GB2349394A - Electrical heating elements - Google Patents

Abstract

A method of achieving an electrically resistive heating layer 48 on a substrate 42,44 formed on an electrically non-conductive surface, and such that the resistance of the resistive heating layer differs in different regions thereof. Particles of a dry metal or metal alloy powder are thermally sprayed onto the surface of the substrate by relative displacement of the substrate and a thermal spraying nozzle 51 whereby the metal or metal alloy powder is partially oxidised to form a composite structure of metal or metal alloy interspersed with metal or metal alloy oxide derived from the oxidation of the same metal or metal alloy. The variation of the resistivity at different locations is achieved by adjusting the distance d of the nozzle 51 from the substrate 42,44 during said relative displacement.

Description

DESCRIPTION ELECTRICAL HEATING ELEMENTS The present invention relates to electrically resistive heating elements and is concerned particularly, but not exclusively with their use in dry toner fusing applications within the business machines industry.

Toners are made from thermoplastics copolymer materials, with the majority incorporating styrene as their primary constituent. Carbon black is added for pigmentation purposes. Xerographic images are composed of numerous toner particles transported and deposited on paper, (or other medium), by the control of adhesive forces. When toner is first transferred from the photoconductor drum to paper, it adheres to the paper fibres because of electrostatic and dispersion forces and in this condition the xerographic image can be easily rubbed off. In order to make an image permanent on paper, (or other medium), toner formulations have been developed which respond, (melt, flow, join together, spread, wet and penetrate), under pressure, heat or a combination of both pressure and heat and thereby become permanently fixed (fused) onto the paper.

By far the most extensively used method of fusing is the simultaneous application of both pressure and heat. This has certain fundamental advantages over direct radiant and pressure fixing methods and is favoured by the majority of Original Equipment Manufacturers (OEMs) as the preferred fixing solution.

A typical configuration for heat/pressure fusing is shown in Fig. 2 of the accompanying drawings. A fuser 10, sometimes known as the hot roller, is in contact with a pressure roller 12, sometimes known as the backup roller, and is responsible for providing the heat requirements of the system. In most applications, the pressure roller 12 is driven by the fuser roller 10 via frictional forces and the two are under compressive load. The fuser roller 10 is heated by the action of an axially disposed heat lamp 14 and requires time to reach operating fusing temperature conditions Thermal losses are such that in this configuration more energy is required towards the axial ends to maintain a constant temperature across the length of the roller. Pressure and heat are applied to the paper/toner combination by passing said combination through the nip formed between these two rollers 10,12. In general, the pressure roller 12 has a conformable coating, (not always), and the fuser roller 10 has a nonconformable coating, (not always).

This method of fusing, although favoured by the majority of OEMs, has certain disadvantages, thermal lag being the most obvious. When copies or prints are required, a person does not want to wait by the side of the machine in order for it to reach fusing temperature ; they want a copy immediately. To reduce the time waiting for the fuser roller 10 to reach operating temperature, (around 180 C), OEMs are obliged to keep this roller well above ambient room temperature, and in some situations only around 20 C less than operating temperature. This requires the roller to be thermally cycled on a continuous basis from initial switch-on of the machine, (stand-by mode), and this wastes energy. The ideal solution would be to have a pressure/heat fusing mechanism with instant heat capability. Another problem associated with the heater rollers which are heated by the conventional method is that the distribution of heat across the surface of the roller can be non-uniform. This can be translated to non-uniform fusing characteristics where offset becomes a problem.

In place of the conventional roller heating system of Figure 2, it is known to provide at least one of the rollers with an electrically resistive coating on or adjacent its circumferential surface through which an electrical current can be passed to generate the required heat. Electrodes are provided at the outer ends of the rollers to enable electrical current to be introduced to the resistive coating usually by way of slip rings.

It is known that a resistive layer can be established by thermally spraying onto an insulating substrate of the roller a ceramic mixture consisting of at least one conductive or semi-conductive ceramic powder and another ceramic which is substantially an insulating material. Blends of these material are applied to achieve a predetermined resistivity. An example of a semi-conductive or lower resistance material is chromium oxide (CrO2 or CrO).

In practical use of rollers heated by integral electrical heating layers/coatings, a problem is that if the resistivity of the heating layer at constant thickness is constant along the length of the roller then the resulting temperature of the roller tends to be less at its two ends due to the enhanced heat losses in these regions due, for example, to their additional exposure to the ambient atmosphere at the roller end faces. Thus, a fuser roll producing a constant power density (W/cm) over it's surface will display a lower operating temperature at the ends due to'end losses'i. e. the endmost sections of the roller are only heated from one side and also have an extra area from which to lose heat. In order to overcome this, the resistor deposited onto the surface should have a higher resistance at the ends so as to generate higher power locally = Pur), offset the end losses and hence display a constant operating temperature across the roller.

It is known to overcome this problem by increasing the resistance of the electrically resistive layer at the end regions of the roller by adjusting the thickness of the resistive layer in these regions. This technique has been adopted in practice because it is not easy to change the resistance by any other means, since this would necessitate adjustment of the composition of the ceramic blend along the length of the roller.

The prior art thus suggests achieving the required higher resistance at the ends of the roller by thinning the resistive coating in these regions. For a material of constant resistivity e. g. 80/20NiCr, where p = 108yQcm, this can result in a higher resistance using the relationship R = (p. 1)/A where: R = resistance of section of resistor (Q) p = resistivity of material (Q. m) 1 = Length of current carrying path (m) A = cross sectional area of current carrying path (m2) Thinning the layer reduces A and increases R and hence P (= Fur).

It is an object of the present invention to provide a process for achieving a variable resistance per unit length along a heating layer on a member such as a roller, without the necessity for significantly adjusting the thickness of the heating layer.

In accordance with the present invention there is provided a method of achieving an electrically resistive heating layer on a substrate formed of an electrically insulating material, or formed of an electrically conductive material having an electrically insulating coating, whereby in both cases the substrate presents an electrically non-conductive surface, and such that the resistance of the resistive heating layer differs in different regions thereof, the method comprising thermal spraying particles of a dry metal or metal alloy powder onto the surface of the substrate by relative displacement of the substrate and a thermal spraying nozzle whereby the metal or metal alloy powder is partially oxidised to form a composite structure of metal or metal alloy interspersed with metal oxide derived from the oxidation of the same metal or metal alloy, the variation in the resistivity at different locations being achieved by adjusting the distance of the nozzle from the substrate during said relative displacement.

In one embodiment, the substrate is cylindrical and the nozzle and cylindrical substrate are relatively displaced both axially and circumferentially whereby the cylindrical substrate is sprayed progressively along its length, not necessarily in a direction normal to the circumferential surface.

In the latter case, in order for the axial end regions of the heating layer to be of higher resistivity than the central region, the nozzle and cylindrical substrate are further relatively displaced such that the thermal nozzle lies closer to the cylindrical substrate as its distance from one end of the substrate increases and lies further away from the cylindrical substrate as its distance from the other end of the substrate reduces, whereby the longitudinal end regions of the heating layer applied to the cylindrical substrate are of higher resistivity, and hence give greater heat output for a given electrical current, than the central region of the heating layer.

In all such cases, a heating element of constant, or substantially constant, thickness can be obtained having controlled non-uniform power density over its area.

In some embodiments, two spaced slip ring means are formed over the longitudinal end regions of the cylindrical resistive layer to enable electrical current to be passed through the resistive layer in use.

The slip ring means can be formed by combustion spraying conductive metal or metal alloy powder, or by shrink fitting respective sleeves.

In some embodiments, the two spaced contact means are mounted in relation to the cylindrical substrate carrying the resistive layer so as to engage directly with the resistive layer to enable an electrical current to be passed through the resistive layer in use.

Advantageously, the resistive layer is coated with a material having high dielectric strength, good release characteristics with respect to toner particles, and small heat capacity.

Preferably, said coating is either a thin sleeve made from anti-static heat shrinkable PFA fluoro resin, a liquid or dry formulation of PTFE or PFA sprayed onto the roller surface and cured, or a gas phase deposition of a polymer.

The invention also provides an electrically resistive heating element formed by the method as hereinbefore described.

In some embodiments, the contact means comprise respective conductive rollers or wheels, coil springs, flat springs, carbon brushes or wire brushes which electrically engage the periphery of the electrically resistive surface at respective spaced locations, or conductive liquid baths into which respective portions of the roller dip.

Advantageously, said conductive rollers are conformable.

The cylindrical substrate can be either solid or in the form of a hollow tube.

The contact means may be selectively displaceable relative to said substrate to enable the mutual spacing of the locations of said contact means to be adjusted.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 illustrates diagrammatically a method in accordance with the invention; Fig. 2 is a diagrammatic perspective view of a conventional heat/pressure fusing arrangement for dry toners; Fig. 3 is a diagrammatic view of one embodiment of a heating element in accordance with the present invention in the form of a heating roller; Fig. 4 is a perspective view of a practical embodiment similar to that of Fig. 3; Figs. Sa and 5b are diagrammatic side and plan views of a second embodiment in accordance with the present invention; Fig. 6 is a diagrammatic side view illustrating the principle of operating of the second embodiment; Fig. 7 is a diagrammatic view illustrating the operation of the embodiment of Fig. 5a and 5b ; Fig. 8 illustrates various types of contact arrangements to the wheels; Fig. 9 illustrates types of contact arrangements; Fig. 10 illustrates the principle of movable contacts; Figs l la, l lb illustrate the effect of displacing the contacts angularly around the element; and Fig. 12 illustrates a practical embodiment utilising displaceable roller contacts; Fig. 3 illustrates diagrammatically a first embodiment of a heater element in accordance with the present invention in the form of a fuser roll comprising a layer of resistive oxide 36 deposited onto a tubular roller 38 with contacts made using thermally sprayed conductive metal slip rings 40a, 40b at either end of the deposit. In this configuration, the heated zone of the roller is defined by the positions of the slip rings 40a, 40b and as such encompasses the whole resistive deposit.

A specific example of such a heating element is shown in the perspective view of Fig. 4.

The heating element of Fig. 4 is in the form of a heating roller comprising a tubular substrate 42, ideally of low thermal mass and preferably thermally insulating, made of either electrically non-conductive material or electrically conductive material with a co-axial electrically non-conductive coating material 44 applied to the surface. Laterally disposed conductive contacts in the form of annular slip rings 46 are thermally sprayed, or by any convenient method attached, directly onto the non-conductive substrate 42 or onto the electrically non-conductive coating 44. The conductive contacts 46 comprise a metal with high electrical conductive properties such as silver, copper, aluminium, nickel or gold. An electrically resistive oxide layer 48 is disposed over the remaining surface of the roller between and in electrical contact with the slip rings 46. Alternatively, the oxide layer 48 can be applied first, with the slip rings applied second so as to overlap the edges of the oxide layer. The annular volume of oxide 48 between the slip rings 46 constitutes the directly heated portion of the roller, electrical current paths extending axially through the oxide between the slip rings 46.

An alternative method of providing for electrical contact with the sprayed oxide layer is to"shrink fit"or by any suitable method attach geared, constant mesh conductive bushes onto each end of the roller.

Electrical energy can thus be transferred to the roller via any number of suitably applicable methods, including carbon brushes 50, (as shown in Fig. 4), spring metals contacts, and via constant mesh conductive gearing as described above.

As shown in Fig. 1, the resistive oxide layer 48 is applied by combustion spraying particles of a dry metal or metal alloy powder onto the surface of the substrate 42/44 by displacing a combustion spraying nozzle 51 over the substrate whereby the metal or metal alloy powder is partially oxidised in the nozzle flame 53 to form a composite structure 48 of metal or metal alloy interspersed with metal oxide derived from the oxidation of the same metal or metal alloy, with the effective resistivity of the combustion sprayed deposit 48 at any particular location on the substrate being determined substantially by the amount of oxidation of the metal or metal alloy powder particles sprayed onto said surface of the substrate at that location. Typical metal powders which can be used are, for example, nickel chrome powders. The metal oxide may be either conductive, semi-conductive or insulating. In all cases, a chemical reaction is performed upon the metal or metal alloy powder, whilst the latter resides in the flame, in order to generate the conductive, semi-conductive or insulating second phase which appears in the final thick-film deposit 48. The resistivity of the resulting material is raised above that of the original metal or metal alloy in some cases by virtue of it taking a smaller proportion of the volume of the final deposit. In addition, the deposit resistivity can be affected by the nature of the second'oxide'phase, ie conducting, semi-conducting or insulating.

In addition to combustion/flame spraying, the present technique could equally well be effected by plasma, HVOF or D-gun processes, or other appropriate thermal spraying technique.

As explained hereinbefore, a fuser roll producing a constant power density (W/cmZ) over it's surface will display a lower operating temperature at the ends due to'end losses'and in order to overcome this the resistor deposited onto the surface should have a higher resistance at the ends so as to generate higher power locally (P = I2 R), which will offset the end losses and hence produce a constant operating temperature along the roller.

This is obtained in the case of the roller of Fig. 4 by adjusting the resistivity of the deposit 48 across the width of the roller such that the resistance at the ends of the roller, and hence the power density, are higher than at the centre and thus overcome the end losses. This is achieved by varying the nozzle to substrate distance as the nozzle passes axially along the length of the roller, as illustrated diagrammatically in Fig. 1, and hence by varying the deposit resistivity. Thus, the distance d of the combustion spraying gun 51 from the roller substrate 42 on which the resistive layer 48 is being deposited is changed gradually from a maximum of d, at the roller ends to a minimum of d2 in the central region of the roller. Any suitable gun trajectory can be selected to suit the particular resistive characteristic required along the length of the roller. Thus, for example, spraying is not necessarily in a direction normal to the circumferential surface of the roller.

The sprayed oxide layer 48 is preferably coated with a material 52 which has three fundamental properties viz: 1. High dielectric strength.

2. Good release characteristics with respect to toner particles.

3. Small heat capacity with reasonably good thermal conduction characteristics.

Coatings which conform with the above can be applied to the surface of the fuser roller in a variety of ways including the following methods: a. A thin sleeve made from, for example, anti static heat shrinkable PFA fluoro resin. b. Liquid or dry formulations of, for example, PTFE or PFA which can be sprayed onto the roller surface and subsequently heat cured. c. Vapour (Gas Phase) deposition of polymer coatings.

A tubular heating roller constructed as described above overcomes the time lag problem inherent in the known system of Fig. 2, provides for substantially uniform temperature distribution across the heated surface and gives substantially instant heat capability. It has a structure which is robust, simple and cost effective. It can achieve the power requirements for all practical machines and, because of its better efficiency, it is able to offer power savings over conventional halogen lamp fusing systems.

As an extension of the basic structure shown in Figs 4 and 5, the"heated zone"of the resistor may be defined as the zone delineated by areas defined by the common area described by the placement of a pair of moveable contacts on the resistive oxide deposit. In the case of a fuser roll, this can, for example, be implemented using a pair of conductive wheels 54 as shown in Fig. 5a and 5b which are in contact with the resistive deposit 48 at the ends of the deposit i. e. at the same axial position as the slip rings in the embodiment of Figs 3 and 4, but which by virtue of the shape of the wheels only contact a limited fraction of the circumference of the tube. In a static situation, this can be considered to be equivalent to having two incomplete slip rings 56 (Fig. 6), each covering only a fraction of the circumference of the tube, at opposite ends of the oxide deposit 48, but at the same angular position. In the dynamic case, the wheels rotate around the circumference of the tube, thus defining an axial (end to end of the tube) heated zone which moves around the circumference of the tube. This can be considered to be equivalent to a heated zone defined by continually removing and reapplying the incomplete slip rings (Fig. 7). Contact to the conductive wheels can be made by means of brushes or flat springs etc (Fig. 8).

The heated zone is a parallel combination of paths through the resistive deposit and can result in greater heating at the ends of the zone due to the greater current density. The temperatures observed across the roller may be more uniform, however, due to the enhanced heat losses at the ends of the roller. This effect can be mitigated by the ability of the above-described spraying process to produce controllably varied resistivity deposits in such a way as to achieve the required uniformity of temperature across the roller.

The areas of the contacts define the"heated zone"and as such this zone is dependent on the nature of the contact. This can be a resilient conductive wheel, e. g. carbon or metal loaded polymer, solid metal wheel e. g. copper, a wheel with a coiled spring around its circumference, a flat spring, a carbon (or similar) brush arrangement, a wire brush arrangement e. g. brass etc. The contact may even be made by allowing the roller to partially dip into a conductive liquid e. g. mercury or an electrolyte e. g. saline solution (Fig. 9).

One advantage of such a contact arrangement is that the power applied to the fuser roll can be concentrated into the"heating zone"and as such the power density can be much higher than for a roller where the whole surface is heated.

This can be used to advantage in either providing a faster heat-up time for the same power consumption or alternatively reducing the power consumption for the same heat-up time.

A further advantage is that if the contact wheels 54 are so arranged that their positioning along the axis of the roller can be adjusted (see Fig. 10), for example electromechanically by the printer itself, the heated zone can be lengthened or shortened in such a way as to optimise the heated zone for the width of paper onto which toner is being fused. This may be used to advantage in the fusing of toner to envelopes where current systems result in either incomplete fusing or in over-temperature excursions of the roller and hence in wasted energy.

In addition, the contacts can be placed at non-coincident positions around the circumference of the roller. This results in the heated zone taking a spiral path around the roller, possibly in two directions if the contacts are on opposite sides of the circumference. This leads to point heating at the nip of the fusing system which may be used to advantage in some industries. This arrangement can also be used to provide the same effect as moving the contacts along the axis of the roller and so provides a further means of solving the"envelope"problem (see Figs. lla and llb).

Fig. 12 shows an example of an embodiment of a roller made in accordance with the present invention using roller contacts/wheels as described above. In accordance with this embodiment of the present invention there is provided an electrically resistive heating element for the fixing of dry toners, comprising a cylindrical substrate 60 formed from an electrically insulating material, (Vycor 7913, Pyrex 7740 or similar), or formed of an electrically conductive material, (aluminium, steel or similar), provided with an electrically insulating coating. In both cases, the substrate 60 presents a robust electrically non-conductive surface on which a thermally sprayed resistive oxide layer 62 is applied to at least part of the said electrically non-conductive surface as described above. Contacts in the form of conductive conformable rollers 64 with a predetermined footprint, shape and size contact the thermally sprayed electrically resistive layer 62 thus forming a virtual fusing strip. The conductive contacts 64 define a strip-like electrically conductive path 66 between the contacts locations through the resistive oxide layer 62. The width of this contact path related to the contact area and shape formed by the conductive conformable roller contacts 64 effectively forms the heating element and has the advantage of producing power and thus heat in this region only.

The conductive conformable rollers 64 are mounted on a carriage (not shown) which enables one or both rollers to be moved laterally across the surface of the fuser roller. This enables the length of the virtual element to be reduced so that only a portion of the overall length is utilized. To enable this reduced region of the heating element to provide for the same temperature requirements as the full strip element, the potential across the two conductive contacts can be reduced accordingly. This topology could be utilized, for example, when items with a width less than A4 paper need to be fused, thus eliminating the aforementioned fusing difficulties regarding printing of envelopes etc.

As described above, the embodiment of Fig. 12 has the advantage of allowing energy to be transferred preferentially in the region of the fuser nip, thus reducing wasted radiated heat to the environment thus saving power and also of allowing energy to be transferred differentially across the length of the heating element thus compensating for differential heat removal from same.

The present invention is by no means limited to the construction of heated rollers for use in dry toner fusing applications.

The arrangement might additionally be extended into other areas where rollers require heating. A number of industrial processes use heated rollers as a means e. g. of drying the materials passing over or between rollers e. g. the paper industry. Traditionally, these are hollow roller heated internally by steam. These rollers could be made in the form of a scaled up fuser roll and the contact arrangement could be made such that only that proportion of the surface of the roller which makes contact with the material, be heated.

In all embodiments described above, the resistive oxide layer may be, but not essentially, formed using the flame spraying procedures set out in our EP-A-302586 and US-A-5039840 to which reference is hereby directed for details.

The substrate can in all cases be any suitable material, including steel, aluminium, ceramics and glass.

Claims (17)

1. A method of achieving an electrically resistive heating layer on a substrate formed of an electrically insulating material, or formed of an electrically conductive material having an electrically insulating coating, whereby in both cases the substrate presents an electrically non-conductive surface, and such that the resistance of the resistive heating layer differs in different regions thereof, the method comprising thermal spraying particles of a dry metal or metal alloy powder onto the surface of the substrate by relative displacement of the substrate and a thermal spraying nozzle whereby the metal or metal alloy powder is partially oxidised to form a composite structure of metal or metal alloy interspersed with metal or metal alloy oxide derived from the oxidation of the same metal or metal alloy, the variation in the resistivity at different locations being achieved by adjusting the distance of the nozzle from the substrate during said relative displacement.

2. A method as claimed in claim 1, wherein the substrate is cylindrical and wherein the thermal spraying nozzle and cylindrical substrate are relatively displaced both axially and circumferentially whereby the cylindrical substrate is sprayed progressively along its length.

3. A method as claimed in claim 2, wherein the nozzle and cylindrical substrate are further relatively displaced such that the thermal nozzle lies closer to the cylindrical substrate as its distance from one end of the substrate increases and lies further away from the cylindrical substrate as its distance from the other end of the substrate reduces, whereby the longitudinal end regions of the heating layer applied to the cylindrical substrate are of higher resistivity, and hence give greater heat output for a given electrical current density, than the central region of the heating layer.

4. A method as claimed in claim 2 or 3, wherein two spaced slip ring means are formed on the substrate prior to the formation of the resistive coating, the ends of the sprayed resistive coating being applied over the slip ring means whereby the slip ring means enable electrical current to be passed through the resistive layer in use.

5. A method as claimed in claim 2 or 3, wherein two spaced slip ring means are formed over the longitudinal end regions of the cylindrical resistive layer to enable electrical current to be passed through the resistive layer in use.

6. A method as claimed in claim 4 or 5, wherein the slip ring means are formed by combustion spraying conductive metal or metal alloy powder, or by shrink fitting respective sleeves.

7. A method as claimed in claim 2 or 3, wherein the two spaced contact means are mounted in relation to the cylindrical substrate carrying the resistive layer so as to engage directly with the resistive layer to enable an electrical current to be passed through the resistive layer in use.

8. A method as claimed in any of claims 1 to 7, wherein the resistive layer is coated with a material having high dielectric strength, good release characteristics with respect to toner particles, and small heat capacity.

9. A method as claimed in claim 8, wherein said coating is either a thin sleeve made from anti-static, heat shrinkable PFA fluoro resin, a liquid or dry formulation of PTFE or PFA sprayed onto the roller surface and cured, or a gas phase deposition of a polymer.

10. A method as claimed in any of claims 1 to 9, wherein the thermal spraying comprises combustion/flame or plasma spraying or an HVOF (high velocity oxy-fuel) or D-Gun process and the nozzle is a combustion/flame or plasma spraying nozzle.

11. An electrically resistive heating element formed by the method of claim 1, comprising an electrically resistive heating layer on a substrate formed of an electrically insulating material, or formed of an electrically conductive material having an electrically insulating coating, whereby in both cases the substrate presents an electrically non-conductive surface, and such that the resistance of the resistive heating layer differs in different regions thereof, the electrically resistive heating layer comprising a layer of substantially uniform thickness of metal or metal alloy powder, partially oxidised to form a composite structure of metal or metal alloy interspersed with metal or metal alloy oxide derived from the oxidation of the same metal or metal alloy.

12. An electrically resistive heating element as claimed in claim 11, wherein said electrically resistive heating layer is tubular and is of greater resistance adjacent its ends than in its central region.

13. An electrically resistive heating element as claimed in claim 12, wherein the resistance of the electrically resistive heating element reduces progressively from its ends towards its central region, at least in respective end regions.

14. An electrically resistive heating element formed by the method of claim 7, wherein the contact means comprise respective conductive rollers or wheels, coil springs, flat springs, carbon brushes or wire brushes which electrically engage the periphery of the electrically resistive surface at respective spaced locations, or conductive liquid baths into which respective portions of the roller dip.

15. An electrically resistive heating element as claimed in claim 14, wherein said conductive rollers are conformable.

16. An electrically resistive heating element as claimed in claim 14 or 15, wherein the cylindrical substrate is either solid or is in the form of a hollow tube.

17. An electrically resistive heating element as claimed in any of claims 14 to 16, wherein said contact means are selectively displaceable relative to said substrate to enable the mutual spacing of the locations of said contact means to be adjusted.