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 parked 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 take-off 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 parked 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.

The present invention seeks to solve these and other problems of known printers for transferring ink from a moving ink ribbon to a continuous moving substrate and does so by using the ribbon drive motor to drive movement of the print head. In this way, the invention provides a clear cost saving over the known printer outlined above because it halves the number of motors to one. This also saves on the cost of associated parts.

Additionally, a single-motor printer can, of course, be made more compact than the known twin-motor printer. Furthermore, 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.

Applicant is aware that printer manufacturers have already proposed single-motor printers to drive both an ink ribbon and a moveable print head. Examples are the image recording devices disclosed in EP 0274266 and US 5169247, both to Canon KK, but these devices are designed for less demanding low-speed printing applications, basically typewriters.

Analysis of EP 0274266 and US 5169247 reveals that both devices employ an intricate mechanism of gears, cams and levers to drive the ink ribbon and the print head from a single motor. The essential features of these very complex mechanisms are that: (i) the print head is pivotably mounted and spring-biased into a parked position; (ii) the print head is urged into the printing position by a lever or cam that presses against the print head and acts against the biasing force of the spring; and (iii) the lever or cam is driven by gearing coupled to the motor, the gearing also driving, inter alia, the ink ribbon.

Mechanisms appropriate for low-speed personal printers such as typewriters would be quite unable to cope with the heavy duty of the high-speed printers contemplated by the present invention. The tiny components of the mechanisms would be likely to fail and servicing of those components would be practically impossible.

For these reasons, the Canon documents do not lead the way to the present invention which 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, a ribbon drive motor and print head drive means for applying torque derived from the ribbon drive motor to the print head.

By applying a torque to the print head, the print head can be moved by means of a simple mechanism that is robust and easily serviced. This arrangement also facilitates an advantageous mode of operation in which the print head is continuously urged against a platen to ensure consistent print quality. Preferably, this is achieved by a clutch that resists, but does not prevent, relative movement between the print head and the print head drive means, the torque thus transmitted acting to urge the print head against the platen.

The platen preferably has a shore A hardness rating of 40 ± 5 and is of resilient construction to reduce bounce or chatter.

The clutch may be a friction clutch including a plurality of friction plates or pads that transmit torque while slipping with respect to one another. It is also possible to use other types of clutch such as a fluid clutch.

Where a friction clutch is used, it preferably includes means for compensating for wear of the friction materials that are employed. For example, a friction element for transmitting torque to a shaft may be movable axially with respect to the shaft and may be biased into mutual abutment with a corresponding friction element.

In a preferred embodiment, the print head drive means includes a directional coupling acting upon the print head, the coupling being directional in the sense that it transmits substantially different torques to the print head depending upon the direction in which torque derived from the motor is applied to it.

More specifically, this preferred embodiment contemplates a printer in which 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, preferably transmitting very little if any of the torque derived from the motor. However, 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, the unidirectional bearing transmits substantially all of the torque derived from the motor to the print head shaft, preferably by locking the print head drive means to the print head shaft.

Conveniently, 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.

The ribbon spool is suitably a take-up spool that draws the ribbon from a take-off spool . The take-up spool may be driven by a take-up shaft and a take-up spool drive means may be coupled to the take-up shaft by a directional coupling acting upon the take-up shaft such as a unidirectional bearing arranged such that the take-up 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.

A ribbon spool can be removably keyed to a spool mounting by, for example, a bayonet- style arrangement. The ribbon spool may have a core shaped to define interlocking formations.

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-up 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 such as 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 rubber of the platen may be extruded around a rigid core, and preferably has a shore A hardness rating of 40 ± 5.

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 longitudinal axis of 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 an anti-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 anti-clockwise as illustrated (denoted by the arrow Cl), the drive belt 14 also circulates anti-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 anti-clockwise.

This anti-clockwise movement of the print head drive pulley 16, acting via the aforementioned slipping clutch, urges the print head from an initial parked 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 anti-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 first friction pads 52 which are in sliding frictional face- to-face contact with second and third friction pad assemblies 54 and 55. Friction pad

assembly 54 is fixed to the print head shaft 48 and friction pad assembly 55 is located on the shaft 48 via a pin and groove arrangement which permits axial movement of the assembly 55 along the shaft but prevents rotation of the assembly 55 around the shaft 48. The friction pad assembly 55 is biased toward the friction pads 52 by a spring 68, thus 5 allowing the clutch assembly to compensate for any wear that may occur to the friction material and hence to keep a constant frictional force on the clutch assembly. Roller bearings 56 provide support for the shaft.

In a printing operation, anti-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

10 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 slide with respect to friction pads 54 and 55. Specifically, rotation of the print head pulley 16 causes rotation of the first friction pads 52 and this in turn transmits torque to the second

15 and third friction pads 54 and 55 across their mutually abutting faces, causing the second and third friction pads 54 and 55 to rotate as well. Since the second and third friction pads 54 and 55 are 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 0 head pulley 16 anti-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 pads 52 and the second and third friction pads 54 and 55. In this way, the print head pulley 16 continues to exert a torque upon the print head 5 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 parked 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. clockwise as illustrated.

This in turn rotates the print head pulley 16 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 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 parked 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 anti-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 anti-clockwise too. Conversely, when the take-up pulley 40 is driven clockwise as illustrated in Figures 1 and 2, 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 clockwise to move the print head 30 into the parked or rest position, the take-up spool pulley 40 does not then unwind the used ink ribbon from the spool.

Referring finally to Figures 5(a) and 5(b), a mounting 58 for the take-up spool 18 comprises a generally tubular wall 60, the outer surface of which has diametrically- opposed J-shaped slots 62 formed therein. The slots 62 are shaped and dimensioned to receive inwardly-extending diametrically-opposed lugs formed in a tubular wall (not shown) associated with the take-off spool 18, which wall is dimensioned to receive the wall 60 of the mounting 58.

To install a new take-up spool 18, the spool 18 is aligned with the mounting 58 such that the inwardly-extending lugs can enter the open ends of the respective J-slots 62. By turning the spool 18, the lugs move up the slopes of and along the J-slots 62 eventually reaching their respective closed ends whereupon the spool 18 is locked into place in a bayonet- sty le 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.