Method and device for determining the degree of curing of printing inks

Abstract

Method for determining the degree of curing of printing inks (CMYK) on a printing substrate (1), wherein first color separations (30) for at least 4 printing colors (CMYK) are generated according to a printing template, according to the provided color separations (30), printing inks (CMYK) are transferred to the printing substrate (1) in printing units (19, 20, 21, 22), the color separations (30) comprise at least one control field in addition to a desired print image and the degree of curing of at least one position in the printed image is determined by means of a measuring device (10), characterized by that the least one control field is a hardening degree control field (3, 3'), that the degree of hardening control field (4, 4') is generated in at least 4 color separations (30), the degree of curing control field (3, 3') is produced by overprinting at least 4 printing inks (CMYK) on the printing material (1), wherein the color value of the individual printing inks (CMYK) of the degree of curing control field corresponds to a solid tone in the respective color values ​​and the color black (K) is transferred to the printing material (1) as the lowest printing ink, and the degree of curing of the printing inks (CMYK) is determined by measuring the degree of curing control field (3, 3') after printing on the printing material (1) with the measuring device (10), taking into account the degree of curing of at least the lowest printing ink (K).

Description

[0001] The invention relates to a method and a device for determining the degree of curing of printing inks and / or varnishes on a printing substrate. Color separations for at least four printing inks and / or varnishes are first generated according to a printing template, and the corresponding printing inks or varnishes are transferred to the printing substrate in printing and / or varnishing units according to these color separations. For this purpose, a printing press with the corresponding printing and / or varnishing units for printing the printing substrate with at least four printing inks or varnishes is provided in particular. In addition to a desired print image, the color separations have at least one control field, and the degree of curing of the printing inks is determined at at least one position in the printed image using a measuring device.

[0002] In known printing processes such as offset, flexographic or gravure printing processes as well as in industrial inkjet printing processes, it is often necessary for printed printing inks and / or varnish layers to be dried or fully cured. This applies in particular when UV inks or UV varnishes are used. Curing or drying here refers in particular to the polymerization of the UV inks or varnishes. Such polymerization, i.e. full curing and / or drying of the printing inks and / or varnishes, is usually achieved by means of UV dryers that are arranged inline in the printing press. When other printing inks and / or varnishes are used, a different drying process can also be provided, e.g. using infrared dryers.

[0003] The corresponding printing processes for printing the substrate, preferably using UV varnishes or inks, can be used in sheet-fed or web-fed printing presses. The prerequisite for high-quality prints is sufficient drying or curing of the printed printing inks and / or varnishes. This is particularly true if the inks and / or varnishes have been printed at least partially on top of one another. Printing inks or varnishes that are not completely dried or cured can result in smearing or blocking of the printed roller webs or sheets, especially if they are stacked on top of one another in the delivery. This at least reduces the quality of the printed image itself and can make further processing of the printed product more difficult or impossible.

[0004] To ensure the best possible print quality and the smoothest possible further processing of the printed products, it is often necessary to determine or monitor the degree of drying or curing of the printing inks and / or varnishes. In the following, we will refer to the degree of curing of the printing inks and / or varnishes. Depending on the type of ink or varnish used, the degree of drying is also included.

[0005] In order to determine the respective degree of curing of the inks or varnishes, it is known, for example from DE 10 2007 030 566 A1, to determine the degree of drying or curing by generating surface waves and measuring the energy density of these waves on the printing material.

[0006] From EP 0 025 878 A1 it is known to determine the degree of hardening at least locally by means of a colour density measurement.

[0007] From DE 10 2006 041 721 A1 it is also known to determine the degree of drying globally by measuring the moisture content of the draft air and the exhaust air of the drying device itself.

[0008] DE 10 2008 041 825 A1 describes a non-destructive testing method for the degree of curing or drying of paints and varnishes using an SPR sensor.

[0009] US 2009 / 0 315 963 A1 describes a method for determining the drying state of a print image that has been printed with an inkjet printing device.

[0010] The methods and devices known from the prior art generally do not allow for the conclusive determination of the degree of drying for all combinations of printing inks that are created one above the other in the printed image, in such a way that it can be concluded that all printing inks applied to the substrate have completely dried. These measurements either involve complex measurements of global parameters that do not allow for any conclusions about specific local conditions, or they involve complex measurements of local parameters that, in turn, do not allow for any conclusions about the global condition of all locally printed printing ink combinations.

[0011] The object of the present invention is therefore to provide a simpler method or a simpler device in order to obtain at least a more reliable statement about the degree of drying or the degree of hardness of printing inks or varnishes.

[0012] This object of the invention is achieved by a method for determining a degree of curing according to claim 1 and a device for determining the degree of curing according to claim 4.

[0013] In the method according to the invention, it is provided that the at least one control field is a degree of curing control field. With a dedicated degree of curing control field, it is now sufficient for this one degree of curing control field to be measured exclusively by a suitable measuring device and, solely by determining the degree of curing of this degree of curing control field or by the values ​​measured by the measuring device on this degree of curing control field, to determine whether the degree of curing of the entire printed image is sufficient for a desired state or to determine what degree of curing has been achieved. For this purpose, it is provided in particular that the degree of curing control field is generated by overprinting printing inks and / or varnishes on the printing substrate. The degree of curing of the printing inks and / or varnishes is then determined by measuring the degree of curing control field after printing on the printing substrate with the measuring device.When determining the degree of curing, the degree of curing of at least one of the printing inks or varnishes used, preferably the lowest printing ink or varnish layer, is determined. Since the sequence of the printing inks or varnishes on the substrate is determined by the arrangement of the printing units within the sheet-fed press, the degree of curing of the printing inks and / or varnishes on the substrate is preferably determined by determining the degree of curing of the first printed ink or varnish layer.

[0014] On the one hand, the degree of curing of the measured printing ink or varnish can be determined. This degree of curing provides overall information about the degree of curing of the printing inks or varnishes used, particularly when the measured printing ink or varnish is the bottom layer of ink. This is particularly advantageous when an interdeck dryer is provided within the sheet-processing machine, e.g. a sheet-fed printing press in offset operation, after the first three printing inks, for example the KCM printing inks, i.e. black, cyan and magenta. If these three printing inks are printed on top of each other, the degree of curing of the bottom printing ink provides good information about the degree of curing of all the printing inks. Of course, different printing inks can also be printed and measured, especially spot colors.

[0015] For this purpose, the invention provides a device of the generic type, in which a printing press is equipped with printing and / or coating units for printing a substrate with at least four printing inks and / or varnishes. The device is to have a printing substrate with the curing degree control field described above, and furthermore, the device is to have a measuring device for determining a curing degree of the curing degree control field. This measuring device, as described above, is to preferably determine the curing degree of at least one of the printing inks or varnishes used, preferably the first printing ink or varnish printed.

[0016] A printing process control strip is defined by comprising at least one control field, wherein this one control field is a degree of curing control field for determining the degree of curing of printing inks and / or varnishes on the printing substrate. In particular, a single control field, which is the defined degree of curing control field, should also be understood as a complete printing process control strip within the meaning of this invention. Otherwise, it is entirely conceivable that in a conventional printing process control strip, which has different measuring fields with printing inks of different tonal values ​​printed one above the other, the degree of curing control field according to the invention is integrated as an additional measuring field.

[0017] Furthermore, it can be provided that the printing process control strip is provided in a computer program product which is stored on a data carrier.

[0018] The print process control strip comprising the cure level control field can be provided on a printing plate or other intermediate carrier for a printed image. The print process control strip can also be provided on a final printed product or on a printed substrate, e.g., a sheet of paper.

[0019] This printing process control strip can be printed onto a printing substrate according to the method according to the invention and measured using a measuring device. This printing and measuring can be carried out by the device according to the invention. Furthermore, it can be provided that a statement about the degree of curing of the printing inks or varnishes used is determined and, based on this degree of curing, at least one parameter of the printing press is controlled such that an optimal degree of curing is achieved to improve the quality of the printed product. The adjustable parameters can be, for example, the intensity or wavelength of the drying device or the flow rate of the printing substrate through the drying device.Such a modified method is no longer exclusively a method for determining the degree of curing of printing inks and / or varnishes, but rather a method, a device and a printing process control strip for controlling the degree of curing of these printing inks or varnishes within a printing process.

[0020] The method according to the invention provides for the cure level control field to be generated in at least four color separations. These at least four color separations then serve as the basis for generating the printed image. In offset printing, these color separations are initially used to produce printing forms, preferably printing plates, which are then used in the actual printing process to transfer ink to the printing substrate.

[0021] Starting from these at least four color separations, at least four printing inks are then printed on top of each other in the printing process itself according to the at least four color separations, thus creating the cure level control field on the printing substrate. Combinations of at least four printing inks and / or varnishes are of course also possible here. It is particularly preferred that the individual printing inks or varnishes be printed on top of each other in such a way that the color values ​​of the individual printing inks or varnishes each correspond to a solid tone, i.e. a color value of 100%. The cure level control field is then created in such a way that at least four solid tone printing inks are printed on top of each other. A cure level control field created in this way then corresponds to an ink coverage of at least 400%. Overprints of four printing inks are not provided for in conventional process control strips.In particular, it is not intended that three colored printing inks and the printing ink black are printed on top of one another in solid tones. According to the invention, it is precisely this overprinting of at least three printing inks over the printing ink black that has the advantage that the printing ink black now serves as an indication of complete curing of all printing inks in the printed image. Therefore, it is further provided that, in a preferred development, the curing degree control field is generated at least from the printing inks CMYK, with the color black being transferred to the printing material as the lowest printing ink. By measuring the printing ink black, i.e. by measuring the lowest ink layer, a reliable value can then be determined as to whether curing has taken place and / or what degree of curing the printing inks or varnishes have.For this purpose, a corresponding curing level control field is provided within the print process control strip, which is created by overprinting at least four printing inks or varnishes in solid color. When using additional or different printing inks, a printing ink other than black may also be printed at the bottom.

[0022] The bottom color is preferably an ink that dries slowly, i.e. a slow-drying printing ink or a slow-drying varnish. In particular, it should ideally be the slowest-drying printing ink in the printing process. In this way, by determining the degree of curing of the bottom printing or varnish layer, i.e. preferably the black ink, a particularly reliable value can be obtained for the curing status of all the colors. This is especially true because an interdeck dryer is generally only provided after the first three colors. In conventional printing process control strips, the color black is not found in combination with three other printing colors in a solid color. Only such a massive structure can then provide appropriate information about the degree of curing of the printing inks.Known control fields of a print process control strip generally feature the overprinting of three colored printing inks, with two printing inks, particularly when UV printing inks, being transferred to the printing substrate before the first interdeck dryer. The degree of curing of the individual printing inks is then determined by the effect of the dryer on two ink layers, with a black ink layer in particular never being dried on its own. In an actual print image, a combination of different ink layers is often applied over black. In order to obtain the degree of curing of this combination of printing inks, including the color black, the invention provides for the overprinting of all printing inks, including the color black, in the print process control strip, each in solid color.

[0023] Based on the resulting hardness control field, a measuring device is then used to determine the degree of cure. An IR measuring device that emits radiation in the infrared range is advantageously provided for this purpose. The reflected radiation is then measured by the IR measuring device, and the quotient of emitted and reflected radiation provides information about the degree of cure of the printing inks used. To measure the degree of cure, a measuring head of the IR measuring device is first brought into contact with the hardness control field. A corresponding control device is provided to establish contact between the measuring head and the hardness control field. Depending on the intensity of the emitted infrared radiation, the thickness of the printing inks, and the degree of cure of the printing inks, different quotients between reflected and emitted radiation are determined.The penetration depth of the measuring head should be selected such that reflection from the printing substrate itself is not measured. This ideal penetration depth can be determined using an iterative experimental procedure in which different thicknesses of printing inks are applied on top of one another in order to determine the influence of the printing substrate on completely dried printing ink surfaces in relation to the reflected IR radiation. By using the appropriately adjusted IR measuring head, which has been brought into contact with the cure level control field, a statement can then be made about the degree of curing, i.e. the degree of curing of the cure level control field. The measuring method described here works according to the principle of attenuated total internal reflection (ATR).

[0024] In a further development of the invention, the control device of the IR measuring device should be designed such that the contact pressure is advantageously adjusted so that partial mixing of at least some of the printing inks occurs at least in a boundary area of ​​the individual ink layers below the measuring head. In this way, the curing of various ink layers can be easily determined in the curing degree control field. However, a better result is always achieved when the inks are dry to the point where they just do not mix.

[0025] The measuring head of the IR measuring device should be set up so that the reflection on / in the lowest printing ink or the lowest varnish layer is used to determine the degree of curing. The measuring device should be set up so that there is as little reflection as possible on the surface of the printing material. An IR measuring device can then be set up exactly as described so that there are no reflections on the surface of the printing material and the lowest printing ink or the lowest varnish layer is used as the basis for determining the degree of curing if a previously determined ideal distance between the IR measuring device and the printing material surface, i.e. an optimal penetration depth of the measuring head into the printing ink layers, is set.Of course, radiation is always reflected by the upper layers of paint, but it is assumed here that even if the upper layers are cured, the lower layer may still not be fully dry. This layer is therefore the most relevant.

[0026] To ensure the appropriate positioning of the IR measuring device's measuring head, a control device is provided within the device. This device adjusts the position of the measuring head in such a way that at least reflections from the bottommost ink or varnish layer are measured without causing reflections on the substrate. Furthermore, the control device can alternatively or additionally adjust the contact pressure of the measuring head on the curing level control field or on the associated ink layers in such a way that at least some of the inks or varnishes mix, at least in the boundary areas of the individual layers. Such mixing should then no longer occur once the ink has completely dried.

[0027] A possible alternative would be a non-contact measurement method. In this case, IR radiation is emitted into the ink layers in such a way that reflections from the substrate are always measured. These substrate emissions can be determined in a test measurement and subtracted from the measurement when measuring the cure control field. The measurement results obtained in this way would then provide information about the cure level of all printing inks and / or varnishes.

[0028] An example of the invention, from which further features essential to the invention can also arise and to which the invention is not limited, is shown in the following drawings.

[0029] They show: Fig. 1 a printed sheet with curing control fields Fig. 2 a color separation with curing degree control fields Fig. 3 a sketch of the device for determining the degree of curing of printing inks.

[0030] In the Fig. 1 shows a printed sheet 1. This sheet 1 is initially provided with a print image, which is not further described here, and further with a print process control strip 2. The print process control strip 2 can have different measuring fields, which can ensure print control and control of the printing process itself. Possible uses of a print process control strip include, for example, determining a dot gain. This print process control strip can have a curing degree control field 3 according to the invention. Such a curing degree control field 3 can also be provided as a separate, independent curing degree control field 3', which can be provided on the printed sheet 1 apart from further used print process control strips 2. This individual, separate curing degree control field 3' can then itself be regarded as a special case of a print process control strip 2.

[0031] The Fig. 2 shows an individual color separation 30, which is assigned to one of the printing inks used, e.g. CMYK. For reasons of clarity, the representation of a possibly printed print image has been omitted, and only measuring fields 5, 6, 7 of the printing process control strip 2 are shown, which specify different tonal values ​​of the printing ink representing the color separation 30. These measuring fields 5, 6, 7 with different tonal values ​​of the printing ink used in the color separation 30 then result in different measuring fields in the printed print sheet 1, e.g. in the overprint of different printing inks or also for a tonal value wedge of the individual printing ink of the color separation 30 itself. The printing process control strip 2 also comprises a hardening degree control field 4, which represents a solid tone of the corresponding color separation 30, i.e. a tonal value 100% of the printing ink used.Additionally or alternatively, the degree of curing control field 4 in the color separation 30 can also be provided here as a separate degree of curing control field 4' with a solid coverage of the corresponding printing ink apart from the printing process control strip 2 used, thus establishing an independent printing process control strip 2 comprising only this degree of curing control field 4'.

[0032] The degree of hardening control fields 3, 3', which are located in the Fig. 1 are produced by overprinting the different printing inks in a printing machine 18, as shown in Fig. 3. The tonal values ​​with which the printing ink is transferred to the printing substrate 1 are predetermined by the color separations 30, which represent the individual printing inks used within the printing machine 18. As shown in the Fig. 2, the color separations 30 for the curing degree control field 4, 4' each have a solid tone in each individual printing color (CMYK), i.e., an ink coverage of 100%. The curing degree control fields 3, 3' are therefore produced by overprinting solid tone areas of the individual CMYK printing colors of the printing machine 18. A printing machine 18, as is conventionally used when using UV inks, is shown in the Fig. 3. Here, printing inks KCM are applied in the sequence black, cyan, and magenta in the first three printing units 19, 20, and 21 to a printed sheet 1 fed through the printing press 18. In a UV dryer 23 following the printing units 19, 20, and 21, these printing inks are dried, i.e., the UV inks are polymerized so that they harden. In a subsequent printing unit 22, the last color, yellow (Y), is then transferred to the printed sheet 1 and finally dried again with a UV dryer 23.

[0033] In previous print process control strips and previously known control patches on a printed sheet 1, individual solid patches of the individual printing colors are first applied, i.e., patches with 100% coverage of the individual printing colors. These 100% patches are always reliably dried by the UV dryer 23, which follows the third printing unit 21, especially if another dryer 23 follows the last printing unit 22.

[0034] In known process control strips, the color black (K) is never printed on top of other colors. Therefore, as a single color in the solid tone, it is always completely dry. Furthermore, the color black (K) is generally the most difficult to dry, which is particularly true when individual printing colors are printed on top of each other. In known process control strips, combinations of the solid tones of the individual CMY printing colors are printed on top of each other. They each achieve a coverage of 100% if the colors are printed individually, 200% if two solid colors are combined and printed on top of each other, or a maximum of 300% if all the solid colors are printed on top of each other. Combinations with the color black are not taken into account here. An ink coverage of 400% or similar is never intended.

[0035] As shown, in a conventional offset printing process, the ink black is the first to be applied and thus, in the case of overprinting, lies beneath other inks. In the case of overprinting, the ink black receives a radiation dose for the first time when the inks cyan and magenta have been printed over it in the subsequent inking units 20, 21. This makes black the ink that is inherently the most difficult to dry and, due to the specified ink sequence KCM, the ink that receives the least radiation dose, since the dryer radiation acting on the black ink layer first dries the ink layers above it, which are printed in the subsequent printing units 20, 21 onto the black ink applied in printing unit 19.If, as described, the degree of curing control field 3, 3' is formed in that all color separations 30 of the CMYK printing inks each have a degree of curing control field 4, 4' with a tonal value of 100% solid tone, the degree of curing control field 3, 3' has a color density of 400% solid tone, with the bottommost ink layer being formed by the black printing ink (K). Measuring such a degree of curing control field 3, 3', in particular in such a way that the degree of curing of the bottommost ink layer, i.e. the printing ink (K) printed first in the first printing unit 19, is determined, enables a final assessment of whether all printing inks have been dried by the UV dryers 23. If a measurement of the bottommost (black) layer determines that it is fully cured, a degree of curing for all printing inks used is reliably determined such that all printing inks are cured.

[0036] The Fig.3 now shows a setup for determining the degree of curing of the individual printing inks (CMYK), in particular by determining the degree of curing of the lowest ink layer (K) of the degree of curing control field 3.

[0037] The sketch shows a degree of curing control field 3 consisting of KCMY ink layers printed one above the other. This degree of curing control field 3 lies on a printing substrate 1. The degree of curing of the bottommost ink layer K is then measured using an IR measuring device 10. For this purpose, the measuring head 13 of the IR measuring device 10 is brought into contact with the degree of curing control field 3. The measuring head 13 of the IR measuring device 10 then emits IR radiation 11. The position of the measuring head 13 with respect to the different KCMY ink layers of the degree of curing control field 3 is determined such that the IR radiation 11 has a penetration depth into the degree of curing control field 3 that is sufficient to reach the bottommost ink layer K without causing reflections on the printed sheet 1. Based on this penetration depth of the IR radiation 11, electromagnetic radiation is now reflected by the individual color layers.This reflected radiation 12 is measured by the measuring head 13 of the IR measuring device 10.

[0038] In order to achieve a corresponding positioning of the measuring head 13 with respect to the KCMY color layers, the IR measuring device 10 is connected to a control device 14, which enables a lowering or raising of the IR measuring device 10 or even of the measuring head 13 alone in order to achieve an optimal position.

[0039] The measuring head 13 can, for example, be a germanium crystal, which penetrates the CMYK ink layers by a distance of less than 0.6 µm. The contact pressure specified by the control device 14 can lead to mixing at least between the lowest ink layer K and the overlying ink layer C if these are not fully cured. If such mixing is detected, it always means that the ink layers are not fully cured. The corresponding positioning of the germanium crystal, ieof the measuring head 13 can be determined by a measuring method in which different penetration depths are specified and different contact pressures are tried out, the IR radiation 11 and the reflected radiation 12 are measured and then information is collected about the actual penetration depth by means of scanning electron microscopy images and the effective quality of the measurement is evaluated and values ​​are determined from this in order to achieve an optimal contact pressure and optimal positioning of the measuring head 13. A standard measuring device can itself be used as the IR measuring device 10. In principle, an inline measurement on the hardening degree control field 3, 3' is conceivable with corresponding modifications to the measuring method and the measuring device itself, but an offline measurement using the standard measuring device, ie the IR measuring device on a separate work table, is preferred.An inline measurement would always be preferable without contact. To determine the optimal positioning of the measuring head 13, knowledge of the characteristic reflection behavior of the printing substrate can be used.

[0040] The data thus determined by the IR measuring device 10 are transmitted to an evaluation device 15. This evaluation device 15 uses the measured values ​​to determine the degree of curing of at least the bottommost ink layer (K) and then infers the degree of curing of all remaining ink layers. In particular, by determining a completely cured bottommost ink layer K, it is possible to reliably infer that the CMY ink layers are cured. This result determined by the evaluation device 15 can then be transmitted to an output device 16, e.g., a monitor. Furthermore, this result of the evaluation device 15 can also be transmitted to a control device 17 for controlling a printing press 18.If the evaluation device 15 determines that the degree of curing of the lowest ink layer K, and thus the degree of curing of all ink layers, does not reliably result in complete curing of all ink layers on the printed sheet 1, the control device 17 can adjust the printing press 18 to such an extent that either the flow rate of the printed sheets 1 through the individual UV dryers 23 is reduced and / or the intensity of the individual UV dryers 23 is increased. By subsequently measuring the degree of curing control fields 3, 3' using the IR measuring device 10, it can then be determined whether the changes made by the control device 17 have been successful.In this way, simply by using a curing degree control field 3, 3', which overprints all the printing inks used, in particular the black printing ink, in particular at the bottom, in full tone, it can be reliably concluded that all the printing inks in the print image on the printing sheet 1 are completely cured. If this complete curing is not detected, the control device 17 can control and modify the printing press 18 to such an extent that the complete drying of all the printing inks is achieved. List of reference symbols 1 sheet 2 print process control strips 3, 3' Hardness level control field 4, 4' hardness control field 5, 6, 7 measuring fields 10 IR measuring device 11 IR radiation 12 reflected radiation 13 Measuring head 14 Control device 15 Evaluation device 16 Output device 17 Control device 18 printing press 19, 20, 21, 22 printing units 23 UV dryers 30 color separation

Claims

[1] Method for determining the degree of curing of printing inks (CMYK) on a printing substrate (1), wherein first color separations (30) for at least 4 printing colors (CMYK) are generated according to a printing template, according to the provided color separations (30), printing inks (CMYK) are transferred to the printing substrate (1) in printing units (19, 20, 21, 22), the color separations (30) comprise at least one control field in addition to a desired print image and the degree of curing of at least one position in the printed image is determined by means of a measuring device (10), characterized by , that the least one control field is a hardening degree control field (3, 3'), that the degree of hardening control field (4, 4') is generated in at least 4 color separations (30), the degree of curing control field (3, 3') is produced by overprinting at least 4 printing inks (CMYK) on the printing material (1), wherein the color value of the individual printing inks (CMYK) of the degree of curing control field corresponds to a solid tone in the respective color values ​​and the color black (K) is transferred to the printing material (1) as the lowest printing ink, and the degree of curing of the printing inks (CMYK) is determined by measuring the degree of curing control field (3, 3') after printing on the printing material (1) with the measuring device (10), taking into account the degree of curing of at least the lowest printing ink (K). [2] Method according to claim 1, characterized by , that an IR measuring device (10) which measures in the infrared range is used as the measuring device, the IR measuring device (10) has a measuring head (13), the measuring head (13) is brought into contact with the hardening degree control field (3, 3') by a control device (14) and a degree of curing of the printing inks (CMYK) is detected based on the reflected infrared radiation (12) of the IR measuring device (10). [3] Method according to claim 2, characterized by that the contact pressure of the measuring head (13) is regulated by the control device (14) so ​​that the penetration depth of the infrared radiation (11) of the IR measuring device (10) is sufficient to determine the degree of curing of the lowest ink layer (K) and / or the penetration depth of the infrared radiation (11) is limited so that there are no reflections by the printing material (1) itself. [4] Device for determining a degree of curing of printing inks (CMYK) on a printing material (1) for carrying out a method according to at least one of the preceding claims, comprising a printing machine (18) with printing units (19, 20, 21, 22) for printing a printing material (1) with at least 4 printing inks (CMYK), characterized by , that the printing material (1) has at least one hardness control field (3, 3') and a measuring device (10) is provided for determining a degree of curing of the degree of curing control field (3, 3'), that the measuring device is an IR measuring device (10) which measures reflections in the near infrared range, the IR measuring device (10) has a measuring head (13), the measuring head (13) is configured such that at least the reflections of the lowest printing ink (K) are used to determine the degree of curing without reflections occurring on the surface of the printing substrate (1), that a control device (14) is included which adjusts the position of the measuring head (13) in a controlled manner such that reflections of the lowest ink layer (K) are measured without reflections occurring on the printing substrate (1), and that the curing degree control field (3, 3') is formed by overprinting at least 4 printing inks (CMYK) or varnishes in full tone.

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