Incremental printing of symbolic information – Ink jet – Fluid or fluid source handling means
Reexamination Certificate
2001-02-02
2003-08-05
Hsieh, Shih-Wen (Department: 2861)
Incremental printing of symbolic information
Ink jet
Fluid or fluid source handling means
Reexamination Certificate
active
06601950
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a melting device for melting a unit of ink for use in an inkjet printer, comprising a melting chamber provided with a wide end for dispensing the unit of ink to the melting chamber and to a narrow end thereof. The melting chamber has a form such that the ink unit moves, as a result of melting, in a direction from the wide end to the narrow end, the ink unit being laterally enclosed with respect to this direction by one or more walls of the melting chamber. The present invention also relates to an inkjet printer provided with a melting device of this kind.
This melting device is known from U.S. Pat. No. 5,030,972. The melting device is used to provide the inkjet printer printhead, which uses hot-melt melt ink, with liquid ink. Hot-melt ink, also known as phase-change ink, is an ink which is solid under normal ambient conditions but liquid at an elevated temperature. To enable this ink to be transferred to a receiving material by means of the inkjet printer printhead, the ink must be made liquid. During printing, the liquid ink is ejected in the form of individual droplets by the printhead in the direction of the receiving material. In this way, an image built up of a number of separate dots is formed on the receiving material. In order to melt the solid ink as quickly as possible without the need for considerable power, a solid unit of ink in the known device is brought into direct contact with a heater, which also keeps the ink liquid in the printhead. To achieve this, the melting chamber is formed into a constriction by a first vertical wall which acts as a heater (hereinafter referred to as the melting wall) and a second wall, and at an angle thereto which serves to keep the ink unit in contact with the melting wall. When an ink unit is dispensed into the melting chamber via the wide end of the constriction it is enclosed by the two walls. The ink unit melts there where it is in contact with the vertical melting wall, which is heated to above the temperature at which the ink is a liquid. As a result of this melting, the dimensions of the ink unit decrease, so that after the melted ink has been discharged it moves in the direction of the narrow end. In these conditions the melted ink is discharged through the small perforations in the melting wall. The transport of the ink through these perforations takes place by capillary forces.
A melting device of this kind has a significant disadvantage. Since the passage of melted ink to the inkjet head is dependent on the capillary action of the perforations in the vertical melting wall, the maximum speed at which melted ink can be supplied to the printhead is relatively low. This gives rise to problems, particularly if there is a considerable demand for liquid ink, for example if the inkjet printer has to print an illustration with a high degree of coverage, particularly a colored poster. An inadequate supply of melted ink can lead to the liquid ink in the printhead becoming exhausted, so that printing must be temporarily interrupted, which is a disadvantage with respect to inkjet printer productivity. Another problem that may arise in this connection is the inclusion of air bubbles in the liquid ink, which has a very adverse effect on the printing behavior of the printhead. An additional disadvantage of the low speed at which liquid ink is fed through the perforations is that a thin layer of liquid ink will form between the vertical melting wall and the solid ink unit. A layer of liquid ink of this kind forms a thermal barrier, so that the ink will melt more slowly under otherwise identical conditions. It is also a disadvantage with respect to the supply of melted ink to the printhead.
SUMMARY OF THE INVENTION
The object of the melting device according to the present invention is to obviate these disadvantages. To this end, a melting device is invented in which each of the one or more walls of the melting chamber is heated, during the melting process, to above the temperature at which the ink is liquid. It has surprisingly been found that in this way the supply of melted ink increases very considerably. In a melting device according to the invention in which the solid ink unit is heated to above its melting point at all places where it is enclosed by the one or more walls of the melting chamber, extra driving forces are apparently brought into play which cause the melted ink to be discharged at an accelerated rate. The reason for this highly accelerated supply is not completely clear, but closer research has indicated a number of possible causes. Firstly, an ink unit melted from a number of sides will decrease in its structure more rapidly, so that the unit can move more rapidly in the direction of the narrow end of the melting chamber. In the melting device according to the present invention, this movement is made possible by the fact that the unit of solid ink is enclosed only laterally with respect to the direction of movement, i.e. there is no support in a plane perpendicular to the direction of movement which would obstruct the movement of the ink unit. The movement results in a driving force which presses the already melted ink out of the contact surface between the solid ink unit and the melting wall. In addition, as a result of the more rapid movement, the ink unit will come into contact with a “fresh” melting wall surface at an earlier time. Since the thermal conductivity coefficient of the melting wall has a finite value, this means that the fresh melting wall surface has a higher temperature than the melting wall surface which has previously already given up its heat. Here again, the supply of melted ink can be further increased. An additional advantage of this is that the value of the thermal conductivity coefficient of the melting walls is less critical. In addition to the above-mentioned effects which already reinforce one another, it has been found that when an ink unit is laterally enclosed by one or more walls of the melting chamber, the solid ink unit is frequently pressed into contact on at least a part of the melting wall surface with a force greater than the force of gravity acting on the ink unit. A more powerful contact pressure force of this kind ensures that the melted ink is pressed more rapidly out of the contact surface, and this means an extra driving force for the transport of the already melted ink together with a reduction of the thermal barrier between the solid ink unit and the melting wall. These effects appear to reinforce one another in such a manner that a considerable supply of melted ink can be obtained. In addition to these effects, there are probably other causes whereby the supply of melted ink in a melting device according to the invention is considerable. However, knowledge of this is of secondary importance to the successful application of the invention.
In a preferred embodiment, the solid ink unit moves in the melting chamber by the force of gravity. This can be achieved by so disposing the melting chamber with respect to the gravity field that a nett force forms on the solid ink unit in the direction of the narrow end of the constriction. In this way no additional means are required to move the ink unit in the direction of the narrow end. Such means, for example in the form of a spring, not only increase the cost price of the melting device but also have the disadvantage that the dispensing of a following solid ink unit is rendered difficult. In the example given, the spring will have to be pressed in to place a new ink unit between the one or more melting walls of the melting chamber and the spring itself.
In a further, preferred embodiment, the melting chamber comprises, in the vicinity of the narrow end, at least one passage opening for the passage of melted ink. As a result of the provision of the passage opening, the melted ink which will move towards the narrow end under the influence of the force of gravity can leave the melting chamber. Transport of the melted ink will then take place so that it will finally reach th
Hollands Peter Joseph
Mooren Gerardus Johannes Catharina
Reinten Hans
Hsieh Shih-Wen
Oce--Technologies B.V.
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