Laser Levels Playing Field

The widespread use of digital printing has revolutionized the folding carton industry in terms of both processing speed and flexibility. In turn, this puts pressure on downstream finishing processes to deliver compatible speed and automated flexibility. A new generation of digital finishing machines is doing just that, thanks to the availability of compact, sealed lasers at the kilowatt power level.

Digital Printing-Flexibility To Maximize Visual Impact

Ideally, the folding carton is an industrial product with high aesthetic impact (see Figure 1). In today's competitive marketplace, innovative and eye-catching packaging gives a product an edge, allowing it to stand out on the shelves among dozens of comparable items.

With the rise of technological advancements in laser processing capabilities, folding cartons are no longer limited to simple paperboard containers. Laser processing creates new possibilities to add visual impact to folding cartons without decreasing processing speed.

Traditional die constraints hinder packaging design options, whereas lasers can offer flexibility to create both simple and intricate embellishments, patterns, and display windows in a wide array of materials in varying thicknesses. These design features add interest and showcase the package's contents or unique attributes, creating a lasting impression on consumers. Moreover, the final product enables improved interaction with the user and involves intricate cuts and tight angles that cannot be cut by die boards.

Fortunately, design innovation no longer requires a sacrifice of production efficiency. Laser modules offer multiple processing capabilities to create an array of features in a single workstation without the need for additional tooling or dies.

This includes adding features such as perforations, slits, display windows, and easy-open tear strips in addition to traditional score lines. Laser processing takes folding carton manufacturing to a new level of visual appeal while maintaining functional elements in a cost-effective manner, even when producing short runs.

Today, manufacturers often create a small run of a few thousand cartons to assess their real market impact. This small run flexibility and market agility have been enabled by high throughput digital printers. The latest models can print 22.5×14.3-in. single-sided sheets at 22 sheets/min. Some carton manufacturers are running two such printers in parallel for a top speed of 44 sheets/min. With lasers, maximum finishing speed is die-line dependent and also a function of paper thickness, but digital finishing can only become the absolute tool of choice if it can at least match 44 sheets/min, with headroom for future speed increases. The bulk of the folding carton market consists of paper products with a thickness of 9 mils or more, but plastic sheets and plastic-coated papers also are used, and these also must be processable at high speed.

By digital finishing, we mean finishing that is performed according to stored CAD/CAM designs accessed and used in real time. Because smarter digital machines incorporate a bar code reader, a bar code then can be printed on every sheet indicating its particular finishing pattern. Contrast this to traditional physical tooling with die boards, in which new tooling is required for even the smallest change.

Digital Finishing with Lasers

The laser is the critical technology that has enabled digital finishing. It is an omnidirectional cutter with no tool wear issues that can be moved very quickly under computer control using low inertia mirrors. Plus, by changing the speed at which the beam is scanned across the carton, it is simple to use the same tool for cutting and scoring in a single process. Also, by switching the beam on and off, perfs can be created.

Which laser? There are now many types of industrial-grade lasers and the digital finishing of folding cartons needs a laser that can deliver several key benefits simultaneously. A laser cuts materials such as paper and plastic by applying intense local energy to vaporize or melt the target material. So first and foremost, it is critical that the laser beam is absorbed efficiently rather than reflected or transmitted.

But the absorption of light by paper and plastics is very dependent on wavelength. Therefore, the most important criterion is that the laser wavelength is one that can be absorbed efficiently by all the common materials used in folding cartons. This need is best met by the carbon dioxide (CO2) laser, a very mature technology dating back nearly 50 years with eye-safe infrared output at a wavelength of 10 microns.

HIGH POWER LASERS WITH SMALLER FOOTPRINT

Since one of the main values of a folding carton is aesthetics, the edge quality is very important. However, some infrared lasers like the CO2 laser easily can create discolored and even charred edges that would be cosmetically unacceptable. In “laser-speak,” this is called the heat-affected zone (HAZ) and must be minimized at all costs. It represents material that has been intensely heated by the laser but not quite heated enough for removal.

This HAZ can be virtually eliminated by using a laser that offers two characteristics — high beam quality and fast pulse rise time. Beam quality is important because the cutting/heating power of a laser is increased by focusing the beam to a small spot on the material. This spot must have clean edges rather than a fuzzy area with low power that would create a HAZ around the cut.

To avoid charred edges, it is equally important that the laser output be pulsed with fast rise and fall times. Sealed “slab discharge” CO2 lasers meet both these criteria as well as providing a very high power-to-size (and weight) ratio. As shown in Figure 2, their fast pulse rise time means most of the laser power is delivered well above the threshold for cutting, rather than just heating and charring.

Surprisingly, until recently there was no perfect laser for high-speed finishing of folding cartons. Most cartons are made primarily of paper, which is a relatively tough material to cut with lasers, particularly thicker paper and high-end papers with a high clay content. To deliver the 44 sheets/min target described earlier requires at least 500W and preferably a kilowatt of laser power, in order to confidently handle 160-lb and heavier paper stocks. But sealed slab discharge lasers were limited to 500W, limiting their speed in this application.

One solution was to use two slab discharge lasers, thereby increasing the size and cost of the machine, often to unacceptable levels. Another solution was to use a traditional flowing gas CO2 laser, which made for an even larger and more complex system, negating many of the size, agility, and simplicity advantages of digital finishing.

Fortunately, this situation completely changed in 2010 with the development of sealed lasers at the 1 kW level. In addition to providing the necessary power and fast pulsing characteristics, these new lasers are more integrated than traditional carbon dioxide lasers, with the RF power supply incorporated entirely within the compact laser head.

As a result, one of these lasers can power a compact digital finishing machine (see Figure 3) that delivers all the advantages of edge quality, complex shapes, and single sheet flexibility and that can keep pace with two high-speed digital printers — even with thick and coated paper stocks.

Digital printing has had a huge impact on the folding carton industry. Now a new generation of laser-powered digital finishing machines is poised to have a similar far-reaching impact with software flexibility replacing hard tooling and the die-board going into the same obsolescence as the print blanket in this market.

SUPPLIER INFO

William Dinauer is the owner and founder of LasX Industries Inc., St. Paul, MN, manufacturers of laser digital converting equipment and services for processing non-metallic materials at high production speeds. Dinauer holds an M.S. degree in Manufacturing Systems Engineering and a B.S. degree in Agricultural Engineering from the Univ. of Wisconsin-Madison. Contact him atwdinauer@lasx.com

Eduardo Arteaga, commercial print digital finishing engineer, has been with LasX Industries for five years. He holds an M.S. degree in Mechanical Engineering from the Central Univ. of Venezuela and an MBA from Hamline Univ. Contact him at earteaga@lasx.com.

Andrew Held is product marketing manager, CO2 lasers, for Coherent, Santa Clara, CA, manufacturers of the Diamond E-1000 and other CO2 sealed laser systems. He holds a Ph.D. in Physical Chemistry from the Univ. of Pittsburgh. Contact him at andrew.held@coherent.com.

The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to the author(s).


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