GB2342891A - Fixing roller for electrophotographic printer with heating resistors on its surface - Google Patents

Abstract

A fixing heating roller 1 comprises a plurality of heating resistors 14,15,16,17 located on a thermally conductive surface 12 of roller 1. In the embodiment shown the four resistors extend over four respective zones of layer 12 of roller 1 and a thermally conductive non-stick layer 13 covers layer 12 and the resistors and prevents adherence of paper 20 to the roller 1. Circuit 22 controls the current supply to each of the resisitors.

Description

2342891 Oven to fix printing powder for a printer To reproduce the image

of a document using a printing apparatus such as a photocopier or fax machine with ordinary paper, 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 to place them, depending on the image, on a sheet to be printed in contact with the drum. The sheet then passes into an oven in which the particles of powder are cooked to encrust them in the fibres of the paper and thus fix the image.

Classically, the cooking oven comprises a rolling press with a heating roller and a counter-bracing roller. The heating roller is formed from a hollow cylinder, in thermal conductive material, which is heated indirectly, by radiation, using a halogen lamp housed inside.

This arrangement has the disadvantage of having relative high thermal inertia and therefore a relatively significant heating time when the printer is switched on. It is possible to imagine. temporarily increasing the heating pcwer, but this would require a lamp and a supply which are far more powerful, which is economically excluded.

JP 08-110723A, Patent Abstracts of Japan, vol 096, No 008, shows a printed heating resistor on the outer surface of the roller.

2 The heating time is thus reduced because of the reduction in the thermal inertia and the direct supply of thermal energy to the place where it is consumed by the printing support. However, as the image zones comprise several layers of superimposed ink, which therefore requires a significant thermal input, they cannot receive all the thermal energy needed because the heating resistor has limited inertia. Ink fixing is therefore imperfect.

The present invention aims to deal with this difficulty.

To this end, the invention concerns an oven to fix printing powder for a printer comprising a fixing heating roller, with heating means on the outer surface of the roller, to heat a printing support, comprising a plurality of heating resistors located on the outer surface of the roller, characterised by the fact that selective supply means for the heating resistors are planned, equipped to be controlled by means to analyse an image to be fixed.

Thus, the thermal energy is supplied selectively on the outer surface of the roller, and therefore to the printing support, depending on the requirements in the various zones of the image to fix. Moreover, the means to analyse the image can be adapted to the thermal characteristics of various printing powders.

3 The invention will be better understood using the following description of a form of realising the fixing oven in the invention, with reference to the attached drawing, in which:

FIGURE 1 is an axial section representing partially the roller in the oven and a counter-bracing roller, and FIGURE 2 is a developed schematic view of the periphery of the fixing roller and the command electronics.

The fixing roller, reference 1, belongs here to a fax and is carried by a chassis, not shown, of the latter. A counter-bracing roller 2 is opposite a generator for roller 1 and plated elastically on it by release springs which are not shown. The two rollers 1 and 2, with respective axes 19 and 21, are assembled so that one can roll on the other and thus plate on roller 1 a succession of bands or lines from a support 20 to print, here a sheet of paper which has already received the printing powder, or toner, depending on the desired image. In this example, the two rollers 1 and 2 are, to this effect, both assembled in a rotating manner on the chassis and the sheet 20 is driven by two pulleys below the extraction, not shown, which thus drive in a rotating manner the rollers 1 and 2 clamping the sheet 20 by friction. Roller 1, assembled in bearings 10, could be motorised. In other examples, it is possible that roller 1 is fixed and that the counter-roller 2 turns around axis 19 of the latter by rolling on its surface, to plate on it like a poster, the 4 whole surface of a sheet thus fixed in relation to the roller 1.

Roller 1 here is hollow and comprises an inner cylindrical tube or layer 11 forming the mechanical support of a layer 12 in electrical insulating material but thermally conductive, here enamel, which itself has a thin outer layer 13 which is thermally conductive, of non-stick coating on the roller 1, here Teflon, used to avoid any sticking of the paper 20 to the roller 1. Layer 12 comprises a heating resistor and more precisely, in this example, four resistors 14, 15, 16, 17 therefore placed on the outer surface of roller 1. Similarly, four thermistors are planned here respectively associated to each resistor 14-17, and therefore only thermistor 18, associated with resistor 14, has been shown.

As shown in figure 2 (compressed vertically in relation to figure 1), the four resistors 14 to 17 and thermistor(s) extend over four respective zones of the periphery or outer active surface of roller 1, here forming four sections which are axially end to end. They are formed here by thick layers of material paste with a determined resistivity, silk screen printed on the layer of enamel 12 and then covered with an electrically insulating layer, such as enamel or varnish. Each resistor 14-17 is formed here from a track which winds over the whole surface of the section in question, so that the heat is properly distributed. As a variant, it is possible to have planned, for each section of roller 1, a conductive layer covering the whole of the outer surface of the section and preferably supplied at two opposite points, radially and axially, to distribute best the current flow.

In this example, the cylindrical support 11 is in thermally conductive material, here aluminium, and with a sufficient mass to make the thermal buffer function, without however "short-circuiting" thermally resistors 1417, in order to allow for quick heating. Thus, the sheet of paper 20 is heated directly, through the layer of Teflon 13, by resistors 14-17 and by the layer of enamel 12 in which the calories from resistors 14-17 are diffused as well as by support 11. The latter contributes to distributing the heat over the whole surface of the layer of enamel 12 and temporarily provides moreover a complementary thermal input if the calorific absorption power of the sheet 20 exceeds the power of resistors 14-17, for example during high speed or prolonged printing. A supply inlet ring 15 and earth return ring 16 are solid with roller 1, by silk screen printing, and linked respectively to two extremities of resistor 14. The other resistors 15 to 17 and thermistors such as that referred to as 18 are also supplied by rings (not shown) as that referred to as 15, earth ring 16 being common to resistors 14 to 17. The various rings are shown, in figure 2, in dark lines at each axial extremity of roller 1.

6 The supply return rings (not all shown) of the thermistors are individual and linked to a first inlet of an individual circuit to measure the current as shown by reference 22. Circuit 22 is in fact a comparator which receives, through a second inlet, an Ic command current supplied by a microprocessor circuit 30, to supply at the outlet a signal representing the difference between the command circuit Ic and the current going through thermistor 18. Depending on the sign for this difference, circuit 22 commands a circuit breaker 24 linking the outlet of a supply 23 to ring 15, in order to close circuit breaker 24 when the current from thermistor 18 is lower than command current ic. This indicates in effect that the temperature of the layer of enamel is too low, the thermistor 18 here having a negative temperature coefficient (CTN). When the heating thus produced has sufficiently reduced the value of thermistor 18, its current jumps upwards over threshold Ic and therefore commands the opening of circuit breaker 24. Thermistors such as 18 are permanently supplied here by supply 23.

It is possible to plan to sub-divide each resistor 14-17 into several, to control individually and thermally the angular fractions of the periphery of roller 1 and thus create meshing formed from juxtaposed elementary zones of the outer surface of roller 1. For example, when an image calls, in one of these elementary zones, for several superimposed layers of different primary coloured ink or toner to form a composite colour, the reference 7 temperature, translated here by current Ic, can thus be temporarily increased to cook the extra thickness of ink at the correct temperature, taking into account the increased cooling of resistor 14-17 in question, due to the extra thickness. The microprocessor circuit 30 manages the printing after analysing the image to be fixed, that is, partial images with various primary colours. It. then detects, by comparison between positions of the pixels of partial electronic images, that such a superimposition of primary colours will occur and increases even more the temperature reference, therefore current Ic, which has raised the number of superimposed layers of ink to cook, therefore the total thickness of toner.

Thus, generally, the heating resistor, mentioned at the start of the detailed description, is divided into a plurality of heating resistors distributed here axially and/or at angles, over the outer surface of roller 1, as illustrated by resistors 14-17 and sub-divisions of the latter mentioned hereinabove, with a determined position in relation to the support to print 20 and therefore in relation to the image to fix. The microprocessor printing circuit 30 analysing the electronic image to fix, that is, the position of various pixels for each primary colour, determines a card, in the format of the image, representing the number of layers of colours to fix, for each pixel, here from zero to four layers. It thus selectively commands, by means of a plurality of selective supply circuit breakers such as 24, the plurality of

8 heating resistors depending on the cumulative number of layers of pixels, various colours, the image zone to be fixed by the heating resistor 14-17 in question.

Scanners and scanner holders globally referred to as 17 and 19, fixed to the chassis, and others which are not shown, provide the desired electrical links between rings such as 15 and 16 and supply or measuring wires. A dual assembly can similarly be planned, with tracks or rings which are solid with the chassis and scanners which are solid with roller 1.

As a variant, it is possible to plan to integrate thermistor 18 with resistor 14. In all cases of assembly, the resistance measurement of thermistor 18 (or 14) can be carried out by comparison between the voltage at the terminals of a low value series resistor (little loss of voltage) and a reference voltage which could be adjustable and supplied for example by the outlet of a transistor from a command temperature adjustment circuit. Similarly, it is possible to envisage analogue regulation, and not a hit or miss governor, for supply 23, by means of a command to adjust its outlet voltage. It could thus comprise for example a corrector bridge with a smoothing filter supplying the NPN transistor collector, in series with the charge (14-17), the basic voltage of which can be adjusted. The drop in the collector-emitter series voltage would thus vary at will and the subsistence outlet voltage 9 (emitter/mass), at the level of ring 15, would thus vary inversely.

It can be imagined that rings 15, 16 and others which are solid with roller 1 could be assembled otherwise and elsewhere on the latter, for example on the layer of enamel 12 and in particular at the axial extremity possibly with an axial overflow of the enamel layer 12 for this purpose. Roller 1 could also be full, since its heating elements are directly heated, on the level of the peripheral surface 12, 13 which co- operates with sheets 20. Rings 15, 16 and others have an axial symmetry around roller 1 and can be oriented as desired, for example radially towards the outside or they could even be placed on radial planes or even on a single radial plane. They can also be cylindrical, parallel and opposite, in concentric overlapping rings carried by two opposite surfaces of a same support 11 or 12. As mentioned previously, such sliding links are not necessary if roller 1 is fixed and is covered by roller 2 rolling above by applying the sheet 20 like a poster.

Claims (5)

1. Oven to fix printing powder for a printer comprising a fixing heating roller, with means to heat the external surface of the roller, to heat a printing support, comprising a plurality of heating resistors located on the external surface of the roller, characterised by the provision of selective supply means for the heating resistors which are arranged to be controlled by means to analyse an image to be fixed.

2. Oven according to claim 1 in which the resistors are silk-screen printed on the heating roller.

3. Oven according to claim 1 or 2 in which, thermally, the heating roller is in conductive material and is connected to said heating resistors.

4. Oven according to any one of claims 1 to 3 in which a regulation thermistor is integrated into at least one of said resistors.

5. An oven for fixing printing powder for a printer substantially as hereinbefore described with reference to the accompanying drawings.