GB2027628A - Method and system for laser perforation of sheet material - Google Patents

Abstract

In the perforation of sheet material eg: cigarette filter paper by light energy, a continuous focused laser beam 38 is reflected from different locations along the beam axis to provide separate pulsed beams 52a, 54a and the beams are issued onto the sheet material with the same beam cross- sectional area. The lengths of respective different light paths 42, 44 for conveyance of light from the point of focus of the laser to final image locations are made equal and different focusing elements may be included in light paths to provide for sameness of beam cross-sectional area at the final image locations. The light paths are preferably provided in part by light conducting apparatus having light- reflective elements mounted for movement, such that different perforation matrices may be readily attained. <IMAGE>

Description

SPECIFICATION Method and system for laser perforation of sheet material This invention relates generally to perforating material by the use of light energy and pertains more particularly to methods and systems providing spatially precise matrices of perforations in sheet material.

In perforating sheet material, a twodimensional hole matrix is frequently sought with rigorous limits on perforation spacing uniformity as between rows and columns of the matrix. An illustrative field of current interest is that of perforating cigarette filter tipping paper where hole matrix uniformity enables consistency of cigarette dilution characteristics. In various known mechanical puncture and electric arc perforating practices, row spacing is rendered precise by providing an individual perforating device for each row. Uniformity in the spacing of perforations made in each row and hence precise column spacing is achieved by synchronizing operation of each perforating device. Since the perforating devices, e.g., pin or electrode pair, are physically limited in size, these practices can readily accommodate quite close spacing of adjacent rows of the matrix.

The prior art has also encompassed perforating practices involving lasers providing pulsed or continuous light energy in row-column perforation. In these efforts, however, there generally has been an apparent preference, for economic and physical size reasons, for use of a single laser serving both row and column perforation. Known single laser practices of type affording spacing uniformity have involved splitting of the laser beam into plural beams, one for each row, and the focusing of light onto a sheet member by use of an individual lens for each row. Spacing of perforations by precise limits within each row has been sought by inclusion of a movable reflective element in each of the-plural beam paths.

Complexity attends precision movement, e.g.

vibration or pivoting, of such reflective element into and out of its reference plane, to uniformly locate holes in rows, and the present state of the art is accordingly limited.

The foregoing prior art practices and references iltustrating same and other practices are further discussed in the statement filed herein pursuant to 37 CFR 1.97 and 1.98.

The present invention has, as its primary object, the provision of improved methods and systems for perforating sheet material by the use of light energy.

A more particular object of the invention is to provide for expeditious perforation of cigarette filter tipping paper by laser.

In attaining these and other objects, methods of the invention provide for the focusing of a continuous beam of light energy and reflection of the focused beam at locations spaced from one another along the beam axis to generate pulsed Tightbeams. The beams are conducted to material to be perforated in like beam cross-sectional area, thereby to render hole size uniform.

In a particularly preferred embodiment, the system of the invention employs commonly rotative reflective discs for generation of the pulsed light beams, and uses variably positionable light-conducting apparatus, whereby different perforation matrices are made readily available.

The foregoing and other objects and features of the invention will be further understood from the following detailed description of preferred methods and systems and from the drawings wherein like references numerals identify like parts throughout.

In the drawings: Fig. 1 is a block diagrammatic showing of a preferred system embodiment.

Fig. 2 is a perspective view. of the reflective discs of Fig. 1 , the discs being shown side-by-side for purpose of explanation.

Figs. 3 and 4 are optical diagrams applicable to the Fig. 1 system.

Fig. 5 depicts a further system embodiment expanded from the Fig. 1 system to include additional reflective discs.

Fig. 6 shows the respective configurations of the reflective discs of the expanded system.

Fig. 7 is a schematic drawing explanatory of variable perforating matrices attainable with the expanded system of Fig. 5.

Referring to Fig. 1, a web 10 of sheet material is collected by take-up drum 12 following horizontal transport from a payout drum, not shown. Take-up drum 12 is rotated by drive unit 14 with drum speed being established by a control signal on line 1 6 as furnished by potentiometer 18.

A further signal is derived from potentiometer 20 and applied to line 22 for control of drive unit 24 of light-reflector assembly 26, which comprises shaft 28, rotated by drive unit 24, lightreflective discs 30 and 32 and spacer 34, keyed to shaft 28 with the discs for rotation therewith.

Laser 36 generates a continuous output beam 38 which is focused by lens 40 at a location adjacent discs 30 and 32. Light beams reflected by the discs are conducted by apparatus 42 and 44, which include respective issue focus elements 46 and 48, and are supported by fixed frame 50 for independent rotation about the axes 52a and 54a of the pulsed beams.

Fig. 2 shows in side-by-side perspective disc 30 and disc 32, as the latter would be seen rightwardly of disc 30 in Fig. 1. The discs are keyed to shaft 28 in position wherein lines 56 and 58 are in a common plane with shaft axis 60. In the illustrative embodiment wherein two discs are used and are intended to confront beam 38 (Fig.

1) alternately, the discs have light transmissive uniformly spaced peripheral portions 62 and 64 which are mutually staggered, defining reflective facets 66 and 68 therebetween. Forty-five such facets are typically employed with each facet subtending four degrees of arc (angles 70 and 76) and each transmissive portion also subtending four degrees of arc (angles 74 and 72). With transmissive portion 62a having its leading edge aligned with line 56 and transmissive portion 64a spaced from line 58 by facet angle 76, the discs are properly aligned for alternate reflection of the laser beam, the beam passing through transmissive portion 62a to be reflected by the facet clockwise of transmissive portion 64a. The light-transmissive portions are typically openings in the discs of size sufficient to freely pass the laser beam.Spacer 34 is selected of extent along axis 60 to space discs 30 and 32 to position as desired the locations of the origins of modified beams reflected by the disc facets. While disc 32 might be constructed with no light-transmissive portions since it is the last disc from the laser, the described construction mitigates against spurious reflection of the laser output beam by disc 32 during confrontation of facets of disc 30 with the laser beam, i.e., laser output beam spillage beyond disc 30 simply passes through disc 32 openings.

Referring to Fig. 3, each confrontation of a facet of disc 30 with beam 38 will give rise to the propagation of a modified version of the laser output beam, such modified beam being shown at 52 and having central axis 52a, i.e. axis of symmetry, which is made parallel to the optical axis 42a of apparatus 42 by the orientation of disc 30. Beam 52 has outer rays 52b and 52c which diverge respectively oppositely from beam central axis 52a. The virtual object or origin location of beam 52 is shown at 52d.

On each confrontation of a facet of disc 32 with beam 38, further modified beam 54 is generated, having central axis 54a (axis of beam symmetry), made parallel to optical axis 44a of tube 44. Beam 54 has divergent outer rays 54b and 54c and has virtual object or origin location 54d. Distance d20 identifies both the distance between disc 30 and beam origin 380 along axis 38a and the distance between disc 30 and origin location 52d along axis 52a. Likewise, distance d32 identifies both the distance between disc 32 and beam origin 38o along axis 38a and the distance between disc 32 and origin location 54d along axis 54a.The spacing of disc 30 from disc 32 along axis 38a is defined by da Referring to Fig. 4, apparatus 42 includes plane reflective elements 42b and 42c. Element 42c is aligned with issue focus element 46, having entry plane 46a. Apparatus 44 includes plane reflective elements 44b and 44c, the latter aligned with issue focus element 48, whose entry plane is also plane 46a.

If one were to move focus elements 46 and 48 into alignment directly with discs 30 and 32, respectively, and dispense with reflective elements 42b, 42c, 44b and 44c, a distance D would then apply to each of the light paths extending from disc to issue focus element. The light path from laser beam origin 380 for the disc 30-reflected beam would then be d30 plus D, and for the disc 32-reflected beam would then be d30 plus da plus D. Considering reduction factors, i.e.

the ratio of image size to object size, the arrangement under discussion yields different reduction factors based on such different length light paths. Attainment of equal perforation hole size for each beam is evidently unattainable in such arrangement where compensation for the different length light paths is not introduced, e.g.

by different focusing characteristics of issue focus elements 46 and 48. While such arrangement and compensation are within the purview of the subject invention, the use of apparatus 42 and 44, now discussed in detail, is preferred as facilitating attainment of reduction factors of like magnitude for each disc-reflected beam and equality in perforation hole size without need for greatly different optics in the issue focus elements or elsewhere.

A distance da is selected as between disc 30 and element 42b along axis 52a. A distance db is selected along an axis parallel to beam axis 38a between elements 42b and 42c. A distance dc is selected along an axis parallel to axis 52a between element 42c and the entry plane 46a of issue focus element 46. As the divergence of beam 52 is constant throughout its passage through apparatus 42, and is prescribed by the divergence of beam 38, one may simply lay off distances along axis 38a corresponding to the location of reflective elements 42b and 42c to determine the divergence of beam 52 in the course of its passage through apparatus 42. For example, element 42c is disposed distant from origin location 52d by the sum of distances d30, d a and db. Line 42c', struck across beam 38 at such composite distance identifies divergence which will occur at element 42c.Divergence at plane 46a is obtained by laying off along axis 38a the composite distance d30, dat d b and dc, such divergence being indicated by line 46a' along axis 38a. Virtual object 52d is distant from element 42c by the sum of distances d30, da and db.

Distances dal, d,' and dc' of apparatus 44 correspond in type to distances da, db and dc.

Divergence at reflective element 44b is indicated by line 44b' along axis 38a, i.e. at distance from origin 38o equal to the sum of d and dal.

32 a With virtual object 54d at identical distance from focus entry plane 46a as virtual object 52d, like beam cross-sectional area will occur for each beam at plane 46a. Otherwise stated, like reduction factors will apply to light conducted from beam origin 38o to the conveyance plane of the sheet material, whether such light is reflected by disc 30 or disc 32. To this end, the composite path length for disc 32-reflected beams, i.e., sum off30, d5, dal, db' and dc', is made equal to the above-noted composite path length for disc 30reflected beams, namely, the sum of d30, dat db and dcs As will be seen from the foregoing, the invention realizes a common cross-section, at the transport plane of material to be perforated, for beams generated alternately at different locations along the path of a focused light beam by equalizing reduction factors thereof.

By way of further example, assume issue focus element 48 to be in registry with disc 32 (elements 44b and 44c omitted). Direct reflectance path length is now D for the beam reflected by disc 32 to issue focus element 48.

The composite path length for light yielding such beam is d30 plus da plus D. To now conform to such length, the composite path length for light yielding the disc 30-reflected beam, one causes the latter length to exceed the direct reflectance path from disc 30 to issue focus element 46. For equal light path lengths, one simply spaces reflectors 42b and 42c by distance dam whereby the composite path length for light yielding the disc 30-reflected beam is also d30 plus da plus D.

Where different length beam light paths exist from the point of focus of the laser beam to the issue focus elements (or conveyance plane), compensation is made by different focusing optics. The path lengths are preferably forced to related lengths, however, to minimize disparities in focusing optics.

In Fig. 5, four reflective discs 30', 32', 78 and 80, are spaced along shaft 28 by spacers 34, 82 and 84. Additional light conducting apparatus 86 and 88 have issue focus elements 90 and 92.

Modified beams 94 and 96 are propagated respectively by the facets of discs 78 and 80.

Beam 82 is divergent about a central symmetry axis coincident with optical axis 86a of apparatus 86. Beam 88 is divergent about a central symmetry axis coincident with optical axis 88a of apparatus 88.

Each of the light conducting apparatus 42, 44, 86 and 88 is in the form of a tube. Tube 88 is typical of all tubes, comprising vertical conduits 88b and 88c, horizontal conduit 88d and reflector assemblies 88e and 88f. The horizontal conduit has threaded end connections with reflector assembly blocks 88e-1 and 88e2, thus providing a vernier-type adjustment of overall tube length and enabling the practice of varving the lenqth of the light path from its predetermined length to permit like cross-sectional areas of beams as they issue from the tubes. Plane reflector blocks 88e-3 and 88e-4 are releasably secured to the reflector assembly blocks.Issue focus element 92 has lens holder 92a secured in housing 92b which is threadably secured to vertical conduit 88c, thus permitting adjustment of lens position relative to web 10. Tubes 40, 42, 86 and 88 are commonly supported by housing 98, the horizontal and lower vertical conduits being rotatable about the upper vertical conduit. The tubes are selected to have internal diameter in excess of the maximum crosssection of the beam conveyed thereby, i.e., the tube walls do not intercept or reflect the beams.

Thus, the tubes function as housings for supporting the reflective elements therein such that successive elements, e.g., 42b and 42c (Fig.

4), are maintained in mutually fixed spatial relation and both thereof may rotate jointly about the central axis of the beam incident on the first such element. As a practical safety matter, the tubes serve also to contain the beams and minimize operating hazards.

Fig. 6 shows the configurations of discs 30', 32', 78 and 80. with all discs keyed to common plane keying lines 100, 102, 104 and 106 and assuming forty-five facets per disc as in the system of Figs. 1-3, facets of all discs each subtend two degrees of arc and openings thereof each subtend six degrees of arc. Facet 108 of disc 32' has its leading clockwise edge coincident with keying line 102. Facets 110, 12 and 114 of disc 30', 78 and 80 have their leading clockwise edges spaced from keying lines 100, 104 and 106 respectively by two, six and four degree angles 116,118 and 120. By this configuration, it will be seen that clockwise rotation of shaft 28 will give rise to successive propagation of modified beams 54, 52, 96 and 94 (Fig. 5).Such firing order is chosen simply for illustration and any firing order may be employed by modifying the registry of facets and transmissive portions of the discs. As noted for the two disc embodiment above, the last successive disc may be arranged without lighttransmissive portions, but same are preferred to mitigate against spurious light energy reflections from such last disc The laser beam is focused to its divergence origin 36o (Fig. 4), such that the beam cross-section clears the openings of the penultimate disc (disc 78), thereby assuring that the full beam can be incident on each disc.

Referring to Fig. 7 circular paths 122, 124, 126 and 128 define the possible locations of the issue focus elements of tubes 42, 44, 86 and 88, respectively. As indicated, the paths interfere with one another in the leftward extents thereof but are non-interfering in remaining rightward positional range. In an illustrative perforation matrix setting, the tubes are set as indicated, such that tube 42 provides perforation row 130, tube 44 provides row 132, tube 86 provides row 134 and tube 88 provides row 136. Spacing S1 between rows 130 and 134 is established by the setting of tubes 42 and 86 relative to one another. Spacing S2 between rows 1 32 and 136 is established by the setting of tubes 44 and 88 relative to one another.

The settings of tubes 42 and 44 further provides spacings S3 and S4 of respective rows 130 and 132 from the center line of web 10. The illustrated perforation arrangement is applicable, for example, in cigarette making for perforation of filter tipping paper. Typically opposed tobacco rod sections and an intervening double filter link are brought to end-to-end abutting relation and perforated filter tipping paper (web 10) is applied thereto for joining the tobacco rod sections and the intervening double filter link. Subsequently, a cut is made summetrically of the assembly, ie., along the center line of web 10 paper. Two independent cigarettes are thus made, each having concentric spaced rows of perforations equally spaced from the filter end.

Perforation pattern change may be achieved simply by repositioning the participating tubes.

Perforation density in such rows is controllable by the adjustments to the speed of rotation of the reflective disc assembly and the transport speed of the web. Hole size is rendered quite uniform among the rows by the practice discussed above for establishing path lengths and/or employing compensatory focusing optics. Light path definition and enclosure by devices other than the illustrated tube assembly may be undertaken. The invention contemplates the stacking of plural lenses in the issue focus elements for size reduction of holes.

Claims (36)

1. A method for perforating sheet material by light energy comprising the steps of: (a) projecting a focused beam of light energy; (b) establishing a conveyance plane for said sheet material; (c) reflecting said focused beam from first and second locations spaced successively along the axis of said focused beam from the point of focus thereof, thereby issuing respective first and second pulsed beams from said first and second locations; and (d) conducting said first and second pulsed beams separately to said conveyance plane in manner equalizing respective cross-sectional areas thereof in said conveyance plane.

2. The method claimed in claim 1 wherein said step (d) is practiced by preselecting a first light path extending from said first location to said conveyance plane and having length exceeding the direct reflectance path length between said first location and said conveyance plane.

3. The method claimed in claim 2 wherein said step (d) is further practiced in preselecting a second light path extending from said second location to said conveyance plane and having length less than said length preselected for said first light path.

4. The method claimed in claim 3 wherein the length difference between such preselected light paths is made equal to the distance between said first and second locations along the axis of said focused beam.

5. The method claimed in claim 1 wherein said step (d) is practiced by preselecting lengths of light paths extending (i) from said point of focus of said focused beam to said first location and thence to said conveyance plane, and (ii) from said point of focus of said focused beam to said second location and thence to said conveyance plane.

6. The method claimed in claim 5 wherein said preselected light path lengths are equal and wherein said step (d) is further practiced by like separate focusing of said first and second pulsed beams prior to conducting the same to said conveyance plane.

7. The method claimed in claim 5 wherein said preselected light path lengths are unequal and wherein said step (d) is further practiced by mutually different separate focusing of said first and second pulsed beams prior to conducting the same to said conveyance plane.

8. The method claimed in claim 3 wherein said first and second pulsed beams are each subjected to plural reflections in said first and second light paths.

9. The method claimed in claim 3 including the further steps of preselecting perforating positions in said conveyance plane and displacing said first and second light paths while retaining such preselected lengths thereof to provide for issuance of said first and second pulsed beams onto such preselected positions.

10. The method claimed in claim 3 including the further step of enclosing said first and second light paths along lengths thereof.

11. The method claimed in claim 1 wherein said step (a) is practiced by use of a continuous laser beam.

12. The method claimed in claim 1 wherein said step (b) is practiced by the use of first and second light-reflective means moved respectively alternately into confronting relation to said focused beam at said first and second locations.

13. The method claimed in claim 12 wherein each of said first and second light-reflective means comprises a plurality of light-reflective elements disposed in mutually spaced relation in a circular locus, said step (b) being further practiced by rotation of each such circuiar locus through said focused beam about a rotational axis in intersecting relation with said axis of said focused beam.

14. A system for providing a matrix of perforations in sheet material in the course of transport thereof through a conveyance plane, comprisinq: (a) source means for generating a focused beam of light energy; (b) light-reflective means for confrontation with said focused beam at first and second locations along the axis of said beam for generation of first and second pulsed beams; (c) light conducting means for conveying said first and second pulsed beams separately through first and second light paths from said first and second locations to said conveyance plane and for equalizing respective beam cross-sectional areas in said conveyance plane; and (d) means disposed between said light conducting means and said conveyance plane for separately focusing said first and second pulsed beams onto said sheet material.

15. The system claimed in claim 14 wherein said light conducting means coniprises first and second light-reflective elements disposed in said first light path and supported for joint rotation about the axis of said first pulsed beam.

16. The system claimed in claim 15 wherein said light conducting means also comprises first and second light-reflective elements disposed in said second light path and supported for joint rotation about the axis of said second pulsed beam.

17. The system claimed in claim 15 whrerin said light conducting means further supports said first and second light-reflective elements for translational displacement relative to one another.

18. The system claimed in claim 15 wherein said light conducting means includes tube means for enclosing said first light path and for supporting said first and second light-reflective elements.

19. The system claimed in claim 18 wherein said means (d) is supported by said tube means for translational movement relative thereto.

20. The system claimed in claim 18 wherein said tube means includes a first extent receiving such first pulsed beam from said means (b), a third extent in light beam issuing relation to said sheet material and a second extent between said first and third extents.

21. The system claimed in claim 19 wherein said first and second light-reflective elements are supported respectively at junctions of such first and second extents and of said second and third extents of said tube means.

22. The system claimed in claim 21 wherein said second extent of said tube means is adjustable to adjust the length of the light path therethrough.

23. The system claimed in claim 22 wherein said means (d) is supported by said third extent of said tube means for translational movement relative thereto.

24. The system claimed in claim 14 wherein said means (d) comprises a shaft supported for rotation about an axis intersecting said beam axis, said light-reflective means comprising for each of said first and second locations a plurality of lightreflective elements supported at angularly spaced common radial positions about said shaft, the light-reflective elements for such locations being respectively mutually offset angularly of said shaft.

25. The system claimed in claim 24 including drive means for rotating said shaft and spacer means for mutually separating said light-reflective means along such shaft axis such said lightreflective elements are moved into said mutually different locations along said beam axis upon such shaft rotation.

26. The system for providing perforations in sheet material in the course of transport thereof through a conveyance plane, comprising: (a) source means for generating a focused beam of light energy; ' ' (b) light-reflective means for confrontation with said focused beam at first and second lodations along the axis of said focused beam for generation of first and second pulsed beams; (c) first and second light conducting means respectively for receiving said first and second pulsed beams and each comprising first and second light-reflective elements supported for joint rotation about the axis of the beam thereby received for issuing the same in registry with any one of a variably selectable plurality of positions in said conveyance plane.

27. The system claimed in claim 26 wherein said light conducting means further supports said first and second light-reflective elements for translational displacement relative to one another.

28. The.system claimed in claim 27 wherein said light conducting means includes tube means for enclosing light paths therethrough and for supporting said first and second light-reflective elements.

29. The system claimed in claim 28 wherein said tube means includes a first extent receiving such first pulsed beam from said means (b), a third extent in light beam issuing relation to said sheet material and a second extent between said first and third extents.

30. The system claimed in claim 29 wherein said first and second light-reflective elements are supported respectively at junctions of such first and second extents and of said second and third extents of said tube means.

31. The system claimed in claim 30 wherein said second extent of said tube means is adjustable to adjust the length of the light path therethrough.

32. The system claimed in claim 26 wherein said means (b) comprises a shaft supported for rotation about an axis intersecflng said beam axis, said light-reflective means comprising for each of said first and second locations a plurality of lightreflective elements supported at angularly spaced common radial positions about said shaft, the light reflective elements for such locations being respectively mutually offset angularly of said shaft.

33. The system claimed in claim 32 including drive means for rotating said shaft and spacer means for mutually separating said reflective means along such shaft axis such that said reflective elements are moved into said mutually different locations along said beam axis upon such shaft rotation;

34. The system claimed in claim 26 including means disposed between said light conducting means and said conveyance plane for separately focusing said first and second pulsed beams onto said sheet material.

35. The system for perforating sheet material constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

36. A method of perforating sheet material substantially as hereinbefore described with reference to the accompanying drawings.