WO2010011227A1

WO2010011227A1 - Method and system for laser marking

Info

Publication number: WO2010011227A1
Application number: PCT/US2008/071138
Authority: WO
Inventors: Scott R. Sabreen
Original assignee: The Sabreen Group, Inc.
Priority date: 2008-07-25
Filing date: 2008-07-25
Publication date: 2010-01-28

Abstract

A system and method for direct laser marking of plastics capable of achieving improved dark-on-light marking, light-on-dark marking, and tone-on-tone color marking. The system and method includes a laser source and a laser beam focusing and steering device for directing a laser beam onto a plastic substrate in a predetermined pattern, wherein the laser source has a wavelength in the range of 1060 nm and 1070 nm, a pulse width in the range of 10 ns and 120 ns, and a beam quality mode with an M² factor near 1. A laser beam generated by such a laser source can be diverged while still maintaining the advantageous laser marking characteristics, thus increasing the working distance. Laser marking according to the present invention yields improved cosmetic and aesthetic properties, micro-marking capabilities, and improved machine readability characteristics for techniques such as bar codes and data matrix codes.

Description

METHOD AND SYSTEM FOR LASER MARKING

FIELD OF THE INVENTION

[0001] This invention relates generally to a laser marking system and, more particularly, to an improved laser marking system for dark marking on plastic objects having light surfaces. This invention is also applicable to light marking on plastic objects having dark surfaces and tone-on-tone color marking on plastic objects having dark surfaces.

BACKGROUND OF THE INVENTION

[0002] Plastics, used here in its broadest sense to include at least thermoplastics and thermoset polymers, are used across virtually every industry. One category of plastics that has a particularly broad application due to its unique modulus properties, including physical and chemical, is a thermoplastic polymer composition referred to as polyoxymethylene ("POM"), which can also be referred to as acetal and/or polyacetal. By way of example and not limitation, POM is commonly used in industries such as automotive, medical/health care, plumbing, gears, consumer products, heavy-duty industrial, conveyor belt handling, food containers, aerospace/military, and many others.

[0003] Following is a representative list of potential POM applications in various industries. In the automotive industry, POM can be used for in-tank fuel modules, fuel level measuring devices, seat-belt components, steering columns, window-support brackets and handles, speaker grilles, washer pump housings, door opener gears, and window lift motors. In the medical/health care industry, certain grades of POM can be used for medical fittings and medical components requiring sterilization or gamma radiation. In plumbing applications, POM can be used to replace brass or zinc components such as shower heads, ballcocks, faucet cartridges, washing machine gears, and various other fittings that benefit from the lubricity, corrosion and hot-water resistance, and lighter weight of POM. Consumer products incorporating POM include keyboards, toys, garden sprayers, butane lighter bodies, zippers, telephone components, sink sprayers, and bicycle components. Other heavy-duty industrial applications for POM can include couplings, pump impellers, conveyor plates, conveyor chains, gears, bushings, sprockets, and springs and other mechanical components that require dimensional stability. Applications of POM in the food industry include milk pumps, coffee spigots, filter housings, food conveyors, and other applications requiring manufacturers to provide certain grades of POM that may be FDA-approved. Applications further include injection molding, cast and extrusion POM in rod, bar, sheet, and tube stock shapes for machining of preformed shapes. As can be seen, the beneficial applications for POM are virtually limitless, thus the scope of this invention is not limited to any particular industry or application.

[0004] There are two basic types of acetal that are compounded, sold, and used in a particularly broad range of applications: (a) homopolymer; and (b) copolymer. One example of a commercially available homopolymer version of POM is offered by DuPont (Delrin®). Examples of commercially available copolymer versions of POM are offered by Ticona (Celcon®, Hostaform® and Duracon®). In addition to DuPont and Ticona, many companies worldwide sell their own versions of homopolymer and copolymer acetals, including but not limited to BASF (Ultraform®), DSM Engineering Plastics (Plaslube®), Korea Engineering Plastics (Kepital), Mitsubishi Gas Chemical Company (Lupital), and LNP GE Plastics (Fulton®).

[0005] In most instances, it is desirable to mark plastics, including but not limited to POM, with information or data for utilitarian and/or aesthetic purposes. The marking can be any information or data ("indicia") whatsoever, including but not limited to product specifications, product identification, warning-caution, serialization, alphanumeric information, bar codes/data matrix codes for component traceability, graphics, schematic diagrams, personal or company logos, trade names, trademarks, data or batch codes, symbols, patterns, personalized signatures, and the like. Over the years, a number of techniques have been used to print on plastics, including pad and screen, ink jet and sublimation printing, two-shot molding, labeling, ink or paint filling, embossing and hot stamping. However, each of these methods has one or more significant disadvantages such as high cost, complexity of operation, long set-up and process time, limited durability, and/or inability to easily vary the indicia being marked. As an example, the physical and chemical resistance properties of many plastics, including but not limited to POM₅ polyolefins, and nylons, often make it extremely difficult to obtain adhesion of printing inks or labels. The difficulties of ink printing on acetals such as POM are well known in the industry. Accordingly, companies are often faced with the decision of either using expensive surface pretreatment processes that may improve the adhesion of printing inks or labels to certain desirable plastics such as POM, polyolefins, and nylons (of course, this adhesion is not permanent and not indelible), or selecting an entirely different resin that is more expensive and may not have the same desirable properties but has better adhesion capability. Another concern is that some of the widely used surface pretreatments emit ozone and other VOCs that are not environmentally friendly. A further concern is that many surface pretreatments are affected by variation in temperature, humidity, and other environmental factors. Moreover, the results of the surface pretreatment may only be effective for a limited time and surface pretreatment processes can have deleterious effects on downstream manufacturing operations.

[0006J Laser marking is an alternative marking process that can potentially have many advantages when compared to conventional ink printing processes and labeling, including: indelible marking; high resolution capability; variability of indicia being marked; non-contact; lower overhead cost; fast cycle tune; lower piece cost; reduction of consumables cost; reduced risk of forgery; reduction and/or elimination of surface pretreatments; no curing time; reduction and/or elimination of inks and solvents; reduced environmental impact; and reduced maintenance. Given the many parameters that must be selected and controlled, however, development and implementation of an effective system for laser marking plastic materials requires knowledge and skill in many different technological disciplines, including but not limited to polymer materials science, laser enhancing additives, chemistry/chemical interactions, laser technology (hardware and software), optics, and systems integration and machine vision readable codes. For this reason, it is common for individuals or entities to combine their efforts in an attempt to better understand and implement the many disciplines critical to developing an improved laser marking system.

[0007] The quality and utility of a laser marking method or system can be measured by parameters such as contrast of indicia to substrate background color, sharpness of indicia, speed, uniformity, ease of use, cost-efficiency, reliability, variability, and adaptability. The relative importance of each of these parameters depends on the application. For example, in laser marking applications wherein the indicia must be machine visible (such as component traceability applications utilizing bar codes and data matrix codes), the contrast relative to the substrate, sharpness, and unifoπnity of the detailed laser marking lines, borders and adjacent spaces ("quiet zones") are critical. If the edge details (i.e., the sharpness) of the marking relative to the spaces and quiet zones are not easily distinguishable, then the purpose of the laser marking is defeated. As the machine code size decreases, the indicia becomes more condensed and the sharpness requirement becomes more and more important. For example, data matrix codes can be 1/8" square and smaller, which can result in exceptionally small cell sizes. In laser marking applications requiring sequential serialization or other variable information wherein each piece receives a unique indicia, the speed and ease of varying the indicia between each piece is of primary importance. If the content of the indicia cannot quickly and easily be modified, then the laser marking method or system may not be feasible. Specific to data matrix machine vision codes, small cell size is important to help maximize the information capacity, whether the information is in the form of numeric only, alphanumeric, ASCII, or some other form. For example, a data matrix comprised of 24 rows by 24 columns (Error Correction Code (ECC) 200) with one data region can contain approximately 52 alphanumeric characters or 72 numeric only characters or 34 8-bit ASCII characters. Information capacity of this level and greater is desirable for unit level component traceability, including but not limited to products subject to United States Department of Defense MIL-STD 130 and/or pharmaceutical e-pedigree laws, which are becoming more prevalent in the United States and elsewhere. Of course, a laser marking method or system that offers all of these advantages will result in the broadest range of applications and will offer the greatest commercial value.

[0008] Another important consideration when selecting a laser marking method or system is the color of the substrate and the desired color of the marking. For instance, it is well-known in the art of laser marking that light marking can be obtained on POM objects having dark surfaces ("light-on-dark marking"). However, even though light-on-dark marking of alphanumeric information or graphics on POM objects may be readable to the human eye, machine vision codes that require contrast, sharpness, and uniformity of the detailed laser marking lines, borders and quiet zones may not be satisfied. It is also well-known in the art of laser marking that dark marking on POM objects having light surfaces such as natural, white, and light-to-medium color tones ("dark-on-light marking") is exceptionally more difficult and, prior to the present invention, has yielded results that are unsatisfactory for most applications.

[0009] When using POM, high quality dark-on-light laser marking would have great value to many industries because it would remove any limitations when choosing the color or grade of the POM to be marked. For instance, an entity seeking to laser mark a POM product in a manner that is machine- visible is often forced to select a dark POM in order to take advantage of the improved contrast ratio possible with light-on-dark marking, as opposed to the prior art dark-on- light marking systems. Prior art techniques often also require that the dark POM have a smooth high gloss surface texture and no resin filler. Alternatively, an entity could be forced to abandon the use of POM altogether and to select a different plastic that could achieve machine-visible dark-on-light marking. As a result, a method or system of laser marking that provides high quality dark-on-light laser marking of POM would advantageously remove any constraints in choosing whether to use a dark or light POM. Without these constraints, designers and manufacturers can select from various grades of POM having specific properties desirable for particular end-use applications such as unfilled, reinforced fill (including mineral-filled, glass- filled, or fiber-filled), anti-microbial, appearance (high gloss, low gloss, stipple texture, smooth texture), and more. Using prior art techniques, these various grades and appearance factors would adversely affect the quality, sharpness, and uniformity of the detailed laser marking lines on the POM.

[0010] Over the last several years, various attempts have been made to achieve a true direct dark-on-light laser marking but none have yielded superior marking quality on POM combined with an efficient and reliable system. For instance, certain prior art systems claim to achieve relatively high contrast, but require such a high loading level of laser-enhancing doping additives that the bulk modulus properties of the plastic substrate could be adversely affected and could lead to delamination problems. Other prior art systems make similar claims but require that the laser source operate at or near its maximum power output. Operation of certain lasers at or near their maximum power output is likely to damage the laser over time, cause inconsistent marking results, decrease the sharpness of the marking results, and increase the cost of operation, thus such a high power output would not be practically feasible in many production environments. Still more prior art systems seek to maximize contrast of dark-on-light marking, but such contrast comes at the expense of sharpness and edge detail that is critical to optimizing machine vision readability of the laser marking. High contrast alone is not sufficient when seeking to maximize the utility of laser marking. As already discussed, a laser marking can have high contrast yet still not be sufficiently machine vision code readable such that it cannot achieve acceptable fidelity. Moreover, high sharpness and line edge detail in laser marking is also important for micro-marking (i.e., scaleability), human readability, informational schematics and diagrams, and aesthetically pleasing graphics and logos in general. Some prior art systems also claim to achieve dark-on-light marking but really only create a colored hue rather than a laser marking that approaches a true black on a light substrate. Some prior art systems also claim laser ablation removal of a pigmented coating layer which is an indirect laser marking method and multi-step process. Examples of prior attempts to achieve a true dark-on-light marking system can be found in U.S. Patent Nos. 6,489,985; 6,518,542; 6,627,299; 6,903,153; and others. It has thus long been desired in the laser marking field of technology to obtain direct dark-on-light marking that has both high contrast and high sharpness while also being capable of high fidelity, with a laser marking system or method that is both efficient and reliable and does not affect polymer performance.

[0011] What is needed is an improved laser marking method or system that allows for high quality dark-on-light marking of plastics such as POM, wherein quality is measured by parameters such as contrast, sharpness (i.e., edge detail), resolution (i.e., dots per inch ("dpi")), fidelity in machine vision applications, speed, uniformity, ease of use, cost-efficiency, reliability, variability, and adaptability.

[0012] It is therefore an object of this invention to provide a laser marking system that yields cosmetically improved dark-on-light marking results, light-on-dark marking results, and tone-on- tone color marking results as compared to prior art laser marking systems utilizing Nd:YAG, frequency-doubled Nd: YAG (532 run), CO₂, Excimer gas lasers, and UV lasers.

[0013] It is another object of this invention to provide a laser marking system that provides improved quality for dark-on-light marking using low loading levels of laser-enhancing doping additives that are unlikely to affect bulk modulus properties of the substrate. Improved quality for light-on-dark marking and tone-on-tone color marking can also be achieved using low loading levels in conjunction with specific colorants, pigments, dyes, and compounds (organic or inorganic).

[0014] It is yet another object of this invention to provide a laser marking system that provides improved dark-on-light marking quality, light-on-dark marking quality, and tone-on- tone color marking quality using a laser operating at a lower power output relative to its maximum available power output.

[0015] It is still another object of this invention to provide a laser marking system that provides improved dark-on-light marking quality having a relatively larger working distance (as measured along the z axis from the lens to the part to be marked) and relatively larger marking field (as measured along the x and y axes of the part to be marked), thus allowing laser marking of larger parts and/or increased numbers of parts per lot.

[0016] It is another object of this invention to provide a laser marking system that provides improved dark-on-light marking quality, light-on-dark marking quality, and tone-on-tone color marking quality at a marking speed that can be equal to or faster than prior art systems.

[0017] It is yet another object of this invention to provide a laser marking system that can be used for dark-on-light marking, light-on-dark marking, and tone-on-tone color marking of plastics without the need for replacing the laser source or other equipment. It is yet another object of this invention to provide a laser marking system that can also be used to engrave metals, including deep engraving, without the need for replacing the laser source or other equipment.

SUMMARY OF THE INVENTION

[0018] According to this invention, a direct laser marking system and method comprises a laser source generating a laser beam and a laser beam focusing and steering device for directing the generate laser beam onto a plastic substrate in a predetermined pattern, wherein certain parameters of the laser source fall within a predetermined range of values. The directed laser beam can be accomplished by "galvanometer" beam delivery (moving mirrors with a fixed lens), by "flying optic" beam delivery (movement of the entire optical assembly), or any other alternatives known in the art now or in the future. Thus, by utilizing certain laser parameters according to a preselected range of values, dark-on-light marking, Hght-on-dark marking, and tone-on-tone color marking may be achieved on plastic articles that is cosmetically improved, provides functional benefits in avoiding drawbacks of alternative printing processes, and has significantly improved machine readability relative to prior art marking systems. Preferably, the laser source has a wavelength in the range of 1060 nm and 1070 nm, a pulse width in the range of 10 ns and 120 ns, and a beam quality mode with an M² value near 1. Use of a laser beam possessing TEMoo beam quality mode with an M factor near 1 allows the beam to be diverged while still maintaining the advantageous laser marking characteristics such as spot size and power of the final focused beam on the work surface, thus increasing the working distance and marking field of the improved laser marking system and method. For instance, in a particularly preferred embodiment of the present invention, the laser beam having an M² factor near 1 is diverged using appropriate focal length lenses and collimation.

[0019] The invention provides a novel laser marking system and method for creating an improved dark-on-light, light-on-dark, or tone-on-tone color marking on a plastic article. The laser marking created according to the present invention has improved cosmetic and aesthetic properties. Moreover, the laser marking has significantly improved machine readability characteristics for both data matrix codes (i.e., smaller data matrices and smaller data matrix cell sizes) and bar codes (i.e., fine edge line details and spacing between bars) while achieving significantly improved fidelity. The quality of the laser marking achieved with the present invention has sufficient contrast, sharpness, and line edge detail that micro-marking is possible in dark-on-light, light-on-dark, and tone-on-tone color applications. The invention further provides a laser marking system and method that utilizes relatively lower loading levels of laser- enhancing doping additives that are less likely to significantly affect bulk modulus properties of the plastic article being marked. Moreover, the invention provides a laser marking system and method that does not require the laser source to operate at close to its maximum power output, thus potentially improving the efficiency and reliability of the system. The invention further provides a laser marking system and method that permits a relatively larger working distance and marking field, thus allowing laser marking of larger parts and/or an increased number of parts in a single lot. The invention also provides a marking speed equal to or faster than previously possible using appropriate vector ordering software programming techniques. In addition, the invention provides a laser marking system and method that can be used for dark-on-light, light- on-dark, and tone-on-tone color marking of plastics without the need for replacing the laser source or other equipment, thus resulting in a more time and cost efficient system. The invention also provides excellent laser marking of metal materials and many other substrate materials for cosmetic indicia, engraving, and deep engraving.

A BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be more fully understood from embodiments of the invention described in the detailed description together with the drawings provided to aid in understanding, but not limit the invention:

[0021] FIG. 1 is a schematic diagram of a laser marking system employing a photo masking technique.

[0022] FIG. 2 is a schematic diagram of a laser marking system employing a beam-steering technique.

[0023] FIG. 3 a is a photograph of a test chip bearing indicia marked with a prior art laser marking system, wherein such test chip does not include any laser material additive and is marked using a Nd: YAG laser.

[0024] FIG. 3b is a photograph of a test chip bearing indicia marked with a prior art laser marking system, wherein such test chip includes a laser material additive at 1.0% load level and is marked using a Nd: YAG laser.

[0025] FIG. 3 c is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive at 1.0% load level and is marked using a fiber laser having a selected wavelength, pulse width, and beam quality.

[0026] FIG. 4a is a close-up view of the test chip of FIG. 3b bearing indicia marked with a prior art laser marking system, wherein such test chip includes a laser material additive at 1.0% load level and is marked using a Nd: YAG laser.

[0027] FIG. 4b is a close-up view of the test chip of FIG. 3 c bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive at 1.0% load level and is marked using a fiber laser having a selected wavelength, pulse width, and beam quality.

[0028] FIG. 5a is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive at 1.0% load level.

[0029] FIG. 5b is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive at 0.75% load level.

[0030] FIG. 5 c is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive at 0.50% load level.

[0031] FIG. 6a is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (BASF Mark-It®) at 0.75% load level with no TiO₂ (natural).

[0032] FIG. 6b is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (BASF Mark-It®) at 0.75% load level with 1.0% TiO₂ load level (white). [0033] FIG. 6c is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (BASF Mark-It®) at 1.0% load level with 1.0% TiO₂ load level (white).

[0034] FIG. 7a is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (DSM Micabs®) at 2.5% load level with no TiO₂ (natural).

[0035] FIG. 7b is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (DSM Micabs®) at 2.5% load level with 1.0% TiO₂ load level (white).

[0036] FIG. 7c is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (DSM Micabs®) at 3.0% load level with no TiO₂ (natural).

[0037] FIG. 7d is a photograph of a test chip bearing indicia marked with a laser marking system according to the present invention, wherein such test chip includes a laser material additive (DSM Micabs®) at 3.0% load level with 1.0% TiO₂ load level (white).

[0038] FIG. 8a is a photograph of a white POM test chip bearing a data matrix code (overall data matrix size of 0.180" x 0.180") marked with a laser marking system according to the present invention, wherein such white POM test chip includes a laser material additive (DSM Micabs®) at 2.5% load level with 1.0% TiO₂ load level (white).

[0039] FIG. 8b is a close-up view of the white POM test chip of FIG. 8a bearing a data matrix code (overall data matrix size of 0.180" x 0.180") marked with a laser marking system according to the present invention, wherein such white POM test chip includes a laser material additive (DSM Micabs®) at 2.5% load level with 1.0% TiO₂ load level (white).

[0040] FIG. 9 is a photograph of a polyester sample part (in this case, an underhood automotive fuel component) bearing a data matrix code (overall data matrix size of 0.180" x 0.180") marked with a laser marking system according to the present invention, wherein such polyester sample part does not include a laser material additive.

DETAILED DESCRIPTION

[0041] FIG. 1 shows the principles of a laser marking system 10 employing a basic photo masking technique according to the prior art. The photo masking technique utilizes a laser source 12 that generates a dispersed, pulsating laser beam 14 and a template/mask 16 to mask off the area that is not to be marked. Oriented behind the template/mask 16 is an optical lens 18 that focuses laser beam 14 onto the surface of the substrate 20 to be marked. The prior art teaches that Excimer lasers can be used when laser marking according to the photo masking technique. Excimer lasers typically operate at a wavelength of approximately 193-355 nm, with a pulse frequency over 100 Hz, and a pulse width under 50 ns. Since the photo masking technique works very rapidly, it offers a solution for repeated labels such as expiration dates on food packaging or logos on small electronic parts, but not variable for sequential information or machine vision codes. However, the photo masking technique has significant disadvantages. For example, the photo mask process has limited flexibility due to the requirement of a set template/mask 16 that defines the indicia to be marked. The photo mask process also reduces the available laser energy per area as the size of the indicia to be marked increases. Moreover, Excimer lasers used in the photo masking technique are typically quite large and bulky devices. Also, gas chemicals used with such lasers can be toxic and must be removed from the facilities in an environmentally safe manner.

[0042] FIG. 2 shows an alternative laser marking system 30 employing a beam-steered technique that is also known in the prior art. Similar to the photo masking technique, the beam- steered technique utilizes a laser source 32 that generates a laser beam 34. However, the beam- steered technique uses two mirrors 36, 38, each manipulated by a computer-controlled galvanometer 40, 42 that enables the mirrors, in combination with an optical lens 44, to steer laser beam 34 onto the surface of the substrate 46 to be marked. By changing the angle of one or both of mirrors 36, 38, the laser beam 34 is directed onto the substrate 46 where the desired indicia is "drawn" similar to using a pen or pencil. In particular, galvanometer 40 is paired with mirror 36 to steer laser beam 34 in the x-axis direction and galvanometer 42 is paired with mirror 38 to steer laser beam 34 in the y-axis direction. The beam-steered laser marking technique is thus extremely flexible compared to the photo masking technique because the galvanometer- controlled mirrors 36, 38 are accurate and can be directed to "draw" any graphic pattern, logo, label or font anywhere within the range of the marking field. Examples of devices used for beam-steering are disclosed in U.S. Pat. Nos. 5,225,923; 5,719,372; and 5,734,412, which patents are incorporated herein by their reference. It should also be noted that another form of the beam-steered laser marking technique can use a "flying optic" system instead of the galvanometer-controlled mirrors 36, 38. In contrast to the galvanometer-controlled system in which only the mirrors move, the entire optical assembly moves in a "flying optic" system.

[0043] The beam-steered technique is sometimes used with a Continuous Wave (CW) CO₂ laser, but an Nd: YAG laser is most commonly used. CW CO₂ sealed gas lasers operate at 10600 nm but these lasers do not generate sufficient peak power to yield a contrasting mark; instead, the result is a colorless engraving. Nd: YAG lasers typically operate at a wavelength of 1064 nm, with a pulse frequency typically between 0.3 and 50 kHz, and a pulse width of 120-200 ns. The wavelength of an Nd: YAG laser can be doubled to 532 nm or tripled to 355 nm. However, according to the prior art, for the laser marking of plastics, the beam-steered Nd: YAG (1064 nm) laser offers the best compromise between material absorption, speed, flexibility, and marking quality.

[0044] Another laser sometimes used for the beam-steered technique is an air-cooled UV laser operating at 355 nm. This UV laser can produce gray contrast on POM containing TiO₂ without any other laser material additives, but can achieve virtually no contrast on unpigniented or natural POM. One advantage of the beam-steered UV laser is that the toxic gases are not used; however, there are numerous disadvantages, including a small marking field (typically in the range of 2-5 inches square), expensive laser procurement costs, and very slow marking speed. It is thus unlikely that a UV laser could be practically implemented in a manufacturing setting.

[0045] The marking quality (i.e., contrast, sharpness (edge detail), uniformity, speed, ease of use, cost-efficiency, reliability, etc.) yielded by a laser marking system depends on at least three fundamental factors: laser material additives; laser type; and laser set-up (including software and vector optimization). Moreover, many secondary factors must also be considered in order to optimize the marking quality, including but not limited to: polymeric material substrate; "as- molded" surface finish texture and gloss (stipple, high gloss, matte finish); and material fillers. Proper consideration and selection of both the fundamental factors and the secondary factors are critical in achieving optimal marking results.

[0046] Laser Material Additives

[0047] Depending on color of the substrate, some plastics (ABS is one example) can yield an average quality laser mark without the need for chemical additives using the most well-known and popular 1064 run beam-steered Nd: YAG laser. However, most non-pigmented (natural) plastics and pigmented (white, black, blue, green, red, etc.) plastics, including POM and polyolefins, are not laser markable or are only slightly laser markable if such plastics are not compounded with a laser material additive. It is therefore well known in the art that chemical additives can be added to the plastic material through a doping process. These laser material additives improve the plastic material's absorption of the laser energy and thus increase the laser markability.

[0048] FIGS. 3a and 3b illustrate the improved laser marking quality achieved by using laser material additives with prior art laser marking systems. In particular, FIG. 3a depicts a test chip made of POM that is not doped with any laser material additive, wherein the test chip was laser marked using an Nd: YAG laser according to the prior art. As can be seen, the laser marking is extremely faint and barely, if at all, legible. Such results are unsatisfactory in general, but especially in applications requiring machine vision. FIG. 3b, on the other hand, depicts a test chip made of POM that is doped with a laser material additive, wherein the test chip was again laser marked using an Nd: YAG laser (industry standard 70-100 W raw output power and M² greater than 2) according to the prior art. The improved quality of laser marking is apparent, but the brownish color of the marking remains far from ideal. As these Figures make clear, doping the POM with a laser material additive yields greater contrast but the dark-on-light marking is clearly not a black or any color close thereto. Accordingly, there remains a long-felt need for a laser marking system that produces a higher contrast and higher sharpness dark-on-light mark (as well as a light-on-dark mark and a tone-on-tone color mark), which, among other advantages, results in a mark capable of higher fidelity.

[0049] As demonstrated by FIG. 3c, the present invention solves the long-felt need for high quality dark-on-light laser marking. FIG. 3c depicts a test chip made of POM that is doped with a laser material additive, wherein the test chip was laser marked using the laser marking system of the present invention. The improved contrast and improved sharpness is easily apparent. Notably, the amount of laser material additive in the test chips of both FIGS. 3b and 3c is the same - 1.0% load level of Engelhard's Mark-It® brand of additive and 0.5% of TiO₂. The improved marking results achieved by the present invention is further illustrated by the close-up views provided in FIGS. 4a and 4b.

[0050] It is visibly apparent that the contrast and sharpness (edge detail) of the laser marking performed according to the system and method of the present invention is significantly improved. This improved sharpness of the marking relative to the proportional spaces and quiet zones improves the machine readability of the marking and thus achieves higher fidelity. For instance, in the 1 -dimensional bar code 50, 52 of the test chips of FIGS. 4a and 4b (as well as FIGS. 3b and 3 c), the spaces between the lines of the bar code are more clearly visible in the test chip of FIG. 4b marked according to the present invention; thus a mere visual inspection reveals the significantly improved edge detail achieved with the present invention. Similarly, the 2- dimensional data matrix code 56 of the test chip of FIG. 4b also has a significantly improved visible clarity compared to the data matrix code 54 of the prior art test chip of FIG. 4a. As discussed in more detail below, the machine readability of both the 1 -dimensional bar code 52 and the 2-dimensional data matrix code 56 marked according to the present invention are thus significantly improved over the prior art.

[0051] Another advantage of the laser marking system and method of the present invention is that it yields improved contrast, sharpness (edge detail), and uniformity for a wide range of commercially available light-absorbing laser material additives. Interestingly, the present invention actually allows one or more laser material additives to be used for light-on-dark applications even though such additives are believed by their manufacturers to be only suitable for dark-on-light applications. When a laser material additive is selected that absorbs light in the wavelength range of the laser beam used, the light-absorbing laser material additive can improve the marking characteristics of the plastic material with which the laser material additive is combined. As discussed below, the wavelength of the laser beam used in the present invention is in the near infrared spectrum, namely, in the range of 1060 nm to 1070 nm. Accordingly, in a preferred embodiment of the present invention, a laser material additive that allows light absorption in the near infrared spectrum is added to the plastic to be marked. In a still more preferred embodiment, a laser material additive that allows light absorption in the wavelength range of 1060 nm to 1070 nm is added to the plastic to be marked. Even when using the laser light-absorbing laser material additives in the 1060 nm to 1070 nm range, the material substrate color and surface finish affect the marking quality. This is especially true for certain colored substrates, including but not limited to green, red, and blue. Thus, to achieve the improved results of this invention, the type of laser material additive, the concentration thereof, and the laser setup must be carefully selected on a case-by-case basis.

[0052] It has been found by the inventor that antimony-based laser material additives have light-absorbing characteristics in the near infrared spectrum, thus such antimony-based laser material additives are particularly well-suited for achieving high contrast, high sharpness dark- on-light marking using the present invention. Many examples of antimony-based laser material additives have been disclosed in the prior art. For instance, U.S. Pat. No. 6,903,153 discloses the use of antimony trioxide as a laser material additive, and also discloses that the additive could also contain copper and/or phosphate to improve its laser marking performance. Another example in the prior art is U.S. Pat. No. 6,693,657, which discloses the use of calcined powder consisting of co-precipitated mixed oxides of antimony and tin. Yet another example of an antimony-based laser material additive comes in the form of mica platelets that are coated with one or more metal oxides of antimony and/or tin and/or tin dioxide. Although not all have been tested, it is believed that the present invention will achieve improved laser marking using these laser material additives as well as any other antimony-based laser material additives.

[0053] It is further believed that some non-antimony-based laser material additives having light-absorbing characteristics in the near infrared spectrum will also achieve high contrast, high sharpness dark-on-light marking using the laser marking system and method of the present invention (although such additives may not be effective for all plastics). There are potential environmental and health-related benefits to avoiding the use of heavy metals such as antimony when producing a laser marked product, thus non-antimony-based laser material additives have been introduced into the market. So long as such non-antimony-based laser material additives absorb light in the near infrared spectrum, it is believed that the present invention can be used with such additives to achieve improved laser marking for one or more plastics.

[0054] In addition, it is believed that the present invention would achieve similar improved results with additive technologies developed in the future, such as niobium additives and/or additives implemented using principles of nano-technology (i.e., nano-technology particle sizes).

[0055] Yet another advantage of the present invention is that the required load level of the laser material additive is significantly reduced relative to the load level required when practicing prior art laser marking systems. The load level is the percentage by weight of the laser material additive relative to the overall weight of the finished plastic material (inclusive of the additive therein). A reduction in the required load level is beneficial for several reasons. The first benefit, of course, is that less laser material additive is required, thus costs of purchasing laser material additive are reduced. Second, some laser material additives can have a grayish tint, thus additional coloring components are often added to the plastic compound to counteract or mask the graying effects associated with the laser material additive. A common example of a coloring component is TiO₂. Since less laser material additive is required with the present invention, the required amount of coloring components is reduced and the costs associated therewith are similarly reduced. Third, additional components such as laser material additives may have different bulk modulus properties than the POM (or other plastic) to which they are added. One example is DSM Micabs® which is an antimony trioxide encapsulated in a polyolefinic carrier. Since POM is typically selected because of its beneficial chemical and physical properties, it is clearly beneficial to minimize any modification of those properties. By using less laser material additive (and, in turn, less coloring components), the bulk modulus properties (chemical, physical, etc.) of the POM are less affected by the doping process. Fourth, an excessive amount of additives can cause delamination. Prior art techniques have been directed toward using high levels of additives to achieve increased contrast while not addressing the deleterious effects caused by such high levels of additives. These are just a few benefits associated with the reduction in the required load level of laser material additive provided by the present invention. Other additional benefits will be apparent to those of ordinary skill in the art.

[0056] Laser Types

[0057] The type of laser used in a laser marking system, and more specifically the beam characteristics of such laser, has significant effects on the marking quality achieved. Prior art laser marking systems employing the beam-steering technique typically use Nd: YAG and/or CO₂ lasers. However, it has been discovered that fiber lasers, vanadate lasers, and thin-disc lasers generate laser beams having characteristics that are particularly advantageous when developing a laser marking system for marking POM substrates. It is to be noted, however, that the present invention is not limited to fiber lasers, vanadate lasers, and/or thin-disc lasers and in fact encompasses any laser types that generate laser beams having the selected characteristics discussed below.

[0058] The term fiber laser generally refers to lasers in which the lasing medium is an optical fiber doped with low levels of rare-earth halides to make it capable of amplifying light. Laser diodes can be used for pumping because of the fiber laser's low threshold power, eliminating the need for cooling. Additional advantages of fiber lasers include but are not limited to a compact design due to the ability to coil fibers, a rugged setup due to the fibers being shielded from the environment, and a diffraction-limited beam quality. Moreover, with no laser crystal or intra- cavity optics near the galvo assembly the entire beam-steering/galvo mount assembly is reduced to a compact, lightweight package. Fiber lasers will focus to smaller, tighter spot sizes, greater beam brightness, and are thus capable of achieving higher power density than comparable (conventional) Nd: YAG lasers given similar optics. Moreover, fiber lasers can achieve high peak power and the spot size does not change as power is varied - for example, if power is varied from 8 W to 12 W, the spot size of the beam does not change. While it is generally known that conventional Nd: YAG lasers should not operate above 80% of maximum power for sustained operations, fiber lasers do not have the same risks in operating at or close to 100% of maximum power. Fiber lasers were developed in the late 1980's but their use was originally focused on telecommunications and military applications. Fiber lasers are now offered for laser marking applications, primarily metal. As of today, there are several different manufacturers of fiber lasers and there are minor variations between the lasers of each manufacturer. For example, some manufacturers incorporate active acoustic optical Q switch for the pulse repetition rate while others do not. There may also be other minor differences such as variations in rise-time and fall-time, peak power, and portability, but it is believed that all fiber lasers would be effective in achieving the unproved laser marking of the present invention.

[0059] Notwithstanding that a fiber laser can technically be considered a solid state laser, a fiber laser has distinctive characteristics that require it to be treated as a separate category from commonly used solid state lasers such as Nd:YAG lasers. For instance, U.S. Patent No. 6,489,985 discloses the significant differences between a fiber laser and more commonly used CO₂ and Nd: YAG lasers. One significant distinction is that an Nd: YAG laser has a gain media that is a doped "crystal" Nd: YAG (i.e., long crystal rod about the length of a pencil), whereas the gain media in a fiber laser is Ytterbium optical fiber. Moreover, adaptation and integration of a fiber laser into a laser marking system requires more than simply plugging in a different type of laser; instead, effective integration requires that the entire laser marking system be modified to take advantage of the distinct characteristics of a fiber laser. Based upon the fiber laser manufacturer, there are various methods of active or passive pulse repetition Q switching. Fiber lasers can be used for generating pulses with durations which are typically between tens and hundreds of nanoseconds. Due to the high gain efficiency of doped fibers, fiber lasers have the potential to obtain very high power efficiencies.

[0060] The term vanadate laser generally refers to lasers based on neodymium-doped vanadate crystals, including but not limited to yttrium vanadate (Nd: YVO₄). Based upon laser design configuration, vanadate lasers generally possess an M² factor less than 2, a pulse repetition rate of 10-100 kHz, and a pulse width in the range of 20-150 ns. In 1966, The American Institute of Physics cited the crystal-field energy levels and laser properties of YVO₄:Nd:YAG vanadates, also called ortho vanadates. Although such lasers did not immediately become popular, vanadates have been used commercially for at least 12-15 years and probably longer. For Nd: YVO₄ the typical laser emission wavelength is 1064 nm, the same as Nd: YAG. However, Nd: YVO₄ does not allow for pulse energies as high as for Nd: YAG because its capability for energy storage is lower than that of Nd: YAG due to the lower upper- state lifetime and the high gain efficiency. On the other hand, Nd: YVO₄ is better suited for high pulse repetition rates where it still allows the generation of fairly short Q-switched pulses.

[0061] A thin-disc laser (not to be confused with rotary disc lasers) is yet another form of laser that is believed to be effective in achieving the improved laser marking results according to the present invention. The thin-disc laser is a special kind of diode-pumped solid state laser introduced in the 1990's. The thin-disc laser produces short pulse widths and high beam quality. Although not yet tested, it is believed that a thin-disc laser operating at 1064 nm will achieve similarly improved results as fiber lasers and vanadate lasers.

[0062] As already noted, it has been discovered that significantly improved laser marking contrast, sharpness, and uniformity can be obtained when utilizing certain fiber lasers, vanadate lasers, and thin-disc lasers in a laser marking system. It has been further discovered that a unique combination of laser beam characteristics is the key to achieving the significantly improved marking quality. In particular, the wavelength of the laser beam, the pulse width of the laser beam, and the beam quality mode of the coUimated final focused laser beam (for determining the spot radius size and power density attributes) are the key considerations when selecting a laser source according to the present invention.

[0063] The wavelength of the laser beam is one important consideration when selecting a laser source according to the present invention. It has been discovered that wavelengths in the range of 1060 nm to 1070 nm, and preferably a wavelength of 1064 nm, provide the significantly improved dark-on-light marking quality. Fiber lasers, vanadate lasers, and thin-disc lasers can all generate laser beams having a wavelength m this range. It should be noted, however, that the mere use of a fiber laser, vanadate laser, or thin-disc laser is not sufficient to achieve the improved marking quality of this invention. For instance, a frequency-doubled vanadate laser, wherein the typical 1064 nm wavelength is modified to 532 nm, does not achieve the unproved marking quality of this invention. Accordingly, one element of the present invention is to select a laser beam having a wavelength in the range of 1060 nm to 1070 nm, and preferably a wavelength of 1064 nm. [0064] The pulse width of the laser beam is another important consideration when selecting a laser source in accordance with the present invention. Pulse width, which is typically measured in nanoseconds, is the lifetime of a laser pulse, generally defined as the time interval between the halfpower points on the leading and trailing edges of the pulse. Pulse width can vary widely. By modulating a continuous- wave light source, pulse widths ranging from tens of picoseconds to arbitrarily high values can be obtained. With respect to the present invention, it has been discovered that relatively short pulse widths in the range of 10 ns to 120 ns provide the significantly improved dark-on-light marking quality, wherein the most preferred pulse width is approximately 70 ns. Fiber lasers, vanadate lasers, and thin-disc lasers can all generate optical pulses in this range. Again, however, it is believed that other lasers that can generate optical pulses in this range can similarly achieve the improved marking quality of this invention. Accordingly, another element of the present invention is to select a laser beam having a pulse width in the range of 10 ns to 120 ns, and preferably a pulse width of 70 ns.

[0065] The beam quality mode of the laser beam is another consideration when selecting a laser source according to the present invention. Beam quality can generally be understood as the energy distribution within the laser beam and how tightly a laser beam can be focused under certain conditions. One quantitative way to measure the beam quality is called the M² factor, wherein M² is a beam quality index that measures the difference between an actual beam and a Gaussian beam. Generally speaking, the beam quality of the laser beam increases as the value of M² draws closer to 1 (less beam divergence). Another quantitative measure of laser beam quality is the TEMnm mode, where n is the number of nodes in the horizontal direction and m is the number of nodes in the vertical direction. The TEM_mn mode is related to the M² factor. A beam in TEMoo mode is the lowest order mode, also known as the fundamental transverse mode of the laser resonator, and has the same form as a Gaussian beam. Accordingly, an ideal Gaussian beam has a beam quality quantitatively measured by a TEMoo mode and an M² factor equal to 1, although practically an M² factor of exactly 1 is not currently attainable.

[0066] It has been discovered that laser beams having a beam quality with an M² value near 1 (preferably in the range between 1 and 2, and even more preferably in the range of 1 and 1.5) and thus a beam quality at or near TEMoo, when combined with the above-referenced wavelengths and pulse widths, provides significantly improved dark-on-light marking quality. In prior art laser marking systems utilizing Nd: YAG lasers, the Nd: YAG lasers typically have an M² value of approximately 2 to 8 or even greater. Prior to the present invention, persons of ordinary skill have explained that M² values of 2 or greater were desirable because M² values near 1 result in very thin laser marks (fine line width), thus increasing the time required to process large areas that need to be filled in. However, the present invention overcomes this problem in the prior art and in fact utilizes the thin laser mark, via collimation, to obtain the desired spot size and power density, which further improves the quality and efficiency of the laser marking system. Specifically, in a preferred embodiment of the present invention, a laser having a beam quality with an M² value near 1 is selected such that the beam spot size is roughly 50% smaller than the beam spot size of an Nd: YAG laser (utilizing similar optics), which allows the laser used in the present invention to generate high power density without requiring the laser to operate near its maximum power output. Preferably, appropriate focal length lenses and collimation can be used to create a laser beam spot size at the work surface that yields improved results. For instance, using a relatively large 330 mm flat field lens, the beam diameter can be made smaller by collimating the beam to achieve minimal divergence and thereby smaller spot size with greater power density. The beam that results from this particularly preferred embodiment thus allows the laser to operate at a power significantly less than maximum raw output power, allows a larger working distance and marking field such that larger parts or a larger number of parts can fit in the work area, and overcomes the problem in the prior art regarding slower processing times for lasers generating beams at or near TEMoo. Accordingly, another element of the present invention is to select a laser beam having an M² value near 1 and a beam quality mode at or near TEMoo- Further, smaller flat field lenses (such as 254 mm, an industry standard size sold with many lasers) can also be used when higher power densities on the workpiece are needed. Such instances requiring higher power density can include materials sensitive to power, including color substrates (such as blue, red, green), to achieve a darker contrasting mark which can be important both for cosmetic/aesthetic purposes and for readability of machine vision codes. The 330 mm lens has a working distance of approximately 15.5 inches and an approximately 9.5 inch by 9.5 inch marking field. A 254 mm lens has a working distance of approximately 12 inches and an approximately 7.4 inch by 7.4 inch marking field. A person of ordinary skill in the art will recognize that various other lens sizes can be used when desirable to vary the power density on the workpiece.

[0067] The present invention therefore achieves dark-on-light laser marking (as well as light- on-dark laser marking and tone-on-tone color laser marking) having significantly improved contrast, sharpness, and uniformity when a laser source is selected that generates a laser beam having a wavelength, pulse width, and beam quality within the above-described ranges. This unique combination of laser beam characteristics provides particularly advantageous dark-on- light marking quality when designing a system for laser marking POM substrates. While this combination of laser beam characteristics can be obtained using certain fiber lasers and vanadate lasers (and likely thin-disc lasers), a person of ordinary skill in the art would recognize that other lasers that can generate laser beams having this unique combination would also fall within the scope of the present invention.

[0068] Laser Set-Up

[0069] Persons of ordinary skill in the art will recognize that the optimal set-up of the laser marking system depends on many factors. First, with respect to the plastic substrate to be marked, at least the chemical composition (e.g., non-pigmented, pigmented, filled, unfilled, antimicrobial), physical geometry (e.g., flat, cylindrical, contoured), and other physical characteristics (e.g., high gloss, low gloss, stipple texture, smooth texture, metallic and/or other appearance effects) need to be considered. Second, with respect to the indicia to be marked, the content thereof affects the detailed laser set-up requirements. The range of indicia that can be marked is virtually unlimited, ranging from selection of text elements, font styles and sizes, fill and outline effects, machine vision codes (including but not limited to bar codes and data matrix codes), micro-marking, circuit diagrams and schematics, logos, and many others. Selection of the indicia to be marked directly affects the optimal laser set-up, including but not limited to the optics configuration (e.g., lens selection, collimation, beam expander, focused spot size, power density, working distance, marking field), laser operating software (e.g., vector ordering, line spacing, fill angle, fill method), and laser parameters (e.g., power, pulse repetition rate, marking speed). Accordingly, the present invention is not limited to any particular set-up of the laser marking system. Nevertheless, a few examples of particularly preferred embodiments will be described below.

[0070] The following laser set-up yielded the significantly improved marking results depicted in the photograph of FIG. 5a:

TESTCHIP COMPOSITION:

Substrate Material POM

Laser Material Additive 1.0% Mark-It® (Eng

Coloring Component 0.5% TiO₂

LASER SET-UP PARAMETERS:

Fiber Laser

Power 8 W - 10 W

Pulse Repetition Rate (Q) 2O kHz

Marking Speed 20"/sec

Working Distance 12+ inches

Wavelength 1064 nm

Beam Quality TEM₀₀

Pulse Width Approximately 70 ns

Flat Field Lens 330 mm

Collimation Yes

Beam Expander Yes

[0071] The test chip of FIG. 5a illustrates the significantly improved marking results when the POM substrate is marked according to the laser marking system and method of the present invention. It should be noted that the test chip of FIG. 5a (as well as FIGS. 5b and 5c) are not to scale - the actual dimensions are: (a) the character height of the alphanumeric "ABCDEFG" is approximately 0.100 inches; (b) the overall width of the bar code is approximately 0.910 inches;

(c) the character height of the micro-marked text "Marking" is approximately 0.020 inches; and

(d) the outer dimensions of the data matrix code are approximately 0.25 inches square. Moreover, the test chips of FIGS. 5b and 5c further demonstrate that the laser marking system and method of the present invention also yield significantly improved marking results for POM having lower loading levels of laser material additives - 0.75% Mark-It® and 0.50% Mark-It®, respectively, in FIGS. 5b and 5c. Although the laser marking on these test chips shows progressively less contrast, the edge detail and clarity remains virtually identical to the preferred embodiment of FIG. 5a having 1.0% Mark-It®. In particular, both the 1 -dimensional bar codes and the 2-dimensional data matrix codes marked on the test chips of FIGS. 5b and 5c can still be successfully and reliably read by a machine. Accordingly, the laser marking system and method of the present invention significantly decreases the required loading level of laser material additive necessary to achieve improved contrast and sharpness. As previously noted, a reduction in the loading level with improved marking results has many benefits, including but not limited to less material costs and the reduced risk that the bulk modulus properties of the POM will be adversely affected.

[0072] The above-described embodiments yield excellent results for a simple white POM substrate on which relatively uniform markings are to be made. However, in view of the many variables present in any given application, the above embodiments may not always be the best solution. For instance, as noted above, variables affecting the particular application can include: selection of POM based polymer resin (or other plastic resins), presence and type of filler materials, selection of uncolored natural, white, or colored substrate, surface texture, gloss, type of marking (e.g., alphanumerics, thick lines, thin lines, graphics, etc.), and many others. Recognizing these variables, the following laser setup yielded significantly improved marking results as depicted in the photographs of FIGS. 6a through 6c and 7a through 7d:

TESTCHIP COMPOSITION:

Substrate Material POM (Natural or White)

Laser Material Additive 0.75% to 1.0% Mark-It® (Engelhard/BASF), or

2.5% to 3.0% Mϊcabs® (DSM) Coloring Component Natural or White (1.0% TiO₂)

LASER SET-UP PARAMETERS:

Fiber Laser

Power 8 W - 12 W Pulse Repetition Rate (Q) 20 - 25 kHz

Marking Speed 15" - 30"/sec

Working Distance 15+ inches

Wavelength 1064 nm

Beam Quality TEM₀₀

Pulse Width Approximately 70 ns

Flat Field Lens 330 mm

Collimation Yes

Beam Expander Yes

[0073] The above laser set-up parameters reflect the actual test results to date for a fiber laser. However, the significantly improved results achieved using the present invention would also occur hi a preferred embodiment of a fiber laser using a power of 5 - 20 Watts, a pulse repetition rate of 20 - 30 kHz, and a marking speed of 15" - 40" per second. Moreover, although FIGS. 6a through 6c and 7a through 7d depict dark-on-light applications, note that this same preferred embodiment can be used to achieve similarly improved results for light-on-dark applications and tone-on-tone color applications.

[0074] As can be seen from the small sampling of indicia marked on the test chips depicted in the figures, there is a virtually unlimited number of variations and possibilities in the types and forms of indicia to be marked. As a result, there is no single universal laser set-up that will yield optimal results in every circumstance. However, persons of ordinary skill in the art will recognize that the focused spot size and power density can be adjusted through the selection of appropriate flat field lens size and collimation, power, pulse repetition rate, and marking speed. Moreover, many commercially available laser software applications enable a user to define each unique object to be marked and to define the laser set-up parameters for each unique object to be marked. Accordingly, combining the disclosure of the present invention with commercially available tools and ordinary skill in the art will achieve significantly improved laser marking results. [0075] Although not depicted in the figures, results to date indicate that a vanadate laser can also be successfully used to implement the present invention. The following laser set-up with a vanadate laser is thus expected to yield significantly improved marking results:

TEST CHIP COMPOSITION:

Substrate Material POM (Natural or White)

Laser Material Additive 0.75% to 1.0% Mark-It® (Engelhard/BASF), or

2.5% to 3.0% Micabs® (DSM) Coloring Component Natural or White (1.0% TiO₂)

LASER SET-UP PARAMETERS:

Vanadate Laser (20 Watt)

Power 5 W - 20 W

Pulse Repetition Rate (Q) 20 - 50 kHz

Marking Speed 15" - 30"/sec

Working Distance 9+ inches

Wavelength 1064 nm

Beam Quality TEM₀₀

Pulse Width Approximately 70 ns

Flat Field Lens 254 mm

Collimation Yes

Beam Expander No

[0076] Another important advantage of the present invention is the machine readability of the laser marking. Of course, the entire purpose of laser marking a part with a machine readable code (bar code, data matrix code, and others, whether currently existing or later developed) is to allow a machine vision system to successfully and reliably read the code at a later date (what is commonly referred to as "fidelity"). To illustrate the improved machine readability achieved with the present invention, various test chips were marked with 1 -dimensional bar codes and 2- dimensional data matrix codes using both the prior art laser marking system and the laser marking system of the present invention (the test chips with 1.0% Mark-It® are depicted in FIGS. 3b and 3c, respectively). The bar codes were tested using an industry-standard handheld bar code reader and the data matrix codes were tested using an industry-standard data matrix code reader referred to as Readrunner™. Following is a chart showing the test results for a prior art system utilizing an industry-standard Nd: YAG laser compared to the test results for the improved laser marking system of the present invention. As can be seen, the present invention achieves laser marking that yields significantly improved machine readability:

[0077] With reference to the 1 -dimensional bar code, the present invention yielded a laser marking that was successfully read in all cases. In contrast, the prior art yielded a failure to read in all three instances. This inability to read a 1 -dimensional bar code using a prior art system for direct dark-on-light laser marking of POM is significant given the widespread use of bar codes and bar code readers hi many applications worldwide. Accordingly, the present invention has significant value for improved dark-on-Hght laser marking of POM for a virtually unlimited number of bar code applications.

[0078] With reference to the 2-dimensional data matrix code, the improved machine readability of the present invention is most clearly illustrated by comparing the results of the 0.50% Mark-It® test chips. Whereas the 2-dimensional data matrix code marked using the prior art system yielded a failure to read ("Locator Failed - First Edge Not Found") for 100% of the test cycles, the 2-dimensional data matrix code marked using the system and method of the present invention yielded a successful read ("ABCDEFG") for 100% of the test cycles. [0079] A 2-dimensional data matrix code has important advantages over conventional 1 - dimensional bar codes, including but not limited to: (a) the ability to encode information digitally; (b) very high information density allowing the placement of much information in a small area; (c) scaleability down to smaller sizes; (d) built-in error correction techniques; and (e) ability to be read at any orientation. By way of example and not limitation, industries that make significant use of 2-dimensional data matrix codes include automotive, aerospace, electronics, semiconductor, medical devices, pharmaceuticals, electrical products (fuse boxes, schematics & diagrams, connectors, relay switches, circuit boards, etc.), animal identification tags, beverage closures, food packaging, and other unit-level manufacturing applications requiring traceability. As a result of these and other factors, it is expected that use of data matrix codes will grow exponentially over the coming years, thus further increasing the importance of a reliable method of laser marking such codes in dark-on-Hght POM applications as well as light-on-dark applications, tone-on-tone color applications, and applications requiring plastics other than POM.

[0080] The above chart illustrates the fidelity or reading accuracy advantages of the present invention over the prior art when marking a data matrix code. The present invention yields additional significant advantages over the prior art. First, one very important consideration in evaluating methods for marking data matrix codes is termed "growth factor" which can be understood as the ability to control cell fill such that an actual data matrix cell size is marked in accordance with the intended data matrix cell size (i.e., the data matrix cell is not over-filled or under-filled beyond a predetermined threshold). A laser marking technique which allows control over the laser beam spot size and power density as well as the material science of the substrate allows precise control over the data cell and the amount of cell fill and thus results in a desirable growth factor. A desirable growth factor, in turn, significantly improves the ability to mark data matrix codes having high fidelity. Testing results for data matrix codes laser marked according to the present invention indicate a very desirable growth factor, thus demonstrating the utility of the present invention in laser marking high fidelity data matrix codes. Second, the speed of the machine readability is also improved. In view of the improved contrast, sharpness, and uniformity, a machine vision reader can more efficiently identify, focus on, and capture the bar code or data matrix code. This faster machine readability gains importance when laser marking in high volume applications as even a slight gain in speed can have significant effects. Third, a higher quality data matrix code generally requires less error correction. Other advantages associated with the present invention will be apparent to those of ordinary skill in the art.

[0081] While the above chart relates to a relatively large size data matrix code (¹A" x ¹A") that contains only 7 characters ("ABCDEFG"), the present invention can achieve significantly smaller data matrix codes while still maintaining the same high fidelity results. As depicted in FIGS. 8a, 8b, and 9 (none of which are to scale), the present invention has a demonstrated capability of laser marking data matrix codes on both POM and polyester with an overall size of only 0.180 inches by 0.180 inches (although not yet tested, the present invention should also be equally effective in laser marking other plastics). The data matrix code laser marked on the polyester underhood automotive fuel component depicted in FIG. 9 (note the component is less than 0.75 inch wide) is a 32 x 32 data matrix that contains 72 alphanumeric characters and spaces (although only 72 characters were attempted, a 32 x 32 data matrix has a potential information capacity of 91 alphanumeric characters and spaces). The data matrix code laser marked on the white POM test chip depicted in FIG. 8a is a 36 x 36 data matrix that contains 93 alphanumeric characters and spaces (although only 93 characters were attempted, a 36 x 36 data matrix has a potential information capacity of 127 alphanumeric characters and spaces). The close-up view of this data matrix code depicted in FIG. 8b illustrates the distinct data matrix cells having individual cell sizes of 0.005 inches, which results in a resolution of 834 dpi (dots per inch). For comparison purposes, it is believed that high-end desktop laser printers for paper normally operate at a resolution in the range of 600 dpi. Another comparison is that human hair thicknesses are typically measured in the range of approximately 0.003 inches to 0.004 inches. Moreover, it is believed that the present invention is capable of laser marking even smaller data matrix codes containing even more data at even higher resolutions.

[0082] The micro-marking capabilities of the present invention are also clearly illustrated by the close-up views contained in FIGS. 4a and 4b (which are close-ups of the test chips depicted in FIGS. 3b and 3c, respectively). For instance, FIGS. 4a and 4b illustrate examples of micro- marking 58, 60 hi dark-on-light applications achieved by the prior art system and the system of the present invention, respectively. The improved quality and readability of the micro-marking achieved using the present invention is visibly evident in these close-up views. The improved ability to micro-mark using the present invention is further illustrated by the test chips of FIGS. 6a, 6b, 6c, 7a, 7b_s 7c, and 7d, wherein the dark-on-light text marked in relation to the circuit diagrams has a character height of approximately 0.015 inches. The ability to micro-mark in many applications, whether dark-on-light, light-on-dark, or tone-on-tone color, has significant value in the industry where there is limited marking space and/or a desire to minimize the size of marking for aesthetic, anti-counterfeit/forgery, or other purposes. Scaleability while maintaining quality and readability (both human and machine) of the laser marking is a valuable feature and will only become more valuable in the future.

[0083] The laser marking system of the present invention also allows a working distance of 12+ inches in certain applications. This relatively large working distance results from the high beam quality. In particular, increased beam quality means that beam quality does not significantly degrade (or diverge) to an unacceptable level until the beam has traveled a greater distance. In comparison to prior art Nd: YAG laser marking systems, the acceptable working distance in the preferred embodiment of the present invention utilizing an optical beam expander and a larger flat field lens is approximately 1.5 times greater. Whereas Nd: YAG laser marking systems typically operate with a working distance of approximately 7-8 inches, the working distance of a laser marking system according to a preferred embodiment of the present invention can be at least 12-15 inches. This larger working distance permits larger workpieces to be marked and/or a larger number of smaller workpieces to be processed in a single lot.

[0084] In a particularly preferred embodiment of the present invention, the speed of the beam-steered laser marking process can be increased by utilizing a serpentine or bi-directional optimized vector path rather than isolated discrete straight line vectors. In its most basic form, lasers plot vector lines to form indicia relative to attributes of outline, fill, and outline with fill. Single-pass fill angles can range from 0 to 90 degrees, i.e., horizontal, vertical or any angle in between. Vector lines are often termed uni-directional or bi-directional. Given a predetermined line separation width, uni-directional lines have start points on the same side. In a unidirectional fill, the formation of the vector line commences when the laser beam turns on at the start point and shuts off at the end point. During the "laser beam off mode," the beam-steering device (whether galvanometer-controlled or flying optics) repositions the laser beam spot to the next start location on the same side to draw the next line. In a bi-directional fill, the formation of vector lines is back and forth, such that start and end points are on different sides of the shape to be drawn. Serpentine vector optimization fill path utilizes a bi-directional routine, but further it joins the end of the first vector with a short segment line to the start of the next vector with the laser beam continuously in the "on" mode. This eliminates the inefficiency and time loss when the laser beam shuts off to reposition, and significantly improves marking speed.

[0085] Yet another advantage of the present invention is that the described system for creating dark-on-light laser marking can also be used for creating light-on-dark laser marking and tone-on-tone color laser marking. Although certain laser parameters such as the power may need to be adjusted when switching from dark-on-light to light-on-dark and/or tone-on-tone color laser marking, chemical foaming (white), thermal chemical carbonization (black), and degradation of a colorant (color) can all be achieved using a single laser to obtain the desired color result. This capability permits an efficient use of time and equipment resources.

[0086] Although the invention has been described in conjunction with one or more preferred embodiments, it will be apparent to those skilled in the art that other alternatives, variations and modifications will be apparent in light of the foregoing description as being within the spirit and scope of the invention.

Claims

WHAT IS CLAIMED:

1. A laser marking system comprising: a laser source for generating a laser beam; and an optical device for focusing and steering the laser beam to a surface of a plastic article, wherein the laser beam has a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns, and an M² factor between 1 and 2.

2. A laser marking system in accordance with claim 1 , wherein the laser beam has an M² factor between 1 and 1.5.

3. A laser marking system in accordance with claim 1 , wherein the laser source is a fiber laser.

4. A laser marking system in accordance with claim 1 , wherein the laser source is a vanadate laser.

5. A laser marking system in accordance with claim 1 , wherein the laser source is a thin- disc laser.

6. A laser marking system in accordance with claim 1 , wherein the laser beam has a wavelength of approximately 1064 nm.

7. A laser marking system in accordance with claim 1 , wherein the laser beam has a pulse width of approximately 70 ns.

8. A laser marking system in accordance with claim 1, wherein the plastic article is doped with a laser material additive for increasing a light absorption characteristic of the plastic article.

9. A laser marking system in accordance with claim 8, wherein the laser material additive absorbs light in a near infrared spectrum.

10. A laser marking system in accordance with claim 9, wherein a load level of the laser material additive is approximately 2.5%.

11. A laser marking system in accordance with claim 1 , wherein the surface of the plastic article has a light color and is marked with a dark color.

12. A laser marking system in accordance with claim 1 , wherein the plastic article is formed of polyoxymethylene.

13. A laser marking system in accordance with claim 1 , wherein the plastic article is formed of nylon.

14. A laser marking system in accordance with claim 1, wherein the plastic article is formed of polyester.

15. A laser marking system in accordance with claim 1, wherein the plastic article is formed ofpolyolefin.

16. A laser marking system in accordance with claim 1, wherein the optical device comprises a galvanometer-controlled beam-steering device.

17. A laser marking system in accordance with claim 1, wherein the optical device comprises a flying optic beam-steering device.

18. A laser marking system comprising; a laser source for generating a laser beam; and an optical device for focusing and steering the laser beam to a polyoxymethylene substrate, wherein the laser beam has a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns₅ and an M² factor between 1 and 2.

19. A laser marking system in accordance with claim 18, wherein the laser beam has an M² factor between 1 and 1.5.

20. A laser marking system in accordance with claim 18, wherein the laser marking system can also be used for marking a metal substrate.

21. A laser marking system in accordance with claim 18, wherein the laser source is a fiber laser.

22. A laser marking system in accordance with claim 18, wherein the laser source is a vanadate laser.

23. A laser marking system in accordance with claim 18, wherein the laser source is a thin- disc laser.

24. A laser marking system in accordance with claim 18, wherein the polyoxymethylene substrate is doped with a laser material additive for increasing a light absorption characteristic of the polyoxymethylene substrate.

25. A laser marking system in accordance with claim 24, wherein a load level of the laser material additive is in the range of 0.5% to 3.0%.

26. A laser marking system in accordance with claim 18, wherein the optical device comprises a galvanometer-controlled beam-steering device.

27. A laser marking system in accordance with claim 18, wherein the optical device comprises a flying optic beam-steering device.

28. A laser marking system in accordance with claim 18, wherein the optical device comprises a collimator and a flat field lens.

29. A laser marking system in accordance with claim 28, wherein the optical device further comprises a beam expander.

30. A laser marking system comprising: a fiber laser for generating a laser beam; and an optical device for focusing and steering the laser beam to a surface of a plastic article.

31. A laser marking system comprising: a fiber laser for generating a laser beam; and an optical device for focusing and steering the laser beam to a surface of a plastic article, wherein the plastic article is compounded with a laser material additive with a load level in a range of 0.50% to 3.0%.

32. A laser marking system in accordance with claim 31 , wherein the plastic article is compounded with a laser material additive with a load level in a range of 0.75% to 2.5%.

33. A laser marking system comprising: a fiber laser for generating a laser beam; an optical device for focusing and steering the laser beam to a polyoxymethylene substrate, wherein the polyoxymethylene substrate is compounded with a laser material additive with a load level in a range of 0.50% to 3.0%.

34. A laser marking system in accordance with claim 33, wherein the polyoxymethylene substrate is compounded with a laser material additive with a load level in a range of 0.75% to 2.5%.

35. A laser marking system in accordance with claim 33, wherein the optical device comprises a collimator and a flat field lens.

36. A laser marking system in accordance with claim 35, wherein the optical device further comprises a beam expander.

37. A laser marking system comprising: a vanadate laser for generating a laser beam; an optical device for focusing and steering the laser beam to a polyoxymethylene substrate, wherein the polyoxymethylene substrate is compounded with a laser material additive with a load level in a range of 0.75% to 3.0%.

38. A laser marking system comprising : a thin-disc laser for generating a laser beam; an optical device for focusing and steering the laser beam to a polyoxymethylene substrate, wherein the polyoxymethylene substrate is compounded with a laser material additive with a load level in a range of 0.75% to 3.0%.

39. A laser marking system comprising; a fiber laser for generating a laser beam, wherein the laser beam has a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns, and an

M² factor between 1 and 2; a beam-steered optical device for focusing and steering the laser beam, wherein the optical device includes a collimator and a flat field lens; and a polyoxymethylene substrate forming the surface to be marked, wherein the polyoxymethylene substrate is compounded with a laser material additive with a load level in a range of 0.75% to 2.5%.

40. A laser marking system in accordance with claim 39, wherein the laser beam has an M² factor between 1 and 1.5.

41. A method of laser marking comprising : generating a laser beam having a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns, and an M² factor between 1 and 2; steering and focusing the laser beam to the surface of the plastic article; and marking the surface of the plastic article with selected indicia.

42. A method of laser marking in accordance with claim 41, wherein the laser beam has an M² factor between 1 and 1.5.

43. A method of laser marking in accordance with claim 41 , wherein the step of generating a laser beam is performed using a fiber laser.

44. A method of laser marking in accordance with claim 41 , wherein the step of generating a laser beam is performed using a vanadate laser.

45. A method of laser marking in accordance with claim 41 , wherein the step of generating a laser beam is performed using a thin-disc laser.

46. A method of laser marking in accordance with claim 41, wherein the laser beam has a wavelength of approximately 1064 nm.

47. A method of laser marking in accordance with claim 41, wherein the laser beam has a pulse width of approximately 70 ns.

48. A method of laser marking in accordance with claim 41 , wherein the plastic article is formed of polyoxymethylene.

49. A method of laser marking in accordance with claim 41 , wherein the plastic article is formed of nylon.

50. A method of laser marking in accordance with claim 41 , wherein the plastic article is formed of polyester.

51. A method of laser marking in accordance with claim 41 , wherein the plastic article is formed of polyolefϊn.

52. A method of laser marking in accordance with claim 41, wherein the plastic article is doped with a laser material additive for increasing a light absorption characteristic of the plastic article.

53. A method of laser marking in accordance with claim 52, wherein a load level of the laser material additive is in a range of 0.5% to 3.0%.

54. A method of laser marking in accordance with claim 52, wherein a load level of the laser material additive is in a range of 0.75% to 2.5%.

55. A method of laser marking comprising: generating a laser beam having a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns, and an M² factor between 1 and 2; steering and focusing the laser beam to the surface of the plastic article; and marking a data matrix code on the surface of the plastic article.

56. A method of laser marking in accordance with claim 55, wherein the laser beam has an M² factor between 1 and 1.5.

57. A method of laser marking in accordance with claim 55, wherein the data matrix code has a uniform cell size less than 0.006 inches.

58. A method of laser marking in accordance with claim 55, wherein the data matrix code has a uniform cell size in the range of 0.005 inches to 0.006 inches.

59. A method of laser marking in accordance with claim 55, wherein the data matrix code includes at least 32 rows and at least 32 columns.

60. A method of laser marking in accordance with claim 55, wherein the data matrix code includes at least 36 rows and at least 36 columns.

61. A method of laser marking in accordance with claim 55, wherein the data matrix code has an overall size of approximately 0.180 inches by 0.180 inches or less.

62. A method of laser marking in accordance with claim 55, wherein the step of generating a laser beam is performed using a fiber laser.

63. A method of laser marking in accordance with claim 55, wherein the laser beam has a wavelength of approximately 1064 nm.

64. A method of laser marking in accordance with claim 55, wherein the laser beam has a pulse width of approximately 70 ns.

65. A method of laser marking in accordance with claim 55, wherein the plastic article is formed of polyoxymethylene.

66. A method of laser marking in accordance with claim 55, wherein the plastic article is formed of nylon.

67. A method of laser marking in accordance with claim 55, wherein the plastic article is formed of polyester.

68. A method of laser marking in accordance with claim 55, wherein the plastic article is formed of polyolefin.

69. A method of laser marking in accordance with claim 55, wherein the plastic article is doped with a laser material additive for increasing a light absorption characteristic of the plastic article.

70. A method of laser marking in accordance with claim 69, wherein a load level of the laser material additive is in a range of 0.5% to 3.0%.

71. A method of laser marking in accordance with claim 69, wherein a load level of the laser material additive is in a range of 0.75% to 2.5%.

72. A method of laser marking comprising: generating a laser beam having a wavelength between 1060 nm and 1070 nm, a pulse width between 10 ns and 120 ns, and an M² factor between 1 and 2; steering and focusing the laser beam to the surface of the plastic article; and marking an indicia having a resolution of at least 700 dots per inch on the surface of the plastic article.

73. A method of laser marking in accordance with claim 72, wherein the laser beam has an M² factor between 1 and 1.5.

74. A method of laser marking in accordance with claim 72, wherein the step of generating a laser beam is performed using a fiber laser.

75. A method of laser marking in accordance with claim 72, wherein the laser beam has a wavelength of approximately 1064 nm.

76. A method of laser marking in accordance with claim 72, wherein the laser beam has a pulse width of approximately 70 ns.

77. A method of laser marking in accordance with claim 72, wherein the plastic article is formed of polyoxymethylene.

78. A method of laser marking in accordance with claim 72, wherein the plastic article is formed of nylon.

79. A method of laser marking in accordance with claim 72, wherein the plastic article is formed of polyester.

80. A method of laser marking in accordance with claim 72, wherein the plastic article is formed of polyolefin.

81. A method of laser marking in accordance with claim 72, wherein the plastic article is doped with a laser material additive for increasing a light absorption characteristic of the plastic article.

82. A method of laser marking in accordance with claim 81 , wherein a load level of the laser material additive is in a range of 0.5% to 3.0%.

83. A method of laser marking in accordance with claim 81 , wherein a load level of the laser material additive is in a range of 0.75% to 2.5%.