EP3436271B1 - Printing press having an infrared dryer unit - Google Patents

Description

Technical background

The invention relates to a printing press with a printing unit for applying solvent-containing printing ink to a printing material, a transport device for transporting the printing material from the printing unit to a dryer unit, which comprises at least one infrared radiator for drying the printing material.

State of the art

Offset printing machines, lithographic printing machines, rotary printing machines or flexographic printing machines are used, for example, for printing sheet-shaped or web-shaped printing materials made of paper, cardboard, foil or cardboard with printing inks. Typical ingredients of printing inks are oils, resins and binders. In the case of UV-curable printing inks, curing and adhesion to the printing material are based on polymerization, which is triggered by photoinitiation using UV light. In the case of printing inks and varnishes containing solvents and, above all, water, drying is required, which can be based on both physical and chemical drying processes. Physical drying processes include the evaporation of solvents and their diffusion into the substrate, which is also known as "knocking away". Chemical drying means the oxidation or polymerization of printing ink ingredients.

There are transitions between physical and chemical drying. For example, knocking away the solvents can cause monomeric resin molecules to converge, so that they may polymerize more easily. Drying devices for drying the printed substrate thus serve to remove solvents and / or to trigger crosslinking reactions.

The DE 10 2005 046 230 A1 describes a rotary printing press with a printing unit for printing a printing sheet with printing ink, a coating device for applying a coating on the printed printing sheet. In the area of the sheet path, drying devices in the form of infrared emitters, which can also be designed as carbon emitters, are arranged after the printing unit and the coating device.

EP 0 495 770 A1 relates to an infrared radiator with a carrier plate on which at least one conductor track made of a material that generates heat by electrical energy is arranged.

Technical task

In such infrared radiators, a heating filament made of carbon or tungsten in the form of a coil or ribbon is enclosed in an inert gas-filled radiator tube, which is usually made of quartz glass. The heating filaments are connected to electrical connections that are inserted through one end or both ends of the radiator tube.

The heating filaments themselves have a very low thermal mass and therefore a fast response time in the range of 1 to 2 seconds. However, it can take several minutes until the entire IR dryer system consisting of quartz tube, filament, electrical connections and a reflector is in thermal equilibrium.

Since the printing stock runs at a web speed of 3 to 5 m / s in modern rotary printing machines and this speed is already available at the beginning, up to 1500 m of printing stock can be lost until thermal equilibrium is reached. With changing individual printing processes, these losses arise again with each printing process.

The higher the electrical power of the quartz tube emitters, the faster they reach the temperature of the IR dryer system. The increase in power not only increases the amount of energy emitted by the infrared radiator, which can lead to overheating of the printing material, but also changes the main wavelength of the emitted radiation, which shifts in the direction of the short-wave spectral range.

In the case of water-based printing inks, it is desirable that the main emission wavelength of the infrared radiators matches the absorption characteristics of the water, that is to say about 2.75 μm. The previous commercial infrared heaters therefore either have an adapted emission spectrum; then they have a low electrical power and require a comparatively large radiation area and accordingly a large heat capacity for a sufficiently large radiation power, which in turn requires comparatively long heating and cooling times of the infrared radiator and thus inertness of the dryer unit. Or the infrared emitters have a high electrical output and low inertia; then their emission spectrum is not optimally adapted to the absorption characteristics of the water.

Often, several infrared emitter tubes lying side by side form a surface emitter. In order to obtain a homogeneous radiation on the substrate, the distance between the surface heater and the substrate should be at least 1.5 times the center distance between the individual heater tubes if the heater tube longitudinal axes are aligned in the transport direction of the substrate. This comparatively high minimum distance between the surface emitter and the substrate leads to a low effective radiation intensity on the substrate level, which extends the reaction time within which the required radiation power is applied to the substrate.

However, a fast response time is particularly necessary with multi-color printing, before the substrate is either printed with the next color or finished with a varnish or turned in the printing machine for the purpose of printing the back. Because of the relatively short time in which the printing material stays between the printing units, the required radiation power must act on the printing material without the print image being damaged by overheating.

In addition, both short- and medium-wave infrared emitters with an emission wavelength in the range of about 1000-2750 nm must be actively cooled, in particular in narrow installation spaces, as are typical for printing presses, in order to protect them from overheating. A cooling air stream is often generated for this purpose, which blows the infrared radiators directly. However, it has been shown that cooling air flowing past the infrared radiator interacts with warm process air, which among other things serves to remove moisture and thereby change the temperature at the printing material and reduce the removal of moisture.

The invention is therefore based on the object of providing a printing press with a dryer device which is improved for drying solvent-based and in particular water-based printing ink with regard to homogeneity and speed of drying and in which the dryer unit does not require active cooling of the infrared radiator.

General description of the invention

This object is achieved on the basis of an infrared radiator of the type mentioned in the introduction in that the infrared radiator is designed as a flat heating element made of a dielectric heating element material which emits infrared radiation when heated and which has a heating surface facing the printing material to be dried and a contacting surface on which a heat conductor conductor made of an electrically conductive, noble metal-containing resistance material is applied, which is connected to an electrical contact to an adjustable power source.

In the printing press according to the invention, the infrared dryer unit comprises at least one heating element which has a heating surface facing the printing material to be dried. The heating surface emits infrared radiation in the direction of the substrate. It is flat and, in the simplest case, flat, but it can also have a structure and a flat geometric shape that deviates from flatness. The flatness of the heating surface results in an equally flat radiation field and enables the setting of a short distance between the substrate and the heating element. This contributes to the homogeneity and speed of drying; as explained in more detail below.

The heating element consists at least partially of a dielectric material. This is not electrically conductive and can therefore not be easily heated by direct current flow, but by heat conduction via the conductor of the heating conductor. The conductor track thus serves directly to heat the heating element. As a result of heating, the heating element material emits infrared radiation in the medium-wave wavelength range, which matches the absorption characteristics of water as closely as possible.

The heating element forms the actual element that emits infrared radiation. It can be made in several layers, but it is preferably made entirely of the dielectric heating element material. It is essential that the surface areas covered with conductor tracks consist of electrically insulating material in order to reliably prevent flashovers and short circuits between adjacent conductor track sections.

The heating element is contacted with the heating conductor, for example, via a contacting surface opposite the heating surface. This is in direct contact or in indirect contact - via an electrically insulating and heat-conducting intermediate layer - with the conductor track made of a resistance material.

1. (1) von einem edelmetallhaltigen Widerstandsmaterial. Das in dieser Hinsicht bevorzugte Widerstandsmaterial besteht zu mindestens 50 Atom-%, vorzugsweise zu mindestens 95 At.-% aus Elementen der Platingruppe. Die Platingruppe umfasst die folgenden Edelmetalle: Ru, Rh, Pd, Os, Ir, Pt. Diese liegen in reiner Form oder als Legierung untereinander oder mit einem oder mehreren anderen Metallen vor, insbesondere mit Au, Ag.
2. (2) von Widerstandsmaterial aus hochtemperaturfestem Stahl, Tantal, einer ferritischen FeCrAl-Legierung, einer austenitischen CrFeNi-Legierung, Siliziumcarbid, Molybdändisilicid oder einer Molybdän-Basislegierung. Dieses Werkstoffe, insbesondere Siliziumcarbid (SiC), Molybdändisilicid (MoSi₂), Tantal (Ta), hochwarmfester Stahl oder eine ferritische FeCrAl-Legierung wie Kanthal® (Kanthal® ist eine eingetragene Marke der SANDVIK INTELLECTUAL PROPERTY AB, 811 81 Sandviken, SE) sind an Luft oxidationsbeständig und kostengünstiger als Platingruppenmetalle.

The resistance material is infrared-capable in the sense that it is temperature-resistant up to at least 1000 ° C, ideally even in an oxidative environment, that it is electrically conductive, and that its electrical conductivity does not change significantly with temperature or the change in resistance is known. These conditions are met in particular:

1. (1) from a noble metal-containing resistance material. The preferred resistance material in this regard consists of at least 50 atom%, preferably at least 95 atom%, of platinum group elements. The platinum group includes the following precious metals: Ru, Rh, Pd, Os, Ir, Pt. These are in pure form or as an alloy with one another or with one or more other metals, in particular with Au, Ag.
2. (2) Resistance material made of high-temperature steel, tantalum, a ferritic FeCrAl alloy, an austenitic CrFeNi alloy, silicon carbide, molybdenum disilicide or a molybdenum-based alloy. This material, in particular silicon carbide (SiC), molybdenum disilicide (MoSi ₂ ), tantalum (Ta), high-temperature steel or a ferritic FeCrAl alloy such as Kanthal® (Kanthal® is a registered trademark of SANDVIK INTELLECTUAL PROPERTY AB, 811 81 Sandviken, SE) are resistant to oxidation in air and less expensive than platinum group metals.

The conductor track is preferably used as a thick film layer, for example made from resistance paste by means of screen printing or from metal-containing ink using ink jet printing generated and then baked at high temperature. The conductor track runs, for example, in a spiral or meandering line pattern. The high absorption capacity of the heating element material enables homogeneous radiation even with a comparatively low conductor occupancy density of the heating surface. A low occupancy density is characterized in that the minimum distance between adjacent conductor track sections is 1 mm or more, preferably 2 mm or more. A large distance between the conductor track sections prevents flashovers, which can occur in particular when operating at high voltages under vacuum. The conductor track can be at least partially covered with a cover layer made of an electrically insulating and / or optically scattering material. The cover layer serves as a reflector and / or for mechanical protection and for stabilizing the conductor track.

The heat conductor conductor track is connected to an electrical contact via which it can be connected to a circuit. The electrical contact can preferably be releasably connected to a circuit via the electrical contact, for example via a plug, screw or clamp connection.

The flat shape of the heating element and the infrared emission enable a flat-homogeneous radiation of infrared radiation and the associated reduction in the distance between the printing material and the heating element. This makes it possible to provide a higher radiation output per unit area and to produce a homogeneous radiation and a uniform temperature field even with thin heating element wall thicknesses and / or with a comparatively low conductor occupancy density.

Due to the uniform radiation and high emissivity, the distance between the substrate and the heating element can be small, which increases the radiation intensity and increases the efficiency accordingly. The distance is preferably less than 15 mm.

The short distance enables high power densities of more than 100 kW / m ² and even more than 200 kW / m ² on the substrate and leads to a reduction in waste in modern high-performance printing machines. The heating element is preferred to achieve a power density above 180 kW / m ² , preferably to achieve a power density in the range from 180 kW / m ² to 265 kW / m ² . The area power is defined as the electrical connection power of the conductor track in relation to the base body area occupied by the conductor track.

The temperature on the substrate is regulated and moisture is removed by forced flow of warm process air. The removal of moisture depends on the absorption capacity of the process air (mainly determined by the temperature) and its degree of influence on the substrate (mainly determined by flow properties). Thin heating elements have a low heat capacity and enable rapid temperature changes. Active cooling by means of cooling air flowing past the infrared radiator is therefore not necessary. As a result, when using the printing press according to the invention, interactions with the warm process air are avoided which affect their temperature and flow behavior and which reduce the temperature of the printing material and the warm process air and thus slow down the removal of moisture.

In view of the shortest possible response time, the printing press according to the invention is therefore preferably equipped with a plate-shaped heating element with a plate thickness of less than 10 mm. The transport device has a maximum format width for transporting the printing material, in the preferred case the heating element for irradiation over the entire format width consisting of several heating element sections which can be controlled electrically independently of one another.

The heating element sections span the maximum possible format width of the printing press. For example, they are juxtaposed. Because they can be switched on and off separately, individual heating elements can be switched on or off as required. Additional thermal separation can reduce heat loss due to heat conduction from the heating element (s) switched on to the heating element (s) that are not switched on.

It has proven to be advantageous if the heating element material comprises an amorphous matrix component and an additional component in the form of a semiconductor material.

The amorphous material, such as quartz glass, can easily be brought to the geometric shape suitable for the application, for example in the form of flat, curved or corrugated plates. The additional component embedded in it forms its own amorphous or crystalline phase made of semiconductor material, such as silicon. The energy difference between the valence band and the conduction band (band gap energy) decreases with increasing temperature. On the other hand, with sufficiently high activation energy, electrons can be lifted from the valence band into the conduction band, which is accompanied by a significant increase in the absorption coefficient. The thermally activated occupation of the conduction band means that the semiconductor material can to a certain extent be transparent to certain wavelengths (such as from 1000 nm) at room temperature and become opaque at high temperatures. With increasing temperature of the heating element material, absorption and emissivity can increase. This effect depends, among other things, on the structure (amorphous / crystalline) and doping of the semiconductor. Pure silicon, for example, shows a noticeable increase in emissions from around 600 ° C, which saturates from around 1000 ° C.

If the semiconductor material is heated sufficiently, it can therefore assume an energetic, excited state in which it emits infrared radiation with a high power density. In this state, the semiconducting additional component decisively determines the optical and thermal properties of the heating element; more precisely, it causes absorption in the infrared spectral range (that is, in the wavelength range between 780 nm and 1 mm) and in particular absorption in the wavelength range around 2750 nm. With such a heating element, power densities above 180 kW / m ² , preferably power densities in Range from 180 kW / m ² to 265 kW / m ² , achievable.

Such a heating element material thus shows an excitation temperature which must at least be reached in order to maintain the thermal excitation of the material and thus a high radiation emission. The additional component then causes the heating element material to emit infrared radiation. The spectral emissivity ε _λ can be calculated as follows for known directional-hemispherical spectral reflectance R _gh and transmittance T _gh : $${\mathrm{\epsilon }}_{\mathrm{\lambda }}=1-{\mathrm{R}}_{\mathrm{gh}}-{\mathrm{T}}_{\mathrm{gh}}$$

The "spectral emissivity" is understood to mean the "spectral normal emissivity". This is determined on the basis of a measurement principle which is known as "Black Body Boundary Conditions" (BBC) and is published in " DETERMINING THE TRANSMITTANCE AND EMITTANCE OF TRANSPARENT AND SEMITRANSPARENT MATERIALS AT ELEVATED TEMPERATURES "; J. Manara, M. Keller, D. Kraus, M. Arduini-Schuster; 5th European Thermal-Sciences Conference, The Netherlands (2008 ).

The matrix doped with the additional component has a higher heat radiation absorption than would be the case without the additional component. This results in an increased proportion of energy transmission by radiation from the conductor track into the heating element, a faster distribution of the heat and a higher radiation rate on the substrate. This makes it possible to provide a higher radiation output per unit area and to produce a homogeneous radiation and a uniform temperature field even with thin heating element wall thicknesses and / or with a comparatively low conductor occupancy density.

In the heating element material, the additional component is preferably at least partly in the form of elemental silicon and is stored in an amount that has a spectral emissivity ε of at least 0.7 at a temperature of 600 ° for wavelengths between 2 and 8 μm in the heating element material C and a spectral emissivity ε of at least 0.8 at a temperature of 1000 ° C.

The semiconductor material and in particular the preferably used elementary silicon therefore cause the glassy matrix material to turn black, at room temperature, but also at an elevated temperature above, for example, 600 ° C. This achieves good radiation characteristics in the sense of broadband, high emissions at high temperatures. The semiconductor material forms an elementary semiconductor phase dispersed in the matrix. This can contain several semiconductor elements or metals (metals, however, up to a maximum of 50% by weight, better not more than 20% by weight; based on the proportion by weight of the additional component).

The heat absorption of the heating element material depends on the proportion of the additional component. In the case of silicon, the weight fraction should preferably be at least 0.1%. On the other hand, a high silicon content can impair the chemical and mechanical properties of the quartz glass matrix. In view of this, the weight fraction of the weight fraction of the silicon additional component is preferably in the range between 0.1 and 5%.

In a preferred embodiment of the printing press according to the invention, the dryer unit comprises a plurality of heating elements which are arranged one behind the other in the transport direction of the printing material.

A pressure unit is assigned to each dryer unit. The larger number of printing units enables a high printing speed and a high printing quality.

In this embodiment of the printing press in particular, it has proven to be advantageous if a device for supplying process air into the intermediate space between the printing material and the heating elements is provided.

The desire to process is used to dry the printing material and to remove the printing ink from the solvent, for example water. In order to achieve a drying of the printing material which is uniform over the web width of the printing material and which is constant over time, the aim is to achieve a reproducible, laminar flow of the process air. The flat, preferably flat heating surface of the heating elements and the narrow gap between the heating surfaces and the printing material contribute to this in the printing press according to the invention.

The printing press according to the invention can be used for rotary printing, offset printing, planographic printing, letterpress printing, screen printing or gravure printing. However, it has proven particularly useful if the printing unit comprises an inkjet print head, with at least one traction roller equipped with a drive motor being arranged downstream of the dryer unit when viewed in the transport direction of the printing material.

In the inkjet printing process or inkjet printing process, the image-forming device is designed as an inkjet printing head which has one or more nozzles by means of which ink drops are transferred to the printing material. In particular when using water-based ink, it can happen that the printing material deforms, for example waves, which can lead to poor print quality, damage to the print head and printing material and to uneven drying of the printing material. The latter is particularly noticeable when - as is adjustable in the printing press according to the invention - the distance between the printing material and the dryer unit is very small. In order to counteract this and to ensure the most uniform and reproducible flatness of the printing material, viewed in the transport direction of the printing material, at least one draw roller equipped with its own drive motor is arranged downstream of the dryer unit.

If the pull roller is also designed as a cooling roller, the printing material can be cooled after the dryer unit, which can be helpful in particular in view of the potentially high energy input in order to minimize damage to the printing material.

embodiment

Figur 1

einen Ausschnitt einer erfindungsgemäßen Druckmaschine mit dem Transportweg für den Bedruckstoff durch ein Druckaggregat und eine Infrarot-Trocknereinheit in schematischer Darstellung,

Figur 2

eine Ausführungsform des erfindungsgemäßen Heizelements mit einer Reflektorschicht in schematischer Darstellung und in einer Seitenansicht,

Figur 3

zeigt ein Diagramm zum Anschaltverhalten eines Heizelements der Trocknereinheit,

Figur 4

ein Diagramm mit Emissionsspektren eines kachelförmigen Heizelements im Vergleich zu einem herkömmlichen Infrarotstrahler mit Quarzglas-Hüllrohr und Kanthal® -Wendel,

Figur 5

ein Diagramm zur Verdeutlichung des Bestrahlungsprofils der auf dem Bedruckstoff auftreffenden Infrarotstrahlung bei Einsatz der erfindungsgemäßen Druckmaschine, und

Figur 6

anhand zweier Diagramme (a) und (b) einen Vergleich von Homogenität und Intensität der Bestrahlung von Bedruckstoff mittels kachelförmigem Heizelement und mittels Infrarot-Flächenstrahler nach dem Stand der Technik.

The invention is explained in more detail below on the basis of an exemplary embodiment and a patent drawing. The drawing shows in detail:

Figure 1

a section of a printing machine according to the invention with the transport path for the printing material through a printing unit and an infrared dryer unit in a schematic representation,

Figure 2

An embodiment of the heating element according to the invention with a reflector layer in a schematic representation and in a side view,

Figure 3

shows a diagram of the switching behavior of a heating element of the dryer unit,

Figure 4

a diagram with emission spectra of a tiled heating element in comparison to a conventional infrared radiator with quartz glass cladding tube and Kanthal® coil,

Figure 5

a diagram to illustrate the radiation profile of the infrared radiation impinging on the printing material when using the printing press according to the invention, and

Figure 6

Using two diagrams (a) and (b), a comparison of the homogeneity and intensity of the irradiation of printing material by means of a tiled heating element and by means of an infrared area heater according to the prior art.

press

Figure 1 schematically shows an embodiment of a printing machine according to the invention in the form of a roll inkjet printing machine, to which reference number 1 is assigned overall. Starting from an unwinder 2, the material web 3 from a printing material, such as paper, for example, arrives at a printing unit 40. This comprises a plurality of ink jet print heads 4 arranged one behind the other along the material web 3, through which solvent-based and in particular water-containing printing inks are applied to the printing material.

Viewed in the transport direction 5, the material web 3 then arrives from the printing unit 40 via a deflection roller 6 to an infrared dryer unit 70. This is equipped with a plurality of infrared heating elements 7, which are designed for drying or knocking off the solvent into the material web.

The further transport path of the material web 3 goes to a take-up roll 9 via a pull roller 8 which is equipped with its own pull drive motor and via which the web tension is adjusted.

Several - in the exemplary embodiment there are eight - heating elements 7 are combined in a heating block which extends over the maximum format width of the printing press 1. The individual heating elements 7 are arranged in a row in the heating block and can be controlled separately from one another in accordance with the dimensions and color assignment of the printing material. An electrical and thermal insulator is located between the individual heating elements 7. The free distance between the heating surface of the heating elements and the top of the material web 3 is 10 mm.

The transport speed of the material web 3 is set to 5 m / s. This is a comparatively high speed, which is made possible by optimizing the individual processing steps, and which in particular requires a high drying rate. The dryer unit 70 required to achieve this requirement is described below with reference to FIG Figures 2 to 5 explained in more detail.

If the same reference numbers are used in other figures as in Figure 1 are used to designate identical or equivalent components and components, as are explained in more detail above with reference to the description of the printing press according to the invention.

heating element

At the in Figure 2 The schematically shown embodiment of a heating element 7 is an infrared radiator with a tiled base body 20 with a flat radiation surface (underside 26) and also a flat top side 25. A conductor track 23 is applied to the base body top side 25, which in turn is embedded in a reflector layer 24.

The base body 20 has a rectangular shape with a plate thickness of 2.0 mm and lateral dimensions of 10 cm x 20 cm. It consists of a composite material with a matrix of quartz glass in which phase areas made of elemental silicon are homogeneously distributed. The weight fraction of this Si phase is 2.5% and the maximum dimensions of the Si phase areas are on average (median) in the range from about 1 to 10 µm. The composite material is gas-tight, it has a density of 2.19 g / cm ³ and it is stable in air up to a temperature of around 1200 ° C. It shows a high absorption of heat radiation and a high emissivity at high temperature.

The conductor track 23 is produced from a platinum resistance paste on the upper side 25 of the base body 20. Lines or clamps for feeding in electrical energy are welded onto both ends. The conductor track 23 shows a meandering course, which covers a heating surface of the base body 20 so densely that there is a uniform distance between adjacent conductor track sections of 2 mm remains. In the cross section shown, the conductor track 23 has a rectangular profile with a width of 1 mm and a thickness of 20 μm. As a result of the small thickness, the proportion of material of the expensive conductor track material (platinum) in the infrared radiator is small compared to its efficiency. The conductor track 23 has direct contact with the top 25 of the base body 20, so that the greatest possible heat transfer into the base body 20 is achieved. The opposite underside 26 serves as an emitting surface for the heat radiation when using the infrared radiator. The direction of radiation is indicated by the directional arrow 27.

The reflector layer 24 consists of opaque quartz glass and has an average layer thickness between 1.0-1.5 mm. It is characterized by freedom from cracks and a high density of about 2.15 g / cm3 and it is thermally resistant up to temperatures above 1100 ° C. The reflector layer 24 covers the entire heating area of the base body 20 and it completely covers the conductor track 23 and thus shields it from chemical or mechanical influences from the environment.

Measurement of the switch-on behavior

A quick response time of the dryer unit 70 after the printing machine is switched on is a prerequisite for low waste in the printing process. The diagram of Figure 3 shows the temperature profile over time after switching on the Figure 2 described heating element 7. On the y-axis, a temperature T _rel (in%) normalized to a maximum temperature, which arises during operation with the maximum electrical connected load, is plotted against the operating time t in seconds. T _rel is measured at a distance of 5 mm from the heating surface using a thermopile measuring sensor.

When the maximum electrical connected load of up to 200 kW / m ^{2 is applied} to the conductor track 23, the maximum temperature, which remains essentially constant in the further heating process, is established after a short time in comparison with conventional medium-wave infrared radiators. The short reaction time compared to conventional medium-wave infrared emitters reduces waste. In addition, in the printing press 1 according to the invention, there is no need to implement air cooling for the heating elements 7. This increases the process efficiency, since cold cooling air reduces the temperature of the printing material 3 and hinders the removal of moisture. The combination of uncooled heating elements 7 and warm convective process air for moisture transport optimizes the printing process in modern high-performance printing machines.

Measurement of emissivity

The composite material shows a high absorption of heat radiation and a high emissivity at high temperature. At room temperature the emissivity of the composite material is measured using an integrating sphere. This allows the measurement of the directional-hemispherical spectral reflectance R _gh and the directional-hemispherical spectral transmittance T _gh , from which the normal spectral emissivity is calculated. The emissivity at elevated temperature is measured in the wavelength range from 2 to 18 µm using an FTIR spectrometer (Bruker IFS 66v FTIR), to which a BBC sample chamber is coupled using additional optics, using the BBC measuring principle mentioned above. The sample chamber has temperate blackbody environments and a beam exit opening with detector in the half-spaces in front of and behind the sample holder. The measurement samples with a thickness of 2 mm are heated to a specified temperature in a separate oven and placed in the beam path of the sample chamber with the blackbody surroundings set to the specified temperature for measurement. The intensity detected by the detector is composed of an emission, a reflection and a transmission component, namely intensity that is emitted by the sample itself, intensity that falls on the sample from the front half space and is reflected by it, and intensity that falls on the sample from the rear half-space and is transmitted by it. Three measurements must be carried out in order to determine the individual variables of emissivity, reflection and transmittance.

The emissivity measured on the composite material in the wavelength range from 2 to about 4 µm depends on the temperature. The higher the temperature, the higher the emission. At 600 ° C, the normal emissivity in the wavelength range from 2 to 4 µm is above 0.7. The normal temperature is 1000 ° C Emissivity in the entire wavelength range between 2 and 8 µm above 0.8.

Figure 4 shows the emission spectrum of the heating element 7 (curve A) compared to the emission spectrum of a conventional infrared radiator with quartz glass cladding tube and heating coil made of Kanthal® (curve B) with the same power. The emitted power P _rel (as a _relative value related to the maximum value in%) is plotted on the left y-axis and the wavelength λ (in nm) on the x-axis. In addition, the transmission spectrum of water is entered in the diagram (curve C), the right y-axis indicating a relative value T _H2O .

The temperature of the conductor track 23 on the base body 20 is set to 1000 ° C. The comparison heater with a Kanthal® filament is also operated at a temperature of around 1000 ° C. It can be seen that the tiled heating element 7 has an emission maximum in the wavelength range of 1,500 nm to about 2,000 nm that better matches the transmission maximum of water at 2750 nm than the emission curve of the standard radiator. This results in an approximately 25% higher power density on the substrate 3 compared to the standard infrared emitter with the same electrical power and the same distance.

Measurement of the spatial homogeneity of the emitted radiation

The spatial homogeneity of the emitted radiation is tested in accordance with IEC 62798 (2014). For this purpose, the infrared area heater is installed in a test device and mounted on a movable table. At a predetermined working distance of 10 mm from the radiation surface of the infrared radiator, the optical power is detected by means of a thermoelectric detector. The irradiance is determined at several measuring points in steps of 5 mm. The irradiance is defined as sufficiently homogeneous if it deviates from the maximum value measured at 10 measuring points around the center of the sample by no more than +/- 5%. This type of measurement is also referred to below as "axial measurement",

The diagram of Figure 5 illustrates the result of axial measurements when using the tile-shaped heating element 7. A normalized optical power L (in%) is plotted on the y-axis, and the lateral distance A (in mm) from the axis zero point on the x-axis extending center line, which relates to the lateral dimension of the heating element 7.

The lateral profile of the optical power is measured at a working distance of 10 mm. This is comparatively homogeneous over a larger area around the center line at almost 100%. This is shown by the fact that in a work area with more than 10 measuring points around the center line, the optical power does not drop below 95% compared to the maximum value (100%).

Diagrams (a) and (b) of Figure 6 illustrate schematically the relationship between irradiation homogeneity or intensity and the distance between the radiator and the substrate, as well as differences in this regard between an infrared area radiator composed of several individual radiators (diagram (a)) and the tiled heating element 7 for use in the printing press 1 according to the invention ( Diagram (b)). On the ordinate of diagrams (a) and (b), the homogeneity "H" or the radiation intensity "I" incident on the heating material against the distance "A" (also in relative unit) between the radiator and Printed material applied. The surface radiator 71 in diagram (a) is represented by a plurality of medium-wave or short-wave radiant heaters arranged side by side, the cladding tubes of which are indicated by three circles. The tile-shaped heating element 7 of the printing press according to the invention is indicated in diagram (b) by a hatched rectangle. The tile-shaped heating element 7 and the flat arrangement 71 of the carbon radiators have the same electrical connection power.

The course of the homogeneity H with the distance A is indicated by the dashed curve line H, and the course of the intensity I by the solid curve line. Accordingly, both in the case of the standard area radiator 71 and in the case of the tiled heating element 7, the radiation intensity I decreases with the distance A to approximately the same extent, but the homogeneity of the radiation is in the heating element 7 largely independent of the distance A, whereas in the standard infrared area heater 71 it is small at a short distance.

The gray hatched area schematically defines a "work area" in which there is an acceptable irradiation homogeneity on the substrate. It becomes clear that this homogeneity can be achieved with the standard infrared surface radiator 71 by maintaining a certain distance, but for this a significant loss in radiation intensity has to be accepted. In contrast to this, the tile-shaped heating element 7 enables a sufficiently high homogeneity even at very small distances, at which the radiation intensity is also high. Thus, the efficiency of the heating element 7 1 compared to the surface radiator 71 made of carbon single radiators - is significantly improved.

Claims (10)

1. A printing machine comprising a printing unit for applying a solvent-containing printing ink onto a printing substrate, a transport facility for transporting the printing substrate from the printing unit to a drier unit comprising at least one infrared heater for drying the printing substrate, characterised in that the infrared heater is formed as a plane heater element made of a dielectric heater element material that emits infrared radiation when being heated, said heater element having a heating surface facing the printing substrate to be dried and a contact surface onto which a heating conductor path made of an electrically conductive resistor material containing a noble metal has been applied, said heating conductor path being connected to a point electrically contacting an adjustable power source.
2. The printing machine in accordance with Claim 1, characterised in that the heater element is formed as a plate having a plate thickness of less than 10 mm.
3. The printing machine in accordance with Claim 1 or 2, characterised in that the transport facility has a maximum format width for transporting the printing substrate and that the heater element consists of a plurality of heater element sections for radiation across the format width, wherein said heater element sections can be electrically energised independently of each other.
4. The printing machine in accordance with any one of the preceding claims, characterised in that the heater element material comprises an amorphous matrix component as well as an additional component having the form of a semiconductor material.
5. The printing machine in accordance with any one of the preceding claims, characterised in that the drier unit comprises a plurality of heater elements which are arranged in series in the transport direction of the printing substrate.
6. The printing machine in accordance with Claim 5, characterised in that a facility for supplying process air into the space between the printing substrate and the heater elements is provided.
7. The printing machine in accordance with any one of the preceding claims, characterised in that the printing unit comprises an ink jet printing head and that, viewed in the transport direction of the printing substrate, at least one draw roller equipped with a drive motor is provided downstream of the drier unit.
8. The printing machine in accordance with Claim 7, characterised in that the draw roller is formed as a cooling roller.
9. The printing machine in accordance with any one of the preceding claims, characterised in that the heater element is configured to achieve a power density of more than 180 kW/m².
10. The printing machine in accordance with Claim 9, characterised in that the heater element is configured to achieve a power density within a range from 180 kW/m² to 265 kW/m².

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