JPH03234535A - Retention of ink drop in constant size - Google Patents

Abstract

PURPOSE: To maintain a size of an ink droplet in a relatively wide temperature range constant by detecting a temperature of a printhead, and altering a parameter of a pulse to be applied to a heater in response to a detected temperature. CONSTITUTION: A temperature of a printhead is detected, and a parameter of a pulse to be applied to a heater is altered in response to a detected temperature. Accordingly, a droplet size is altered, and a different halftone effect can be created. For example, if a brighter tone is desired, to reduce a size of the ink droplet at the predetermined detected temperature, a pulse continuing time is shortened, and a pulse voltage is enhanced. Thus, predetermined droplet size can be maintained. In the case of a decided mass and size thermal storage unit, a possible burst mode printing capacity is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION The present invention relates to a method and apparatus for controlling the printhead of a thermal inkjet printer, and more particularly to a method and apparatus for controlling the printhead of a thermal inkjet printer, and more particularly for maintaining an approximately constant ink droplet size or The present invention relates to a method and apparatus for modifying the duration and voltage of a pulse applied to a heating element of a printhead in response to a measured temperature of the printhead.

BACKGROUND OF THE INVENTION The printhead of a thermal inkjet printer responds to electrical pulses to heat ink in contact with a heating element, causing it to evaporate and form bubbles that eventually direct the ink droplets from the nozzle onto the recording medium, i.e., paper. It is equipped with a series of heating elements that are emitted towards the target. To produce high quality prints, the temperature of the print head must stabilize within a specified range when the print head of a thermal ink jet printer is driven with pulses of predetermined duration and power. This specified temperature range is generally determined by considering the following two factors. First, ink drop ejection failure (i.e.
The specified temperature range covers a wide temperature range (approximately 10 °C to 70 °C).

Second, since the spot size of the ink droplets depends on the temperature of the printhead, the specified temperature range is limited to a narrow temperature range of about 10° C. to 20° C. within the wide temperature range mentioned above. For example, at the lower end of a narrow temperature range, the spot size will be small and not all the dead areas will be filled with ink. Furthermore, at the upper limit temperature of a narrow temperature range, the spot size becomes large, resolution decreases, and excessive ink adheres to the paper. Considering that in experiments, the temperature range that can prevent ink droplet ejection failure is 40°C to 50°C, and that the size of ink droplets depends on the temperature of the print head, the optimal temperature range is: 10″C
It turned out to be.

However, this method of operation has several problems. 1st
Even if the printhead is equipped with a high power external heater, a relatively long time is required to warm the printhead to the minimum acceptable operating temperature to obtain the desired spot size. (i.e., there is a long initial printout time.) This drawback can be eliminated by heating the printhead even when it is not in use (i.e., preheating to reduce warm-up time). Yes, but it requires constant heating of the print head. Second, the print head and its heat sink must be able to print at high temperatures in burst mode (printing operations that require large amounts of ink) before reaching the maximum allowable operating temperature for the desired ink drop spot size. The number of covered pages is quite small.

Improving burst mode printing capability requires increasing the mass of the printhead heat sink, with the penalty of increased printer cost and size.

U.S. Pat. No. 4,712,930 discloses an energy control means for changing the voltage and/or pulse width of a signal pulse applied to a print head in order to control the area of a print dot corresponding to a heating element. Discloses a print head. If the printhead gets too hot, the pulse width is reduced and/or the voltage is reduced. This US patent does not disclose controlling the thermal inkjet printhead, i.e., changing the pulse width and voltage, to maintain a constant ink drop size or to change the ink drop size. U.S. Pat. No. 4,679,055 discloses a halftone thermal inkjet printing method and apparatus that varies pulse width and/or voltage based on calculated printhead temperature. If the printhead temperature becomes too hot, the pulse width is reduced and/or the voltage is reduced. This US patent also does not disclose controlling the thermal ink side printhead, ie, changing the parameters of the pulses based on measured temperature fluctuations.

U.S. Pat. No. 4,719,783 discloses a temperature compensated thermal printing device that measures the temperature of the printhead to vary the pulse width. This US patent also does not disclose controlling the thermal ink printer, ie, changing the voltage.

U.S. Pat. No. 4,704,618 discloses a signal processing circuit that uses a temperature sensor to modify the signal applied to a resistor. This US patent also does not disclose controlling the thermal inkjet printer, ie, changing the voltage.

U.S. Pat. No. 4,636,812 discloses a temperature control device for a thermal printer that maintains a printhead at a predetermined temperature. U.S. Pat. No. 4,651,166 also discloses a temperature control device for a thermal printer. U.S. Pat. No. 4,633,269 also discloses a method and apparatus for heating a thermal print head. but,
The present invention is not described or suggested in any of the above US patents.

SUMMARY OF THE INVENTION A first object of the present invention is to improve the print head of a thermal inkjet printer in order to maintain a constant ink droplet size over a relatively wide temperature range or to vary the ink droplet size. The aim is to eliminate the above-mentioned drawbacks by providing a method and apparatus for controlling the invention.

The second objective is to control the print head of thermal inkjet printers by removing the constraint of a constant print head temperature and enabling instant start-up of the print head. It is an object of the present invention to provide a method and apparatus for performing II.

A third objective is to improve the printhead of thermal inkjet printers by reducing the mass and size of the printhead's regenerator, or by using a regenerator of a defined mass and size to improve the ability to print in burst mode. An object of the present invention is to provide a method and apparatus for controlling the invention.

SUMMARY OF THE INVENTION To the above and other objects, the present invention provides a method and method for controlling the printhead of a thermal inkjet print having a plurality of heating elements that generate ink droplets in response to electrical pulses. Disclose the device. The method and apparatus of the present invention detects the temperature of the print head and, in response to the detected temperature, changes the parameters of the pulses applied to the heating element to maintain the ink droplet size approximately constant; Change the size of the drops. Changes in pulse parameters include changes in pulse width and voltage. For example, when the temperature of the print head exceeds a predetermined temperature, the pulse width is shortened and the voltage is increased to maintain a constant ink droplet size.

Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings.

EXAMPLE It has been experimentally found that the operating characteristics of the thermal inkjet printer are affected by temperature fluctuations in the print head. If the print head temperature is too low, print quality defects may occur such as inconsistent jetting, poor character contours, or low print density. Furthermore, if the temperature of the print head is too high, defects in print quality may occur due to reduced resolution, insufficient drying, or inconsistent operation. Therefore, if the temperature of the print head is too high or too low, the operation of the gutter for printing will be inconsistent, but the temperature range is relatively wide (10 to 70 degrees Celsius). Within this wide temperature range, there is a narrow temperature range in which good print quality can be obtained. Experience has shown that the width of this narrow temperature range is 10-20°C, although the location of this narrow temperature range may be affected by differences in printhead and ink composition. When the temperature of the print head falls outside of this narrow temperature range, print quality deteriorates. Specifically, when the temperature of the print head becomes lower than the lower limit of the narrow temperature range, the characters are not completely filled in and the density of the print decreases, resulting in a decrease in the quality of the print. On the other hand, when the temperature of the print head becomes higher than the upper limit of the narrow temperature range, the lines become enlarged and the resolution of the print decreases, so that the quality of the print also deteriorates. Since printing is performed by applying electrical pulses to selected heating elements, the printing operation results in an increase in printhead temperature. Therefore, continuous high-density printing operations can cause temperature increases that exceed an acceptable range.

FIG. 1A is a graph showing how printhead temperature variations affect the volume of ink droplets ejected from a thermal inkjet printer. Changes in the volume of the ink drop cause a corresponding change in the spot size created when the ink drop impinges on the paper. Thermal inkjet printheads are designed to produce ink droplets of a size that overlaps the ink spots on the paper so that no blank spaces are left in the areas that are to be completely covered.

If the drop volume is not sufficient to completely cover the print, the density of the print will be unacceptably thin and the text will appear jagged. On the other hand, if the volume of the drop is large enough to form a spot on the paper that is considerably larger in size than that required for complete coverage;
Print resolution will be reduced.

The variation in droplet volume with temperature can be explained to some extent with reference to FIG. 1B. FIG. 1B is a graph showing the temperature of the ink layer on the heating element of a thermal inkjet printhead at the moment the ink layer in direct contact with the heating element reaches its nucleation temperature. For typical water-based inks, the nucleation temperature is about 280°C. The nucleation temperature used here is the temperature at which liquid ink suddenly becomes vapor (nucleation of bubbles is generated from nothing). The several curves shown in FIG. 1B show ink temperature (y-axis) as a function of distance from the heating element surface (y-axis) for different ambient temperatures.

For each ambient temperature shown in FIG. 1B, the temperature of the ink is at ambient temperature prior to the start of the electrical pulse. When an electric pulse is applied to the heating element in contact with the ink layer, the temperature of the heating element begins to rise. The heat flowing from the heating element to the lower temperature ink layer heats the ink layer that is in direct contact with the heating element. Those skilled in the art will appreciate that transient heat flowing into a vast medium produces the temperature profile illustrated. Figure 1B shows that the higher the ambient temperature, the higher the temperature profile during nucleation, rotating counterclockwise around the nucleation temperature of 280°C. Therefore, when the interface between the heating element and the ink reaches the nucleation temperature, the temperature of the ink at a predetermined distance from the heating element increases as the ambient temperature increases, as shown in FIG. 1B.

When the ink layer in contact with the heating element reaches the nucleation temperature, the ink layer suddenly turns into vapor. The vapor layer, or bubble, is initially very thin, but its high internal pressure causes the bubble to expand rapidly. At the liquid-vapor interface in the vapor layer, more liquid can become vapor, but the expanding bubble isolates the heating element and ink. Since air bubbles have low thermal conductivity, heat flow from the heating element to the ink layer is significantly impeded. However, as long as there is thermal energy available to supply heat of vaporization to the liquid changing to the gas phase, vaporization at the liquid-vapor interface can continue to supply bubbles. 1st B
As shown, the heat stored in the ink layer in contact with the heating element prior to bubble nucleation provides this thermal energy. However, not all of the thermal energy stored in the heated ink layer is available for further vaporization; only the boiling temperature of the ink can provide the heat for further vaporization. Only the ink layer that exceeds it.

For water-based inks, the boiling temperature (at atmospheric pressure) is 100°
It is C. Therefore, FIG. 1B shows the temperature profile above about 100'' of the respective temperature profile (point F).

The outline of the area (above the dotted line indicated by G and H) is shown.

These so-called superheated ink layers provide the thermal energy that causes bubble growth, which in turn jets the ink layer into the thermal inkjet printer. Thermal energy stored in the superheated ink layer is proportional to the y-axis, the temperature profile curve, and the area enclosed by the dotted line parallel to the y-axis through 100° C. in FIG. 1B. for example,
The area for the 55°C ambient temperature curve is bounded by points F, H, I, and the area for the 25°C ambient temperature curve is point F.

It is surrounded by G and I. Therefore, the higher the ambient temperature (FIG. 1B), the wider the region defined above and the greater the stored energy that causes bubble growth.

Therefore, although there may be other contributing factors,
As the temperature of the printhead increases, more energy is stored in the superheated ink layer, and it is this stored energy that drives the process, so thermal inkjet printheads produce larger drop volumes (and larger spots). on paper).

Figure 2A is a graph illustrating the experimental results obtained by measuring the volume of ink droplets produced by a thermal inkjet printhead when the ambient temperature was maintained constant and the duration of the pulse applied to the heating element was varied. It is. As will be explained later, for the measurements of FIG. 2A, when changing the duration of the pulse, the voltage of the pulse also needs to be adjusted. FIG. 2A shows that the ink layer volume varies depending on the duration of the drive pulse, showing that a short duration drive pulse produces a small volume ink drop and a long duration drive pulse produces a large volume ink drop. The changes can be explained with reference to FIG. 2B.

Similar to FIG. 1B, FIG. 2B shows the temperature profile within the ink layer at the moment when the ink layer in direct contact with the heating element reaches the nucleation temperature (280° C.). However, in Figure 2B, the individual curves are
5°C), representing the temperature profile for different drive pulse durations. As discussed with respect to FIG. 1B, it will be appreciated from FIG. 2B that longer duration drive pulses cause a greater amount of thermal energy to be stored in the superheated ink layer than shorter duration drive pulses. For example, 4μ
The area for a second pulse is bounded by points A, D, E and is larger than the area for a 2 μsec pulse surrounded by points A, B, E. Therefore, for long duration drive pulses, a large amount of thermal energy stored in the superheated ink layer causes large bubbles to form after nucleation, resulting in large volume ink drops. Conversely, short duration drive pulses result in droplets of smaller volume because less thermal energy is stored in the superheated water layer.

Droplet volume control mechanism on the heating element surface shown in Figure 2A (
The variation in drop volume with drive pulse duration can be applied to thermal inkjet printers by varying the drive pulse duration to maintain constant drop volume over a range of printhead temperature variations. This control is accomplished by measuring the temperature of the printhead and modifying the duration of the drive pulse to compensate for variations in printhead temperature. Therefore, if the print head temperature increases due to a print command or an increase in ambient temperature, the duration of the drive pulse will be shortened. Conversely, if the print head temperature decreases due to fewer print commands or lower ambient temperatures, the duration of the drive pulse will be extended.

However, when the above control method is applied to a thermal inkjet printer, it is impossible to maintain a constant spot size over a range of fluctuations in print temperature, and when the print head temperature exceeds a certain value,
It was found that the print head was unable to form ink drops.

The reason why this result occurs can be explained with reference to FIG. Figure 3 shows the temperature of the heating element (y-axis) versus time (
It is a graph expressed as a function of the y-axis). In Figure 3, the unit of the y-axis (time) is μseconds, and the unit of the y-axis is
It is °C. From Figure 3, the temperature of the heating element is the ambient temperature (25°C) at time = 0 when the heating pulse starts.
It can be seen that it starts at , and continues to rise during the time the heating pulse is on. Further, from FIG. 3, it can be seen that the temperature of the heating element initially increases rapidly as time progresses, but as time progresses, the rate of change in temperature decreases. However, at about 3 microseconds, the rate of change of the temperature of the heating element becomes high again. The reason why the gradient changes at 3 μs is that the temperature of the heating element reaches the nucleation temperature and the ink in contact with the heating element evaporates, and the thermal conductivity of air bubbles is low, so the heat does not transfer sufficiently to the ink. This is because it is not transmitted to the layers. As a result, the heat that had flowed to the ink layer in contact with the heating element remains within the heating element, increasing its temperature. (It goes without saying that heat is still flowing from the heating element to the supporting structure below.

) Thus, because of the low thermal conductivity of the bubble, the power that is subsequently supplied to the heating element after the bubble has formed has no effect on the growth of the bubble size and, therefore, on the size of the drops ejected by the printhead. No effect.

It can be seen that for thermal inkjet printheads, simply increasing the duration of the drive pulse does not have the desired effect of increasing the volume of ejected drops. It can also be seen from FIG. 3 that if the duration of the drive pulse is shortened to 2 microseconds or less, the temperature of the heating element does not reach the required nucleation temperature, so no bubbles or ink droplets are formed. Dew.

FIG. 4 is a graph depicting the temperature of the heating element of a thermal inkjet printhead as a function of time. Each curve represents three different power levels (voltages) applied to the heating element. From FIG. 4, it can be seen that at different power levels, the duration of the heating pulse is different. That is, for the curve corresponding to the highest input power level, the pulse duration is 2 μs, whereas for the curve corresponding to the lowest input power level,
The pulse duration is 4 μsec. Each of the curves in FIG. 4 shows a characteristically rapid increase in the temperature of the heating element near the end of the heating pulse, which is indicative of bubble formation. Those skilled in the art will appreciate that this heating time before bubble nucleation controls the amount of energy stored in the superheated ink layer at a given temperature. Therefore, by controlling the combination of drive pulse duration and power level, the bubble nucleation time and the energy stored in the superheated ink layer at a given temperature can be controlled as desired. This control of stored energy allows the drop volume to remain constant even as the printhead temperature varies.

Next, the effectiveness of this control method will be demonstrated in the following example. Two thermal inkjet prints (printheads A and B) with somewhat different internal geometries and materials were used with an ink consisting of water, ethylene glycol, dispersant, and Raven 5250 carbon blank as the colorant. It was tested. It was found that the optimum spot size for this printing was 122 μm in diameter. As can be seen from the table below (created by interpolating the measured data points), the optimal spot size of 122 μm is
This was achieved at different temperatures depending on whether driven with a microsecond drive pulse or a 3 microsecond heating pulse. The applied power level was adjusted as described above so that bubble nucleation occurred near the end of the heating pulse.

A 35°C45°C824°C
Although the value of the 40"C spot size is determined by the detailed structure of the printhead and the composition of the ink, it is clear that spot size control is effective even as the printhead temperature varies.

Note that, as shown in FIG. 3, the present invention varies the power or voltage of the pulse as well as the duration of the pulse, since the time required to reach the nucleation temperature is determined by the input power.

For example, for a given printhead ambient temperature, the longer the pulse duration, the more energy is available (see Figure 2B), and for a given pulse duration, the higher the printhead ambient temperature. , a lot of energy can be used (see Figure 1B), so if the temperature of the print head is constant, the changes in Figure 1B and the changes in Figure 2B can be traded off to increase heating without adding wasted energy. Shorter durations can be combined with increased voltages to reach the nucleation temperature near the end of the pulse (Figure 3); in other words, relatively short pulses require relatively high voltages. , and relatively long pulses require relatively low voltages.

Based on the above description, the method and apparatus of the present invention detects the temperature of the printhead, applies a pulse of predetermined power and duration to the heating element based on the detected temperature, and the resulting spot size is Controls the print head of a thermal inkjet printer so that it is the optimal size for high-quality prints. It is not necessarily necessary to directly measure the temperature of the heating element chip; it is sufficient to measure the temperature of the substrate that is thermally coupled to said chip. Once the printhead temperature is detected, the programmed routine searches a stored table or map for a predetermined pulse duration and voltage for the detected printhead temperature;
The retrieved pulse duration and voltage can be applied to the heating element.

If the detected temperature is higher than a predetermined temperature (the temperature at which the desired droplet size is obtained) (as compared by the normal comparison mechanism), the pulse duration is reduced to maintain the desired droplet size. , the pulse voltage is increased. If the detected temperature is below a predetermined temperature, the pulse duration is increased and the pulse voltage is decreased to maintain the desired ink drop size. If the detected temperature matches the predetermined temperature, the previously applied pulse duration and voltage will maintain the desired ink drop size.

It will be appreciated that using the present invention, by varying the voltage and duration of the pulses, temperature fluctuations can be compensated for and the desired ink droplet size maintained, thereby considerably extending the operating temperature range of the printhead. . Furthermore, by increasing the pulse duration or increasing the pulse voltage,
It can be seen that the minimum operating temperature can be lowered because the desired ink drop size can be produced at a lower temperature.

Therefore, two advantages are obtained.

(1) The print head is an instant-on type that does not require background heating. That is, initial printout time is reduced because the pulse duration and voltage can be varied to compensate for warm-up time.

(2) For a given mass and size of the regenerator, the possible burst mode printing capability is increased.

In other words, the duration and power of the pulses can be used to suppress temperature fluctuations, thus significantly reducing the mass and size of the regenerator.

Although the case where a constant droplet size is maintained has been described above, the present invention can also be used when changing the droplet size by changing the parameters of the pulse applied to the heating element according to the detected temperature. It is therefore possible to vary the droplet size to produce different halftone effects. For example, if a brighter tone is desired, the pulse duration is shortened and the pulse voltage is increased to reduce the size of the ink drop for a given detected temperature. Conversely, if a darker tone is desired, the pulse duration 10 is lengthened and the pulse voltage is lowered to increase the size of the ink drop for a given detected temperature.

Although preferred embodiments have been described, the described embodiments are intended to be illustrative and not limiting. From the above description, many modifications A and changes can be easily considered, but all of these modifications and changes B should be included in the scope of the present invention.

[Brief explanation of drawings]

FIG. 1A is a graph showing changes in the volume of ink droplets depending on the temperature of the print head. FIG. 1B is a graph showing the temperature profile in the ink layer in contact with the heating element when the print head temperature is different. FIG. , a graph showing the change in ink droplet volume depending on the pulse duration, FIG. 2B is a graph showing the temperature profile in the ink layer in contact with the heating element when the pulse duration is different, and FIG. 3 is a graph showing the temperature profile of the heating element surface. Graphs shown as a function of time and FIG. 4 are graphs showing the heating element surface temperature for different pulse durations and power levels. Volume of droplet (p0 Temperature (°C) Relationship between temperature of heating element and time FIG. 3

Claims (1)

[Claims] (1) A method for controlling a printhead of a thermal inkjet printer having a plurality of heating elements that generate ink droplets in response to electrical pulses, the method comprising: detecting the temperature of the printhead; and responding to the detected temperature. and changing the parameters of the pulse applied to the heating element so as to maintain the size of the ink droplet substantially constant.