You know that situation. You are desperately trying to open a package of peanuts when sitting in an airplane and you knock over your neighbor's coffee cup. Or you buy freshly packaged salad in the supermarket, and it is already decaying the next day.
Figure 1. For laser structuring, the selective absorption of the different materials allows ablating the individual material layers without harming the layers underneath.
These are examples where the quality of the packaging is almost more important than the actual product. In such cases, lasers open up many possibilities to further optimize the quality of packaging: selective weakening of single packaging layers (easy opening) or localizing tiny perforation holes for ventilation (easy ventilation).
Selective micro-weakening
Almost all packaging films are multi-layer structures that consist of several films, one on top of the other, each with a thickness of several 10 µm. Each layer has a different function: PET is for stiffness and aroma preservation, PE for sealing and tear-proofness, PP for vapor impermeability, aluminum for general light hermetic sealing, and paper for stiffness. Easy opening means selective weakening of the mechanically supporting layer without affecting other functionalities.
Mechanical scribing or punching methods are fast and considerably simple, however, they cannot differentiate between the individual layers. The result may be that either the mechanically supporting layer is not completely separated (scribing depth too low) or the light or humidity hermetic layer is harmed (scribing depth too high). For some time research has been going on to find a dependable method that selectively acts on the different layers. Laser structuring with CO2 lasers offers a new solution. This technology benefits from the different optical properties of the single film layers.
Polymers, metals, and paper layers show significantly different absorption and reflection reactions in the wavelength range of CO2 lasers (9.4 to 11 µm). Most of the polymer materials in the packaging industry, such as PET, PS, or OPA, absorb the laser beam efficiently at the typical wavelength of 10.6 µm. The result is local heating, and the polymer layer is completely ablated in the small scribing area. A few polymers such as OPP or even LLPE are, however, somewhat transparent to the central wavelength of 10.6 µm. Aluminum layers act as perfect mirrors for all wavelengths of the CO2 laser at low laser powers, that is, they neither absorb nor transmit the beam.
The laser method allows for localization of the desired scribing structures on almost all kinds of packaging materials, provided the correct wavelength is used and the scribing is applied from the right side. Two examples illustrate this. The first, PET-Al-PE, is a classic material mix in food packaging, such as is used for coffee pouches. The selective absorption of the CO2 laser allows the complete ablation of the mechanically supporting PET layer without affecting the two layers below. The mirror effect of the aluminium layer helps to use the laser light most efficiently.
The second, OPP-PE, is a film structure often used for detergent packagings. As the absorption of the laser beam in PP is stronger than in PE, the PP layer can be structured selectively without affecting the other layer. In this case, the laser beam can be applied from either side, that is, it could also penetrate the PE layer first and then ablate the PP layer.
Figure 2. Example of a laser-structured web-shaped packaging with metal coating (a), application from medical device packaging (b), and example of a packaging film for food industry (c).
Depending on the application, either fixed optics or scanner optic systems may be used for structuring. In most cases processing is done on web-shaped material. For scribing structures in the web direction, the laser with fixed optics is exactly positioned above the web. Due to production technology reasons, most bags and pouches require cross web scribing structures. Here lasers with fast scanner heads are used compensating the web speed and applying almost any structure even at high speeds with on-the-fly-technology. Also semicircles or diagonals of opening structures on the edge of a bag may be scribed.
Typical scribing speeds may be 10-15 m/s, depending on material and technology applied. This leads to material speeds of 100-250 m/min. The minimal scribing kerf width is about 100 µm with fixed optics and 200 µm with scanner optical systems. Thus cuts that are almost invisible to the human eye can be scribed into many materials.
Laser structuring advantages
The range of typical applications of laser structured easy openings is wide. Tiny traces of laser structuring can be found on packagings of power-muesli bars, animal food, detergents, and even cosmetics and medical products. For these applications laser technology competes with mechanical methods using rotating knives or barrel punches. Laser technology features a number of advantages.
The special optical selectivity of the different film materials allows structuring of single layers, individually, without affecting other layers. The process is so stable that it can be integrated into industrial high-volume production without great cost and effort.
Lasers work contact- and wear-free, which guarantees high process reliability even at high processing speeds. Laser technology allows large freedom in packaging design and thus opens up new possibilities for easy opening. Due to good focusability of the laser beam, mechanical weakening is low and almost invisible. If desired, structures can be localized either crosswise or in flexible curves. Some methods even allow for lines to be scribed at small distances, which opens up new kinds of easy opening.
Micro air holes
The shelf-life of perishable food has great influence on its economic value. A typical example is so-called convenient food packagings, that is, pre-cut and ready-for-use fresh meals containing salads or vegetables, which may be prepared quickly. A longer shelf-life can make the difference between profit or loss of such products. The freshness of these products is basically determined by air-ventilation and preservation of humidity.
Laser perforation technologies open up new possibilities to make micro-holes into packagings selectively. The right size and number of holes can guarantee air circulation and preservation of humidity.
Figure 4. Laser-perforated polymer film. The melted edge of the holes (insert) avoids microcrack formation.
Here a CO2 laser (10.6 µm) with high pulse intensities is used. Different from selective structuring of easy openings, the laser intensity is so high that all layers are ablated thermally, and therefore the laser perforates a micro hole. Lasers with optimal beam quality are used with laser power depending on the material—in the range of 100-2000 W.
The perforation pattern is generated either via special polygon mirrors for high-volume production or via fixed optics with synchronous pulse triggering. This procedure only allows perforations in lines. Up to 16 lines can be made in parallel with the laser system on a single web. In one line, 50 holes/cm can be perforated—thus allowing not only for ventilation but also for easy-tear-off applications.
Special optical set-ups like the patented variable pattern generator (VPG) allow the user to choose the distance between holes flexibly in one line (distance between holes 4-500 mm).
Typical hole sizes in films without aluminum layer range from 60 to 400 µm. Aluminum-coated films require considerably more laser power, and the thermal input is correspondingly higher.
Advantages of laser perforating
For perforating polymer films, laser technology competes with needle technologies and flame perforating.
The advantages of the laser technology include smaller holes, which are crack-proof due to the micro melted edge. Moreover, lasers work contact- and wear-free, whereas needles get dull or break. Due to the special polygon technology and the extremely short laser pulses, it is possible to perforate holes at high web speeds (for example, 300 m/min).
Rofin/Baasel Lasertech (Starnberg, Germany) offers several lasers for use in the packaging industry. The StarScribe Easy, a mobile turnkey CO2 laser, can be integrated into existing production lines and winding systems. The flexible articulated arm allows fast beam positioning without considerable additional effort.
The StarScribe Easy S(cribing), a quasi-CW-laser with 100 W maximum output, is used for scribing applications in web transport direction with continuous or interrupted lines.
The StarScribe Easy P(erforation), an air-cooled pulsed CO2 laser with 100 W maximum output, is used for perforating few holes at low to medium winding speeds. The StarScribe WD (web direction) is a high-performance integration module that can be integrated into existing slit-cutters. The StarScribe CW (cross web) is for flexible scribing structures that are not parallel to web direction. And the Perfolas Film perforates packaging films. Most applications are for easy ventilation, but easy opening is also a field of application.
Klaus Wanner ([email protected]) is in sales and Dr. Thomas Renner ([email protected]) is the marketing director of Rofin/Baasel Lasertech, Starnberg, Germany.