Melt-on-demand solid ink thermal ink jet printhead

Abstract

 The present invention is a melt-on-demand printhead (20) that keeps the entire solid ink supply (24) at room temperature until it receives a print command, then it rapidly applies a distributed heat over a large surface area of the ink supply (24) without the aid of thermal conductors to transport the heat. A portion of the solid ink (24) melts and travels into the drop ejector (28) that expels an ink drop to the medium within seconds of when the printhead (20) received a print command. The melt-on-demand thermal ink jet printhead (20) has a solid ink supply (24), a drop ejector (28) located near the solid ink supply (24), and a thin-film solid ink heater (38) located near a large surface area of the solid ink supply (24) that it rapidly heats.

Description

Field of the Invention

[0001] This invention relates generally to thermal ink jet printheads and more particularly to thermal ink jet printheads that use solid ink.

Background of the Invention

[0002] Thermal ink jet printers have gained wide acceptance. W.J. Lloyd and H.T. Taub in "Ink Jet Devices," Chapter 13 of Output Hardcopy Devices (Ed. R.C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Patents 4,490,728 and 4,313,684 describe these printers. Thermal ink jet printers produce high quality print, are compact and portable, and print quickly but quietly because only ink strikes the paper. The typical thermal ink jet printhead uses liquid ink (i.e., ink having colorants dissolved or dispersed in a solvent). It has an array of precisely formed nozzles, each having a chamber that receives liquid ink from the ink reservoir. Each chamber has a thin-film resistor, known as a thermal ink jet firing resistor, located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the thermal ink jet firing resistor, a small portion of the ink, next to the thermal ink jet firing resistor, vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.

[0003] Liquid inks have several disadvantages and one of them is a reduction in the throughput (i.e., the speed at which the printhead prints a page) when printing in color. The printhead must print slowly or make multiple passes across the page to reduce color bleeding of the inks before they dry. Single pass printing that prints all colors concurrently could dramatically improve the printer throughput.

[0004] Additional disadvantages of liquid inks include low surface tension and high spreading. They cause the ink to wick along paper fibers and produce dots whose size depends on the media and that have a high degree of feathering. The amount of spreading depends on the contact angle of the droplet with the media, the wettability of the media, and the evaporation rate of the solvent from the ink. This feathering reduces print quality. Some thermal ink jet printheads use clay-coated paper to reduce feathering. Although this paper has the look and feel of fine quality paper, it is not plain paper and it is not readily available. Liquid inks can reduce feathering and dependency of the dot size on the media by having properties of high surface tension and low spreading, but they can obtain this only by decreasing the throughput of the printhead. Thus, liquid inks result in either low printhead throughput or in printed dots that have a high-degree of feathering and a size that depends on the print media.

[0005] Solid inks eliminate the problems of multi-pass color printing, dependency of the dot size on the media, and feathering. Solid inks strike the paper in liquid form and shortly thereafter thicken. Once an ink drop thickens, it cannot blend with other ink drops so the printer can print several different colors during a single pass of the printhead. Dot size no longer depends on the media; instead, it depends on the thickening properties of the ink. Feathering of the ink dots no longer occurs because penetration of the solid ink into the paper is precisely controlled. Another advantage of solid ink is that its operating temperature dictates its thickening properties. By controlling the thickening properties of the ink, one controls how much it spreads, how much it stands above or penetrates the print media, how much the dots of various colors blend, and the sharpness of the edges. An increased contrast is a further advantage of solid ink. Thus, solid inks have the advantages of increased printer throughput and higher quality print because they make the dot size independent of the print media, they have better print contrast, and they give the designer better control over the ink.

[0006] Presently existing thermal ink jet printheads that use solid inks have a lengthy warm-up cycle because they melt a large quantity of ink (i.e., a large pellet of solid ink) before printing and because they use localized heat sources to melt the ink. The localized heat sources generate heat at a point or over a small area. They need thermal conductors, such as metals, or heat pipes, to transfer heat to the ink supply. The localized heat sources and thermal conductors increase the mass and the heat capacity of the printhead. This increases the warm-up time of the printhead, quantity of heat needed to melt the ink, and the expense of the power supply that drives the heater.

[0007] The warm-up time of presently existing solid ink printheads can be thirty minutes long. This is unacceptable to most users. The delay can be reduced to sixty seconds by storing the ink at elevated temperatures when the printhead is idle, but many users would find this delay unacceptable. Even if this shortened delay is acceptable, the elevated temperatures cause ink degradation. This results in clogged nozzles; requirements for servicing the nozzle, such as wiping the nozzles; lower quality print; and stringent requirements on the materials and the design of both the printhead and the ink that results in higher costs.

Summary of the Invention

[0008] For the reasons previously discussed, it would be advantageous to have a solid ink thermal ink jet printhead that stores solid ink at room temperature and that begins printing a few seconds after receiving a print command. The present invention is a method and apparatus for melt-on-demand thermal ink jet printing that provides these and other advantages.

[0009] The present invention is a melt-on-demand printhead that keeps the entire solid ink supply at room temperature until it receives a print command, then it rapidly applies a distributed heat over a large surface area of the ink supply. A portion of the solid ink melts and travels into the drop ejector that expels an ink drop to the medium within seconds of when the printhead received a print command.

[0010] The melt-on-demand thermal ink jet printhead has a solid ink supply, a drop ejector located near the solid ink supply, and a thin-film solid ink heater located near a large surface area of the solid ink supply that it rapidly heats.

[0011] The thin-film solid ink heater that rapidly applies a distributed heat to a large surface area of the ink supply can be a stand alone device or it can be part of another device in the printhead such as the drop ejector. If it is part of the drop ejector, it can be a special purpose thin-film layer that is dedicated to the heater, or it can be an entire thin-film layer that serves an additional purpose, or it can be part of an etched thin-film layer that has other parts, such as thermal ink jet firing resistors and conductors.

[0012] The present invention provides an important advance in the state of the art of thermal ink jet printing because the printhead can store the entire solid ink supply at room temperature and begin printing within seconds of receiving a print command. This is accomplished primarily through the thin-film solid ink heater that applies heat to a large surface area of the solid ink supply without the aid of thermal conductors to transport heat. The low mass of the thin-film solid ink heater and the absence of thermal conductors produces a heater with a low mass and heat capacity that rapidly reaches its operating temperature and rapidly heats a large surface area of the solid ink supply. Thus, the printhead can begin printing within seconds of receiving a print command without heating the solid ink to high temperatures for prolonged intervals.

[0013] Additionally, printheads not realistically possible with other solid ink heaters are possible with the present invention. A page-wide array of printing elements is desirable because entire lines can be printed at once. A conventionally designed solid ink thermal ink jet printhead with a page-wide array may have power demands during the warm-up cycle that exceed the output of ordinary wall sockets. A page-wide thermal ink jet printhead designed according to the present invention can run on the power supplied by ordinary power sockets because the thin-film solid ink heaters have a low mass and heat capacity that reduces the power requirements of the printhead.

[0014] An exemplary embodiment of the invention will now be described with reference to the following drawings:

Figure 1 is a cut away drawing of the preferred embodiment of the melt-on-demand thermal ink jet printhead and shows the placement of the ink supply, the thin-film solid ink heater, and the drop ejector.

Figure 2 is a cut-away drawing of the drop ejector shown in Figure 1.

Figure 3 is an exploded drawing of the drop ejector shown in Figure 2 that shows the preferred embodiment of the thin-film solid ink heater.

Detailed Description of the Invention

[0015] Person skilled in the art will readily appreciate the advantages and features of the disclosed invention after reading the following detailed description in conjunction with the drawings.

[0016] Figure 1 shows a cross section of the preferred embodiment of the melt-on-demand thermal ink jet printhead 20. The printhead has a housing 22, a solid ink supply 24, a layer of molten ink 26, and a drop ejector assembly 28 which has a thin-film solid ink supply heater 38. Solid ink supply 24 sits on top of a the drop ejector assembly 28. The solid ink used in the preferred embodiment is a wax-type of ink, such as stearic acid, that melts at temperatures above room temperature. The thin-film solid ink heater 38 rapidly applies a distributed heat to the bottom of solid ink supply 24. The ink near the bottom area of solid ink supply 24 melts and forms the layer of molten ink 26. When the temperature of this ink reaches about 100°C and has a thickness of 2mm, the molten ink travels through input port 30 into firing chamber 32 where thermal ink jet firing resistor 34 heats it and ejects it through nozzle 36 to the medium (i.e., paper, plastic or any other substance printheads form printed characters on). Approximately, five to ten seconds will elapse from when the printhead receives a print command to when it ejects a drop.

[0017] Gas bubbles form in solid ink supply 24 as it cycles from a liquid to a solid and visa versa. The embodiment shown in Figure 1 is preferred because the gas bubbles will tend to rise upwards and away from firing chamber 32 where they would cause the printhead to malfunction. Alternate embodiments of the present invention may place drop ejector assembly 28 at the side of, or on top of, or at another location about solid ink supply 24.

[0018] As ink leaves the printhead, a void will form between the top of the drop ejector and the bottom of the ink supply. The entire ink supply must melt periodically to fill that void with ink. Another advantage of the preferred embodiment is that convection currents in the molten ink help melt the entire ink supply. As the molten ink at the bottom of the solid ink supply 24 gets hotter, its density becomes lower then that of the surrounding ink. The lower density ink rises upwards and creates convection currents that transfer heat to higher elevations of the solid ink supply. If convection currents were not available to melt the ink, then it would be necessary to make housing 22 from metal. The metal would increase the cost of the housing. Additionally, a metal housing would conduct heat away from the bottom surface of the ink supply so that the solid ink supply would need a more powerful and expensive heater.

[0019] Printhead 20 has a rigid, thermally insulating plastic housing 22 that forms the top and sides of the printhead. Alternate embodiments of the invention may use a foamed plastic housing. Either type of plastic housing minimizes the heat loss of the printhead and protects the user who touches the housing from the high temperatures of the printhead.

[0020] Figure 2 shows the drop ejector assembly 28 in more detail. In the preferred embodiment, substrate 62 in Figure 2 is a plastic film, preferably formed of a polyimide plastic such a Dupont Kapton. The base have any required thickness, but it is preferably from about 25 micrometers to about 3000 micrometers and most preferably from about 50 micrometers to about 300 micrometers thick. Plastic sheets of this thickness are commercially available on rolls. Alternate embodiments may use any plastic that does not melt at the operating temperature of the ink (approximately 140°C) and that is chemically stable in the presence of the ink.

[0021] Plastic is a poor heat conductor and plastic substrate 62 needs a heat spreader layer 64 to prevent the heat generated by thermal ink jet firing resistors 68 from forming hot spots on the plastic substrate 62. Heat spreader layer 64 consists of a layer of chromium and a layer of titanium. Commonly used direct current sputtering processes create both layers. This process places substrate 62 and either chromium or titanium in a vacuum and places a potential difference near 400 to 600 volts between them. The process creates a chromium layer having a thickness of approximately 1000 Å and later creates a layer of titanium having a thickness of approximately 3000-4000 Å. Heat spreading layer 64 aids in bonding the overlying structure to plastic substrate 62. Another alternate embodiment of the invention may use a silicon substrate instead of the plastic substrate. That embodiment would not need heat spreader layer 64.

[0022] A radio frequency sputtering technique deposits an insulating layer 66 of glass (silicon dioxide) on heat spreader layer 64. Insulating layer 66 electrically isolates heat spreader layer 64 from the thermal ink jet firing resistors, conductors, and contacts. In the preferred embodiment, the glass layer has a thickness of .6 micrometers and is flexible.

[0023] A tantalum aluminum layer 70 is created on insulating layer 66 using the direct current sputtering process that formed heat spreader layer 64. Tantalum aluminum layer 70 has a thickness of approximately 1600 Å and has a very thin layer of gold sputtered on it.

[0024] Figure 3 shows the drop ejector with the nozzle plate 122 and the insulating layer 120 removed from it. Their removal exposes the firing resistors 110, the firing resistor conductors 116, the thin-film solid ink heaters 108, the thin-film solid ink conductors 109, the input ports 112, and the insulating barriers 118. The molten ink flows from the solid ink supply to the firing chambers by passing through input ports 112. One firing resistor 110 resides in each firing chamber and power that flows on conductor 116 energizes it.

[0025] In the preferred embodiment of the invention, the printhead has a large array of thermal ink jet firing resistors such as 300 to 600 thermal ink jet firing resistors 110 per inch. A printhead with a page-wide array of thermal ink jet firing resistors may have as many as 6,000 resistors. As Figure 3 shows by the placement of the firing resistors and as Figure 2 shows by the location of input ports 80, the thermal ink jet firing resistors are not in a single line but divided into two rows or even three rows. By dividing the resistors into two or more rows the thermal ink jet firing resistors can be placed closer together.

[0026] In the preferred embodiment of the invention, thin-film solid ink heaters 108 and thin-film solid ink conductors 109 are etched into the same thin-film layer as firing resistors 110 and firing resistor conductors 116. In previously known thermal ink jet printheads, the tantalum aluminum layer that forms thin-film solid ink heater 108 and its conductors 109 was etched away.

[0027] Standard photolithographic techniques such as those described by Peter Van Zant, Microchip Fabrication: A Practical Guide to Semiconductor Processing, 2d ed.,(McGraw-Hill, New York, 1990) form thin-film solid ink heaters 108 and firing resistors 110 in tantalum aluminum layer 106. Additionally, these techniques remove the gold layer from the tantalum aluminum that make-up thin-film solid ink heaters 108 and firing resistors 110.

[0028] The dimensions of thin-film solid ink heater 108 and the resistivity of tantalum aluminum determine the resistance of the thin-film solid ink heater. Specifically,

where ρ is the resistivity of tantalum aluminum, L is the length of thin-film solid ink heater 108 and A is the cross sectional area of the thin-film solid ink heater. In the preferred embodiment of the invention, thin-film heater 108 is driven with 60 to 70 watts of power. If thin-film solid ink heater 108 is driven with 30 volts, then it should have a resistance of 15 ohms.

[0029] In the preferred embodiment of the invention, thin-film solid ink heater conductors 109 and firing resistor conductors 116 have a layer of copper electroplated over the gold layer to improve conductivity. Since copper readily corrodes, the copper is coated with a very thin layer of gold.

[0030] The preferred embodiment of the invention can estimate the temperature of the layer of molten ink 26. An electronic circuit accomplishes this by momentarily shutting-off the power to thin-film solid ink heater 108 and measuring its resistance. From the resistance, the temperature can be calculated because the resistance of tantalum aluminum is proportional to its temperature. In the preferred embodiment, drop ejector assembly is thin enough (approximately 3mm) and conductive enough that the temperature of thin-film solid ink heater 108 can be used to estimate the temperature of the layer of molten ink 26 in Figure 1. In alternate embodiments, the resistance of heat spreader layer 102 or even the firing resistors 110 maybe measured to estimate this temperature. The printhead drives firing resistors 110 with pulses having a duration of 5 µsec. every 500 µsec. This leaves 490 µsec. for measuring the resistance of firing resistors 110. Alternate embodiments of the invention may estimate the temperature by measuring the time that has elapsed from the print command.

[0031] A nozzle plate 122 in the preferred embodiment of the invention is made from the same type of plastic that substrate 100 is made from. Radio frequency sputtering techniques deposit an insulation layer 120 of silicon dioxide on nozzle plate 122. Standard photolithographic techniques (described by Van Zant, Microchip Fabrication) etch firing chamber 32 shown in Figure 1 into this layer. A heat staking process bonds the nozzle plate assembly, shown in Figure 3, to the substrate 100 and its super structure. Glue is applied to either or both the nozzle plate assembly and the structure on substrate 100. Then, the pieces are pressed together and heated. An exicmer laser drills input port 112 and nozzles 124.

[0032] An alternate embodiment of the invention uses the thermal ink jet firing resistors 34 in Figure 1 to rapidly apply a distributed heat to the solid ink supply. If one would remove thin-film solid ink heaters 108 and thin-film conductors 109 from Figure 3, the resulting apparatus would be identical to this embodiment.

[0033] When the printer receives a print command, the printhead drives each firing resistor 34 with an ink melting wattage of approximately .1 to .001 watt and preferably .01 watt to form a layer of molten ink 26. The heat travels through the insulation to the heat spreader layer 102 in Figure 3. Heat spreader layer 102 further spreads out the heat. While the printhead is responding to the print command, the printhead periodically measures the temperature of the layer of molten ink 26 shown in Figure 1 and when it detects a low temperature it drives each firing resistor with approximately .01 watt of power when it is not driving the firing resistor with a printing pulse. As stated earlier, printing pulses last for about 5µ sec. and occur no more frequently then once every 500 µsec. By using the firing resistors 34 to heat the solid ink supply, the printhead gains an ink supply heater without increasing the heat capacity of the printhead.

[0034] In another alternate embodiment of the invention, heat spreader layer 64 in Figure 2 has an additional function, it is the thin-film solid ink heater that rapidly applies a distributed heat to solid ink supply 24 in Figure 1. The heat spreader layer that functions as a thin-film solid ink heater is identical to the heat spreader layer described earlier in this document with the addition of electrical contacts so that the power supply can drive heat spreader layer 64 with a low wattage power. Standard photolithographic techniques (described by Van Zant, Microchip Fabrication) etch a hole through insulating layer 66, tantalum aluminum layer 70 and the overlying gold layer. Standard sputtering techniques (described by Van Zant, Microchip Fabrication) fill the hole with a conductive metal, such as copper, so that the printhead can drive heat spreader layer 64. The printhead should drive heat spreader layer 64 with between 3 and 300 watts and more likely between 60 and 70 watts. By building the thin-film solid ink heater into heat spreader layer 64, the printhead gains a solid ink heater without increasing the heat capacity of printhead 20 shown in Figure 1. This is important because an increase in the heat capacity of the printhead increases the amount of heat that must be delivered to printhead 20 to melt solid ink supply 24. This lengthens the warm-up time of the printhead. Additionally, it increases the power output requirements of the power supply. This directly increases the cost of the power supply.

[0035] The thin-film solid ink heater that instantly applies a distributed heat over a wide expanse of the ink supply is a vast improvement over the commonly used cartridge heater that is a point source of heat and that relies on thermal conductors, such as heat pipes, to transfer its heat to a wide expanse of the ink supply. The thermal conductors, like the single purpose heat cartridge, add mass and heat capacity to the printhead that lengthens the warm-up time and increases the printhead's heat requirements. This increases the cost of the power supply since the price of the power supply is proportional to the wattage of the power supply.

Claims

1. A melt-on-demand thermal ink jet printhead (20) in a printer that receives a plurality of print commands and that prints on a medium, comprising:

a. a solid ink supply (24) that has a surface;

b. a means, that resides near the surface of the solid ink supply (24), for ejecting drops (28);

c. a means (38 positioned within the drop ejection means (28), for rapidly applying a distributed heat to a section of the surface of the solid ink supply (24) to form a layer of molten ink (26) that can travel into the drop ejecting means (28); and

d. a means for driving the rapid acting, distributed heating means with

2. A melt-on-demand thermal ink jet printhead (20) in a printer that receives a plurality of print commands and that prints on a medium, further comprising:

a. a solid ink supply (24) that has a surface;

b. a means, that resides near the solid ink supply (24), for ejecting drops (28);

c. a plastic substrate located near a section of the surface of the solid ink supply (24);

d. a means, attached to the plastic substrate, for rapidly applying a distributed heat to the section of the surface of the solid ink supply (24) to form a layer of molten ink (26); and

e. a means for driving the rapid acting, distributed heating means with a power signal.

3. A melt-on-demand thermal ink jet printhead (20), as in claim 1 or 2, further comprising a means for storing the solid ink supply (24) and a quantity of ink in the drop ejection means (28) at room temperature until the printer receives a print command.

4. A melt-on-demand thermal ink jet printhead (20), as in claim 1 or 2, wherein the rapid acting, distributed heating means (38) forms within a time interval less than 20 seconds a layer of molten ink (26) that has a volume that allows the printer to begin printing less than 20 seconds after the printer receives a print command.

5. A melt-on-demand thermal ink jet printhead (20), as in claim 1 or 2, wherein the driving means further comprises:
   a means for estimating a temperature of the surface of the solid ink supply (24) and producing a temperature signal; and
   a means for varying the power signal driving the rapid acting, distributed heating means in response to the temperature signal.

6. A melt-on-demand thermal ink jet printhead (20), as in claim 2, wherein the rapid acting, distributed heating means (38) is an array of firing resistors (34).

7. A melt-on-demand thermal ink jet printhead (20), as in claim 2, wherein the rapid acting, distributed heating means (38) is a thin-film metal layer that has a contact for the driving means.

8. A melt-on-demand thermal ink jet printhead, as in claim 2, wherein:
   the surface of the solid ink supply (24) is a bottom surface of the solid ink supply (24);
   the rapid acting, distributed heating means (38) applies the distributed heat to the bottom surface of the solid ink supply (24) and forms the layer of molten (26) ink near the bottom surface of the solid ink supply (24);
   the means for ejecting drops (28) has an input port (30) positioned to receive the molten ink (26) as the molten ink (26) flows downward from the solid ink supply (24);
   the means for ejecting drops (28) is positioned above the medium; and
   the means for ejecting drops (28) has a nozzle (36) directed downward to the medium so that the molten ink (26) is ejected downward towards the medium.

9. A melt-on-demand thermal ink jet printhead (20), as in claim 2, wherein:
   the drop ejection means (28) has a thin-film metal layer; and
   the rapid acting, distributed heating means is a section of the thin-film metal layer (108) that has a contact for the driving means.

10. A method for melt-on-demand thermal ink jet printing, comprising the steps of:

a. storing an entire solid ink supply (24) at room temperature;

b. receiving a print command;

c. applying rapidly, a distributed heat to a section of a surface of the solid ink supply (24) in response to the print command to form a layer of molten solid ink (26); and

d. ejecting drops of molten ink (26).

Drawing