EP0734876A2

EP0734876A2 - A printer for printing on a continuous print medium

Info

Publication number: EP0734876A2
Application number: EP96301983A
Authority: EP
Inventors: Michael Cameron Green; Anthony James Palmer; Alan Hall
Original assignee: ITW Ltd
Current assignee: ITW Ltd
Priority date: 1995-03-31
Filing date: 1996-03-22
Publication date: 1996-10-02
Also published as: GB9506651D0; BR9601209A; EP0734876A3; NO961275D0; AU5033196A; NZ286279A; NO961275L; CA2173083A1; MX9601215A

Abstract

A thermal printer for printing on a continuous print medium by ink transfer from a thermal print ribbon has a print head which is pivotally mounted and which is driven in an oscillatory nodding motion by means of a stepper motor so as repeatedly to bring a linear array of energisable printing elements to bear against a platen roller. Both the element array and the platen extend transversely to respective paths of travel of the print medium and the ribbon. The print medium is fed through the printer from an inlet region, between the platen and print head, and thence to an outlet region. The instantaneous rate of travel of the print medium past the print head is substantially the same as the rate of feed of print medium to the printer. Typically this rate is of the order of 250 to 400 mm per second. The ribbon also travels between the print head and the platen, overlying the print medium and is driven in such a manner that it travels at the same rate as the print medium during each printing operation. This is achieved by driving the ribbon with a motor the speed of which is controlled according to the sensed speed of travel of the print medium, e.g. by coupling the motor to processing circuitry which takes an input signal from a shaft encoder associated with the platen roller. Alternatively, the ribbon may be driven by frictional contact with the print medium. These methods of driving the ribbon have the advantage that the ribbon speed can be varied to take account of different print medium speeds, e.g. due to the requirements of differing packaging processes, and also in response to transient variations in print medium speed during each printing operation.

Description

This invention relates to a thermal printer for printing on a continuous print medium by thermal ink transfer from a print ribbon.
It is known to print continuous packaging material and other continuous print media such as label bearing substrates with alphanumeric information and other symbols using a thermal transfer printer. A print head having a row of electronically driven heating elements is brought to bear against an ink-carrying thermal transfer ribbon lying over the print medium while the print medium is driven perpendicularly to the row of print elements. In one known printer, the ribbon is supplied from a take-off spool and then passes along a ribbon path which extends between the print head and the path of the print medium, and thereafter is fed onto a take-up spool, the ribbon travelling across the print head at at least approximately the same speed as the print medium whilst printing is taking place. The path followed by the print medium extends around movable rollers which deflect the print medium by variable amounts both upstream and downstream of the print head. These rollers impose significant stresses on the print medium and complicate threading when the print medium is loaded into the printer. Such a printer operates typically at print medium speeds up to 200 mm per second.
It is an object of this invention to provide a more reliable and faster printer for continuous printing.
According to a first aspect of this invention there is provided a thermal printer for printing on a continuous print medium by ink transfer from a thermal print ribbon, comprising means defining a print medium path between inlet and outlet regions of the printer, a platen extending transversely of the said path, a thermal print head having energisable print elements and located in an opposing relationship with respect to the platen on the other side of the print medium path from the platen, means defining a ribbon path which, between the print head and the platen, runs in the same direction as and lies adjacent the print medium path, a printing actuator operable to bring the print head and the platen together in successive printing operations, and ribbon drive means operable to drive the ribbon along the ribbon path at variable rates during the printing operations, the print medium path defining means being arranged such that the print medium travels past the print head at an instantaneous rate which is substantially the same as that with which it enters the inlet region.
Preferably, the speed with which the ribbon is driven during each printing operation is variable in response to the print medium speed of travel so as to match the speed with which the print medium is fed past the print head. This may be achieved, for example, by forming the platen as a roller around which the print medium is wrapped so that the speed of rotation of the roller is a measure of the speed of passage of the print medium. A shaft encoder, typically an optical encoder, is used to provide an input to processing means forming part of the printing actuator to control movement of the print head and energisation of the printing elements. In addition, the ribbon may be driven by a stepper motor coupled to a ribbon drive roller, the speed of operation of the stepper motor being governed by the sensed speed of rotation of the platen roller. Alternatively, the printer may be arranged such that the ribbon is pressed against the printing medium so that, providing the ribbon is fed from and taken up on spools at a sufficient rate, the speed of the ribbon across the print head during the printing operation is determined entirely by frictional drive from the print medium, no separate ribbon drive motor being required.
In this way it is possible to operate the printer over a wider range of speeds than prior art printers, with the print medium passing the print head at substantially the same rate as it is fed to and extracted from the printer. Typically, the printer is capable of operating at print medium speeds up to 400 mm per second.
According to a method aspect, the invention includes a method of printing on a continuous print medium, comprising providing a printer having a print head with energisable print elements, and a platen, the print head and the platen being located in an opposing relationship with respect to each other, feeding the print medium to the printer at a continuous feed rate and passing it between the print head and the platen in the same direction as and adjacent the print medium, feeding a print ribbon between the print head and the platen, and periodically bringing the print head and the platen together and energising the print elements with the ribbon in contact with the print medium to cause transfer of ink from the ribbon to the print medium in a printing operation, wherein the print medium is passed between the print head and the platen at a rate corresponding to the rate at which the print medium is fed to the printer during and between successive printing operations and wherein, during each printing operation, the ribbon is passed between the print head and the platen in contact with the print medium, the rate at which the ribbon moves during the printing operation being variable in response to the rate of travel of the print medium in the printer.
The ribbon path may be defined by guides and, between the print head and the platen, runs in the same direction as and lies adjacent the print medium path, the printing actuator being operable to move the print head towards and away from the platen in successive printing operations, to compress the ribbon and the print medium together along a line of printing elements on the printing head. The print head may be mounted on a print head carrier which is pivotable about an axis running transversely to the print medium path, the print elements being spaced from this axis to execute the above-mentioned movement towards and away from the platen as a nodding motion.
The platen is preferably a roller having a diameter not greater than 20 mm.
Advantageously, the print head carrier is linked to the shaft of a stepper motor coupled to the printing actuator, with the printing elements spaced from the axis of rotation of the motor so that they follow an arcuate locus which passes through the surface of the platen roller at a location where it supports the print medium. Operation of the stepper motor over a small angular range successively in opposite directions moves the print head towards and away from the platen roller at the beginning and end respectively of each successive printing operation. Rigid coupling of the print head carrier to the motor shaft (e.g. by means of cranks and a connecting rod or by direct co-axial connection) results in accurate positioning of the print head elements with respect to the print medium as it travels over the platen roller and with respect to the platen roller axis of rotation.
The invention also includes, according to a third aspect thereof, a printer for printing on a continuous print medium by ink transfer from a print ribbon, comprising means defining a print medium path, a platen extending transversely of the path, a print head having energisable print elements and located in an opposing relationship with the platen on the other side of the print medium path from the platen, means defining a ribbon path which, between the print head and the platen, runs in the same direction as and lies adjacent the print medium path, a ribbon take-off spool, a ribbon take-up spool which is belt driven via a slipping clutch drive, and a printing actuator operable to bring the print head and the platen together in successive printing operations.
The printer may further comprise a ribbon drive pulley located in the ribbon path between the print head and the take-up spool, and a ribbon drive motor coupled to the drive pulley, the take-up spool being belt-driven by the said motor. The take-up spool may be belt-driven directly from the print medium or from the roller driven by the passage of the print medium. Thus, the belt drive may include a driven roller arranged to bear against either the print medium where it lies over the platen or an alternative supporting surface, or against a roller which is rotated by the passage of the print medium. The driven roller may be mounted on the print head carrier so as to drive the take-up spool only during a print operation, i.e. when the print head bears against the ribbon, the print medium and the platen.
The invention is applicable primarily to printing variable information on continuous plastics film packaging material, with each print operation being triggered by, for example, sensing the position of products to which the packaging material is to be applied as they travel along an adjacent conveyor. Typically, the information includes sell-by dates, serial numbers, pricing information, and bar codes.
The invention will be described below by way of example with reference to the drawings in which:-

Figure 1 is a diagrammatic exploded side view of a printer in accordance with the invention;
Figure 2 is a front view of a base unit printer of Figure 1;
Figure 3 is a rear view of a ribbon cassette of the printer of Figure 1;
Figure 4 is a block diagram of electrical parts of the printer;
Figure 5 is a plan view of a ribbon drive roller for the printer of Figure 1; and
Figure 6 is a diagrammatic front view of an alternative printer in accordance with the invention.

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 10 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 mounted on a front plate 16 ofthe unit 10, and a ribbon drive stepper motor 18 similarly mounted on the front plate 16. Coupled to the motor shaft 14S of the print head stepper motor 14 is a pivotable print head carrier 20 which supports a print head 22.
Coupling of the print head carrier 20 to the motor shaft 14S is by way of a parallelogram linkage lying behind the front plate 16 and comprising a first crank 24 fixed to the motor shaft 14S, a connecting link or rod 26, and a second crank 28 generally in the form of a semi-circular plate which is mounted on a shaft 30 supporting the print head carrier 20. Shaft 30 takes the form of a boss 30B on the front side of plate 16, with an axially extending tongue 30T to which the print head carrier 20 is bolted.
The motor shaft 18S ofthe ribbon drive stepper motor 18 is attached to a drive spindle 32 which, like the print head carrier 20, projects perpendicularly from the front plate 16 of the base unit 10.
For clarity, the cassette unit 12 is shown in Figure 1 spaced from the base unit 10. In practice, when fitted to the base unit 10, the cassette unit 12 is closer to the base unit 16, such that ribbon spools 34, 36, which are rotatably mounted on a front plate 38 of the cassette unit 12, are coextensive with the print head 22 in terms of their location in a direction perpendicular to the front plate 38 of the base unit 40. Also attached to the cassette unit front plate 30 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 the 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. Certain items shown in Figure 1 are not shown in Figure 2, and vice versa. In particular, a platen roller 44 and a deflection roller 46 are shown in Figure 2 but not in Figures 1 and 3. These components are mounted on the printer frame or other apparatus which the printer is associated.
Referring to Figure 2, continuous film material to be printed (shown by reference numeral 50) enters the printer in an inlet region 52, passes over and wraps around platen roller 44 from where it follows a downwardly inclined path to pass underneath and wrap around a deflection roller 46 before passing to an outlet region 54 of the printer. The positioning of platen and deflection rollers 44 and 46 is such that the film substrate 50 is nowhere deflected through an angle greater than 60° by any one guiding element, and preferably not greater than 45°. The function of the deflection roller 46 may be performed instead by any deflecting support for the film substrate 50 positioned to cause the substrate to be wrapped partly around the platen roller. 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 film 50, and both axes are fixed in position so that the path of the substrate film 50 remains substantially constant during and between successive printing operations. As a result, the instantaneous rate at which the film 50 passes over the platen 44 always matches the rate at which it is supplied to and extracted from the printer through the inlet and outlet regions 52 and 54.
The thermal transfer ribbon 42 follows a ribbon path as follows. Firstly, a supply of the ribbon is provided on a feed spool 36 which is mounted by means of a bearing (not shown) fixed to the cassette unit front plate 38. A degree of friction is built into this bearing to maintain tension in ribbon 42. From spool 36, the ribbon 42 passes over a break detector roller 58 attached to the end of a break detector arm 60 which is rotatable about the rotation axis of the feed spool 36 and biased in a clockwise direction as seen in Figure 3 by a spring 62. From roller 58, the ribbon 42 passes over guide rollers 64 and 66 attached to the cassette unit front plate 30 and thence through a 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 a further guide roller 68. The head and the platen are seen in Figure 2, as are also guide rollers 66 to 68, so that the location of the ribbon path relative to the head and platen can be seen. Where the ribbon 42 passes over platen 44 it is in frictional contact with the substrate film 50. The ribbon 42 is held in contact with substrate film 50 only between the start and finish of each printing operation, during which the lower surface ofthe print head 22 bears against the platen 44 through the 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.
From the print head 22, the ribbon 42 travels over guide roller 70 and is then wrapped around the drive roller 40. A pinch roller 72, mounted on a pivotable support arm 74, maintains the ribbon 42 in gripping contact with drive roller 40. Drive roller 40 has a rubber sleeve and is driven by motor 18 via a toothed belt 75 and toothed pulleys 75P behind base unit front plate 16 on the motor shaft 18S and the spindle 32, (shown in Figures 1 and 2) so that the ribbon 42 is pulled through the space between the print head 22 and the platen 44. From the drive roller 40, the ribbon 42 passes to a take-up spool 38 which is belt-driven by a belt 76 from a pulley 78 (see Figure 1) mounted on the shaft of drive roller 40. The mounting bearing (not shown) of the take-up spool 34 is mounted on a shaft fixed to the cassette unit front plate 30 and, like the mounting bearing of the feed spool 36, has a degree of friction built in. The diameter of the pulley 34P associated with take-up spool 34 together with the diameter of the pulley associated with drive roller 40 are such that the shaft bearing the take-up spool 34 is always driven faster than the speed of rotation necessary to take up the ribbon 42 from the drive roller 40, regardless of the diameter of the ribbon reel. The friction slip built into the connection between spool 34 and the belt-driven shaft allows the respective speeds of rotation of the drive roller 40 and the take-up spool 34 to be different from each other.
The print head 22 has side-facing printing elements 82 (Figure 1) 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 which, in the printing position of the print head 22 is tangential to the platen roller 44, as shown by the chain lines in Figure 2. The arcuate locus followed by the line of printing elements 82 when the print head 22 is pivoted about axis 84 passes through the intersection of a tangent parallel to the print head lower surface 86 and the platen roller surface. Consequently, 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. When these elements are heated under electronic control, and the film 50 and ribbon 42 are passed together over the element, 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.
During printing, the ribbon 42 is in contact with film 50 and normally travels at the same speed. This is achieved by mounting an optical shaft encoder on a shaft bearing the platen roller 44. The output of the encoder is representative of the speed of the film 50, and by processing this output signal, the stepper motor 18 driving ribbon drive roller 40 is adjusted such that the ribbon is driven at the correct speed. This synchronisation between ribbon 42 and film 50 can be maintained over a wide range of speeds.
The preferred embodiment is capable of operating at a film speed of 400 mm per second. 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. These are a first output 96 coupled to a first motor driver circuit 98 for moving the print head between its inactive retracted position and its active extended position (respectively shown in Figure 2) by means of stepper motor 14 and its associated linkage.
A second output of the processor unit 94 is a multi-wire input 100 coupled to the energisable elements 82 of the print head 22.
The third output 102 is coupled to a second motor driver 104 to control stepper motor 18, thereby stopping and starting the ribbon, and controlling the ribbon speed during each printing operation.
Other inputs to the processor include trigger input 106 which receives a trigger signal initiating each printing operation. Typically, the trigger signal is generated by sensing the position of products to which the substrate film is to be applied as packaging, as the products travel along an adjacent conveyer. Another input 108 receives the information to be printed from a memory 110. 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.
Whilst the printing operation is progressing, the speed at which the ribbon is driven via driver 104 and motor 18 is determined according to the film speed signal input received by the processor at input 92, so as to drive the ribbon at the same speed as the film. The rate at which the print head elements are driven (i.e. the rate at which the pattern or characters are printed) is also varied by processor unit 94 according to the film speed signal input.
When the processor senses that all of 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. A processor 94 then waits for the next trigger signal on 106 before repeating the above process. Further inputs 112 and 114 of the processor 94 are called respectively to a ribbon status sensor 116 and a ribbon break sensor 118 which are respectively associated with a spring loaded pivotable arm 120, seen in Figure 3. This arm 120 has a roller 122 at its distal end contacting the periphery of the ribbon supply on ribbon feed spool 36, so that when the ribbon supply runs low, an alarm can be activated and/or operation of the packaging apparatus of which the printer is part can be halted. Similarly, the break sensor 118 is responsive to excessive clockwise movement of arm 60 (see Figure 3) to sense breakage of the ribbon 42 which, during normal operation, keeps roller 58 approximately in the position shown in Figure 3.
Further details of the preferred printer in accordance with the invention will now be described. Limits on the movement of print carrier 20 and print head 22 are determined firstly by the striking of the print head elements 82 against the platen 44 (see Figure 2) through the ribbon 42 and film 50, and, in the retractive position, by an adjustable stop (not shown) associated with the semi-circular plate 28 behind the front plate 16 of the base unit.
Drive to the ribbon drive roller 40, which, it will be seen, is mounted on the cassette front plate 30, is transferred from the base unit 10 to the roller by means of drive spindle 32 shown in Figure 1. Referring to Figure 5, roller 40 contains a clutch bearing 40C which is so mounted within the roller 40 that it is allowed to float in the sense that the centre of bearing 40C need not coincide exactly with the centre of the roller 40. When the cassette unit 12 is mounted on base unit 10, the drive shaft or spindle 32 attached to ribbon drive motor 18 (see Figure 1) enters clutch bearing 40C (Figure 3). Needle rollers of the clutch bearing, which are self-locking when driven in one rotary direction, engage the outer surface of shaft or spindle 32 and drive is transferred from spindle 32 to the bearing 40C and thence via pins 40P to the roller 40. The floating nature of the clutch bearing 40C within the roller 40 allows for a degree of mismatch between the axis 128 of drive spindle 32 and that 130 of roller 40 when the cassette unit 12 is mounted on the base unit 10.
The cassette unit 12 is located on base unit 10 by means of a retention pin 132 and a tubular socket 134, as shown in Figure 1.
An alternative embodiment is shown in Figure 6. In many respects this alternative printer corresponds to that shown in Figures 1, 2, and 3, and corresponding parts are correspondingly numbered in Figure 6. Figure 6 is diagrammatic in the sense that it is a front view of the printer base unit 10 with certain components of the ribbon cassette unit shown superimposed so that the interaction of both units can be seen.
In this alternative embodiment, the printer has no ribbon drive roller. Instead, synchronisation of the ribbon travel and ribbon speed with film travel and film speed is achieved solely by frictional contact between ribbon 42 and film 50 between the print head 22 and platen roller 44. This frictional contact is sufficient to overcome the resistance to movement of the ribbon 42 presented by the frictional bearing mounting 36B of feed spool 36 and the frictional resistance produced by break detector pin 58 and roller 66 defining the ribbon path upstream of the print head 22. Drive for the take-up spool 34 is derived from a roller 140 (shown in dotted lines in Figure 6) which is mounted on the print head carrier 20 in such a position that it contacts the substrate film 50 alongside the ribbon 42. Indeed, film 50 is pinched between drive roller 140 and platen roller 44. A pulley (not shown) is mounted for rotation with roller 140 and a belt 142 is threaded around this pulley to transfer rotational drive to the take-up spool 34. As in the embodiment of Figures 1 and 2, there is a clutch element between the shaft driven by belt 142 and spool 34 to allow the speed of rotation of spool 34 to vary as the diameter of the taken up ribbon reel increases with use of the ribbon 42. By taking belt drive for the take-up spool 34 directly from substrate film 50, the applicants have, in this second embodiment, dispensed with the need for a ribbon drive motor and the floating bearing arrangement of Figure 5. The drive belt 142 for the ribbon take-up spool 34 is tensioned by means of a roller 144 mounted at the end of an arm 146 which pivots about the bearing 34B ofthe spool 34 and is spring-loaded so as to be biased in the anti-clockwise direction as seen in Figure 6.
Other differences which the second embodiment exhibits, compared with the embodiment of Figures 1, 2, and 3 include the mounting of the print head carrier 20 directly on the output shaft of the print head stepper motor 14, the latter being mounted behind the front plate of the base unit (as shown by dotted lines in Figure 6). The print head 22 is resiliently mounted on the carrier 20 by means of a coil spring 148.
In this case, tapered positioning pins 150 are attached to the cassette unit front plate and engage in apertures (not shown) in the base unit 10.

Claims

A thermal printer for printing on a continuous print medium by ink transfer from a thermal print ribbon, comprising means defining a print medium path between inlet and outlet regions of the printer, a platen extending transversely of the said path, a thermal print head having energisable print elements and located in an opposing relationship with respect to the platen on the other side of the print medium path from the platen, means defining a ribbon path which, between the print head and the platen, runs in the same direction as and lies adjacent the print medium path, a printing actuator operable to bring the print head and the platen together in successive printing operations, and ribbon drive means operable to drive the ribbon along the ribbon path at variable rates during the printing operations, the print medium path defining means being arranged such that the print medium travels past the print head at an instantaneous rate which is substantially the same as that with which it enters the inlet region.
A printer according to claim 1, including a print medium speed sensor and control means coupled to the speed sensor to receive a sensor output signal therefrom, and wherein the ribbon drive means comprises a ribbon drive roller connected to a motor, the control means having an output coupled to the motor and being operable to drive the motor at a rate dependent on the sensor output signal.
A printer according to claim 2, wherein the control means are operable to cause the motor to run at a rate which is dynamically variable during the printing operations in response to the sensor output signal.
A printer according to claim 1, wherein the ribbon drive means are arranged such that the ribbon is driven by frictional contact with the print medium.
A printer according to any preceding claim, wherein the ribbon drive means are operable to drive the ribbon at the same speed as the print medium.
A printer according to any preceding claim, wherein the print medium path is substantially fixed.
A printer according to any preceding claim, wherein the platen has a cylindrical surface and the print medium path defining means are arranged to cause the print medium to wrap around the said surface.
A printer according to claim 7, wherein the platen is a roller and wherein the print medium path defining means include a print medium deflector parallel to and adjacent the platen to cause the print medium to wrap around the platen.
A printer according to any preceding claim, wherein the platen is fixed in position and the print head is movable towards and away from the platen in response to operation of the printing actuator.
A printer according to claim 9, wherein the print head is mounted in a print head carrier, and wherein the printer further comprises a motor coupled to the print head carrier and operable to drive the carrier and the head in an oscillating motion.
A printer according to claim 9 or claim 10, wherein the print head is mounted on a print head carrier which is pivotable about an axii running transversely to the print medium path, the print elements being spaced from the said transverse axis to execute the movement towards and away from the platen and to follow an arcuate locus intersecting the platen.
A printer according to claim 11, including a stepper motor coupled to the print head carrier for pivoting the print head carrier about the transverse axis, the print head being moved towards and away from the platen at the start and finish respectively of each printing operation.
A printer according to claim 1, including a print medium speed sensor, the printing actuator being responsive to an output from the speed sensor.
A printer according to any of claims 2, 3 and 13, wherein the sensor is a shaft encoder associated with a roller positioned with respect to the print medium path so as to be in contact with the print medium as it passes along the said path with the surface speed of the roller matching the speed of the print medium.
A method of printing on a continuous print medium, comprising providing a printer having a print head with energisable print elements, and a platen, the print head and the platen being located in an opposing relationship with respect to each other, feeding the print medium to the printer at a continuous feed rate and passing it between the print head and the platen in the same direction as and adjacent the print medium, feeding a print ribbon between the print head and the platen, and periodically bringing the print head and the platen together and energising the print elements with the ribbon in contact with the print medium to cause transfer of ink from the ribbon to the print medium in a printing operation, wherein the print medium is passed between the print head and the platen during each printing operation at a rate corresponding to the rate at which the print medium is fed to the printer, and wherein, during the printing operation, the ribbon is passed between the print head and the platen in contact with the print medium, the rate at which the ribbon moves during the printing operation being variable in response to the rate of travel of the print medium in the printer.
A method according to claim 15, wherein the relative approaching movement of the print head and the platen takes place as movement of the print head, the platen remaining fixed in position.
A method according to claim 16, wherein the print head is pivoted about an axis extending transversely to the direction of travel of the print medium.
A method according to claim 15, including sensing the rate of travel of the print medium and driving the ribbon during the printing operation at a rate which is dynamically variable in response to the sensed rate of travel of the print medium.
A method according to claim 15, wherein the ribbon is driven between the print head and the platen by frictional contact between the ribbon and the moving print medium during the printing operation.
A method according to any of claims 15 to 19, wherein the ribbon is driven at a rate which, during each printing operation, is substantially the same as the rate of travel of the print medium.
A printer for printing on a continuous print medium by ink transfer from a print ribbon, comprising means defining a print medium path through the printer, a platen extending transversely of the said path, a print head having energisable print elements and located in an opposing relationship with the platen on the other side of the print medium path from the platen, means defining a ribbon path which, between the print head and the platen, runs in the same direction as and lies adjacent the print medium path, a ribbon take-off spool, a ribbon take-up spool which is belt-driven via a slipping clutch drive, and a printing actuator operable to bring the print head and the platen together in successive printing operations.
A printer according to claim 21, further comprising a ribbon drive pulley located in the ribbon path between the print head and the take-up spool, and a ribbon drive motor coupled to the drive pulley, wherein the take-up spool is belt-driven by said motor.
A thermal printer for printing information on a moving web by ink transfer from a thermal print ribbon, comprising:
a frame;

a print head mounted on the frame and movable between an inactive retracted position and an active extended position, the print head including an elongate array of energisable printing elements;

a printing actuator operable to move the print head between the retracted and extended positions;

means defining a ribbon path extending across the print head in a direction laterally of the printing element array;

ribbon drive means operable to drive the ribbon along the ribbon path; and

electronic control means having a web speed sensor input, a printing actuator output and a ribbon drive output, and arranged to operate the ribbon drive means at a speed dependent on a signal received at the web speed sensor input.
A printer according to claim 23, wherein the print head is rotatable about an axis extending parallel to the array of printing elements, the printing actuator and the control means being operable to cause the print head to oscillate about the axis to bring the print head repeatedly into the extended position for respective successive printing operations in which ink is transferred from the ribbon to the web whilst both ribbon and web are moving past the head and whilst the head is held stationary in the extended position.
A printer according to claim 23, wherein the control means are operable to cause the ribbon drive means to drive the ribbon across the head at the same speed as the rate of travel of the web as indicated by the web sensor input signal.
A printer according to claim 23, including:
a roller associated with a web path extending in the same direction as the ribbon path across the print head, the roller being located to receive the web therearound; and

a web speed sensor comprising a shaft encoder connected to the roller.