Abstract:
Multiple images are printed utilizing lateral sets of print elements on a printhead and lateral portions of a ribbon.

Description:
BACKGROUND TO THE INVENTION 
     This invention relates to a method of printing. 
     In pixel based printing systems such as dot matrix ribbon printing, or thermal transfer printing which utilises a carrier or carrier which carries print medium such as ink, (known in thermal printing, as ribbon or foil), one major expense for a user is the cost of the ribbon or foil. 
     SUMMARY OF THE INVENTION 
     According to the invention we provide a method of printing utilising a printing apparatus having a print head with an array of printing elements each of which is individually selectable in a plurality of pixel row positions along an adjacent substrate to transfer a pixel of print medium from a carrier onto the adjacent substrate, the array extending laterally with respect to a direction of relative movement between the carrier and substrate, and the print head, characterised in that the method includes the steps of 
     (a) carrying out a first printing operation by means of causing relative movement between the substrate and carrier, and the print head, such that the print head moves relative to a first area of the carrier from a start position to an end position whilst printing elements from a first set of adjacent printing elements of the laterally extending array are selected to transfer a first set of pixels of print medium from the area of the carrier onto the substrate to produce an image having height less than one half of the width of the carrier; 
     (b) causing relative movement between the print head and the carrier to reposition the print head at the start position of the carrier; 
     (c) causing relative movement between the carrier and the substrate to present fresh substrate adjacent to the area of the carrier, and 
     (d) carrying out a second printing operation by means of causing relative movement between the fresh substrate and carrier, and the print head, such that the print head moves again relative to the area of the carrier from the start position to the end position whilst printing elements from a second set of adjacent printing elements laterally disposed with respect to the first set of adjacent printing elements are selected to transfer a second set of pixels of print medium from the area of the carrier onto the fresh substrate, to produce a second image having a height less than one half of the width of the carrier. 
     The invention offers a way for a user to save the cost of thermal printing ribbon or foil, or other carrier and print medium where the image to be printed is substantially narrower than i.e. at least half of the width of the carrier. 
     By “fresh substrate” we mean an entirely fresh substrate, such as a different label, or a further part of the same substrate, onto which pixels of print medium have not previously been transferred from the carrier. 
     By means of the invention, two separate substrates or separate areas of substrate can be printed for example, with the same information, but the printing apparatus only consumes one area of ribbon or foil. 
     Particularly where the image is very narrow compared to the width of the carrier, the method may be repeated several times for the same area of carrier, with each relative movement between substrate and carrier, and the print head, utilising a different set of printing elements to transfer different pixels of print medium onto substrate. 
     After each printing operation the printing head may be moved e.g. laterally, away from the carrier and substrate, and held a short distance away from the carrier whilst the carrier and/or substrate are moved in preparation for the next printing operation, and then moved e.g. laterally, back towards the carrier and substrate. 
     In one embodiment, the relative movement between the substrate and carrier, and the print head, is produced by movement of the print head whilst the substrate and carrier are held generally stationary relative to a base. 
     In another embodiment, the relative movement between the substrate and carrier, and the print head, is produced by movement of the substrate and carrier whilst the print head is held generally stationary relative to a base. 
     The invention is particularly but not exclusively applicable to thermal transfer printing, where the print medium comprises ink carried on a carrier comprising a continuous backing carrier, and the printing elements are energised to produce heat to transfer pixels of ink from the carrier onto a substrate. 
     In such an application, there are typically at least six, commonly eight or twelve or more printing elements per millimetre of printing head, arranged in a single line array. The printing elements may, however, be arranged in a multiple line, or other non-single line array. 
     However the invention may be applied to any other dot based printing system such as a dot matrix printer which utilises a woven ribbon as a carrier for ink and where printing elements are arranged in an array. 
     According to a second aspect of the invention we provide a printing apparatus adapted for performing the method of the first aspect of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 is a side illustrative view of a printing apparatus which may be operated by a method in accordance with the invention, without a print medium carrying carrier being shown, for clarity; 
     FIG. 2 is a top plan view of the printing apparatus of FIG. 1, showing the print medium carrying carrier; 
     FIG. 3 is a front illustrative view of the printing apparatus of FIG. 1 again without the print medium carrying carrier for clarity; 
     FIG. 4 is an illustrative view of a length of print carrying medium after fifteen printing operations according to the invention have been carried out, and 
     FIG. 5 is a plan view of part of an alternative embodiment of a printing apparatus in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 to  3 , there is shown a printing apparatus  10  comprising a print head assembly  11  which mounts a plurality of individually energisable thermal printing elements, preferably provided at an edge of the print head assembly  11 , in a single line array. The print head assembly  11  is movable relative to carrier, being a carrier  12  which carries print medium comprising ink, whilst the thermal printing elements are individually selectively energised under computer control, wherein the elements will become hot, thus to cause pixels of ink to be removed from the carrier  12  and deposited onto a substrate  22  to the right hand side of the apparatus  10  as seen in FIG.  1 . The substrate may for example be a label which is subsequently applied to an article, or packaging material, or may be the article itself, which substrate moves past the printing apparatus  10  and is temporarily halted at the printing apparatus  10  whilst printing thereon is effected. 
     In this way, information can be printed, in ink, on the substrate. 
     The information usually is, one or more alpha-numeric characters, to indicate for example, a sell-by date. The or each such character is defined by a plurality of pixels of print medium i.e. ink, transferred from the carrier  12  by the energised printing elements of the printing head assembly  11  as the print head assembly  11  is moved relative to the carrier and substrate. 
     The carrier  12  carrying the ink is provided on a supply spool  14  carried on a hub  15 , the carrier  12  passing around a carrier guide path comprising idler rollers  16 , 17 , 18 , around a further roller  19  between the roller  19  and a drive roller R and then on to a take up spool mounted on a hub  20 . The drive roller R and take up spool are driven, as hereinafter explained, from a motive means  21  which is in this example, a stepper motor. 
     The hub  15  and hence spool  14  provides some resistance to carrier  12  being paid out therefrom, this being provided by a friction means being a clutch material W and a spring S configured as is well known in the art. The take up spool is also mounted on a hub  20  having a similar friction means. 
     The print head assembly  11  is driven for movement relative to the carrier  12  by the motor  21  via a transmission. The transmission comprises a pair of generally parallel spaced apart flexible drive members comprising belts  23 , 24 , which are entrained respectively about pairs of rollers  25 , 26 , and rollers  27 , 28 . 
     The first pair of rollers  25 , 26 , are mounted on respective generally parallel and vertical drive shafts  30 , 31 , with shaft  31  being driven via a belt  32  or chain drive or otherwise as required, from an output shaft  33  of the stepper motor  21 . 
     The second pair of rollers  27 , 28 , are each mounted on respective generally parallel and vertical shafts  34 , 35 , via bearings so that the rollers  27 , 28 , are free to rotate relative to their respective shafts  34 , 35 . 
     Drive shaft  30  has secured to it, a gear  30   a  which meshes with a gear  30   b  on a shaft L on which roller R is provided. 
     As can be seen from FIG. 1, the print head assembly  11  is of generally rectangular configuration, and is secured to a mounting structure T which is clamped at screws  36 , 37 , (see FIG. 3) to the belts  23 , 24 . Upon operation of the motor  21  drive is transmitted from the drive shaft  33  of the motor to each of the belts  23 , 24 , via the shaft  31 , and hence the print head assembly  11  is caused to move either in the direction indicated by arrow A, relative to the carrier  12 , or an opposite direction depending upon the sense of rotation of the output shaft  33  of the motor  21 . 
     The structure T comprises a slider element V and a bearing Be and which is fixed relative to the print head assembly  11  and is slidable relative to the slider element V. Hence the print head assembly  11  can slide in the direction of arrow B and in an opposite direction, relative to the slider element V. 
     The mounting structure T is also clamped at its rear edge  40  to a third belt  41  as shown at  42  in FIG. 2, the third belt  41  being driven in synchronism with belts  23 , 24 , from shaft  31 , but being entrained only about the shafts  31  and  30 . 
     The print head assembly  11  also carries at its rear edge, a guide roller  44  which is rotatable about a generally vertical axis  45  transverse to the direction A of movement of the print head assembly  11  during printing. The roller  44  bears on a generally horizontal post  46  of generally circular cross section, the post being mounted via a lever arm  47  for rotation about a horizontal axis  48  generally parallel to but spaced from the post  46 , on a bearing  50  which is fixed relative to a body of the printing assembly  10 . 
     Hence as the print head assembly  11  moves from side to side, in the direction of arrow A or oppositely, the print head assembly  11  is guided for movement via the guide roller  44  and post  46 . 
     A strong spring  47   a  is provided between the post  46  and a frame part P of the apparatus  10  to bias the post  46  about axis  48  away from the print head. assembly  11 . The print head assembly  11  carries a hook formation H which engages with post  46  so that as the post  46  moves in the direction generally opposite to that of arrow B, the print head assembly  11  is moved with it, and slides relative to the mounting structure T. 
     The amount that the post  46  can be moved by the spring  47   a  is restricted by means of an air cylinder  50  which is positioned behind the post  46 . 
     In the figures, the print head assembly  11  is shown in a start position spaced away from a substrate  22 , but with the carrier  12  carrying the ink, entrained over an edge of the print head assembly  11  mounting the thermal printing elements. 
     To bring the print head assembly  11  towards the carrier  12  and substrate to effect printing, the print head assembly  11  is moved in a direction indicated by arrow B, i.e. laterally, which is transverse to the direction of movement of the print head assembly  11  during printing, as indicated by arrow A. 
     Movement of the post  46  and hence of the print head assembly  11  in direction B is achieved by means of the air cylinder  50  and its piston  51 , which, when actuated, rotates the guide post  46  about axis  48 , thus to urge the print head assembly  11  towards the substrate  22 , against the restoring force of the spring  472 . The piston  51  is arranged to retain the print head assembly  11  in its extended position against the restoring force of the springs  47   a , whilst the print head assembly  11  moves from the beginning, to end of printing positions in direction of arrow A, to effect printing on the substrate. 
     At the end of printing, when the print head assembly  11  is in its end of printing position, the piston  51  is deactuated and the print head assembly  11  is moved in an opposite direction to arrow B by the restoring force of the spring  47   a  away from the substrate and, by actuating the motor  21  in an opposite sense of rotation, the print head assembly  11  is moved back to the start position shown in the drawings in a direction opposite to the direction of arrow A. 
     The hub  20  of the take up spool carried by hub  20  is driven from the motor  21  via a drive belt  80  shown in dotted lines in FIG. 2, which is fixed to rotate with the drive roller R. Between drive roller R and the shaft L which is rotated by gear  30   b , there is a mechanical one-way clutch which permits the shaft L to rotate relative to the roller R as the stepper motor  21  rotates in one sense of rotation (clockwise in FIG. 2) during a printing operation. Thus the carrier  12  and take-up spool  20  remain stationary during a printing operation as the extended print head  11  moves downwardly as seen in FIG. 2. A one-way clutch suitable for this purpose is well known in itself and is a purely mechanical unit. 
     Of course, when the stepper motor  21  is rotated in an opposite sense of rotation, in the absence of any other means, the one-way clutch would cause the drive roller R to rotate clockwise as seen in FIG. 2, and thus drive the carrier  12  which is entrained about it, as well as the take up spool  20 , so that the carrier  12  advances as the print head assembly  11  is moved back to the start of print position indicated in the drawings. 
     To enable the apparatus  10  to operate in accordance with the present invention, there is provided a further clutch between the gear  30   b  and shaft L so that during the return movement of the printing head  11  to the start of print position, the shaft L and hence the drive roller R can be prevented from rotating with the gear  30   b . Such a clutch preferably comprises an electrically operated clutch which is under the control of the computer control of the apparatus. 
     Further features of the printing apparatus are as follows. 
     In this embodiment described, the spools  14  and spool carried by hub  20  as well as the drive roller R (but not its shaft L) and idler rollers  19 ,  18  and  17  are carried by a cassette  55  which can be removed from the body of the printing apparatus  10  to facilitate replenishing the printing apparatus  10  with carrier  12 . 
     The carrier guide path includes a peeler bar P′ behind which the carrier  12  passes immediately after passing over the print head assembly  11 , the bar P′ being operable to ensure proper separation of ink deposited on the substrate, and remaining carrier  12 . 
     The belt  41  is maintained under tension by means of a tensioning roller  59  and the belts  23 , 24 , can also be kept under constant tension by tensioning rollers  60 . 
     When the cassette  55  carrying the spools  14  and  20  is removed, a micro switch  61  which feeds power to the stepper motor  21  is tripped so that there is no risk of the mechanism of the printing apparatus  10  being actuated without the cassette  55  being in position. 
     In the event that the carrier feed spool  14  becomes empty, an electronic sensor carried by a clamp  62  past which the carrier  12  passes, will signal the lack of carrier  12  to an operator, and/or disable printing apparatus  10 . 
     The amount of movement of the print head assembly  11  in a direction opposite to that of arrow A i.e. the return movement, is restricted by means of a microswitch carried on a clamp means  63  which senses the print head assembly  11  when returned to its start position, immediately to stop motor  21 . 
     It will be appreciated that by virtue of the print head assembly  11  being mounted on the flexible belts  23 , 24 , and  41  via the mounting structure T, the assembly  11  is able to float to a smaller degree about the central axis of post  46 . The roller  44  mounted at the rear of the printing assembly  11  engages with the post  46  to restrict other movements. 
     Hence in the event that the substrate onto which print medium is to be transferred is not exactly at right angles to the array of printing elements mounted by the print head assembly  11 , the assembly  11  can move slightly about the central axis of post  46  as the print head assembly  11  is moved towards the substrate by the actuator  50  to accommodate such slight misalignment. 
     Hence, improved quality of print can be achieved throughout the entire printing operation. In the absence of some means to accommodate misalignment of the substrate, quality of print would tend to suffer over at least some of the area of the substrate onto which information is printed. 
     The printing apparatus described above may be operated by a method in accordance with the first invention as follows. 
     In the apparatus described, the print head assembly  11  may comprise at least six, but possibly eight, twelve, or more energisable printing elements per millimetre width of the print head assembly  11 , with all of the energisable print elements arranged in a single line array across the printing head assembly  11 . 
     Rather than utilizing all of the printing elements for printing, as the print head assembly  11  is traversed relative to the carrier  12  and substrate, a first set only of the printing elements may be utilized on a first printing operation. For example, when the height of the image to be printed is small (i.e. at least less than half of the width of the carrier  12 ) in the direction of the print head movement only a first set of adjacent printing elements are utilized whilst the print head assembly  11  is traversed over or otherwise moves over an area of the carrier  12  from its start to end of print positions to transfer pixels of ink from the carrier  12  onto the substrate  22 . 
     At that stage, rather than advancing the carrier  12 , the print head assembly  11  is moved as hereinbefore described relative to the carrier  12  back to the start of print position, but the electronically operated clutch between the gear  30   b  and its shaft L is operated so as to isolate the roller R so that the carrier  12  is not advanced. The substrate  22  may be advanced e.g. where on a web, or an entirely fresh substrate may be presented adjacent to the same area of the carrier  12  which was traversed by the print head assembly  11  immediately previously. 
     To print a second image having a height less than half the width of the carrier  12 , the print head assembly  11  is operated to traverse the same area of the carrier  12 , but a second set of adjacent printing elements, laterally disposed with respect to the first set, are utilised during printing to transfer pixels of ink from the carrier  12  onto the substrate. 
     Thus only some of the printing elements, a first set, are utilised the first time the print head assembly  11  traverses the area of the carrier  12 , and only some, a second laterally disposed set, different to the first set of printing elements are utilised the second time the print head assembly  11  traverses the same area of the carrier  12 . Hence two laterally disposed portions of the area of the carrier  12  are used in the two consecutive printing operations. 
     At this stage, if the full width of the area of carrier  12  has now been used, when the print head assembly  11  is returned to the start of print position, the clutch between the gear  30   b  and its shaft L is operated to cause the roller R and the take-up spool  20  to rotate so that the carrier  12  is advanced to provide a fresh area of carrier  12  for subsequent printing operations. 
     The take up spool  20  may have a slipping clutch which permits differential movement between the spool  20  and the drive roller R as the spool  20  becomes filled with used carrier  12 . 
     Thus the amount of carrier  12  utilised for printing will be reduced by half, in this example, assuming that the carrier  12  is advanced after the print head assembly  11  has relatively traversed the area of the carrier  12  for a second time. 
     Referring now to FIG. 4, when the height of the image to be printed is sufficiently small for more than two images to be printed one on top of the other using the same area of carrier  12  it might be possible for the print head assembly  11  to traverse or otherwise move over the same area of the carrier  12  more than twice. If this is the case, on each traverse of the same area of the carrier  12  a different set of laterally disposed printing elements will be utilized, thus using different laterally disposed portions of the area of the carrier  12 , with a consequent saving in carrier  12 . 
     FIG. 4 illustrates a length of carrier  12  comprising three areas R 1 , R 2  and R 3 , each of which has been used for printing five images, in five printing operations, thus utilising five laterally disposed ribbon portions P 1  to P 5  of each area R 1  to R 3  respectively. 
     However, it should be noted that in accordance with the invention repeated printing operations may only be performed where the width of the substrate  22  between sides S 1  and S 2  onto which the image is to be printed is sufficient. Thus before commencing printing the width of substrate  22  available for printing may be determined and those printing elements, if any, which would print outside the available width disabled. The printing operation process may then be repeated until the number of adjacent printing elements available for a further printing operation is too few to print an image of the required width, and then the carrier  12  may be moved on to provide a fresh carrier area. 
     Where the print head assembly  11  traverses the same area R 1  to R 3  of the carrier  12  more than twice, it will be appreciated that for each such traverse, fresh substrate  22 , being either a fresh area of substrate  22 , or an entirely different substrate  22 , would need to be presented adjacent to that area of the carrier  12 . 
     Various modifications may be made to the apparatus described with reference to the drawings, as follows. 
     For example, although the printing apparatus  10  described has been of the type which utilises a carrier  12  carrying ink which is deposited by means of thermal printing elements onto a substrate, the invention may be applied to any other printing apparatus having a plurality of selectively operable printing elements to effect printing, such as a dot matrix printer. The print head assembly  11  may incorporate an array being a single line of printing elements as described, or an array being a matrix i.e. multiple lines of such elements. 
     Although in the arrangement described, the print head assembly  11  is carried via the mounting structures T by three drive belts  23 , 24 , 41 , to move relative to a base B 1  of the apparatus, in another arrangement, less than three drive belts, or more than three drive belts, may be provided. 
     In place of drive belts, any other suitable endless loop members, such as chains, could be used to provide a transmission and mounting for the print head assembly  11 , or indeed any other suitable flexible or rigid drive member or members which is/are able to provide drive to, and a means of mounting the print head assembly  11 , could be used. 
     Although it is preferred for single stepper motor  21  to be used as a motive means for the printing apparatus  10 , with suitable logic control e.g. utilising a computer, if desired more than one stepper motor  21  or other motive means may be provided. For example a separate motor may be provided to drive the drive roller R and take up spool  20  for the carrier  12 . 
     Any alternative means to the piston and cylinder arrangement  50  for effecting movement of the print head assembly  11  towards the substrate, may be provided. 
     Although the invention has been described with reference to an apparatus in which the print head assembly  11  moves relative to the carrier  12  of print medium, and substrate during printing i.e. relative to a base B 1 , the invention may be applied to an apparatus of the type in which the print head is at a fixed position relative to a base B 1 , and the carrier  12  carrying print medium, and the substrate are together moved relative to the print head during printing. In such an embodiment, rather than a print head assembly  11  moving back to a start position of an area of the carrier in order relatively to traverse or otherwise move relative to the carrier a second time, the carrier may be arranged to be moved back relative to the print head assembly whilst fresh substrate is presented adjacent that area of the carrier, and the carrier and fresh substrate is traversed past the fixed print head assembly a second, and where appropriate, further, times. 
     Referring now to FIG. 5, a partial view of one embodiment of such an apparatus  100  shows carrier path and drive components. A carrier  112  carrying the ink is provided on a supply spool  114  carried on a hub  115 , the carrier  112  passing around a carrier guide path comprising guide roller  116 , print head roller  117  against which the print head  111  exerts a force during printing, guide roller  118 , carrier drive roller  119 , which is operable to drive the carrier  112  and is solely responsible for the amount of carrier  112  movement in either direction, as hereinafter explained. The carrier is then guided on to a take-up spool  120  carried on a hub  121 . Supply spool  114 , carrier drive roller  119 , and take-up spool  120  are driven from a single motive means  122 , which in this example is a two-way stepper motor, via a drive and timing belt  123 . Spool  114  is driven through a one-way clutch and slip clutch and spool  120  is driven through a one way clutch and slip clutch, the one way clutches operating in tandem such that the two clutches are operable so that when the stepper motor  122  is operated so as to move the timing belt  122  in a clockwise direction as seen in FIG. 4, the take up spool  120  is driven, whilst spool  114  is not driven. Thus carrier  112  may be paid out from the supply spool  114  and taken up onto spool  120 . Conversely, if stepper motor  122  is operated so as to move the timing belt  123  in an anti-clockwise direction as seen in FIG. 4, the supply spool  114  is driven so as to rotate anticlockwise and take-up carrier  112  onto it, whilst spool  120  is not driven and carrier  112  can be paid out from spool  120  for a purpose hereinafter described. 
     Additionally, slip clutches are provided for each of these spools  114  and  120  to accommodate differential movement between the spools  114  and  120  as increasingly, carrier is fed out from the supply spool  114  onto the take-up spool  120 . The slip clutches also provide slight resistance (drag) when the respective spools  114 , 120 , are paying out carrier  112 . 
     If desired, at least the one-way clutches may be electrically operated, although simple mechanical devices are adequate to perform this function. 
     A substrate  124  is supplied from a supply spool (not shown) and passes between the carrier  112  and print head roller  117 . Particularly if the substrate  124  consists of labels on a carrier, the path can continue around the print head drive roller  117 , around a nip roller  125  and a guide roller  126 . If the substrate is of another form such as polythene film, the path may continue in substantially the same direction, as indicated by chain line  127 . The substrate  124  is driven by a second motive means (not shown) so that the substrate  124  moves in synchronism with the carrier  112  past the print head assembly which is indicated by arrow  111 . 
     Movement of the substrate  124  may be continuous or intermittent as desired. 
     During printing, the stepper motor  122  drives the timing belt  123  in a clockwise direction, the one-way clutch and slip clutch of spool  114  offers only slip/drag resistance to clockwise rotation and spool  114  acts as a supply spool. At the same time, the one way clutch and slip clutch of spool  120  allow spool  120  to be driven with carrier drive roller  119  in a clockwise direction so that the carrier  112  is taken up on to spool  120 . By virtue of the slip clutch on the take-up spool  120 , the actual amount of carrier  112  which traverses the print head  111 , is governed entirely by the carrier drive roller  119  which is directly driven via belt  123  from the motor  122 , and preferably comprises a rubber coated roller which gives good stiction with the carrier  112 . 
     After completion of the first printing operation using an area of carrier  112 , the print head assembly  111  is pulled back a small distance, in the order of half to one millimetre, from the carrier  112  in the direction of arrow C, thus releasing the pressure exerted on roller  117  during printing. This is achieved as the print head assembly  111  is mounted on an arm  130  which is rotatable about axis  130 a of idler roller  16 . The arm  130  is spring biased by a spring wound about the central axis  130  of idler roller  116 , or otherwise, to urge the arm  130  away from the reaction roller  117 . 
     The arm  130  and hence the print head  111 , can be moved against the force of that spring by a pneumatically operated actuator which acts on the arm  130  in the direction of arrow D. Other suitable arrangements are no doubt possible. 
     The substrate  124  is then driven on so that an area of fresh substrate is provided adjacent to the print head  111 . At the same time, the stepper motor  122  drives the timing belt  123  in an anticlockwise direction, the one way and slip clutches of spool  120  offering only slip/drag resistance to carrier  112  being paid out from spool  120  so that spool  120  acts as a supply spool whilst the one-way and slip clutches of spool  114  causes the spool  114  to be driven so that spool  114  acts as a pick-up spool. However, the amount of carrier  112  driven is again governed by the carrier drive roller  119 . By this means, the same area of carrier  112  from which pixels of ink were removed during the previous printing operation can be aligned with the print head  111  and fresh substrate in preparation for a second printing operation. 
     This process may be repeated as often as required for an area of carrier  112 . When that area of carrier  112  has been fully used, the carrier  112  is not wound back as the substrate  114  is wound on, but a first printing operation is carried out using a fresh area of carrier  112 . 
     The operation of the two-way stepper motor  122  and the second stepper motor which drives the substrate  124 , must be accurately coordinated. This may be achieved by mechanical means but is most conveniently provided by means of computer control. Alternatively, the stepper motor  122  may be arranged to drive the substrate. 
     In each case, the print head assembly  111 , where the printing elements are energised thermally to transfer pixels of print medium i.e. ink from the carrier carrier  112  onto the substrate, control is preferably achieved by a computer, together with the relative movements of the print head and/or carrier and/or substrate as appropriate to cause either selective printing elements to be energised during each print operation, or for all or substantially all of the printing elements to be used during each printing operation but the printing elements are only energised in selected pixel row positions during each printing operation to enable the same area of carrier  112  or other carrier respectively to be used to print information, by a method as described in detail above with reference to the embodiment of FIGS. 1 to  3 . 
     The mechanism of FIG. 4, although ideal for performing a method of the first aspect of the invention, may be used in other apparatus where it is desired to move carrier in an appropriate direction to the direction the carrier and substrate move during printing.