Copyright Reed Business Information, a division of Reed Elsevier, Inc. Mar 2003

At a time when trends in converting and printing are moving toward small-part features and intricate patterns, advances in technology are increasing the versatility of laser applications.

Carbon dioxide (CO ₂ ) laser converting has long been recognized as the premier technique for precision slitting, cutting and kiss-cutting of intricate patterns and closely spaced features on thin film materials. Now, leading equipment manufacturers are combining sophisticated laser-beam steering with X-Y tables, machine-vision systems, software control and mechanical dies to bring the precision of lasers to large-format converting of printed materials, and high-speed contour/shape-cutting on roll-to-roll systems.

Laser Converting 101

Laser converting systems generally consist of a CO ₂ laser and a beam-steering system. Modern equipment uses 100- to 500-watt, sealed, no-maintenance lasers. These new sealed lasers feature slab-discharge technology and offer the benefits of compact design, simple operation, no maintenance and low operating costs. Manufacturers have demonstrated that these lasers need no scheduled maintenance to the laser head over more than 2- 1/2 years of continuous operation.

The simplicity of a slab laser's design makes it very easy to install, integrate and operate. Moreover, these lasers are far smaller than older types of CO ₂ lasers, but without a reduction in output power. For example, a unit capable of producing up to 500 watts of average output power (with 1.5-kW peak power) measures just 11 x 9.5 x 47 in. and weighs only 135 lbs.

A further advantage of slab-discharge technology is its fast pulsing characteristics. Compared to conventional CO ₂ lasers, the high-frequency pulses of slab-discharge lasers have fast "rise-and-fall" times relative to the duration of the pulse. Therefore, the laser power level can be tightly controlled and is efficiently delivered for cutting or drilling, rather than for merely heating the material to be processed. The result is a clean cut or hole, with little heat-induced collateral damage. This efficiency, coupled with fast pulsing (up to 100 kHz), allows slab lasers to process materials far more quickly.

Advent of beam steering

These advances in laser design have led directly to the development of modern compact laser-converting stations that feature beam steering. High-speed beam steering consists of mirrors mounted on computer-controlled galvanometers to guide the laser beam over the surface of the material to be processed ( See Figure 1 ). Software allows the design of complex shapes and parts with intricate detail and sharp corners that are difficult and time-consuming to create by other manufacturing methods.

Processing thin-film materials using a laser requires accurate and reliable control of both the amount of energy delivered by the laser and the location to which it is delivered. Advanced techniques coordinate beam-steering motion, web speed and laser power to deliver the right amount of energy for every speed. The result is a consistently high cut-quality with no burn marks along any shaped path. By adjusting laser-operating parameters, different processes such as cutting, kiss-cutting and perforating can be carried out in a single operation.

Large-format converting

Galvanometers have been used to steer laser beams in converting systems for many years. The great value of galvo-systems lies in the precision (2 to 8 mil) and speed (60 to 300 in./sec) with which they operate. However, their shortcoming lies in the small area over which they can operate effectively. Although these systems are specified to operate at up to a 20-in. field of view, the quality of the cuts at the edge of the field is markedly inferior to those at the center, even with optical compensation.

The difference is due to laser-beam divergence and angle of incidence changes from the center to the edge of the field of view. Consequently, in practice, converters typically use only a 16-in., or less, field of view. In cases where high precision, such as 2-mil., is desired, the field of view can be as small as 4 in. Such a small field of view is impractical for most converting applications.

So how can converters cut in a large-sheet format and still take advantage of the speed and precision of a galvo-system? In the past, converters would switch to linear X-Y tables that move under a stationary laser beam, to cut large sheets. This is an unsatisfactory solution because even though linear X-Y tables operate over a large area, their cutting speeds are well below what a galvo can achieve.

Tile-to-tile precision

A far better solution combines a high-speed galvo-system with a large area X-Y table and cuts patterns using a tiling method. In this method, the large work surface is divided into smaller areas (or tiles) that are laser-processed using a galvo-system. Once a tile is completed, the X-Y table steps over to an adjacent tile, which is also laser-processed using the galvo. Software control automatically performs the tiling function and ensures perfect tile-to-tile registration on such Galvo-X-Y systems.

A further advantage of using this method is that it can be combined with vision systems to cut, kiss-cut or perforate preprinted materials. In preprinted materials, registration problems sometimes occur because prints are not always in the same location on each sheet. By combining machine vision with this method, each tile can be viewed individually and cutting parameters can be adjusted through software to produce perfect cuts for each print. For example, during the manufacture of television remote controls, electroluminescent (EL) screen-printed features must be accurately positioned with respect to cut holes. Galvo-X-Y is combined with vision systems to ensure perfect registration of holes to screen-printed features.

Shape cutting at high speed

For high-volume converting applications, mechanical dies are the preferred cutting tools because of their lower cost, high throughput and speed. However, the trend today is toward more intricate patterns and smaller features that cannot be cut using mechanical dies. Consequently, a method that combines the high-throughput of diecutting systems with the high precision of galvo-based laser cutting would enable the converting industry to meet evolving market needs.

Using a similar approach to large-format converting, high-speed precision cutting can be achieved via a laser module with a galvo-based beam-steering system united with a mechanical diecutter. In roll-to-roll and sheetfed applications where both standard and intricate features are present, the mechanical die cuts non-precision features while the laser cuts precision features. A vision system aligns precision features to a pre-determined reference.

For example, during the converting of flexible circuitry for microelectronics, some features are too small and too closely spaced to be cut by mechanical dies. These features must be precision-cut by a combination laser/die system to achieve both speed and accuracy. Cutting speeds for combination systems depend on the number and intricacy of the small features to be cut, but typically, an all-laser system can cut 30 to 150 fpm, while a combination laser/die system can cut 200 to 600 fpm.

In roll-to-roll cutting applications, rotary dies stamp out shapes from a variety of thin-film materials. This technique is fast and convenient, but it has no flexibility in that a different rotary die must be made for each unique pattern. Furthermore, in cases where odd shapes are to be cut, rotary dies frequently lead to large amounts of wasted material.

In contrast, because galvo-based laser-cutting systems are software-controlled, changes to a cut or score pattern can be made quickly and inexpensively. In addition, recent advances in galvo-based systems have enabled roll-to-roll laser contour slitting and shape-cutting to achieve cutting speeds of up to 1,000 fpm.

One of the most significant advantages of using galvo-systems is that odd shapes can be interlaced using nesting software to minimize material waste. For example, in the simple case of slitting rolls of circular labels, a rotary-die pattern arranges the labels side by side across and along the web ( See Figure 2 ). In this pattern, material between the labels is unused and, therefore, wasted.

In contrast, using software-controlled laser cutting, the cut pattern can be modified such that each row of labels is staggered with respect to the one next to it ( See Figure 3 ). A galvo-based laser system then contour slits between the labels to create multiple rolls of labels from one large roll. These smaller rolls can then be kiss-cut by another galvo-set, or they can be saved for rotary cutting. In this way, maximum use is made of the material and process speeds remain high. This technique, also applicable for contour scoring, is currently being used to score films for resealable pouches.

Conclusion

Although laser-converting systems are ideal for precision applications, they cannot replace mechanical dies in all applications. However, new systems that combine laser modules with mechanical dies, X-Y tables and vision systems give converters the tools they need to serve evolving customer needs.

Chris Chow is v.p. of engineering for Preco Laser Systems, LLC, in Somerset, WI. He can be reached at 715/247-3285, fax: 715/247-5650, E-mail:cchow@precolaser.com , www.precolaser.com

David Clark is director of marketing for the Materials Processing Business Unit of Coherent, Inc. He can be reached at 530/889-5386, fax: 408/764-4825, E-mail:david.clark@coherentinc.com , www.CoherentInc.com