Lasers impact food packaging

The availability of small-footprint sealed CO2 lasers and the development of shorter output wavelengths are spurring growth in thin-film converting applications

Advances in sealed CO2 laser technology continue to increase the performance, extend the reliability, lower the size, and reduce the overall cost of ownership of these lasers. Several recent articles in this magazine explain how sealed CO2 lasers, as rugged, reliable tools, have enabled applications across a diverse spectrum of industries. One of the newer application areas is in converting-turning paper, plastic film, and other web-type materials into pre-formed packaging. This article describes how recent advances in CO2 laser technology are enabling a fast-growing segment of this market-laser converting of flexible food packaging.

Laser converting

Flexible packaging for processed food and drinks can take a variety of forms and shapes, including envelopes and pouches, with reclosable packages being a particularly high growth area. These packages are made from thin-film materials such as polyester (PE), aluminum foils, and polypropylene (PP)-often in the form of biaxially oriented polypropylene (BOPP). These materials are frequently combined as composite structures that incorporate two or more layers. A particularly common film composite is a bi-layer laminate of PP and PE, where the PE provides the barrier layer, in contact with the food, and the graphics-grade PP is the outer, printed layer. Total film thicknesses are usually on the order of 100 microns, even for stiffer stand-up pouches. In the converting process, these films are initially handled on a fast-moving master roll and lasers may be used for scoring, cutting, trimming, and perforating this material. For example, laser scoring is used to create easy-open packages and laser perforation is used for breathable packages.

The non-contact laser beam provides advantages compared to traditional methods using mechanical tooling, such as no tool wear, leading to consistent results. Flexibility is also important because today’s product cycles are short. Mechanical methods with tooling costs do not lend themselves to short runs and quick switches between film types and packaging formats. And with real-time control of laser parameters, the flexibility of laser converting also allows manufacturers to obtain consistent scribing results even when they may receive film materials that vary in thickness.

Obviously in a high-volume/low-margin business such as film converting, laser reliability and overall cost of ownership are critical considerations. It’s also important for the laser to have a small footprint so that it can be buried into the converting equipment. In terms of laser output, these applications usually only require modest power levels in the 30- to 100-watt range, because of the relatively thin multi-laminate film structures involved.


FIGURE 1. This compact, sealed laser delivers 30 watts of power but weighs only 6 kg (13.5 lb).
Click here to enlarge image

This makes today’s small CO2 lasers the ideal tools for the flexible fold packaging market. Coherent’s low-power sealed Diamond CO2 lasers offer a combination of high reliability, low cost per watt, and small package size-around the size of a typical shoebox (see Figure 1). These folded waveguide lasers are ideal for processing thin plastic (polypropylene and polyester) films at speeds greater than 300 meters/minute (1000 ft/min). In addition to providing a cost-enabling solution for these medium power applications, the small size of these new lasers enables multiple lasers to be arrayed across a wide web (e.g. a 1.5-meter/60-inch wide master roll), delivering unprecedented process flexibility. Moreover, these lasers have a maintenance-free lifetime in the 10,000-hour to 20,000-hour range, which meets the needs of the typical high-throughput converter.

Higher speed

The advent of compact CO2 lasers with alternative output wavelengths is another development that is expected to have a significant impact on the flexible food packaging market. The reason is that most polymer films have an infrared absorption spectrum that consists of numerous sharp peaks. Consequently, small shifts in laser wavelength can have a dramatic impact on absorption efficiency, because the films used in flexible packaging involve very thin films that often only absorb a small percentage of the incident laser power. For this reason, increasing the absorption coefficient of the thin film(s) by optimizing the laser wavelength can dramatically increase the processing speed for a given laser power level.

The output of most CO2 lasers is centered at 10.6 microns but the use of wavelength-selective optics inside a folded resonator configuration can produce other wavelengths, such as 10.2 microns. The impact of commercial 10.2-micron lasers on thermoplastics was recently investigated in Coherent’s applications laboratory, summarized in a presentation last year.1 For unstretched polypropylene films, there is only a small difference in processing speeds between 10.6 and 10.2 microns. But with the commonly used thin (30-50 micron) BOPP films, the difference was dramatic (see Figure 2).


FIGURE 2.Comparison of PP film cutting speeds at 10.2 and 10.6 micron laser wavelengths.
Click here to enlarge image

Incidentally, by using different isotopes of carbon dioxide, laser output wavelength can be shifted even further, which can impact other applications. For example, Coherent has found that the 9.4-micron laser produces optimum results when processing Kapton (polyimide) films commonly used in electronics and other industries.

Ganging multiple lasers

Alcan Packaging, (Chicago, IL), is a leading supplier of flexible packaging for the meat, dairy, and juice markets, as well as for health and beauty products (see Figure 3). As far back as 1985, the company began laser scoring of films. It now makes extensive use of sealed CO2 lasers for converting thermoplastic films and foils. Alcan Packaging’s Sr. Process Engineer Finian Flood explains, “The advent of compact, rugged CO2 lasers has enabled us to replace die boards and other mechanical tools for slitting, perfing, and scoring. Often we create smaller packages in a jumbo roll (up to 400 mm/15.75 inches wide). The small footprint of today’s sealed lasers is a major advantage for working even wider rolls, where we use multiple lasers across the web, usually with fixed optics, for slitting and scoring along the web. We also have a line that uses optics to scan the beam across the web for complex shapes.”


FIGURE 3. The advent of compact CO2 lasers allows converters to use multiple lasers across a wide web.
Click here to enlarge image

Given the availability of sealed CO2 lasers with powers up to 500 watts and beyond, why not use one of these high-power lasers rather than multiple low-power lasers? Flood notes that the use of multiple lasers provides maximum flexibility because it allows real-time, independent control of each beam. As an example, he cites scoring. “Scoring is increasingly popular for easy-opening food packages. Just a simple thing like small variations in film thickness could cause problems with mechanical scoring, but our laser processing can easily accommodate these variations.”

Laser digital converting

Since its inception in 1998, LasX Industries has grown to become one of the market leaders in laser converting. The company provides both LaserSharp converting equipment solutions and contract processing. Founder and President Bill Dinauer explains, “It seemed clear to us that U.S.-based converting could not stay indefinitely with mechanical tooling. Product cycles are now too short to justify tooling for on-shore converting operations. In addition, the advent of compact CO2 lasers has enabled the development of ‘digital converting’-computer-controlled laser converting. This is a particularly exciting development and we believe that laser digital converting is now roughly at the same stage as sheet metal laser cutting in the early 1980s, that is, on the cusp of widespread acceptance in production applications.”

Much of the equipment sold by LasX for flexible packaging applications is in the form of retrofits of existing slitter-rewinders. This cost-effective strategy integrates existing functions of the slitter-rewinder with laser digital converting processes such as laser scoring and perforating, thereby providing multiple processes at no additional labor/time surcharge. Two retrofit configurations are available, one taking the form of fixed-beam down web processing using multiple lasers, similar to the Alcan Packaging machines, and the other as dynamic (scanner-based) cross web processing. Here the laser beam is rapidly scanned both across and along the roll to produce any pattern required. Depending on the web width, pattern complexity, and repeat pattern spacing, a typical scanner-based retrofit incorporates between two and four laser assemblies. LasX also has integrated its equipment into digital printers for digital converting of narrow webs (e.g. labels).

Very recently, LasX has been one of the first companies to implement 10.2-micron lasers for use on BOPP and laminated (PP/PE) polymer films. The company has demonstrated large increases in production speed with 10.2-micron lasers, allowing smaller, less-costly lasers to be specified, a significant factor in justifying the economics of laser digital converting.

Conclusion

For high-volume, low-margin processing such as converting, compact, sealed CO2 lasers now offer the ideal combination of low cost per watt, high reliability, and small footprint. This allows laser converting to be embedded in a process line for minimum cost/labor impact. Because many of the packaging films are organic polymers, the ability to supply these lasers at optimized wavelengths will only serve to make these lasers even more attractive tools for systems integrators and converters alike.

Reference

Hoult T., and Dinauer, W., “Improvements to laser processing of thin polymer films - using non-standard Novel Laser wavelengths,” SPE Conference, ‘ANTEC 04’, May 2004, Chicago, USA.

David Clark (david.clark@coherent.com) is with Coherent Inc., Santa Clara, CA, (www.coherent.com).

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