EP0092106A1 - Electrophotographic copying process for removing the developer liquid from a photoconductor surface - Google Patents

Abstract

The invention has a drum (1) with a photoconductor (21). Around the circumference of the drum (1) are a direct current corona with a high voltage supply circuit (15), an exposure station (3), a development station (22) with an electrode (4) and a tub (5) with developer liquid, a metering roller (6), an image transmission station (16) with transmission corona (9) and heating device (10), a cleaning device (11, 12) and a further charging device (13), for example an AC corona or extinguishing lamp, are arranged. A wiper lip (7) cleans the metering roller (6), which is set to a voltage of approximately + 1.5 to + 2.0 kV if the photoconductor (21) consists of silk. In the case of an organic photoconductor, a voltage of the same order of magnitude, but with a negative polarity, is applied to the metering roller (6). The individual sheets of paper (8) are removed from a container (25) in the transfer station (16).

Description

The invention relates to an electrophotographic copying method for removing developer liquid from a photoconductor surface, which is electrostatically charged, exposed for informational purposes, the latent charge image obtained with the developer liquid is developed into a visible toner image and the excess developer liquid is removed with a metering element arranged at a short distance from the photoconductor surface. to which a bias voltage of the same polarity as that of the charged photoconductor surface is applied, the toner image is transferred from the photoconductor to an image receiving material, fixed thereon and the photoconductor is cleaned and / or discharged.

DE-OS 3 018 241 discloses such a method for removing excess developer liquid of a liquid developer consisting of an insulating developer liquid and charged toner particles suspended therein from a photoconductive surface which bears an electrostatic charge image developed by means of the liquid developer. For this purpose, a drying element in the form of a squeeze roller or a suction roller is brought into contact with the photoconductive surface, the squeeze roller or the suction roller being kept at a potential whose polarity is equal to that Polarity of the charge of the charged toner particles and in addition the relative movement between the photoconductive surface and the squeeze roller or the suction roller is controlled so that the relative speed in the contact area becomes zero. The outer surface of the squeeze roller or the suction roller consists of an elastomeric material which has a Shore A hardness of less than 45 and a resistance value of less than 10 ⁹ Ohm.cm. The photoconductive surface is on a drum which runs counterclockwise past a metering or stripping roller which is suitable for limiting the amount of liquid remaining on the photoconductor after the latent charge image has developed. This dosing. or stripper roller does not touch the developed charge image, so that neither streaks nor distortions are generated. A developer liquid layer with a thickness between 10 and 15 μm then remains on the photoconductor surface. After passing through the _D osier- or wiper roll runs the drum surface to the crushing or Aufsaugwalze over, which is maintained at such a bias that results in an electric field through which the toner is adhered to the photoconductor surface. The bias applied to the nip roller has the same polarity as the toner particles in the developer liquid. In this way it is achieved that the developed image adheres to the surface of the photoconductor without streaking, without smearing and without the Toner is transferred to the nip roller. The remaining liquid developer layer on the photoconductor surface is reduced after the passage of at the nip roll to a thickness of 2 to 3, so that the layer thickness of the _E ntwicklerflüssigkeit on the photoconductor as a whole is lowered to about one fifth of the initial value.

In addition to the advantages, such as high resolution and small energies for fixing the copies, compared to dry development, the liquid development has the disadvantage that the copies have to be heated, since after the transfer of the toner image from the photoconductor surface to the image-receiving material together with residual developer liquid , this must be evaporated by heating when fixing the copies. As a result, on the one hand, developer liquid is lost in large quantities and must be replenished again and again in the copying machine, and on the other hand, the air in the vicinity of the copying machine undesirably accumulates with evaporated developer liquid. Although the usual developer liquids as such are not toxic, these are predominantly aliphatic hydrocarbons such as i-decane, in which the charged toner particles are dispersed, the large discharge of developer liquid is also undesirable for reasons of low environmental impact.

In the prior art, as described, for example, in DE-AS 23 61 833 (US Pat. No. 3,907,423), To reduce the discharge of developer liquid after the development of the charge image on the photoconductor layer by electrophoretic deposition of charged toner particles, excess and thus excess developer liquid is reduced in layer thickness before the transfer of the toner image to the image-receiving material by means of a stripping roller rotating in opposite directions to the photoconductor. For this purpose, it rotates at a distance of only about 50 in order to counteract the movement of the photoconductor layer at a high circumferential speed. The toner images deposited on the photoconductor layer are not blurred, but the excess amount of developer liquid is only partially removed, so that moist copies are ejected.

The stripping or dosing roller is separated from the photoconductor surface by a gap of 0.05 to 1 mm in width. Due to the counter-rotation of the metering roller with a greater peripheral speed than that of the photoconductor drum, the developer liquid in the metering gap between the photoconductor and the metering roller is divided into two oppositely directed flows, one flow being sheared off by the metering roller and removed by a downstream wiper blade. With a metering gap of 100 _/ um, the discharge of developer liquid through an A4 copy is approximately 0.2 g, so that because of the linearity between the developer liquid discharged and the metering gap can be assumed that with a gap of 50 _/ um the discharge will be about 0.1 g of developer liquid.

The metering roller is held in insulated bearing disks and a preload of approximately 300 V is formed on the roller by induction. The image areas on the photoconductor surface have an electrical potential of 900 to 950 V, while the non-image background areas have an electrical potential of about 150 V. Since the electrical potential of the metering roller is lower than that of the image area and higher than that of the non-image areas on the photoconductor, the toner flows from the non-image areas to the image areas and is held on to them.

It can also be provided to apply a bias voltage to the metering roller by means of a DC voltage source, which is set in such a way that it is lower than the electrical potential of the image areas and higher than that of the non-image areas of the photoconductor surface. By applying the bias voltage the only thing that is achieved is that the background tint of the copies is reduced and a strong contrast of the image is achieved.

Attempts have been made until recently, both with suction rolls made of a foamed, open-pore polymer and with excess developer liquid using squeeze rolls remove the development of the electrostatic charge image from the photoconductor surface and thereby further reduce the discharge of developer liquid through the copies. The squeeze roller technology for this purpose for easier drying of liquid-developed charge images on photoconductor layers is described in its beginnings, for example, in US Pat. No. 3,299,787, from which it emerges that a squeeze roller with associated cleaning element is used to remove the excess developer liquid from a photoconductor belt.

A reduction in the discharge of developer liquid due to a higher peripheral speed of the dosing valve is hardly possible because of the asymptotic decrease in the discharge depending on the peripheral speed, since the peripheral speed required for a noticeable discharge would be very difficult to achieve in conventional copying machines. A noticeable reduction in the gap between the metering roller and the photoconductor surface below 50 .mu.m, which would lead to a reduction in the discharge of developer liquid, cannot be achieved because of the precision tolerances with regard to the straightness of the photoconductor drum and the metering roller. Compliance with certain precision engineered Tole _- tolerances becomes more difficult the longer the photoconductor drum and the metering roller to prepare, for example, copies in DIN Al. For the production of large-scale copies are because of the larger construction gaps of the drums and metering rollers larger than 50 in order to aim at reducing the discharge of developer liquid at the same time.

The object of the invention is to improve a method of the type described above so that the discharge of developer liquid in copies up to DIN Al formats is reduced without a reduction in the gap between the metering element and the photoconductor surface and an increase in the peripheral speed of the Dosing element is required.

This object is achieved in that the bias of the metering element is greater than 1 kV and the maximum voltage on the photoconductor surface is selected to be equal to or less than the bias of the metering element.

The difference between the maximum voltage on the photoconductor surface and the bias of the dosing element is preferably up to 200 V.

The further embodiment of the invention results from the features of claims 3 to 8.

The invention achieves the advantage that the simple measure of applying a large electric field in the gap between the metering element and the photoconductor surface while maintaining the rest Copy conditions, the discharge of developer liquid is significantly reduced.

There is no valid explanation for the processes involved. There is no metering of developer liquid by means of charges which are sprayed onto the liquid surface by a free corona which is not wetted with developer liquid and which could then bring about an adjustment of the liquid layer on the photoconductor, as described in DE-OS 2 739 104 is. This is a surprising effect which was not to be expected from the person skilled in the art, since it was only known from the prior art to apply a potential for background-free development which is of the order of magnitude of 300 V and considerably smaller than that Potential of about 950 V of the image points on the photoconductor layer.

The invention is explained below with reference to the drawings.

• Fig. 1 eine schematische Seitenansicht eines elektrofotografischen Kopiergeräts zur Durchführung des Verfahrens nach der Erfindung, und
• Fig. 2 die Abhängigkeit der ausgetragenen Entwicklerflüssigkeitsmenge von der an das Dosierelement angelegten Spannung und dem Spalt zwischen dem Dosierelement und der Fotoleiteroberfläche.

Show it:

• Fig. 1 is a schematic side view of an electrophotographic copier for performing the method according to the invention, and
• Fig. 2 shows the dependency of the amount of developer liquid discharged on the voltage applied to the dosing element and the gap between the dosing element and the photoconductor surface.

The structure of a copying machine with which the method according to the invention can be carried out corresponds to the prior art and is shown schematically in FIG. 1. A drum 1 is provided with a photoconductor 21 and is rotated counterclockwise by a drive source, not shown, at a predetermined speed. Around the circumference of the drum 1 there are an electrical charging device 2, for example a corona, an exposure station 3, a development station 22, a metering roller 6 for excess developer liquid, an image transfer station 16, a cleaning device 11, 12 and a further charging device 13, for example an AC corona, and / or extinguishing lamp arranged.

If the photoconductor 21 is made of organic material, for example poly-N-vinylcarbazole / trinitrofluorenone, the photoconductor is negatively charged by the electrostatic charging device, while a positive charge takes place in the case of a photoconductor 21 made of selenium. The charged photoconductor 21 is exposed for informational purposes in the exposure station 3 via its optics, ie with a radiation image of a Original exposed. The electrostatic latent charge image obtained is developed in the development station 22 into a visible toner image by means of the developer liquid. The development station 22 consists of a curved plate 4, which is adapted with its curvature to the peripheral surface of the drum 1 and a trough 5, which is filled with developer liquid. The plate 4 serves as a development electrode and is supplied with a certain voltage by a voltage source, not shown. Instead of the curved plate 4, a roller can also be provided. In organic photoconductor layers, the toner particles dispersed in the developer liquid are positively charged, while in selenium layers they are negatively charged. Excess developer liquid is largely removed by the removal device, which consists of the roller 6 with a scraper 7.

At the transfer station 16, image-receiving material, for example a paper sheet 8, is fed to the drum 1 from a container 25. The transfer station 16 contains a charging device 9, for example a corona, which charges the paper sheet 8 electrostatically from the rear. In the case of a selenium photoconductor 21, the paper sheet 8 is charged positively. Instead of the charging device 9, a pressure roller can also be provided, which rests on the peripheral surface of the drum 1 and is connected to a voltage source which applies the pressure roller to a suitable potential for transmission charges. After the transfer of the toner image from the photoconductor 21 to the paper sheet 8, the latter is detached from the peripheral surface of the drum 1 and passed over a heating device 10 which dries the still moist toner image.

The cleaning device comprises a roller 11, for example a foam roller, and a wiper lip 12 which is arranged in the immediate vicinity of the roller 11. The roller 11 is wetted with developer liquid and, together with the wiper lip 12, cleans the photoconductor surface of residual toner.

The charging device 13 removes all residual charges of the photoconductor 21, which is completely discharged.

In the known copying machines, such copying conditions prevail that the DC corona 2 is supplied with an operating voltage of +6.3 kV when the photoconductor 21 is selenium. In accordance with the amount of light supplied in the exposure device 3, the approximately 50 μm thick selenium photoconductor layer charged to approximately +1150 V is discharged and toner particles are deposited in accordance with the residual charge then present on the photoconductor layer in order to develop the latent charge image into a toner image.

The charging device 14 consists of the DC corona 2, which is connected to a high-voltage circuit 15, which is designed to carry out the method according to the invention for a continuous operating voltage of 8 kV of the DC corona 2.

The metering roller consists of aluminum, which is anodized on the surface and rotates at a distance of 50 µm in the opposite direction to drum 1 at three times the peripheral speed. The metering roller 6 carries, in a manner not shown, at the ends of its projecting stub shafts, discharge rollers made of insulating material with built-in roller bearings which are rotatably mounted in shields. By selecting the diameters of these discharge rollers, the desired column S between the metering roller 6 and the photoconductor surface is predetermined. The drain rollers are in fixed contact with the photoconductor surface on the drum l. A voltage source 17 is connected to the metering roller 6.

In Fig. 2, the amounts of developer liquid discharged per A4 copy are plotted as a function of the size of the gap between the metering roller 6 and the photoconductor surface, the parameter of these curves being the voltage applied to the metering roller 6. If Isopar M is used as developer liquid with Infotec ^(R) toner particles dispersed therein, the curve profiles shown in FIG. 2 are obtained. Isopar M is a liquid isoparaffinic hydrocarbon that boils at 223 ° C. To determine the discharge of Isopar M from the copies, the fusing station of the copying machine is switched off and a larger number of copy sheets are weighed before and after they have passed through the copying machine. Different voltages between 0 and +2 kV are applied in succession to the metering roller 6. As can be seen, the amount of developer liquid discharged increases linearly with the width of the gap S when the metering roller 6 is without tension. Almost the same situation arises at a voltage of +1 kV on the metering roller 6. These two curves in FIG. 2 show that there are no significant effects for reducing the discharge of developer liquid at voltages below +1 kV on the metering roller 6. Only at voltages greater than +1 kV, for example, +1.5 kV results in a considerable reduction of the discharge of developer liquid for widths of the gap S between 50 _/ um and 130 microns. The discharge is in the range of the gap of 50 to up to 110 _/ um about reduced by half and even up to a width of the gap S of 130 / um there is still a reduction of 42% compared to an operation with a metering 6, to which no voltage is applied.

Particularly favorable conditions result when the metering roller 6 is operated with a voltage of +2 kV, since the discharge of developer liquid is then reduced to gap widths of 150 μm to 1/5 and less of the value which occurs at a voltage loose metering roller 6 results. Even with a gap of 200 _/ um results in even a reduction of the discharge by about 30% compared to an operation with voltage-free metering. 6

Similar results are obtained with an organic photoconductor if negative voltages are applied to the metering roller 6, which is necessary since the copier in this case is operated with positively charged toner.

The influence of the speed of the counter-rotation of the metering roller 6 with respect to the photoconductor on the discharge of developer liquid is reduced by the high voltage on the metering roller 6. For example, the discharge becomes only 20 to 30% smaller if the metering roller 6 rotates at 1.5 times the peripheral speed instead of at 3 to 3 1/2 times the peripheral speed in the opposite direction to the photoconductor. Even with the circumferential speed of the metering roller 6 and the photoconductor aligned, the reduction in the discharge of developer liquid when the high voltage is applied is considerable if only it is ensured that a developer, such as the wiper blade 7 in FIG. 1, removes the developer liquid drawn from the metering roller 6 by the latter Surface is removed.

When developing with liquid toner from developer liquid and negatively charged ones dispersed therein Toner particles are obtained when positive voltage is applied to the metering roller 6, background-free copies with discharge values, as indicated for different voltages and gap widths in FIG. 2. While the copies are flat when the voltage is applied at +1.5 kV and are still legible, the contrast of the copies is no longer sufficient when the voltage is applied at +2 kV. In the event of negative voltages on the metering roller 6, black copies are obtained over the entire area under the specified conditions with a positively charged selenium photoconductor and the developer liquid discharge is also reduced. It follows that the amount of developer liquid discharged is largely independent of the polarity of the voltage applied to the metering roller 6, but not the copy quality or the toner density of the copies.

Correspondingly, in the case of photoconductors made of organic materials, such as poly-N-vinylcarbazole and trinitrofluorenone, which are to be negatively charged, negative voltages are to be applied to the metering roller 6 in order to obtain black copies free of charge over the entire area with little discharge of developer liquid.

To improve the image contrast, measures are suitable which enable fast, stable and dense deposition of toner particles on the charged areas of the photoconductor. For this purpose, it can be provided that one instead of a flat electrode 4 To provide application roller for the toner, which is at a distance of about 50 microns from the photoconductor and this constantly supplies fresh developer liquid, so that no premature depletion due to already deposited toner particles occurs in the developer liquid layer in direct contact with the photoconductor. The strong electric field, as a result of the short distance of only 50 µm, favors a quick separation of the toner particles. This results in easy-to-read, groundless copies with a density of 0.65. The toner density is determined by the logarithm of the ratio of the amount of light reflected on the copy and the amount of light reflected from the developed toner image. There is a voltage of +1.5 kV on the metering roller at 50 µm from the selenium photoconductor. The discharge of developer liquid through the copies is about 0.065 g per A4 copy. If the metering roller is earthed, ie there is no voltage on it, the discharge is approx. 0.16 g per A4 copy.

The stability of the toner images deposited on the photoconductor can be further increased by an additional development electrode, not shown, between the application point of the developer liquid and the metering roller. An effective measure for _increasing the stability of the toner images on the photoconductor is to charge the photoconductor _layer up to and beyond the charging _voltage U _maxD Voltage specifies the copying conditions under which copies of maximum toner density are obtained according to the prior art. In the prior art, the maximum charging voltage for a photoconductor layer made of 50 μm selenium is +1150 V. If the corona voltage for charging is increased from +6.3 kV to +8 kV, the photoconductor layer made of selenium is charged to approximately +1800 V. The deposited toner images are not removed even by a metering roller with a potential of +2 kV. Under these conditions, the voltage of the transmission corona in the transmission station only has to be increased from +6.3 kV to +7.5 kV. If a transfer roller is used instead of a transfer corona, its potential must be increased accordingly. The copies made under such copying conditions with a gap of 50 µm have optical densities between 0.9 and 1.1. The copies themselves are dry because the Isopar M content is only about 0.015 g per A4 copy.

Without voltage applied to the metering roller 6, 0.16 g per A4 copy of developer liquid are discharged.

With a metering gap of 110 μm, 0.035 g per _D IN A4 copy of developer liquid is discharged if there is a potential of +2 kV on the metering roller 6 and 0.350 g per DIN A4 copy if the metering roller 6 is operated without voltage.

Similarly specific discharge values for the developer liquid are obtained on DIN Al copies if the photoconductor drum has a length of 80 to 105 cm.

The reductions in discharged developer liquid that can be achieved with pure developer liquids are somewhat higher under otherwise identical copying conditions than with liquid developers with toner particles. This can be due to the different conductivities, since pure developer liquids have specific conductivities between 10 ^-13 ohm ^-1 cm ^-1 to 10 ^-15 ohm ^-1 cm ^-1 , whereas developer liquids with dispersed toner particles have specific conductivities in the range of 10- ¹¹ ohm ^-1 cm ^-1 possess conductivity in the range of possess.

The output of developer fluid of 0.015 g per A4 copy with a metering gap of 50 µm is remarkably low. The template for such a copy has a coverage of 7% with black characters or symbols. In the case of a completely white original, the discharge of developer liquid under otherwise identical conditions is only 0.003 g to 0.004 g per A4 copy.

A discharge of developer fluid of approximately 0.01 g per A4 copy for a 7% coverage original appears to be the lower amount required to remove the toner deposited on the photoconductor to impart the paste-like consistency required for the transfer to the image-receiving material.

Claims (8)

l. The electrophotographic copying process for removing _E ntwicklerflüssigkeit from a photoconductor surface which is electrostatically charged, information-wise exposed, developed, the resulting latent image with the developer liquid to a visible toner image and the excess developer liquid is removed by a at a small distance arranged on the photoconductor surface metering element to which a Bias of the same polarity as that of the charged photoconductor surface is applied, the toner image is transferred from the photoconductor to an image receiving material, fixed thereon and the photoconductor is cleaned and / or discharged, characterized in that the bias of the dosing element is greater than 1 kV and the maximum voltage on the photoconductor surface is chosen to be equal to or less than the bias of the dosing element. 2. Electrophotographic copying method according to claim 1, characterized in that the difference between the maximum voltage on the photoconductor surface and the bias of the metering element is up to 200 volts. 3. Electrophotographic copying method according to claims 1 and 2, characterized in that the bias of the metering element is in the range of 1.5 to 2.0 kV. 4. Electrophotographic copying method according to claim 1, characterized in that the metering element is designed as a metering roller and is arranged at a smaller distance from the photoconductor surface than the developer liquid layer thickness on the photoconductor surface. 5. Electrophotographic copying method according to claim 1, characterized in that the photoconductor surface is charged to a voltage higher than the charging voltage U _maxD for maximum toner density of the copies. 6. Electrophotographic copying method according to claim 4, characterized in that a metering gap S of 50 to 130 _/ um between the photoconductor surface made of selenium and the metering roller and a bias of the metering roller of +1.5 kV are provided. 7. Electrophotographic copying method according to claim 4, characterized in that a metering gap S of 50 to 200 _/ um between the photoconductor surface of selenium and the metering roller with a bias of the metering roller of +2 kV. 8. Electrophotographic copying method according to one or more of claims 1 to 7, characterized in that the developer liquid is removed along a metering gap S up to 105 cm in length.

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