Printer for printing on a continuous print medium

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 a stepper motor so as to repeatedly to bring a linear array of energizable 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, for example, 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, for example, due to the requirements of differing packaging processes, and also in response to transient variations in print medium speed during each printing operation.

FIELD OF THE INVENTION 
This invention relates to a thermal printer for printing on a continuous 
print medium by thermal ink transfer from a print ribbon. 
BACKGROUND OF THE INVENTION 
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. 
OBJECT OF THE INVENTION 
It is an object of this invention to provide a more reliable and faster 
printer for continuous printing. 
SUMMARY OF THE INVENTION 
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 energizable 
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 to 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 energization 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 energizable print 
elements, and a platen, the print head and the platen being located in an 
opposing relationship with respect to each other, feeding a print ribbon 
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 to the print 
medium, feeding the print ribbon between the print head and the platen, 
and periodically bringing the print head and the platen together and 
energizing the print elements with the ribbon in contact with the print 
medium so as 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 to 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 so as to 
execute the above-mentioned movement towards and away from the platen as a 
nodding or pivotal 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, for example, 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 energizable 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 to the print medium path, a ribbon take-off spool, a 
ribbon take-up spool which is belt driven by means of 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, that is, 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 plastic 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.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
Referring to FIGS. 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 of the 
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 of the 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 FIG. 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 10, such that ribbon spools 34, 36, which 
are rotatably mounted on a from 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 cassette unit 12. 
Also attached to the cassette unit front plate 38 is a ribbon drive roller 
40 visible in FIG. 1 below ribbon spool 34, and also in FIG. 3. 
The relative positioning of the ribbon spools 34, 36, the prim head 22, and 
the ribbon drive roller 40 may be ascertained by comparison of FIG. 1 with 
FIG. 3. The ribbon 42 itself is shown in full lines in FIG. 3, but in 
phantom lines in FIG. 1 for clarity. Certain items shown in FIG. 1 are not 
shown in FIG. 2, and vice versa. In particular, a platen roller 44 and a 
deflection roller 46 are shown in FIG. 2 but not in FIGS. 1 and 3. These 
components are mounted on the printer frame or other apparatus with which 
the printer is associated. 
Referring to FIG. 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 which it follows a downwardly 
inclined path so as to pass underneath and wrap around the 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.degree. by any one guiding element, and preferably not greater than 
45.degree.. 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 44. 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 roller 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 42 is provided from 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 so as 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 FIG. 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 38 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 roller 44. The ribbon 42 then passes over a further guide 
roller 68. The print head 22 and the platen roller 44 are seen in FIG. 2, 
as are also guide rollers 66 to 68, so that the location of the ribbon 
path relative to the print head 22 and platen roller 44 can be seen. Where 
the ribbon 42 passes over platen roller 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 of the print head 22 bears 
against the platen roller 44 through the ribbon 42 and film 50, as shown 
in FIG. 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 by means of a toothed belt 75 and toothed pulleys 75P mounted 
behind base unit front plate 16 2nd on the motor shaft 18S and the spindle 
32, (shown in FIGS. 1 and 2) so that the ribbon 42 is pulled through the 
space between the print head 22 and the platen roller 44. From the drive 
roller 40, the ribbon 42 passes to the take-up spool 34 which is 
belt-driven by a belt 76 from a pulley 78 (see FIG. 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 38 
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 78 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 (FIG. 2) extending 
along a line parallel to the axis of rotation 84 of the print head carrier 
20. These printing elements 82 project from a lower surface 86 of the 
print head 22 which, in the printing position of the print head 22, is 
tangential to the platen roller 44, as shown by the chain lines in FIG. 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 82 are heated under 
electronic control, and the film 50 and ribbon 42 are passed together over 
the elements 82, ink is transferred from the ribbon 42 to the film 50 so 
as 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 42 is driven at the correct speed. This 
synchronization 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 shalt encoder associated with the platen roller 44 is shown in FIG. 4 
by reference numeral 90. Encoder 90 provides an input signal 
representative of the film speed to an input 92 of a processor unit 94. 
The processor unit 94 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 22 between its inactive retracted 
position and its active extended position (respectively shown in FIG. 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 energizable 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 42, 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 50 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 18, and to 
initiate printing by energizing the elements of the print head 22 in 
accordance with the information stored in the memory 110 so as to thereby 
print the information as a pattern or a series of characters. 
Whilst the printing operation is progressing, the speed at which the ribbon 
42 is driven by means of 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 42 at the same speed as the film 50. The rate at 
which the print head elements 82 are driven, that is, the rate at which 
the pattern or characters are printed, is also varied by processor unit 94 
according to the film speed signal input 92. When the processor 94 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 
14 to withdraw the print head 22 to its retracted, inactive position. The 
processor 94 then waits for the next trigger signal at input 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 FIG. 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 FIG. 3) to sense breakage of 
the ribbon 42 which, during normal operation, keeps roller 58 
approximately in the position shown in FIG. 3. 
Further details of the preferred printer in accordance with the invention 
will now be described. Limits on the movement of print head carrier 20 and 
print head 22 are determined firstly by the striking of the print head 
elements 82 against the platen roller 44 (see FIG. 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 10. 
Drive to the ribbon drive roller 40, which, it will be seen, is mounted on 
the cassette front plate 38, is transferred from the base unit 10 to the 
roller 40 by means of drive spindle 32 shown in FIG. 1. Referring to FIG. 
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 FIG. 1) enters clutch 
bearing 40C (FIG. 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 by means of 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 the axis 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 FIG. 1. 
An alternative embodiment is shown in FIG. 6. In many respects this 
alternative printer corresponds to that shown in FIGS. 1, 2, and 3, and 
corresponding parts are correspondingly numbered in FIG. 6. FIG. 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 primer has no ribbon drive roller. 
Instead, synchronization of the ribbon travel and ribbon speed with film 
travel and film speed is achieved solely by frictional contact between the 
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 roller 
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 FIG. 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 so as to transfer rotational 
drive to the take-up spool 34. As in the embodiment of FIGS. 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 increases with use of the ribbon 42. By taking belt 
drive for the takeup 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 beating arrangement of FIG. 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 of the spool 34 and is spring-loaded so as to be biased in the 
counter-clockwise direction as seen in FIG. 6. 
Other differences which the second embodiment exhibits, compared with the 
embodiment of FIGS. 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 FIG. 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.