FR2784759A1 - PRINTING POWDER FIXING OVEN - Google Patents

Abstract

The printing powder fixing oven for a printer comprises a heating fixing roller (1), with heating resistors (14-17) arranged on the external surface (12, 13) of the roller for heating a printing medium. (20).

Description

ô: 2784759

  Printing powder fixing oven for printer For the reproduction of the image of a document in a printing device such as a photocopier or a plain paper fax machine, a latent image of electric charges is formed on the periphery of a rotating drum to attract particles of printing powder, or toner, and then deposit them, according to the image, on a sheet to be printed in contact with the drum. The sheet is then passed through an oven in which the powder particles are baked to embed them in the fibers of the

paper and thus fix the image.

  Conventionally, the baking oven comprises a rolling mill with a heating roller and a counter-rest roller. The heating roller consists of a hollow cylinder, made of thermally conductive material, which is heated

  indirectly, by radiation, by a halogen lamp housed inside.

  This arrangement has the drawback of having a relatively high thermal inertia and therefore a relatively long heating time when the printer is switched on. We could consider temporarily increasing the heating power, but this would require a much more powerful lamp and power supply, which is

economically excluded.

  The present invention aims to address this difficulty.

  To this end, the invention relates to a printing powder fixing oven comprising a heating fixing roller, with means for heating the external surface of the roller to heat a printing medium, characterized in that the means heaters are arranged at the

outer surface of the roller.

  Thus, there is direct heating, which overcomes any path in the air, thermally resistant, between the heating means and the outside of the roller. Thermal energy is thus directly supplied to

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  where it is consumed by the print media, which

  allows a very rapid increase in power on power up.

  The manufacturing is thus very simple and the screen-printed layer can be evenly distributed on the external surface of the roll (if it is a roll

  not full), which ensures good heat distribution.

  The invention will be better understood using the following description of a

  embodiment of the fixing oven of the invention, with reference to the accompanying drawing, in which: - Figure 1 is an axial section partially showing the oven roller and a backing roller, and - Figure 2 is a schematic developed view of the circumference of the wheel

  mounting and control electronics.

  The fixing roller, referenced 1, belongs here to a fax machine and is

  carried by a chassis, not shown, thereof. A counter roller 2

  support is opposite to a generator of the roller 1 and pressed elastically thereon by return springs not shown. The two rollers 1 and 2, with respective axes 19 and 21, are mounted in such a way that one can roll over the other and thus press on the roll 1 a succession of strips or lines of a support 20 to be printed. , here a sheet of paper having already received printing powder, or toner, depending on the desired image. In this example, the two rollers 1 and 2 are, for this purpose, both rotatably mounted on the chassis and the sheet 20 is driven by two downstream extraction rollers, not shown, which thus drive the rollers 1 and 2 in rotation. pinching the sheet 20 with friction. The roller 1, mounted on bearings 10, could be motorized. In other

  examples, it could be provided that the roller 1 is fixed and that the counter

  roller 2 rotates around the axis 19 of the latter by rolling on its surface, to flatten there in the manner of a poster, the entire surface of a sheet then

fixed relative to roller 1.

  The roller 1 is here hollow and has an inner cylindrical layer or tube 11 forming a mechanical support for a layer 12 of material

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  electrically insulating but thermally conductive, here enamel, which itself carries a thin external layer 13, thermally conductive, of non-stick coating of the roll 1, here in Teflon, serving to prevent any sticking of the paper 20 to the roll 1 The layer 12 comprises a heating resistor and more precisely, in this example, four resistors 14, 15, 16, 17, therefore arranged on the external surface of the roll 1. Similarly, there are provided here four thermistors respectively associated with each resistance 14-17, and of which only the

  thermistor 18, associated with resistance 14, has been shown.

  As shown in FIG. 2 (compressed vertically with respect to FIG. 1), the four resistors 14 to 17 and thermistor (s) extend over four respective zones of the periphery or external active surface of the roll 1, here forming four axially end-to-end sections. They are here made up of thick layers of paste of material with a determined resistivity, serigraphed on the enamel layer 12 and then covered with an electrically insulating layer, such as enamel or varnish. Each resistor 14-17 is here formed of a track which winds over the entire surface of the section considered, to distribute the heat well. As a variant, a conductive layer could have been provided covering the entire external surface of the section and preferably supplied at two opposite points,

  radially and axially, to best distribute the current flow.

  In this example, the cylindrical support 11 is made of thermally conductive material, here aluminum, and has a mass sufficient to act as a thermal buffer, without however "thermally short-circuiting" the resistors 14-17, in order to allow a rapid heating. Thus, the sheet of paper 20 is heated directly, through the teflon layer 13, by the resistors 14-17 and by the enamel layer 12 in which the calories of the resistors 14-17 are diffused as well as by the support 11 The latter contributes to distribute the heat over the entire surface of the enamel layer 12 and also temporarily temporarily provides additional heat if the heat absorption power of the sheet 20 exceeds the power of the resistors 14-17, for example. when printing at high speed or for extended periods of time. A supply inlet ring and a ground return ring 16 are

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  integral with the roll 1, by screen printing, and connected respectively to the two ends of the resistor 14. The other resistors 15 to 17 and the thermistors like that referenced 18 are likewise supplied by rings (not shown) like that referenced 15, the ring 16 of mass being common to resistors 14 to 17. The various rings are mentioned, in FIG. 2, by the reinforced lines at each end

axial of the roller 1.

  The supply return rings (not shown) of the thermistors are individual and connected to an individual current measurement circuit like that referenced 22. The circuit 22 is in fact a comparator which receives a setpoint current Ic on an input to supply at the output a signal representing the difference between the setpoint current Ic and the current passing through the thermistor 18. According to the sign of this difference, it controls a switch 24 connecting the output of a power supply 23 to the ring 15, in order to close the 'switch 24 when the current of the thermistor 18 is lower than the setpoint current. This in fact indicates that the enamel layer temperature 12 is too low, the thermistor 18 here having a negative temperature coefficient (NTC). When the heating thus produced has sufficiently reduced the value of the thermistor 18, its current crosses the threshold Ic upwards and therefore commands the opening of the switch 24. The thermistors like 18 are here supplied with

permanence through food 23.

  Provision may be made to separate each resistor 14-17 into several, to thermally control fractions of the periphery of the roll 1. For example, when an image involves several superimposed layers of ink, or toner, of different primary colors for forming a composite color, the reference temperature, translated here by the current Ic, can then be temporarily increased to bake the ink thickness at the right temperature, taking into account the increased cooling by the thickness. In this case, a microprocessor controlling the printing detects such a superimposition of primary colors and increases the temperature reference (Ic) the more the higher the number of layers of ink to be cooked, therefore the thickness of toner. . Thus, in general, it is advantageous to divide the heating resistance, mentioned

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  at the beginning of the detailed description, in a plurality of resistances of

  14-17 heating distributed, or even subdivisions thereof as mentioned above, having a determined position relative to the support to be printed 20 and therefore relative to the image to be fixed. The above printing microprocessor, having the analysis of the image to be fixed, that is to say the position of the various pixels for each color, determines a map, in the format of the image, representing the number of color layers to be fixed, for each pixel, here from zero to four layers. It then selectively controls, by a plurality of selective supply switches like 24, the plurality of heating resistors as a function of the cumulative number of layers of the pixels, with the various colors, of the image area to be fixed by the resistance.

heating 14-17 considered.

  Brush holders and brushes generally referenced 17 and 18, fixed to the chassis, and others not shown, provide the desired electrical connections between the rings like 15 and 16 and the supply or measurement wires. A dual assembly is also possible, with tracks or

  rings secured to the chassis and brushes secured to the roller 1.

  As a variant, provision could be made to integrate the thermistor 18 with the resistor 14. In all mounting cases, the resistance of the thermistor 18 (or 14) can be measured by comparison between the voltage across a series resistance of low value (little loss of voltage) and a reference voltage which can be adjusted and supplied for example by the output of a transistor of a circuit for adjusting a set temperature. Likewise, provision could be made for analog regulation, and not all-or-nothing, of the power supply 23, by a command for adjusting its output voltage. It can thus include for example a rectifier bridge with smoothing filter supplying the collector of an NPN transistor, in series with the load (14-17), whose base voltage is adjustable. The collector-emitter series voltage drop thus varies at will and the output voltage (emitter / ground) that remains, at

  the ring 15 therefore varies in the opposite direction.

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  It is understood that the rings 15, 16 and other integral with the roller 1 could be mounted otherwise and elsewhere on it, for example on the enamel layer 12 and in particular at the axial end with possibly an axial overflow of this layer 12 to this effect. The roller 1 could likewise be full, since its heating elements are directly heated, at the peripheral surface 12, 13 which cooperates with the sheets 20. The rings 15, 16 and others have an axial symmetry on the roller 1 and can be oriented as desired, for example radially outward or they can be arranged in radial planes or even a single radial plane. They can also be cylindrical, parallel and opposite, in nested concentric rings carried by two opposite surfaces of the same support layer 11 or 12. As mentioned above, such sliding connections are not necessary if the roller 1 is fixed and is traversed by roller 2 rolling on it to apply sheet 20 to it

way of a poster.

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Claims (7)

  1.- Printing powder fixing oven for a printer comprising a heating fixing roller (1), with means (14-17) for heating the external surface (12, 13) of the roller for heating a support   printing (20), characterized in that the heating means (14-   17) are arranged on the outer surface of the roller (1).   2. Oven according to claim 1, in which the heating means   have at least one electrical resistance (14-17).   3.- Oven according to claim 2, wherein the resistance (14-17) is   screen printed on the heating roller (1).   4.- Oven according to one of claims 2 and 3, wherein,   thermally, the heating roller (1) is made of conductive material and is   connected to the heating resistor (14-17).   5.- Oven according to one of claims 1 to 4, wherein there is provided a   plurality of heating resistors (14-17).   6.- Oven according to claim 5, wherein there is provided means for selective supply of heating resistors (14-17), arranged   to be controlled by means of analysis of an image to be fixed.   7.- Oven according to one of claims 2 to 6, wherein a   regulation thermistor (18) is integrated into the electrical resistance (14- 17).