EP4023449B1 - Method for multicolour thermal direct printing and printer for multicolor thermal direct printing - Google Patents

Description

The present invention relates to a method for multi-colour, in particular two-colour, direct thermal printing and a printer for multi-colour, in particular two-colour, direct thermal printing.

Direct thermal printing papers that change color in response to different temperatures are known to those skilled in the art. For example, the direct thermal printing paper KLRB 46B from Kanzan is a paper that turns red at a temperature of 75° C to 80° C and black at a temperature of 95° C to 105° C.

The EP1866161B1 also shows a two-colour thermal direct printing paper. The EP1866161B1 further shows a printer for printing on such a thermal direct printing paper, which comprises two print heads, wherein the first print head applies a temperature to the thermal direct printing paper that triggers a discoloration in the first color and the second print head applies a temperature to the thermal direct printing paper that triggers a discoloration in the second color. From the EP3175993B1 is a thermal direct printing paper on which three-color printing is possible.

Direct thermal printers with a print head whose thermocouples can generate two different temperatures are familiar to the expert from the DE60207488T2 and the DE60036515T2 This allows two-colour printing to be produced on thermal direct printing paper. These print heads and their control are expensive and complex. WO-A-2020/013820 discloses a direct thermal printer for printing on multi-colored direct thermal printing paper. The paper comprises three different heat-sensitive layers, which each form different colors when heat is applied. At a first temperature, the corresponding heat-sensitive layer turns yellow. At a second temperature lower than the first temperature, the corresponding heat-sensitive layer turns magenta. At a third temperature lower than the second temperature, the corresponding heat-sensitive layer turns cyan.

The object of the invention is to provide a more cost-effective thermal direct printer for multi-coloured thermal direct printing and a method for operating a thermal direct printer for multi-colour direct thermal printing.

• Mit einer Transportwalze wird das Thermodirektdruck-Papier zeilenweise entlang einer Druckrichtung bewegt. Dabei passiert das Thermodirektdruck-Papier den Druckkopf. In einer Ausführungsform wird das Thermodirektdruck-Papier fortlaufend, das heißt ohne Halt, bewegt. In einer Ausführungsform wird das Thermodirektdruck-Papier schrittweise bewegt. In einer Ausführungsform ist die Bewegungsgeschwindigkeit, das heißt die Transportgeschwindigkeit des Thermodirektdruck-Papiers zwischen 50 mm/s und 400 mm/s, insbesondere zwischen 100 mm/s und 200 mm/s.
• In jeder Zeile des Thermodirektdruck-Papiers werden ausgewählte Wärmequellen des Druckkopfes elektrisch aufgeheizt. Der Fachmann versteht, dass die Wärmequellen, die aufgeheizt werden, von dem gewünschten Druckbild abhängen. Es ist natürlich auch möglich, dass Zeilen vorhanden sind, in denen keine Wärmequellen aufgeheizt werden, weil in diesen Zeilen keine gedruckten Pixel vorhanden sind. An den Stellen, an denen das Thermodirektdruck-Papier die zweite Farbe ausbilden soll, werden mehrere aneinandergrenzende Wärmequellen auf eine Drucktemperatur erhitzt. An den Stellen, an denen das Thermodirektdruck-Papier die erste Farbe ausbilden soll, werden von mehreren aneinandergrenzenden Wärmequellen ein Anteil auf die Drucktemperatur erhitzt und der andere Anteil der Wärmequellen wird nicht erhitzt.

This object is achieved by a method according to claim 1. According to the invention, a method for operating a direct thermal printer is proposed. The direct thermal printer is designed for printing on direct thermal printing paper. The direct thermal printing paper comprises at least one thermo-reactive layer. At least two colors can be formed with the at least one thermo-reactive layer. When a first temperature is applied, that is, when a point on the direct thermal printing paper is heated to a first temperature, a first color is formed. When a second temperature is applied, a second color is formed. The second temperature is higher than the first temperature. The direct thermal printing paper is moved by the direct thermal printer along a paper path from a paper holder to a paper output. In one embodiment, the paper holder is a space for a direct thermal printing paper roll from which direct thermal printing paper can be unwound as a continuous strip. The direct thermal printer also comprises a print head which comprises electrically controllable heat sources arranged next to one another transversely to the paper path. In one embodiment, the heat sources are electrical resistors that heat up when current flows through them. The heat sources supply heat to the direct thermal printing paper at specific points. The method comprises the following steps:

• The thermal direct printing paper is moved line by line along a printing direction using a transport roller. The thermal direct printing paper passes the print head. In one embodiment, the thermal direct printing paper is moved continuously, i.e. without stopping. In one embodiment, the thermal direct printing paper is moved step by step. In one embodiment, the movement speed, i.e. the transport speed of the thermal direct printing paper, is between 50 mm/s and 400 mm/s, in particular between 100 mm/s and 200 mm/s.
• In each line of the direct thermal printing paper, selected heat sources of the print head are heated electrically. Those skilled in the art will understand that the heat sources that are heated depend on the desired print image. It is of course also possible that there are lines in which no heat sources are heated because there are no printed pixels in these lines. At the points where the direct thermal printing paper is to form the second color, several adjacent heat sources are heated to a printing temperature. At the points where the direct thermal printing paper is to form the first color, one portion of several adjacent heat sources is heated to the printing temperature and the other portion of the heat sources is not heated.

The two-colour thermal direct printing paper, in particular the aforementioned paper from Kanzan, turns red when heat is applied in a first temperature range of 75° C to 80° C and black at a temperature of 95° C to 105° C. Since the paper slides along under the print head and the heat sources are covered with a protective layer or glass, for example, and the paper, as linerless paper, has a protective layer, in particular a silicone layer, over the thermoreactive layer, the heat sources during printing may have to have slightly higher temperatures than the above-mentioned discolouration temperatures. The thermal direct printing paper is designed so that two discrete temperatures are supplied for two-colour printing. However, thermal direct print heads whose heat sources can generate two different temperatures are technically complex and sometimes unreliable. The method according to the invention has the advantage that only one discrete temperature, the printing temperature, has to be generated at the heat sources. If several adjacent pixels are heated to the printing temperature, the printing temperature is transferred and the thermal direct printing paper under these pixels heats up to the higher temperature range in which the second color is formed. If some of a group of adjacent heat sources, i.e. a group of pixels next to each other, are heated to the printing temperature and others are not, an average temperature is formed that is transferred to the thermal direct printing paper, which falls into the lower temperature range of the thermal direct printing paper and forms the first color. It can be observed that the active heat sources, i.e. those heated to the printing temperature, form pixels in the second color on the thermal direct printing paper. In the vicinity of these pixels in the second color, spots in the first color form because the temperature on the thermal direct printing paper drops slightly in these areas and corresponds to the lower temperature range. The process takes advantage of the effect that a point heat source has heat radiation in the immediate vicinity. By combining active and inactive heat sources, a pixel pattern is created on the thermal direct printing paper that has a few pixels in the second color and pixels in the first color around them. However, due to the small pixel size, the human eye does not perceive the individual pixels. Rather, depending on the ratio between active and inactive heat sources and their geometric arrangement, the area appears like a surface in the first color.

In the sense of the invention, the thermal direct printing paper comprises a thermo-reactive layer that can form both the first and the second color. In the sense of the invention, a thermal direct printing paper is also a paper that comprises a first thermo-reactive layer that can form a first color and a second thermo-reactive layer arranged above or below that can form a second color. Furthermore, a thermal direct printing paper is also included that has, for example, a flat coating of one color and a non-transparent cover layer on top of that, wherein the cover layer becomes transparent and the colored coating becomes visible. In this way, both the first color and the second color can be formed. Furthermore, thermal direct printing papers are also included that can form more than two colors. The person skilled in the art understands that the unprinted thermal direct printing paper is usually white or has a whitish color. This is not to be understood as the first or second color in the sense of the invention.

In one embodiment, the second color is black. In one embodiment, the first color is red. Those skilled in the art will understand that the first color and the second color refer to a reaction of the thermoreactive layer. The thermoreactive layer changes color at the pixel to which heat is applied. By combining various pixels of the first color with the second color and with the light, in particular white, color of the unprinted paper, color effects can be achieved, such as a light red area or a dark red area. However, these consist of individual red, white and black pixels and are not to be regarded as separate colors within the meaning of this disclosure.

In one embodiment, the printing temperature is equal to or higher than the second temperature. The heat source must transfer heat to the paper at the higher temperature range. This is the second temperature. Due to the effects that may occur due to the protective layer on the print head and the protective layer on the direct thermal paper, the printing temperature may need to be slightly higher than the second temperature.

In one embodiment, the step of moving the thermal direct printing paper line by line is carried out during a line time from a line n to a next line n+1. In one embodiment, the movement of the thermal direct printing paper is continuous, i.e. the paper is moved past the print head at a constant or almost constant speed. In one embodiment, the Movement of the direct thermal printing paper step by step, that is, the direct thermal printing paper is moved to the next line n+1 and stopped until the line time has elapsed and then moved to line n+2.

In one embodiment, when printing an area, the area on the direct thermal printing paper comprises several rows and several columns. From the perspective of the print head, the rows correspond to a time at which this row is located under the heat sources of the print head. From the perspective of the print head, the columns of the area correspond to certain heat sources of the print head. If an area on the direct thermal printing paper is printed in the second color, all heat sources corresponding to the columns of the rows are heated to the printing temperature over the times corresponding to the rows of the area. Thus, all heat sources corresponding to the projection onto the area on the direct thermal printing paper are heated to the printing temperature and the area changes color to the second color. In other words, to print an area on the direct thermal printing paper in the second color, where the area comprises several rows (n, n+1, n+2) and several columns, during the printing of the several rows (n, n+1, n+2), all heat sources corresponding to the several columns are heated to the printing temperature.

If, however, the surface is printed in the first color, a portion of the heat sources mentioned in the previous section must be heated to the printing temperature (active heat sources) and another portion is not heated (inactive heat sources), i.e. the other portion of the heat sources is not electrically controlled. In other words, to print an area on the thermal direct printing paper in the first color, where the area comprises several rows (n, n+1, n+2) and several columns, a portion of the heat sources corresponding to the several columns are heated to the printing temperature during the printing of the several rows (n, n+1, n+2), so that the projection of the heat sources onto the surface corresponds to a regular grid of active and inactive heat sources, with the same number or more inactive heat sources than active ones. As already explained above, a pure print in the first color, i.e. a surface that only has pixels in the first color, is not possible with the method according to the invention. However, the human eye perceives the surface in the first color if the pixels of the first color and the pixels of the second color are arranged in a regular grid and the pixels of the first color predominate. Therefore, the same number or more inactive heat sources must be present in the grid on the projection of the surface than active heat sources.

In one embodiment, for printing an area on the thermal direct printing paper in a color that corresponds to an overlay of at least two of the first color, the second color and the paper color, the area comprising several rows (n, n+1, n+2) and several columns, the heat sources corresponding to the several columns are heated to the printing temperature during the printing of the several rows (n, n+1, n+2) in such a way that the projection of the heat sources onto the area corresponds to a regular grid of active and inactive heat sources. The paper color is white, whereby the white can have a slight reddish tinge. If the grid is arranged in such a way that the distance between two heat sources that are heated to the printing temperature exceeds a minimum distance in the row direction and column direction, a white pixel appears on the area. It is thus possible to design a grid that represents an overlay of pixels in the first color, in the second color and of white pixels. In one embodiment, the second color is black. Shades of the first color, especially red, can be printed in different brightness levels. A screen that includes more active heat sources will result in a darker shade of the first color being printed on the direct thermal paper than a screen with fewer active heat sources. Are for example, the first color is red and the second color is black, then by selecting the appropriate grid with white and black pixels, shades of red in different brightnesses can be produced, from a bright red with a large white component to a dark red that turns into black. The fact that the printing of red pixels using the inventive method always results in a black pixel in the middle, which is surrounded by red pixels, means that there can be no pure red area and that an area perceived by the human eye as a red area consists of a plurality of red pixels and a smaller number of black pixels and in particular also a smaller number of white pixels.

In one embodiment, the regular grid is a grid that repeats row by row and/or column by column or is arranged diagonally or alternately. Different effects can be achieved with different regular grids and, in particular, different brightnesses of the first color can be printed. A repeating grid over several pixels in the row direction and in the column direction results in a small area with a certain brightness of the first color. Various geometric shapes can be put together from many small areas, resulting in many shapes and a large degree of freedom for prints in the first and second colors.

In one embodiment, the thermal direct printing paper is linerless paper. In one embodiment, the thermal direct printing paper comprises a silicone layer on the top. Linerless paper is a paper made from a continuous strip that does not comprise a carrier tape. The back paper layer has an adhesive material, in particular an adhesive coating, in order to be able to stick labels separated from the continuous strip onto an object. To ensure that the label web does not stick to itself when rolled up into a continuous roll, the top of the paper is provided with a silicone layer from which the adhesive material can be detached. This silicone layer on the thermal direct printing paper has the advantage in the method according to the invention that the heat from the heat source is distributed somewhat in the silicone layer and not only penetrates the paper at certain points under the heat source, but also in its surroundings. In the area surrounding the heat source, heat still penetrates the paper, but at a slightly lower temperature. The effect that a heat source that is heated to the printing temperature produces a black pixel and red pixels appear around it is particularly pronounced in linerless paper. In addition to the properties of the silicone material, this effect is also caused by the fact that the thickness of the silicone layer ensures a distance between the heat sources and the thermoreactive layer. This distance also leads to a certain spread of heat under the heat source. The inventive method makes particular use of this effect, which is undesirable in itself and negative in normal printing processes. The inventive method is therefore particularly advantageous in linerless paper.

In one embodiment, an active heat source is electrically controlled by the control device during a printing interval. The printing interval is divided into a saturation interval and a subsequent cooling interval. The saturation interval begins with a waiting time. The heat source is only supplied with power during the time after the waiting time has elapsed until the end of the saturation interval. The waiting time in line n depends on the status of the heat source in at least one previous line n-1. In one embodiment, the printing interval corresponds to the line time. In one embodiment, the saturation interval is the same for all heat sources of the print head. In one embodiment, the waiting time can be selected differently for each heat source and depends on the previous state of this heat source. For example, if a heat source in a previous line n-1 is set to the printing temperature, it will still have a certain amount of residual heat in the following line. If this heat source is to be heated up to the printing temperature again in this line, somewhat less power is required than with a cold heat source due to the residual heat. For this reason, no power is supplied during a waiting period. The power supply therefore does not cover the entire saturation interval. This type of control of heat sources is known to experts as history control. In one embodiment, the print head has a temperature sensor on its surface. The saturation interval, which is the same for all heat sources, is determined depending on the temperature of the temperature sensor. For example, a longer saturation interval is selected in a very cold environment than in a warm environment.

According to the invention, a thermal direct printer is proposed for printing on thermal direct printing paper. The thermal direct printing paper comprises at least one thermo-reactive layer. At least two colors can be formed with the at least one thermo-reactive layer. When a first temperature is applied, a first color is formed and when a second temperature is applied, a second color is formed. The second temperature is higher than the first temperature. The thermal direct printer comprises a transport roller that moves the thermal direct printing paper along a paper path from a paper intake to a paper output. The thermal direct printer comprises a print head that comprises electrically controllable heat sources arranged next to one another transversely to the paper path. The print head supplies heat to the thermal direct printing paper at specific points. The thermal direct printer comprises a control device that controls rotation of the transport roller and uses the transport roller to move the thermal direct printing paper line by line along the print head in a printing direction. Each heat source of the print head is electrically connected to the control device and can be heated to a printing temperature.

The control device is designed to heat selected heat sources of the print head to the printing temperature in each line of the direct thermal printing paper. At the locations where the second color is to be formed on the direct thermal printing paper, several adjacent heat sources are heated to the printing temperature. At the locations where the first color is to be formed on the direct thermal printing paper, a portion of several adjacent heat sources is heated to the printing temperature and the other portion of the heat sources is not heated.

In one embodiment, the print bar is a print bar based on thick film technology. Examples of print bars based on thick film technology are the KD2004-DC91B from Rohm or the KPW-104-BZR from Kyocera. In one embodiment, the print bar is a print bar based on thin film technology.

In one embodiment, the printing temperature is equal to or higher than the second temperature.

In one embodiment, the transport roller is a pressure roller that presses the direct thermal printing paper against the print head.

In one embodiment, the direct thermal printer is a linerless printer and the direct thermal printing paper is a continuous paper made of linerless paper with a silicone layer on its top.

In one embodiment, the control device controls the transport roller so that the transport roller moves the direct thermal printing paper from a line n to a next line n+1 during a line time, wherein the movement occurs stepwise or continuously during the line time.

Fig. 1

schematische Darstellung eines Thermodirekt-Druckers,

Fig. 2

schematische Darstellung eines Druckkopfes für einen Thermodirekt-Drucker,

Fig. 3

schematische Darstellung eines Temperaturverlaufs an einer Wärmequelle während eines Druckintervalls und Darstellung des durch die Steuervorrichtung angelegten Stromes an der Wärmequelle,

Fig. 4

schematische Darstellung eines Thermodirektdruck-Papiers für zweifarbigen Druck in einer ersten Ausführungsform,

Fig. 5

schematische Darstellung eines Thermodirektdruck-Papiers für zweifarbigen Druck in einer zweiten Ausführungsform, und

Fig. 6-15

verschiedene Raster zum zweifarbigen Thermodirektdruck und zugehörige Druckergebnisse.

Some embodiments of the invention are shown by way of example in the drawings and described below. They show, in each case in a schematic representation:

Fig. 1

schematic representation of a direct thermal printer,

Fig. 2

schematic representation of a print head for a direct thermal printer,

Fig. 3

schematic representation of a temperature curve at a heat source during a pressure interval and representation of the current applied to the heat source by the control device,

Fig. 4

schematic representation of a thermal direct printing paper for two-color printing in a first embodiment,

Fig. 5

schematic representation of a thermal direct printing paper for two-color printing in a second embodiment, and

Fig. 6-15

different screens for two-color direct thermal printing and corresponding print results.

Fig. 1 shows a thermal direct printer 30 in a schematic representation. The printer comprises a paper path that leads from a paper holder 46, in which a paper roll 32 is stored, via a printing roller 40 and a print head 42 to a paper output 44. The paper is wound onto a paper roll 32 in the form of a continuous strip of thermal direct printing paper 34. The thermal direct printing paper 34 comprises a bottom side, on which an adhesive layer is applied, and a top side 36, on which a silicone layer is applied. The printing roller 40 serves as a transport roller for the continuous strip of thermal direct printing paper 34. The printing roller presses the thermal direct printing paper 34 with its upper side 36 against the print head 42. The thermal direct printer 30 comprises a control device 48 which is electrically connected to the print head 42 and the printing roller 40. The control device 48 controls the printing roller 40, which moves the thermal direct printing paper 34 line by line past the print head 42. The control device 48 also controls the print head 42, in particular the control device 48 controls the heat sources 52 of the print head 42, so that the desired print is produced in line n, which is located directly under the print head 42 due to the transport of the thermal direct printing paper 34. The coordinated control of the printing roller 40 and the printing head 42 by the control device 48 results in line-by-line printing on the direct thermal printing paper 34. The pixels in the column direction correspond to the individual heat sources 52, the pixels in the row direction n, n+1, n+2 correspond to the respective point in time t0, tz, 2tz at which the corresponding row n, n+1, n+2 is located under the printing head 42. The control device 48 is designed to heat each individual heat source 52 to a printing temperature or not to heat it.

Fig. 2 shows a print head 42 in a view from below. On this side, the thermal direct paper 34 is moved along a printing direction 60 by the printing roller 40 past the print head 42 line by line. The print head 42 comprises a printing field 56 with a plurality of heat sources 52 arranged next to one another. The heat sources emit heat at specific points. There is a distance 54 between the heat sources 52. The distance 54 between the heat sources 52 is shown in Fig. 2 relatively wide. In practice, the distance 54 between two adjacent heat sources 52 is very narrow. It should be Fig. 2 It should only be indicated that each heat source 52 is mounted independently of the adjacent heat sources 52 and can be controlled separately by the control device. The width of the printing field 56 corresponds at least to the maximum printable Paper width. The printing field 56 is covered with a preferably good heat-conducting cover in order to protect the heat sources 52 from mechanical damage. The heat sources 52 are in particular heating resistors. The print head 42 comprises a temperature sensor 58 which measures the temperature 58 on the top of the print head and passes it on as a parameter to the control device 48.

Typical values for the thermal direct paper width are 60 mm, 80 mm or 120 mm. A typical resolution of the applicant's linerless thermal direct printing paper is 200 dpi, in particular 300 dpi. A typical printing speed, i.e. a typical transport speed at which the paper is moved under the print head, is 100 mm/s to 400 mm/s, in particular 120 mm/s, 150 mm/s or 250 mm/s.

To achieve a resolution of 200 dpi, approximately 8 dots (pixels) per mm are required, i.e. 8 heat sources 52 per mm. To achieve a resolution of 300 dpi, approximately 12 dots (pixels) per mm are required, i.e. 12 heat sources 52 per mm. With a paper width of 80 mm, the print head contains at least 960 heat sources 52.

At a resolution of 300 dpi, 12 dots/mm, i.e. 12 lines/mm in the transport direction are necessary. At a typical transport speed, i.e. printing speed, of 150 mm/s, the printing roller must make 1800 lines/s, i.e. 1800 steps per second.

Fig. 3 shows a schematic diagram for the control of a heat source 52 by the control device 48 (lower part) and the associated heat development at the heat source 52 (upper part). A printing interval ti is shown. During a printing interval ti, a line n of the thermal direct printing paper is printed. The printing interval ti is typically at least 300 µs long. In any case, the printing interval ti must not be longer than a line time tz and is in particular the same length as a line time tz. At the beginning of the printing interval, the thermal direct printing paper 34 is the printing roller 40 has been moved from line n-1 to line n. In the case of a step-by-step transport of the direct thermal printing paper 34, the direct thermal printing paper 34 is on line n at the beginning of the printing interval. The printing interval consists of a saturation interval ts, during which the heat source 52 is supplied with power by the control device 48. Furthermore, the printing interval ti consists of a cooling interval ta following the saturation interval ts. If the printing interval ti is shorter than the line time tz, the direct thermal printing paper 34 is transported to the next line n+1 following the printing interval ti and until the end of the line time tz. If the printing interval ti is the same length as the line time tz, the direct thermal printing paper 34 is transported to the next line n+1 in the late part of the cooling interval ta and until the end of the line time tz/the printing interval ti.

Depending on whether the heat source was energized in the previous line n-1 or in the previous lines n-1, n-2, n-3 or not, the saturation interval begins with a waiting time tw, tw ', tw". When the waiting time tw, tw ', tw" has elapsed, a current is applied to the heat source 52 by the control device 48. The heat source 52 begins to heat up to the pressure temperature TD and maintains the pressure temperature TD until the end of the saturation interval ts. The saturation interval ts is the same length for all heat sources 52 of the print head, with the waiting time tw, tw', tw" being calculated separately for each heat source 52 of the print head based on the previous lines n-1, n-2, n-3 (history control). In the cooling interval ta, no current is applied to the heat source 52 by the control device 48 and the temperature at the heat source 52 falls to its initial value during this time. Based on the print head temperature determined by the temperature sensor 58 on the print head, the control device 48 determines the length of the saturation interval ts.

For two-colour thermal direct printing according to the method according to the invention, a heat source is always the printing temperature is heated. The method does not include a first temperature for printing the first color and a second temperature for printing the second colors. In one row, the active heat sources 52 are heated to the printing temperature TD in each case. The other heat sources 52 are inactive heat sources 52 and are not heated.

Fig. 4 shows a schematic representation of a thermal direct printing paper 34 for two-color printing in a first embodiment. A cross-section of the thermal direct printing paper 34 is shown. The thermal direct printing paper comprises a paper layer 12. An adhesive layer 14, i.e. an adhesive layer, is applied to the underside of the paper layer 12. A first thermoreactive layer 22 is applied above the paper layer 12, which forms a first color, in particular red, when heat is applied. The first thermoreactive layer 22 turns red at a temperature of 70° C to 85° C, in particular 75° C to 80° C. A second thermoreactive layer 20 is applied to the first thermoreactive layer 22, which forms a second color, in particular black, when heat is applied. The second thermoreactive layer 20 turns black at a temperature of 90° C to 110° C, in particular 95° C to 105° C. A silicone layer 16 is applied above the second thermoreactive layer 20, which prevents the adhesive layer 14 from adhering and enables the adhesive layer 14 to be released when an endless strip of the direct thermal printing paper 34 is rolled up into a roll.

Both the first thermoreactive layer 22 and the second thermoreactive layer 20 are transparent when no heat has been applied to them.

If a first temperature, in particular 75° C to 80° C, is applied to the thermal direct printing paper 34, the first thermo-reactive layer 22 turns red and the second thermo-reactive layer 20 remains transparent. This creates a red print. If a second temperature is applied to the thermal direct printing paper 34, the second temperature being higher than the first temperature, in particular 95° C to 105° C, the first thermoreactive layer 22 turns red and the second thermoreactive layer 20 turns black. The second thermoreactive layer 20 covers the first thermoreactive layer 22 so that only the black color in the second thermoreactive layer 22 is visible. A black print is created.

Fig. 5 shows a schematic representation of a thermal direct printing paper 34 for two-color printing in a second embodiment. In comparison to Fig. 4 There is no separate first thermoreactive layer and second thermoreactive layer. A thermoreactive layer 18 is applied to the paper layer 12, which turns red when a first temperature is applied and turns black when a second temperature is applied.

Fig. 6 to 15 show various grids for controlling the heat sources of the print head 42 (right graphic in landscape format) and the resulting print image on the thermal direct printing paper 34 (left graphic in landscape format). The drawings of the print head 42 show 14 heat sources 52 lying next to each other, which are controlled by the control device 48 independently of each other, regularly and depending on the line time. A 1 means an active heat source, i.e. the heat source 52 has a current and temperature profile as shown in Fig. 3 is shown schematically. A 0 indicates an inactive heat source 52, which means that no heating power is provided by this heat source. The printed image on the thermal direct printing paper 34 is shown schematically. A black area in the drawings indicates a black pixel. A hatched area in the drawings indicates a red pixel. A white area indicates an unprinted, i.e. white, area on the thermal direct printing paper 34.

Fig. 6 to 12 show 14 consecutive lines n, n+1, ..., n+13, which are printed at the times t=0, tz, 2*tz, ..., 13*tz, where the times indicate the start times of the printing interval ti. Regular grids are shown in each case, which are taken from a larger area. This is therefore a zoom view of an area that appears to be the same color. As previously shown, a resolution of 300 dpi requires printing with approximately 12 dots/mm. This means that the Fig. 6 to 12 The fields shown have a size of approximately 1.16 mm * 1.16 mm (-1.36 mm ² ).

Fig. 6 shows a grid with a high proportion of black dots. 75% of the heat sources are active. This means that only 25% of the heat sources are inactive. A black color forms at the active heat sources, which is overlaid with the red color that forms at the inactive heat sources. The area shown has a dark red tone due to the overlay of the black and red colors. The human eye does not recognize the individual pixels, but only an area that forms this red tone. Fig. 7 to 12 show areas that form different shades of red with different proportions of black pixels, red pixels and white pixels. Individual heat sources in a row are activated by the control device. A black pixel is formed under these heat sources, red pixels are formed in its surroundings and no discoloration of the white thermal direct printing paper occurs at a greater distance. Different color effects can be achieved with different grids. The human eye sees red tones of different brightness.

Fig. 13 to 15 do not show square print areas, but are extended by one or two lines to complete the display. No regular grids are selected that extend beyond the edges of the The drawings shown can be mentally extended. Rather, they depict complete forms.

Fig. 13 shows a square black dot of 4x4 pixels (0.3 mm * 0.3 mm at 300 dpi). At a high printing speed, the thermal direct printing paper is moved quickly under the print head by the printing roller. The Fig. 3 The cooling interval ta shown is not large enough for the heat sources to have cooled down sufficiently at the time (6*tz, 7*tz - lines n+6, n+7). This is why the red color runs a little behind and after the black pixels have been printed in the transport direction, not only the pixels of the next line but also the pixels of the line after that are colored red. The same effect occurs in Fig. 14 on.

Fig. 14 shows a section of a barcode consisting of two bars, printed in black. The black bars are different lengths. This is therefore the bottom left edge of a barcode. The bars at the edge of the barcode are slightly longer than in the middle, as the number corresponding to the barcode is printed in the middle of the barcode. Due to the reaction of the thermal direct printing paper, it is advantageous to position the bars of a barcode lengthwise in the direction of transport. The red fade effect on the printed paper is therefore at the end of the bars and not across the barcode. This makes the barcode easier to scan.

Fig. 15 shows a schematic of the printing of a number in black. The number has very few pixels and would in reality be printed with significantly more black pixels. The red component is then negligible in reality, so that the black printed number is only surrounded by a very thin red border that is barely visible to the human eye.

The drawings serve solely as a schematic representation to illustrate the invention. In order to produce individual geometric structures in a real size for printing To depict a character, significantly more pixels must be used, but these cannot be shown here for reasons of clarity. If in reality more pixels are used to print a character, then the red portion for a black character is significantly lower than, for example, in Fig. 14 and 15 displayed and therefore not disturbing.

The functions of various elements shown in the drawings, including the function blocks, may be implemented by dedicated hardware or by generic hardware capable of executing software in conjunction with the corresponding software. If the functions are provided by means of a processor, they may be provided by a single dedicated processor, a single shared processor, or multiple generic processors which may in turn be shared. The functions may be provided by, without limitation, a digital signal processor (DSP), network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) with stored software, random access memory (RAM), and non-volatile memory.

Claims (11)

1. Method for operating a direct thermal printer for printing on direct thermal printing paper, wherein the direct thermal printing paper comprises at least one thermoreactive layer, wherein at least two colours are formable with the at least one thermoreactive layer, wherein a first colour is formed when a first temperature is supplied and a second colour is formed when a second temperature is supplied, wherein the second temperature is higher than the first temperature, wherein the direct thermal printing paper is movable through the direct thermal printer along a paper path from a paper receptacle to a paper outlet, and the direct thermal printer comprises a printhead comprising electrically controllable heat sources which are arranged next to one another transversely with respect to the paper path and at points supply heat to the direct thermal printing paper, and wherein the direct thermal printer comprises a control device which is designed to heat selected heat sources of the printhead to a printing temperature, wherein the method comprises the following steps:

   - moving, by means of a transport roller, the direct thermal printing paper line by line past the printhead in a printing direction,

   - in each line of the direct thermal printing paper, electrically heating selected heat sources of the printhead, wherein, at the locations where the second colour is intended to be formed on the direct thermal printing paper, multiple adjacent heat sources are heated to the printing temperature, and at the locations where the first colour is intended to be formed on the direct thermal printing paper, one proportion of multiple adjacent heat sources are heated to the printing temperature and the other proportion of the heat sources are not heated.
2. Method for operating a direct thermal printer according to Claim 1, wherein the printing temperature is equal to or higher than the second temperature.
3. Method for operating a direct thermal printer according to Claim 1 or 2, wherein, in the step of moving the direct thermal printing paper line by line, the direct thermal printing paper is moved during a line time from one line to the next line and wherein the movement during the line time is carried out in steps or continuously.
4. Method for operating a direct thermal printer according to one of Claims 1 to 3, wherein, in order to print an area on the direct thermal printing paper in the second colour, the area comprising multiple lines and multiple columns, all heat sources corresponding to the multiple columns are heated to the printing temperature during the printing of the multiple lines.
5. Method for operating a direct thermal printer according to one of Claims 1 to 4, wherein, in order to print an area on the direct thermal printing paper in the first colour, the area comprising multiple lines and multiple columns, a proportion of the heat sources corresponding to the multiple columns are heated to the printing temperature during the printing of the multiple lines, such that the projection of the heat sources on the area corresponds to a regular grid of active and inactive heat sources, the number of inactive heat sources being equal to or greater than the number of active heat sources.
6. Method for operating a direct thermal printer according to one of Claims 1 to 5, wherein, in order to print an area on the direct thermal printing paper in a colour which corresponds to a superposition of at least two of the first colour, the second colour and the paper colour, the area comprising multiple lines and multiple columns, the heat sources corresponding to the multiple columns are heated to the printing temperature during the printing of the multiple lines in such a way that the projection of the heat sources on the area corresponds to a regular grid of active and inactive heat sources.
7. Method for operating a direct thermal printer according to Claim 6, wherein the regular grid is a grid which repeats in lines and/or in columns or is arranged obliquely.
8. Method for operating a direct thermal printer according to Claim 6 or 7, wherein the second colour is black and shades of the first colour are printed in different brightness levels, and a grid containing more active heat sources results in the printing of a darker shade of the first colour on the direct thermal printing paper than a grid with fewer active heat sources.
9. Method for operating a direct thermal printer according to one of Claims 1 to 8, wherein the first colour is red and the second colour is black.
10. Method for operating a direct thermal printer according to one of Claims 1 to 9, wherein the direct thermal printing paper is a linerless paper which has a silicone layer on its top side.
11. Method for operating a direct thermal printer according to one of Claims 3 to 10, wherein an active heat source is electrically controlled by the control device during a printing interval, wherein the printing interval is divided into a saturation interval and a subsequent cooling interval, wherein the saturation interval begins with a waiting time, wherein the heat source is supplied with power only in the time after expiry of the waiting time until the end of the saturation interval, and wherein the waiting time in a line is dependent on the status of the heat source in at least one preceding line, wherein the printing interval in particular corresponds to the line time or is shorter than the line time.

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