A method of operating a thermal printer

Abstract: In a thermal printer in which ink is transferred from a single-use thermal print ribbon to a substrate such as packaging material by energising selected print elements of a thermal print head (22), a first print run is executed by energising only a group of print elements in registry with one half of the ribbon width so that ink is depleted only from one half of the ribbon, and a second print run is executed by energising the same group of elements using the same ribbon but with the ribbon supply and take-up spools interchanged in order that ink is depleted only from the other half of the ribbon. This allows printing on comparatively narrow printing areas using ribbon which is at least twice the width of the printed area without undue ribbon wastage and with reduced ribbon breakage frequency compared to the breakage frequency with a ribbon of a width nearer to the width of the printed area.

Description: This invention relates to a method of operating a thermal printer in which ink is transferred to a substrate from a thermal print ribbon. European Patent Specification No. 0734876A discloses a thermal printer for printing information such as dates and bar codes on packaging material, label-bearing films and other substrates by thermal transfer of ink from a thermal print ribbon using a thermal print head having a line of selectively energisable print elements which are brought to bear against a moving ribbon with the ribbon being sandwiched between the print head and the substrate which is to bear the printed information. Conventionally, the ribbon is fed to the printing region from a supply spool via one or more deflection rollers, and is collected via one or more further deflection rollers on a take-up spool, and once the full length of the ribbon is depleted of ink, a new spool of unused ribbon is fitted to the machine. To ease the ribbon replacement operation, spindles for receiving the supply and take-up spools are mounted on a removable cassette unit which includes the above-mentioned deflection rollers. Ribbon replacement is effected by removing the cassette unit from the printer body, removing the supply and take-up spools from the cassette unit, fitting a new supply spool with unused ribbon to the relevant spindle on the cassette unit, threading the free end of the ribbon around the deflection rollers and on to an empty take-up spool, and then sliding the cassette unit back onto the printer body, whereby the ribbon automatically takes up the correct position with respect to the thermal print head and the path of travel of the substrate material. Ribbons of different widths may be used, depending on the transverse width of the area to be printed, i.e. in the direction transversely of the ribbon. However, the applicants have found that the tendency for the ribbon to break increases as the ribbon width is reduced. In addition, excessive compression of the take-up spool can occur, leading to difficulty in removing it from the printer. GB-A-2272669 discloses a printer that uses a double width ink ribbon in a cassette, which is mounted initially in a first orientation to enable transfer of ink from a first half of the width of the ribbon. After the fall length of the ribbon has been used, the cassette is mounted in a reversed orientation so that ink can be transferred from the other half of the width of the ribbon. The elements of the print head of the printer extend across substantially the entire width of the ribbon. US-A-4641149 discloses a thermal transfer printer for printing with an ink ribbon in plural columns, in which initially printing is carried out in the upper half of the ribbon, then the ribbon cassette is reviewed to effect printing in the other half of the ribbon. It is an object of the present invention to provide an improved method of operating a thermal printer which reduces the incidence of ribbon breakage. According to a first aspect of this invention, there is provided a method of operating a thermal printer in which ink is transferred from a single- use thermal print ribbon to a substrate by energising print elements of a thermal print head at a printing station, wherein the ribbon is fed along a path from a supply position to a take-up position through the printing station in a first run whereby print elements are energised such that ink is depleted only in a longitudinal strip of the ribbon, the width of the strip being no greater than half the width of the ribbon, wherein the ribbon is inverted and is fed through the printing station in a second print run whereby the said print elements are energised such that ink is depleted from a longitudinal strip in the other half of the ribbon, characterised in that in the first run the ribbon is fed from a first spool acting as a supply spool to a second spool acting as a take-up spool, the second spool containing the partially-depleted ribbon is removed, turned through 180 degrees to invert the ribbon and mounted in place of the first spool to act as a new supply spool, and in the second print run the ribbon is fed from the new supply spool to a new take-up spool; in that the printer is arranged such that the print head has print elements extending across substantially the entire width of the ribbon; and in that for said first and second print runs only selected ones of the print elements that are confined to a group arranged in a line extending transversely of the ribbon over no more than half the width of the ribbon are energised. Advantageously, the first and second spools are positioned on a ribbon-mounting structure of the printer on opposite sides of the printing station, wherein an unused reel of ribbon is wound on the first spool in the supply position with one edge of the ribbon adjacent the ribbon-mounting structure for the first printing run, and wherein subsequently the second spool bearing the partially-depleted ribbon is removed from the take-up position and mounted in the supply position with the said one edge of the ribbon remote from the ribbon-mounting structure, in preparation for the second print run. Preferably, the group of print elements energised are of such an extent and position with respect to the ribbon as to define an ink depletion strip which is between the respective ribbon edge and a centre line of the ribbon and spaced from both that edge and the centre line by respective guard bands. This allows printing on comparatively narrow printing areas using ribbon which is at least twice the width of the printed areas without undue ribbon wastage and with reduced ribbon breakage frequency compared to the breakage frequency with a ribbon of a width nearer to the width of the printed area. In addition, since it is not necessary for the operator to collect as new, unused ribbon between each print run, printer down-time is saved. In the conventional use of a thermal printer, it is normal to confine the energised print elements to a width which is somewhat less than the total width of the ribbon so as to provide guards bands adjacent each edge of the ribbon, thereby allowing for ribbon alignment tolerances. When the printer is used in accordance with the above-described method, the guard band adjacent the centre line of the ribbon may be much reduced in width, effecting a further reduction in ribbon wastage. It will be appreciated that use of the preferred method in accordance with this invention will result in at least half of the print head elements remaining unenergised throughout operation of the printer. To make better use of the print head, it is possible within the scope of the invention to reconfigure the printer after a predetermined period to use a second group of print head elements and to shift the relative position of the printer with respect to the substrate so that the printing location on the substrate remains the same. In this way, a first series of first and second print runs may be performed until the energised print elements show signs of wear, then the printer may be configured and shifted so as to use the so far unenergised print elements until they too, show signs of wear, whereupon the print head is replaced. Thus, it is possible to double the useful life of the print head. The invention also includes a thermal printer and a print ribbon carrying out any of the above described methods, the printer comprising a print head having print elements that extend across substantially the entire width of the ribbon when passing therethrough, and means confining energisation of the print elements to only those print elements in registry with part of the width of the ribbon, the said part having width no greater then one half of the width of the ribbon. The invention will be described below by way of example with reference to the drawings. In the drawings: Figure 1 is a diagrammatic exploded side view of a thermal printer for performing a printing method in accordance with the invention; Figure 2 is a front view of a base unit of the printer of Figure 1; Figure 3 is a rear view of a ribbon cassette unit of the printer of Figure 1; Figure 4 is a block diagram of electrical parts of the printer; Figure 5 is a simplified view of the printer ribbon during a first print run; Figure 6 is a simplified view of the ribbon during a second print run; Figure 7 is a diagram showing a portion of the ribbon and the manner in which ink is depleted from the ribbon; and Figure 8 is a simplified side view of the printer mounted in packaging apparatus. Referring to Figures 1, 2, and 3 together, a printer for the continuous printing of a continuous print medium by transfer from a thermal transfer print ribbon has a base unit and a removable ribbon cassette unit 12. The base unit, which is mounted to a frame of the printer (not shown) contains a print head stepper motor 14 and a ribbon drive stepper motor 18. A pivotable print head carrier 20 supports a print head 22 and is coupled to the motor 14 by way of a parallelogram linkage including a connection link 26. Ribbon spools 34, 36 are detachably and rotatably mounted on a front plate 38 of the cassette unit 12, and when the cassette unit 12 is fitted to the base unit 10, they are coextensive with the print head 22 in terms of their location in a direction perpendicular to the plate 38. Also attached to the cassette unit front plate 38 is a ribbon drive roller 32 visible in Figure 1 below ribbon spool 34, and also in Figure 3. The relative positioning of the ribbon spools 34, 36, the print head 22, and a ribbon drive roller 40 may be ascertained by comparison of Figure 1 with Figure 3. The ribbon 42 itself is shown in full lines in Figure 3, but in phantom lines in Figure 1 for clarity. A platen roller 44 and a deflection roller 46 shown in Figure 2 are mounted on the printer frame or other apparatus with which the printer is associated. Referring to Figure 2, a continuous film substrate 50 on which information is to be printed enters the printer in an inlet region 52, passes over and wraps around the platen roller 44, from where it follows a downward inclined path to pass underneath and wrap around the deflection roller 46 before passing to an outlet region 54 of the printer. Both platen roller 44 and deflection roller 46 have axes of rotation which extend at right angles to the direction of travel of the substrate 50. The thermal transfer ribbon 42 travels in the same direction as the substrate 50 and follows a ribbon path from supply spool 36 via deflector rollers 58, 64, and 66, thence through a printing region which, when the cassette unit 12 is loaded into the base unit 10, lies between the print head 22 and the platen 44. The ribbon then passes over further guides 68, 70, and via drive roller 40 and pinch roller 72 onto the take-up spool 38, which is belt driven by a belt 76 from a pulley 78 mounted on the shaft of drive roller 40. Where the ribbon 42 passes over the platen 44, it is in frictional contact with the substrate film 50. The ribbon 42 is held in contact with the substrate film 50 only between the start and finish of each printing operation during which the lower surface of the print head 22 bears against the platen 44 through ribbon 42 and film 50 as shown in Figure 2. At other times, the print head 22 is raised by operation of its stepper motor 14. The print head 22 has side-facing printing elements 82 (Figure 2) extending along a line parallel to the axis of rotation 84 of the print head carrier 20. These printing elements 82 project from a lower surface 86 of the print head 22. The ribbon 42 and the substrate film 50 are pinched between the print head 22 and the platen roller 44 precisely at the line of printing elements 82, and when these elements are heated under electronic control and the film 50 and ribbon 22 are passed together in the same direction over the elements, ink is transferred from the ribbon 42 to the film 50 to print characters and symbols according to pre-programmed information incorporated in the signals fed to the print head 22. An optical shaft encoder on a shaft bearing the platen roller 44 produces a signal representative of the speed of the film substrate 50 and by processing this output signal, the stepper motor 18 driving the ribbon drive roller 40 can be adjusted such that the ribbon is driven at a required speed. The shaft encoder associated with the platen roller 44 is shown in Figure 4 by reference numeral 90. Encoder 90 provides an input signal representative of film speed to an input 92 of a processor unit 94. The processor unit has at its heart a microprocessor, and has three outputs comprising output 96 for a first motor driver circuit 98 for raising and lowering the print head, a second output which is a multiwire output 100 coupled to the energisable elements 82 of the print head 22, and a third output 102 for a ribbon motor driver 104 for controlling ribbon stepper motor 18. Other inputs to the processor include a trigger input 106 for receiving a trigger signal initiating each printing operation which is typically generated by sensing the position of products to which the substrate film 50 is to be applied as packaging as the products travel along an adjacent conveyor. Another input 108 receives the information to be printed from a memory 110 according to an input design. Thus, on receipt of a trigger signal at input 106, the processor is programmed firstly to move the print head 22 to its extended position, to start the ribbon drive motor, and to initiate printing by energising the elements of the print head 22 in accordance with the information stored in the memory 110 thereby to print the information as a pattern or a series of characters in a designated print area on the substrate film 50. When the processor senses that all the information relating to the required design has been supplied from memory 110 and has been fed to the print head 22, it issues a stop signal to the ribbon driver 104 to stop ribbon travel, and the driver 98 for the print head motor 14 receives a signal causing the motor to withdraw the print head to its retracted, inactive position. These operations are repeatedly performed on successive printing areas of the substrate 50 as part of a print run comprising many printing operations during which, the ribbon is progressively fed from supply spool 36 to take-up spool 38. A ribbon status sensor 116 associated with a pivotable arm 120 (see Figure 3) in the cassette unit detects when the ribbon supply runs low so that an alarm can be activated and/or operation of the packaging apparatus of which the printer is part can be halted. There is also a break sensor 118 responsive to excessive clockwise movement of arm 60 (see Figure 3) to sense breakage of the ribbon 42. The width of the ribbon 42 depends on the width of the area to be printed. However, in accordance with the invention, it is advantageous in the case of a comparatively narrow printed area to provide a ribbon 42 of a width which is at least twice the width of the printed area and then to use two longitudinal halves of the ribbon successively, as will now be explained with reference to Figures 5 to 7. By confining the energisation of the elements of the print head 22 to those elements which are in registry with a strip of the ribbon occupying one half of the ribbon width, it is possible to make a particularly economical use of the ribbon in a way which minimises ribbon breakage. In a first print run, the limited transverse extent of the energised elements creates a strip 120 of depleted ribbon which occupies just under one half of the width of the ribbon 42 adjacent a first edge 42A, as shown in Figure 5. This mode of operation is continued until the supply spool 36 is exhausted and substantially all of the ribbon is wound onto the second spool 34, here acting as a take-up spool. At this point, the take-up spool 34 is removed from its mounting on the cassette unit 12, the empty supply spool 36 is also removed, and then the second spool 34 is turned through 180° about an axis perpendicular to its axis of rotation and placed on the spindle formerly occupied by the supply spool 36 so that the second spool 34 becomes the new supply spool as shown in Figure 6. Since the ribbon has been inverted by the rotation of the spool 34 through 180°, the first edge 42A now lies in the opposite position with respect to its position during the first print run shown in Figure 5, i.e. at the other end of the print head 22. The undepleted portion of the ribbon 42 wound on spool 34 is now available for printing, using the same group of printer head elements as was used during the first print run. Accordingly, a second print run may now be commenced, as shown in Figure 6, whereby the above-mentioned group of elements is in registry with the undepleted part of the ribbon 42 adjacent its second edge 42B, and printing may be carried on until the ribbon is once again exhausted, this time the ribbon 42 being wound up on an empty spool 40 mounted on the cassette unit 12 in the position formerly occupied by the second spool 34, as shown in Figure 6. The relative positions of those parts 120, 122 of the ribbon 42 depleted during the first and second print runs respectively is shown in Figure 7. It will be seen that energisation of the elements during the first print run is such that a first guard band G1 and a second guard band G2 adjacent the first edge 42A of the ribbon 42 and the centre line 42C of the ribbon 42 respectively are left undepleted during the first print run and, similarly, guard bands G3 and G4 adjacent edge 42B and centre line 42C respectively are left undepleted during the second print run. Since guard bands G2 and G4 on either side of the centre line 42C are remote from the edges of the ribbon 42, they may be much narrower than guard bands G1 and G3. This results in a particularly economical use of ribbon 42 in that, compared with printing from a narrow ribbon in the conventional way, a higher percentage of the ribbon area is used for printing. It will be noted from the drawings that both the supply spool and the take-up spool rotate in the same direction throughout, in this case in an anticlockwise direction as seen in Figures 5 and 6. It will be appreciated that the path of the ribbon 42, instead of extending from the sides 124, 126 (see Figure 5) of the ribbon reels which are remote from each other, may extend from the inner sides 128, 130, with the spools rotating in the clockwise direction. In the latter case, it is still possible to obtain the dual use of the ribbon illustrated in Figure 6. The spools 34, 36 are open-ended in the sense that they may be fitted in either of two orientations on the printer mounting spindles, i.e. with either end of the tubular core facing the ribbon mounting structure from which the spindles project. For clarity, the print head 22 is shown diagrammatically in Figures 5 to 7. Referring to Figure 7, the print head elements 82 are shown by dotted lines. Figure 7 is, effectively, an underside view of the ribbon during the second print run, and also shows the print head elements 82A which are energised during the first and second print runs. As will be seen, these extend over no more than one half of the width of the ribbon 42. Since according to the above-described method of use, the first and second print runs are performed with only elements of the group 82A being energised, it is possible to extend the life of the print head when it is used solely for printing in print areas of a width which is less than half of the ribbon width by using instead a second group 82B of elements when the elements of group 82A show signs of wear. This may be carried out by, firstly, moving the position of the design entered into memory 110 (see Figure 4) so as to have coordinates corresponding to the half of the ribbon which is shown as the rear half of Figure 5. Concurrently with the repositioning of the design coordinates, the printer is moved on its mounting with respect to the apparatus in which it is positioned so that printing occurs in the same position on the substrate 50 as when the group 82A of printing elements was used. This repositioning of the printer is shown diagrammatically in Figure 8 in which the printer is shown by references 10, 12 and positioned above a substrate 50. Printer 10, 12 is mounted on support rails 130 associated with, for example, packaging apparatus by means of sliding bearings 132. The centre line of a box image to be printed on the substrate is indicated by reference 50C and with the printer 10, 12 in the position shown, the image is printed using the left-hand side of the print head 22 as seen in Figure 8. When the print head elements on the left-hand side of the print head 22 are worn, bearing lock 134 is released and printer 10, 12 is moved leftwards on rails 130 so that the centre line 42C of the ribbon, which is also the centre line of the print head 22, is moved to the other side of the box image centre line 50C, thereby bringing the elements of the right-hand side of the print head 22 into registry with the box image position. The bearing lock is then operated again to lock the printer 10, 12 in the new position on rails 130. Throughout this description, reference has been made to a thermal printer in which the print head 22 remains fixed in its position longitudinally over the ribbon and both the ribbon and the substrate move relative to the head during printing. In particular, the substrate 50 is a continuous film of material which moves at a continuous rate. However, the invention may also be put into effect in an intermittent printer, i.e. one in which the substrate and the ribbon remain stationary during printing whilst the print head 22 is caused to move longitudinally of the ribbon during each printing operation. The exchange and inversion of the ribbon spools is the same as described above.

Claims: A method of operating a thermal printer in which ink is transferred from a single- use thermal print ribbon to a substrate by energising print elements of a thermal print head at a printing station, wherein the ribbon is fed along a path from a supply position to a take-up position through the printing station in a first run whereby print elements are energised such that ink is depleted only in a longitudinal strip of the ribbon, the width of the strip being no greater than half the width of the ribbon, wherein the ribbon is inverted and is fed through the printing station in a second print run whereby the said print elements are energised such that ink is depleted from a longitudinal strip in the other half of the ribbon, characterised in that in the first run the ribbon is fed from a first spool acting as a supply spool to a second spool acting as a take-up spool, the second spool containing the partially-depleted ribbon is removed, turned through 180 degrees to invert the ribbon and mounted in place of the first spool to act as a new supply spool, and in the second print run the ribbon is fed from the new supply spool to a new take-up spool; in that the printer is arranged such that the print head has print elements extending across substantially the entire width of the ribbon; and in that for said first and second print runs only selected ones of the print elements that are confined to a group arranged in a line extending transversely of the ribbon over no more than half the width of the ribbon are energised. A method according to claim 1, wherein the first and second spools are positioned on a ribbon-mounting structure of the printer on opposite sides of the printing station, wherein an unused reel of ribbon is wound on the first spool in the supply position with one edge of the ribbon adjacent the ribbon-mounting structure for the first printing run, and wherein subsequently the second spool bearing the partially-depleted ribbon is removed from the take-up position and mounted in the supply position with the said one edge of the ribbon remote from the ribbon-mounting structure, in preparation for the second print run. A method according to claim 1 or claim 2, wherein the group of elements is of such an extent and position with respect to the ribbon as to define an ink depletion strip which is between the respective ribbon edge and a centre line of the ribbon and spaced from both the said respective edge and the centre line by respective guard bands. A method according to claim 3, wherein the width of the guard band adjacent the centre line is less than the width of the guard band adjacent the edge. A method according to claim 4, wherein the centre line guard band width is less than or equal to half of the edge guard band width. A method according to any of claims 2 to 5, wherein after the said first and second print runs are performed, the printer is reconfigured to use a second group of the print head elements, not including elements of the first group, to perform further first and second print runs, the relative position of the printer and the substrate being shifted to compensate for the change in printing position brought about by the reconfiguration. A method according to claim 6, wherein the reconfiguring of the printer is carried out after a plurality of successive pairs of the first and second print runs have been performed. A thermal printer and a print ribbon for carrying out a method as claimed in any of claims 1 to 7, the printer comprising a print head having print elements that extend across substantially the entire width of the ribbon when passing therethrough, and means confining energisation of the print elements to only those print elements in registry with part of the width of the ribbon, the said part having a width no greater than one half of the width of the ribbon.

Inventors: GREEN MICHAEL CAMERON, GREEN, MICHAEL CAMERON
Applicants: ITW LTD, ITW LIMITED
Application year: 1997
Main classification: B41J33
IPC codes: B41J33, B41J17