Vision Inspection Systems Drive Profits

With their fast changeover and high-speed capabilities, the latest generation of presses certainly provide the platform for a lean production environment in which waste is driven out of the workflow. But without reliable, high-performance press controls that enable the press to run uninterrupted at full speed throughout the run, these advances in press technology cannot be exploited to full potential. Solutions are urgently needed, not only because of demands for higher quality brand presentation, but problems such as errors on pharmaceutical or medical packaging can have serious consequences.

Thankfully, a new generation of systems are able to offer a standardized, objective means of measuring color and precision inspection, in-line, without compromising performance. Furthermore, systems are available that can handle the full variety of film and paper substrates, including transparent, supported, unsupported, and reflective types.

Color standardization is vital

A common industry color standard is increasingly necessary as brand owners insist on identical color consistency for product packaging at a global level. Spectophotometric measurement, using spectral response and L*a*b* values, provides the most objective way to describe color. But it is the frequency with which results can be taken that makes in-line solutions so much more reliable. While hand-held devices may be capable of taking a few readings during the course of a production run, the latest in-line systems have software powerful enough to take readings, from as many as 72 image targets, continuously.

Configuration

In flexographic and gravure printing applications, an in-line inspection and color measurement system comprises a spectral camera, a video camera, a lighting system (to illuminate the target areas and ensure the substrate image is captured under suitable, uniform conditions), high-powered software to process the image, and a central data repository to enable at-a-glance viewing of print defect information and registration and spectral data, as well as communication with other systems in the workflow. The sensor and lighting are located after the final printing station.

Why in-line?

An in-line inspection and color measurement system provides a number of important benefits:

• The ability to monitor quality throughout the production run, for 100 percent of the printed web, not just at the end of each roll;
• Elimination of much human intervention;
• A platform for a reliable, stable means of monitoring quality and fast remedial action;
• Ability to reach color targets faster, thus reducing material waste.

Camera specifications

Faster cameras are available to enable the latest generation of gravure and flexo presses to perform without quality compromise at higher speeds. Recommended inspection camera resolutions are 250 dpi, although a maximum of 2,500 dpi is possible; cameras must be able to detect defects from

Cross-substrate resolution is critical. The printing process is a dynamic process. For instance, an ink splash, spherical as it approaches the substrate, becomes a long, oval stain due to the substrate's relative velocity. In gravure, doctor blade defects are serious, potentially running for hundreds of feet, but may be just 0.1 mm wide. The narrowest field of view (FOV) per camera is 180 mm, although optics to allow for narrower FOVs are possible. There is no limit on the maximum width.

Lighting

Typically, several cameras are needed to carry out the inspection, with solid-state lamps providing a constant illumination level on the substrate's target inspection area. The key criteria that determine the camera and lamp types for a packaging press are the mix of substrates used, maximum printing speed, maximum print width, and the desired resolution.

Lighting systems with adjustable intensity and specific angles enable in-line inspection of transparent, translucent, reflective, and opaque substrates. Three lights would be included in the configuration: a high-intensity top light for non-reflective and non-transparent surfaces; a low-intensity foil light for reflective surfaces and inks; and a back light, both illuminating and providing contrast for transparent or semi-transparent substrates. White LED temperature-controlled, actively cooled lamps offer longer life, more reliable consistency, and can even can be configured to illuminate color in the UV spectrum. Invisible inks are increasingly incorporated into packaging for anti-counterfeit or safety purposes.

High definition detection

Thanks to software advancements, on-the-fly high-definition viewing of dot-profiles is possible. Images are captured at up to 2,500 dpi and made available to the operator via the user interface. As a result, a host of defects common to central-impression (CI) flexo, such as pin-holing, bridging, doughnuts, and halos, can be identified and remedied without production stoppages.

Golden Template

An important topic in flexo printing is the Golden Template Comparison. A Golden Template is a master image that is captured by the inspection system when the press is running good copy. Every printed repeat is compared to that original Golden Template. High-performance software and hardware is needed to track movements in the print direction and compensate for web stretch, as well as to make accurate comparisons between printed output and the target on the fly. There are two important benefits of a Golden Template. First, it results in a fixed reference with a stable, repeatable quality. Secondly, it is more sensitive to slow-growing defects such as register, streaking, and color changes.

PDF verification

For added assurance, a PDF verification program makes structural comparisons between the Golden Template and the customer-approved digital artwork file. This alerts the operator about cylinder set-up or plating errors, without the need for visual checks and resolves the problem of inspecting packages that include multi-language information or different alphabets. A fixed PDF reference eliminates the problem of emerging defects such as subtle color changes that can be missed by alternative systems that even periodically change the reference image, let alone roll from one image to the next with every impression.

Software integration

A waste management system should allow control of the slitter/rewinder speed, waste removal, and roll splicing. This frees the operator from relying on error-prone, time-consuming manual detection methods. New tag-less defect tracking features use weight to determine waste removal and avoid the need to contaminate a roll with tags.

A central repository audits, stores, and generates reports of both current and historical data, enabling trend analysis and defect traceability. Important benefits of such a system are ease of integration within the corporate IT network for maximum accessibility and the ability to store job settings, thus simplifying set-up times.

In addition to color and web data, it can store a fixed Golden Template reference image for each job, and pick up every small defect deviating from this perfect image taken at the beginning of the run in good copy. Full integration of the inspection equipment into the press and the company communication network is also possible. The central database provides a data connection between the inspection system and slitter/rewinder equipment for fast, efficient marking and removal of defective roll material. Performance data is stored and made available for analysis via the database server.

Closed-loop color measurement enables feedback to seamlessly affect the output of other systems in the print workflow. Industry players are collaborating to allow data to be easily exported between compatible software systems, to extend the closed loop from color measurement to color formulation. Customer-defined targets (e.g., supplied by a color standard library, a Color Exchange format (.CxF), or color samples), are evaluated. Spectral curve data is obtained and loaded into an ink formulation software, to create the spot-color formulae, which are used by an in-house dispensing system to dose the ink stations. The color targets are loaded into the color measurement system software. The color target data is referenced with live color measurements that can be taken at full press speeds. Any color deviations are automatically captured and reported in ∆E values. This measurement data can be sent back automatically to the ink formulation software, via .CxF file, where any ink recipe changes can be generated.

Conclusion

The modern in-line, spectrophotometric, integrated color measurement and inspection system has come of age. It reaches quality targets faster, reducing start-up material waste; it gives reassurance of accuracy throughout the production run; it provides a means of minimizing labor costs. This technology, therefore, is an essential factor in an industry where getting perfect results first time, every time is not a luxury, but a given. pP