Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Reexamination Certificate
2002-05-20
2004-01-27
Tran, Huan (Department: 2863)
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
C257S712000, C257S713000, C372S034000
Reexamination Certificate
active
06683639
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a printhead for an image-forming apparatus, containing a substrate, a row of light-emitting elements disposed on a first side of the substrate, and a cooling element disposed on a second side of the substrate opposite the first side. The present invention also relates to an image-forming apparatus provided with such a printhead.
A printhead and apparatus of this kind are known from U.S. Pat. No. 4,703,334. The known printhead is constructed from a ceramic substrate on which a row (array) of light-emitting diodes (LED's) is disposed. On the first side where the LED's are located, the printhead is also provided with an image-forming element provided with a selfoc lens array. At the back of the substrate, i.e. the second side remote from the LED's, there is a cooling element. The latter is constructed as a support plate made from a material having a high thermal capacity, for example aluminium, so that this element can serve as a heat sink to absorb heat. The cooling element is provided with a number of projecting longitudinal ribs which serve to enable the absorbed heat to be transferred to an air flow taken along the ribs. When the printhead is printing, the LED's produce relatively considerable heat. This heat must be dissipated because the LED temperature must not be too high. A high LED temperature results in a drop in light emission and changes in the wavelength of the emitted light. In addition, the life of the LED's falls off if they are kept at a high temperature. In the known printhead, the heat generated by the LED's is discharged via the thermally conductive ceramic substrate to the cooling element which is in turn cooled by a forced air flow. In this way it is possible to prevent the LED temperature from becoming too high during the operation of the printhead so that the optical image-forming characteristics of the printhead remain constant as far as possible. In addition, the low operating temperature means that the printhead life is also sufficiently long.
A printhead of this kind is also known from German patent 38 22 890. Here again, the printhead is constructed around a thermally conductive substrate, in this case a body made from solid copper. The cooling element is constructed from a large number of rod-shaped elements made from a material having a high thermal capacity and conduction. These rod-shaped elements in turn give up the absorbed heat to an air flow which is conducted along the rod-shaped elements by means of a fan.
The known printheads have a number of significant disadvantages. The thermally conductive substrates required to be able to discharge the relatively considerable quantities of heat to the cooling element are speciality products which are expensive, difficult to obtain and often difficult to machine. For example, it is very difficult using such substrates to make structures having a number of layers and mutual connections. Also, the known materials are often brittle or have little shape stability, which further makes printhead production difficult. All this means that the known printheads are expensive to produce, so that the printhead also has a relatively considerable influence on the total production costs of the image-forming apparatus.
A subsequent disadvantage of the known printheads is that the heat produced by the light-emitting elements is discharged uncontrollably as a result of the very intensive but uncontrollable heat discharge via the conductive substrate. One of the results of this is that the array of light-emitting elements may have too great a spread in temperature and hence also in light yield. For example, if the temperature is locally lower than nominal, so that the light yield there is too high, a visible print artefact may form, such as the disappearance of thin lines. Another disadvantage is that the uncontrolled heat discharge always results in uncertainty concerning the form of the substrate (which is temperature dependent) and hence the print characteristic of the print head. A small deformation can in fact, result in defocusing of an LED so that it is no longer possible to obtain sharp illumination of the photoconductor. This has an adverse effect on print quality.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a printhead which is inexpensive, for example made from relatively standard materials and with relatively standard processes, and with which it is possible to obtain good and controllable cooling of the light-emitting elements. To this end, a printhead has been developed wherein the substrate is thermally insulating and is provided with at least one thermally conductive track which extends through the substrate from the first side to the second side and is disposed at a predetermined location with respect to the light-emitting elements in order to conduct heat from the first side to the second side in such manner that the elements are maintained substantially at a predetermined temperature during operation of the printhead.
According to the present invention, it is possible to use cheap standard materials as the substrate, for example a glass fiber reinforced epoxy plate. A material of this kind is thermally insulating, but this does not mean that overall, no heat can be dissipated by this material, but rather that the coefficient of thermal conduction is so small that when this material is used the temperature of the light-emitting elements might rise to an unacceptably high level if further steps were not taken with respect to cooling. According to the present invention, the provision of one or more thermally conductive tracks through the material at predetermined locations enables sufficient heat to be discharged from the environment of the light-emitting elements to the cooling element. At the same time, a correct choice of the location where these tracks are provided enables the heat dissipation to be accurately controlled. In this way it is possible not only to prevent the temperature of the light-emitting elements from reaching a specific top limit, but also the temperature of the light-emitting elements can be maintained substantially at a predetermined temperature so that adequate uniformity in the temperature is ensured. As a result, the light emission of the elements will also be sufficiently uniform over the length of the array and the substrate will acquire a form known in advance. The predetermined temperature of the light-emitting elements is typically 30-60° C. but, depending on the application, instantaneous load, type of LED's, wear, and so on, can also be outside that range. In addition, this does not have to be a fixed value but can be adjusted in dependence on the above and other factors so that good print quality can be obtained under all conditions.
Thus using a printhead according to the present invention it is possible to obtain an image-forming apparatus with which it is possible to produce images with a very high print quality and wherein the long life of the printhead helps to reduce service costs. In addition, using the printhead according to the present invention enables the printhead costs themselves to have a reduced influence on the total production costs of the image-forming apparatus.
A printhead is also known from U.S. Pat. No. 5,113,232 which is provided with a row of light-emitting elements disposed on a thermally insulating substrate. In this printhead, the heat is discharged via a conductive metal layer disposed over an appreciable part of the surface of the substrate. In this way, the heat produced by the LED's is discharged via lateral transport to a heat sink which in this way acts as a cooling element. A construction of this kind has the significant disadvantage that the heat-dissipating power is relatively small, because the heat has to be transported over a relatively large distance by a thin layer. As a result, the temperature of the LED's can rise to relatively high values. In addition, the substrate itself is heated very non
Driessen-Olde Scheper Lamberdina
Ramackers Henrikus G. M.
Van Acquoij Catharinus
Birch & Stewart Kolasch & Birch, LLP
Oce--Technologies B.V.
Tran Huan
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