EP0458997B1 - Method of operating a thermal ink jet printer - Google Patents

Description

The invention relates to a method for operating an ink writing device working according to the thermal converter principle and a recording head for such a device according to the features of the patent claim.

A method according to the preamble of the claim is known from JP-A-63199652.

Known ink heads that work according to the thermal converter principle (bubble jet principle) and are described, for example, in DE-OS 30 12 698, have a large number of individual nozzles from which defined individual droplets are ejected under the action of an electronic control. A characteristic feature of this technology is that an electrical resistor designed as a heating element is located in a capillary filled with ink, in the vicinity of its opening. If a certain amount of thermal energy is supplied to this heating element by means of a short current pulse, extremely rapid heat transfer to the ink liquid (film boiling) first creates a rapidly expanding ink vapor bubble (bubble), which then collapses relatively quickly after the energy supply is lost by cooling the ink liquid . The pressure wave generated by the ink vapor bubble in the interior of the capillaries causes an ink jet of limited mass to emerge from the nozzle opening onto the surface of a nearby recording medium.

An advantage of this bubble jet principle is that by utilizing the phase change liquid-gaseous-liquid of the ink liquid, the relatively large and rapid volume change necessary for ink ejection is obtained from a very small active transducer area (typically approx. 0.01 mm²) . The small transducer areas, in turn, when using modern manufacturing processes, such as high-precision photolithographic processes in layer technology, allow a relatively simple and inexpensive construction of ink writing heads which are distinguished by a high density of traces and small dimensions.

Such ink writing heads are therefore advantageously used in ink writing devices which operate according to the so-called drop-on-demand principle, i.e. The structure of the characters and / or the graphic patterns is carried out by the controlled ejection of individual ink droplets which, with a corresponding relative movement between the recording medium and the writing head, build up the characters or patterns in a grid in a predetermined character matrix. To generate an acceptable typeface, it is necessary to eject spherical droplets of the same mass and speed at precisely defined points in time.

In the known bubble jet writing heads, the surrounding ink liquid is heated up to evaporation by a suitable current pulse through the resistance heating element. The resulting ink vapor bubble briefly generates a high pressure in the ink channel, which leads to the ejection of an ink droplet, hereinafter referred to as primary drop, from the nozzle opening. During the collapse of the vapor bubble, there is negative pressure which draws liquid from the ink channel and leads to the withdrawal of the ink liquid in the outlet nozzle. If the two liquid fronts from the ink channel and the outlet nozzle now meet, overpressure arises again, which can lead to the ejection of another droplet, the so-called micro-jet, especially at high speeds of the primary drop. These unintentionally ejected droplets adversely affect the typeface because different ink masses are applied to grid points of the matrix grid.

The formation of these micro jets in bubble jet writing mechanisms can be suppressed by reducing the drop flight speed of the primary drop and thus the flow speeds in the ink channel as a whole. However, the lowering of the flying speed of the primary drop requires a lowering of the working speed (spray frequency) of the entire ink writing device.

The invention is therefore based on the object of specifying measures for an ink writing device of the type mentioned at the outset with which these micro-jets can be safely prevented in a simple manner without impairing the printing speed.

This object is achieved according to the features of the patent claim.

The formation of additional drops is prevented by reducing the excess pressure which arises when the vapor bubble collapses. This can be achieved by delaying the condensation of the vapor bubble, thereby reducing the flow rate at which the liquid fronts from the exit nozzle and the ink channel meet. This has the advantage that the speed of the primary drop remains unaffected.

The heat that is added or removed during this process has the greatest influence on the condensation of the ink vapor bubble. Therefore, the condensation of the ink vapor bubble can be delayed either by increased heat supply to the electrothermal transducer element or by reduced heat dissipation not according to the invention.

The increased supply of heat is achieved according to the invention by reheating the heating element by means of one or more current pulses of suitable duration but of the same amplitude. This subsequent heat supply is adjusted in terms of the time and the amount so that the collapse of the ink vapor bubble is only delayed, but not a new one is generated, which in turn would lead to the ejection of a further drop.

The invention is explained below using an exemplary embodiment, for which reference is made to the illustrations.

Fig. 1

in Prinzipdarstellung den Aufbau eines Tintenschreibkopfes,

Fig. 2

die Dampfblasenerzeugung nach einem Stand der Technik und

Fig. 3

die Dampfblasenerzeugung nach der Erfindung.

Show there

Fig. 1

in principle the structure of an ink writing head,

Fig. 2

the vapor bubble generation according to a prior art and

Fig. 3

the vapor bubble generation according to the invention.

1 shows a schematic representation of a print head structure as used in an ink printing device that works according to the so-called bubble jet principle. The ink writing head 10 is built up in layers and the ink is supplied through an opening in the cover plate. A series of ink channels 12 are formed on a substrate 11, for example made of silicon, by means of separating webs 13. Each ink channel 12 ends on the side facing a recording medium (not shown here) in an outlet nozzle 14. All ink channels 12 are connected to an ink chamber 15 in the interior of the print head 10. Each ink channel 12 is assigned an electrothermal transducer element in the form of a heating resistor 16, which can be contacted and controlled individually from the outside via contact connections 17, which run in the form of conductor tracks on the substrate 11. A protective layer 18 covering the heating resistors 16 and the contact connections 17 serves to reduce interfering influences between the ink liquid 23 and the resistance or contact material. There is a cover plate 19 above the substrate 11 and the channel structure 12 built thereon, which is preferably consists of glass or a similar material and in which the ink chamber 15 is formed. The ink is supplied to the ink chamber 15 via an opening 20 through which the ink liquid 23 is introduced into the ink chamber 15, for example via a line or via a channel 21, additional connecting and sealing devices 22 being provided.

The supply of ink liquid 23 into the ink chamber 15 can alternatively take place through an opening in the substrate 11.

The left half of FIG. 2 shows a current pulse 26 suitable for the control of the electrothermal transducer elements 16, which is characterized by its relative amplitude I / I _max under the pulse duration T. The right half of FIG. 2 shows the various stages of the droplet ejection with subsequent condensation of the ink vapor bubble 28 at time t = 0. The pulse is applied to the transducer element 16. This begins to heat up and the ink liquid 23 is heated locally. Pressure begins to build up within the ink channel 12 and the ink vapor bubble 28 begins to develop. After approximately 6 μs, the pulse is switched off again, the ink vapor bubble continues to grow, and a primary drop 24 is expelled from the outlet nozzle 14. The ink vapor bubble 28 condenses very quickly, in the ink channel 12 there is negative pressure when the vapor bubble collapses, through which ink liquid 23 is sucked out of the ink channel 12 and ink liquid 23 is withdrawn from the outlet nozzle 14. The collision of these two liquid fronts from the ink channel and the nozzle again leads to excess pressure, which causes an additional, troublesome drop, the so-called "micro-jet" 25, to emerge from the nozzle 14.

genügt ein Nachheizimpuls 27 gleicher Amplitude von ca. 3 µs Impulsdauer, der ca. nach 35 µs dem Hauptheizimpuls folgt. Dadurch wird eine Verzögerung der Kondensation der Dampfblase erreicht, ohne daß eine neue Dampfblase erzeugt wird, die ihrerseits zum Ausstoß eines weiteren Tropfens führen würde.3 explains how, according to the invention, these additional ink droplets 25 emerging from the outlet nozzle can be avoided. In contrast to FIG. 1, the condensation of the vapor bubble is delayed, as a result of which the flow velocities in the ink channel 12 are reduced, with which the liquid fronts from the nozzle and the nozzle Part of the ink channel 12, which is connected to the ink reservoir, meet. The speed at which the primary drop 24 leaves the outlet nozzle 14 remains unaffected. The delay in the condensation of the ink vapor bubble 28 is achieved by increased heat supply to the transducer element 16. A reheating pulse 27 is used for this purpose, which is applied to the heating element shortly before the ink vapor bubble finally collapses. In a typical application with a heating pulse 26 which triggers the primary drop 24 and has a pulse duration of approximately 6 μs and a relative amplitude ${\text{I / I}}_{\text{Max}}\text{= 1}$

a reheating pulse 27 of the same amplitude of about 3 µs pulse duration is sufficient, which follows the main heating pulse after about 35 µs. This achieves a delay in the condensation of the vapor bubble without creating a new vapor bubble, which in turn would lead to the ejection of another drop.

In order to achieve the desired delay in the collapse of the vapor bubble, it is also possible to apply several post-heating pulses of the same amplitude.

Claims (1)

1. A method for operating an ink printer which operates according to the thermal converter principle, in which

   - ink (23) is heated in localized manner until it evaporates by means of current pulses (26) through electrothermal converter elements (16) located in ink channels (12) of an ink print head (10),

   - the resulting ink vapour bubble (28) in the ink channel (12) briefly causes a high pressure which causes a drop of ink (24) to be ejected from the outlet nozzles (14) at the end of the ink channels (12),

   - the underpressure prevailing upon the subsequent collapse of the ink vapour bubble (28) draws ink (23) out of the ink channel (12) and draws ink (23) back from the outlet nozzles (14), and

   - the overpressure occurring upon the meeting of the two liquid fronts in the ink channel (12) and in the outlet nozzle (14) is reduced such that the condensation of the ink vapour bubble (28) is delayed by an increased supply of heat to the converter element (16) and thereby once the ink drop (24) has been ejected no further, unwanted, ink drop (25) is ejected, and

   - the increased supply of heat to the converter element (16) takes place by at least one subsequent heating pulse (27) following the current pulse (26) triggering the evaporation,

   characterised in that the subsequent heating pulse has the same amplitude as the current pulse.

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