EP0372097A1 - Arrangement for producing varying size ink droplets in an ink jet printer - Google Patents

Abstract

The generation of ink droplets of varying size in an ink jet printer operating according to the principle of the thermotransducer is achieved by suitable geometry of the thermotransducers (1) in cooperation with control pulses matched to said thermotransducer geometry. In this arrangement, the thermotransducers (1) are subdivided into a plurality of heat-generating segments (W1...W3) of varying width (bi), varying layer thickness (di) or layer materials ( rho i, ai). To improve the cross-talk behaviour, the heat-generating segments (W1...W2) are separated from one another by regions which, in relation to said heat-generating segments (W1...W3), have lower heat- generating rates or a smaller thermal conductivity (ai). <IMAGE>

Description

The invention relates to an arrangement for producing ink droplets of different sizes in an ink printing device according to the preamble of patent claim 1.

Known ink print heads that work on the principle of thermal conversion (bubble jet) have a matrix of individual nozzles from which defined individual droplets are ejected under the action of an electronic control. Each nozzle is connected to an ink channel, in which pressure waves are generated in the ink liquid by means of an actuator. The process of building pressure in the ink liquid is based on the creation of small microbubbles. An electrothermal transducer element (heating element) in the form of a thin-film resistor is located under each ink channel at a certain distance from the outlet nozzle as an actuator. By briefly energizing one of these transducer elements, the ink liquid immediately above is heated in a thin layer to high excess temperatures. In the course of the subsequent evaporation of the heated ink liquid, a vapor bubble with high internal pressure is formed above the heating element, the expansion of which causes the ink liquid located in the corresponding ink channel to be expelled through the nozzle. In addition to fluid mechanical influences, which are given by the channel and nozzle geometry, there is also a proportionality between the ink droplet volume and the surface of the heating element, which also determines the size of the corresponding vapor bubble.

If such ink printing devices are to be used to produce fonts of different font qualities, for example fonts in a so-called draft quality (DQ) and in a so-called beautiful font (Near Letter Quality, NLQ), it is advantageous to add ink droplets of different sizes and thus different droplet volumes produce. This means on the one hand an improved font quality (degree of blackening) at high printing speed and on the other hand a reduced resolution due to larger ink droplets, e.g. possible in "draft" mode. In addition, such ink printing devices are used as output devices for graphics, which requires that gray or color levels must be able to be represented. In particular, the generation of ink droplets of different sizes in color printing makes it possible to avoid oversaturation of the printing paper with solvents.

To produce ink droplets of different sizes, it is known from EP-A1-0203534 to control them with an adjustable number of control pulses in an ink writing device with piezoelectric transducer elements which are each assigned to the ink channels of the writing head. The repetition frequency of the drive pulses is matched to the resonance frequency of the ink channel and the drive pulses follow one another in time in such a way that an ejection of a small amount of ink from the outlet opening of the ink channel caused by subsequent drive pulses always occurs before the ink droplet caused by the first drive pulse is detached from the outlet opening occurs.

WO 87/03363 describes an ink printing device based on the thermal converter principle, with which eight different shades of gray or semitones can be produced.

For this purpose, three droplet generators are provided which, due to the different area of the thermal transducers, eject different droplet volumes, which are binary-weighted. The eight possible grayscale or halftones are generated by sequentially spraying droplets from the differently sized nozzle openings onto one and the same pixel. In addition, a droplet generator for emitting colorless ink liquid is provided, which is applied directly to an ink droplet ejected from the other nozzles, whereby soft contours of the colored ink droplets are achieved.

The object of the invention is to provide measures for controlling the ink droplet volumes for an ink printing device of the type mentioned at the outset, which produce halftone images or gray or color levels from individual dots of different sizes in a simple manner and without reducing the printing speed and the resolution capacity.

This object is achieved according to the features specified in the characterizing part of patent claim 1.

Advantageous embodiments are characterized in the subclaims.

Appropriate geometry of the thermal transducers in conjunction with control pulses matched to this thermal transducer geometry can be used to specifically eject droplets of different volumes from the outlet nozzles. Since the surface of the thermal transducers also determines the size of the resulting vapor bubble and the evaporation of the ink liquid begins abruptly only above a critical evaporation temperature, thermal transducer geometries with an inhomogeneous heat generation rate are used according to the invention, the heat generation sections contributing to the vapor bubble formation, for example, both via the amplitude and over the duration of the control pulses the effective area of the thermal converter can be controlled.

If the thermal transducers are divided into several bubble generation sections, they can be used on the one hand to generate droplet volumes that differ according to the number of these sections, and on the other hand, in addition to the generation of grayscale levels, it is also possible in the so-called draft mode to achieve a degree of blackening that can be achieved in Schönschreib Modus (NLQ) to achieve and thus to achieve optimal color coverage without oversaturation of the recording medium with solvents in the production of color prints by reducing the droplet mass.

The variation of the thermal transducer geometry according to the invention for the controlled generation of different ink droplet volumes is characterized by a particularly simple technological feasibility, since additional electrical control lines on the thin film substrate can be dispensed with.

• Figur 1 Beispiele von Thermowandlern zur Erzeugung von Tintentröpfchen unterschiedlicher Volumina,
• Figur 2 Temperaturverläufe von Wärmeerzeugungsabschnitten der Thermowandler nach Figur 1 bei verschiedenen elektrischen Ansteuerimpulsen,
• Figur 3 weitere Beispiele von Thermowandlern zur Erzeugung von zwei verschieden großen Tintentröpfchen und
• Figur 4 Beispiele für eine Heizelementgeometrie zur Erzeu­gung von drei verschiedenen Tröpfchengrößen.

Exemplary embodiments of the ink printing device according to the invention are described below with reference to drawings. Show there

• FIG. 1 examples of thermal converters for producing ink droplets of different volumes,
• FIG. 2 shows temperature profiles of heat generating sections of the thermal converter according to FIG. 1 with different electrical control pulses,
• Figure 3 further examples of thermal converters for producing two different sized ink droplets and
• FIG. 4 examples of a heating element geometry for producing three different droplet sizes.

FIG. 1 shows top views of differently geometrically designed thermal converters 1, each of which has power supply lines 2, 3 in the form of Conductor tracks can be contacted. Thermal converter 1 and conductor tracks 2, 3 can be produced, for example, by vacuum deposition of suitable electrically conductive alloys (for example hafnium diboride HfB₂ for the thermal converter) or metals (for example aluminum Al for the conductor tracks) on a base oxide with subsequent photo-technical structuring.

Eine Modulation der Tintentröpfchenmasse ist bei einer solchen homogenen Wärmeerzeugungsrate durch Verändern der Amplitude und/oder der Impulsdauer der Ansteuerspannung nur in einem sehr geringen Maße möglich, da nach dem Einsetzen der Verdampfung der gesamte Thermowandler von einer Dampf­schicht bedeckt und damit von dem Thermowandler thermisch entkoppelt ist.FIG. 1 a shows a conventional, rectangular thermal converter 1, which is primarily suitable for producing ink droplets of one size. The rectangular shape of the thermal converter results in a homogeneous heat generation rate (heat generation rate = thermal power / area =

At such a homogeneous heat generation rate, modulation of the ink droplet mass is only possible to a very small extent by changing the amplitude and / or the pulse duration of the control voltage, since after the onset of evaporation, the entire thermal converter is covered by a vapor layer and is therefore thermally decoupled from the thermal converter .

A thermal transducer is used to generate ink droplets of continuous size, the geometric configuration of which is explained with reference to FIG. 1b. This thermal converter has a length 1 and, starting from a right power supply 2, runs conically to a left power supply 3, so that the area of the thermal converter decreases steadily in the direction of the left power supply 2. The width of the thermal converter on the right power supply 2 is denoted by b₁, the width on the left power supply 3 by b₀. If you also define the base point of a current coordinate x on the front side of the left power supply 3, the following relationship is obtained for the width b (x):
b (x) = b₀ + ((b₁ - b₀) / 1) x with b₀ <b₁.

Because of the resulting uneven current density distribution, the temperature T _V necessary for evaporation is first reached in the vicinity of the left power supply 3 with a constant pulse duration and a relatively small amplitude of the control voltage. Small-volume ink droplets are emitted. As the drive voltage or the pulse duration increases, the effective area for bubble formation above the thermal converter increases in the direction of the right power supply 2, ie ink droplets with a larger volume are ejected. In principle, a thermal converter of conical geometry enables the ink droplet volumes to be continuously adjusted over the amplitude and / or pulse duration of the drive voltage within certain limits given by the channel and nozzle geometry.

While continuous-size ink droplets can be produced with the thermal transducer according to FIG. 1b, the thermal transducers according to FIGS. 1c to 1e are suitable for producing two discrete, different droplet volumes. The thermal converters are divided into areas of different widths with the same layer thickness (FIGS. 1c, 1d) or into areas of different thermal or electrical properties (FIG. 1e).

The thermal converter according to FIG. 1c has a stepped contour within its longitudinal extent, so that the surface of this thermal converter is composed of two rectangles of different widths. The rectangle adjacent to the left power supply 3 has a width b 1, the rectangle adjacent to the right power supply 2 has a width b 2, the width b 1 being smaller than the width b 2. Such a division of the surface of the thermal converter with the same layer thickness creates two different heat generating sections W 1, W 2.

In Figure 1d, the thermal converter is shown in the form of a simple rectangle which has an insulation gap 4 for generating two different droplet volumes. The length of the insulation gap 4 within the thermal converter defines the two heat generating sections W₁, W₂ with the effective width 2 · b₃ in the heat generating section W₁ and the width b₂ in the heat generating section W₂.

Another possibility for producing a non-uniform heat distribution in the thermal converter is shown in FIG. 1e. The thermal converter consists of two heat generating sections W₁, W₂ different layer thickness (d _i ) and / or layer materials, ie materials with different electrical (specific resistance ρ _i ) and / or thermal properties (thermal conductivity a _i ).

All of the exemplary embodiments of thermal converters shown with reference to FIGS. 1b to 1e have in common that the absolute droplet sizes are determined by the individual areas of the heat generating sections W 1, W 2. A small heated area causes a small droplet to be ejected, the entire area is used to generate large droplets, while the crosstalk, i.e. the quality of the separation of small and large droplets is given by the ratio of the heat generation rates or the surface temperatures of the individual heat generation sections W 1, W 2.

The operation of the thermal converter is shown in more detail in Figure 2 based on the time-dependent temperatures of the heat generating sections W₁, W₂ with various control pulses.

In Figure 2a, the amplitude and pulse duration Δt₁ of the control voltage for the thermal converter according to Figure 1 are chosen so that the temperature T _V necessary for the onset of bubble formation only from the section W₁ of the thermal converter, which, for example, has the higher heat generation rate due to its smaller width (Fig. 1c) is exceeded. After the onset of evaporation at time t _v , the temperature T of the section W 1 rises more strongly until the end of the heating pulse at the time t 1, since the heat flow between the section W 1 and the ink liquid is greatly reduced by the formation of bubbles. The course of the temperature from the time of the start of evaporation t _V until the onset of condensation at the time t _K characterizes the expansion of the vapor bubble, this mechanical work being carried out and the ink jet being expelled from the nozzle. Since the section W₂ of the thermal converter does not reach the temperature necessary for initiating the evaporation due to its lower heat generation rate (for example due to the greater width of this section compared to the section W₁, see FIG. 1c), this does not contribute to the formation of droplets. The volume of the small droplet emitted is thus determined by the area of the heat generating section W 1.

In order to produce larger ink droplets, the heat generating section W 2 must also exceed the temperature T _V necessary to initiate the evaporation. This can be achieved in a simple manner by increasing the amplitude of the control voltage and / or by extending the heating pulse duration.

In FIG. 2b, a drive voltage is selected to generate a larger ink droplet, the pulse duration of which is identical and the amplitude of which is greater than the amplitude of the drive voltage in the previous example. The evaporation Temperature T _V is therefore achieved by both heat generating sections W 1, W 2, so that the entire surface of the thermal converter contributes to the formation of droplets. The same effect is achieved if the amplitude of the control voltage is not increased, but the heating pulse duration compared to the heating pulse duration according to Figure 2c is extended (Δt₂> Δt₁).

With regard to the life of the thermal converter and to rule out a second uncontrolled evaporation, unnecessary thermal overloading of the section with the higher heat generation rate (heat generation section W 1) or surface temperature is to be avoided.

Since the bubble formation in the ink liquid which begins first in the heat generation section W 1 may initiate evaporation in the heat generation section W 2, it is advantageous to use thermal converter geometries in order to improve the crosstalk behavior, in which the bubble generation sections through colder areas, i.e. Areas with lower heat generation rates or lower thermal conductivities than the bubble generation sections are separated.

FIG. 3 shows exemplary embodiments of thermal converters whose heat generating sections W 1, W 2 are thermally decoupled from one another by separating areas T 1, T 2. As shown in Figure 3a, the thermal separation of the two heat generating sections W₁, W₂ can be achieved by a locally widened resistance geometry. The two heat generating sections W₁, W₂ with their widths b₁ and b₂ do not form a boundary layer at which they meet directly (see FIG. 1c), but are widened with the aid of a heat generating section W₁, W₂ rectangular web 5, which has a width b₃, thermally decoupled (b₃> b₁, b₂).

Another possibility for improving the crosstalk behavior is explained with reference to FIG. 3b. The thermal separation of the heat generating sections W₁, W₂ is done by inserting a low-resistance material between the two heat generating sections W₁ and W₂. The thermal converter is divided into two sections of rectangular shape, which have the same thermal and electrical properties (specific resistance ρ₁, layer thickness d₁ and thermal conductivity a₁), but are of different widths (b₁> b₂). The separation area T₁ is characterized by a material with a specific resistance ρ₂ <ρ₁. The same effect, namely a thermal separation between the heat generating sections W₁, W₂ can also be achieved in that the material in the separation area T₁ has a greater than the heat generating sections W₁, W₂ layer thickness d₂> d₁ or a lower than the heat generating sections W₁, W₂ lower thermal conductivity a₂ < has a₁.

Since the position of the thermal converter in the ink channel also has an influence on the emitted droplet mass, spatial separation of the heat generating sections W 1, W 2 according to FIG. 3c is also possible. A between the individual heat generating sections W₁, W₂ with their different widths b₁, b₂ inserted separation area T₂ has a length Δ1 and can be realized, for example, by a material from which the power supply lines 2,3 are made (eg aluminum Al). In addition, combinations of the thermal converter geometries shown with reference to FIGS. 3a and 3b with such an additional spatial separation of the heat generating sections W 1, W 2 are also possible.

FIG. 4 shows that the thermal transducers can also be used to produce more than two different ink droplet volumes by appropriately dividing them into bubble generation sections. 4a and 4b show thermal transducer geometries which are suitable for ejecting three ink droplets of different sizes. The individual heat generating sections W₁, W₂ and W₃ are either generated analogously to the arrangement according to FIG. 1c by different widths b₁, b₂ and b₃ with the same layer thickness d₁ alone (b₁ <b₂ <b₃) or in addition to the thermal decoupling serving locally widened resistance geometries according to FIG. 3a (b₁ <b₂ <b₃, b₄ <b₅). Of course, even if the thermal converter is subdivided into a large number of heat generation sections, all of the above-mentioned measures for improving the droplet separation and combinations thereof can be used.

As the primary prerequisite for the generation of different droplet sizes in the methods presented is that the surface temperatures of the individual heat-generating sections develop differently over time, we would like to point out another way of achieving this. For this purpose, the thermal transducers of conventional geometry (cf. FIG. 1a) with a constant layer thickness and layer width are covered and structured with a cover layer of low thermal conductivity (for example silicon dioxide SiO₂) in such a way that regions of different cover layer thicknesses arise above the thermal converter or the thermal converters are partially with other poorly heat-conducting ones Layers covered. The areas created in this way also correspond in their function to the heat generation sections already described and can be used in the same way for producing different droplet sizes.

Claims (10)

1. Arrangement for producing ink droplets of different sizes in an ink printing device working according to the thermal converter principle with the following features: a) a plurality of electrothermal transducer elements (1) are arranged in ink channels of the ink printing device, which generate an ink vapor bubble through individual control pulses in the relevant ink channel, which eject a certain volume of ink as droplets from an outlet nozzle closing the ink channel, b) the electrothermal transducer elements (1) are divided into different heat generation sections (W₁ ... W₃) such that an inhomogeneous heat generation rate can be generated. c) the amplitude and / or the duration of the control pulses, the heat-generating sections (W 1 ... W 3) of the electrothermal transducer elements (1) that contribute to the formation of vapor bubbles are controlled. 2. Arrangement according to claim 1, characterized in that the heat generating sections (W₁ ... W₃) are formed by different widths (b₁, b₂) with the same layer thickness (d _i ) of the electrothermal transducer elements (1). 3. Arrangement according to claim 1, characterized in that the heat generating sections (W₁ ... W₃) from regions of different layer thicknesses (d _i ) and / or layer materials (ρ _i , a _i ) of the electrothermal transducer elements (1) are formed. 4. Arrangement according to claim 1, characterized in that the heat generating sections (W₁ ... W₃) are formed by an insulation gap (4) within part of the electrothermal transducer elements (1). 5. Arrangement according to claim 1, characterized in that the heat generating sections (W₁ ... W₃) are thermally separated from one another by inserting a separation region (T₁) with a lower heat generation rate. 6. Arrangement according to claim 5, characterized in that the heat generating sections (W₁ ... W₃) are thermally decoupled by a locally widened resistance geometry of the electrothermal transducer elements (1). 7. Arrangement according to claim 5, characterized in that the heat generating sections (W₁ ... W₃) are spatially separated by inserting a separation area (T2). 8. Arrangement according to claim 1, characterized in that the surface of the electrothermal transducer elements (1) has a conical shape. 9. The arrangement according to claim 1, characterized in that the amplitude of the drive pulse is selected so that the temperature (T _V ) necessary for evaporation of the ink liquid is first achieved only in the heat generation section (W₁) with the greatest heat generation rate and by increasing the amplitude of the drive pulse this temperature (T _V ) is reached in the remaining heat generating sections (W₂, W₃). 10. The arrangement according to claim 1, characterized in that the pulse duration of the drive pulse is chosen so that the temperature (T _V ) necessary for evaporation of the ink liquid is first achieved only in the heat generation section (W₁) with the greatest heat generation rate and by extending the heating pulse duration this temperature (T _V ) is reached in the remaining heat generating sections (W₂, W₃).

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