Electrophotographic printing apparatus having a corotron-polarised transfer roller

Abstract

 Electrophotographic printing apparatus in which a transfer roller (15) of the toner from a photoconductive drum (2) to a print medium (13) is provided with a resilient, resistive external layer (16) and a conductive core (17) which is earthed by a resistive element (19) having controllable resistance and is charged by a corotron (20) with a specific surface charge which gradually discharges across the resistive layer (16) and the controllable resistor (19) in a way determined by the value of the controllable resistor (19) such that the residual charge transferred from the transfer roller (15) to the print medium (13) can be easily controlled, dependable and repeatable in order to obtain an optimal value.

Description

[0001] The present invention concerns electrophotographic printing apparatus with a transfer roller electrically polarised by a corona effect discharge or "corotron".

[0002] It is known that in electrophotographic printing apparatus, the transfer of powdered pigment or "toner" from a photoconductive drum on which the image is formed onto a print medium occurs by means of the mutual contact between one face of the print medium (usually a sheet of paper) and the photoconductive drum, along a generatrix of the latter.

[0003] The transfer of the electrically charged toner from the photoconductive drum to the print medium occurs with the help of an electrostatic discharger or corotron facing the contact generatrix of the face of the print medium opposite the face in contact with the photosensitive drum and the toner.

[0004] This face is electrically charged by the corotron with a charge opposite to that of the toner which is therefore attracted to the print medium.

[0005] The process is very effective and widely used but has the disadvantage that because the transfer takes place between moving surfaces, even if they are accurately synchronized, there is relative microscopic slippage between the print medium and the photoconductive drum which degrades the print quality.

[0006] This disadvantage is particularly apparent in modern high-resolution electrophotographic printers.

[0007] A further disadvantage lies in the fact that the regions of the photosensitive drum devoid of toner are also electrically charged with an opposite charge to that of the print medium, such that a not insignificant force of attraction arises between the photosensitive drum and the print medium which, although it locally reduces the microscopic slippage, tends however to wrap the print medium onto the drum and impede its correct conveyance towards a toner fixing station, with possible jamming of the apparatus.

[0008] In order to avoid these disadvantages electrophotographic printing apparatus has been proposed and is available on the market in which the corotron is replaced by a resilient transfer roller, pressed against the photosensitive drum along a generatrix thereof to form a nip through which the print medium passes.

[0009] In this way the print medium is forced to move in a way which is rigorously synchronised to the photosensitive drum without the possibility of relative microscopic slippage.

[0010] Moreover, the print medium is subjected to elastoplastic forces which encourage it to separate from the photosensitive drum.

[0011] The fundamental problem of these transfer devices having a transfer roller is that of correctly charging the print medium.

[0012] To this end various solutions have been proposed.

[0013] For example, the surface of the transfer roller can be conductive and connected to an appropriate voltage source and, beneath the conductive surface, there is a resilient, insulating layer which ensures the necessary elasticity of the roller.

[0014] However, in the usual arrangement the transfer roller is made with a conductive cylindrical core and an outer resilient layer having suitable resistivity.

[0015] The core is connected to a voltage supply, such that a charging current flows from the conductive core through the resilient layer to the surface of the print medium.

[0016] The circuit is closed by means of the capacitances, in series, formed by the print medium, of the photosensitive layer of the photoconductive drum, and the conductive core of the photoconductive drum, which is usually earthed.

[0017] Both of the arrangements have different limitations and their manufacture is highly critical.

[0018] In the first case it is difficult to connect a conductive layer whose conductivity does not vary over time to an underlying resilient support.

[0019] In the second case, it is extremely difficult to ensure that the optimal electric charge accumulates on the surface of the transfer roller for transfer of the toner to the photoconductive drum to the print medium; a weak current in the transfer roller causes a surface charge to develop which is insufficient to transfer the toner completely, while a high current neutralises the electric charge of the toner particles which are not transferred to the print medium as they are no longer correctly charged.

[0020] There is therefore a critical window of electric charge and of associated current which generates it, within which the transfer system functions.

[0021] This critical operative condition is obtained either by calibrating the force with which the transfer roller presses against the photoconductive drum, or by making the transfer roller from material having adequate electrical conductivity which is consistent with the mechanical situation so as to allow the desired quantity of electrical charge to be deposited onto the print medium, that is onto the paper.

[0022] Extensive research has been carried out into the problems associated with this approach and has lead to the identification of optimal dimensions for the diameter of the transfer roller and the photoconductive drum, as well as their relative ratio, of preferred compositions characterised by a predetermined resistivity for the manufacture of the transfer roller, of mechanical arrangements to ensure a predetermined contact pressure, independent of possible eccentricities of the roller, and of various combinations of these characteristics.

[0023] All of these arrangements which start from the supposition that the charging current is injected into the conductive core of the transfer roller and flows through the resistance of the resilient layer, are susceptible to variation and adaptation only in the design phase and severely restrict the characteristics of the transfer process, and therefore of the printing process, by limiting the possibility of using print media with different characteristics such as thickness, resistivity and dielectric constant in the same electrophotographic apparatus, or of varying the speed of the print process in the same apparatus.

[0024] These limitations are overcome by the electrophotographic printing apparatus which is the subject of the present invention, comprising a transfer roller with a conductive core and an external resilient, resistive cylindrical layer, in which the external surface of the transfer roller is polarised and electrically charged by an electrostatic discharger or corotron, the surface electrical charge partially discharging in a controlled way and in an inverse sense with respect to that usually adopted, through the series of resistors constituted by the resilient resistive layer of the transfer roller and an adjustment element having variable resistance.

[0025] The residual charge is transferred in a controlled manner to the print medium along the nip between the transfer roller and the photoconductive drum.

[0026] It has been found that in this arrangement the electrical charge transferred to the print medium is largely insensitive to variations in the contact pressure of the transfer roller and of the consequent variations in the resistivity of the material and, in practice, is a function only of the discharge voltage of the corotron, the variable adjustment resistance and of the peripheral velocity of the transfer roller.

[0027] Therefore the charge transferred is easily controlled and regulated as a function of specific requirements and the possible different modes of operation of the electrophotographic printing apparatus.

[0028] The characteristics and advantages of the present invention will become clearer from the following description and from the accompanying drawings in which:

Figure 1 is a schematic block diagram of a preferred embodiment of an electrophotographic printing apparatus according to the present invention;

Figure 2 is a schematic view of a transfer device commonly used in the prior art;

Figure 3 is an equivalent electric circuit of the device in Figure 2;

Figure 4 is a schematic view of a transfer device for the electrophotographic apparatus according to the present invention;

Figure 5 is an equivalent electrical circuit of the device of Figure 4;

Figure 6 is a qualitative timing diagram of the variation of the specific surface charge of the surface on the transfer roller of the electrophotographic apparatus of Figure 1 and of the device of Figure 4; and

Figure 7 shows a variant of the apparatus of Figure 1.

[0029] With reference to Figure 1, an electrophotographic printing apparatus according to the present invention includes a photoconductive drum made from a conductive core 1 and an external photoconductive layer 2, rotated by a motor 3 and controlled by a control unit 4 which causes the drum to move at predetermined peripheral velocity in the direction of rotation indicated by the arrow 5.

[0030] Along generatrixes of the photoconductive drum are disposed in a known way, and in order with reference to the direction of rotation of the photoconductive drum:

• a cleaning device with a cleaning blade 6;
• a lamp 7 for neutralising the residual electrical charge and normalising the photoconductive layer 2;
• a device for electrically charging the photoconductive layer, for example an electrostatic discharger having a control grid or "scorotron" 8 supplied by a high negative voltage supply 9, of the order of -5KV, -10KV with respect to earth, to charge the photosensitive surface of the drum to a potential of the order of -700V,
• a scanning device 10 and selective exposure device 11 (generally a laser diode) controlled by the unit 4;
• a developer 12 for selectively applying the toner to the photosensitive surface of the drum; and
• a device for transferring the toner which is selectively deposited on the surface of the photoconductive drum to a print medium 13, interposed between the transfer device and the photoconductive drum and in contact with this latter along its generatrix.

[0031] The print medium is moved forward to a fixing station 14 at a controlled speed which is equal to the peripheral velocity of the photoconductive drum.

[0032] All of these aspects are conventional and well known and do not require further explanation.

[0033] According to the present invention the transfer device includes a transfer roller 15 in pressure contact with the photoconductive drum along a generatrix, forming a nip through which the print medium 13 passes.

[0034] The transfer roller is rotated by the motor 3 with a peripheral velocity coordinated (equal to or slightly greater than) with the peripheral velocity of the photoconductive drum.

[0035] The transfer roller 15 is made from a resilient external layer 16 having a high resistivity of the order of 10⁸ Ω.cm or more and a conductive core 17, rotatably supported by supports 18 electrically insulated from ground.

[0036] The conductive core 17 is earthed via a variable resistor 19 or equivalent means, for example a field-effect MOS device.

[0037] The transfer device also includes a corotron 20 supplied by a source 21 of positive voltage +V of an appropriate value, for example +5/10KV, facing a generatrix of the transfer roller 15 in order to charge the surface with a predetermined electric charge.

[0038] The angular velocity of rotation of the transfer roller multiplied by the angle of rotation necessary to carry the electrical charge along the generatrix into contact with the print medium defines the delay with which the electrical charge is transferred.

[0039] The simple combination of a transfer roller 15 of the type described, a corotron 20 and a variable resistor 19 allows a predetermined electrical charge to be repeatedly applied to the print medium 13, independently of variations in the contact pressure.

[0040] In addition, the different conductivity of the transfer roller which can arise from the productive processes or from the use of different materials can be compensated within a broad range, by releasing the transfer device from the design and production criticality of known devices.

[0041] It also allows the charge applied to the print medium to be regulated within a broad range as a function of variations in the print process parameters such as the thickness and humidity of the print medium.

[0042] As illustrated in Figure 1, upstream of the transfer station constituted by the nip formed from the photosensitive drum and the transfer roller there is a thickness detector 22 which, by utilising known electrical capacitive techniques, is also able to detect the humidity of the print medium and, by means of a regulatory unit 23, to send an adjustment command to the variable resistor 19.

[0043] It is however evident that the automatic control of the resistor 19 can be replaced with a manual calibration based upon the knowledge of the weight of the paper used and of the environmental storage conditions of the print media.

[0044] Before describing a further advantageous application of the possibility of adjustment of the electrical charge applied to the print medium, it is appropriate to consider a qualitative explanation of the functioning of the described transfer device in comparison with known transfer devices.

[0045] Figure 2 schematically represents a known transfer roller formed from an internal conductive core 24 and an external resilient, resistive layer 25 pressed against a generatrix of a photoconductive drum 26, with an interposed print medium 27.

[0046] A voltage generator 28 applies a suitable voltage V to the core 24 of the transfer roller.

[0047] The conductive core of the photosensitive drum 26 is earthed.

[0048] The equivalent circuit of the structure formed in this way is represented to a first approximation by the circuit in Figure 3 in which the generator 28 is connected in series to a resistor 29 and a capacitor 30.

[0049] The resistor 29 represents the resistance of a limited cylindrical arc 31 of the resistive layer 25, and the capacitor 30 represents the capacity formed by a limited cylindrical arc 32 of the conductive core of the photosensitive drum and of the juxtaposed external surface of the cylindrical arc 31 separated from a dielectric constituted by the print medium 27 and the photoconductive layer of the drum 26.

[0050] It is clear that the time constant RC of the circuit varies as a function of the resistance 29.

[0051] Since the rotation of the drum 26 and the transfer roller continually renews the circuit elements, the capacitor 30 is charged during the short and finite transit period of juxtaposition with the arc 31 to a voltage which depends on the time constant RC of the circuit and which varies with R.

[0052] Unfortunately the arc 31 of the resistive layer is subject to variations in resistivity caused by variations in compression, the charge of the capacitor 30 is therefore to a large extent unpredictable, the more as the time constant RC increases.

[0053] The variability of the charge can to some extent be limited by making the resistive layer from low resistivity materials, which is difficult to achieve.

[0054] Figure 4 shows schematically the structure of the transfer device according to the present invention and Figure 5 shows the equivalent electrical circuit.

[0055] A corotron 33 applies a specific charge Qs to every surface element 34 of the transfer roller 36 which, to a first approximation, depends only on the voltage V applied to the corotron 33 by a voltage generator 35 and on the exposure time Δt of the surface element to the discharge from the corotron (it is therefore inversely proportional to the peripheral velocity of the transfer roller).

[0056] The charge Qs gradually discharges, with an exponential law, through the series connected resistance 37 of the resistive, resilient layer and the variable resistance 38 which earths the transfer roller core.

[0057] In parallel with the surface element 34 and the resistor 37 there is naturally a plurality of other surface elements with associated resistance, schematically represented by the element 40 and by the resistor 39, each being charged at different times with the same specific charge.

[0058] By means of a suitable choice of the resistivity of the resilient layer of the transfer roller, which must be high and which is therefore easily reconciled with the resilience requirement of materials such as synthetic rubbers, and by a suitable choice of the value of the controllable resistor 38, it can be ensured that the residual specific charge transferred from the transfer roller has the desired optimal value in the contact zone with the print medium.

[0059] Figure 6 is a qualitative timing diagram of the value of the specific charge Qs.

[0060] The initial value Qs1 depends as mentioned on the corotron voltage supply, the peripheral velocity of the transfer roller and, to a certain extent, also the resistivity of the resilient layer and the value of the variable resistance.

[0061] The specific charge Qs falls exponentially over time as defined by the time constant of the discharge circuit (Figure 5) and shown by the line 41.

[0062] By increasing the value of the variable resistance 38 (Figure 5) the time constant increases, as does the initial specific charge Qs1, and the specific charge decreases according to the line 42.

[0063] By decreasing the value of the variable resistance 38 the time constant diminishes, as does the initial specific charge and the specific charge decreases according to the line 43.

[0064] If t1-t0 represents the transit time of the surface element from the charging station facing the corotron to the transfer station, the variable resistance 38 can be easily calibrated in such a way that, at the transfer station, the specific charge has an optimal value Qs0 to cause the transfer of toner.

[0065] It is clear that during the time interval t1-t0, the resilient, resistive layer of the transfer roller is not subjected to elastic deformation, therefore its resistivity is invariable and causes no uncertainty in the value of Qs0.

[0066] The elastic deformation which occurs at time instant t1 and the possible consequent variations in resistivity have an entirely negligible effect on the value of Qs0.

[0067] It is therefore clear that the value of Qs at the transfer station can be easily regulated and controlled as a function of the process parameters, in particular as a function of the speed of the print process.

[0068] For example, if the speed of the print process is doubled, it causes a substantial reduction, virtually a halving, of the initial specific charge of the transfer roller.

[0069] However, by increasing the resistance 38 (Figure 5) and consequently the time constant of the discharge circuit it is possible (line 44) that at time t1/2 the residual specific charge still has the value Qs0.

[0070] It is therefore very easy to make electrographic printing apparatus able to operate at different speeds, for example at low speed to give high print resolution and at high speed, for example double, to give greater throughput, although with inferior resolution.

[0071] Figure 7 schematically represents apparatus of this type.

[0072] In Figure 7 the control unit 50 receives from a bistable control button 51 or from a system processor 52 such as a PC, a signal which selects the operating mode, high or low speed and, in dependence on this signal, controls the speed of rotation of the motor 53 which drives the moving parts of the apparatus (photoconductive drum 54, transfer roller 55, fixing station 56).

[0073] In addition it controls a variable resistor, preferably made from an electronic device such as a metal-oxide-semiconductor field-effect transistor (MOSFET) 57 across a control circuit 58.

[0074] The MOSFET constitutes a controlled variable resistor which earths the conductive core of the transfer roller.

[0075] It is clear that it is possible to replace the MOSFET 57 with a bipolar transistor.

[0076] It is also clear that the preceding description is given with reference to electrophotographic printing apparatus of the type commonly known as a "laser printer", electrophotographic printing apparatus meaning any printing apparatus which uses electrophotographic techniques for forming a latent image on a photoconductive element and its transfer to a print medium, such as diode array printers or photocopiers.

Claims

1. Electrophotographic printing apparatus of the type in which a latent image is formed on a photoconductor (2), the latent image is developed with the selective application of toner to said photoconductive drum (2) and transferred onto a print medium (13) interposed between the said photoconductive drum (2) and a transfer roller (15) in pressure contact with the said photoconductive drum (2) along a generatrix of said photoconductive drum, the said transfer roller (15) having a rigid, internal, conductive, cylindrical core (17) and an external resilient, resistive layer (16), characterised in that it includes a corona effect electrostatic charge generator (20, 21) to apply a positive electrical charge to the said external layer (16) and in that the said core (17) is electrically insulated and earthed through resistive means (19, 38, 57) having adjustable resistance.

2. Electrophotographic apparatus as in Claim 1 including a sensor (22) to measure a parameter which influences a transfer operation performed by said transfer roller (15), and control means (23) connected to said measuring sensor (22) and to said resistive means (19) having adjustable resistance, in order to vary said adjustable resistance as a function of said parameter measured by said measuring sensor.

3. Apparatus as in Claim 2 in which said parameter is the moisture content of said print medium.

4. Apparatus as in Claim 3 in which said parameter is the thickness of said print medium.

5. Electrophotographic apparatus as in Claim 1, 2, 3 or 4 including control means (50, 51, 52), which causes said photosensitive drum (2, 54) and said transfer roller (15, 55) to move at a peripheral velocity chosen from at least two peripheral velocities and which varies said adjustable resistance as a function of the chosen peripheral velocity.

Drawing