EP3867076B1 - Method for printing on a surface of a nonabsorbent substrate with an ink to be applied by an inkjet printing device - Google Patents

Description

The invention relates to a method for printing a surface of a non-absorbent substrate with an ink to be applied by an inkjet printer according to claim 1.

through the EP 2 617 577 A1 An ink jet recording method is known in which an image is recorded on a polyolefin-containing target recording surface by ejecting an ink composition containing water, a colorant and resin from nozzles of an ink jet recording head.

through the WO 2018/168 675 A1 a method is known for applying an oil-based ink to a substrate having a resin layer, wherein the substrate is pre-treated to improve wetting properties before application of the oil-based ink and post-treated after application of the oil-based ink.

through the U.S. 2017/0190188 A1 an image recording method is known in which a recording substrate is subjected to a surface treatment by exposing an image recording surface of the recording substrate to light from an excimer emission using a xenon gas, the recording substrate being a substrate made of non-absorbing or weakly absorbing fibrous materials, and in which a ink composition is applied by an ink jet method onto the image recording surface of the recording substrate after the surface treatment.

through the US 2018/0236787 A1 an ink jet recording method is known which comprises applying a treatment liquid in which there is a content of a nitrogenous solvent, which content is larger than that in an ink composition, wherein there is little or no absorptivity to the recording medium, wherein surface asperities are equal to or larger than 10 µm, wherein the ink composition containing a nitrogen-containing solvent is carried out by ejecting the ink composition onto the recording medium from an ink jet head, the processing liquid having been previously applied onto the recording medium.

through the U.S. 9,573,349 B1 a method for printing non-absorbent substrates with a water-based ink is known, in which ink drops are applied to the respective substrate, in which substrates are used, each of which has a surface tension of less than 45 mN/m, in which prior to the application of the ink drops the surface tension of the substrates to be printed is increased to a value of at least 45 mN/m, at which point the ink droplets are only then applied to the respective substrate having the increased surface tension.

Of the US 2012/0026264 A1 can be seen, a printed substrate by heat, z. B. to dry by infrared radiation, a range between 40 ° C and 100 ° C and for the exposure time a range between 0.2 s and 10 s is proposed for the drying temperature.

through the CH 703 704 A1 discloses a printing device for printing flat structures in the form of tin plates with at least one printing ink, the surface tension of this flat structure being less than the surface tension of the printing ink, comprising at least one printing unit that can be activated and a final drying system downstream of this at least one printing unit that can be activated, for drying the at least one printing unit printing ink applied to the surface structure in an activatable printing unit, with at least one irradiation device having at least one electromagnetic radiation source each for irradiating and increasing the surface tension of the surface structure before it is fed to the first activatable printing unit being configured upstream of the first printing unit that can be activated in the process sequence, and with the radiation source being an elongated UV -Gas discharge tube or an elongated infrared heater.

through the DE 10 2012 017 538 A1 a method for treating the surface of objects with fluids is known, in which the fluid is applied using a first tool that can be moved under computer control and then further treated with the second tool that can be moved under computer control, the time delay between the application of the fluid to the object and the subsequent further treatment is adapted to the spreading behavior of the fluid.

The object of the invention is to create a method for printing a surface of a non-absorbent substrate with an ink to be applied by an inkjet printing device, with which non-absorbent substrates are printed with a print image of high print quality, in particular while avoiding the ink droplets spreading, despite the use of a water-based ink will.

The object is achieved according to the invention by a method having the features of claim 1. The dependent claims relate to advantageous refinements and/or developments of the solution found.

The advantages that can be achieved with the invention are, in particular, that knowledge of the properties of the surface of the substrates to be printed that are actually present before printing and that influence wetting is not required, but that the substrates to be printed are preferably brought to a good, i.e . H. at least partial to very good, i. H. Complete wettability can be added, while at the same time possible negative effects on the printed substrates are avoided, or at least limited, by this measure.

Another advantage is a very flat print with a low haptic ink structure, ie a low relief formation, and a print with a high degree of gloss, which z. B. frequently asked requirements in the packaging market be fulfilled in a simple way. Moreover, substrates printed in this way can be formed without disruptive abrasion in the forming machine. This is also a good prerequisite for overcoating and a small and therefore environmentally friendly application of ink with water as an environmentally friendly solvent.

The proposed solution is illustrated on the basis of the single figure.

There is e.g. B.in packaging printing, the need to print non-absorbent substrates in an industrial printing process with at least one water-based ink. These substrates are z. B to preferably consist of a solid especially flat substrates, z. B. substrates each made of a metallic material, in particular sheet metal, z. B. from a steel sheet or from a tinplate or from an i. i.e. R. surface-oxidized aluminum material. Alternatively, non-absorbent substrates made of, in particular, pre-lacquered wood, of a plastic or of a composite material can also be considered, provided these substrates are not damaged by heating during the printing process, e.g. B. dimensionally stable. The substrates are preferably in the form of plates or slabs; B. but also be formed in web form or each as a round body.

In particular, metallic substrates are usually treated with a primer, e.g. B. with a white ground, and / or pre-coated with a varnish. These substrates have z. B. an oiled or greased surface or treated with an anti-corrosion agent. The surface of the substrates to be printed is in particular hydrophobic. For printing on these substrates, a z. B. pigment-based, especially soluble ink with a water content of at least 70%, especially at least 80% is used. If the substrate in question is intended to be further processed into packaging for food, an ink without photoinitiators is preferably used. To carry out the present invention z. B. substrates with a non-polar surface and to print on this surface, a polar ink is used.

The respective ink is applied to the respective substrate by an inkjet printing device in the form of ink drops, in particular through the use of at least one inkjet print head that ejects the ink drops. B. during a monodirectional movement (single pass method) or during a bidirectional movement (multi pass method) of the push button and / or the substrate in question are applied to the same. The ink drops z. B. in a halftone printing process z. B. each in a dot density of at least 360 dpi, z. B. of 600 dpi or 1,200 dpi applied to the respective substrate. In order to generate a predetermined print image, the ink drops are usually applied in a grid having a plurality of positions in a precisely positioned manner on the relevant substrate. It can be provided that ink drops of variable size and/or that ink drops with different hues and/or that ink drops with different brightness intensities are respectively applied to the respective substrate. For a multicolored printed image structure, color points from a basic color set, e.g. B. CMYK used.

In order to be able to print the substrates successfully, the surface tension of the substrates in question must be at least as high as the surface tension of the ink used. Only in this way can at least partial wetting of the respective surface of the relevant substrates with the intended ink take place. Otherwise, the ink drops applied to the non-absorbent surface of the substrates in question will contract and wetting will not be possible. If at all, if the surface tension of the substrates in question is insufficient, the result is at least an unclear, in particular blurred or pale or color-untrue and therefore unusable printed image. The surface tension of the relevant substrates and the surface tension of the ink used are usually temperature-dependent. Other environmental conditions such. B. the humidity or certain surface properties of the substrates in question, such as their roughness and / or their pretreatment and / or their degree of contamination affect the wettability of the respective surface of these substrates.

The surface tension of solid substrates can e.g. B. be determined with commercially available test inks. The surface tension of a water-based ink can e.g. B. using the ring method by Lecomte De Noüy or using the plate method by Wilhelmy or using the ironing method by Lenard with a tensiometer or by the capillary effect or approximately by a comparative measurement with test inks.

At room temperature, e.g. B. in the surrounding air with a temperature of 20 ° C can be present for metallic substrates in solid form, ie in particular for sheet metal z. B. assume the following values for their surface tension: untreated steel 29mN/m aluminum (oxide) 33-35mN/m tinned steel approx. 35 mN/m phosphated steel 43-46mN/m

Oily or greased surfaces have a surface tension of around 30 mN/m.

In the following it is assumed that substrates are used which each have a surface tension of less than e.g. 50 mN/m, preferably less than 45 mN/m. Therefore i. i.e. R. each at the same room temperature of z. B. 20°C the surface tension of the substrates intended for printing is lower than the surface tension of the water contained as a solvent in the ink and i. i.e. R. also lower than the surface tension of the ink used for printing itself.

At standard pressure (SATP), i.e. at 0.1 MPa corresponding to 1 bar and at a temperature of 20°C, water has a surface tension of 72.74 mN/m and at a temperature of 50°C a surface tension of 67.95 mN/m m.

In order to achieve good printing results, it is therefore provided that in the machine executing the printing process, e.g. B. in a printing machine, in particular in a printing formless working digital printing machine having at least one inkjet print head, before applying the ink drops to the surface of the substrate to be printed, the surface tension of this surface z. B. by at least 10%, preferably by more than 40% in each case to a value of at least 45 mN/m, in particular at least 50 mN/m, preferably to a value of over 70 mN/m. Only after this increase in surface tension, i. H. after the surface of the substrate to be printed has a sufficiently high surface tension, the ink drops are applied to the respective i. i.e. R. applied to the substrate having increased surface tension.

In order to improve the wettability of non-absorbent surfaces intended for printing with a water-based ink, in particular metallic surfaces, the surface tension of the substrates in question can be increased, e.g. B. by phosphating these substrates and / or by applying a suitable coating, z. B. be increased by applying a white base and / or a special coating. Alternatively or additionally, the surface tension of the substrates in question can be increased by pretreating them with a corona method or with a plasma method or with a chemical method or by flame treatment or by UV irradiation or with other methods of surface activation. When increasing the surface tension of the substrates to be printed, a value is sought and set that not only reaches a minimum value for the preferably complete wettability of the surface of the substrates in question, but i. i.e. R. significantly exceeds.

The surface tension of the substrates in question, which is increased beyond the minimum value for wettability, leads to a mostly strong spreading of the ink drops applied to their surface, ie to an undesired spreading of these ink drops on the surface of the substrates in question. The spreading of the ink drops has a very rapidly changing color property in the printed image Consequence and / or leads to an unwanted flow of ink z. B. into micro-recesses of the substrate in question. Another effect when using ink drops with different color tones is that these ink drops bleed, which can be caused by e.g. B. by a merging of different color dots and thus leads to completely undesirable color effects in the printed image, not only in color, but also in the sharpness of contours of printed structured surfaces such. B. in a font or on an image edge.

It is therefore advantageous, preferably directly, i. H. within just a single second, preferably within 0.5 seconds after the printing of the substrates, by a particularly shock-like (over) drying of the ink droplets, stopping them from spreading or even largely preventing them. This happens in the immediate vicinity of the inkjet printer. In the case of a binder-based ink, this drying causes the at least one binder to gel, which also prevents or stops the ink droplets from spreading on the substrate in question. The shock-like (over) drying of the ink drops to stop or prevent them from spreading is also referred to as pinning. By pinning, ink drops applied to the substrate are fixed in their respective position on the surface of the substrate to be printed.

the 1 illustrates the proposed method for printing a surface of a non-absorbent substrate 01 with an ink to be applied by an inkjet printing device 02. The substrate 01 is printed in an industrial printing process, preferably using a digital printing machine that has at least one inkjet printing device 02 and, for this purpose, e.g. B. transported along a transport route 09. In the transport direction T of the substrate 01, which is indicated by directional arrows, there is generally first a device 03 for pretreating the substrate 01 to be printed, in the form of a device that increases the surface tension of the surface of the substrate 01 to be printed, with this pretreating device 03 increasing the surface tension of the surface to be printed Surface of the substrate 01 increased at least to the value of the surface tension of the Inkjet printing device 02 corresponds to the ink to be applied to the surface of substrate 01. This device 03, which increases the surface tension of the surface of the substrate 01 to be printed, pretreats the substrate 01 to be printed, e.g. B. with a corona method or with a plasma method or with a chemical method or by flame treatment or by UV radiation.

The substrate 01 to be printed is placed on a z. B. table-shaped workpiece carrier 04 z. B. laid flat. The inkjet printing device 02 and the workpiece carrier 04 are designed and arranged relative to one another in such a way that the substrate 01 to be printed on, which is carried by the workpiece carrier 04, moves relative to the inkjet printing device 02, or such a relative movement can at least be carried out, with ink being applied to the surface by the inkjet printing device 02 during this relative movement of the substrate 01 to be printed is applied. The ink can be applied to the surface of substrate 01 by either inkjet printing device 02 or workpiece carrier 04 during a monodirectional movement (single pass method) or during a bidirectional movement (multipass method).

According to the invention, the workpiece carrier 04 is heated to a temperature above the air temperature surrounding it and the substrate 01 carried by the workpiece carrier 04 is in turn z. B. by thermal conduction of the workpiece carrier 04 within a very short time, z. B. in the range of less than 1 second, preferably even in the range of a few milliseconds, i.e. in less than 50 ms, in particular in less than 10 ms, with the ink applied to the surface of substrate 01 subsequently also being heated, in particular by thermal conduction of substrate 01 heated by the workpiece carrier 04 in practically just as short a time, possibly with a permissible deviation, ie a tolerance of at most 5° C., preferably at most 2° C., in particular to e.g. B. at least 90% of the elevated temperature of the heated workpiece carrier 01 is heated. The ink applied to the substrate 01 is a temperature that is significantly higher than the temperature of the air surrounding the substrate 01 is thereby applied practically suddenly. Although the temperature of the ink applied to the substrate 01 is above the temperature of the air surrounding the substrate 01, it is below a boiling temperature of the solvent contained in the ink, with water being used as the solvent contained in the ink. The heating of workpiece carrier 04, substrate 01 and ink applied thereto is in the 1 by e.g. B. obliquely upward arrows indicated. The workpiece carrier 04 is preferably heated by a heating device, with which the temperature to be set for it is preferably regulated or at least can be regulated.

This heating of the ink applied to the substrate 01 results in a vapor layer above the liquid phase of this ink. This is because volatile components of the ink, which are primarily the solvent contained in the ink, i.e. in particular the water, increasingly tend to change from the liquid phase of the ink into a gaseous phase as the temperature rises, resulting in the formation of vapor. Evaporation below the boiling point of the solvent contained in the ink, i.e. in particular the water, is also referred to as evaporation. The evaporation rate and the vapor pressure prevailing in the vapor are on the one hand u. also depends on the material composition of the ink and its rheological properties. On the other hand, in the arrangement described above, they can also be influenced by the surface tension of the ink and/or by the surface tension of the substrate 01 carrying the ink and its temperature.

In the embodiment according to the invention, it is provided that the vapor layer formed above the liquid phase of the ink is transported away by an air stream and thereby removed. The air flow is through at least one nozzle unit 06; 07 provided, the respective blowing nozzle unit 06; 07 ejects the air flow towards the substrate 01 under machine environmental conditions. In this case, from the respective nozzle unit 06; 07 air is usually heated and/or dehumidified beforehand. So e.g. B. in the inflow of the respective nozzle unit 06; 07 in each case a heating device that is preferably controlled or at least controllable in terms of the temperature to be set. The respective blowing nozzle unit 06; 07 with at least one z. B. line-shaped heating device can be supplemented, the respective heating devices z. B. are designed as a radiation source in the IR or near IR range and / or are designed as LED emitters. The relative humidity of the respective nozzle unit 06; 07 expelled air flow is preferably less than 65%. Of the respective nozzle unit 06; 07 provided air flow has a flow rate of z. B. more than 25 m / s, preferably more than 30 m / s, in particular more than 50 m / s or even more than 100 m / s. This air flow is directed at an angle of z. B. 0° to 90°, preferably in a range between 30° and 45°, onto the substrate 01 carrying the ink, with this air flow preferably being directed in the same direction as the transport direction T of the substrate 01 in question. A blow-out opening of the blow-out nozzle unit 06 in question, which is preferably designed in the form of a slot transversely to the transport direction T of the substrate 01 in question; 07 is arranged at a distance of 0.1 mm to 10 mm, preferably 2 mm, above the printed surface of the substrate 01 in question. The blowing nozzle unit 06 in question; 07 removes the vapor layer formed above the liquid phase of the ink, but does not displace the respective ink droplets applied to the substrate 01. In the case of a monodirectional relative movement (single pass method) between substrate 01 and inkjet printing device 02, e.g. g. a single blower nozzle unit 07 in the direction of transport T of the substrate 01 in question immediately after the inkjet printing unit 02. In the case of a bidirectional relative movement (multi-pass method) between the substrate 01 and the inkjet printing unit 02, it is advantageous to have an intermediate drying device immediately before and after the inkjet printing unit 02 respective substrate 01 and/or the ink applied to the surface of substrate 01 has a blowing nozzle unit 06; 07 to arrange ( 1 ). In a further embodiment, the blowing nozzle units 06; 07 not only in front of and behind the inkjet printing device 02, but also between individual segments and/or between different colored ones The nozzles of this inkjet printing device 02 that emit ink can be or will be arranged in order to enable even more immediate intermediate drying of the individual ink jobs.

As described above, it is advantageous that the ink applied to the surface of the substrate 01 is e.g. B. is dried by applying heat to the substrate 01 in question and/or to the applied ink in an intermediate drying process that differs from the heat conduction of the workpiece carrier 04, with this intermediate drying preferably being carried out by means of at least one blower nozzle unit 06; 07 is running. The substrate 01 carrying the intermediately dried ink is then fed to a (hot air and/or IR) final dryer 08, which is usually of conventional design and is spatially separated from the inkjet printing device 02, and e.g. B. through this final dryer 08, with only this final drying brings about a complete physical drying of the ink and, if necessary, its crosslinking. The intermediate drying of the ink application takes place during the printing process, i.e. practically at the same time as the printing process immediately after or before the printing passes, in particular by the respective blowing nozzle unit 06; 07 and is inextricably linked to the printing process. In the preferred embodiment, the intermediate drying of the application of ink is supported and/or intensified by heating the substrate 01. From the respective nozzle unit 06; 07 discharged, preferably jet-shaped air flow settles on the surface of the substrate 01 and usually dries an entire area on this substrate 01. The flow rate of the blowing nozzle unit 06; 07 determines the intensity of the drying of the ink applied to the surface of the substrate 01, this intensity increasing with the distance from the preferably slit-shaped blow-out opening of the respective blower nozzle unit 06; 07 decreases. In a preferred embodiment, the respective blowing nozzle unit 06; 07 in each case a plurality of layers lying one behind the other in the transport direction T of the substrate 01 in question, preferably transverse to the transport direction T of the substrate 01 in question, e.g. B. slit-shaped exhaust openings. The intermediate drying of the substrate 01 in question and/or the ink applied to the surface of the substrate 01 can be intensified by one or more heating devices arranged in particular in the area of the inkjet printing device 02. B. formed as a radiation source in the IR or near IR range and / or as an LED radiator and z. B. in the transport direction T of the substrate 01 in question immediately before or after the inkjet printing device 02 inside and/or outside of the respective blowing nozzle unit 06; 07 is arranged.

Reference List

01

substrate

02

inkjet printing device

03

pre-treatment facility

04

workpiece carrier

05

-

06

air nozzle unit

07

air nozzle unit

08

dryer

09

transport route

T

transport direction

Claims (15)

1. A method for printing on a surface of a nonabsorbent substrate (01) with an ink to be applied by an inkjet printing device (02), in which an ink containing water as a solvent is used to print on the substrate (01), wherein an ink with a water proportion of at least 70% is used, in which the substrate (01) to be printed is supported by a workpiece support (04) and is moved relative to the inkjet printing device (02), wherein during this relative movement by the inkjet printing device (02) ink is applied to the surface of the substrate (01) to be printed, wherein the ink applied to the surface of the substrate (01) is heated to a temperature which is above the temperature of the air surrounding the substrate (01) and below a boiling temperature of the solvent contained in the ink, wherein a vapor layer consisting of the solvent contained in the ink is formed above a liquid phase of the ink, wherein the vapor layer formed above the liquid phase of the ink is transported away by an air flow discharged by at least one blowing nozzle unit (06; 07), wherein the air output by the respective blowing nozzle unit (06; 07) is heated and/or dehumidified, characterized in that the workpiece support (04) is heated to a temperature above the surrounding air temperature, wherein the substrate (01) supported by the workpiece support (04) and the ink applied to the surface of the substrate (01) are respectively heated by thermal conduction of the workpiece support (04) with a permissible deviation to the increased temperature of the workpiece support (04), wherein the substrate (01) supported by the workpiece support (04) and the ink applied to the surface of the substrate (01) are heated to the increased temperature of the workpiece support (04) in each case by thermal conduction of the workpiece carrier (04) with a permissible deviation of no more than 5°C and/or to at least 90% of the increased temperature of the workpiece support (04).
2. The method according to claim 1, characterized in that the air flow is discharged from the respective blowing nozzle unit (06; 07) with a relative humidity of less than 65% and/or that the air flow is discharged from the respective blowing nozzle unit (06; 07) at a flow rate of more than 25 m/s.
3. The method according to claim 1 or 2, characterized in that the air flow discharged from the respective blowing nozzle unit (06; 07) is oriented at an angle of 0° to 90° proceeding from the contact plane of the relevant substrate (01) supported by the workpiece support (04) to the substrate (01) supporting the ink and/or that the air flow discharged from the respective blowing nozzle unit (06; 07) is aligned with the transport direction (T) of the relevant substrate (01).
4. The method according to claim 1 or 2 or 3, characterized in that the air flow discharged from the respective blowing nozzle unit (06; 07) is discharged from a plurality of air outlet openings arranged in tandem in the transport direction (T) of the relevant substrate (01) and/or that the air flow provided from the respective blowing nozzle unit (06; 07) is discharged from at least one slot-shaped air outlet opening respectively perpendicular to the transport direction (T) of the relevant substrate (01) and/or that the air flow provided from the respective blowing nozzle unit (06; 07) is discharged from air outlet openings arranged between individual segments and/or from air outlet openings arranged between nozzles of the inkjet printing device (02) applying differently colored inks.
5. The method according to claim 1 or 2 or 3 or 4, characterized in that the relevant substrate (01) supported by the workpiece support (04) is blown on by at least one respective air outlet opening of the blowing nozzle unit (06; 07) respectively arranged at a distance of 0.1 mm to 10 mm above the printed surface of the relevant substrate (01).
6. The method according to claim 1 or 2 or 3 or 4 or 5, characterized in that a heating device arranged in the respective blowing nozzle unit (06; 07) is regulated in the temperature to be set.
7. The method according to claim 1 or 2 or 3 or 4 or 5 or 6, characterized in that an intermediate drying of the relevant substrate (01) and/or of the ink applied to the surface of the substrate (01) is executed from the respective blowing nozzle unit (06; 07).
8. The method according to claim 7, characterized in that the intermediate drying of the relevant substrate (01) and/or of the ink applied to the surface of the relevant substrate (01) is enhanced by one or a plurality of heating devices arranged in the region of the inkjet printing device (02).
9. The method according to claim 8, characterized in that heating devices arranged directly after the inkjet printing device (02) are used in the transport direction (T) of the relevant substrate (01).
10. The method according to claim 8 or 9, characterized in that heating devices arranged within and/or outside of the respective blowing nozzle unit (06; 07) are used.
11. The method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10, characterized in that, after the printing process the substrate (01) supporting the ink is fed to an end dryer (08) spatially separated from the inkjet printing device (02), wherein a hot air end dryer and/or IR end dryer (08) is used as the end dryer (08).
12. The method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11, characterized in that the ink is applied to the surface of the relevant substrate (01) during a monodirectional relative movement between the substrate (01) and the inkjet printing device (02), single pass method, or that the ink is applied to the surface of the relevant substrate (01) during a bidirectional relative movement between the substrate (01) and the inkjet printing device (02), multi pass method.
13. The method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12, characterized in that the heating of the workpiece support (04) is performed by a heating device, in which the temperature to be set for the workpiece support (04) is regulated.
14. The method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13, characterized in that the surface tension of the surface of the substrate (01) to be printed is increased at least to the value which corresponds to the surface voltage of the ink to be applied to surface of the substrate (01) by the inkjet printing device (02), wherein the surface voltage of the surface of the substrate (01) to be printed is increased to the surface voltage of the surface of the substrate (01) in a device (03) pretreating the substrate (01) to be printed, wherein this device (03) carries out a pretreatment on the substrate (01) to be printed with a Corona process or with a plasma process or with a chemical process or by a flame treatment or by a UV irradiation.
15. The method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14, characterized in that at least one inkjet print head discharging the ink respectively in the form of ink drops is used as the inkjet printing device (02), wherein in a raster printing process in each case the ink drops are applied in a dot density of at least 360 dpi to the respective substrate (01).

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