Abstract:
A method of protecting a laser unit from dust during laser processing of a target in a processing area includes establishing an essentially ambient pressure at the processing area, and directing a longitudinal gas flow towards the processing area to establish in a first region intermediate the processing area and the laser unit, thereby effectively preventing dust from moving towards the laser unit and at the same time minimizing the forces acting on the target. A corresponding dust protection device defines a channel extending from a radiation inlet opening, facing the laser unit, to a radiation outlet opening, facing the target. A gas control unit communicates with the channel to feed gas thereto through at least one gas inlet aperture spaced from the radiation outlet opening. Simultaneously, the gas control unit removes gas from the channel through at least one gas outlet aperture adjacent to the radiation outlet opening.

Description:
This aplication claims the benefit of U.S. Provisional Application No. 60/194,031, filed Apr. 3, 2000. 
    
    
     TECHNICAL FIELD 
     The present invention relates to laser material processing, and more specifically to a method and a device for protecting a laser unit from dust or debris produced during laser processing of a target or substrate in a processing area. 
     The present invention is especially useful in connection with an apparatus for laser engraving or marking. Therefore, the technical background of the invention, and objects and embodiments thereof, will be described with reference to such laser engraving apparatus. However, the invention may also be applicable in connection with equipment for any other type of laser material processing, such as other laser surface treatment, laser cutting and laser welding. 
     BACKGROUND ART 
     Generally, for example as disclosed in DE-A1-43 38 774 or DE-A1-44 05 203, a laser engraving apparatus comprises a laser unit and a target guiding device which is disposed below the laser unit and adapted to guide a target past the laser unit. The laser unit includes a laser head being adapted to generate laser radiation, and a control head being adapted to direct and focus the laser radiation on the target to be provided with laser engraved markings. When the target is being laser processed, material is removed from the target in the form of small particles. These particles, or debris, collectively produce dust that either adheres to the surface of the target or, by the impact of the laser radiation, flies away from the target. In such an apparatus, it is important to prevent dust from entering the laser unit, since the presence of dust potentially could lead to deterioration or destruction of any optical components included therein. 
     In a laser engraving apparatus disclosed in EP-A1-0 085 484, a not further described air-curtain arrangement is used in connection with a depending bellow intermediate a laser unit and an underlying engraving area to prevent ingress of dust into the laser unit. Such an arrangement might be sufficient to prevent dust contained in the surrounding air from entering the laser unit. However, dust or debris which is produced at the engraving area, as a result of material being removed from the target, might still enter the laser unit. This problem is enhanced in high-precision engraving, when the distance from the laser unit to the engraving area has to be reduced, thereby bringing the source of dust generation closer to the sensitive laser unit. 
     Evidently, the situation would be even worse if the laser unit were to be arranged beneath the engraving area, since gravity would promote an accumulation of dust on the laser unit. When two opposite sides of a target are to be provided with engraved markings, especially when the markings on the two sides must be precisely located with respect to one another, it is advantageous, or even necessary, to engrave the target from both sides. If the target has to be turned upside-down between the engraving operations, the positional relationship between the opposite sides is easily lost. This problem is accentuated when a continuous web of material is being engraved, since the turning operation will be complicated to achieve and requires a great deal of space. Such a turning operation is often inconsistent with high production speeds. Also, in using two consecutive engraving stations to engrave one respective side of the web, and an intermediate turning station for turning the web, the distance between the engraving stations must be so large that it is difficult to maintain a positional relationship between the markings on the opposite sides of the web. 
     Further, if the target to be provided with laser engraved markings is small, light-weight and/or made of flexible material, a conventional air-curtain arrangement might undesirably alter the position or shape of the target during the engraving operation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to eliminate, or at least alleviate, the drawbacks mentioned above and to provide an improved method and device for protecting a laser unit from dust during processing of a target or substrate in a processing area. More specifically, the invention aims at essentially eliminating ingress of dust into the laser unit while still allowing for high-precision laser processing of all kinds of targets. 
     It is also an object of the invention to provide for minimum influence on the position and shape of the target during processing thereof in the processing area. 
     A further object of the invention is to provide for laser processing from either side, or both sides, of a target. 
     These and other objects, which will appear from the following description, have now been achieved by a method and a device as defined in the appended independent claims. Preferred embodiments of the invention are set forth in the dependent claims. 
     By establishing an ambient pressure at the processing area, forces acting on the target can be minimized. Thus, with respect to the dust protection measures, any kind of feasible target can be processed, even targets that are small, light-weight and/or flexible. By establishing a longitudinal gas flow directed towards the processing area in a first region intermediate the processing area and the laser unit, dust is effectively prevented from moving towards the laser unit. Further, the method and device of the invention allows for laser processing from beneath a target, since the longitudinal gas flow in the first region can be adjusted to counteract the gravitational driving force of the dust produced on the target during processing. 
     In one embodiment, gas is simultaneously removed adjacent to the processing area and fed to the first region intermediate the processing area and the laser unit, thereby establishing the ambient pressure at the processing area and the gas flow directed towards the processing area. Preferably, the longitudinal gas flow is accelerated towards the processing area, to further increase the ability of the gas flow to counteract the gravitational driving force of the dust. 
     In another embodiment, a housing is arranged intermediate the laser unit and the processing area. A channel extends within the housing from a radiation inlet opening which is facing the laser unit, and a radiation outlet opening which is facing, and generally is located close to, the processing area. Gas is simultaneously fed to and removed from the channel at separated locations therein. By balancing the feeding and removal rates of gas in the housing, it is assured that the pressure at the radiation outlet opening, and in practice also at the processing area, is maintained at essentially ambient pressure. In a simple and efficient arrangement, the balancing of the feeding and removal rates of gas is effected by controlling the gas removal rate such that essentially ambient pressure is established at the processing area. 
     It is preferred that the removal of gas is effected adjacent to the radiation outlet opening, since such removal will facilitate the control of the pressure at the radiation outlet opening. Furthermore, generated dust can be removed from the region around the radiation outlet opening. Thus, the target can be cleaned from dust while being processed. 
     Preferably, the longitudinal gas flow is established over essentially a full cross-section of the channel in the first region. In one embodiment, this is achieved by feeding gas into the first region in at least one pair of opposite inlet gas flows, which cooperate to form the longitudinal gas flow directed towards the processing area. Preferably, the opposite inlet gas flows together sweep a full cross-section of the channel in the first region. On entering the channel, the opposite inlet gas flows are preferably directed essentially perpendicular to the longitudinal center line of the channel, so that they meet and together form the longitudinal gas flow over the whole cross-section of the channel. 
     In another embodiment, a peripheral gas flow is separately established along a channel periphery in the first region. Preferably, this is achieved by directing one or more gas jets from the channel periphery onto the processing area. Such gas jets will counteract the formation of any wake regions close to the channel walls, particularly in any corners thereof. Additionally, such gas jets can assist in moving the generated dust from the target to the radiation outlet opening where it is efficiently removed. 
     Preferably, gas is removed in at least two diametrically opposite directions adjacent to the radiation outlet opening. This arrangement allows for sufficient control of the pressure at the processing area, as well as efficient removal of dust from the processing area. 
     In a further embodiment, a pressure barrier is established in a second region in the channel, preferably intermediate the first region and the laser unit. The pressure barrier, i.e. a region of higher pressure as seen from the processing area, will assist in preventing dust from reaching the laser unit. 
     In an additional embodiment, a lateral gas flow is established in a third region in the channel, preferably intermediate the first region and the laser unit. The lateral gas flow is established between opposite openings in a side wall portion of the channel, preferably in the form of a sheet of gas that sweeps a full cross-section of the channel in the lateral direction. Any dust passing the first region, and the second region if present, has been significantly decelerated and will be effectively removed by such a lateral gas flow. Preferably, the lateral gas flow is established by balancing the associated gas feeding and removal rates, so that the lateral gas flow is prevented from interacting with, and potentially disturbing, the flow field in the first region. 
     In one embodiment, the channel has a non-circular cross-section, at least in the first region. This configuration will prevent any formation a swirling gas motion in the first region. Such a swirling gas motion, for example around the longitudinal axis of the channel, would exhibit a central low-pressure zone, in which dust could move towards the laser unit. Preferably, the cross-section is polygonal. To counteract any formation of wake regions close to the channel walls, particularly in the corners of the polygon, it is preferred that jet-generating openings are formed in a side wall portion of the channel, preferably at the corners. Each jet-generating opening has a shape and location to provide for a gas flow along a periphery portion of the channel towards the processing area. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, a presently preferred embodiment of the invention will be described, reference being made to the accompanying schematic drawings. 
     FIG. 1 is a side view of a laser engraving apparatus in a system for manufacturing opening tabs for can ends, the laser engraving apparatus including a dust protection device according to the invention. 
     FIG. 2 is a side view of the dust protection device of FIG.  1 . 
     FIG. 3 is a cross-sectional view taken along the center line of the dust protection device of FIG.  2 . 
     FIG. 4 corresponds to FIG.  3  and schematically indicates regions of different gas characteristics in the interior of the dust protection device. 
     FIG. 5 is a perspective view of the dust protection device of FIGS. 2-4. 
     FIG. 6 is a slightly inclined end view of the dust protection device of FIG.  5 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows part of a system for manufacture of marked tabs or opening rings to be included in ends for beverage cans (not shown). A thin metal strip S is fed from a supply  1  to a laser unit  2  supported by a supporting member  3 , and finally fed to a tab forming unit  4  which is of a type known per se and which forms tabs by punching and stamping the strip S (see for instance the pamphlet “This is PLM Fosie” issued by Applicant&#39;s company PLM Fosie AB in the mid nineties). The strip S is guided by a guiding device  5  when passing the laser unit  2 , and fed from the supply  1  by a feeding means (not shown) arranged in association with the tab forming unit  4 . The laser unit  2  is of a high-power and high-speed type and is capable of providing engravings or markings in the surface of the strip S. This laser unit  2  comprises a laser head  2 ′, which is adapted to generate laser radiation at a suitable wavelength, and a so-called scanner head  2 ″, which is adapted to receive the laser radiation from the laser head  2 ′ and focus and direct the generated radiation to a given location on the surface of the strip S. Thus, a laser processing area S′ (FIGS. 2-4) is formed at the surface of the strip S. 
     Since the tab surface available for the markings is very small, the laser radiation must be accurately positioned on the strip S, and the strip S must be also be accurately positioned during the laser engraving operation. In general, the strip S is made of aluminum with a thickness of about 0.24 mm and a width of about 67 mm. Such a strip S will flex even when relatively small forces are applied to its surface. 
     Intermediate the scanner head  2 ″ and the guiding device  5 , there is provided a dust protection device  6 . The dust protection device  6  is in fluid communication with a unit  7  controlling and effecting simultaneous feeding and removal of air to and from the dust protection device  6 , as will be further described below with reference to FIGS. 2-4. The air control unit  7  comprises a main control device  7 A, such as a computer, an air pumping device  7 ′, such as a fan or pump, an air sucking device  7 ″, such as a fan or pump, and a high-pressure device  7 ′″, such as a compressor. The main control device  7 A could also be connected to one or more pressure sensors, as will be further described below. 
     As shown in more detail in FIGS. 2-4, the dust protection device  6  comprises a housing  8  having a flange  9  at a first end to be directly fastened to the scanner head  2 ″ (indicated with ghost lines in FIGS.  2 - 4 ), by means of bolts or the like (not shown) extending through holes (not shown) in the flange  9 . A longitudinal channel  11  extends from a radiation inlet opening  12  at the first end to a radiation outlet opening  13  at an opposite second end. When installed, the radiation outlet opening  13  faces the processing area S′, and radiation is transmitted through the channel  11  between the scanner head  2 ″ and the surface of the strip S (indicated with ghost lines in FIGS.  2 - 4 ). The housing  8  has an inspection window W to allow for inspection of the channel  11  during operation of the dust protection device  6 . The second end may or may not be connected to the guiding device  5  (only shown in FIG.  1 ). The distance from the radiation outlet opening  13  to the strip surface is generally less than one or two centimeters. 
     As shown in FIG. 3, two opposite air inlet openings or apertures  16 ,  17  are formed in a side wall portion of the channel  11 . In the illustrated embodiment, as shown in FIG. 6, the channel  11  is delimited by four side walls and has a generally square or rectangular cross-section in the lateral direction. The air inlet openings  16 ,  17 , which are mutually identical in shape and dimensions, extend across one respective side wall  14 ,  15  of the rectangular channel  11 . The openings  16 ,  17  communicate with a respective air inlet chamber  18 ,  19 , each having a spigot  20 ,  21  for connection to the air pumping device  7 ′ by means of hoses H 1  or the like (FIG.  1 ). 
     Also referring to FIG. 3, two opposite air outlet openings or apertures  22 ,  23  are formed in the side walls  14 ,  15  adjacent to the radiation outlet opening  13 . The air outlet openings  22 ,  23 , which are mutually identical in shape and dimensions, extend across one respective side wall  14 ,  15  of the rectangular channel  11 . The openings  22 ,  23  communicate with a respective air outlet chamber  24 ,  25 , each having a spigot  26 ,  27  for connection to the air sucking device  7 ″ by means of hoses H 2  or the like (FIG.  1 ). 
     In operation, the air pumping device  7 ′ continuously and symmetrically feeds air to the air inlet openings  16 ,  17 . Thus, two opposite symmetrical air flows, laterally directed towards the center of the channel  11 , as shown by arrows in FIG. 3, are set up through the air inlet openings  16 ,  17 . Simultaneously, air is symmetrically sucked in two opposite directions, as shown by arrows in FIG. 3, from the region close to the radiation outlet opening  13  through the two opposite air outlet openings  22 ,  23 . Thus, the air sucking unit  7 ″ continuously sucks air through the air outlet openings  22 ,  23 , thereby establishing a region R 0 , schematically indicated in FIG. 4, of essentially ambient pressure. Thus, ambient pressure is established at the radiation outlet opening  13  and at the processing area S′. The air flows entering through openings  16 ,  17  meet and form, in a region R 1  indicated in FIG. 4, a combined air flow in the longitudinal or vertical direction towards the radiation outlet opening  13  and the processing area S′. Since the openings  16 ,  17  extends over a respective side of the rectangular channel  11 , the longitudinal air flow is established over essentially the whole cross-section of the channel. 
     In the illustrated embodiment, the channel walls  14 ,  15  in region R 1  are inclined towards the center line L of the channel  11  so that the cross-section continuously decreases towards the radiation outlet opening  13 . In this configuration, the longitudinal air flow is accelerated towards the radiation outlet opening  13  in region R 1 . This has been found to improve the ability of the device  6  to protect the scanner head  2 ″ from dust produced at the processing area S′. 
     For optimum performance, it has been found that the air outlet openings  22 ,  23  should be inclined towards the center line L, preferably at angle of about 30-60°, so that the openings  22 ,  23  to some extent face the radiation outlet opening  13 . This has been found to enhance the ability to remove dust from the processing area, as well as the ability to establish an essentially uniform and ambient pressure in region R 0  at the opening  13 . To further promote the uniformity of the pressure distribution in region R 0 , the total surface area of the openings  22 ,  23  should be about ⅔ of the surface area of the radiation outlet opening  13 . 
     As indicated in FIG. 4, a region R 2  of stagnant air at a comparatively high pressure is established in the channel  11 , immediately beneath region R 1 . Thus, a pressure barrier is established in region R 2 , i.e. a pressure gradient of increasing pressure towards the radiation inlet opening  12 . This pressure barrier assists in preventing dust from reaching the scanner head  2 ″. 
     In some channel geometries, wake regions can be formed, particularly in any corners of the channel  11 . In such wake regions, or stagnant regions, dust might slide along the walls of the channel  11 , thereby passing region R 1 . To this end, as illustrated in FIGS. 2 and 6, a number of jet-generating passages  28 , which open into the channel  11 , are provided between the radiation inlet opening  12  and the air inlet openings  16 ,  17 . The passages  28  are arranged to form jets J that are directed towards a central point P in the processing area S′, i.e. on the strip surface, as indicated by single line arrows in FIGS. 3 and 4. More specifically, five passages  28  are formed at each lateral corner of the channel  11 , as shown in FIG.  5 . The twenty passages  28  communicate with a common ring-shaped chamber  29  which is connectable to the high-pressure device  7 ′″ of the air control unit  7 , for example by means of a hose H 3  (FIG.  1 ). Due to an inherent spread in the generated air jets J, a flow of air will be established along the channel walls. This peripheral flow of air will eliminate the above-mentioned wake regions. The jets J have the additional function of breaking away dust particles formed at the processing area S′ during the engraving operation. 
     The dust protection device according to the preferred embodiment of FIGS. 1-6 comprises an auxiliary dust protection arrangement  30 . This arrangement  30  sets up a lateral sheet of air across the full cross-section of the channel  11  in a region R 3  (indicated in FIG. 4) intermediate the region R 2  and the radiation inlet opening  12 . This lateral sheet of air is arranged to capture and remove any dust that passes the regions R 1  and R 2 , for example heavy particles. As shown in FIG. 3, two opposite openings or apertures  31 ,  32  are formed in a side wall portion of the channel  11 . The openings  31 ,  32 , extend across one respective side wall of the rectangular channel  11 . The openings  31 ,  32  communicate with a respective air chamber  33 ,  34 , each having a spigot  35 ,  36  for connection to the air control unit  7  by means of a respective hose H 1 ′, H 2 ′ or the like (FIG.  1 ). The opening  31  is connected to the pumping device  7 ′ and has a delivery snout  31 ′ extending into the channel  11 . The opening  32  is connected to the air sucking device  7 ″ and has a reception snout  32 ′ extending into the channel  11 . The delivery snout  31 ′ is designed to form the entering air into a lateral sheet, and the reception snout  32 ′, being slightly larger in the longitudinal direction, is designed to receive essentially all air leaving the snout  31 ′. In operation, the air feed and removal rates through the openings  31 ,  32  are essentially balanced, so that the lateral air flow is established with minimum interaction with the other regions R 0 -R 2  in the channel  11 . 
     The embodiment shown on the drawings has been used in a laser processing apparatus with satisfactory results. Here, the air pumping device  7 ′ delivers air at a rate of approximately 10,000-30,000l/min to the openings  16 ,  17 ,  31 , equally distributed between the three. The high pressure device  7 ′″ feeds air at a pressure of 0.15-0.4 MPa (1.5-4 bar) to the ring-shaped chamber  29 , thereby equally distributing air to the jet-generating passages  28 , each having a diameter of approximately 1.5 mm. Simultaneously, the air sucking device  7 ″ is controlled by means of the main control device  7 A to remove air from the openings  22 ,  23 ,  32 . The air flows are controlled in such manner that essentially ambient pressure is established at the radiation outlet opening  13 , and thereby also at the processing area S′ on the strip S. In one embodiment, this is done without feed-back control by simply balancing the air flows entering and leaving the housing  8 . In another embodiment, the removal of air is actively controlled by the control unit  7 A based on the output of one or more pressure sensors (the sensors  37 ,  38  in FIG.  3 ), which are arranged in association with the channel  11 . Alternatively, all air flows into and out of the housing  8  could be individually and actively controlled by the control device  7 A. 
     The amount of pressure deviation from ambient that can be tolerated at the opening  13  depends on the type of target. With the present target, the pressure at the radiation outlet opening  13  is preferably controlled within ±1 kPa (±10 mbar) of ambient, in order not to undesirably affect the position or shape of the strip S during the engraving operation. 
     As shown in FIGS. 3-6, the dust protection device  6  includes a supplementary cleaning arrangement  40 . The cleaning arrangement  40  is provided at the first end of the housing  8 , i.e. facing the scanner head  2 ″. The cleaning arrangement  40  comprises two lateral, hollow pipes  41 ,  42  which are connected to an exterior coupling  43  (FIG. 6) of the housing  8 . The coupling  43  is in turn connected to the high pressure device  7 ′″ by means of a hose H 3 ′ or the like (FIG.  1 ). In the periphery of each pipe  41 ,  42 , there is provided one or more rows of holes  44 . Each row of holes  44  is offset with respect to the lateral direction, thereby being capable of directing air towards the radiation inlet opening  12 , to remove any dust deposited on the scanner head  2 ″. Preferably, the air control unit  7  is operated to intermittently feed air through the cleaning arrangement  40 , so that air is emitted in short bursts from the rows of holes  44  (FIG.  5 ). The cleaning arrangement  40  is advantageously used whenever dust might have been deposited on the scanner head  2 ″, for example when restarting the laser engraving apparatus after a shut-down or breakdown in production. 
     The dust protection device  6  can be modified in numerous ways without departing from the scope defined in the appended claims. For example, any number of openings could be provided to generate the longitudinal air flow in region R 1  and the lateral air flow in region R 3 . Also, further air outlet openings could be provided to improve the pressure control at the radiation outlet opening  13 . Likewise, a different arrangement of the air openings might be used to achieve the desired flow distribution within the channel  11 . The above also applies to the jet-generating openings  28 . 
     It should be appreciated that the air control unit  7  might include some kind of filter (not shown) to remove dust particles in the air flow from the dust protection device  6 . Further, other gases than air might be used. 
     In addition to the dust protection device  6  described above, the invention also relates to a method of protecting a laser unit  2  from dust during laser processing of a target, in this embodiment a strip S, in a processing area S′. In its broadest aspect, this method comprises the steps of establishing an essentially ambient pressure at the processing area S′, and establishing, in region R 1  intermediate the processing area S′ and the laser unit  2 , a longitudinal gas flow which is directed towards the processing area S′. 
     It should also be appreciated that the inventive device and method could be used in protecting other types of marking units for non-mechanical processing of targets.