EP0118640A2

EP0118640A2 - Method of adjusting thermal ink jet printers

Info

Publication number: EP0118640A2
Application number: EP83306808A
Authority: EP
Inventors: John David Meyer
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1982-11-22
Filing date: 1983-11-09
Publication date: 1984-09-19
Also published as: EP0118640A3; JPS5995154A

Abstract

A method for designing a thermal ink jet printer for minimum cavitation damage of the printhead resistors during bubble collapse and, consequently, maximum lifetime, is shown. The method comprises the steps of selecting a solvent, resistor size and operating temperature so that a calculated B-factor is minimized. The selected parameters can be changed to minimize the calculated B-factor and to achieve a maximum lifetime.

Description

This invention is concerned with a method of designing thermal ink jet printers.
Application of a current pulse to a thermal ink jet printer as described for example in UK Patent Application No. 8217720, causes an ink droplet to be ejected from a printhead nozzle by heating a resistor located within an ink supply. This resistive heating causes an ink vapor bubble to form over the resistor and the resultant pressure increase in the ink supply forces the desired ink droplet from the printhead. Thermal ink jet printer lifetime is dependent upon resistor lifetime and a majority of resistor failures result from cavitation damage which occurs during bubble collapse.
In order to make multiple nozzle, e.g., page width, arrays, economically feasible, it is important that thermal ink jet printer lifetimes exceed at least one billion droplet ejections. Thermal ink jet printer design techniques in the prior art have attempted to optimize various parameters other than lifetime and attainment of minimum cavitation damage susceptibility and long printhead lifetime has, typically, not been pursued.
Florschuetz and Chao, in "Transactions of the ASME" (Journal of Heat Transfer), May 1965, at 209-220, disclose a calculation for a "B-factor" which characterizes the mechanics of a collapsing bubble. Florschuetz and Chao also teach that heat transfer domination of the bubble collapse changes to liquid inertia domination within a B-factor range of 0.05 to 10.
In accordance with the illustrated preferred embodiment of the present invention, a method is shown which utilizes the B-factor calculation in designing thermal ink jet printers having minimum cavitation damage susceptibility and long lifetimes.
The present invention provides a method for designing a thermal ink jet printer for ejecting ink droplets from an ink supply by heating a printhead resistor and creating a bubble in said ink supply, the method being characterized by the steps of selecting an ink, selecting a maximum radius of the bubble, selecting an operating temperature of the thermal ink jet printer, and calculating a B-factor of the thermal ink jet printer to be less than or equal to a preselected value.
A method as set forth in the last preceding paragraph may be further characterized by the step of selecting a printhead resistor having a long dimension and a short dimension so that an average of the long dimension and the short dimension is substantially equal to the maximum bubble radius.
In carrying out a method as set forth in either one -of―- the last two immediately preceding paragraphs, it is preferred that the preselected value of the B-factor is 50.
A method as set forth in the last preceding paragraph may be further characterized by the step of decreasing the density of the ink so that the B-factor of the thermal ink jet printer is decreased.
In carrying out a method as set forth in the last preceding paragraph, it is preferred that the step of decreasing the density of the ink comprises adding a material to the ink which has a density less than the density of the ink.
In carrying out a method as set forth in the last preceding paragraph, the preferred material may comprise particulate material or pigment.
A method as set forth in the last preceding paragraph may be further characterized by the step of increasing the operating temperature of the thermal ink jet printer so that the B-factor is decreased.
In carrying out a method as set forth in any one of the last seven immediately preceding paragraphs, it is preferred that the ink has a density (p), a specific heat (c) and a latent heat of vaporization (L), and the step of selecting the ink comprises selecting the ink for a minimum value of pc/L.
In carrying out a method as set forth in the last preceding paragraph, it is preferred that the step of selecting the ink further comprises selecting the ink for a minimum boiling temperature.
In carrying out a method as set forth in the last preceding paragraph, it is preferred that the step of selecting the ink further comprises selecting the ink for a maximum gram molecular weight.
There now follows a detailed description which is to be read with reference to the accompanying drawings of a method according to the invention; it is to be clearly understood that this method has been selected for description to illustrate the invention by way of example and not by way of limitation.
In the accompanying drawings:-

Figure 1 is a part-sectional perspective view of a printhead of a thermal ink jet printer which was designed in accordance with the method of the preferred embodiment of the present invention; and
Figure 2 is a flow chart of the method used to design the thermal ink jet printer of Figure 1.

Referring to Figure 1, ink is received from a reservoir through a supply tube 3 and is supplied to a capillary region 11. When a current pulse is applied to a resistor 5 (through conductors which are not shown), resistive heating causes a bubble to form in the ink overlying the resistor 5 and an ink droplet is forced from a nozzle 9. Multiple nozzles may be located on the printhead 1 and barriers 7 are used to eliminate crosstalk between nozzles. A thermistor- controlled heater 13 is mounted on the printhead 1 and is used to maintain the ambient temperature of the printhead 1 at a preselected operating temperature. The operation of the printhead 1 is described in more detail in the above- discussed UK Patent Application.
Figure 2 is a flow chart of the method used to design the printhead 1 with reference to a B-factor calculation. The B-factor used herein is defined, in the above-referenced article by Florschuetz and Chao, as:
In Equation 1, p is the density of the ink used, L is the latent heat of vaporization of the ink used, c is the specific heat of the ink used, AT is the difference of the ambient temperature and the boiling point of the ink being used, K is the thermal diffusivity of the ink being used, and Δp is the difference of the ambient pressure and the vapor pressure of the ink being used. Persons of ordinary skill in the art will be able to derive these parameters for a given ink under consideration.
Further, R_o is the maximum radius of a bubble created in the ink supply by heating of the resistor 5. _Pv is the integrated average vapor density which is defined as:
where ρ_v is the vapor density of the ink being used and T_SATis the saturation temperature of the ink being used. ψ is a temperature difference correction factor which is defined as:
where p_v is the equilibrium vapor pressure of the ink being used, and p
is the final system pressure.
In the flow chart of Figure 2, step 21 is typically performed first, although the step sequence may be changed if necessary, because the size of the resistor 5 is usually determined by the required resolution. For instance, if a resolution of 200 dots per inch is desired, the total width of the barrier 7 and the resistor 5 must be less than .127mm (.005 inch). To a very close first approximation, the radius of the resistor 5 (or an average of a long dimension and a short dimension if the resistor 5 is other than round) is equal to the maximum bubble radius, R_o. A higher degree of certainty can be achieved by actually measuring R_o with the aid of a microscope and a strobe light. Note that in Equation 1, the B-factor decreases only linearly with increases in R_o.
In step 23, a solvent which comprises the base of the ink being used in the printhead 1 is selected with reference to the B-factor calculation discussed hereinabove. It is usually necessary to select a solvent from a restricted group of solvents which have suitable characteristics such as viscosity and volatility. It is important to select a solvent which has a minimum ^pc/L since the B-factor decreases as the square of pc/L. It is also important that the solvents selected have a minimum boiling temperature since the B-factor is directly proportional to the square of AT. Finally, the solvent selected should have as large a gram molecular weight as possible so that ρ _v, which decreases the B-factor by an inverse square law relationship, is maximized. All of the parameters required to make the solvent selection are available in standard reference textbooks.
In step 25, the operating temperature of the heater 13 is selected. For a first calculation, the operating temperature will usually be chosen as room ambient for conven- - ience. Nevertheless, since the B-factor increases as the square of AT, the operating temperature should be maximized to minimize the B-factor.
In step 27, the B-factor of equation 1 is calculated and in step 29 a decision is made with regard to the B-factor. If the B-factor is 50 or less it can be assumed that a life time in excess of one billion droplet ejections will be achieved and design may be stopped at this point if further optimization is not necessary.
If the B-factor is greater than 50, or if further optimization is desired, the operating temperature of the heater 13 is increased in step 31. This decreases AT and, consequently, the B-factor as the square of AT. Again, in step 33, a determination is made about the magnitude of the calculated B-factor. If the B-factor is still too high, the density, p, of the total ink being used can be decreased, in step 35, since the B-factor decreases as the square of p. This can easily be done by adding to the solvent pigments or other, particulate, matter which have a lower density than that of the solvent alone. In step 37, the B-factor is again evaluated and various of the steps 21-35 can be performed to optimize the B-factor as needed.
In actual practice, a printhead 1 was designed in accordance with the method shown in Figure 2 and no evidence of cavitation damage was detected after 16 billion droplet ejections. A 2 kilohertz drive pulse having a 25 microsecond precursor section at .33 amperes and a 2 microsecond .63 ampere trigger section was used. A .003 inch (.0076mm) square resistor 5 and made of metallic glass on a silicon substrate and having a resistance of 4 ohms was used in conjunction with a .003 inch (.076mm) diameter nozzle 9. Isopropyl alcohol was used as the solvent and 20 degrees centigrade was the operating temperature.

Claims

1. A method for designing a thermal ink jet printer for ejecting ink droplets from an ink supply by heating a printhead resistor and creating a bubble in said ink supply, the method being characterized by the steps of:

selecting an ink;

selecting a maximum radius of the bubble;

selecting an operating temperature of the thermal ink jet printer (1); and

calculating a B-factor of the thermal ink jet printer to be less than or equal to a preselected value.

2. A method according to claim 1, further characterized by the step of selecting a printhead resistor (5) having a long dimension and a short dimension so that an average of the long dimension and the short dimension is substantially equal to the maximum bubble radius.

3. A method according to either one of claims 1 and 2, characterized in that the preselected value of the B-factor is 50.

4. A method according to claim 3, further characterized by the step of decreasing the density of the ink so that the B-factor of the thermal ink jet printer is decreased.

5. A method according to claim 4, characterized in that the step of decreasing the density of the ink comprises adding a material to the ink which has a density less than the density of the density of the ink.

6. A method according to claim 5, characterized in that the added material comprises particulate matter.

7. A method according to either one of claims 5 and 6 characterized in that the added material comprises a pigment.

8. A method according to either one of claims 6 and 7 further characterized by the step of increasing the operating temperature of the thermal ink jet printer so that the B-factor is decreased.

9. A method according to any one of the preceding claims characterized in that the ink has a density (p), a specific heat (c) and a latent heat of vaporization (L), and the step of selecting the ink comprises selecting the ink for a minimum value of pc/L.

10. A method according to claim 9, characterized in that the step of selecting the ink further comprises selecting the ink for a minimum boiling temperature.

11. A method according to claim 10, characterized in that the step of selecting the ink further comprises selecting the ink for a maximum gram molecular weight.