EP3590723A1 - Method for printing and drying - Google Patents

Abstract

The invention relates to a method for printing and drying, wherein an ink containing water and humectant is printed on a substrate using the ink printing method, whereby the moisture of the substrate is increased, and first of all during a first drying step (3a) the water of the printed ink is essentially is completely evaporated and then the humectant is evaporated during a second drying step (3b). The invention makes it possible to optimally dry substrates printed with ink and to prevent problems which can be caused by humectants on the substrate.

Description

The invention relates to a method having the features of claim 1.

Technical field and state of the art

The invention lies in the technical field of the graphics industry and there in particular in the area of printing and drying of inks containing water and humectants, thus in the area of so-called inkjet printing.

Inks are known which contain humectants (or evaporation inhibitors or "humectants") which ensure that the ink on the ink print head or its nozzle surface does not dry out. Such humectants undesirably get onto the substrate, e.g. on paper.

From the two documents EP 3 012 110 A1 and EP 3 034 309 A1 an inkjet printing machine with a dryer is known, whereby the combined treatment with high frequency and infrared (or hot air) is disclosed with the aim of uniformly printing printed and unprinted areas of the substrate, ie drying the substrate free of tension. However, the problem can arise here that humectants remain on the substrate and interfere with its further processing, for example by coating.

task

It is therefore an object of the invention to provide a technical solution which is improved over the prior art and which, in particular, makes it possible to optimally dry substrates printed with ink, ie in particular to prevent problems which can be caused by moisturizers on the substrate.

Solution according to the invention

This object is achieved according to the invention by a method with the features of claim 1. Advantageous and therefore preferred developments of the invention result from the subclaims and from the description and the drawings. The features of the invention, the developments of the invention and the exemplary embodiments of the invention also represent advantageous developments of the invention in combination with one another.

In the course of extensive printing laboratory investigations to solve this task, it was found that optimal drying takes place in two steps, the water and then the humectant being evaporated first.

The invention relates to a method for printing and drying, in which an ink containing water and humectant is printed on a substrate using the ink printing method, the moisture of the substrate being increased, and first of all the water of the printed ink being evaporated substantially completely during a first drying step and then the humectant is evaporated during a second drying step.

The invention advantageously enables substrates printed with ink to be optimally dried. Problems that can be caused by humectants on the substrate can be prevented.

The second drying step can preferably take the form of a brief burst of heat which essentially completely removes the humectant from the substrate surface and thereby protects the substrate.

The substrate can be an absorbent substrate, for example paper or cardboard; Water from the ink can get onto such a substrate and then into the substrate. Furthermore, the substrate can be a laminated substrate, for example laminated paper or laminated cardboard; Such a substrate may already contain water.

Developments of the invention

A preferred development of the invention can be characterized in that the evaporation of the water takes place during a first predetermined period of time with hot air and / or with radiation primarily coupling to the water molecules or to pigments contained in the ink.

A preferred development of the invention can be characterized in that the water is evaporated i) with hot air or ii) with hot air and infrared radiation, or iii) with ultraviolet radiation, with high-frequency radiation or with microwave radiation.

A preferred development of the invention can be characterized in that the duration of the first predetermined time period is in the range between 1 and 5 seconds.

A preferred development of the invention can be characterized in that the evaporation of the humectant takes place during a second predetermined period of time with hot air and / or with radiation primarily coupling to the humectant or to pigments contained in the ink.

A preferred development of the invention can be characterized in that the evaporation of the humectant takes place with hot air and / or with ultraviolet radiation.

A preferred development of the invention can be characterized in that the evaporation of the humectant with hot air and / or with ultraviolet radiation with at least one wavelength in the range from 365 to 405 nanometers or with at least one wavelength in the range from 365 to 395 nanometers or with a Wavelength of 385 nanometers.

A preferred development of the invention can be characterized in that the duration of the second predetermined time period is in the range between 0.1 and 0.5 seconds.

A preferred development of the invention can be characterized in that the first drying step is carried out at a temperature in the range between 50 ° C. and 100 ° C., preferably just below 100 ° C.

A preferred development of the invention can be characterized in that the second drying step takes place at a temperature in the range between 150 ° C and 250 ° C.

A preferred development of the invention can be characterized in that after the second drying step the substrate is cooled with blown air, preferably with ambient air sucked in and particularly preferably with cooled ambient air. This can advantageously prevent heat from entering the substrate, or deeper layers of the substrate, from heat conduction and from undesirably increasing its mean temperature.

Figures and embodiments of the invention

The invention and its preferred developments are described below with reference to the drawings, ie Figures 1 to 3 , described in more detail using a preferred embodiment. Features corresponding to one another are provided with the same reference symbols.

Figur 1:

Ablaufplan eines bevorzugten Ausführungsbeispiels des erfindungsgemäßen Verfahrens;

Figur 2:

Temperaturverläufe bei dem bevorzugten Ausführungsbeispiel des erfindungsgemäßen Verfahrens; und

Figur 3:

Tintendruckmaschine bei der Durchführung des bevorzugten Ausführungsbeispiels des erfindungsgemäßen Verfahrens.

The figures show:

Figure 1:

Flow chart of a preferred embodiment of the method according to the invention;

Figure 2:

Temperature profiles in the preferred embodiment of the method according to the invention; and

Figure 3:

Ink printing machine in the implementation of the preferred embodiment of the method according to the invention.

The in Figure 1 The flowchart shown of a preferred exemplary embodiment of the method according to the invention shows the following steps:
Method step 1 (provision): The substrate and the ink to be printed on the substrate are provided for carrying out the method according to the invention, in particular in a printing press. The substrate can be paper, cardboard, cardboard or corrugated cardboard. The ink is a water-based ink, ie it contains water as a solvent. The ink also contains at least one humectant.

Process step 2 (printing): The ink provided is applied to the substrate provided in the ink printing process by image-wise transfer of ink drops. Here, the ink also gets water and humectant on the substrate. The water increases the moisture of the substrate or its moisture content.

Process step 3 (drying) and in particular 3a (first drying step): First of all, the water of the printed ink is essentially completely evaporated. This evaporation is preferably carried out by exposure to i) hot air or ii) hot air and infrared radiation, or iii) ultraviolet radiation, high-frequency radiation or microwave radiation.

Method step 4a (stipulation of the first stipulated time period): The first stipulated time period t ₁ is stipulated for the duration of drying 3, ie the first drying step 3a. This specification can be done by a computer of the printing press and corresponding control of a dryer and / or the printing press or their transport speed. For example, in the case of a dryer which can be switched in segments, its drying path can be controlled. Alternatively, the transport speed of the substrate can be controlled. Another alternative is that Providing a dryer with a fixed drying section, which corresponds to the time span t ₁ at a fixed transport speed.

Process step 3 (drying), in particular second drying step (3b): after step 3a, the humectant is evaporated. This evaporation is preferably carried out by applying hot air and / or ultraviolet radiation.

Method step 4b (specification of the second specified time period): The second specified time period t ₂ is specified for the duration of the drying 3, ie the second drying step 3b. The specification can be made in accordance with step 4a and the alternatives described there.

In the Figure 2 temperature curves shown are described below. The diagram has a time axis and a temperature axis above it.

The diagram in Figure 2 shows the temperatures of the substrate 8 on its surface (temperature profile 5) and on average (temperature profile 6) during the drying process. In the first phase during the period t ₁ , energy is continuously supplied to the substrate. This phase preferably lasts about 1 to 2 seconds. The surface temperature rises noticeably. Because of the heat conduction of the substrate, the temperature in the deeper layers of the substrate also rises, and thus on average with a time delay.

With increasing temperature, the evaporation of the water increases exponentially. Especially in the last part of the first phase, when the target temperature T ₁ for evaporating the water is exceeded at about 90 ° C, the largest amount of water evaporates. In this first phase, the energy cannot be supplied as quickly as desired without the substrate bursting due to the water vapor forming inside. Hence the relatively long drying time of 1 to 2 seconds. At the end of this first phase, the water transferred to the substrate has essentially completely evaporated.

If one continued with a uniform supply of energy (curve 5, dashed section in phase t ₃ ), the temperature would now rise more quickly. Since water no longer evaporates, there is no cooling effect (enthalpy of vaporization). This would continue until the target temperature T ₄ of approximately 130 ° C. for the evaporation of the humectants was sufficiently exceeded, so that sufficient evaporation inhibitors can evaporate during the time of the high surface temperature. The average temperature (curve 6, dashed section in phase t ₃ ) of the substrate would also increase. The substrate would lose much of its natural moisture. It would shrink and become brittle. A lot of energy would be used in this way, especially with thick substrates, just to heat the top layer of the substrate.

Better (and provided according to the invention) is a so-called "heat shock" in a second phase t ₂ . The drying energy still required is introduced in as short a time as possible. A bursting of the paper as in phase t ₃ is not to be feared here, since hardly any steam can form because the substrate is already dry. The phase t ₂ is preferably only a few tenths of a second long. The surface temperature (curve 5) rises so quickly that the average temperature (curve 6) increases only slightly due to the limited thermal conductivity of the substrate. Only the top layer is preferably briefly heated, the lower layers in the substrate heat up less. Accordingly, the surface temperature suddenly drops again after phase t ₂ because it is cooled down by the deeper layers of the substrate. Overall, less energy is introduced into the substrate. Especially with thick substrates (eg 200 to 800 g / m ² ), the drying according to the invention saves a lot of energy. So that the same amount of humectant can evaporate again in the steeper and shorter temperature peak as in the slower process corresponding to phase t ₃ , the peak temperature is preferably somewhat higher.

The "heat shock" can be done by a selective or uniform drying process. In a selective process, the energy only couples in where ink has been applied. That would be, for example, ultraviolet radiation with a wavelength of 385 nm. Several UV lamps can be arranged in such a way that they light up focus on one spot. Alternatively, the light from several UV lamps can be concentrated using suitable lenses or mirrors.

With a uniform drying process, the energy is injected everywhere, including at non-printed areas. However, drying of the unprinted areas is not desirable. However, since essentially only the uppermost layer is heated during the "heat surge", the drying takes place only in the uppermost layers. For example, hot air can be blown onto the sheet. The air is preferably between 200 and 500 ° C warm. Large amounts of air are preferably blown onto the paper from small nozzles (diameter <5mm) and from a short distance (less than 20mm). Because of the high air velocities, this results in a good heat transfer coefficient. Together with the high air temperature, the necessary energy can be introduced within a few tenths of a second.

T ₂ denotes in Figure 2 the mean peak temperature with heat shock, T ₃ the mean peak temperature with normal heating.

In the Figure 3 The ink printing machine shown is described below.

Fig. 3 shows a printing machine 14 with a feeder 7 for an arcuate substrate 8. After the application of the water-based inkjet inks in the printing unit 9 with the print heads 9a, the drying (first drying step 3a) is carried out by means of several first dryers 10. These can preferably be operated with hot air , combined with IR and hot air, or with UV, or microwave or high-frequency fields. The dryers are arranged on cylinders. The drying (second drying step 3b) then takes place with the second dryer 11 for the heat shock (preferably immediately after the last first dryer, with the sheet still warm). The dryer 11 can be followed by a cooling device 11a or a cooler 11a, preferably for blowing air or cooled desire on the substrate 8.

Alternatively, the sheet can also be guided on belts or in grippers that are guided linearly. Alternatively, a sheet-like substrate can be dried.

LIST OF REFERENCE NUMBERS

1

Provision (substrate, ink)

2

To press

3

dry

3a

first drying step / evaporation of the water

3b

second drying step / evaporation of the humectant

4a

Specify the first specified period of time

4b

Specifying the second specified time period

5

temporal temperature profile of the surface temperature

6

temporal temperature profile of the mean temperature

7

investor

8th

substratum

9

printing unit

9a

printheads

10

several first dryers

10a

Hot air and / or radiation

11

second dryer

11a

Cooler / condenser

12

boom

13

computer

14

press

t ₁

first predetermined time period / first phase

t ₂

second predetermined period / second phase

t ₃

third period / third phase

T ₁ - T ₄

temperatures

Claims (11)

Process for printing and drying, wherein an ink containing water and humectant is printed on a substrate by means of the ink printing process, whereby the moisture of the substrate is increased, and wherein firstly during a first drying step (3a) the water of the printed ink is evaporated substantially completely and then the humectant is evaporated during a second drying step (3b). Method according to claim 1,
characterized,
that the evaporation of the water (3a) takes place during a first predetermined period (t ₁ ) with hot air (10a) or with radiation (10a) primarily coupling to the water molecules or to pigments contained in the ink. Method according to claim 2,
characterized,
that the evaporation of the water (3a) i) with hot air or ii) with hot air and infrared radiation, or iii) with ultraviolet radiation, with high-frequency radiation or with microwave radiation. Method according to one of claims 2 or 3,
characterized,
that the duration of the first predetermined time period (t ₁ ) is in the range between 1 and 5 seconds. Method according to one of the preceding claims,
characterized,
that the evaporation of the humectant (3b) takes place during a second predetermined period (t ₂ ) with hot air (11a) and / or with radiation (11a) primarily coupling to the humectant or to pigments contained in the ink. Method according to claim 5,
characterized,
that the evaporation of the humectant (3b) takes place with hot air and / or with ultraviolet radiation. Method according to claim 5,
characterized,
that the evaporation of the humectant (3b) takes place with hot air and / or with ultraviolet radiation with at least one wavelength in the range from 365 to 405 nanometers or with at least one wavelength in the range from 365 to 395 nanometers or with a wavelength of 385 nanometers. Method according to one of claims 5 to 7,
characterized,
that the duration of the second predetermined time period (t ₂ ) is in the range between 0.1 and 0.5 seconds. Method according to one of the preceding claims,
characterized,
that the first drying step (3a) takes place at a temperature in the range between 50 ° C and 100 ° C. Method according to one of the preceding claims,
characterized,
that the second drying step (3b) takes place at a temperature in the range between 150 ° C and 250 ° C. Method according to one of the preceding claims,
characterized,
that after the second drying step (3b) the substrate is cooled with blown air.

Images