GB2311492A - A printer having a printhead movable between a parked position and a printing position and a motor for simultaneously driving the printhead and an ink ribbon - Google Patents

Description

PRINTERS The present invention relates to printers, particularly printers suitable for applying indicia to packaging material.

In typical printers of the aforementioned type, ink from an ink ribbon is transferred at a printing station onto a substrate. The printer is arranged such that the ink ribbon and the substrate travel continuously past a print head situated at the printing station. Commonly, ink is transferred by a thermal transfer process in which the print head has electrically-energisable heating elements arranged to contact the ribbon and to transfer ink from the ribbon to the substrate during a printing operation.

Thermal transfer printers are widely used for printing indicia like bar codes, prices and sell-by dates on substrates such as packaging material or labels. Typically, the substrate is a continuous web of material which moves past the print head at high speed, for example 0.5 metres per second.

A known thermal transfer printer comprises a print head facing a cylindrical rotary platen, the print head being pivotally mounted for movement about a pivot axis towards and away from the platen between a printing position pressing against the platen and a park or rest position remote from the platen.

In use, a continuous substrate is fed through the printer between the print head and the platen. The axis of rotation of the platen, its longitudinal axis, is oriented transversely with respect to the direction of motion of the substrate as the substrate passes between the print head and the platen.

That axis of the platen is parallel to the pivot axis of the print head.

A continuous ink ribbon is fed within the printer between the platen and the print head at a speed controlled to match the speed of the substrate as the substrate passes over the platen. The ribbon overlays the substrate as the ribbon and the substrate pass over the platen, with the substrate between the ribbon and the platen. The ribbon is supplied from a takeoff spool within the printer and is drawn onto a take-up spool after it has passed over the platen. A ribbon drive motor acts on the take-up spool to drive the motion of the ribbon within the printer.

A free end of the print head carries a straight line array of printing elements. The array is oriented transversely with respect to the direction of movement of the substrate, and in parallel to the rotational axis of the platen and the pivot axis of the print head. In this way, the array contacts the platen, in use, tangentially with respect to the surface of the platen, thereby defining a line of contact.

Each printing element is separately energisable to apply heat to the ink ribbon when the print head is in the printing position, thereby to transfer ink from the ribbon to the substrate. By energising different elements at different times during a printing operation under electronic control, the shape of the desired indicia is developed as the substrate passes through the printer.

In the known printer outlined above, the print head is moved between the rest position and the printing position by a dedicated stepper motor. The use of a dedicated stepper motor gives rise to several disadvantages.

Perhaps the greatest disadvantage stems from the use of two motors within the printer. Apart from the increased cost and mechanical complexity involved in having two motors, there is a considerable problem with undesirable heat emission from the motors, causing difficulties in heat dissipation. If the temperature of the printer becomes excessive, this can have a detrimental effect on ink transfer from the ribbon.

A stepper motor, in particular, also suffers from the disadvantage that the print head can "bounce" or "chatter" upon making contact with the platen. This can lead to a significant deterioration of the print quality because, clearly, reliable contact between the print head and the platen is imperative, and in serious cases it can even lead to the stepper motor flipping back to the print head's rest position, totally preventing a printing operation. Unless such a condition is quickly detected, the result can be a series of unprinted packages, labels and so on.

It is crucial to position and orient the print head in relation to the platen such that the desired line of contact between print head and platen is obtained. If there is any significant misalignment, the line of contact will be reduced ultimately to a point with disastrous effects on print quality. A disadvantage of known printers is that they require laborious, time-consuming and costly setting-up together with regular checks and adjustment. All of this increases downtime of the production line including a printing station, and involves skilled labour with all of its costs.

The present invention seeks to solve these and other problems of known printers.

Accordingly, in one aspect, the invention resides in a printer for transferring ink from a moving ink ribbon to a continuous moving substrate, the printer comprising a print head movable between a parked position and a printing position, and a motor for simultaneously driving the movement of the ribbon and the movement of the print head.

Advantageously, in the printing position the print head is continuously urged against a platen.

The invention provides a clear cost saving over the known printer outlined in the introduction, by halving the number of motors to one (which also saves on the cost of associated parts). The printer can also be made more compact than the known twin-motor printer.

In addition, with only a single motor driving the movement of both the ink ribbon and the print head, not only can the power consumption be decreased but also problems owing to heat dissipation are reduced.

In another aspect, the invention provides a printer comprising a print head carrying a linear array of printing elements, a platen having a curved surface against which the linear array bears during a printing operation, and alignment means for aligning the array of printing elements in relation to the platen to ensure a line of contact parallel to an axis of curvature of the curved surface of the platen.

Where the platen is cylindrical, the line of contact is tangential to the curved surface and parallel to the longitudinal axis of the platen.

Preferably, the alignment means bears against the platen.

Where the platen is cylindrical, the alignment means conveniently comprises a plurality of location rollers that bear against the platen. If the platen is designed to rotate, as is preferred, the location rollers can rotate with it.

To provide a simple means of triangulation, first and second location rollers are preferred, the rollers defining a gap or groove that accommodates the platen.

In a further aspect the invention provides a platen for a printer constructed from a material having a shore hardness rating of 40 or less.

Preferably, the platen is constructed from foam rubber.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic front view showing the main components of a preferred embodiment of the invention; Figure 2 is a detailed front view corresponding to and elaborating upon the schematic view of Figure 1; Figure 3 is a side view corresponding to Figure 2; Figure 4 is an enlarged detail view of a print head shaft assembly illustrated in Figure 3; and Figures 5(a) and 5(b) are front and side views respectively of a mounting for a take-off spool.

Referring initially to Figure 1, a printer 10 according to the present invention comprises a motor 12 which is coupled via a drive belt 14 to a print head drive pulley 16 and an ink ribbon take-up spool pulley 40 which is connected to an ink ribbon take-up spool 18. Ink ribbon 20 is dispensed from a take-off spool 22 and is then guided by a series of rollers 24 to the take-up spool 18. A substrate 26 in the form of a continuous web of plastics packaging material is supplied to the printer 10 and is guided over a cylindrical rotary platen 28 before leaving the printer 10. The rotational axis of the platen 28 is oriented transversely with respect to the direction of motion of the substrate 26 as the substrate 26 passes over the platen 28.

In this preferred embodiment, the platen 28 is constructed from a substantially resilient material, which in one form is foam rubber. This material has been found to reduce deterioration of print quality through bounce and chatter, particularly in conjunction with the other anti-bounce measures that feature in the invention. The foam rubber of the platen may be extruded around a rigid core, and preferably has a shore hardness rating of 40 or less.

The printer 10 is arranged such that the ink ribbon 20 and the substrate 26 travel continuously past a print head 30 which faces the platen 28. In use, the substrate 26 is fed between the print head 30 and the platen 28, and the ink ribbon 20 is also fed between the print head 30 and the platen 28 at a speed controlled to match the speed of the substrate 26 as the substrate 26 passes over the platen 28. The series of rollers 24 is arranged such that the ribbon 20 overlays the substrate 26 as the ribbon 20 and the substrate 26 together pass over the platen 28, with the substrate lying between the ribbon 20 and the platen 28 at that point.

The print head 30 is pivotally mounted for movement about a pivot axis 32 (which axis is shared by the print head drive pulley 16) towards and away from the platen between a printing position pressing against the platen and a rest position remote from the platen. The pivot axis 32 of the print head 30 is parallel to the axis the platen 28.

The print head 30 is connected to the print head drive pulley 16 via a slipping clutch. The operation of the clutch will be described in more detail with reference to Figures 3 and 4 but, for present purposes, suffice it to say that the clutch frictionally resists relative movement between the print head 30 and the print head drive pulley 16. Thus, as the motor 12 drives the print head drive pulley 16 in a clockwise direction, as illustrated, the print head 30 is driven downwardly towards the platen 28 into a printing position. Not only that, however; the print head 30 is positively held down in the printing position by virtue of the frictional force exerted through the slipping clutch as the motor 12 drives the print head drive pulley 16 in the direction indicated.

A free end 34 of the print head 30 carries a straight line array of printing elements (not shown). The array is oriented transversely with respect to the direction of movement of the substrate 26, and in parallel to the axis of the platen 28 and the pivot axis 32 of the print head 30. In this way, the array contacts the platen 28, in use, tangentially with respect to the surface of the platen 28, thereby defining a line of contact.

Each printing element is separately energisable under the control of printer electronics (not shown) to apply heat to the ink ribbon 20 when the print head 30 is in the printing position, thereby to transfer ink from the ribbon 20 to the substrate 26 to form the shape of the desired indicia as the substrate 26 passes through the printer 10.

The preferred embodiment is shown in more detail in Figures 2 and 3 and reference numerals have been retained where appropriate. The motor 12 has a motor shaft 36 carrying a motor pulley 38. The drive belt 14 encircles the motor pulley 38, the print head drive pulley 16 and a take-up spool pulley 40 attached to the take-up spool 18. The drive belt 14 thereby couples together the three pulleys 38, 16 and 40 so that when the motor 12 is activated, its torque is transmitted to each of the three pulleys 38, 16 and 40.

The basic operation of the printer will now be explained with reference where necessary to Figure 1. As the motor turns the motor shaft 36 clockwise as illustrated (denoted by the arrow C1), the drive belt 14 also circulates clockwise (arrow C2), which in turn drives the take-up spool pulley 40/take-up spool 18 (arrow C3), and the print head drive pulley 16 (arrow C4), both clockwise.

This clockwise movement of the print head drive pulley 16, acting via the aforementioned slipping clutch, urges the print head from an initial park or rest position into a printing position in which the free end 34 of the print head 30 is urged against the platen 28. When the free end 34 can move no further as the platen blocks its movement, the clutch starts to slip but, by virtue of the frictional force thereby generated, a firm holding force is applied to the print head to hold it in the printing position and thus to resist bounce and chatter.

Rotation of the take-up pulley 40 in the clockwise direction draws the ink ribbon 20 from the free-wheeling take-off spool 22 and then between the platen 28 and the print head 30 and onto the take-up spool 18. At the same time, the substrate 26 is driven over the platen 28 where ink from the ribbon 20 is transferred onto the substrate 26 by the printing elements carried by the print head 30.

Turning to the print head shaft assembly 46 which can be seen in Figures 2, 3 and in greater detail in Figure 4, the principal components thereof are carried on a print head shaft 48. The print head 30 is offset with respect to and fixed to the print head shaft 48 and the print head pulley 16 is coupled to the shaft 48 via a unidirectional bearing 50. The print head pulley 16 carries a first friction pad 52 which is in sliding frictional face-to-face contact with a second friction pad 54 that is fixed to the print head shaft 48.

Roller bearings 56 provide support for the shaft.

In a printing operation, clockwise rotation of the motor shaft 36 causes the print head pulley 16 to turn in the sense denoted by arrow C4 and the unidirectional bearing 50 is so arranged in the print head shaft assembly 46 that the print head pulley 16 can rotate around the print head shaft 48.

However, rotation of the print head pulley 16 around the print head shaft 48 is not entirely free because the friction pads 52 and 54 slide with respect to one another. Specifically, rotation of the print head pulley 16 causes rotation of the first friction pad 52 and this in turn transmits torque to the second friction pad 54 across their mutually abutting faces, causing the second friction pad 54 to rotate as well. Since the second friction pad 54 is fixed to the print head shaft 48, the shaft 48 also turns, thereby urging the print head 30 against the platen 28 in the printing position.

As ink ribbon 20 is drawn across the platen, the drive belt 14 continues to turn the print head pulley 16 clockwise.

However, when the print head 30 is hard against the platen, sandwiching the ribbon 20 and the substrate 26, the print head shaft 48 must cease turning. Continued rotation of the print head pulley 16 is accommodated by slippage between the first friction pad 52 and the second friction pad 54. In this way, the print head pulley 16 continues to exert a torque upon the print head shaft 48 and so continually and positively urges the print head 30 against the platen 28 while the print head 30 is in the printing position.

When the print head 30 is to be lifted away from the platen 28 into the park or rest position at the end of a printing operation, the drive direction of the motor 12 is reversed causing the drive belt 14 to circulate in the opposite sense, i.e. anticlockwise as illustrated. This in turn rotates the print head pulley 16 anti-clockwise. In this direction of rotation, the unidirectional bearing locks the print head pulley 16 to the print head shaft 48, consequently turning the shaft 48 anti-clockwise (denoted by arrow A4), which results in the print head 30 being pivoted off and away from the platen 28. It will be noted that this movement is positively driven, thereby ensuring that the print head 30 is reliably and, above all, quickly returned to the park or rest position when desired.

The take-up spool assembly is best seen in Figure 3. In Figure 3, the take-up pulley 40 is coupled to a take-up shaft 42 via a unidirectional bearing 44; the take-up spool 18 is fixed to the take-up shaft 42. The unidirectional bearing 44 is arranged such that, when the take-up pulley 40 is driven clockwise (as illustrated in Figures 1 and 2) the pulley 40 is locked to the take-up shaft 42 thereby causing the shaft 42 to rotate clockwise too. Conversely, when the take-up pulley 40 is driven anti-clockwise as illustrated, the bearing 44 allows the take-up shaft 42 to remain static while the pulley 40 turns freely. The significance of this is that when the motor 12 turns the drive belt 14 anti-clockwise to move the print head 30 into the park or rest position, the take-up spool pulley 40 does not then unwind the used ink ribbon from the spool.

A further aspect of the invention is embodied by location rollers 58 which, in the preferred embodiment illustrated, are provided in pairs. As can be seen in the side view of Figure 3, the pairs of location rollers 58 are disposed one either side of the print head. As can be seen in the front views of Figures 1 and 2, each pair consists of a lead roller 58A and a trailing roller 58B, the lead roller 58A being disposed ahead of the platen 28 and the trailing roller 58B being disposed behind the platen 28, having regard to the direction of motion of the substrate 26 through the printer 10. The axes of rotation of the rollers 58A and 58B are parallel to one another and to the axis of rotation of the platen 28, and pairs of rollers 58 disposed on respective sides of the print head 30 share a common axis.

The spacing between the rollers 58A and 58B is such that the platen 28 is effectively received in a recess or groove defined between the rollers 58A and 58B, with the platen 28 contacting the roller 58A along a first line of contact and contacting the roller 58B along a second line of contact.

The rollers 58A and 58B are in fixed positional relationship with the chassis of the printer 10 and so, when in contact with the platen 28 as illustrated, they ensure accurate location and alignment of the printer 10 in relation to the platen. In particular, the rollers 58A and 58B ensure that the free end 34 of the print head 30 bears against the platen along the desired line of contact.

It will be appreciated that the triangulation between the platen 28 and the rollers 58 ensures simple and easily repeatable positioning of the print head 30 with respect to the platen 28. This considerably simplifies the setting-up task, reducing lost production time and removing the skill element from the job. Indeed, correct location is automatically maintained because the rollers 58 and the platen 28 will, as they rotate in use, tend to cause the printer 10 to adopt the correct position with respect to the platen 28.

Referring finally to Figures 5(a) and 5(b), a mounting 60 for the take-off spool 22 comprises a generally tubular wall 62, the outer surface of which has diametrically-opposed J-shaped slots 64 formed therein. The slots 64 are shaped and dimensioned to receive inwardly-extending diametricallyopposed lugs formed in a tubular wall (not shown) associated with the take-off spool 22, which wall is dimensioned to receive the wall 62 of the mounting 60.

To install a fresh take-off spool 22, the spool 22 is aligned with the mounting 60 such that the inwardly-extending lugs can enter the open ends of the respective J-slots 64. By pressing and then turning the spool 22, the lugs move along the J-slots 64 eventually reaching their respective closed ends whereupon the spool 22 is locked into place in a bayonet-style arrangement.

Other keyed or interlocking attachments of a spool to its mounting are possible, and may be developed, although a bayonet fitting is simple and is currently preferred.

As the quality of the ink ribbon is crucial to printing quality, the use of a sub-standard ink ribbon may be avoided by adapting the mounting so that it employs bespoke interlocking shapes unique to the suppliers of good-quality ribbons.

To achieve whatever shapes may be desired for attachment of the spool to its mounting with accurate registration, the core of the spool may be moulded from plastics such as ABS as opposed to the cardboard cores used in prior art printers.

Cardboard can deform before and during use, obviously causing particular problems in the high-speed printers with which the invention is particularly concerned. These problems can be manifested as vibration and as 'wandering' of the ribbon with respect to the spool mounting and thus with respect to the printer as a whole, to the detriment of print quality.

The present invention may of course be embodied in other specific forms. Accordingly, reference should be made to the appended claims and other general statements herein rather than to the foregoing specific description as indicating the scope of the invention.

Claims (21)

1. A printer for transferring ink, in use, from a moving ink ribbon to a continuous moving substrate, the printer comprising a print head movable between a parked position and a printing position, and a motor for simultaneously driving the movement of the ribbon and the movement of the print head.

2. A printer according to claim 1, wherein the print head is continuously urged against a platen when the print head is in the printing position.

3. A printer according to claim 2, wherein the print head is connected to the motor via print head drive means and a slipping clutch, the clutch frictionally resisting relative movement between the print head and the print head drive means.

4. A printer according to claim 3, wherein the print head is driven by a print head shaft and the print head drive means is coupled to the print head shaft via a unidirectional bearing, the unidirectional bearing permitting the print head drive means to rotate around the print head shaft in a printing operation but, when the print head is to be moved into the parked position at the end of a printing operation by reversing the direction of movement of the print head drive means, locking the print head drive means to the print head shaft.

5. A printer according to any preceding claim, wherein the print head drive means and a ribbon spool are coupled to the motor by a drive belt or chain for transmission of torque from the motor to the print head drive means and the ribbon spool.

6. A printer according to claim 5, wherein the ribbon spool is a take-up spool that draws the ribbon from a take-off spool.

7. A printer according to claim 6, wherein the take-up spool is driven by a take-up shaft and a take-up spool drive means is coupled to the take-up shaft via a unidirectional bearing, the unidirectional bearing being arranged such that the takeup spool drive means is locked to the take-up shaft during a printing operation but, when the direction of movement of the take-up spool drive means is reversed, the take-up shaft can remain static while the take-up spool drive means rotates.

8. A printer according to any preceding claim, wherein a ribbon spool is removably keyed to a spool mounting.

9. A printer according to claim 8, wherein the ribbon spool is keyed to the spool mounting by a bayonet-style arrangement.

10. A printer according to claim 8 or claim 9, wherein the ribbon spool has a core shaped to define interlocking formations.

11. A printer comprising: a print head carrying a linear array of printing elements; and alignment means for aligning the linear array in relation to a platen, the platen having a curved surface against which the linear array bears during a printing operation; wherein the alignment means is arranged to ensure a line of contact with the platen that is substantially parallel to an axis of curvature of the curved surface of the platen.

12. A printer according to claim 11, wherein the platen is cylindrical and the line of contact is tangential to the curved surface and parallel to the longitudinal axis of the platen.

13. A printer according to claim 11 or claim 12, wherein the alignment means bears against the platen.

14. A printer according to claim 13, wherein the platen is cylindrical and the alignment means comprises a plurality of location rollers that bear against the platen.

15. A printer according to claim 14, wherein the platen is mounted for rotation and the location rollers are mounted for rotation with the platen.

16. A printer according to claim 15, wherein one or more pairs of location rollers consist of a lead roller disposed ahead of the platen and a trailing roller disposed behind the platen having regard to the direction of motion of the substrate through the printer, the axes of rotation of the rollers being parallel to one another and to the axis of rotation of the platen.

17. A printer according to claim 16, wherein pairs of location rollers are disposed on respective sides of the print head and wherein counterparts of each pair share a common axis.

18. A printer according to any of claims 14 to 17, wherein first and second location rollers define a gap or groove that accommodates the platen.

19. A platen for a printer, the platen being constructed from a material having a shore hardness rating of 40 or less.

20. A platen according to claim 19, constructed from foam rubber such as polyurethane foam rubber.

21. A printer, substantially as hereinbefore described with reference to or as illustrated in any of the accompanying drawings.