WO1991001886A1

WO1991001886A1 - Heat transfer printer

Info

Publication number: WO1991001886A1
Application number: PCT/EP1990/001291
Authority: WO
Inventors: Hubert Mugrauer
Original assignee: Siemens Nixdorf Informationssysteme Aktiengesellschaft
Priority date: 1989-08-07
Filing date: 1990-08-07
Publication date: 1991-02-21
Also published as: EP0485487A1

Abstract

A printer has a printing mechanism in which at least two linear printing combs (19) are arranged symmetrically to each other with their longitudinal axis perpendicular to the direction of transport of a record carrier (7) to be printed. A continuously rotating ink carrier (10) is pressed against the printing combs (19). A feed element (24) for the record carrier (7) and for the ink carrier is associated with the printing combs. The elements of each pair composed of a printing comb and a feed element can be displaced relative to each other. During printing, the printing combs are selectively activated for line by line printing.

Description

Thermal transfer printing device

The invention relates to a thermal transfer printing device according to the preamble of claim 1.

In modern printers, which are preferably used as peripheral devices of data processing systems, electrophotographic printers, among others, have also become established. However, the electrophotographic printing principle is relatively complicated and therefore also expensive, but today it delivers printing results of good to high quality, which is dependent on the selected image resolution.

Another known, albeit less successful, non-mechanical printing principle is thermal transfer printing. According to this principle, a corresponding amount of energy is to be applied locally, ie in microdots close to or on the surface of a recording medium, which generates an image point on the recording medium. For this purpose, it was also proposed to use special papers as the recording medium, which change color when radiation energy strikes them. This variant of thermal transfer printing has such considerable technological and qualitative disadvantages that it has not become established.

If one wishes to implement the principle of thermal transfer printing without special papers, dye particles according to the desired pattern, triggered by local supply of energy, must be transferred from a dye carrier to the recording medium in individual pixels. The dye carrier is therefore decoated locally during the printing process. In most printing processes, however, the colored area portion, based on the total area of the recording medium, is usually only a few percent. A single-use dye Fabric backing is therefore too expensive, even for printers with low output, which only print a few A4 sheets per minute.

There has been no shortage of attempts to solve this problem. For example, EP-Al-02 53 300 discloses a thermal transfer printing device with a regenerable dye carrier which is designed as an endless belt and is coated with a thermoplastic printing ink with a low melting point. After passing through a printing station in the thermal transfer printing device, the dye carrier is passed through a heated inking unit in order to completely regenerate the dye layer there. For this purpose, the printing ink remaining on the dye carrier after the printing process is melted in the inking unit and collected in a heated tub with liquefied printing ink. Printing ink is again fed from this trough to the surface of the dye carrier via a system of application rollers and doctor blades, and the dye carrier is thus continuously recoated before it passes through the printing station again.

A thermoplastic printing ink, known in printing technology as cold set or hot carbon ink, is suitable as the printing ink. These printing inks consist of waxes or wax-like products, in which color pigments and carbon blacks are finely dispersed in the liquid state during production. Printing inks of this type are conventionally used for copy form sets. Printing inks of this type are also so interesting for thermal transfer printing, because on the one hand they are fast drying inks. This is one of the prerequisites that make it appear suitable for a regenerable dye carrier. On the other hand, these printing inks a relatively low melting point, preferred wise to below 100 ^* C, ie it is sufficient a relatively overall rings amount of energy pixel to these ink locally in a Mikro¬ to plasticize and pressurized to in the printing station to to transfer the recording medium adjacent to the dye medium. This property makes it possible to design a print head for such a thermal transfer printing device as an optical character generator, as is also known from EP-Al-02 53 300. Switch cells which are known from the optoelectronics as zip code elements are used in this known print head. These switching cells change their transmission properties by applying an external electric field and can be switched practically without a memory effect.

In principle, it is now possible to remove the obstacles that prevent the introduction of the long-known thermal transfer printing principle in practice. Nevertheless, it must be made clear that a printing device designed according to this principle must be competitive with other printers already on the market. This applies above all with regard to the printing performance and printing quality in relation to the production and printing costs. Electrophotographic printers are today in a wide range of services from, for example, a few sheets of A4 per minute to more than 10 times this value

Market launched. In doing so, efforts are also made to constantly increase the print quality, which can only be achieved by an ever finer resolution of the print image. Ten halftone dots per mm and more are no longer unusual for standard electrophotographic printers.

Since the print image is also composed of micro-image dots arranged in a raster in thermal transfer printing, the print head for a thermal transfer printing device must also be able to produce such a fine image raster »But this means that the total energy consumption of the print head for a therm - mo-transfer printing device is no longer uncritical, even if the amount of energy required for local plastification of the printing ink in a micro pixel appears at first to be relatively small. The energy converted and, above all, as heat loss increases even more if a high pressure output is simultaneously sought. The invention is therefore based on the object of providing, in a further development of known solutions, a thermal transfer printing device of the type mentioned at the outset which, as a graphics-capable page printer, can even be used in a medium performance range of up to about 50 sheets per minute and at the same time is competitive with the electrophotographic printers prevailing in this product area.

According to the invention this object is achieved in a thermal transfer printing device according to the preamble of claim 1 with the features described in the characterizing part of this claim.

Functionally, this means that in the solution according to the invention, for example with two pressure combs, the

Printing capacity of the printing device is in principle evenly distributed over these two printing combs. This could be organized, for example, in such a way that one of the printing combs in alternation with the other is activated for printing an entire page. During its inactive phase, the pressure comb can cool down and is therefore less sensitive to temperature, since longer recovery phases are systematically prescribed to dissipate the heat loss.

However, the printing process can be organized in such a way that, in the example of two printing combs, the successive printing lines are alternatively generated by one of the printing combs. In other words, one printing comb generates the even-numbered printing lines and the other printing comb generates the odd-numbered printing lines. Of course, this function requires a corresponding spatial arrangement of the printing combs with respect to the transport direction of the recording medium to be printed and an actuation of the printing combs adapted to them based on the supplied printing information. This adjustment could be done in the printer control. For example, the print information could be supplied to each print comb individually with the proviso that the line information for the in Direction of transport of the recording medium at the rear of the printing comb is supplied with a delay corresponding to the spatial distance between the two printing combs.

In principle, this last-mentioned possibility means that a conventional printing comb, which is technologically designed for a printing performance of n sheets / min, could achieve a printing performance of 2n sheets / min according to the solution according to the invention according to the assumed example that therefore a new technology would have to be developed. In addition to the two alternatives outlined, a plurality of intermediate forms or other variants are conceivable.

In conjunction with a regenerable dye carrier, in which the printing ink is used practically without loss, these solutions also make it possible to advance into performance ranges in thermal transfer printing devices for which thermal transfer printing previously seemed entirely unsuitable. Low printing costs are advantageous here, since the printing ink is used optimally during the printing process and the transfer printing - relatively considered - takes place with a low energy consumption. Since this relatively low energy requirement solves a problem of inadequate wear resistance of the printheads, which previously opposed the introduction of the thermal transfer printing principle in practice - this also applies in particular when using optical character generators - a high level of reliability is achieved liquidity, combined with minimal maintenance. The manufacturing costs are low compared to the electrophotographic printers considered to be competing products, although a comparable print quality can be achieved.

A thermal transfer printing device according to the invention is also very environmentally friendly. The environmentally friendly hot carbon ink used as a printing ink in the solution according to the invention consists of waxes in which dye particles are finely dispersed. Because of the regenerability of the dye fully used. In comparison to electrophotographic printers, therefore, neither dust-like dyes are used nor photoconductors that have to be disposed of when they are replaced. A thermal transfer printing device constructed according to the solution according to the invention also operates quietly and can be constructed very compactly. It is therefore suitable for normal office operations. Further advantages result from the developments of the invention described in the subclaims and from the detailed description below.

An embodiment of the invention is explained below with reference to the drawing n ago. It shows

1 shows a side view of a thermal transfer printing device with an input or output compartment arranged on each side of a printing unit and a paper turning device placed over the printing unit, in a schematic representation;

Figure 2 is a side view of the printing unit of the thermal transfer printing device according to Figure 1 u. a. with two symmetrically arranged pressure combs and

3 shows a schematic representation of one of the optical switching cells from which the printing combs are constructed.

A printing unit 1 is arranged centrally in the thermal transfer printing device shown schematically in FIG. 1. Viewed in the processing direction running from left to right, there is an input compartment 2 in front of the printing unit 1 for the recording media to be printed, for example single sheets of A4 format. A device control 3 is arranged below this input compartment 2. On the other side of the printing unit 1 there is an output compartment 4 for the printed recording medium, which is designed as an automatic stacking device. A turning device 5 is arranged above the input compartment 2 and the printing unit 1. In the Wen- The device 5 is provided with pivotable guide fingers 6 which, depending on the setting, effect the transport of a recording medium 7 running out of the printing unit 1 directly into the output compartment 4 or for printing on both sides into the turning device 5. A recording medium 7 inserted into the turning device 5 runs into a swivel plate 8 arranged above the input compartment 2, which is at this time in the upper end position shown with broken lines. After the recording medium 7 has entered, the swivel plate 8 is swiveled downward. The direction of rotation of a pair of transport rollers 9 is reversed and conveys the record carrier 7, with its back side still unprinted, back into the printing unit 1.

In order to give an idea of the dimensions of the thermal transfer printing device shown in this overview sketch of FIG. 1, essential device dimensions are indicated in FIG. The widths for the input and output compartments 2 and 4 are predetermined by the dimensions of the record carriers 7 to be printed. If the thermal transfer printing device is designed in such a way that the recording media 7 are transported and printed in the transverse position, then a width BL for the input and output compartments 2 and 4 of approximately 25 cm with a paper format DIN A4 is constructive possible. The printing unit width is still smaller, so that the total width b2 is of the order of about 72 cm. The height h1 of the thermal transfer printing device, which is due to the construction of the printing unit to be described, is approximately 22 cm without the optional turning device 5. Together with the turning device 5, the total height h2 of the thermal transfer printing device is approximately 32 cm. The device depth, which cannot be seen in the side view of FIG. 1, is in turn due to the format of the record carrier 7 and is therefore approximately 40 cm in the case of a DIN A4 format.

In Figure 2 is now an embodiment for the printing unit 1 of the thermal transfer printing device explained in principle shown. The recording media 7 run through the printing unit in a transport direction indicated by an arrow 9. Underneath the transport path of the recording medium 7, a dye carrier 10 designed as an endless belt is arranged over rollers. The dye carrier 10 loops around two deflection rollers 11 arranged below the paper web of the recording medium 7 on the input or output side of the printing unit 1. These deflection rollers 11 are assigned pressure rollers 12 located above the paper web of the recording medium 7. These press the record carriers 7 in the area of the printing unit 1 against the dye carrier 10, so that the record carriers 7 are carried along in this area by the circulating dye carrier 10.

In addition, the dye carrier 10 is guided over a pressure roller 13, which is assigned to an inking unit 14. The inking unit 14 serves to regenerate the dye carrier 10 and has an inking roller 15, which is partially immersed in a lowerable ink pan 16. The pressure roller 13 and the ink roller 15 are preferably heated in the same way as the ink tray 16 and are arranged with respect to one another in such a way that they exert a predetermined pressure force P 1 on the dye carrier 10 running between them.

As a result of these measures, the outer side of the dye layer of the dye carrier 10 rotating in the clockwise direction is heated when it hits the pressure roller 13. Thus, the printing ink, a hot-carboxylic color, squeezes plasticized and off in Einlaufbe¬ range between the platen roller 13 and the inking roller 15 with a relatively low melting point below 1Ö0 ^* C. The counterclockwise ink roller 15 wets when immersed in the liquefied printing ink tray 16 with the printing ink and leads this to the dye layer side of the dye carrier 10. The dye feed is metered through a doctor blade arrangement 17, the knife 18 of which is guided over the circumferential surface of the inking roller 15. In the process, superfluous printing ink is dripped off and drips back into the ink tray 16. The design and the arrangement tion of the pressure roller 13 and the inking roller 15 are designed with respect to the dye carrier 10 so that the dye carrier 10 preferably formed from a polyimide tape is completely and evenly covered with a new dye layer when leaving the inking unit 14. The initially plastic dye layer cools down quickly. It is already frozen before the dye carrier 10 comes into contact with the recording medium 7, running over the deflection roller 11 on the input side.

In the area of the centrally located actual printing station of the printing unit 1, two printing combs 19 are arranged mirror-inverted to one another within the rotating dye carrier 10. Both pressure combs 19 are pivotally mounted on an axis 20 oriented perpendicular to the transport direction 9 of the recording medium 7, as is indicated schematically by double arrows 21. The undersides of the pressure combs 19 are designed as heat sinks, as shown in FIG. 2 by cooling fins 22 projecting downwards. These cooling fins are in the air flow of a schematically indicated cooling fan 23.

A pressure roller 24 located above the transport path of the recording media 7 is assigned to the end region of the printing combs 19 remote from the bearing. Each pairing of a printing comb 19 with the associated pressure roller 24 is designed such that the contact surface of the printing comb exerts a predetermined contact force on the dye carrier 10 and the recording medium 7 adjacent to it.

The printing combs could be constructed as conventional thermal printing combs or else as optical character generators. In the latter embodiment, both printing combs 19 have a plurality of switch cells, which are not shown in FIG. 2 and which are arranged transversely to the transport direction 9 of the recording media 7 and by means of which a line of micropixels can be generated on the recording media 7. In FIG. 3, a possible embodiment for such a switching cell of the pressure combs 19 is shown schematically on an enlarged scale. A radiation beam 25 is indicated, the wavelength of which may lie, for example, in a range from 300 to 1200 μm. This radiation generated by a radiation source (not shown) is converted into parallel radiation using known optical means, the scattering angle of which should be less than 1 '. In the beam path of this parallel beam 25 there is an optical switching element 26. B. a so-called PLZT element known from optoelectronics can be used. PLZT elements are transparent, ferroelectric ceramic plates made of lead zirconate titanate (Pb, Ld) (Zr, Ti) 0 ₃ doped with lanthanum, which are coated on both sides with transparent electrode surfaces. These electrodes have connections via which a pulse-shaped voltage V can be applied as a control voltage. PLZT elements have the property that their transmission properties change with regard to a rotation of the polarization plane as well as the scattering of the transmitted radiation when such an external electric field is applied. In the present case, it is not the rotating effect, which is often interesting in optoelectronic applications, but the scattering effect that is used. A continuous, transmitted radiation beam is indicated in solid lines. This occurs as long as the control voltage V is zero. If, on the other hand, there is an external electric field, predetermined by a control voltage not equal to zero, the transmitted radiation beam now shown in broken lines diverges with a total opening angle of up to approximately 6 ^* .

In the beam path behind the optical switching element 26, an optical imaging element 27 is arranged at a predetermined distance, which has spherical surfaces and is spherical, for example, as in FIG. 3. This optical imaging element 27 is manufactured, for example, by injection molding from an organic glass. An arrow P2 indicates that the optical imaging element is provided with a the contact pressure of approximately 0.2 N presses the dye carrier 10 with its ink layer side 100 against the recording medium 7. The assigned pressure roller 24, which forms the counter bearing for the contact pressure P2, is shown partially in section.

Considered functions, the switching cell shown in Figure 3 has two operating states. If the effective control voltage V at the switching element 26 is zero, then the incident parallel beam 25 is transmitted without geometric transformation. The optical imaging element 27 focuses this parallel bundle of rays in a focal point in the plane of the dye layer 100 of the dye carrier 10. The radiation energy thus concentrated in a micro picture point locally plastifies the dye layer 100. The melted dye particles become under the influence of the contact pressure P2 transferred to the recording medium 7.

In the second operating state, the control voltage V is present at the switching element 26 with an amplitude which changes its crystal lattice and so causes the transmitted beam to diverge, as indicated by broken lines in FIG. 3. This scattered radiation only partially captures the optical imaging element 27 and focuses it in a degenerate focal spot of considerably larger diameter. Thus, the energy density effective in this focal spot is reduced to such an extent that the dye layer 100 of the dye carrier 10 is no longer plasticized, so that transfer printing onto the recording medium 7 does not take place.

In the print combs 19, a large number of such switch cells are now arranged in integrated form in their contact area with the dye carrier 10 together with a corresponding electronic control such that a print line can be generated transversely to the transport direction 9 of the record carriers 7 . On the basis described with reference to FIG. 3 and with his knowledge of the structure of optical time Chen generators, for example for modern electrophotographic printers, it is possible for a person skilled in the art to construct a geometrical arrangement of switch cells for a print cell structure in such a way that each of the print combs 19 is suitable for simultaneously producing a line of micro picture elements in one print cycle. It can also be imagined, inter alia, that the switching cells of the printing combs 19 are combined in a modular manner in several groups, which are then arranged next to one another in the direction of the printing lines.

This structure also per se includes a control circuit for the individual optical switching elements 26 in order to supply them individually with the control voltages V which are derived from pressure information. This control for the pressure combs 19 is only of subordinate importance in the present case. The control of optical character generators based on supplied print information is also, at least according to its principle, well known from the field of application in electrophotographic printers. For the sake of completeness, it should therefore only be hinted at here that the print information which is processed in the device control 3 consists of one information bit for each micro pixel, in accordance with the raster origin of the print image to be created. This print information is processed in the control circuit for the print combs 19 as a control information such that, in synchronism with the transport of the recording medium 7, the radiation energy corresponding to the picture information strikes the line in all micro picture elements of a print line.

In the present embodiment _^ is assumed that the two Drucftkämme 19 constructed and arranged side by side ange¬ and are driven such that they produce alternatively successive print lines. In other words, this means that one of the two printing combs 19 generates the even-numbered, the other the odd-numbered printing lines. Because of the spatial distance between the two printing combs, a synchronization problem must be solved for the printing process. Because too When a sheet begins to be printed, the recording medium 7 first runs past the front of the printing combs 19, as seen in the transport direction 9, which, for example, produces the odd-numbered printing lines in the form of micro-image lines. This front print comb already prints out several print lines in succession until the recording medium 7 has been transported to such an extent that the field for the first even-numbered print line finally faces the rear print comb. The second printing comb therefore works with a delay, the delay depending on the distance of the printing combs in the transport direction 9.

If the device controller 3 outputs the print information bit-by-row print line by print line, then a delay circuit must be assigned to the control of the rear print comb in order to compensate for the geometric spacing of the print combs. On the other hand, the preparation of the print information in the device control 3 could also be designed such that the line information for the rear print comb is already transmitted with a delay.

This configuration of the printing unit 1 is advantageous since a printing dot raster in the order of 10 micro-pixels per mm is necessary for the required print quality. Since the printing comb 19 is intended to sweep over a longitudinal edge of the recording medium 7, it is readily apparent that several 1000 optical switching cells are integrated in each printing comb if the printing comb is designed as an optical character generator. Even if the amount of radiation transmitted in the individual switching cell is relatively small per se, because of the high integration density, even with optical character generators there is a not inconsiderable heat loss which has to be dissipated. This applies in particular when the thermal transfer printing device described is to be used as a printer for the medium performance range, ie is to be able to print approximately 20 to 50 A4 sheets per minute. In this performance range, due to the systematic conditionally, the printing cycles for the printing of one microprinting line are defined in a range from 0.5 to a maximum of 1.4 ms, as can be easily calculated. The remaining recovery time for dissipating the heat of loss occurring at the pressure comb 19 is so short that, despite the low energy expenditure in the thermal transfer printing device described, additional measures are necessary to ensure that the print quality is high at the pressure comb ensure the low operating temperature required.

However, if, as described, a hot carbon ink with a low melting point is used as the printing ink, the locally required amount of energy for plasticizing the printing ink is reduced in a micro pixel. Then, firstly, both conventional thermal print heads and optical character generators can be used to design the print combs. Secondly, by using more than one pressure comb, the heat loss occurring per pressure comb can be kept within such limits by a longer cooling phase that the thermal load neither negatively influences the print quality nor reduces the overall operating time for such pressure combs. The exemplary embodiment described provides for the use of two pressure combs. However, it would be obvious to the person skilled in the art that the number of pressure combs could be increased even further, if with increasing design effort, if this appears necessary. For example, an arrangement with four pressure combs would be created if the mounting of the pressure combs is fork-shaped, two pressure combs each being mounted on one axis and the bearing arms of one of the pressure combs encompassing the other, internal pressure comb. This design principle, which is mentioned by way of example, is at the expense of the device depth that is then required, but in itself can easily be increased in the number of printing combs.

Furthermore, in the exemplary embodiment described, the printing process is organized in such a way that the two printing combs, at position swiveled in at the same time, alternatively generating even-numbered or odd-numbered print lines, ie both print combs are essentially simultaneously active during printing. Compared to a printing device of the same output and with only one printing comb, the period duration for a printing cycle is doubled, ie the recovery phase within a printing cycle increases accordingly. However, one could also imagine activating only one printing comb for a longer period of time at half the printing cycle times and cooling the second printing comb in this position in the pivoted-out position; for example, the printing combs could be selectively alternated in cycles for printing one side of the page Activate record carrier. Shorter activation periods would also be conceivable as further variants. However, it must be taken into account that the printing combs must then also be designed such that shorter printing cycle times can be fully mastered. In the embodiment described, however, the printing output could also be doubled by simply doubling the printing combs - without new technological development.

In connection with the measures for the continuous regeneration of the dye carrier, the solution described thus offers the possibility of creating a simple, efficient and low-maintenance thermal transfer printing device at low manufacturing costs for a performance range for which thermal transfer printing has hitherto been the case seemed inappropriate.

Claims

Claims;

1. A thermal transfer printing device, in which printing ink melted selectively by local energy supply in a printing station is transferred from a rotating, continuously regenerated dye carrier (10) to a recording medium (7) in a printing station, characterized in that: that at least two cell-shaped pressure combs (19), arranged parallel to one another and with their longitudinal axis transverse to the transport direction (9) of the recording medium (7) or the dye carrier (10), are arranged, and that these pressure combs are assigned as well as A pressure element (24) facing the record carrier is provided, the respective pairings of print comb and associated pressure element being able to be individually activated for a line-by-line, transverse to the transport direction (9) of the record carrier (7).

2. Thermal transfer printing device according to claim 1, since - characterized in that the self-contained dye carrier (10) with external Farbstoff¬ layer (100) on two, on both sides of the printing combs (19) arranged deflection rollers (11) and a pressure roller (13) is guided, which is arranged on an inking unit (14) for regenerating its dye layer and that is assigned to the deflecting rollers, two further pressure rollers (12) are provided which hold the recording medium (7) against the dye layer side (100) of the dye carrier (10).

3. Thermal transfer printing device according to one of claims 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the elements of each pair consisting of a printing comb (19) and one of the first pressure rollers (24) are each adjustable to each other.

4. Thermal transfer printing device according to claim 3, since - characterized in that the pressure combs (19) each about an axis (20) transverse to the transport direction (9) the record carriers (7) to be printed are pivotally arranged.

5. Thermal transfer printing device according to claim 4, d a - d u r c h g e k e n n z e i c h n e t that the pivot axes (20) of the printing combs (19) are each one of the deflection rollers (11) for the dye carrier (10) arranged adjacent.

6. Thermo-transfer printing device according to one of claims 1 to 5, d a d u r c h g e k e n n z e i c h n e t that the pressure combs (19) have a heat sink.

7. thermal transfer printing device according to claim 6, d a - d u r c h g e k e n n z e i c h n e t that the pressure combs (19) on the side facing away from the dye carrier (10) have a plurality of cooling fins (22).

8. Thermo-transfer printing device according to claim 6 or 7, so that a cooling fan (23) is arranged in the effective range of the pressure combs (19).

9. Thermal transfer printing device according to one of claims 1 to 8, characterized in that the printing combs (19) are each formed as an optical character generator having a plurality of optical switching cells (26, 27) controlled by pressure information, selectively focus a parallel light bundle (25) on the dye carrier (10) in each pixel, so that the printing ink can be plasticized in these printing pixels by the transmitted and focused radiation energy Pressure on the record carrier (7) is transferable.

10. Thermal transfer printing device according to claim 9, d a - d u r c h g e k e n n z e i c h n e t that a hot carbon ink is used as a printing ink to reduce the locally required amount of radiation.