Abstract:
A device forms grooves or slits in a material web as it moves in a longitudinal direction. The grooves or slits, which are spaced apart from one another in the longitudinal direction, are formed by a laser beam. A laser beam source generates a continuous laser beam and a deflection mirror directs the laser beam onto the material web. The mirror rotates about an axis of rotation and it is displaceable along the axis. The mirror has a first mirror face, the line of intersection of which with a sectional plane perpendicular with respect to the axis of rotation forms a circular arc which varies depending on the axial position of the sectional plane. A second mirror face deflects the laser beam towards an absorber. Focusing optics focus the laser beam, after reflection from the first mirror face, onto the moving material web.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit, under 35 U.S.C. §119(e), of provisional patent application No. 61/563,658 filed Nov. 25, 2011; the prior application is herewith incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a device for forming grooves or slots, which are spaced apart from each other in a longitudinal direction, by means of a laser beam into a material web, which is moved in the longitudinal direction. 
     Packaging films, in particular packaging films for packaging foodstuffs, are provided in a plurality of applications with longitudinal grooves spaced apart from each other, which serve as a tear-open aid. The longitudinal grooves extend only up to a certain depth such that the barrier properties of the packaging film are maintained in order to protect the packaged foodstuffs from light, oxygen, moisture, drying out, contamination or loss of aroma. 
     The insertion of these types of grooves is effected in many cases by way of a focused laser beam while the packaging film is being wound from a storage roll onto a take-up roll. 
     In order to be able to insert grooves at a constant length, constant depth and constant spacing into a moving material web of this type by means of a laser beam, it is known to use a laser beam source which generates a pulsed laser beam, the frequency and pulse duration of which is varied with the changing speed of the material web, at the same time the laser output per pulse having to be kept constant. In this case, it has been shown that a method of this type cannot fulfill the demands made in practice—variation in the web speed of 2-10 m/s, adjustment of the duty cycle (product of pulse duration and pulse frequency) within a frequency range of 5-100 kHz as well as for pulse durations of between 5 and 100 μs at the same time maintaining the laser output per pulse—or can only fulfill them with very high amounts of technical expenditure. 
     As an alternative to this, it is also possible to generate a pulsed laser beam from a continuous laser beam with the aid of a rotating diaphragm disk, for example a finger or apertured disk. In this case, the laser beam is focused onto the diaphragm disk through a lens. The diaphragm disk is set at an acute angle to the beam axis. If the beam contacts a finger, the energy is directed, by reflection at the finger, into an absorber and is thermally nullified. The resulting pulsed laser beam is parallelized once more through a lens arranged downstream of the diaphragm disk and is able to be directed to the operating point via the deflection mirror. A further lens then focuses the parallelized laser beam onto the material web. The ratio between finger and slot width produces the duty cycle, a changeable web speed being balanced out via entrainment of the rotational speed of the diaphragm disk. 
     A disadvantage of that method is that the edges of the diaphragms are subject to an extreme thermal load such that the service life of the diaphragm disks is severely reduced in the case of output in the kW (kilowatt) range. Over and above this, stepless adjustment of the duty cycle, frequently demanded in practice, can only be realized with difficulty. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a device for forming longitudinally spaced-apart grooves or slits into a material web, which device overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which makes it possible to fulfill the afore-depicted practical demands with a small amount of technical expenditure. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a device for forming grooves or slits in a material web while the material web is being moved in a longitudinal direction, the device comprising: 
     a laser beam source configured for generating a continuous laser beam; 
     a deflection mirror rotatably mounted about an axis of rotation and displaceable in a direction of the axis of rotation; 
     the deflection mirror having at least one first mirror face, a line of intersection of the mirror face with a sectional plane defined perpendicular with respect to the axis of rotation forming a circular arc about the axis of rotation, with an angular extent of the circular arc varying in dependence on an axial position of the sectional plane, and the first mirror face deflecting the laser beam into a first direction; 
     focusing optics disposed along the first direction for focussing the laser beam, after reflection from the first mirror face, onto the material web while the deflection mirror having at least one second mirror face disposed to deflect the laser beam in second directions, different from the first direction; and 
     an absorber disposed material web is being moved in the longitudinal direction, for forming grooves or slits in the material web spaced apart from one another in the longitudinal direction; 
     along the second directions to receive the laser beam after reflection from the second mirror face. 
     In other words, the objects of the invention are achieve by way of a device that includes a laser beam source for generating a continuous laser beam and a deflection mirror, which rotates about an axis of rotation, is arranged so as to be displaceable in the direction thereof and has at least one first mirror face. The line of intersection of the first mirror face with a sectional plane perpendicular with respect to the axis of rotation forms a circular arc about the axis of rotation, the angle of curvature of which varies in dependence on the axial position of the sectional plane, and which deflects the laser beam into a first direction. The deflection mirror has at least one second mirror face, which deflects the laser beam in second directions, which deviate from said first direction, towards an absorber. After reflection from the first mirror face, the laser beam is focused onto the material web by way of a focusing optics. 
     Through these measures it is possible, simply by displacing the deflection mirror in an axial manner, to alter the duty cycle of the laser beam striking the material web, as the angle of curvature of the first mirror face depends on the axial position, in which the laser beam, focused preferably in an at least approximate manner onto the first mirror face, strikes the deflection mirror. 
     If the angle of curvature of the first mirror face reduces continuously in a direction parallel to the axis of rotation, the duty cycle is able to be altered in a stepless manner. 
     First and second mirror faces preferably directly adjoin each other. In other words: There are no intermediate regions between the mirror faces so that the laser beam always strikes either the first or second mirror face. 
     If the second mirror face is planar and oriented inclinedly with respect to the axis of rotation, the deflection mirror as claimed in the present invention is able to be produced in a particularly simple manner by corresponding faces being inserted into a circular cylindrical base body by means of surface milling. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a device for inserting grooves or slots, which are spaced apart from each other in a longitudinal direction, by means of a laser beam into a material web, which is moved in said longitudinal direction, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic diagrammatic sketch of a device according to the invention in a first position of the deflection mirror, in which the laser beam is directed onto the material web; 
         FIG. 2  shows the deflection mirror in a second position in which the laser beam striking it is directed towards an absorber; 
         FIG. 3  shows a perspective representation of a deflection mirror as claimed in the present invention; 
         FIG. 4  shows a partial side view and a partial longitudinal section of the deflection mirror; 
         FIG. 5  shows a cross section of the deflection mirror; 
         FIG. 6  shows one embodiment where two deflection mirrors are arranged on one common shaft; 
         FIG. 7  shows a perforated or incised pattern generated by means of the arrangement according to  FIG. 6 ; 
         FIG. 8  shows a further advantageous development of the invention with an acousto-optic modulator arranged in the beam path of the laser beam; 
         FIG. 9  shows an example of a perforated or incised pattern generated by means of the arrangement according to  FIG. 8 ; 
         FIG. 10  shows an embodiment with a deviation mirror arranged so as to be pivotable in the laser path of the laser beam; 
         FIG. 11  shows a perforated or incised pattern generated by means of the arrangement according to  FIG. 10 ; 
         FIG. 12  shows an embodiment where a rotating polygonal mirror is arranged in the beam path of the laser beam; and 
         FIG. 13  shows a perforated or incised pattern generated by means of the embodiment according to  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a device according to the invention that includes a laser beam source  2  which generates a laser beam LS by way of focusing optics (not shown in any detail). The laser beam is focused in a first focal spot F 1 . In the beam path of the laser beam LS there is disposed a deflection mirror  4 , which has a plurality of mirror faces. In the exemplary embodiment the mirror  4  has four first mirror faces  6  as well as four second mirror faces  8 , which, when the deflection mirror  4  rotates about an axis of rotation  10  that is perpendicular to the drawing plane, pass alternately into the beam path of the laser beam LS. The first mirror faces  6 , in a sectional plane—the drawing plane—perpendicular to the axis of rotation  10 , have the shape of a circular arc, the central point of which lies on the axis of rotation  10 . In addition, the first mirror faces  6  extend parallel to the axis of rotation  10  such that they lie on an outer surface of a circular cylinder, the cylinder axis of which coincides with the axis of rotation  10 . At least the first mirror faces  6  are situated approximately in the first focal spot F 1  of the laser beam LS. 
     The second mirror faces  8  are planar and are inclined with respect to the axis of rotation  10 , i.e. with an inclination relative to the drawing plane. The first and second mirror faces  6  and  8  alternate in the circumferential direction and directly adjoin each other. 
     The laser beam LS striking the circular first mirror faces  6  is deflected into a first fixed direction D 1  to a collimation optics  12 , in which it is parallelized. With a focusing optics  14 , the laser beam LS is focused in a second focal spot F 2  onto a material web B, which moves forward in a longitudinal direction  16 . In addition, more planar deviation mirrors can be located between collimation optics  12  and focusing optics  14 , by means of which deviation mirrors the collimated laser beam LS can be guided corresponding to the position and orientation of the material web B. 
     The rotating deflection mirror  4  is driven by a drive unit  18 , which is controlled by a control unit  20 . By way of a linear drive  22 , which is also controllable by the control unit  20 , the deflecting mirror  4  can also be displaced in the direction of the axis of rotation  10 , i.e. perpendicular to the drawing plane. 
     The current web speed v of the material web B is detected by way of a sensor  24  and a corresponding measuring signal M is forwarded to the control unit  20 , by way of which the laser performance and the rotational speed w of the deflection mirror  4  is controlled in dependence on the web speed v of the material web B. 
     As explained below, the duty cycle of the laser beam LS is set to the material web B by way of the axial position of the deflection mirror  4 . 
       FIG. 2  shows an operating situation where the deflection mirror  4  has rotated further in relation to the operating situation shown in  FIG. 1 , such that the laser beam LS no longer strikes the circular first mirror face  6  but rather the planar second mirror face  8 . As the planar second mirror face  8  is oriented inclinedly with respect to the drawing plane, the laser beam LS is deflected out of the drawing plane into second directions D 2  which deviate from the first direction D 1 , in which it strikes an absorber  25 . The duty cycle is accordingly determined by the ratio between the angles of curvature α and β, which are associated with the first or second mirror face  6  or  8 . The first mirror faces  6 , in a sectional plane perpendicular to the axis of rotation  10 , have in each case the same angle of curvature α and the second mirror faces have in each case the same angle of curvature β. As first and second mirror faces  6 ,  8  directly adjoin each other, four times the sum of the angles of curvature α and β amounts to 360°. 
     In the perspective representation according to  FIG. 3 , it can be seen that the deflection mirror  4  has a substantially cylindrical basic form, into which planar second mirror faces  8 , which extend inclinedly to the axis of rotation or cylinder axis  10 , are inserted in the form of recesses or indentations. It can be seen from the FIG. that the angle of curvature α or the length of curvature of the first mirror faces  8 , in a sectional plane that is perpendicular to the axis of rotation  10 , varies with the axial position of the sectional plane, in the example, with reference to the figure, increasing in the direction of the axis of rotation  10  continuously from bottom to top such that the duty cycle of the laser beam LS used for processing the material web is dependent on the axial position in which the laser beam LS, generated by the laser beam source, strikes the deflection mirror  4 . Consequently, by displacing the deflection mirror  4  in a direction parallel to the axis of rotation  10 , the duty cycle can be steplessly adjusted in a simple manner. 
     It can also be seen in  FIGS. 3 and 4  that the deflection mirror  4  has a region  26  in which it is circular cylindrical over its entire circumference, such that where a laser beam LS strikes the deflection mirror  4  in this region  26 , a continuous incised line is generated in the material web. In other words: The angle ψ at which the second mirror faces  8  are inclined against the axis of rotation  10  as well as the depth of the recess formed by the second mirror faces  8 , is set in such a manner that there remains a region  26  that is circular cylindrical over the entire circumference. 
     The cross section according to  FIG. 5  is placed into a sectional plane in which the angles of curvature α, β of first or second mirror face are of identical size and in each case are 45°. When a laser beam strikes the deflection mirror in this sectional plane, a duty cycle of 50% is accordingly generated. 
     As the first mirror face  6  is a cylinder face, the laser beam LS, after being reflected on said first mirror face, has, in the drawing plane, a divergence that differs from the perpendicular. The collimation optics  12  is consequently made up by at least two aspheric lenses or one toroidal lens in order to parallelize the laser beam LS in both planes. 
     In the exemplary embodiment according to  FIG. 6 , a plurality of deflection mirrors  4 , in the figure only two are shown for reasons of clarity, are arranged on a common shaft  100  such that they rotate about a common axis of rotation  10 . A laser beam LS strikes each of these deflection mirrors  4  such that a plurality, in the example two, of parallel perforated or incised tracks  30  is generated on the material web B ( FIG. 7 ). The plurality of laser beams LS can either be generated, as shown in  FIG. 6 , by means of a beam splitting device  32  or with the aid of a corresponding number of laser beam sources. 
     The generation of several perforated or incised tracks in parallel with each other can also be effected by way of a diffractive optical element  35 , which is arranged in the beam path of the laser beam LS reflected by the mirror face  6  of a single deflection mirror  4 , which generates a multiple beam from one beam. The use of a diffractive optical element of this type for perforating a paper web is known, for example, from commonly assigned U.S. Pat. No. 5,684,617 and its counterpart European patent application EP 0 734 809 A1. 
     In the exemplary embodiment according to  FIG. 8 , an acousto-optic modulator  34  is provided in the beam path of the laser beam LS that propagates towards the deflection mirror  4 , by means of which modulator, when actuated in a corresponding manner, the laser beam can be deflected such that it is no longer available as a useful beam, but rather strikes an absorber (not shown in the figure). The acousto-optical modulator  34  is preferably arranged in a region of the beam path in which the beam cross section of the laser beam is correspondingly small in order to be able to be completely coupled into the acousto-optic modulator, the geometric dimensions of which are restricted. This is the case for example, in the vicinity of a focal spot of the laser beam LS, for example in front of or after the first focal point F 1  situated approximately on the deflection mirror  4  or between the focusing optics  14  and the material web B ( FIG. 1 ). The broken line in  FIG. 8  shows the acousto-optic modulator  34  in an alternative position in which it is situated in the beam path of the laser beam LS reflected by the first mirror face  6 . The acousto-optic modulator  34  serves as a switch such that an interrupted perforated or incised track  36  is able to be generated by means of said modulator ( FIG. 9 ). 
     The exemplary embodiment in  FIG. 10  provides a deviation mirror  38  which is arranged upstream of the focusing optics  14  between the focusing optics  14  and the collimation optics  12  and is able to be pivoted about an axis of rotation  39  that is oriented parallel to the longitudinal direction  16 . In this case, the pivot angle and the focusing optics  14  are matched to each other in such a way that the laser beam LS is always focused by one and the same focusing optics  14 . Such an arrangement produces an approximately sinusoidal perforated or incised track  40 , as illustrated in  FIG. 11 . The sinusoidal course of the track  40  may be further varied to different undulations by varying the pivoting of the deviation mirror  38 . 
     According to  FIG. 12 , a polygonal mirror  42  is mounted upstream of the focusing optics  14 . The polygonal mirror also rotates about an axis of rotation  43  that is parallel to the longitudinal direction  16  of the material web B. This configuration produces a saw-tooth shaped or scaled perforation track or incision track  44  as it is shown in  FIG. 13 .