Abstract:
An optical scanning system and method for laser engraving a plurality of data subrasters into a substrate to form a raster of engraved data defining an image on the substrate. Each subraster has a length dimension and a width dimension. The system includes a transport assembly having an objective lens and a mirror, the mirror capable of reflecting a substantially collimated scanning beam incident thereon in a direction transverse to an axis of the incident beam such that it is directed to the objective lens. The objective lens is capable of focusing the scanning beam on the substrate to engrave a set of data in the width dimension of the subraster and the objective lens and mirror combination is capable of moving along the axis of the incident beam to allow subsequent engraving of other sets of data in the width dimension until a complete subraster is formed along its length dimension. The objective lens and mirror combination is also capable of returning to its starting position to begin engraving of a subsequent subraster of the plurality of subrasters forming the raster of engraved data. A thermoset plastic substrate is also identified as being particularly suitable for use with the aforementioned system and method.

Description:
TECHNICAL FIELD 
   This invention relates to direct laser engraving of a flat substrate for use in a printing process, such as intaglio printing. More particularly, the invention relates to an optical scanning system and substrate material particularly suitable for high-speed and high-resolution engraving. 
   BACKGROUND OF THE INVENTION 
   Many printing processes utilize substrates, platens or forms as printing surfaces to transfer an image to a printable medium. One such process is called intaglio printing. Intaglio printing involves application of printed indicia or images below the surface of a platen or substrate that is utilized as a printing surface. Traditionally, intaglio substrates have been prepared by mechanically engraving or chemically etching a recessed pattern into the printing surface of the substrate, which defines an image. The pattern may comprise an array of dots in the printing surface of the substrate. The recessed pattern, such as the array of dots, define tiny recesses within which ink is held and transferred to the printable medium, such as a sheet or surface. This intaglio process is typically used in die stamping or in engraved processes, sometimes referred to as copper plate printing. It is also used in connection with pad printing, which is typically used to decorate plastic surfaces, as well as in the gravure printing process. 
   Mechanical engraving and chemical etching techniques are time-consuming processes. Mechanical techniques are typically slow due to limitations of engraving equipment. A mechanical stylus must be used to engrave the image into the substrate, which requires a certain amount of time to penetrate and cut the substrate material. Furthermore, accuracy of the engraving becomes an issue when the stylus becomes worn and dull. On the other hand, chemical etching is time consuming due to the many steps involved. Chemical etching is a multi-step process that first involves producing the image onto a film negative, such as with an imagesetter. Once the film is produced, it becomes a mask that can be laid on top of a copper or steel substrate having a thin film coating of sensitizing material. The substrate and mask combination is exposed to light for subsequent chemical development, which transfers the mask to the copper plate. After development, the substrate is ready for acid etching to complete the process. Accuracy is also an issue with chemical etching, due to the limited controllability of the chemical-etching process. 
   Another technique involves direct laser etching, which is a single-step process that requires much less time than mechanical engraving and chemical etching techniques. In this technique, a laser is used to directly engrave the substrate material. However, because metals have a high reflectance, the laser/metal interaction is not conducive to producing plates having sharp engravings. With metals and a majority of plastics, direct laser engraving causes the material to melt, which creates the recessed areas, but also creates pooling of melted material around these recessed areas. This pooling of material acts as a ridge surrounding the recessed areas, which adversely affects the accuracy and usefulness of the printing surface of the substrate. Thus, accuracy remains an issue. Furthermore, although the direct laser technique is only a single-step process, the speed of the engraving process still remains an issue at higher resolution levels, which require the laser to engrave a higher number of tightly focused dots to achieve such resolutions. With presently known systems, the engraving process time is increased when the resolution level is increased. 
   The system and method of the present invention addresses these and other problems associated with direct laser engraving of substrates. 
   SUMMARY OF THE INVENTION 
   An optical scanning system for laser engraving a plurality of data subrasters into a substrate to form a raster of engraved data defining an image on the substrate. Each subraster has a length dimension and a width dimension. The system includes a transport assembly having an objective lens and a mirror, the mirror capable of reflecting a substantially collimated scanning beam incident thereon in a direction transverse to an axis of the incident beam such that it is directed to the objective lens. The objective lens is capable of focusing the scanning beam on the substrate to engrave a set of data in the width dimension of the subraster and the objective lens and mirror combination is capable of moving along the axis of the incident beam to allow subsequent engraving of other sets of data in the width dimension until a complete subraster is formed along its length dimension. The objective lens and mirror combination is also capable of returning to its starting position to begin engraving of a subsequent subraster of the plurality of subrasters forming the raster of engraved data. 
   In a particular embodiment, an optical scanning system for laser engraving of a plurality of subrasters of data into a substrate to form a raster of engraved data is provided and includes a scanner capable of deflecting an input laser beam incident thereon from a first beam direction to create a scanning beam. The system also includes a beam expander capable of receiving the scanning beam and expanding it to create an expanded scanning beam. A transport assembly of the system has an objective lens and a mirror, wherein the mirror is capable of reflecting the expanded scanning beam in a second beam direction transverse to the first beam direction such that it is incident on the objective lens. The objective lens and mirror is capable of moving along an axis defined by the first beam direction. The objective lens is capable of focusing the expanded scanning beam on the substrate to engrave a set of data oriented in a width dimension of the subraster and is also capable of moving along the first beam axis to allow subsequent engraving of other sets of data oriented in the width dimension until a complete subraster is formed to define a length dimension of the subraster. The objective lens and mirror combination is further capable of returning to its starting position to initiate engraving of a subsequent subraster. 
   According to another aspect of the invention, an optical scanning system is provided that is capable of engraving at two different resolutions. 
   According to another aspect of the invention, a substrate is provided for use with a direct laser engraving process to create an intaglio printing substrate. The substrate consists essentially of a thermoset plastic which substantially vaporizes in response to an impinging laser beam that engraves portions of the substrate, thereby substantially eliminating the formation of slag material adjacent to engraved portions of the substrate. 
   According to yet another aspect of the invention, a method of laser engraving a substrate for use in a printing process is provided. The method comprises the steps of: (a) directing a substantially collimated scanning beam having a beam axis to an objective lens that is movable along the beam axis, wherein the scanning beam defines a scan width; (b) focusing the scanning beam through the objective lens and onto the substrate; (c) engraving onto the substrate a set of subraster width data having a width equal to the scan width of the beam; (d) continuously moving the objective lens along the beam axis to subsequent positions relative to the substrate and engraving subsequent sets of subraster width data to form a complete subraster; (e) incrementing the substrate and engraving an additional subraster adjacent to the previously completed subraster; and (f) repeating the steps of incrementing the substrate and engraving an additional subraster until a complete raster made up of a plurality of subrasters is created that defines an engraved image on the substrate. 
   These and other aspects of the invention will become apparent from the specification, drawings and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an embodiment of a direct laser engraving system in accordance with the present invention. 
       FIG. 2  is a top plan view of a portion of the system of  FIG. 1 . 
       FIG. 3  is a side elevational view of the portion of the system shown in  FIG. 2   
       FIG. 4  is a detailed top plan view of the scanner and first lens of the beam expander shown in  FIG. 2 . 
       FIG. 5  is a schematic diagram of a byte and bit layout of a sampling of data sets of a subraster in a sample 1200 dpi scan in accordance with the principles of the present invention. 
       FIG. 6  is a functional block diagram of data handling of the system of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail one or more preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. 
     FIG. 1  depicts a schematic diagram of an embodiment of an optical scanning system  10  in accordance with the principles of the present invention. The system  10  can be utilized for laser engraving a substrate for use in a printing process. A particular feature of the system  10  is the ability to engrave at two different resolutions with the same optical system. In a preferred embodiment, the system  10  is capable of engraving at 1200 dpi and 2400 dpi. Referring to  FIG. 1 , the system  10  includes a polygon scanner  20 , a beam expander  22  and a transport (or trolley) assembly  24 . A receptor assembly  26  is disposed adjacent to the transport assembly  24  and supports the substrate to be engraved. The receptor assembly  26  is capable of incrementally translating the substrate with respect to the transport assembly  24  via a stepped transport. As will be discussed in more detail below, the transport assembly  24  includes a mirror  27  and an objective lens  28  that are also movable to facilitate the engraving process. 
   The system  10  requires an input beam  30 . A laser assembly  31  is provided to produce the input beam  30 , which is incident on the scanner  20  from a first beam direction. The laser assembly  31  includes a laser  32  (preferably a DC excited CO 2  laser), a beam compressor  34  and a modulator  36  driven by a modulator driver  38 . Referring to  FIG. 1 , the laser  32  produces a beam  40  (having a beam width A) that is shaped by the beam compressor  34  to produce a compressed beam  41  (having a beam width B). The compressed beam  41  is modulated by the modulator  36  to produce the input beam  30  (having a beam width C). Preferably, the beam width A of the beam  40  is 8 mm, the beam width B of the compressed beam  41  is 1 mm, and the beam width C of the input beam is 1 mm. The input beam  30  is scanned by the scanner  20  to produce a scanning beam  42 , which is directed to the beam expander  22 . As will be explained in more detail below, the beam expander  22  produces a substantially collimated expanded scanning beam  50  (having a beam width D), which is directed to the mirror  27  and objective lens  28  combination of the transport assembly  24 . The mirror  27  reflects the beam  50  to the objective lens  28 , which focuses the beam onto the substrate. Preferably, the beam width D of the expanded scanning beam  50  is 20 mm and the mirror  27  is angled at 45 degrees. Essentially, the beam expander  22  expands the scanning beam  42  and relays it to create the expanded scanning beam  50 , which has a substantially constant width over an extended distance. 
     FIGS. 2 and 3  depict the system  10  in more detail.  FIG. 2  is a top plan view of the system  10  and  FIG. 3  is a side view of  FIG. 2 . In a preferred embodiment, the scanner  20  is a polygon scanner having  15  facets and is rotatable in a direction indicated by rotational arrows in  FIGS. 2 and 3 . In a preferred embodiment, the scanner rotates at either 10,000 rpm or 20,000 rpm, depending on desired resolution. Considering the scanner  20  as a nutating mirror executing ±10° mechanical deflection of the input beam  30 , the input beam  30  is deflected ±20° due to mirror doubling to produce the scanning beam  42 . As shown in  FIGS. 2 and 3 , the beam expander  22  includes a first lens L 1  and a second lens L 2 . Referring to the detailed view of the scanner  20  and the first lens L 1  of the beam expander  22  in  FIG. 4 , the scanning beam  42 , which is a substantially collimated beam having a beam width C and deflected ±20°, is directed to be incident on lens L 1 . In a preferred embodiment, lens L 1  has a focal length of 15 mm. The lens L 1  is located one focal length f 1  from the scanner surface and acts telecentrically such that the ±20° beams continue to the right of the lens L 1  and remain substantially parallel to a lens axis  60 , while deflected components of the beams converge to a focal plane  64  one focal length f 1  beyond the lens L 1 . Referring to  FIG. 2 , beyond the focal plane  64 , an upper limit beam  66  and a lower limit beam  68  of the deflected beam components expand. Referring to  FIG. 3 , the focal plane  64  acts as a plane of beam symmetry, as shown in  FIG. 3 . 
   The upper limit beam  66  and the lower limit beam  68  expand until they reach a second lens L 2 , which is spaced a focal length f 2  from the focal plane  64  of the first lens L 1 . In a preferred embodiment, the focal length f 2  is 300 mm. Utilizing the preferred pair of lenses L 1  (15 mm focal length) and L 2  (300 mm focal length), the beam expander  22  acts as a 20×beam expander ( 300/15=20). Thus, the beam expander  22  in the preferred embodiment expands the 1 mm input beam to a substantially collimated 20 mm output beam. A fundamental consequence of beam expansion is a complimentary compression of scan angle of the beam (in the preferred embodiment, compression from ±20° to ±1°). This reduction in scan angle imparts a practical field angle to the objective lens  28 . 
   Another consequence of this configuration in the preferred embodiment is the relaying of a 1 mm beam aperture at the scanner  20  to a 20 mm beam aperture at the 300 mm focal distance of lens L 2 . Thus, as shown in  FIG. 2 , although the beams shift laterally (due to deflection) in the long region between the scanner  20  and the objective lens  28 , the beam shift reduces to zero at a distance E beyond lens L 2  (300 mm in a preferred embodiment). In other words, the stability of the 1 mm beam at the scanner  20  (scanning beam  42 ) is transferred as a stable 20 mm beam (expanded scanning beam  50 ) at the objective lens  28 . As explained in more detail below, in a preferred embodiment, the 300 mm distance is a nominal beam travel length for the objective lens  28  with respect to its movement on the transport assembly  24  along a beam axis  70  of the expanded scanning beam  50 . 
   Referring again to  FIGS. 2 and 3 , the transport assembly  24  facilitates movement of the mirror  27  and the objective lens  28  along the beam axis  70  of the expanded beam  50 . The mirror  27  and the objective lens  28  combination are movable within the transport assembly  24  in both directions for a distance F along the beam axis from the nominal position, for a total travel of 2F. In a preferred embodiment, the transport assembly  24  facilitates movement of the mirror  27  and the objective lens  28  for 4 inches (F=4 inches) in each direction for a total range of travel of 8 inches (2F), which corresponds to a dimension of an image format size, which is 8″×10″ in a preferred embodiment. Of course, other format sizes could be accommodated through adjustment of the system in accordance with the principles of the present invention. 
   Referring to  FIG. 3 , the expanded scanning beam  50  is intercepted by the mirror  27  and reflected, thereby folding the beam  50  so that it is incident on a principal plane  80  of the objective lens  28 . The beam  50  is intercepted by the mirror  27  at a nominal position of G, folding the remaining length (E–G) to complete the nominal distance E (300 mm in a preferred embodiment) to the principal plane  80  of the objective lens  28 . At the objective lens  28 , the beam width of beam  50  (20 mm in a preferred embodiment) is converged over a focal distance (f 3 ) of the lens  28  to an energetic focal point to engrave the surface of the substrate material. The transport assembly  24  transports the mirror  27  and the objective lens  28  over a dimension of the format size ( 2 F). For simplicity of illustration, the limit positions of the mirror  27  and the objective lens  28  are represented in  FIG. 3  by the two mirrors shown in phantom lines. It is understood that the mirror  27  and the objective lens  28  move in combination. 
   Referring to the right side of  FIG. 2 , it is noteworthy that with a collimated beam incident on the objective lens  28 , even though the beam position in the aperture of the lens  28  shifts slightly during transport, the fundamental criterion for accurate placement of the engraving focal point, which is the angle of the collimated beam with respect to the lens axis, remains a constant of the system. As explained below, this establishes uniformity of a width dimension of each of a plurality of subrasters of data engraved to form, in combination, an engraved image. 
   The present invention utilizes a method of engraving the substrate wherein a plurality of data subrasters are engraved to form individual swaths, which, in combination form a complete raster of data representing an image to be engraved. Referring to the schematic diagram of  FIG. 5 , a byte and bit layout of a sampling of data sets  102  of a subraster in a sample 1200 dpi scan is shown in accordance with the principles of the present invention. Each data bit of the data sets  102  represents a point, or dot, of the image to be engraved. As shown in  FIG. 5 , each subraster  104  comprises a plurality of data sets in a width dimension of the subraster. In the 1200 dpi resolution example, each data set  102  comprises 40 dots across the width dimension of the subraster (5 sets of 8 dots). In a 2400 dpi example (not shown), each data set comprises 80 dots across the width dimension of the subraster (10 sets of 8 dots). The expanded scanning beam  50  engraves each data set across the subraster width as the transport assembly  24  moves the mirror  27  and objective lens  28  combination in a direction along the length dimension of the subraster  104  (denoted by X in  FIG. 5 ) until the subraster  104  is completed. The objective lens and mirror combination returns to its starting position and the receptor assembly  26  incrementally translates the substrate in a direction along the width dimension of the subraster  104  (denoted by Y in  FIG. 5 ) to initiate engraving of a subsequent subraster. The translation increment is equal to the width dimension of the subraster  104 . In a preferred embodiment, the width dimension of each subraster is about 33.3 mils and a total of 300 subrasters are utilized to form the raster. Although the movement of the objective lens and mirror combination is described herein in various phases or steps of the engraving process, it is understood that movement of these components is continuous throughout the engraving process. Table  1  below shows the data point content for both 1200 dpi and 2400 dpi images in an 8″×10″ image format. 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Operational Data Point Content for 8″ × 10″ Image Format 
             
           
        
         
             
                 
               Each Data Subraster 
                 
             
           
        
         
             
                 
               Data 
                 
               Total Data Points 
               Total Data Points 
             
             
               Resolution 
               Points 
               Data Sets 
               Per Subraster 
               300 Subrasters 
             
             
                 
             
           
        
         
             
               1200 dpi 
               40 
               9,600 
               384,000 
               115,200,000 
             
             
               2400 dpi 
               80 
               19,200 
               1,536,00 
               460,800,000 
             
             
                 
             
           
        
       
     
   
   During engraving, the laser  32  remains on. Instead of turning the laser  32  on and off, the modulator  36  shifts the beam  30  to a position analogous to ON. An unshifted beam position is analogous to OFF. During engraving, this shift happens at a very high rate. In the OFF position, the beam  30  exits the modulator  36 , strikes a dump mirror (not shown), and deflects into a beam dump (not shown) to absorb unwanted laser power. When the modulator  36  shifts the beam  30  in an ON position, the shifted beam  30  bypasses the dump mirror and impinges on the scanner  20 . Based on these ON and OFF positions, each data point or dot can represent an engraved point (ON) or an unengraved point (OFF) on the surface of the substrate. 
     FIG. 6  depicts a functional block diagram of data handling of the system. As shown in  FIG. 6 , a master clock  200  operates at 480 kHz, which is the data rate for 2400 dpi resolution. The master clock  200  is stepped down to 120 kHz for 1200 dpi resolution. The data rate clocks clock an X–Y transport logic  210 , which in turn drives an X–Y transport controller  212  for the transport assembly  24  (which moves the mirror  27  and objective lens  28  in the X-direction) and the receptor assembly  26  (which incrementally translates the substrate in the Y-direction). The data rate clocks also clock a modulation logic  310 , which in turn operates the modulator driver  38  that drives the modulator  36 . The master clock  200  is stepped down further to provide a motor control, as shown in  FIG. 6 . The scanner  20  rotates at either 10,000 rpm (1200 dpi) or 20,000 rpm (2400 dpi). A motor controller  410  is clocked at 6 kHz for 20,000 rpm and 3 kHz for 10,000 rpm to control a motor  412  that drives the scanner  20 . An encoder signal is provided back to the motor controller from the scanner  20 . S.O.S (Start of Scan) pulses from the scanner  20  and are fed to the modulation logic  310  and and the X–Y transport logic  210 . Each S.O.S. pulse initiates the first data point of each data set in the width dimension of the subrasters. Each S.O.S pulse also triggers the modulation sequence for each data set. 
   As already mentioned, a particular advantage of the system  10  is its ability to provide two different resolutions with the same optical system (1200 dpi and 2400 dpi in a preferred embodiment). This is accomplished by narrowing the modulation pulse width at 2400 dpi to half of the modulation pulse width at 1200 dpi, while doubling its repetition rate, which doubles the dot count along the subraster width from 40 to 80. Correspondingly, the motor speed of the scanner is doubled from 10,000 rpm to 20,000 rpm, which provides a full double-resolution dot array (data set) across the width dimension of the subraster. The modulation pulse width is narrowed by reducing the intensity of the beam. Since the dots are formed by a Gaussian focused beam contour, reducing the pulse duration by one half reduces the dot exposure (intensity) on the substrate, which, in turn, sufficiently reduces the dot width to facilitate the double-resolution engraving. The repetition rate of the modulation pulse width is changed via software control. Since the exposure (intensity) is reduced at the 2400 dpi resolution, the total energy remains the same as that at 1200 dpi resolution. Since the total energy is the same, the total engraving duration is the same. Thus, the system is capable of doubling its engraving resolution without increasing engraving time. 
   From the foregoing description, it is apparent that changing the resolution of the system is rapidly accomplished without the need for critical mechanical changes, such as changing the objective lens to focus to a smaller dot size, which can be very costly. Furthermore, two different resolutions can be engraved with the same optical system, which creates the same subraster format to cover the same total area during the same total time, and the same laser providing the same optical power. 
   As yet another aspect of the present invention, it has been found that the use of a thermoset plastic material as the substrate substantially eliminates unwanted slag formation around the engraved points of the substrate. The thermoset plastic material substantially vaporizes in response to the impinging laser beam that engraves points of the substrate, thereby substantially eliminating the formation of the slag material. Desirable results have been achieved by including a mineral filler with the thermoset material. Preferably, the mineral filler has a grain size smaller than a smallest feature of the engraved portions of the substrate. Preferably, the grain size is in the range of about 3 to 5 microns. However, the grain size can be varied to match a particular resolution. The filler adds strength to the substrate material, thereby maintaining the accuracy and detail of the engraved portions of the substrate. In a preferred embodiment, silica is utilized as a filler for the thermoset material. Additionally, a flame retardant can be included to minimize flame and smoke formation from the impinging laser beam. 
   Thermoset plastics provide for more accurate laser engraving due, in part, to their strength and resistance to flow. The polymer component consists of molecules with permanent cross-links between linear chains that form a rigid three-dimensional network structure which cannot flow. The tightly cross-linked structure of thermosetting polymers immobilizes the molecules, providing hardness, strength at relatively high temperature, insolubility, good heat and chemical resistance, and resistance to creep. The use of a thermoset plastic material for a substrate has a significant impact on the cost of printing processes that utilize such substrates. Thermoset plastic substrates are much less expensive than copper or steel substrates and they do not sacrifice engraving accuracy, and hence, printing accuracy. 
   It is contemplated that a variety of thermoset plastic materials can be utilized in accordance with the principles of the present invention. Such materials include epoxies, unsaturated polyesters, phenolics, amino resins (such as urea- and melamine-formaldehyde), alkyds, allyl family (such as diallyphthalate), silicone molding compunds, and polyimides (such as bimaleimides). 
   It is understood that, given the above description of the embodiments of the invention, various modifications may be made by one skilled in the art. Such modifications are intended to be encompassed by the claims below.