EP0583622B1 - Thermal transfer printer - Google Patents

Description

The invention relates to a thermal transfer printing device for a postage stamp printer, with two printing stations at which a record carrier can be printed in a grid pattern during a transport movement in order to produce a print image composed of these prints from columns lying transversely to the transport direction.

A printing device of this type is e.g. B. from US Pat. No. 4,580,144. The print image of a postage stamp is composed of two partial prints, which can increase the printing speed for the overall stamp. A constant amount of information is printed on an envelope at one printing station and a variable amount of information predetermined by electronic data processing is printed on an envelope at the other printing station. If the correct print sequence and assignment are observed between these two printing processes a composite print image results from the constant and the variable print share. The increased printing speed is possible by dispensing with a special electronic control for printing the constant information and by using a printing device for this purpose which has a pictorial form corresponding to this information. The variable information, on the other hand, is fed electronically to a thermal transfer print head. The time required for printing can be reduced by a faster operation of the printing device for the constant information.

An increase in the printing speed with a predetermined division of the printed image into a constant and a variable part is, however, inappropriate because this limits the possible uses of a thermal transfer printing device. It would be desirable to have a printing device that works at high speed and thereby allows the greatest possible variation of all parts of the printed image by suitable control of the printing elements. However, the implementation of such a principle has so far been limited by the fact that the printing electrodes of the thermal transfer printing heads, which are arranged side by side in a plurality in a row, must be selectively heated in order to be able to print variable information. Since a minimum time is specified for heating and cooling the respective printing electrode, the printing speed is limited accordingly.

It is an object of the invention to provide a way of increasing the printing speed of a thermal transfer printing device, but a reduction in flexibility, for example caused by dividing the information to be printed into different groups, is to be avoided.

The invention solves this problem for a thermal transfer printing device of the type mentioned above in that two identical thermal transfer print heads, each of which generates the columns of the printed image at a mutual spacing of an integer multiple of the raster division, under control of a control device two by one raster division in the transport direction Print raster images offset against each other, each with a column spacing equal to twice the grid pitch.

The invention provides that the two print heads always print only the next but one column of the composite print image, the two raster images thus produced, each having half the resolution of the composite print image, being inserted into one another due to the mutual offset in the transport direction by one raster division. Since each print head supplies only half of all required columns for a print image, the print speed can be doubled. In addition, two identical print heads are provided, each of which allows variable control, so that correspondingly variable images composed of the print points of the two print heads can be printed. The control unit takes into account the Distance between the two printheads, the associated print data is ready and activates the printheads.

The print heads are preferably arranged in such a way that they face a common counter-pressure element at approximately the same inclination angles, which are opposite to the surface normal of the record carrier, and each have print elements located next to one another on the record carrier in the direction of the columns. The opposite inclination of the two printheads allows the smallest possible distance between their print bars, which minimizes image errors due to imprecise mutual alignment and speed fluctuations in the movement of the recording medium.

The control device preferably has a central control unit which selects coded print data stored in a memory for the two print heads, supplies them in addition to control signals and activates the print heads as a function of the respective position of the recording medium determined by a displacement sensor. Thus, print data stored in the memory can be changed easily and quickly, which enables a maximum of flexibility in the construction of the print image. By using a displacement transducer, a fixed association between print data and the respective print position is achieved, which is independent of fluctuations in the transport speed.

In one embodiment of an inventive The printing device is the mutual distance between the columns generated by the two printheads, an odd multiple of the raster division, both printheads being activated by two raster divisions after changes in position. In order to ensure a uniform energy consumption, it can be provided that the two printheads are not activated simultaneously, but in succession.

In a further embodiment, the mutual spacing of the columns generated by the two printheads is selected to be equal to the even multiple of the raster division, with one of the two printheads being activated alternately after changes in position by one raster division. A more even energy consumption is inherent.

If necessary, the print heads are offset in the direction of their print bars by half a print element distance. This results in an optimal resolution of the print image generated.

Fig. 1

die schematische Seitenansicht einer Thermo-Transfer-Druckeinrichtung als Ausführungsbeispiel,

Fig. 2

die Darstellung eines im Aufbau befindlichen Druckbildes, das mit einer erfindungsgemäßen Druckeinrichtung erzeugt wird,

Fig. 3

das Blockschaltbild einer Steuereinrichtung als Ausführungsbeispiel,

Fig. 4

die Darstellung eines im Aufbau befindlichen Druckbildes nach einem ersten Druckverfahren,

Fig. 5

den Ablaufplan für das Druckverfahren nach Fig. 4,

Fig. 6

die Darstellung eines im Aufbau befindlichen Druckbildes nach einem zweiten Druckverfahren und

Fig. 7

den Ablaufplan für das Druckverfahren nach Fig. 6.

The invention is explained below with reference to the drawing. In it show:

Fig. 1

the schematic side view of a thermal transfer printing device as an embodiment,

Fig. 2

the representation of a print image under construction, which is generated with a printing device according to the invention,

Fig. 3

the block diagram of a control device as an embodiment,

Fig. 4

the representation of a print image under construction according to a first printing process,

Fig. 5

4 the flowchart for the printing process according to FIG. 4,

Fig. 6

the representation of a print image under construction according to a second printing process and

Fig. 7

the flow chart for the printing process according to FIG. 6.

In Fig. 1, a thermal transfer printing device is shown schematically, which has a counter pressure roller 1 as a counter pressure element and two thermal transfer print heads 2 and 3 as essential components in the printing area. Between the counter-pressure roller 1, which can be rotated in the direction of the arrow, and the two print heads 2 and 3, there is a recording medium 4 which is transported in the direction of the arrow and over which a thermal transfer ink ribbon 5 is guided in the direction of the arrow in the printing area. The two print heads 2 and 3 have at their lower edge in Fig. 1 a print bar 2 'or 3', which rests on the ink ribbon 5 and, depending on the control of the individual electrodes present in the print bar, the selective transfer of the heat of dissolved paint from the Ribbon 5 on the recording medium 4 causes. These operations are for known the thermal transfer printing process.

The opposite inclination of the two printheads 2 and 3 to the surface normal of the recording medium 4 shown in FIG. 1 ensures that their print bars 2 'and 3' are arranged directly next to one another. The two print heads 2 and 3 have approximately identical inclination angles. It can be seen that due to the opposite inclination, a very small distance between the two pressure bars 2 'and 3' is possible, whereby two printing stations are realized which are very close to each other in a single printing area. As a result, image errors are kept to a minimum by imprecise mutual alignment of the pressure bars 2 'and 3' and by speed fluctuations in the paper transport.

With a suitable control, the two print heads 2 and 3 can generate a print image on the recording medium 4, which is composed of print dots which are generated successively or simultaneously by both print heads 2 and 3. If each of the two print heads generates only half the total number of dots required, the print image is generated with twice the value of the print speed possible for a print head 2 or 3.

FIG. 2 shows how such a print image is composed of the print dots of the two print heads. Here, part of an envelope 10 is shown in a plan view while it is in the direction of the arrow is moved past the printing area. In the course of this movement, the print head 2 and the print head 3 print pressure points on the envelope 10 by selectively actuating their printing electrodes, by means of which a postage stamp 11 is generated. This postage stamp 11 has already been completed with one half 12 at the time in FIG. 2. When the envelope 10 is transported further in the direction of the arrow, the print head 3 will fill the gaps between the print dots previously created with the print head 2 and complete the print image in this way, while the print head 2 is no longer activated after reaching the left end of the postage stamp. In this way, the print image is composed of the print dots generated with the two print heads 2 and 3. The time required for this is half as long as the time required to produce a print image of the same resolution with only one print head.

The two print heads 2 and 3 are operated by a control device 13 which, according to FIG. 3, has as its essential components a central control unit 14 to be started by a print request signal 13 and a data memory 15. For example, a correspondingly prepared microcontroller is provided as the central control unit 14, which selects the print information containing the image information in binary coded form and is stored in the data memory under program control and transmits it to the two print heads 2 and 3. The central control unit uses 14 address words to select the respective print data applied to the data memory 15 via address lines 16 and the associated print data read out via data lines 17. The print data are then forwarded unchanged, for example, to the two print heads 2 and 3 via data lines 16 'and 17', where they are buffered until the two print heads 2 and 3 are activated. In addition, the control unit can also be used, for example, to monitor and control the transport, which is preferably carried out step by step.

Each address stands for a specific column to be printed and the associated print data contain, in coded form, which of the individual electrodes are to be actuated in the respective print head 2 or 3. When the addresses are selected, a fixed sequence is specified, the distance between the two print heads 2 and 3 from one another being taken into account by a corresponding, mutual displacement of the addresses assigned to them by a constant value. Assuming a constant transport speed of the recording medium 4, the activation of the print heads 2 and 3 can then take place at certain time intervals predetermined by the desired grid division and the transport speed. Since it may be difficult to keep the transport speed constant, the current position of the recording medium 4 is determined instead by means of a displacement sensor 18 by measuring the distance already traveled by the recording medium 4 and the actual position thereof from the central control unit 14 determined with regard to printheads 2 and 3. When the position belonging to the respective address is reached, the corresponding printhead 2 or 3 fed from a supply unit 19 is then activated by the central control unit 14 via control lines 20 or 21. By taking the actual position of the recording medium 4 into account, it is achieved that fluctuations in the transport speed have no influence on the print image quality.

The formation of such a print image when using printheads at an interval of an odd multiple of the raster division is shown in FIG. 4. The distance between the two longitudinal axes of the print bars 2 'and 3' is three times a given raster division 22. The recording medium becomes from the left moved to the right past the pressure bars 2 'and 3' indicated by dash-dotted lines in the drawing. If the position a) for printing the first column is reached, a printing element of the printing bar 2 'is actuated in accordance with the printing date stored under the associated address, which consequently leaves a corresponding printing point on the recording medium 4. The print bar 3 'is not activated at this position, since no data word can yet be assigned.

After the recording medium 4 has been transported two raster divisions, the third column is printed by the print bar 2 '.

In this position b) three pressure points are set, for example. After a further rightward movement of the recording medium 4 by two raster divisions, the fifth column with, for example, five pressure points is generated by the pressure bar 2 'and the second column with, for example, two pressure points by the pressure bar 3' in position c). Subsequently, after each further transport by two raster divisions, the seventh and fourth, the ninth and sixth columns, etc. are printed simultaneously by the print bars 2 'and 3'.

The implementation of this printing process is explained in more detail in the flow chart according to FIG. 5. Accordingly, a print request is first waited for after the printing device has been started up. After this has been done, various measures for printing preparation such. B. the starting of the transport mechanism ', the setting of the start address to be assigned to the print head 2 and the calibration of the displacement transducer. The data word stored under the start address is then transferred to print head 2. The address to be assigned to the print head 3 is then derived from this address and the associated data word is transferred to the print head 3. When the correct printing position is reached, the printing process is initiated, ie both print heads 2 and 3 are activated. The next address is then determined, for example, by increasing a counter reading for the print head 2. If this is equal to a given end address, printing is ended and another print request is awaited. Otherwise, the new address put in the place of the start address and the process described above is repeated until the end address is reached.

FIG. 6 shows the formation of the print image according to FIG. 4 for the case that the print heads 2 and 3 are separated from one another by a double, that is to say an even-numbered multiple of the grid division. The record carrier 4 is in turn moved from left to right past the pressure bars 2 'and 3' indicated by dash-dotted lines in the drawing. The printing of the first column with a pressure point takes place at position a) by the print head 2, which also generates the third column with three pressure points after the recording medium 4 has moved by two raster divisions 22. The print head 3 has no print data available until the position c) which is further away by a raster graduation 22 is reached, so that it is only activated at position c) and prints the second column comprising two pressure points. With each further transport by a grid division, print bars 2 'or 3' are then alternately activated. This means that the fifth column is printed next by the print bar 2 '(position d)) and subsequently the fourth column by the print bar 3' and so on.

The associated flow chart is shown in FIG. 7. Here, once the printing device has been started up, a print request is again waited for and, after this has been done, the print is prepared, for example by starting the transport mechanism. the start address to be assigned to print head 2 is set and the displacement transducer is calibrated. The data word stored under the start address is then transmitted to the print head 2, the print head 2 is reached when the print position is reached, and the print head 2 is then activated. Now the address for the print head 3 is calculated from the address intended for the print head 2 by addition with a constant value corresponding to the print head spacing, the associated data word is transferred to the print head 3 and this is activated when its print position is reached. The next address of the print head 2 is then determined, for example, by increasing a counter reading. If this is equal to a given end address, printing is terminated and another print request is awaited. Otherwise, this new address is put in the place of the start address and the process described above is repeated until the end address is reached.

The last-mentioned printing process has the advantage over the former that, due to the principle, both print heads 2 and 3 are not activated at the same time, but alternately one after the other. In this way, load peaks are avoided and a more uniform energy consumption is achieved, which allows a smaller dimensioning of the supply unit feeding the two print heads 2 and 3. However, in order to smoothen the energy consumption even in the case of the first-mentioned printing method, it can be provided that the print heads 2 and 3 are activated one after the other at the common printing position by the control device delaying the control signal for the print head 3 for so long. until the printing process of the print head 2 is completed with certainty.

As has been shown, the printing speed in a thermal transfer printing device according to the invention can be doubled compared to devices with only one print head, with a maximum of flexibility being offered, since all parameters determining the print image are predetermined by the control device and therefore changes are simple and in any Way z. B. can be carried out by means of a data processing device. Since each printhead only prints the line after the next one, in addition to increasing the print speed, it is also possible to generate print images of any length in the direction of transport, with the appropriate data storage capacity. In the event of a print head failure, it is also ensured that a recognizable print image is nevertheless produced, since the print head which is still functioning continues to generate a half-resolution raster image. Thus, in the case of a postage stamp printer with a printing device according to the invention, it can still be determined after such a fault whether the postage fee has been paid.

Finally, the resolution of the printed images can be increased without apparatus changes by reducing the printing speed and reducing the column spacing, which can be easily achieved by changing the corresponding parameters in the control device. If an optimal resolution is sought, it can also be provided that the printing elements of the two printheads are displaced in the direction of their printing strips by half a printing element distance.

Claims (7)

1. Thermal transfer printer for a postal stamp printer, having two printing stations at which a recording medium can be printed in grid-like manner during a transport movement, in order to produce a printed image composed of these prints from columns lying at right angles to the transport direction, characterized in that two similar thermal transfer printing heads (2, 3), which generate the columns of the printed image in each case in the mutual spacing of an integral multiple of the grid division, under the control of a control device (14, 15) print two grid images, offset with respect to each other by a grid division in the transport direction, with in each case a column spacing equal to twice the grid division.
2. Thermal transfer printer according to claim 1, characterized in that the thermal transfer printing heads (2, 3) stand opposite a back pressure element (1) common to both with approximately corresponding angles of inclination directed opposite the surface normal of the recording medium (4) and have in each case on the recording medium (4) printing elements arranged next to each other in the direction of the columns.
3. Thermal transfer printer according to claim 1 or 2, characterized in that with the control device (14, 15) a central control unit (14) selects coded print data deposited in a data memory (15) for the printing heads (2, 3), supplies the data to them and activates the printing heads (2, 3) in dependence upon the respective position of the recording medium (4) determined by a position encoder (18).
4. Thermal transfer printer according to one of claims 1 to 3, characterized in that the mutual spacing of the columns each generated by the two printing heads (2, 3) is equal to an odd multiple of the grid division and in that after position changes of the recording medium (4) by two grid divisions both printing heads (2, 3) are activated.
5. Thermal transfer printer according to one of claims 1 to 3, characterized in that the mutual spacing of the columns each generated by the two printing heads (2, 3) is equal to an even multiple of the grid division and in that after position changes of the recording medium (4) by a grid division one of the two printing heads (2, 3) is activated alternately in each case.
6. Thermal transfer printer according to claim 4, characterized in that both printing heads (2, 3) are activated one after the other.
7. Thermal transfer printer according to one of claims 1 to 6, characterized in that the printing heads (2, 3) are offset with respect to each other in the direction of their pressure pads (2', 3') by half a print-element spacing.

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