% continuous inspection of the web repeat. They can identify either random or repeating flaws in real-time across the full width of the web.
Confusingly, some vendors use the term ‘100% inspection’ to describe certain video web sampling systems. They may inspect all of the web repeat or all printed bar codes – hence 100% – but only within a certain sampling period.
True 100% visual inspection systems use high-resolution line-scan CCD cameras, rather than video web cameras. Compact modules, containing linear arrays of CCD sensors, are mounted across the entire web width. Lighting options include fluorescent tubes to cope with different materials including foils and films.
Random and repeating defects are shown in real-time for specific repeat lengths on colour monitors. Earlier systems had monochrome monitors that identified defects using coloured visual codes or flags that were prioritised in terms of possible action.
Besides print defects, visual inspection systems will also pick up tears, rips or bad web splices, again using a series of algorithm programs to cover specific faults, perhaps as small as 0.25mm in area. As with video web, lower component costs and more powerful processors have resulted in more sensitive systems capable of fine-tuning for specific applications.
Futec Inc pioneered this technology in Japan during the late 70s and, with 3500 installed systems, claims market leadership in this sector. Its quality control line scan systems use web markers that are sufficiently accurate to eliminate the use of register marks.
The range includes EasyMax line scan systems now sold and supported in certain European countries by BST ProMark. Ray Scraggs points out that Futec and similar Japanese suppliers were encouraged to develop sophisticated systems because in Japan quality inspections are made at all stages of packaging production – from the press through to the packing line.
The use of visual web inspection systems with post press operations, notably off-line slitting and rewinding, has gained ground throughout many developed markets.
The German-made Print Inspector systems from Geiger Vision Systems, now part of Advanced Vision Technology (AVT), are retrofitted on many rewinders and presses as OEM equipment.
Geiger developed the free-standing Validator. It has unwind and can run with existing slitter/rewinders. A web buffer unit, or accumulator, allows users to simultaneously inspect, rewind, slit and inkjet number if required.
A new twist is the PDF Print Inspector, an off-line scanner which compares films, plates and other media to master PDF origination files.
It gives automated comparison between original PDF graphics file and the printed label or packaging and is aimed at the pharmaceutical industry.
AVT itself offers the Print-Vision /Apollo and Apollo-Label range of 100% visual inspection systems, which can be mounted on a press or inspection rewinder. Products include the PrintVision/Pro and Genesis range of video web inspection systems.
A recent development is AVT’s reflective support module for inspecting reflective materials across the full web width using an LCCD camera and lamp unit. It is offered as an option for the PrintVision Apollo and Argus systems.
Besides viewing the web and alerting the operator, the latest systems also interact with the main press functions and produce data for analysis. According to Malcolm Lear, managing director of TecScan Electronics, future systems could incorporate the same passive functions of many current systems but include ‘machine vision’ capabilities.
“They could combine video and software programs to control the monitor, as well as the variables associated with press management, such as registration, print colour, web tension, print length control and monitoring. This approach informs TecScan’s Advanced Video Inspection System that compliments the Colour-scan CS7 passive inspection system for narrow and wide-web presses.
This package gives both 100% repeat length inspection and video web inspection. Another long-established supplier is Tectonics International. Its Sabre range is supplied in both a passive inspection mode or as an active system with positive feed-back.
As with the Lynx models the video camera has an asynchronous shutter, enabling full-frame capture of up to 28 frames/sec or 168m/min if the repeat is 100mm. The system alerts operators of colour drift and stops the press or inspection machine if necessary. Tectonics also offers the Cub, a small entry-level video web system but its 60 times magnification is powerful for most defects in its class.
Finally, Van Pamel RGB Systems from Belgium offers the latest digital cameras for its narrow-web inspection system. Features include standard auto-scan, label-check and split-screen capabilities.