Abstract:
A device for guiding a continuous strip past a marking unit, which is arranged to non-mechanically provide markings on at least one strip surface, includes a longitudinal channel for receiving the strip. The channel is at least partly defined by guiding elements that are arranged for abutment against the strip from opposite sides of the channel. At least one of the guiding elements is displaceable and biased towards the channel. In this device, the strip can be positioned with high-precision without being subjected to excessive frictional forces during the marking operation, to minimize interference with the motion of the strip.

Description:
This application claims the benefit of U.S. provisional app. No. 60/176,925, filed Jan. 20, 2000. 

   TECHNICAL FIELD 
   The present invention relates to marking of a continuous strip or web, preferably of metal. More specifically, the invention relates to a device for guiding such a strip past a marking unit that is arranged to provide the strip with markings. 
   The present invention is especially useful in an arrangement for manufacture of marked articles to be included cans, in particular beverage cans. Therefore, the technical background of the invention, and objects and embodiments thereof, will be described with reference to such an arrangement. However, the invention may also be applicable in connection with marking of other articles formed from a continuous strip. 
   BACKGROUND ART 
   In many situations, there is a need for indicative markings on a product, for example, traceability markings indicating the origin of the product, or promotional markings. Such indicative markings can be provided by non-mechanical, i.e. non-contact, marking techniques, such as laser engraving or ink jet printing. These non-contact techniques provide for flexibility in production and high production speeds. Often, the markings must be precisely located in a specific area of the product. To this end, the marking equipment must be accurately controlled in providing the markings. Further, the material to be marked must be carefully positioned during the marking process. This is often inconsistent with high production speeds. 
   One area with a need for high production speeds is the beverage can industry, for example in the production of can ends. Typically, a production line for can ends has a capacity of 2,000 ends per minute. A conventional production line is disclosed in a brochure entitled “This is PLM Fosie” issued by Applicant&#39;s Swedish company PLM Fosie AB in the mid nineties. In a first production stage of such a production line, a thin metal strip, preferably a 0.23-mm-thick aluminium strip, is indexed into a tab forming unit in which the strip is punched and stamped to form opening tabs or opener rings integrated with the strip. In a second production stage, circular shells for forming the can ends are die cut from a thin metal sheet, preferably a 0.23-mm-thick aluminium sheet. Each shell is scored for opening, and a rivet for attachment of the tab is formed at the center of the shell. In a third production stage, the strip with the integrated tabs is joined with the circular shells in an attachment station, in which the tabs are separated from the strip and attached to the shells by riveting. A finished can end is achieved when the tab is fastened to the shell. 
   There is need for indicative markings on the tabs. Such markings could be provided by marking the strip before it is fed into the tab forming unit, in which the thus-marked strip is formed into marked tabs. However, due to the indexing motion of the strip into the tab forming unit, the strip will swing and jump in all directions on its way to the tab forming unit. Thus, to control the position of the strip, a guiding device should be arranged in the area of the marking operation. This guiding device should allow for careful positioning of the strip, but should not interfere with the intermittent progression of the strip into the tab forming unit. Also, stretching of the strip should be avoided, and friction should be minimized. Typically, the strip should be positioned with a precision of at least about 5-15 μm in the lateral, or transverse, direction, when providing markings on the surface of the tabs. The vertical position of the strip should also be carefully controlled within the focal region of marking equipment, typically with a precision of at least about 0.1-0.2 mm. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to at least partly fulfil the above identified needs. 
   This object is achieved by a guiding device and an arrangement according to the appended independent claims. Preferred embodiments are defined in the dependent claims. 
   In the inventive device, by one or more guiding elements being pressed against the strip from at least one side of the channel, the strip can be positioned with high-precision without being subjected to excessive frictional forces during the marking operation. Thus, the inventive device does not significantly interfere with the motion of the strip. 
   According one preferred embodiment, the guiding element has a surface portion to be pressed against the strip, and a shoulder portion adjacent to the surface portion for guiding the strip in the channel. This guiding element performs the dual functions of applying stabilizing forces in the lateral or the vertical direction and guiding the strip in the longitudinal direction. Preferably, the surface portion and the shoulder are located on a freely rotatable body. 
   In a further preferred embodiment, the guiding elements are arranged to be pressed against the opposite longitudinal edges of the strip. This will minimize the bearing surfaces between the guiding elements and the strip, to further reduce friction. Preferably, each guiding element comprises a freely rotatable body having a cylindrical portion for abutment against the longitudinal edges, so that both friction and wear can be minimized. It is also preferred that the rotatable body comprises a circumferential shoulder adjacent to the cylindrical portion for guiding the strip in the channel. Preferably, the shoulder is arranged to guide a portion of one of the upper and lower surfaces adjacent to the longitudinal edges. This guiding element is compact and capable of controlling the position of the strip in both the vertical and the lateral direction. 
   In a further preferred embodiment, the guiding device comprises an intake and an outlet assembly, each including first and second intake rollers which receive the strip and abuttingly engage the upper and lower surfaces thereof, respectively. Such an assembly will isolate the strip portion in the channel from twisting and tugging motions in the strip fed to and from the guiding device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A presently preferred embodiment of the invention will now be described in more detail, 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 guiding device according to the invention. 
       FIG. 2  is a plan view of a portion of a metal strip provided with indicative markings. 
       FIG. 3  is a bottom view of a tab having markings on its bottom surface. 
       FIG. 4  is a plan view of a strip guiding device according to a preferred embodiment of the invention. 
       FIG. 5  is a cross-sectional view taken along the line V—V in FIG.  4 . 
       FIG. 6  is an end view taken in the direction of the arrows VI—VI in FIG.  4 . 
       FIG. 7  is an end view taken in the direction of the arrows VII—VII in FIG.  4 . 
       FIGS. 8   a - 8   d  illustrate alternative embodiments. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows part of an arrangement for manufacture of marked opener rings or tabs T ( FIGS. 2-3 ) to be included in ends for beverage cans (not shown). A blank in the form of a thin, continuous metal strip S is fed from a supply  1  to a laser unit  2  supported by a supporting member  3 . The laser unit  2  is of a high-power and high-speed type and is capable of providing laser engraved markings in the surface of the strip S. The laser unit  2  is adapted to generate laser radiation at a suitable wavelength, and to focus and direct the generated radiation to an engraving area on the surface of the strip S. After the engraving operation in the engraving area, the strip S is fed to an adjacent tab forming apparatus  4  which is of a type known per se and which forms tabs by punching and stamping the strip S. When passing the laser unit  2 , the strip S is guided by a guiding unit  5  secured to the supporting member  3 . Further, a dust protection device  6  is connected to the laser unit  2  to protect the laser unit  2  from dust or debris produced during the engraving operation. 
     FIG. 2  shows a portion of the strip S after the laser engraving operation at the laser unit  2 , but before the punching and stamping operation in the tab forming apparatus  4 . The laser unit  2  has provided the engraved markings WIN, A,  8 ,  9  on one surface of the strip S. The approximate periphery of the tab T to be produced in the following tab forming steps in the apparatus  4  has been indicated with ghost lines in FIG.  2 . In practice, the size of the laser engraved markings is about 1.4×1.2 mm which makes them easily readable. In  FIG. 3 , the final tab T is shown from below after processing in the tab forming apparatus  4 . The markings WIN, A,  9 ,  8  are now precisely located on a respective tab surface, in this case intermediate an opening T 1  and peripheral edge portions T 2 , T 3 , T 4  of the tab T. 
     FIGS. 4-7  shows in some detail a preferred embodiment of the strip guiding device  5 . The device  5  comprises a main block  200 , an intake mechanism  210 , a guiding channel  220 , and an outlet mechanism  240 . 
   The intake mechanism  210  includes an upper and a lower intake roller  211 ,  212 , each comprising a spindle  213 ,  214  having a number of cylindrical, laterally spaced radial projections  215 ,  216 . The intake rollers  211 ,  212  are arranged to receive the strip S with the projections  215 ,  216  abuttingly engaging the upper and lower surfaces of the strip S, respectively. Each spindle  213 ,  214  is mounted in the main block  200  for free rotation therein. The clearance between the projections  215 ,  216  corresponds to the thickness of the strip S with nearly zero tolerance, in order to avoid a twisting motion of the incoming strip S being transmitted to the strip portion received in the guiding channel  220 . The lower spindle  214  is provided with guiding shoulders  217 ,  218  with a mutual distance essentially corresponding to the width of the strip S, typically with a tolerance of about 0.5 mm. 
   In an alternative embodiment (not shown), the clearance between the projections  215 ,  216  is adjustable to accommodate strips of different thickness. Preferably, the upper intake roller  211  is adjustable in the vertical direction, while the lower intake roller  212  is fixed to remain level with the guiding path in the subsequent channel  220 . The upper intake roller  211  could be adjusted by means of an eccentric mechanism (not shown) or be spring-biased towards the lower intake roller  212 . 
   Returning to the embodiment of  FIGS. 4-7 , the guiding channel  220  is defined by a guiding shoe  221 , a number of guiding elements  222 ,  222 ′ arranged in pairs on both sides of the channel  220  in the longitudinal direction, and a guiding cover  223 . The distance between the guiding shoe  221  and the guiding cover  223  is such that the strip can move essentially without interference. The guiding shoe  221  will guide the front end of the strip S in proper place on entering the intake mechanism  210 . An opening or window  224  is defined in the cover  223  so that one surface of the strip S is accessible for engraving by means of the laser unit  2 . 
   The guiding elements  222  on a first side of the channel  220  are mounted for rotation at a fixed location in the main block  220 , whereas the guiding elements  222 ′ on a second, opposite side of the channel  220  are mounted for both rotation and lateral displacement in the main block  200 . Each guiding element  222 ,  222 ′ comprises a rotatable guiding roller  225  which has a cylindrical portion with a circumferential surface  226  for abutment on the longitudinal edges of the strip S and an adjacent circumferential shoulder  227  for defining the guiding path of the strip S in the vertical direction. Thus, a small portion of the strip surface will be carried on the shoulders  227 . 
   The displaceable guiding elements  222 ′ further comprises a mounting block  228 , which receives the guiding roller  225  and is displaceably arranged on a common  1  pin  229  extending in the longitudinal direction of the guiding device  5 . Spring-biased pusher arrangements  230  are arranged to urge the guiding elements  222 ′ towards the first side of the channel  220 . Before the strip S enters the channel  220 , the guiding elements  222 ′ are pressed onto the guiding shoe  221 , which has a slightly smaller width than the strip S. On entering the channel  220 , the strip S will displace the guiding elements  222 ′ against the biasing action of the pusher arrangements  230 , thereby locating the strip S in the lateral direction. Typically, the mounting block  228  allows for a movement of ±3 mm in the lateral direction. As an alternative (not shown), all guiding elements  222 ,  222 ′ can be displaceable and biased towards the center of the channel  220 . 
   The components of the outlet mechanism  240  are illustrated in  FIGS. 4 ,  5  and  7 , and are identified by reference numerals  241 - 248  corresponding to reference numerals  211 - 218  of the identical intake mechanism  210 . The above description of the intake mechanism  210  is equally applicable to the outlet mechanism  240  and will not be repeated. 
   The strip guiding device  5  as described above and shown in  FIGS. 4-7  forms a self-adjusting system for careful positioning of the strip S during the laser engraving operation, independently of the subsequent tab forming unit  4 . The strip S will move in a defined path through the guiding device  5 , the path being delimited in the vertical direction by the guiding shoulders  227 , and to some extent by the guiding shoe  221  and the guiding cover  223 . In the lateral direction, the path is delimited by the circumferential surfaces  226  abutting against the longitudinal edges of the strip S. The intake and outlet mechanisms  210 ,  240  has the ability of precluding any twisting motion from being transferred to the strip portion in the marking area defined by the window  224 . 
   In the illustrated preferred embodiment, the strip S is only subjected to guiding forces at its longitudinal edges. Therefore, friction is minimal between the guiding device  5  and the strip S, and the indexing movement of the strip S is essentially undisturbed. In fact, tests show that the illustrated guiding device  5  improves the operation of the subsequent tab forming unit  4  by eliminating twitches and tugs in the strip S. 
   Friction in the guiding device  5  is also minimized by making the guiding elements  222 ,  222 ′ freely rotatable, thereby eliminating any sliding friction between the strip S and the guiding elements  222 ,  222 ′. This also improves the service life of the device  5 . Further, by applying the guiding forces on the longitudinal edges of the strip S, the area of the strip surface accessible for marking is maximized. 
   An alternative, simplified embodiment is shown in the plan view of  FIG. 8   a . Here, the guiding elements  222 ,  222 ′ are provided in the form of blocks or wall portions. Either one, or both, of the guiding elements  222 ,  222 ′ is displaceable and biased onto a longitudinal edge of the strip S. Although not shown on the drawing, each of the guiding elements  222 ,  222 ′ is preferably provided with a guiding shoulder carrying the strip S in a defined path through the channel  220 . 
   The plan view of  FIG. 8   b  shows another alternative embodiment, which is a combination of the embodiment shown in  FIGS. 4-7  and the one shown in  FIG. 8   a . Each of the guiding elements  222 ′, and/or the guiding element  222 , is biased to abut on a longitudinal edge of the strip S. 
   In a further alternative embodiment, shown in the plan view of  FIG. 5   c  and the side view of  FIG. 8   d , guiding elements  222 ,  222 ′ are arranged in pairs and biased (as indicated by arrows in  FIG. 8   d ) to abut on the strip surface from opposite sides of the strip S in the channel  220 . The guiding elements  222 ,  222 ′ comprise a freely rotating body with a circumferential surface  226  for abutment on the surface of the strip S. One guiding element  222 ′ in each pair has a guiding shoulder  227  adjacent to the surface  226 . Thus, the surfaces  226  are pressed against the strip surface, thereby defining the path of the strip S in the vertical direction, whereas the shoulders  227  define the path in the longitudinal direction. Compared to the embodiment of  FIGS. 4-7 , the bearing surface of the guiding elements  222 ,  222 ′ on the strip S is increased, and consequently friction is increased as well. Further, a larger portion of the strip surface will be blocked by the guiding elements  222 ,  222 ′, thereby also reducing the strip surface accessible for marking. 
   Finally, it should be emphasized that the invention by no means is restricted to the embodiments described in the foregoing, and modifications are feasible within the scope of the appended claims. For example, to allow for marking of both sides of the strip S, the guiding shoe  221  could be substituted for a cover similar to the guiding cover  223 . 
   Although the invention is described in connection to laser engraving equipment, it may also be applicable in connection with equipment for any other type of non-mechanical marking, such as ink jet printing.