Abstract:
The present invention relates to laser marking articles. In particular it relates to laser marking articles by laser ablating a coating applied to the article which reveals the surface of the article underneath, thereby forming the mark by the contrasting appearance between the revealed surface of the article and the adjacent remaining coating. Laser parameters are selected to provide uniform, commercially desirable appearance and avoid damage to the underlying surface while maintaining acceptable system throughput. In particular the laser pulse envelope is tailored to provide desirable appearance while maintaining acceptable system throughput.

Description:
TECHNICAL FIELD 
     The present invention relates to laser marking articles. In particular it relates to laser marking articles by laser ablating a coating applied to the article which reveals the surface of the article underneath, thereby forming the mark by the contrasting appearance between the revealed surface of the article and the adjacent remaining coating. The mark may also be formed by laser ablating the first or topmost coating layer to reveal a second coating layer underneath forming the mark by the contrast between the revealed second coating and adjacent first coating. Laser parameters are selected to provide uniform, commercially desirable appearance and avoid damage to the underlying surface while maintaining acceptable system throughput. In particular the laser pulse envelope is tailored to provide desirable appearance while maintaining acceptable system throughput. 
     BACKGROUND OF THE INVENTION 
     Marketed products commonly require some type of marking on the product for commercial, regulatory, cosmetic or functional purposes. A mark is defined as contiguous region or area on the surface of the article which contrasts visually with the adjacent surface. Desirable attributes for marking include consistent appearance, durability, and ease of application. Appearance refers to the ability to reliably and repeatably render a mark with a selected shape and uniform color and optical density. Durability is the quality of remaining unchanged in spite of abrasion to the marked surface. Ease of application refers to the cost in materials, time and resources of producing a mark including programmability. Programmability refers to the ability to program the marking device with a new pattern to be marked by changing software as opposed to changing hardware such as screens or masks. 
     Of particular interest is creating marks on coated or painted articles. Articles made of metal or various types of plastics are often painted or otherwise covered in various industrial coatings to protect and change the appearance of the article&#39;s surface. Laser ablating the coating in particular patterns to remove the coating and reveal the surface of the article underneath is a desirable way to create a mark on article. Covering an article with two or more layers of coatings and laser ablating a first coating to reveal a second coating underneath is another desirable way to create marks. Marking a product by removing a coating with a laser to reveal the article underneath is discussed in US patent application no. 2008/0152859, inventor Masanori Nagai published Jun. 26, 2008. This method depends upon the brightness of the coating being brighter than the surface of the article. Japanese patent application no. 03-150842, inventor Iwasaki Noboru, published Oct. 29, 1992, describes removing one or more coating layers with a laser to reveal a coating layer underneath. 
     One thing that these references have in common is that in order to remove a coating without removing materials beneath the coating layer being removed, the laser ablation threshold for the material being removed must be lower than the laser ablation threshold of the material underneath. Laser ablation threshold is the minimum energy required to cause removal of material. This removal may be ablative, where enough energy is put into the material by the laser to cause the material to disassociate into plasma, or thermal, where the material is essentially melted and vaporized, or a combination of the two. Related to the ablation threshold is the damage threshold. The damage threshold is the minimum laser energy required to cause an undesirable change in the appearance of the material. The damage threshold for materials is generally lower and sometimes much lower than the ablation threshold. We define damage as any undesirable change in the appearance of the materials that comprise the article or under lying coating following laser removal of the topmost layer. 
     In order to cleanly and completely remove a top layer of material leaving the surface of the article undamaged, laser fluence must be greater than the ablation threshold for the overlying material and less than the ablation or damage threshold for the underlying material or surface of the article. In many cases this difference may be small, requiring precise control of laser fluence. In addition, the actual values for ablation and damage thresholds for these materials may vary depending upon location and slight variations in application. Also, actual values for ablation and damage thresholds for these materials may vary as a function of time; as the article is processed, heat generated by the laser removal process may be retained by the article or coatings and influence the ablation or damage thresholds. Therefore, while a single laser fluence may be selected that permits commercially desirable marking to be performed on a particular article at a particular time with a particular laser, any small change in the article or coatings or laser could result in the process no longer producing desirable results. The difference between the minimum fluence that will result in ablation of desired materials and avoid damage to underlying materials is called a process window. In manufacturing it is highly desirable to increase the process window for a particular article and mark to the greatest practical extent in order to handle variations in materials and processing systems without requiring system adjustments which negatively affect system throughput. 
     What is desired but undisclosed by the art is a reliable and repeatable method for removing material that does not leave undesired material on the surface of the article or require frequent adjustment of the equipment. What is needed then is a method for reliably and repeatably creating marks having a desired appearance on coated articles using a laser to remove a layer of coating without leaving undesired material on the under laying materials while maintaining acceptable system throughput. 
     SUMMARY OF THE INVENTION 
     Aspects of this invention create a mark with desired properties on coated articles using a laser marking system. The laser marking system is provided with a controllable beam attenuator that transmits the laser beam at a first attenuation level and then, after a predetermined time interval, changes the attenuation to a second attenuation level thereby creating a laser mark with desired properties. Adding a controllable beam attenuator permits controllable laser fluence. Fluence is defined as the cumulative laser energy applied per unit area and is measured in Joules/cm 2 . Aspects of this invention provide a first laser fluence associated with creating a mark with desired properties on a first portion of the mark. Aspects of this invention then provide a second laser fluence associated with creating a mark with desired properties on a second portion of the mark, thereby creating marks with desired properties 
     Creating marks on a coated article by ablating a top coating layer to reveal an under layer which may be another coating or the surface of the article requires that the ablation threshold for the material to be ablated be lower than the ablation threshold for the material underneath. In most cases this can be arranged by appropriate selection of materials. For example, a topmost coating or paint that is darker, or less reflective than the under layer will absorb more laser energy and typically will ablate at a lower fluence threshold than the under layer. 
     Aspects of this invention consider damage thresholds when marking in order to create marks with desirable appearance. In order to create marks efficiently, irradiance is adjusted to maximize material removal rates without damaging underlying materials. The irradiance combined with the tool path determines the fluence, since the irradiance measures the rate at which energy is applied to the surface of the article and the tool path indicates how much time the laser beam will directed to each point on the mark. The irradiance and tool path of the laser beam is calculated to be above the ablation threshold of the material to be removed and below the damage threshold of the underlying material while maximizing the rate of travel of the laser beam with respect to the article in order to maximize throughput. The difficulty is that these thresholds can be different for different areas of the mark at different times during the marking process. Laser parameters which provide commercially desirable appearance and acceptable rate of material removal and hence throughput in one area of the mark may damage the underlying material in another area of the mark.  FIG. 2  shows the results of laser marking an article with a single irradiance and rate of travel, wherein the results are uneven and commercially unacceptable. Choosing a single set of laser parameters which results in commercially desirable appearing mark may be possible but the resulting rate of material removal must not exceed the fastest acceptable removal rate for the all portions of the mark, resulting in unacceptably low throughput. Aspects of this invention provide greater laser fluences at the start of each set of laser pulses in order to remove material that typically remains if a single laser fluence is used. 
     The damage threshold for a material at a particular location is not only dependent upon the laser irradiance being currently directed at the location but also on the recent history of exposure to laser radiation. Therefore simply measuring laser fluence will not properly predict the appearance of the material following laser processing. This is because previous irradiation of the location or nearby locations will tend to heat the material. Based on the calculated residual heating, aspects of this invention alter the laser fluence to compensate for the decreased damage threshold caused by previous laser irradiation by increasing the fluence of the initial laser pulses in a group of laser pulses. 
     Aspects of this invention control various laser parameters including laser pulse parameters such as pulse duration or pulse repletion rate, or tool path parameters such as spot size, laser beam location, laser beam speed in order to increase throughput of a laser marking system while avoiding damage to underlying materials. A laser is selected and the power, rep rate, pulse temporal shape and pulse duration are selected to provide desired material removal rates. A tool path, or locations and times at which the laser will irradiate the article to form the mark, is then calculated to provide the desired material removal rate while avoiding damage to the underlying material. One tool path calculation is the spacing between subsequent pulses on the article which is controlled by changing the speed of the relative motion between the laser pulses and the article. Another tool path calculation is spot size which controls the irradiance by moving the focal spot in the Z axis to point above or below the surface of the article. A further tool path calculation calculates the spacing between adjacent rows of pulse locations. A tool path is selected which will cover the area to be marked in lines to be traveled in a raster fashion 
     Aspects of this invention control the output of the laser. In order to facilitate application of tool paths as selected by this invention, laser pulse fluence should be attenuated very precisely under the control of the laser marking system. Aspects of this invention control the laser irradiance with sufficient precision to permit tool paths which create marks with commercially desirable uniformity, color, texture and shape. An optical switch is used to switch the laser beam on and off rapidly without requiring the laser to be turned off and on. Aspects of this invention use an acousto-optic modulator (AOM) to modulate the beam precisely and quickly and thereby direct the beam either to impinge the article or travel harmlessly to a beam dump. 
     Aspects of this invention are implemented by adapting an existing laser micromachining system, the ESI Model ML5900 Laser Micromachining System, manufactured by Electro Scientific Industries, Inc., Portland, Oreg. 97229. This system is described in detail in the “ESI Service Guide ML5900”, part number 178472A, published October 2009, Electro Scientific Industries, Inc., Portland, Oreg. 97229, and is included herein in its entirety by reference. Adaptations include addition of an electro-optic device to permit more precise control of laser fluence in real time along with software to control these changes in fluence. 
     To achieve the foregoing with these and other aspects in accordance with the purposes of the present invention, as embodied and broadly described herein, a method for creating a visible mark with desirable commercial qualities on a coated article and apparatus adapted to perform the method is disclosed herein. Included is a laser processing system having a laser, laser optics, and motion stages all operatively connected to a controller with stored, predetermined laser pulse parameters. Stored laser pulse parameters associated with the desired fluence selected depending upon which region of the mark is being processed to create marks with commercially desirable properties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  Tool path 
         FIG. 2  Prior art mark 
         FIG. 3  Microphotograph of prior art laser mark 
         FIG. 4   a  Prior art laser pulses 
         FIG. 4   b  Laser pulses 
         FIG. 4   c  Laser pulses 
         FIG. 4   d  Laser pulses 
         FIG. 5  Laser mark 
         FIG. 6  Microphotograph of laser mark 
         FIG. 7  Adapted laser marking system 
         FIG. 8  Pulse circuit 
         FIG. 9  Pulse circuit 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     An embodiment of this invention creates a mark with desired properties on coated articles using a laser marking system. The laser marking system has controllable laser fluence or dose. Embodiments of this invention determine a first laser fluence associated with creating a mark with desired properties on a first portion of the mark. Aspects of this invention then determine a second laser fluence associated with creating a mark with desired properties on a second portion of the mark. These fluences are then stored in the laser marking system&#39;s storage. The laser marking system is then directed to mark the article using the stored first laser fluence in the first portion of the mark and using the stored second laser fluence in a second portion of the mark thereby marking the article with desired properties. Embodiments of this invention control laser fluence by controlling various laser parameters including laser pulse parameters such as pulse duration or pulse repletion rate, and tool path parameters such as spot size, laser beam location, or laser beam speed in order to increase throughput of a laser marking system while avoiding damage to underlying materials. Typically, a laser is selected and the power, rep rate, pulse temporal shape and pulse duration are selected to provide desired material removal rates. A tool path is then calculated to provide the desired material removal rate while avoiding damage to the underlying material. In particular embodiments of this invention provide increased laser pulse fluence for the beginning few laser pulses in a set of laser pulses in order to remove material that would otherwise remain on the surface of the article. 
     Embodiments of this invention control the output of the laser. In order to facilitate tool paths as selected by this invention, laser pulses should be attenuated very precisely under the control of the laser marking system. Aspects of this invention control the laser irradiance with sufficient precision to permit tool paths which create marks with commercially desirable uniformity, color, texture and shape. Aspects of this invention use an acousto-optic modulator (AOM) to modulate the beam precisely and quickly without requiring the laser to be turned on and off. An embodiment of this invention employs an AOM to modulate the fluence of the laser beam by diffractively redirecting the laser beam from its normal path to a beam dump where the laser beam energy is harmlessly dissipated rather than be directed to the article surface. An AOM is used because it is capable of modulating the laser beam very quickly. Fast modulation is advantageous for embodiments of this invention because it allows the laser marking system to turn the laser beam on and off rapidly and accurately without disturbing the laser itself. 
       FIG. 1  shows a tool path used to remove a coating from an article. An article  30  is coated with a coating  31  to be removed within a shaped area  32 . The laser begins removing material along the tool path  33  at its start point  34  and removes material as it moves to point  36 , represented by the solid line. At point  36  the laser is turned off and the laser marking system re-positions the laser beam with respect to the article so that when the laser turns on, it begins removing material at point  38 , represented by the dotted line. The laser then proceeds to cut and re-position until the end point  39  is reached.  FIG. 2  shows the results of removing material as shown in  FIG. 1 . The article  40 , with a coating  42  has had the coating removed from the area of the mark  44 , exposing material underneath  46 . Note that some of the overlying coating remains  48 . This is because the particular laser irradiance used, while it is optimal for removing bulk material over the majority of the area to be marked, seems to be less effective in removing material at the beginning of the removal process. Irradiance is the rate at which laser energy is applied to the surface of the article per unit area and is measured in Watts/cm 2 . One prior art solution to this problem is to adjust the fluence and the speed of the laser with respect to the article which may reduce throughput and is hence undesirable. The fact that adjustment itself is required is also undesirable as it negatively affects system throughput.  FIG. 3  is a microphotograph showing this effect. In  FIG. 3  an article  50  has been coated with a silver coating covered by a layer of black paint  51 . A laser has been used to remove the black paint in an area  52 . Note that some black paint residue  54  remains in the area  52  where black paint was supposed to have been removed by the laser. 
       FIG. 4   a  shows is a graph of pulse energy vs. time showing groups of prior art laser pulses. Referring to  FIG. 1 , the laser pulses would start  62  when the laser beam was positioned at  34 , with the pulses ending at  64  when the laser beam reached the end of that stroke  36 . The laser beam is repositioned  38  and the laser pulses started again  66 . With this group pulse profile, material removal is incomplete. Although a pulse fluence might be found that removes the material  51  without leaving debris  54 , the laser fluence used to form the mark in  FIG. 4   a  is outside the process window for that particular article at that particular time with that particular laser. An embodiment of this invention solves this problem by tailoring the group pulse profile to provide more energy at the beginning of a stroke while maintaining the specified energy for the remaining portion of the stroke. Tailoring the pulse group envelope in this fashion to form a tailored pulse profile permits commercially desirable marking while maintaining system throughput by using a higher energy pulses at the beginning of the pulse group before changing to pulses with a preselected lower energy to complete the pulse group. This tailored pulse profile creates wider process windows by permitting the use of lower laser fluences without leaving debris. A wider process window permits higher system throughput by reducing the need for system adjustments. An exemplary tailored pulse profile is shown in  FIG. 4   b . In  FIG. 4   b , two groups of tailored pulses  70  are shown. The pulse group  70  starts at time  74  with three pulses  72  at power P 1 . At time  76  the power is reduced to P 2  and five more pulses  78  are directed to the workpiece until time  80 . Each group of pulses is tailored in this fashion to have a predetermined number of pulses of predetermined energy emitted at the beginning of each stroke of the laser beam. 
       FIG. 5  shows an article  100  coated with a coating  102  with a shape boundary  104  which has been laser processed to remove the coating  106  within the boundary  194  by a tailored pulse groups in accordance with embodiments of this invention. Note that no debris apparently remains nor has damage occurred anywhere on the mark.  FIG. 6  is a microphotograph of a similar coated article  110  which has been coated with black paint  112  that has been laser processed by an embodiment of this invention to cleanly and quickly remove the black paint  112  over a desired area to reveal silver paint underneath  114 . Note the apparent lack of debris or damage to the underlying surface. 
       FIG. 7  shows a diagram of an adapted ESI Model ML5900 laser micromachining system  120  adapted for marking articles as an embodiment of the instant invention. Adaptations include a laser  122 , and an AOM fluence attenuator  124 . Laser pulses are emitted by the laser  122  and directed by a series of mirrors and other optical elements (not shown) to the AOM  124  fluence attenuator and thereafter directed by another series of mirrors and optical elements (not shown) to the optical head  128 . The AOM fluence attenuator includes control electronics which control the fluence of transmitted laser pulses under the direction of the controller  140 . The optical head includes the X, Y and Z motion control elements  130  and the galvanometer block  132 . These elements combine to position the laser beam (not shown) with respect to the article  138  being marked to create the 2-dimensional representation of the mark on the surface of the article  138 . The article  138  is fixtured by the rotary stage element  134  which indexes the article  138  from the load/unload position to a position beneath the optical head  138 , where it is marked, and then to an optional inspection station  136  where it is inspected prior to indexing back to the load/unload station for unloading. All of these operations take place under the control of the controller  140 , which coordinates the operation of the laser  122 , the AOM fluence attenuator  124 , the motion control elements  130 , the galvanometer block  132  and the rotary stage  134  to direct the appropriate laser fluence to the appropriate locations on the article  136  to create a mark with commercially desirable appearance. 
     The adapted laser  122  is a diode pumped Nd:YVO 4  solid state laser operating at a frequency tripled 355 nm wavelength model Vanguard manufactured by Spectra-Physics, Santa Clara, Calif. 95054. The laser  122  is configured to yield up to 2.5 W, but is generally run at an 80 MHz mode locked pulse repetition rate which yields a power of about 1 W. Lasers with power of 0.5 Watts to 100 Watts or more preferably 0.5 Watts to 12 Watts may be used advantageously by embodiments of this invention. Laser repetition rates of 10 KHz to 500 MHz or more preferably 1 MHz to 100 MHz may be used. The laser  122  produces laser pulses with duration of about 1 picosecond to 1,000 nanoseconds or more preferably 100 picoseconds to nanosecond to 100 nanoseconds in cooperation with controller  100 . Pulse temporal and spatial distributions are typically Gaussian. Motion control elements  130  and galvanometer block  132  combine to provide beam positioning capability with respect to the article. Embodiments of this invention use laser spots as measured on the article ranging from 5 microns to 500 microns, or more preferably in the range from 10 microns to 100 microns. The system uses beam speeds, or relative motion between the laser beam and the article in the range of 10 mm/s to 1 m/s, or more preferably in the range of 50 mm/s to 500 mm/s. The pitch, or spacing between adjacent lines of laser pulses can range from 1 micron to 250 microns, or more preferably in the range from 10 microns to 50 microns. 
       FIG. 8  shows an electronic circuit employed to adapt a laser processing system to operate according to an embodiment of this invention.  FIG. 8  shows an input signal  150  from the controller  140  that indicates the start of a laser beam stroke. This signal is typically a logical signal that indicates TRUE/FALSE with voltage levels. When the input signal  150  indicates TRUE, or the beginning of a stroke, the signal is passed to the AOM controller  162  trigger input “T”. This indicates that the AOM is active and should be energized to transmit laser pulses. The AOM controller  162  also has an analog voltage input “A” which causes the AOM controller  162  to emit a signal  164  to the AOM (not shown) to transmit laser energy in proportion to the voltage appearing at “A”. The trigger signal  150  is also sent to a pulse circuit  154  which produces a pulse with a programmable duration. This pulse is passed to an amplifier  156  which amplifies the pulse produced by pulse circuit  154  to a programmable voltage level. This amplified pulse is conditioned by the signal conditioning filter  158  to remove any undesired components and then combined by the summation circuit  160  with the original pulse voltage  152  output by the controller  140  and sent to the “A” input of the AOM controller  162  for eventual output  164  to the AOM (not shown). 
       FIGS. 4   b  and  c  show the tailored laser pulse groups output from a laser controlled by an AOM control circuit such as  FIG. 8 . In  FIG. 4   b , the tailored pulse groups  70  have pulses  72  with energy P 1  at the beginning of the group and pulses  78  with energy P 2  for the rest of the group.  FIG. 4   c  shows the tailored pulse groups resulting when the analog input  152  to the circuit in  FIG. 8  is lowered to a level which produces laser pulses with energy P 3 . Note that with this embodiment, the energy level of the laser pulses at the beginning of the stroke have constant energy depending upon the programmed setting of amplifier  156 . 
       FIG. 9  shows an additional electronic circuit employed to adapt a laser processing system to operate according to an embodiment of this invention. In this circuit, the trigger input  170  from the controller  140  is sent to the trigger input “T” of the AOM controller  182  and to a pulse circuit  172  that generates a programmable duration pulse from the start of the trigger signal. This pulse is sent to an analog, switch  174  which gates the analog signal  152  from the controller  140  that has been amplified by an amplifier  176  with programmable gain. The analog switch  174  transmits the analog signal from  174  when a pulse from pulse circuit  172  is present. The signal then goes to the signal conditioning filter  178  which removes undesired components from the signal and passes it to the combiner  180  which combines the original analog voltage control signal  152  from the controller  140  with the amplified signal from the signal conditioning circuit  178  to form the analog control signal to be sent to the AOM controller  182  for output  184  to the AOM (not shown). This circuit will produce tailored pulses as shown in  FIG. 4   d .  FIG. 4   d  shows tailored groups of pulses  90  where the high energy pulses  92  at the beginning of each group  90  are made to be a pre-determined multiple P 4  of the energy P 3  of the predetermined pulses  94 . 
     It will be apparent to those of ordinary skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.