Claims
- 1. A hot melt ink system comprising a hot melt ink having a melting point, ink jet means for projecting the hot melt ink at elevated temperature toward a substrate to produce ink spots on the substrate, platen means for supporting the substrate, and temperature control means for controlling the temperature of the paten means to maintain the portion of a substrate which receives the hot melt ink from the ink jet means at a temperature in the range from about 25.degree. C. below to about 25.degree. C. above the melting point of the hot melt ink.
- 2. A hot melt ink jet system comprising a hot melt ink having a melting point, ink jet means for projecting the hot melt ink at elevated temperature toward a substrate to produce ink spots on the substrate, platen means for supporting the substrate, and temperature control means for controlling the temperature of the platen means to maintain the portion of a substrate which receives the hot melt ink from the ink jet means at a temperature in the range from about 25.degree. C. below to about 25.degree. C. above the melting point of the hot melt ink wherein the temperature control means controls the temperature of the platen means to maintain the substrate at a temperature which is below the temperature at which the ink viscosity is 200 cp by a temperature difference in the range from about one-quarter to twice the difference between the elevated temperature and the temperature at which the ink has a viscosity of about 200 cp.
- 3. A hot melt ink jet system according to claim 2 wherein the substrate is maintained at a temperature which is below the temperature at which the ink viscosity is 200 cp by approximately one-half the difference between the elevated temperature and the temperature at which the ink viscosity is 200 cp.
- 4. A hot melt ink jet system according to claim 2 wherein the substrate is maintained at a temperature which is below the temperature at which the ink viscosity is 200 cp by a temperature difference which is between about one and two times the difference between the elevated temperature and the temperature at which the ink viscosity is 200 cp.
- 5. A hot melt ink jet system according to claim 2 wherein the substrate is maintained at a temperature which is below the temperature at which the ink viscosity is 200 cp by a temperature difference which is between about one-quarter and one-half the difference between the elevated temperature and the temperature at which the ink viscosity is 200 cp.
- 6. A hot melt ink jet system according to claim 1 wherein the hot melt ink has a viscosity in the range from about 10 cp to about 35 cp at the elevated temperature.
- 7. A hot melt ink jet system according to claim 1 wherein the hot melt ink has a viscosity in the range from about 15 cp to about 25 cp at the elevated temperature.
- 8. A hot melt ink jet system according to claim 1 wherein the hot melt ink has a surface tension in the range from about 10-40 dynes/cm at the elevated temperature.
- 9. A hot melt ink jet system according to claim 1 wherein the hot melt ink is projected from the ink jet means in drops having a volume in the range from about 50-100 picoliters.
- 10. A hot melt ink jet system comprising a hot melt ink having a melting point, ink jet means for projecting the hot melt ink at elevated temperature toward a substrate to produce ink spots on the substrate, platen mans for supporting the substrate, and temperature control means for controlling the temperature of the platen means to maintain the portion of a substrate which receives the hot melt ink from the ink jet means at a temperature in the range from about 25.degree. C. below to about 25.degree. C. above the melting point of the hot melt ink including a fibrous substrate supported on the platen means, a first plurality of ink drops having a selected diameter projected from the ink jet means toward a first region of the substrate, a second plurality of ink drops on the surface of a second region of the substrate immediately adjacent to the first region having a diameter no more than about 50% greater than the selected diameter, and a third plurality of ink spots on the substrate at a third region farther from the first region than the second region, wherein the ink drops in the third region have been absorbed by the fibrous substrate and have a diameter at least twice the selected diameter.
- 11. A hot melt ink for use in an ink jet system having an ink jet head which projects hot melt ink at an elevated temperature in the range from about 110.degree. C. to about 140.degree. C. at a viscosity in the range from about 10 cp to about 35 cp toward a substrate having a lower temperature in the range from about 45.degree. C. to about 91.degree. C. comprising an ink having a viscosity in the range from about 200 cp to about 300 cp in the temperature range from about 73.degree. C. to about 104.degree. C.
- 12. A hot melt ink according to claim 11 having a viscosity in the range from about 200 cp to about 300 cp in the temperature range from about 79.degree. C. to about 104.degree. C.
- 13. A hot melt ink according to claim 11 having a viscosity in the range from about 10 cp to about 35 cp in the range from about 120.degree. C. to 130.degree. C.
- 14. A hot melt ink for use in an ink jet system having an ink jet head which projects hot melt ink at an elevated temperature in the range from about 110.degree. C. to about 140.degree. C. at a viscosity in the range from about 10 cp to about 35 cp toward a substrate having a temperature in the range from about 45.degree. C. to about 91.degree. C. comprising an ink having a viscosity in the range from about 200 cp to about 300 cp at a temperature which is above the substrate temperature by about one-third the difference between the substrate temperature and the elevated temperature.
REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of the copending Spehrley et al. U.S. Application Ser. No. 07/094,664, filed Sept. 9, 1987, now U.S. Pat. No. 4,751,528.
BACKGROUND OF THE INVENTION
This invention relates to hot melt ink jet printing systems and, more particularly, to a new and improved hot melt ink jet printing system providing controlled ink drop spreading and penetration for enhanced image quality.
Ink jet systems using inks prepared with water or other vaporizable solvents require drying of the ink (i.e., vaporization of the solvent) after it has been applied to a substrate, such as paper, which is supported by a platen. To facilitate drying of solvent-based inks, heated platens have previously been provided in ink jet apparatus.
Certain types of ink jet apparatus use inks, called "hot melt" inks, which contain no solvent and are solid at room temperature, are liquefied by heating for jet application to the substrate, and are resolidified by freezing on the substrate after application. In addition, the application of hot melt ink to a substrate by an ink jet apparatus transfers heat to the substrate. Moreover, the solidification of hot melt ink releases further thermal energy which is transferred to the substrate and supporting platen, which does not occur with the application of solvent-based inks. With high-density coverage this can raise the temperature of the paper and the platen above limits for acceptable ink penetration.
In the co-pending Spehrley et al. U.S. Application Ser. No. 094,664 filed Sept. 9, 1987, now U.S. Pat. No. 4,751,528, hot melt ink jet apparatus is described in which the temperature of the platen supporting the print medium is controlled. As described in that application, if the substrate temperature is too low, the ink freezes after a short distance of penetration into a porous substrate such as paper, producing raised ink droplets and images with an embossed characteristic. Such ink droplets or images may have poor adhesion or may easily be scraped off or flake off by action of folding or creasing or may be subject to smearing or offsetting to other sheets. Further, raised images having an embossed appearance and a height exceeding about 0.4 mils are often found to be objectionable in the office printing environment. If the paper temperature is too high, however, the size of the ink spot from each drop will vary depending on the characteristics of the paper and, in some cases, the ink does not solidify before it has penetrated completely through the paper, resulting in a defective condition called "print-through".
To overcome these difficulties in accordance with that co-pending application, the support platen temperature is controlled, and by means of intimate thermal contact thereto, the paper substrate temperature is kept at a desired level so that the resulting image stays constant, independent of ambient temperature changes and independent of other printing conditions such as the amount of ink deposited on the paper surface.
It has been suggested in the co-pending application that substrate temperatures above the melting point of the hot melt ink may produce images with larger-than-normal spot size, fuzzy edges, blooming of fine lines, and the like. In addition, even if the substrate temperature were held constant, the image characteristic might vary according to paper characteristics, such as basis weight, rag content, void content, sizing, filler and roughness, because freezing of the ink would not terminate penetration of the ink into the substrate resulting from the thermal interaction of the ink and paper. It is known in the art that hot melt inks produce much more constant image characteristics than do nonfreezing ink systems based on water or glycol. It has heretofore been believed that, if the substrate temperature is allowed to exceed the melting point of a hot melt ink, the thermal stabilizing effect would be lost, and it would not be possible to provide uniformly high printing quality with hot melt inks on a variety of paper substrates. As a consequence, it has been assumed that many inks which have otherwise desirable characteristics cannot be jetted without objectionable embossing or without adequate smear resistance because the inks do not penetrate optimally.
Accordingly, it is an object of the present invention to provide a new and improved hot melt ink jet printing system which is effective to overcome the above-mentioned disadvantages of the prior art.
Another object of the present invention is to provide a hot melt ink jet printing system which is especially adapted for use with a variety of substrate materials having different characteristics.
A further object of the invention is to eliminate the necessity for precise control of the cooling time for hot melt ink applied to a substrate to avoid insufficient or excessive penetration of the ink into the substrate and the corresponding requirement for continuous, constant-speed printing to maintain a desired substrate temperature.
These and other objects and advantages of the invention are attained by providing a hot melt ink jet printing system wherein the hot melt ink-receiving substrate is maintained at a selected temperature above or below the melting point of the hot melt ink, the selected temperature being dependent upon the jetting temperature of the ink and the melting characteristics of the ink, and particularly the relationship between the viscosity of the ink and its temperature. Surprisingly, for substrate temperatures within a limited temperature range above the ink melting point, which are selected in accordance with those characteristics, the ink does not flow sufficiently through most substrates to cause "print-through", but spreads sufficiently to eliminate any raised or embossed effect and to produce enlarged uniform spots which do not have ragged edges. It has now been found that spreading and penetration is terminated by the combined thermal properties of the ink and the paper, and these thermal characteristics vary only within relatively narrow limits across a wide variety of paper types.
US Referenced Citations (2)
| Number |
Name |
Date |
Kind |
|
4741930 |
Howard |
May 1988 |
|
|
4751528 |
Spehrley et al. |
Jun 1988 |
|
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 56-113462 |
Sep 1981 |
JPX |
Continuation in Parts (1)
|
Number |
Date |
Country |
| Parent |
94664 |
Sep 1987 |
|
Patent Grant
4951067
