PROFILING USER CODING IN THE PACKAGING INDUSTRY

By

Natalie J.

Bruton

Laser marking is becoming the preferred means of coding product packages by many industries, gaining dominance over traditional coding technologies. Marking systems using either sealed CO 2 or TEA CO 2

lasers code an array of package and container materials. The reasons for coding, how laser marking works, the effects on different materials, and which type of laser is most appropriate for a given job are discussed by Bruton, all in terms of real-world applications.

Product marking is essential for carrying out such general activities as lot process control, after-market tracking, anti-counterfeiting, industry-specific identification requirements relating to health and hygiene regulations, and accurate product traceability.

Until 1976, these codes were produced by ink-based printing, hot stamping, or embossing processes. That year, the package coding indus­try was revolutionized by a laser solution that

An a s s o r t m e n t of p a c k a g e s marked with laser c o d e r s (purple frame) are surrounded by 1. A

CO2 laser m a r k i n g s y s t e m and 2. T E A CO2 laser marking s y s t e m s . T h e s e photon s h o w e x a m p l e s of laser c o d i n g on : 3. gold foil of a l iquor bo t t l e : 4 . beer and minera l water labe ls ; 5. b o t t o m of an H D P E p h a r m a c e u t i c a l conta iner : 6. a bl ister p a c k ; 7. a p last ic phar­m a c y conta iner , where the s i l k -sc reened ink has been removed by the laser leav ing expirat ion and lot c o d e informat ion; 8. the bot tom of a per fume bott le , where the c o d e is both e legant and unobtru­sive; 9. a c o s m e t i c cap, where the laser s y s t e m actual ly c r e a t e d the p a c k a g i n g g r a p h i c s by removing the fine metal l ic c o a t i n g of the c a p to e x p o s e the p l a s t i c u n d e r n e a t h ; and 10. a c h o c o l a t e bar wrapper.

addressed the industry's demands for improved reliability, permanent indelible codes, efficient and cost effective processes, and increased concerns for the environment (or decreased desire to use inks and solvents).

Since 1976, approximately 15% of the laser coding sys­tems worldwide are used by the packaging industry. The demand for laser solutions continues to grow at a rapid rate in direct relation to demands for readable, permanent codes that satisfy government and manufacturing account­ability.

How laser coding works In contrast to traditional marking systems, codes produced by lasers are permanent because the mark alters the surface of the package by selectively removing or changing the surface material. This is accom­plished when the focused laser light is transformed to heat energy on the surface of the packaging material. The energy is absorbed very close to the surface, the extent of which is determined by the wave­length of the laser used, the pulse length (which can be as short as one millionth of a second), the number of pulses fired and passes made, and so on. The exact effect the energy has on the area being

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marked depends on the interaction between the package substrate and the laser.

The packaging industry relies mainly on carbon dioxide electrical discharge gas lasers with wavelengths of 10.6 μm, such as pulsed transverse excited atmos­pheric (TEA) CO 2 and modulated continuous wave (cw) sealed CO 2 systems, because they can easily mark common packaging materials (plastic, inked paper or board, foil, glass, etc.). However, Nd:YAG and excimer systems are also used in specific applications where the 10.6 μm wavelength does not affect the substrate (i.e., Tefzel plastic coating and highly reflective materials).

The CO 2 lasers do not have to be high powered—a modest average power of up to 100 W is sufficient to mark most packaging materials. Image 1 depicts an example of a modulated sealed CO 2 system; Image 2 shows a pulsed TEA CO 2 system (pages 24-25).

The principals of operation for the sealed CO 2 and TEA CO 2 are very similar. A pulsed (or continuous) electrical discharge takes place transversely to the opti­cal axis between two specially shaped electrodes that have been precisely positioned along each side of the gas volume. The energy coupled into the gas from the elec­trical discharge creates population inversion and gives rise to laser action. The difference between the two sys­tems is visible in the technology that produces the mark style, the system configuration, and its flexibility. These differences are discussed below.

TEA CO 2

TEA CO 2 lasers have typical energy outputs ranging from 2-6 J. Average power levels are rated from 25-60 W and peak power around 5 MW with a relative­ly short pulse length of 0.2 μsec. Since the lasing takes approximately 1 μsec, the system can easily code on moving products. The imaged mask optical configura­tion used with these systems is identical to the common projection photolithography format: Light passed through a stencil mask is imaged by a lens system onto the sample with a demagnification ratio. These systems, although excellent for high speed applications and for producing solid character codes, are large and once installed do not easily accommodate line changes. The systems require input of gas and cooling water to assure effective and efficient operation. However, technology advances are being made to these systems.

In 1995, a significant technological leap was attained in the design of TEA CO 2 lasers manufactured with a solid-state modulator (SSM) that totally replaced con­ventional gas discharge thyratron high voltage switch­ing circuitry—an industry standard on TEA CO 2 lasers since the mid-1980s. The patented SSM technology does not require a high voltage power supply (1 kV ver­sus 30 kV), and eliminates missed marks due to laser self-firing on start- and warm-up time; the laser is always ready to fire since no charge sensor is necessary. In addition, the new technology is warranted for five years or one billion shots. In contrast, thyratron­equipped systems are warranted for only one year or 100-200 million shots.

Additional advances are being achieved though the use of programmable stencil mask systems capable of providing increased code variability. These automatic stencil systems can have up to 8 stencil mask disks pro­viding 40 code variables per stencil. Other features include steering the beam via galvanometers lined to product location, speed for coding across packaging webs, and beam splitting, which allows for various mark locations on one package.

Modulated, cw-sealed CO 2

Other systems use a modulated cw-sealed CO 2 laser. Although the cw CO 2 laser is not pulsed, it can be mod­ulated at frequencies exceeding 10 kHz, and can be super pulsed as long as the duty cycle is adjusted to maintain the same average power. Since peak power can be an important parameter in generating enough instantaneous temperature rise to mark some materials, the ability to increase the peak power can be critical in achieving a good mark. The two scanning methods used with cw CO 2 laser markers are dot matrix and scribing (also called stroke marking).

Currently, there are two separate dot matrix tech­nologies/philosophies that use modulated cw-sealed CO 2 laser and several variations of the scribing technol­ogy. For this article, we will focus on dot matrix mark­ing systems (vs. scribing), which have more application in the packaging industry.

Both technologies for dot matrix coding put dots on the package in the form of coded messages and use the benefits of the sealed CO 2 lasers (small, compact, and typically requires only electrical input). One design uses a single sealed laser tube with one set of optics. Each dot, as it is produced, is directed/scanned in the desired code format. A spinning multifaceted mirrored polygon directs the dots to the proper loca­tion. With 34 dots achievable per facet, the user has complete flexibility to produce up to four lines of code or graphics in various dot matrix configurations. In addition, the laser tube is designed for longevity and reliability, and provides consistent dot energy. The polygon's position is interlinked with the production line speed, code requirements of size, and dot dual time (pulse length). The system is designed for coding products in motion, but can be used to code stationary products. It can also code at high production line speeds with the addition of a galvanometer, which scans the dots in the x-axis.

An alternative technology uses a discreet laser to pro­duce each dot {i.e., 5 X 7 dot matrix uses five lasers in a vertical line, 7 X 7 matrix uses seven lasers in a vertical line, etc.). Each laser has a set of optics for focusing each specific dot. To produce additional lines of code either a galvanometer system is used to reposition the dots to the proper location or an additional bank of lasers is used for each line. The lasers are interlinked with the moving production line. This design offers high-speed, single-line capability as each dot is discreet—a true "dot-on-demand" system. While this system has the advantage of high speed, the disadvantage of varying

Optics & Photonics News/May 1997 26

dot energy related to the tube life or loss of a tube can be critical to code integrity.

Both systems offer excellent programmability and compete with ink jet technology. The challenge to these systems is not necessarily laser technology, but applica­tion parameters. How is the product to be coded? What is the laser interaction with the material? What happens when the line (start-up or shutdown) or material changes? A laser coding system manufacturer needs to understand his/her technology, and also understand the environment where the system will be installed.

The modulated cw CO 2 "dot matrix" system offers the packaging industry four major advantages over pulsed TEA CO 2 laser coders and scribing systems:

They are compact, portable, and self sufficient, requiring only an electrical input with no external cooling system or CO 2 gas supply; Since the laser is modulated and produces laser dots, the code is computer-generated and controlled, allowing absolute code flexibility; Packages can be marked when stationary or on the fly, depending upon the system; and They feature a highly focused beam (dot) and the highest peak power (225 W) available, which is criti­cal for coding on hard-to-mark materials such as glass and certain plastics. C O 2 laser systems, whether dot

matrix, stencil/mask, or scribing mark­ers, feature very low operating costs, and make precise and permanent codes on irregular, curved, or textured surfaces while preserving the integrity of the most delicate packages. The unique qualities of lasers also provide solutions for difficult applications such as coding on the underside of packages or in lim­ited access places—even coding through certain materials like transparent over­wraps and films. While these laser sys­tems still compete with various ink and mechanical coding processes, it is becoming clear that they are a cost effective and viable choice for package coding.

Nd:YAG lasers As stated above, in the packaging industry CO 2 type laser systems are used far more often than other laser systems. Their speed and low power requirements are perfect for packaging industry applications. How­ever, certain applications require the use of an Nd:YAG laser. Unlike the gas media used in CO 2 lasers, the doped crystals found in Nd:YAG lasers respond to excita­tion by generating near infrared light and thus the reaction between the lased light and the substrate will be different than that realized with the CO 2. Nd:YAG is typ­ically used to mark on hard and/or highly

reflective materials, such as metals, ceramics, and certain plastics. The mark/code is produced by scribing/steering the beam to form the desired characteristics of the code. However, here the product must be stationary, inhibiting the use of these systems for high-speed packaging applica­tions.

Effects of laser light As noted, the package substrate, type of laser, wave­length produced by that laser, and pulse length all effect the final look of a package code. Ultimately, it is the interaction with the material that determines whether or not a laser solution is feasible for a particular applica­tion. Depending on the substrate, the code is achieved by an overcoat being removed to expose a contrasting substrate, a shallow indentation being created in the material, or a color change occurring in the material. On glass, due to the thermal impact, the mark appears as a frosted etching. On coated anodized metal, the coating is removed to reveal a contrasting subsurface. On many plastics and anodized metals, a color change occurs due to energy absorption. For example, laser codes on polyvinyl chloride (PVC) appear in golden col­or tints, while on PET, the laser produces an etched effect. And, in some cases where laser codes are pro­duced on paper labels and ink coated plastics or metals,

a specific colored target area is incorporated on the package design. Here, the inked surface is selectively removed, revealing the original material underneath and thus creating a contrasting mark.

The two laser parameters most critical in determin­ing the form of the mark are laser wavelength (microns) and peak power density (watts per square cm). The peak power of a laser is determined by the pulse energy divided by the pulse length so that lasers with a short pulse length can often have very high peak power (in the megawatt range). The peak power density can be increased further by focusing the beam onto the plastic. Typical TEA C O 2 lasers have a pulse energy of 5 J, pulse length of 1 μsec with peak power of 5 MW. Modulated C O 2 (dot matrix) systems have a pulse energy is 0.01 J, pulse length of 50 μsec, and peak power of 200 W. It would appear from these values that the modulated dot matrix systems would have difficulty marking plastics, but in reality, because the dot is highly focused, they can actually produce codes with higher contrast then the pulsed TEA C O 2 systems.

The wavelength of the laser determines the absorp­tion depth of light in the plastic. For a thermal process, a sample that does not absorb the laser light will not mark. The most common marking mechanism is the thermal process whereby the absorbed laser light causes

a rapid temperature rise locally in the plastic. The absorption of light follows an exponential form such that the absorbed energy E (per unit volume) at depth z is given by

where a is the absorption coefficient at that wavelength for the plastic being marked, 10 is the laser peak power density (per unit area) incident on the plastic, and Δt is the time duration of the laser pulse. A qualitative description of the heating process and temperature rise (per unit mass) for a volume of material at depth z is

Temperature rise = (heat added) -(heat conducted away)/(specific heat of plastic)

where the heat added is determined by the light absorbed in the plastic at depth z. The heat lost is deter­mined by the thermal conductivity of the plastic and the temperature rise is determined by the specific heat capacity of the plastic. Since the laser pulse is finite in duration, this description must be integrated over time to provide an accurate description of the marking process.

Some plastic materials in their pure polymer states— such as polyethylene, polypropylene, and other poly­olefins—are transmissive to the C O 2 wavelength (10.6 μm). To laser code these plastics, the material must be modified with light-absorbing additives, Kaolin being one example. (Adding this clay compound can actually reduce the cost of the product's container since the Kaolin displaces more costly petrochemical materi­als.) In the case of polyethylene containers with Kaolin additives, the color of the laser-produced code depends on the pigment used to tint the packages.

The most common additives used to impart laser markability to polyolefins are mica particles coated with titanium dioxide or other metal oxides. One rea­son for their popularity is that mica products are already used to create pearlescent effects (something that is considered desirable in the cosmetic and toi­letries market) in polyolefins and other plastics. Mica also has long been accepted by regulatory agencies in North America and Europe for use in consumer prod­ucts.

The bottom line is that each material reacts differ­ently with each type of laser. And while ink-coated products, PET, and glass are excellent applications for C O 2 lasers, highly reflective materials may require the use of Nd:YAG lasers. Ultimately, an actual test on the specific material is the true indicator of which type of laser should be used. This is only a starting point. To be an effective supplier of lasers to the packaging industry, the laser system manufacturer must not only work with the material, but must understand the packaging indus­try as a whole, including the key driving factors, the production and environmental parameters, and the spe­cial requirements of each market segment (i.e., cosmet­ics and pharmaceuticals).

Coding solutions for packaging industry The driving factors in the packaging industry are cost, speed, reliability, flexibility, and high uptime; but within this industry there are segments that have their own unique requirements. For example, pharmaceutical codes may show manufacturer, line and lot number, shift and batch code, as well as dosage and expiration date; and controlled substances must also be coded to assure security at all stages of manufacture and distribu­tion. Food and beverage packaging requires identifica­tion that shows batch, best before, and location of man­ufacture data to assure traceability in case of recall and to monitor shelf life of perishable products. As a suppli­er to this industry, a laser system manufacturer must provide systems that meet these requirements and be flexible to adjust to the changing demands of each mar­ket segment. The following is a description of various market segments and how laser coding is used.

Beverage packaging The major consideration in applications such as beer, distilled spirits, and beverage packaging is that the code on the container must not disappear when exposed to water, detergents, solvents, or the container's contents during sterilization and filling operations, or when it comes in contact with any packing materials used dur­ing transport. Whereas ink jet coders are not able to

cope with the pasteurization process of breweries, laser coding thrives in such an environment. Codes are applied either to the label or etched directly onto the glass or plastic bottle in a clear, clean, and indelible manner (see Images 3 and 4, page 25).

Another key consideration of this industry is the speed and reliability of the laser in an industrial envi­ronment. CO 2 lasers can code at 1,200 bottles per minute, 24 hours a day. They, along with the whole assembly line, are hosed down with water after each eight-hour shift, and thus must be waterproof. Fortu­nately, lasers can code wet surfaces while maintaining indelibility.

Milk producers demand a high degree of hygiene and cannot afford the downtime associated with refilling consumables such as inks, nor can they afford the potential hygiene problems of solvent spillage. Lasers offer a hygienic alternative and mark the labels on car­tons and bottles with consecutive codes and Julian and expiration dates.

Pharmaceutical packaging Security and cleanliness are the bywords in the pharma­ceutical industry. Federal agencies dictate the need for permanent readable codes; product traceability and adherence to regulatory standards often require this industry to use vision systems for code recording. A

Optics & Photonics News/May 1997 29

laser's ability to produce machine readable codes meets these regulatory demands and reduces the risk of con­tainers being reused without recycling. Further, as stated above, cleanliness is addressed through the elimination of inks and solvents from the coding process.

Some examples of laser coding in this industry are security codes on

Pharmaceutical containers (see Image 5, page 24). This system runs at a speed of 50 ft/min and shoots the code onto the bottom of the container. This HDPE container uses a material additive that inter­acts by absorbing the laser energy to produce a per­manent code; Blister packs (see Image 6, page 25). Traditionally, blister packages have been coded with embossing sys­tems. These mechanical systems can be inflexible and cumbersome. Laser systems can be easily integrated into packaging equipment, replacing older technolo­gy; and Plastic bottles (see image 7, page 25). Here, an eye care manufacturer uses an inked targeted area on the plastic bottle in which to provide a high contrast expiration and batch code.

Cosmetics packaging The cosmetics industry presents some interesting chal­lenges for laser coding systems. Perfume manufacturers,

for example, are mindful of packaging esthetics. Image 8 (page 24) shows an elegant, unobtrusive laser code near the bottom of a perfume bottle. This code assures both the manufacturer and customer that the product is not counterfeit.

Often, cosmetic and toiletry manufacturers will use the same package, changing only the label or graphics for different styles of the same product. For instance, a popular cosmetic manufacturer desired to replace the adhesive labels and preprinted caps used on its nail pol­ish containers with a method of on-line printing. How­ever, the company also required high contrast and very attractive codes and graphics on the container's metal coated caps. A laser system met the company's demands and helped achieve the additional benefit of inventory reduction. Prior to the installation of the laser system, the company had maintained a cap stock with preprint­ed information. The flexibility of the laser system allowed for on-line changing of the code to the different runs of cosmetics with the look of a printed cap (see Image 9, page 25).

Being transmissive to certain materials is a unique characteristic of CO 2 lasers that is extremely beneficial to some applications. A French perfume manufacturer codes through a clear overwrap onto the cartons contain­ing their perfume, which assures against counterfeiting.

Continued on page 81

Optics & Photonics News/May 1997 30

Laser Coding in the Packaging Industry Continued from page 30

Whether coding plastic bottles, aluminum tube con­tainers, paper labels, or glass bottles, code appearance, indelibility, accuracy, reliability, and flexibility are the guiding factors for this industry. Understanding these requirements and providing the customer with the option of codes in dot matrix or stencil type formats, as well as recognizing inventory control opportunities are musts for the cosmetics and other industries.

Food packaging The greatest opportunities for laser coding exist in the food packaging industry where ink-based coding has long been established. This industry's demands revolve around speed, cost, and uptime. Laser systems have seen tremendous growth in this market segment for two rea­sons. First, the European community has required man­ufacturers to include expiration and product freshness information on all packages and these demands are being translated to the rest of the world. And second, major packaging manufacturers are now accepting this technology.

Food packaging encompasses a wide range of prod­ucts and thus substrates. A popular candy bar uses laser coding exclusively for marking a two-line code with batch and expiration date information at a speed of 800 products per minute (see Image 10, page 25). The flow-wrap film has a target ink area and, when marked, pro­duces a high contrast code that does not smear or smudge during product processing. Prior to. using lasers, this candy manufacturer conducted an analysis of its ink systems and determined that line downtime was 5%—equaling approximately $160,000 per year (three shifts at 5,400 hours)—a rate that was directly attrib­uted to the ink systems being used. Once the laser sys­tems were installed, downtime decreased to under 0.5%, an improvement of 350%.

Laser coding systems do not require the use of inks or solvents and the downtime typically attributed to ink-based systems is non-existent with lasers. It is this cost-benefit relationship that must be established, since the initial investment for a laser coder can be higher than ink-based coding systems. For instance, one com­pany reviewed the consequences of ink-based systems when coding on frozen packages and determined that both rework due to smeared codes caused by package moisture and system downtime was easily eliminated with a laser system, thereby making the initial invest­ment worthwhile.

Other considerations With both pharmaceutical and food packaging, there are concerns about the generation of noxious vapors that occur when lasing on inked materials or plastics, as well as with the presence of glass particulate when cod­ing on glass. Fume and particulate extraction should be a consideration when developing and installing a laser coding system.

There are numerous applications in the packaging industry where laser coding systems can offer the solu­tions needed. As a supplier to the industry, the laser sys­tem manufacturer must be prepared to provide and assist the customer in the cost justification for switching to a laser system, be the expert for on-line integration, and assure that the system can produce the required code.

Laser coding manufacturers should seek to provide total solutions for their customers, starting from inte­gration into packaging equipment, communication with vision systems, fume extraction requirements, and long-term service and support.

In addition, the manufacturer should have a handle on what the customer's future desires are and anticipate this with system selection. Remember, the key factors in the packaging industry as a whole are cost, speed, relia­bility, flexibility, uptime, and long term support. A sup­plier must be able to meet and exceed these demands to be successful.

Outlook for laser markers While it is anticipated that today's technology will satis­fy the packager's requirements through the end of this century, there will be some further advances made dur­ing the interim. Most of these will relate to the system's mechanical configurations that will allow such things as increased throughput and larger area coding capability. One can also look for work being conducted that will eventually further advance the content of the marks themselves. And, it is reasonable to expect that smaller, more compact, and less expensive laser marking systems will be announced. In addition, today's systems will be used not just for coding, but for applications such as "easy open tear packages" and applying contest graphics on beverage caps. This goes well beyond basic batch and lot code applications.

The largest growth market for laser coding is the packaging industry. The present ink-jet market within the packaging industry is where the growth will occur. The market for laser-based systems is estimated to be worth $30 million annually, growing to $74 million by the year 2000. This can only occur if the laser coding industry continues to enhance systems to meet the demands for cost effective coding and provides innova­tions that compete with ink-based systems. This is the key to capturing more market share.

Bibliography 1. T. J . McKee, "How lasers work," Physics in Canada 51 (2),

107-114, (1995). 2. Lumonics World Wide Web Site: www.lumonics.com. 3. Derek Crosley, "Laser coding, making its mark on packag­

ing," Food & Drug Packaging 46(6), (1988). 4. J.H. Bechtel, Journal Applied Physics 46, 1585 (1975). 5. S .C. Hsu et al., Metallurgical Trans. 11B, 29 (1980).

Natalie J. Bruton is product line manager for Xymark and LaserMark laser marking systems at Lumonics/Oxnard Operations, Oxnard, Calif.

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