BACKGROUND

A printing device like an ink-jet printer may comprise a heating system for adjusting a temperature of a print medium in the printing device, for example for drying or curing a printing substance such as ink that has been deposited on the print medium. The heating system may comprise a heating element that is to heat the print medium, e.g. by thermal conduction, convection or radiation.

BRIEF DESCRIPTION OF DRAWINGS

[0001] In the following, a detailed description of various examples is given with reference to the figures. The figures show schematic illustrations of

[0002] Fig. 1 : a heating system for a printing device according to an example in side view;

[0003] Fig. 2: a printing device in accordance with an example in side view; [0004] Fig. 3a: a printing device with a heating system and a media advance system according to an example in side view;

[0005] Fig. 3b: the printing device of Fig. 3a in a perspective view in accordance with an example;

[0006] Fig. 4: a method of operating a printing device in accordance an example;

[0007] Fig. 5a: temperature curves of portions of a print medium heated by one and two heating elements, respectively, according to an example; and

[0008] Fig. 5b: temperature curves of portions of a print medium heated by two heating elements using different parameter values in accordance with an example.

DETAILED DESCRIPTION

[0009] A variety of processes in a printing device may involve adjusting a temperature of a print medium, e.g. post-treatment processes for the print medium after depositing a printing substance thereon. For example, a dye-sublimation ink may be deposited on a print medium such as a textile, which may subsequently by heated to above a sublimation temperature of the ink in order to sublimate the ink and allow for an absorption and fixation of the dye in fibers of the textile. In other examples, a print medium may be warmed up prior to depositing a printing substance thereon or may be heated to dry and cure a printing fluid that has been deposited on the print medium. Such treatment processes may have a certain minimum process temperature and duration or exposure time that is to be reached in order to perform the respective treatment. At the same time, a high temperature may damage the print medium or cause image quality defects and may furthermore decrease the energy efficiency of the printing device. [0010] Fig. 1 depicts a schematic illustration of a heating system 100 for a printing device (not shown) in accordance with an example. The heating system 100 may for example be employed in or part of a printing device such as the printing device 200 or 300 described below with reference to Figs. 2 and 3a, 3b, respectively. The heating system 100 may for example be arranged downstream of a print zone of the printing device. In other examples, the heating system 100 may also be arranged upstream of the print zone or within the print zone of the printing device.

[0011] The heating system 100 comprises a first heating element 102A and a second heating element 102B. The heating elements 102A, 102B are to heat a print medium 104, which may for example comprise textile, paper, cardboard, plastic, or a combination thereof. The second heating element 102B is arranged downstream of the first heating element 102A along a direction of motion in which the print medium 104 is moving relative to the first and second heating elements 102A, 102B. In other words, a given portion of the print medium 104 may first pass by the first heating element 102A and then the second heating element 102B. The direction of motion may for example coincide with the Y axis of Fig. 1 and may also be referred to as the Y direction in the following. In some examples, the print medium 104 may be moving along the direction of motion while the heating elements 102A, 102B are stationary. The heating elements 102A, 102B may for example be attached to a frame or housing of the printing device. In other examples, the heating elements 102A, 102B may also move in addition to or instead of the print medium 104, for example in a direction opposite to the Y direction of Fig. 1 .

[0012] The heating elements 102A, 102B may for example be radiative heating elements that are to generate electromagnetic radiation, e.g. infrared radiation. The electromagnetic radiation may be absorbed by the print medium 104 at least in part, thereby transferring energy to the print medium 104, which may lead to an increase of a temperature of the print medium 104. The heating elements 102A, 102B may for example comprise a resistive element, e.g. a resis- tive element comprising ceramic or carbon, or a heat lamp, e.g. a quartz-based halogen lamp. Additionally or alternatively, the heating elements 102A, 102B may also be to transfer energy to the print medium 104 by convection or thermal conduction. In some examples, the heating elements 102A, 102B may be identical heating elements, i.e. may be heating elements of the same type having the same structure and the same rated power consumption, e.g. the same resistance.

[0013] The heating system 100 further comprises a controller 106 that is to adjust a total power P supplied to the first and second heating elements 102A, 102B. The total power P may for example be the sum of the electrical power that is dissipated in the first and second heating elements 102A, 102B, e.g. by conversion to thermal energy. The controller 106 may be implemented in hardware, software, or a combination thereof. The controller 106 may for example comprise a microcontroller having a processor and a storage medium with instructions that are to be executed by the processor to provide the functionality described herein. Additionally or alternatively, the controller 106 may comprise other analog or digital electronic circuits, e.g. a current or voltage source to supply power to the heating elements 102A, 102B. The controller 106 may be to adjust the total power supplied to the heating elements 102A, 102B by generating a corresponding control signal for the current or voltage source or for an external power supply. The controller 106 may for example be to adjust an amplitude or a frequency of a current or voltage applied to the heating elements 102A, 102B to adjust the total power P. In some examples, the controller 106 may be to adjust a duty cycle of a pulse-width modulated current or voltage applied to the heating elements 102A, 102B to adjust the total power P. In one example, the controller 106 may be to adjust the total power supplied to the heating elements 102A, 102B between 0 W and 2 kW per heating element, i.e. between 0 W and 4 kW for two heating elements as in the example of Fig. 1 .

[0014] The controller 106 is to adjust the total power P while maintaining a power ratio between the second heating element 102B and the first heating el- ement 102A. The controller 106 may for example be to adjust the total power based on a target temperature for the print medium 104, e.g. by executing the method 400 described below with reference to Fig. 4 or at least a part thereof. The controller 106 may adjust the total power P while a ratio between a power PB supplied to the second heating element 102B and a power PA supplied to the first heating element 102A remains within a predetermined range or remains constant, e.g. at a predetermined value. The controller 106 may for example change the powers PA and PB by the same factor in order to change the total power P = P _A + PB.

[0015] The power PA supplied to the first heating element 102A is higher than the power PB supplied to the second heating element 102B. The power ratio between the second heating element 102B and the first heating element 102A, PB/PA, may for example be between 1 :3 and 1 :2, i.e. the power PB supplied to the second heating element 102B may be between 33% and 50% of the power PA supplied to the first heating element 102A. In some examples, the power PB may be between 35% and 45% of the power PA, in one example between 38% and 42% of the power PA and in one example 40% of the power PA. The controller 106 may be to adjust the power PA, PB supplied to the first and second heating elements 102A, 102B independently or may be to adjust the total power P, which may subsequently be split in a constant predetermined ratio, e.g. by a voltage divider or a transformer, to obtain the powers PA, PB.

[0016] In the example of Fig. 1 , the heating elements 102A, 102B are arranged at a distance di , which may for example be measured along the Y direction, i.e. along the direction of motion. The distance di may for example be the distance between a downstream edge of the first heating element 102A, i.e. the right edge of the first heating element 102A in Fig. 1 , and an upstream edge of the second heating element 102B, i.e. the left edge of the for second heating element 102B in Fig. 1. In other examples, the distance di may also be measured between other reference points of the heating elements 102A, 102B, e.g. be- tween a center of the first heating element 102A and a center of the second heating element 102B.

[0017] The first heating element 102A is arranged at a distance d2 from the print medium 104. The distance d2 may for example be the distance between a surface of the print medium 104 and a surface of the first heating element 102A facing the print medium 104, which may e.g. be measured in a direction perpendicular to the Y direction of Fig. 1. The distance d2 may for example be between 2 cm and 30 cm. In some examples, the second heating element 102B may also be arranged at the distance d2 or substantially the distance d2 from the print medium 104. In other examples, the second heating element 102B may be further away from the print medium 104 than the first heating element 102A, for example at a distance that is between 1 .5 times and 3 times as large as di , e.g. to reduce a radiation intensity incident on the print medium 104 from the second heating element 102B. In some examples, the controller 106 may be to adjust a distance between the print medium 104 and one or both of the first and second heating elements 102A, 102B.

[0018] The distance di between the first heating element 102A and the second heating element 102B may for example be between 75% and 125%, in some examples between 90% and 110% of the distance d2 between the first heating element 102A and the print medium 104. In one example, the spacing between the heating elements 102A, 102B may be equal to the spacing between the first heating element 102A and the print medium 104, i.e. di = d2. A width of each of the heating elements 102A, 102B in the direction of motion may for example be between 50% and 300% of di, e.g. equal to di in one example. Accordingly, a distance between the respective centers of the heating elements 102A, 102B may be larger than the distance d2, e.g. between 1.5 and four times, in one example two times as large as d2.

[0019] In some examples, the heating system 100 may comprise more than two heating elements (not shown). The heating system 100 may for example com- prise a plurality of heating elements arranged downstream of the first heating element 102A, e.g. between two and ten heating elements downstream of the first heating element 102A. The plurality of heating elements may comprise the second heating element 102B, which may for example be the heating element adjacent to the first heating element 102A, i.e. the heating element immediately downstream of the first heating element 102A. Each of the heating elements downstream of the first heating element 102A may be an identical heating element as the first heating element 102A.

[0020] Some or all of the heating elements in the heating system 100 may be arranged with a uniform spacing or pitch between adjacent heating elements, e.g. along the Y direction of Fig. 1. Each of the plurality of heating elements downstream of the first heating element 102A may for example be arranged at a distance di from the heating element immediately upstream of the respective heating element. Furthermore, some or all of heating elements in the heating system 100 may be arranged at the same distance from the print medium 104, e.g. at the distance d2. In other examples, some or all of the heating elements downstream of the first heating element 102A may be further away from the print medium 104 than the distance d2.

[0021] The power that is supplied to each of the heating elements downstream of the first heating element 102, e.g. by the controller 106 or a power supply controlled by the controller 106, may be the same or approximately the same. For example, the power PB may be applied to each of the heating elements downstream of the first heating element 102A. In other examples, a different power may be applied to each of the heating elements downstream of the first heating element 102A. The power may for example decrease continuously along the direction of motion, wherein the power supplied to a given heating element may for example be between 5% and 20% lower than the power supplied to the heating element immediately upstream of the respective heating element. [0022] The controller 106 may be to adjust the total power supplied to the heating elements in the heating system 100 while maintaining a constant power ratio between each of the heating elements downstream of the first heating element 102A and the first heating element 102A. The controller 106 may for example be to adjust the total power by changing the power supplied to each of the heating elements by the same factor. In some examples, the controller 106 may be to adjust the power supplied to each of the heating elements in the system 100 independently. In other examples, the controller 106 may be to adjust the total power, which may subsequently be split, e.g. according to a constant predetermined ratio.

[0023] Fig. 2 shows a schematic illustration of a printing device 200 according to an example in side view. The printing device 200 is to deposit a printing substance such as ink, for example a dye-sublimation ink, on a print medium 104, which may for example comprise textile, paper, cardboard, plastic or a combination thereof. For this, the printing device 200 may for example comprise a printing unit 202 such as a printhead or a print bar arranged in a print zone 204 of printing device 200, wherein the printing unit 202 is to deposit the printing substance on the print medium 104.

[0024] The printing device 200 comprises a media advance system 206 that is to advance the print medium 104 along a media advance path 208 through the print zone 204 of the printing device 200, e.g. in a media advance direction as indicated by the arrow 208 in Fig. 2. The media advance system 206 may for example comprise a plurality of rollers or pulleys as illustrated in Fig. 2, which may e.g. be to advance a continuous print medium such as a continuous textile web or paper roll along the media advance path 208. Additionally or alternatively, the media advance system 206 may comprise actuated components such as a transport belt (not shown), which may for example be to advance a sheet-like print medium such as a piece of textile or paper. [0025] The printing device 200 further comprises a plurality of radiative heating elements arranged along the media advance path 208 downstream of the print zone 204. In the example of Fig. 2, the printing device 200 comprises three radiative heating elements 102A, 102B, 102C, each of which may for example comprise a resistive element or a heat lamp, e.g. as detailed above. In other examples, the printing device 200 may comprise a different number of radiative heating elements, for example between two and ten radiative heating elements. The radiative heating elements 102A-102C may be arranged similar to the heating elements of the heating system 100. The radiative heating elements 102A- 102C may for example be arranged with a uniform spacing between adjacent elements and such that a distance between adjacent heating elements is between 75 % and 125 % of a distance between the heating elements 102A-102C and the print medium 104. In some examples, the printing device 200 may comprise the heating system 100 and the radiative heating elements 102A- 102C may be part of the heating system 100. In one example, the printing device 200 may comprise a plurality of heating systems, e.g. two heating systems that are to perform different treatment processes on the print medium 104.

[0026] The printing device 200 also comprises a controller 210. The controller 210 may be implemented in hardware, software, or a combination thereof and may for example be similar to the controller 106 of the heating system 100. In some examples, the controller 210 may be part of a main controller (not shown) of the printing device 200, which may for example be to control the printing unit 202 and the media advance system 206, e.g. to execute a print job. In some examples, the controller 210 may be to execute the method 400 described below or at least a part thereof.

[0027] The controller 210 is to adjust a radiation intensity I incident on the print medium 104 from the heating elements 102A-102C. The controller 210 may for example be to provide corresponding control signals to the heating elements 102A-102C or to a power supply for the heating elements 102A-102C. The radiation intensity I may for example be the total intensity of electromagnetic radia- tion, e.g. thermal radiation, emitted by the heating elements 102A-102C that hits a portion of the print medium 104, i.e. the total radiation power per unit area in the portion, wherein the portion of the print medium 104 may for example be the portion of the print medium 104 that is adjacent to or underneath the heating elements 102A-102C. In the following, the radiation intensity I may thus also be referred to as the total radiation intensity. At least some of the radiation incident on the print medium 104 may be absorbed by the print medium 104 and may thus heat the print medium 104.

[0028] A radiation intensity h incident on the print medium 104 from a first heating element along the media advance path in the media advance direction, i.e. the heating element 102A the example of Fig. 2, is higher than a radiation intensity I2 incident on the print medium 104 from a second one of the heating elements. The radiation intensity h may for example correspond to the contribution of the first heating element 102A to the total intensity I incident on the print medium 104 from the heating elements 102A-102C. Accordingly, the radiation intensity I2 may for example correspond to the contribution of the second one of the heating elements to the total radiation intensity I.

[0029] The second one of the heating elements may for example be adjacent to the first heating element 102A, i.e. the heating element immediately downstream of the first heating element 102A. In the example of Fig. 2, the second one of the heating elements may for example be the heating element 102B, which is used as an example for illustrative purposes in the following and may thus also be referred to as the second heating element 102B. In other examples, the second one of the heating elements may not be adjacent to the first heating element 102A, but may for example be the heating element 102C in the example of Fig. 2.

[0030] In some examples, the radiation intensity h is higher than the radiation intensity incident on the print medium 104 from any of the other heating elements. In some examples, the radiation intensity incident on the print medium 104 from each of the other heating elements may be the same or approximately the same. For example, each of the heating elements 102B, 102C may emit radiation having an intensity I2 at the surface of the print medium 104. In other examples, the radiation intensity incident on the print medium 104 from the other heating elements may decrease along the media advance path, i.e. the radiation intensity incident on the print medium 104 from a given heating element may be lower the further downstream the respective heating element is arranged.

[0031] The controller 210 is to maintain a constant ratio between the radiation intensity I2 and the radiation intensity h when adjusting the total radiation intensity I. In other words, the radiation intensities h and I2 may be changed by the same factor when changing the total radiation intensity I. The radiation intensity I2 may for example be between 33% and 50% of the radiation intensity h, in some examples between 35% and 45%, in one example between 38% and 42% and in another example 40%. In some examples, the radiation intensity from each of the heating elements 102A-102C may be changed by the same factor when changing the total radiation intensity I, i.e. the controller 210 may be to maintain a constant ratio between each pair of radiation intensities. For example, the radiation intensity of each of the heating elements 102B, 102C downstream of the first heating element 102A may be the same or approximately the same, e.g. between 33% and 50% of h.

[0032] The controller 210 may for example be to adjust the total radiation intensity I by adjusting the power supplied to the heating elements 102A-102C, e.g. similar as described above for the heating system 100. In some examples, the radiation intensity incident on the print medium 104 from a given heating element may be proportional or approximately proportional to the power supplied to the respective heating element. The controller 210 may for example be to adjust a total power supplied to the first and second heating elements 102A, 102B while maintaining a power ratio between the second heating element 102B and the first heating element 102A, wherein the power supplied to the first heating element 102A is higher than the power supplied to the second heating element 102B.

[0033] Additionally or alternatively, the controller 210 may be to adjust the total radiation intensity I by adjusting a distance between some or all of the heating elements 102A-102C and the print medium 104. The controller 210 may for example be to move the heating elements 102A-102C closer to or further away from the print medium 104 in order to increase or decrease the total radiation intensity I, e.g. using an actuator coupled to the heating elements 102A-102C. The actuator may for example be to move the heating elements 102A-102C along a linear path, e.g. perpendicular to the media advance path, to adjust the distance to the print medium 104. This may e.g. allow for changing a temperature of the print medium 104 on a faster timescale than when changing the power supplied to the heating elements 102A-102C.

[0034] Figs. 3a, 3b show schematic illustrations of a printing device 300 according to another example. The printing device 300 is depicted in side view in Fig. 3a and in a perspective view in Fig. 3b, wherein the print medium 104 is omitted for illustration purposes in Fig. 3b. The printing device 300 is similar to the printing device 200 described above and also comprises a media advance system 206 and a plurality of radiative heating elements 102A, 102B arranged downstream of a print zone (not shown) of the printing device 300. The printing device 300 further comprises a controller (not shown) that is to adjust a radiation intensity incident on the print medium from the heating elements 102A, 102B, e.g. similar to the controller 210 of the printing device 300 or to the controller 106 of the heating system 100. The printing device 300 is to deposit a printing substance on a print medium 104 and may for example be a large-format dyesublimation printer for textiles. Accordingly, the print medium 104 may for example be a continuous textile web, which may e.g. be provided on a supply roll (not shown). [0035] The media advance system 206 of the printing device 300 comprises a take-up roll 302, which may for example be to take up and store a portion of the print medium 104 that has been printed on. The take-up roll 302 may be actuated, e.g. to unroll the print medium 104 from the supply roll and to advance the print medium 104 through the print zone (not shown) of the printing device 300 towards the take-up roll 302 by rotating the take-up roll 302. In addition, the media advance system 206 comprises a plurality of rotatable pulleys or rollers 304 that are to guide the print medium 104 along a media advance path. In some examples, some or all of the pulleys 304 may also be actuated.

[0036] The printing device 300 comprises a heating system 306, which may for example be similar to the heating system 100 of Fig. 1. The heating system 306 comprises a supporting structure or frame 308, to which a pair of radiative heating elements 102A, 102B is mounted. Each of the heating elements 102A, 102B comprises a plurality of heating panels 310, which may for example be ceramic infrared heating panels. The heating panels 310 of each of the heating elements 102A, 102B are arranged adjacent to each other along a transverse direction, e.g. along the X direction of Fig. 3b, which may for example be aligned with a scanning direction of a print head (not shown) of the print device 300, i.e. may extend across a width of the print medium 104. A length of the heating elements 102A, 102B along the X direction may be adapted to the width of the print medium 104 and may for example be equal to or larger than the width of the print medium 104.

[0037] The first heating element along the media advance path in the media advance direction, i.e. the heating element 102A in the example of Fig. 3a, 3b, is arranged at a distance di from the print medium 104. The second heating element 102B is arranged at a distance d2 downstream of the first heating element 102A and may also be arranged at the distance di from the print medium 104. The heating elements 102A, 102B may be arranged in a similar way as the heating elements of the heating system 100. For example, the distance d2 may be between 75% and 125% of the distance di. In some examples, the controller of the printing device 300 may be to adjust the radiation intensity incident on the print medium 104 from the heating elements 102A, 102B by moving the frame 308 of the heating system 306 towards or away from the print medium 104, e.g. along the Y direction of Fig. 3a.

[0038] Fig. 4 shows a flow chart of a method 400 of operating a printing device. The method 400 may for example be used to perform a treatment process on a print medium, e.g. during execution of a print job. The method 400 may for example be implemented with the printing 200 of Fig. 2, which is used as an example for illustration purposes in the following. This is, however, not intended to be limiting in any way and the method 400 may also be implemented with other printing devices comprising a plurality of heating elements that are to heat a print medium, e.g. a printing device comprising the heating system 100 of Fig. 1 or the printing device 300 of Figs. 3a, 3b. Furthermore, the method 400 is not limited to the order of execution implied by the flow chart of Fig. 4. As far as technically feasible, the method 400 can be executed in an arbitrary order and parts thereof can be executed simultaneously at least in part.

[0039] The method 400 comprises, in block 402, setting a ratio between a power that is to be applied to a first group of heating elements and a power that is to be applied to a second group of heating elements to a predetermined value. Each of the first and second groups of heating elements may comprise one heating element or a plurality of heating elements. The heating elements may for example be radiative heating elements. The power applied to a group of heating devices may for example be the total electric power dissipated in the respective group of heating devices. In some examples, the first group of heating elements may be located upstream of the second group of heating elements in a direction of motion in which the print medium 104 is moving relative to the first and second groups of heating elements. In the printing device 200, the first group of heating elements may for example comprise the heating element 102A and the second group of heating elements may for example comprise the heat- ing elements 102B and 102C. In some examples, there may be no additional heating elements between the first and second groups of heating elements.

[0040] The predetermined value for the power ratio between the first group of heating elements and the second group of heating elements may for example have been determined prior to execution of the method 400, e.g. during manufacture or calibration of the printing device 200. The predetermined value for the power ratio may for example have been chosen so as to obtain a constant or approximately constant temperature profile for the print medium 104 as detailed below with reference to Figs. 5a, 5b, e.g. by performing an optimization using the power ratio as a variable parameter. The predetermined power ratio may be substrate-dependent, i.e. may be different for different types of print media. The controller 210 of the printing device 200 may for example store a predetermined calibration table associating various types of print media to respective power ratio values. In some examples, the method 400 may comprise determining the power ratio value, e.g. prior to execution of block 402.

[0041] The predetermined value for the power ratio may for example be such that a power supplied to a first heating element in the first group of heating elements, e.g. the heating element 102A, is higher than a power supplied to a second heating element in the second group of heating elements such as the heating element 102B, e.g. similar to the heating system 100 of Fig. 1. In some examples, the same power may be applied to all heating elements within a group. The power applied to each of the heating elements in the second group may for example be between 33% and 50%, in one example 40% of the power applied to each of the heating elements in the first group.

[0042] The method 400 further comprises, in block 404, receiving a target temperature Tt for the print medium 104. The target temperature may e.g. be the temperature that a portion of the print medium 104 is to be heated to in block 408. The target temperature Tt may for example be higher than a minimum process temperature To for a treatment process that is to be performed on the print medium 104, but may be lower than a damage threshold of the print medium 104. The treatment process may for example comprise drying, curing, fixing, or sublimating a printing substance deposited on the print medium 104 or a combination of the aforementioned processes. The target temperature may for example be between 50°C and 250°C, in one example between 180°C and 210°C. The target temperature may be substrate-dependent and may for example be stored in the controller 210 of the print device 200 or may be provided by a user of the printing device 200.

[0043] In block 406, a total power that is to be applied to the heating elements 102A-102C is adjusted, wherein the total power may e.g. be the sum of the powers that are to be applied to the first and second groups of heating elements. The total power is adjusted based on the target temperature. The total power may for example be increased for a higher target temperature or decreased for a lower target temperature, e.g. such that the portion of the print medium 104 heated in block 408 may reach the respective target temperature. In some examples, the total power may be adjusted using a predetermined calibration curve that associates a given value of the target temperature with a respective value of the total power. The calibration curve may for example have been determined prior to execution of the method 400, e.g. together with the predetermined power ratio, and may for example be stored in the controller 210. The total power may for example be adjusted from a default or starting value, which may e.g. have been set during startup of the printing device 200, or may be adjusted from a previously used value, e.g. from a previous printing process or a previous execution of the method 400.

[0044] The total power is adjusted without changing the ratio between the power that is to be applied to the first group of heating elements and the power that is to be applied to the second group of heating elements set in block 402. In other words, the same predetermined value for the power ratio between the two groups of heating elements may be used irrespective of the target temperature. For example, the power that is to be applied to the first group and the power that is to be applied to the second group may both be adjusted by the same factor, which may correspond to the factor by which the total power is adjusted. In some examples, the total power may be adjusted without changing the ratio between the power that is to be applied to a first heating element in the first group, for example the heating element 102A, and the power that is to be applied to a second heating element in the second group, for example the heating element 102B. In one example, the power that is to be applied to each of the heating elements in the first and second groups may be adjusted by the same factor.

[0045] The method 400 further comprises, in block 408, heating a portion of the print medium 104 by applying the respective power to the groups of heating elements, e.g. to bring the portion of the print medium 104 to the target temperature. For this, the controller 210 may for example generate a corresponding control signal for a power supply connected to the heating elements. To apply the respective power to the groups of heating elements, the controller 210 may for example be to adjust an amplitude or a frequency of a voltage or current applied to the respective heating elements. In other examples, the controller 210 may e.g. be to adjust a duty cycle of a pulse-width modulated supply voltage or current.

[0046] The portion of the print medium 104 is heated while the print medium 104 and the heating elements 102A-102C are moving relative to each other, e.g. by moving the print medium 104, the heating elements 102A-102C or both the print medium 104 and the heating elements 102A-102C while applying power to the two groups of heating elements. In examples, in which the first group is located upstream of the second group, the portion of the print medium 104 that is to be heated may thus first move past the first group of heating elements before moving past the second group of heating elements.

[0047] In some examples, the respective power applied to the groups of heating elements may be constant while the portion of the print medium 104 is adjacent to the heating elements. In other words, the power applied to the groups of heating elements may be constant throughout an exposure time of the portion, e.g. by maintaining the amplitude or duty cycle of the supply voltage constant. The exposure time may for example be between 5 s and 30 s. In one example, the respective power applied to the groups of heating elements may be constant throughout a print job or may remain constant until the print medium 104 is exchanged or until the printing device 200 is switched off.

[0048] The predetermined value for the power ratio between the first group of heating elements and the second group of heating elements set in block 402 may have been chosen such that the portion of the print medium 104 reaches the target temperature when the portion is adjacent to the first group of heating elements, e.g. adjacent to the heating element 102A, and remains at substantially the target temperature while the portion is adjacent to the second group of heating elements, e.g. adjacent to the heating elements 102B, 102C. The predetermined value may for example have been determined by observing the temperature of the portion of the print medium 104 while the portion moves past the heating elements 102A-102C as detailed in the following with reference to Figs. 5a, 5b.

[0049] Fig. 5a depicts a graph 500 showing examples of two temperature curves 502, 504 of portions of a print medium 104 as a function of time during exposure. The curve 502 corresponds to the temperature of a portion of the print medium 104 that was heated using two heating elements such as the heating elements 102A, 102B of the printing device 300, whereas the dotted curve 504 corresponds to the temperature of a portion of the print medium 104 that was heated using one heating element such as the heating element 102A.

[0050] In both cases, the temperature of the respective portion increases quickly as the portion approaches the heating element 102A. The power supplied to the heating element 102A was chosen such that both portions reach a target temperature Tt, e.g. when being adjacent to the heating element 102A. The target temperature may for example be above a minimum process temperature To of a given treatment process, e.g. between 5°C and 20°C above To, but smaller than a damage threshold of the print medium 104 or of a printing substance deposited on the print medium 104, e.g. between 5°C and 20°C below the damage threshold.

[0051] When using one heating element, the temperature of the print medium 104 starts decreasing once the portion has passed the heating element 102A, falling below the minimum temperature To soon after. To achieve a certain exposure time during which the temperature of the print medium 104 is above To, the target temperature Tt may be increased. This, however, may increase a risk of damaging the sample or causing image quality defects. Furthermore, an energy consumption of the printing device may be increased.

[0052] When using two heating elements, the second heating element 102B, which is operated at a lower power than the first heating element 102A, e.g. at 40% of the power of the first heating element 102A, transfers sufficient energy to the print medium 104 to maintain the temperature of the print medium 104 reached at the first heating element 102A. Accordingly, the temperature of the print medium 104 remains approximately constant as the print medium 104 passes by the heating elements 102A, 102B and the temperature of the print medium 104 may be maintained at or close to the target temperature Tt. Thereby, the amount of time during which the print medium 104 is above the minimum process temperature To may be increased without increasing a peak temperature of the print medium 104, thus reducing the risk of damaging the print medium 104.

[0053] Fig. 5b depicts a graph 510 showing examples of three temperature curves 512, 514, and 516 of portions of a print medium 104 as a function of time during exposure. In all three cases, the respective portion of the print medium 104 was heated using two heating elements, e.g. the heating elements 102A, 102B of the printing device 300, operated at a fixed power ratio, for example at a ratio of 4:10 between the power supplied to the second heating element 102B and the power supplied to the first heating element 102A. The dashed curve 512 corresponds to the temperature of a portion of the print medium 104 that was heated using a first value for the total power supplied to the two heating elements 102A, 102B and a first distance between the print medium 104 and the heating elements 102A, 102B. The dotted curve 514 corresponds to the temperature of a portion of the print medium 104 that was heated using a second value for the total power that is higher than the first value with the heating elements 102A, 102B also being at the first distance from the print medium 104. The solid curve 516 corresponds to the temperature of a portion of the print medium 104 that was heated using a third value for the total power that is between than the first and second values with the heating elements 102A, 102B being at a second distance from the print medium 104 that is smaller than the first distance.

[0054] In all cases, the respective portion of the print medium 104 reaches a target temperature adjacent to the first heating element 102A, which is substantially maintained while the portion is adjacent to the second heating element 102B. The target temperature can be controlled by adjusting the total power, the distance between the heating elements 102A, 102B and the print medium 104 or both of these parameters without affecting the stability of the temperature since the power ratio between the heating elements 102A, 102B is not changed. Using two heating elements or groups of heating elements operated at a fixed power ratio may thus facilitate quickly reaching and substantially maintaining a target temperature for a print medium, while at the same time allowing for changing the target temperature by adjusting the total power or distance of the heating elements without having to perform a new calibration of the heating system to achieve a stable temperature.

[0055] The description is not intended to be exhaustive or limiting to any of the examples described above. The A heating system for a printing device, the A heating system for a printing device, and the A method of operating a printing device disclosed herein can be implemented in various ways and with many modifications without altering the underlying basic properties.