EP0308429B1 - Co- and counter-rotating development system for an electro-photographic installation - Google Patents

Abstract

Development system comprising several development cylinders (16, 17, 18) rotating in the same direction as the charge image carrier and a counter-rotating development cylinder (19). The mixture (11) is dispensed from a store (12) and by means of a bucket wheel (14) to the first co-rotating cylinder (16), and from there is conveyed to the counter-rotating cylinder (19), passing via other co-rotating cylinders (17, 18). Outside said cylinder (19) is provided a light-exposure system consisting of a transparent protection tube (26) containing an LED strip (25), and connected to this system is a collector cylinder (28).

Description

The invention relates to a developer station according to the preamble of patent claim 1.

In copier technology and in non-mechanical high-speed data printers that work on the principle of electrophotography, charge images are stored on a charge image carrier, e.g. generated by a photoconductor drum and then colored with a colored powder (toner) in a developer station. When using a photoconductor drum, the toner images are then transferred to plain paper and fixed there.

Typically, a two-component developer consisting of ferromagnetic carrier particles and colored toner particles is used for development. The developer mixture is guided past the charge image on the charge image carrier, for example by means of a magnetic brush arrangement, to which the toner particles - caused by electrostatic forces - adhere. The magnetic brush arrangement consists of a rotatable hollow cylinder, in the interior of which several rows of stationary permanent magnets are arranged. A plurality of magnetic brush arrangements can be provided in a developer station. For example, a magnetic brush arrangement can be used to transport the developer mixture past the charge image carrier. This magnetic brush arrangement is referred to below as the developer roller. Another magnetic brush arrangement can be used to transport the developer mixture from inside the developer station to the developer roller. Such a magnetic brush arrangement or any other arrangement which effects such a developer mixture transport is referred to below as the transport roller. Developer stations, in which are used with the aid of the magnetic brush principle developer mixture for coloring the charge images on the charge image carrier are known from DE-PS 31 19 010.

If developer rollers are operated in such a way that the developer mixture is applied in the direction of movement of the charge image carrier, one speaks of synchronous developer rollers. If the developer mixture is applied with the aid of the developer rollers against the direction of movement of the charge image carrier, such developer rollers are referred to as counter-rotating developer rollers.

Developer stations that hold both counter-rotating and synchronous developer rollers are e.g. known from US-A 39 12 388 and US-A-38 81 446. A metering device, which consists of an adjustable blade and which serves to impress a defined height or thickness on the mixed carpet on the developer rollers, is additionally assigned to these developer rollers. The developer mixture is offered from the storage room to both rollers at the same time.

Developer stations with synchronous developer rollers, e.g. known from GB-A-1 524 543 have good printing results with line-like patterns, e.g. Characters. With solid areas such as occur with graphical representations or with black bars as e.g. for printing bar codes, the blackening is unsatisfactory.

Numerous attempts have been made to be able to achieve the full-surface coloring satisfactorily.

In this way, better full-surface coloring can be achieved without changing the developer station if the toner and / or the carrier material in the developer mixture are changed. So far, however, there has also been a significant reduction in the life of the mixture in all of these cases, which is a major disadvantage because of the higher consumable and maintenance costs involved.

Another possibility to improve the full-surface coloring is to switch from the reverse development process, in which the unloading areas of the charge image carrier are colored, to the direct development process, which is customary in copying technology, in which the charged areas are colored. With direct development, however, the majority of the surface of the charge image carrier usually has to be exposed. This includes the use of certain character generator principles such as LED lines or the use of laser diodes largely due to the associated thermal problems. However, it is precisely these two character generator principles that come into consideration when high dot screen density at high printing process speed is required.

Further attempts to improve the full-surface coloring have shown that, given the boundary conditions (composition of the toner mixture, speed), it is not possible to achieve a homogeneous surface coloring with a single developer roller. Rather, wash-out effects are observed on the rear edge of the solid surface in the case of synchronous development or on the front edge of the solid surface in the case of reverse rotation development, which occur in critical image patterns, e.g. Inverse printing is particularly disruptive.

A further problem with such developer stations is disturbances in the printed image, in particular light spots which result from the fact that the carrier particles contained in the developer mixture come from the photoconductor layer of the photoconductor drum are carried away to the transfer station and impair the transfer process. The usual magnetic carrier-pickup roller alone is not able to reliably avoid this disturbance.

The object of the invention is to design a developer station of the type mentioned at the outset so that high print quality at high process speed is possible using a developer mixture with a long mixture life.

This object is achieved in a developer station of the type mentioned at the outset in that the developer station is designed as a synchronous-counter-rotating developer station, in which the developer mixture of the photoconductor drum is initially offered via a plurality of developer rollers moving in the same direction, the last developer roller then being designed as a developer roller moving in opposite directions is. The peripheral speed of the synchronous developer rollers is significantly higher than the peripheral speed of the charge image carrier. With this measure, an intensive and uniform coloring of the full areas at high printing speed of e.g. 0.4 m / sec and more reached.

In an advantageous embodiment of the invention, an illumination device for the charge image carrier is provided downstream of the developer rollers in the direction of movement of the charge image carrier. This lighting device can be surrounded by a transparent rotating protective roller which prevents the LED strip or luminescent film serving as exposure elements from becoming dirty.

After the actual development, the charge image is compensated for by this lighting device by illuminating the highly charged and non-stressed areas of the charge image carrier and thereby discharging them. On the one hand, the liability of (negative) carrier particles to the Character-dependent, non-exposed photoconductor areas are reduced so that they can be pulled off the charge image carrier by a subsequent carrier-catching roller and returned to the developer station. The memory device can also be used to prevent the memory effects on the charge image carrier that are interfering with the print image due to charge images that have not been completely deleted.

The distance between the lighting device and the counter-rotating developer roller is adjusted so that the surface of the protective tube made of plexiglass is continuously cleaned of the developer mixture and the light emission is only slightly attenuated by adhering toner dust.

In a further advantageous embodiment of the invention, a suction device is provided between the plexiglass protective tube of the lighting device and the carrier capture roller, which sucks up the free toner mixture - which is not bound to the charge image carrier - so that it cannot be torn up out of the developer station.

The transfer of the developer mix carpet from the last synchronizing roller to the counter-rotating roller offers the additional advantage that non-magnetic particles, e.g. Larger lumps of toner or paper particles, which have accumulated in the developer mixture after a long period of operation, are thrown out of the mixture into the storage chamber after being transferred to the counterflow roller. This significantly reduces the risk of such parts coming into contact with the charge image carrier via the last developer roller and being entrained by the latter due to the electrostatic forces, which can lead to faults in the printed image.

The fact that the last developer roller with the opposite direction of movement with respect to the charge image carrier is able to reduce disturbances also has such disruptive particles which have previously been on the surface of the charge image carrier remained stuck, wiped off and transported with the developer mixture down into the storage area. In a further advantageous embodiment of the invention, the first and the second synchronous roller are arranged in such a way that the developer mixture, after passing through the charge image carrier, falls back onto the transport roller, which can be designed, for example, as a paddle wheel, and is fed from there to the second synchronous roller. This means that the already somewhat depleted developer mixture can be returned from the first developer roller to the storage chamber and well-mixed developer mixture can be transferred to the second developer roller.

• FIG 1 eine schematische Schnittdarstellung der erfindungsgemäßen Entwicklerstation und
• FIG 2 eine schematische Darstellung einer Ausführungsform der Entwicklerstation mit drei Entwicklerwalzen.

Embodiments of the invention are shown in the drawings and are described in more detail below, for example. Show it

• 1 shows a schematic sectional view of the developer station according to the invention and
• 2 shows a schematic representation of an embodiment of the developer station with three developer rollers.

A photoconductor drum 10 is arranged as a charge image carrier in a printing device, which is not shown in detail here and operates on the electrophotographic principle. A charge image is applied to this photoconductor drum in a known manner by means of an exposure device controlled by a character, and this charge image is then colored using the developer station shown. The coloring takes place according to the reverse development principle in which the areas discharged by exposure are colored with the aid of a developer mixture 11 containing toner particles and carrier particles. After passing through the developer station, the charge images consisting of colored toner particles are transferred to paper in the usual way.

The developer station essentially consists of a storage chamber 12, which is fed through a filling opening 13 with a foam roller arranged therein as a metering device, developer mixture 11. At the bottom of the storage chamber 12 there is an electromotive driven transport roller in the form of a paddle wheel roller 14 which has spoke-like blades 15 for transporting the developer mixture 11. The storage chamber 12 is closed off from the photoconductor drum 10 by four developer rollers 16, 17, 18 and 19. These developer rollers, which are arranged along the circumference of the photoconductor drum, are located at a distance of approximately 1 to 2.5 mm from the surface of the photoconductor drum and work according to the magnetic brush principle. They essentially consist of hollow cylinders constantly driven by electromotive devices, e.g. made of aluminum with a knurled surface and magnet arrangements 21 arranged therein. The hollow cylinders 20 are charged with a bias voltage which is approximately the size of 20-50% of the charging potential on the photoconductor drum. When using a selenium photoconductor drum with a charging potential of 400 - 1000 V, the bias voltage is 100 - 500 V.

Depending on the direction of movement of their hollow cylinders, the developer rollers 16, 17 and 18 are designed as so-called synchronous developer rollers. In these synchronous developer rollers, the direction of movement of their hollow cylinders corresponds to the direction of movement of the surface of the photoconductor drum 10 in the region of the development gap 22 formed by the hollow cylinders 20 and the surface of the photoconductor drum 10. The developer roller 19 is designed as a counter-rotating developer roller, in which the hollow cylinder 20 moved in the development gap 22 against the photoconductor drum 10.

The developer mixture 11 is transported in accordance with the arrows shown in FIG. 1 in such a way that from a mixture sump bottom of the storage chamber 12 via the paddle wheel roller 14 the developer mixture 11 of the first synchronous developer roller 16 is offered. A metering knife 23 determines the height of the developer mixture carpet on the first synchronizing roller 16 and thus also on the following synchronizing rollers 17 and 18. After the developer mixture at a significantly higher speed than the photoconductor surface (approx. 1.5 times the process speed), that on the photoconductor surface has contained charge image developed three times, with the help of the synchronous developer roller 16, 17 and 18, the developer mixture changes from the third synchronous roller 18 to the underside of the much slower and in the opposite direction driven fourth counter-rotating developer roller 19. Here, a large part of the developer mixture is wiped off by a further metering doctor 24, the residual developer mixture transported to the surface of the photoconductor drum 10 now develops the charge image one last time in the opposite direction. The distances between the developer rollers are advantageously less than 2.5 mm, the development gap 22 having a width of 1 to 2.5 mm. Through this development gap 22, the developer mixture must be conveyed with the highest possible density. The density of the developer mixture must be chosen so that on the one hand the latent charge image is well colored and on the other hand the surface of the charge image carrier is not damaged by excessive squeezing.

In order to be able to transport the developer mixture on the one hand with the aid of the developer rollers, but on the other hand to enable the toner particles to accumulate on the charge image, the surface of the developer rollers is, as already described, subjected to a bias voltage of approximately 20-50% of the charge potential.

An illumination device in the form of a light-emitting diode strip or foil 25 extending along the photoconductor drum 10 is located above the last developer roller 19 designed as a counter-rotating developer roller 19 a protective roller 26 is housed from a transparent rotating plexiglass tube. Rotation and distance to the counter-rotating developer roller 19 are adjusted so that the surface of the plexiglass tube 26 is continuously cleaned by the developer mixture 11 and the light exit region 27 of the light-emitting diode strip 25 is only slightly damped by adhering toner dust. The LED strip generates a spectral light that approximately corresponds to the light of the character generator (eg LED comb).

Furthermore, above the lighting device there is a carrier catch roller 28 which operates in accordance with the developer brushes according to the magnetic brush principle and which, in cooperation with the lighting device, lift the carrier particles of the developer mixture from the surface of the photoconductor drum and feeds them again to the developer mixture 11 via a correspondingly designed guide channel.

The lighting device compensates for the charge pattern via the light-emitting diode strip 25 by illuminating the highly charged (approx. 400-1000 V) and non-stressed areas of the surface of the photoconductor drum 10 and thereby discharging to a residual voltage of less than 50 V, thus the discharge voltage of the Character generator corresponds. This reduces the adhesion of negative carrier particles of the developer mixture to the unexposed areas of the surface of the photoconductor drum 10, so that they can be pulled off the surface of the photoconductor drum 10 by the subsequent carrier catch roller 28 and returned to the developer station. At the same time, the lighting device can prevent memory effects due to charge images on the photoconductor drum surface that have not been completely deleted.

The lighting device generates a largely homogeneous charge image on the charge image carrier in front of the carrier catching device and the subsequent transfer printing station in the printer.

The charge image carrier thus has a uniform residual charging voltage of approximately 50 V. This "imagewise" discharge not only facilitates the removal of the carrier particles from the photoconductor drum, but also promotes the transfer printing of the toner image onto the paper web in the transfer printing station. In order to ensure the same depth of penetration of the light into the surface of the charge image carrier as the character-dependent controlled light generating the charge image, the light of the illumination device has approximately the same spectral structure as the light of the character generator. E.g. If a light-emitting diode comb is used as the character generator, a similar structured lighting device is recommended. Instead of an LED line, a luminescent film can also be used.

The lighting device is surrounded by a suction device, which acts on the area between the carrier catching roller 28 and the protective roller 26 via a suction channel 29 extending along the lighting device. This suction channel 29 is connected via a suction manifold 30 to a suction fan, not shown here. This air suction between the plexiglass tube (protective roller) 26 with the lighting device and the carrier capture roller 28 generates a local vacuum and thus free toner dust, which is not bound by the charge image, is suctioned off and collected in a container. This means that the free toner dust cannot be torn up out of the developer station by the photoconductor drum 10. Carrier particles caught by the carrier catching roller 28 and developer mixture stripped off by the metering doctor 24 of the counter-rotating developer roller are returned to the storage chamber 12 via guide plates 31.

At the bottom of the storage chamber 12 there is an emptying opening 32 through which used developer mixture is sucked off after a certain operating time.

In the embodiment of the developer station shown in FIG. 2, three developer rollers 33, 34 and 35 are arranged. The developer rollers 33 and 34 are synchronous developer rollers, and the developer roller 35 is a counter-rotating developer roller. In this exemplary embodiment, the developer mixture is initially offered via the synchronous developer roller 33 of the photoconductor drum 10. Then the developer mixture is returned to the paddle wheel roller 14. The developer mixture, which is already somewhat depleted, can be enriched again with new developer mixture, and this newly enriched developer mixture is then offered again to the surface of the photoconductor drum via the developer roller 34. In this case, it is also necessary to assign a metering knife 36 to the developer roller 34. The function of the counter-rotating developer roller 35 arranged below corresponds to that of the counter-rotating developer roller 19.

Claims (9)

1. Developer station for an electrophotographic device containing a charge image carrier (10) with means for developing charge images applied to the charge image carrier (10) with the aid of a developer mix (11), in which the developer mix (11) is taken from a reservoir (12) in a developer station by a transport drum (14) and is transported to mutually counter-rotating developer drums (16 to 19) which ink the charge image carrier (10) and which, with the charge image carrier, form a development gap (22), characterised in that the developer mix (11) is first transported to a co-rotating developer drum (16) moving in the same direction as the charge image carrier in the development gap (22), in that further developer drums (17, 18) moving in the same direction as the charge image carrier are arranged, and in that the developer mix (11) is then forwarded to a counter-rotating developer drum (19) which follows in the direction of movement of the charge image carrier (10) and moves in the opposite direction of the charge image carrier (10) in the development gap (22) and from there is returned into the reservoir (12) again, the co-rotating developer drums (16, 17, 18) having a circumferential speed that is noticeably higher than the circumferential speed of the charge image carrier (10).

2. Developer station according to Claim 1, characterised in that the first co-rotating developer drum (16), arranged in the direction of movement of the charge image carrier, and the second co-rotating developer drum (17) are arranged in such a way that the developer mix (11) is supplied to the transport drum (14) again after passing the development gap (22), and is transported from there to the second co-rotating drum (17).

3. Developer station according to one of Claims 1 or 2, characterised in that the transport drum (14) is designed as a paddle wheel drum having spokes comprising individual paddles (15).

4. Developer station according to one of Claims 1 to 3, characterised in that an illumination device (25, 26) for the charge image carrier (10) is provided after the developer drums in the direction of movement of the charge image carrier (10).

5. Developer station according to Claims 4, characterised in that the illumination device is provided with a transparent, rotating protective drum (26).

6. Developer station according to Claim 5, characterised in that the protective drum (26) is arranged at a close distance from the counter-rotating developer drum (19) so that the counter-rotating developer drum (19) cleans adhering developer mix (11) off the protective drum (26).

7. Developer station according to one of Claims 1 to 6, characterised in that a carrier stripper drum (28) which picks up the carrier particles of the developer mix is arranged after the developer drums (16 to 19) in the direction of movement of the charge image carrier (10).

8. Developer station according to one of Claims 1 to 7, characterised in that a suction device (29, 30) which draws off the parts of the developer mix that are not bonded is provided after the developer drums in the direction of movement of the charge carrier.

9. Developer station according to Claim 8, characterised in that the suction device is arranged between the illumination device and the carrier stripper drum (28).

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