JPH03146349A - Ink jet recording device - Google Patents

Abstract

PURPOSE:To change a drop quantity of an ink in a wide range and make tone recording feasible by a method wherein a first heater which applies one type or more discharge pressure to an ink in an ink passage in response to a tone signal, and one or more second heater which controls the viscosity of the ink in response to the tone signal are provided. CONSTITUTION:The subject ink jet recording device is equipped with an ink passage 9 which holds an ink 10 and an ink discharge port 7 which communicates with the ink passage 9, and is provided with a heater 1A which generates bubbles to jet necessary drops for recording by applying one type or more discharge pressure to the ink in response to a tone signal, and a heater 1B which controls the viscosity of the ink 10 in response to the tone signal. A resistance value of the heater 1B can be made larger than that of the heater 1A. Also, a pulse signal which is applied to the heater 1B may be made smaller than a pulse signal which is applied to the heater 1A. In addition, the heater 1B may be arranged between the heater 1A and the discharge port 7.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to an inkjet recording device, particularly an inkjet recording device using a thermal inkjet head.

(Prior Art) FIG. 8 shows one channel of a conventional thermal inkjet head, in which (A) is a longitudinal cross-sectional view and (B) is a cross-sectional view. In the figure, 1 is a heater, 2 is a heater substrate, 3 is a common electrode, 4 is an individual electrode, 5 is a channel substrate, 6 is a channel partition, 7 is a nozzle, 8 is an ink supply port, 9 is an ink flow path, and 10 is a It's ink. Ink 10 is supplied from an ink supply port 8 and fills an ink flow path 9. When a voltage as a print signal is applied between the common electrode 3 and the individual electrodes 4, the heater 1 is heated, generates a vapor bubble, and ink drops are ejected from the nozzle 7.

Fig. 9 explains the process of ink drop ejection, and Fig. 10 is an explanatory diagram of its characteristics, and the time points are indicated by ■■■... The situation at the time point is shown by ■■■... in FIG. 9. 1st
In Figure 0, ■ indicates the heater drive pulse, T indicates the heater surface temperature, and Q indicates the bubble volume. At point (2), no voltage is applied to the heater 1. Heater 1
When a driving pulse is applied, the heater surface temperature rises rapidly (■), and when the ink on the heater surface reaches superheat temperature, bubbles are generated (■). Further, due to the growth of the bubble (■), ink is pushed out of the nozzle (■) and is ejected from the nozzle as an ink drop, reaching a recording medium such as paper (not shown) (■). As the heater temperature decreases, the bubble contracts and disappears, and ink is supplied from the ink supply port, returning to a steady state (■).

Conventionally, to express the gradation of an image in such an inkjet recording device, it is difficult to change the size of the ink drop itself, so for example, a dither method is used to form a matrix of dots, and the dots in the matrix are Gradation is expressed in a pseudo manner using distribution.

However, this method requires a 4×4 matrix to express 16 gradations, and the actual resolution is 17
There is a drawback that it drops to 4. Therefore, attempts have been made to find ways to express gradations without sacrificing resolution.

The amount of drops ejected is, if the physical properties of the ink are constant,
Depends on the size of the bubbles generated. The size of this bubble depends on the heater-ink interface temperature. Furthermore, this temperature increase depends on the amount of heat generated by the heater, that is, the applied voltage or pulse width.

As a method for changing the drop amount, a method has been proposed in which the drop amount is modulated by changing the voltage value of a drive pulse applied to a heater for generating vapor bubbles. However, with this method, the range of drop amounts that can be modulated is narrow, and a sufficient gradation level cannot be obtained.

The multilevel recording method described in Japanese Patent Application Laid-Open No. 62-261453 arranges a plurality of heating elements in series, and selectively adjusts the timing of the plurality of heating elements according to the gradation in the order of distance from the nozzle. The bubbles generated by heating are transferred toward the nozzle. However, this method has the problem that as the number of gradations to be expressed increases, the number of heaters also increases and the driving method becomes complicated.

Furthermore, the inkjet head described in Japanese Patent Application Laid-Open No. 61-283555 expresses gradation by changing the diameter of a nozzle using a piezoelectric element and changing the area of a recording dot, for example.

Furthermore, there is also a method of changing the drop amount by changing the size of the ink flow path according to the gradation information.

However, devices involving these mechanical means increase the size of the device and are not preferable from the viewpoint of responsiveness. In addition, a method of expressing gradation by overlapping dots has been proposed, but this method has the drawback of requiring sacrifices in printing speed.

Therefore, none of the conventional gradation expression methods can be said to be appropriate.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned circumstances, and uses a relatively simple head structure and driving method to control the drop amount over a wide range without impairing resolution. It is an object of the present invention to provide a thermal inkjet recording device that can express a sufficient level of gradation without reducing printing speed.

(Means for Solving the Problems) The present invention provides an inkjet recording device that includes an ink channel holding ink and an ink discharge port communicating with the ink channel, and ink is discharged from the ink discharge port. An inkjet recording apparatus comprising: a first heater that applies one or more types of ejection pressure to ink in accordance with a tone signal within the ink flow path; and a first heater that controls the viscosity of the ink in accordance with the tone signal. The device is characterized by comprising at least one second heater.

The resistance value of the second heater can be made greater than the resistance value of the first heater. The pulse signal applied to the second heater may be smaller than the pulse signal applied to the first heater.

The second heater may be placed between the first heater and the nozzle.

The pulse signal applied to the second heater can be made to temporally precede the pulse signal applied to the first heater.

(Function) Generally, the amount of drops sprayed depends on the heater driving conditions (
(driving voltage, pulse width, ink temperature, driving frequency).

For example, as shown in FIG. 3, as the drive type and pressure increase, the amount of drops discharged tends to increase. The above-mentioned method of changing the voltage value of the drive pulse attempts to control the drop amount by voltage using this relationship. However, simply changing the size of the bubble by changing the thermal energy generated by the heater using voltage will not work, since the ink viscosity near the nozzle is constant and the ink moves the same, so the ink will not be ejected. There is not much change in the amount of ink, and it is not possible to achieve a sufficient effect.

As shown in FIG. 4, it is also known that even if the driving voltage is the same, the drop amount changes due to a decrease in ink viscosity due to an increase in ink temperature.

Therefore, the present invention focuses on the above two points, and provides a second heater near the nozzle to reduce the viscosity of the ink, and generates heat from this second heater to an extent that does not generate bubbles based on the gradation signal. In addition to lowering the ink viscosity, the electric power applied to the first heater is also changed to change the amount of drops to be ejected.

(Example) FIG. 1 is a longitudinal sectional view of an example of a head in an inkjet recording apparatus of the present invention. Portions similar to those in FIG. 8 are designated by the same reference numerals, and their explanation will be omitted. This embodiment differs from the conventional one in that a second heater IB is installed between the heater IA, which generates bubbles for ejecting the drops necessary for recording, and the nozzle 7. These two heaters are formed of, for example, poly-8i on a Si substrate coated with 5iOz. The second heater resistance value is made larger than the first heater resistance value.

The resistance value can be adjusted by changing the film thickness and the shape of the heater. Of course, materials with different resistivities may be used. A protective layer (not shown) made of 5i3N4Ta is formed on the first heater shape to protect the heater from cavitation damage when the bubble disappears.

FIG. 2 is an example of a drive circuit for these two sets of heaters. The first heater IA and the second heater IB are switching elements (FET) SWIA and 5W2A, respectively.
is connected to the collector of the heater 2A, 2 of the other nozzle.
B is a separate switching element (FET) SW2A, 5W
Connected to the collector of 2B. Heater IA is activated by applying the image signal to the gates of switching elements 5WIA and 5WIB.

IB will be driven. As for the image signal, as shown in FIG. 5, first VB is applied to heater IB, and then VB is applied to heater IB.
VA is applied to heater IA.

The resistance values and voltage application times of the first heater IA and the second heater IB are RA, RB and SA, SB, respectively.
Then, the amount of heat generated QA, QB is QA = (VA / RTA) ” RA 5AQB
= (VB /RTB) 2RB SB (However, RT
A%RTB is the resistance value of the entire circuit including the heater).

QB needs to select RB and TB to the extent that bubbles do not occur. In this example, SA =
3 (μ5ec) SB = 200 (μ5ec) RA = 160 (Ω) RB = 400 (Ω). The surface temperatures of heaters IA and IB at that time are shown as TA and TB in FIG. To obtain the above resistance value with poly-8i film, for example, the thickness of IA should be 0.35μ.
m, the film thickness of IB is 0.25 μm, and the heater area is I
A is 45μm x 15μm, IB is 45μm x 160μm
It can be obtained by setting m.

Figure 6 shows 83% by weight of water, diethylene glycol (D
This is a diagram illustrating the change in viscosity depending on temperature of an ink having a composition of 15% by weight (EG) and 2% by weight of dye. As described above, the higher the ink temperature, the lower the ink viscosity, the better the fluidity, and the easier it is to eject the ink. Therefore, when changing the voltage value applied to the first heater IA to change the bubble size and modulating the drop amount, the second heater IB
By generating enough heat to prevent bubbles from forming and controlling the viscosity (fluidity) of the ink, it is possible to vary the drop amount over a wide range.

In the above-described embodiment, only one second heater was provided for reducing the viscosity of the ink, but it is of course possible to provide two or more second heaters. The installation location also needs to have the effect of reducing the viscosity of the ejected ink.
It is not limited to the above-mentioned positions.

The constituent materials of the heater and head are not limited to those shown in this example. By providing the second heater between the first heater and the ink supply port, the viscosity of the ink near the ink supply port is also reduced, which has the effect of speeding up the resupply of ink when bubbles disappear. The jetting response of the head is improved.

FIG. 7 shows the voltage and amount of drops ejected according to this example,
This figure shows the relationship between the ink droplet and the optical reflection density (black) of the dot on paper, where the solid line represents the volume of the ink drop and the dotted line represents the optical density. In this way, the drop amount can be changed over a wider range and the image density can be changed compared to conventional modulation using only voltage.

(Effects of the Invention) As is clear from the above description, according to the present invention, the amount of ejected ink drops can be varied over a wide range, so gradation recording in a thermal inkjet printer can be performed with a simple configuration. can.

In addition, the gradation is expressed digitally, and the gradation level is set to 3 levels, and when used in combination with the dither method, 1
When expressing six gradations, a 2×2 matrix is sufficient, and the resolution can be doubled compared to the conventional dither method alone.

[Brief explanation of the drawing]

Fig. 2 is a longitudinal sectional view of an embodiment of the head in the inkjet recording apparatus of the present invention, Fig. 2 is a circuit diagram showing an example of the drive circuit for the head of Fig. 1, and Figs. 3 to 7 are ,
FIG. 8, which is an explanatory diagram of the operation of the embodiment, shows a conventional inkjet head, in which (A) is a vertical cross-sectional view, and (B)
) is a cross-sectional view, and FIGS. 9 and 10 are explanatory views of the operation of the inkjet head shown in FIG. 8. IA...First heater, IB...Second heater 2...Heater substrate, 3.4...Electrode, 5...Channel substrate, 6...Channel partition, 7... Nozzle, 8...Ink supply port.

Claims (1)

[Scope of Claims] An inkjet recording device comprising an ink flow path holding ink and an ink discharge port communicating with the ink flow path, and in which ink is ejected from the ink flow path, A first heater that applies one or more types of ejection pressure to the ink according to the gradation signal, and one or more second heaters that control the viscosity of the ink according to the gradation signal. An inkjet recording device featuring: