Colour reproduction machine with development units having unequal and/or adjustable width development nip

Abstract: A printing machine (Fig.4) of the type in which a latent image recorded on an image bearing surface (12) is developed with toner to form a visible image thereof, includes a development apparatus (52) for developing the latent image. The development apparatus (52) includes a housing (100) storing a supply of magnetically-attractable developer material containing toner. A moving flexible donor belt (112) mounted within the housing (100) spaced from the image bearing surface (12) transports developer to a development zone (136) adjacent the image bearing surface. A magnetic member (124) attracts and transports the developer material within the housing to the donor belt (112). A plurality of selectively-actuable electrodes (146) embedded within the donor belt are biased to create a powder-cloud of toner particles within the development zone (136). Within the development zone, the flexible donor belt (112) moves over a pair of spaced apart support rollers (148,150) defining the width of the development nip. The spacing between the support rollers is adjustable to inexpensively create different widths for development nips from a common structure development unit, thereby saving image bearing member real estate within the development zone and improving the quality of image development.

Description: This invention relates generally to electrostatographic reproduction machines, and more particularly concerns a scavengeless development apparatus having a belt type donor member and an adjustable width development nip. The process of electrostatographic reproduction is well known in the art. In electrostatographic reproduction machines for making copies of highlight or full-color images, latent images of color components thereof are formed on a photoreceptor, and developed with different color toner particles. The color component images of different color toners may be formed as such, for example, in successive and superimposed image-on-image registration on the photoreceptor, thus forming the desired composite colored image on the photoreceptor prior to transferring to a receiver sheet. Alternatively, the color component images of different color toners may be formed as four separate toner images and then transferred successively in superimposed registration onto a receiver sheet. A significant problem encountered particularly with performing successive image-on-image development is that a subsequent development may interfere with, or "scavenge," toner particles which had been attracted to the photoreceptor in a previous developing step. A hybrid version of a "scavengeless" development apparatus employs a magnetic brush developer roller (magnetic roller) for transporting from a sump to the donor roll, magnetic carrier beads which have charged toner particles adhering triboelectrically thereto. The charged toner particles are attracted from the carrier beads on the magnetic roller to the donor roll. In the development zone, the electrically biased electrode wires then detach the charged toner particles from the donor roll, thereby forming a toner powder cloud in the development zone for developing latent images as above. A key variation to the powder-cloud-creation techniques which are the essence of scavengeless development is to provide electrodes, not spaced from the donor roll, but rather embedded within the donor roll. US-A-5,172,170 discloses a scavengeless development apparatus each, in which a set of longitudinally-disposed electrodes are mounted on or embedded in a rotating donor roll. A contact brush is used as a commutator to energize those electrodes in the development zone of the development apparatus. When the electrodes are energized, AC electric fields are formed between adjacent electrodes. The electric fields then cause charged toner particles near the electrodes to jump off the donor roll, thus forming the powder cloud for latent image development within the development zone. US-A-3,257,224 discloses an apparatus for developing electrostatic images in which a developer roller transports both toner and a magnetic carrier. The roller is made up of rotor plates having windings to which current is supplied intermittently, and an outer cover of an insulating plastic material. The purpose of the electromagnetic windings with the roller is to attract developer material from a sump to surface of the roller. The electromagnetism is cut off only to clean the roller and recycle the developer, after the given portion of the surface exits the development zone. In roller-type scavengeless development apparatus as above, one disadvantage encountered is that the width of the development nip is limited to the footprint of the roller against the image bearing surface. The distance of the developer material sump from the development zone is also limited by the diameter of the rolls, and the biasing schemes are stacked and complicated in order to avoid electrical field interference, for example, within the development zone. US-A-5157443 a development system using a liquid developer in which, instead of using a roller to attract the toner particles an endless belt is used. Another significant disadvantage encountered in multicolour machines is non-uniform development when performing multiple, successive development steps with toner particles of different colours, for example, cyan, magenta, yellow and black toner particles, each of which may have different development characteristics. One result of the different characteristic of yellow toners is that the human eye is less sensitive to yellow toners. As such, high quality development using yellow toners ordinarily requires less yellow toner mass per developed area than do other colour toners. In addition, the sizes of toner particles, as well as the charge-to-mass ratios for toner particles of each colour may also be significantly different. If not compensated for, such differences can result in non-uniform image development. Therefore, in addition to addressing the "scavenging" disadvantages as above with electroded development, there is also a need for addressing poor, non-uniform development associated with differences among tone, and toner development characteristics. In accordance with a first aspect of this invention a colour reproduction machine including: an image bearing member; means for electrostatically recording a first and at least a second latent images on said image bearing member; and, a plurality of development units each storing developer material containing a different type of toner for developing said latent images on the image bearing member, said plurality of development units including: a first development unit containing a first type of toner for developing said first latent image, said first development unit having a movable first belt donor member forming a first development nip with the image bearing member; and, a second development unit containing second and different type of toner for developing said at least second latent images, said second development unit having a second belt donor member forming a second development nip with the image bearing member; is characterised in that said second width of said second development nip is unequal to said first width of said first development nip. Preferably, the second width of said second development nip is greater than said first width of said development nip. The development system preferably includes a first set of back up rollers defining a first width of said first development nip and a second set of back up rollers defining a second width for said second development nip. The development system preferably includes third and fourth development units each having a belt type donor member forming and additional development nip with the image bearing member, each said additional development nip having a width thereto substantially equal to said second width of said second development nip. Preferably, the toner particles of first colour in said first development unit comprise yellow toner particles. Preferably, the developer material stored in said first development unit includes toner particles each having an average size substantially greater than that of toner particles included in said developer material stored in said second development unit. In accordance with a second aspect of this invention a development unit for developing a latent image recorded on an image bearing member, the development unit comprising: a housing storing a supply of magnetizable two-component developer material containing toner particles; means mounted within said housing for moving developer material thereabout; and a toner transport assembly including: a movable belt donor member forming a development nip with the image bearing member and transporting toner particles into and through said development nip for latent image development; a contoured back supporting plate located across said development nip for spacing said belt donor member from the image bearing member; and, first and second back up rollers mounted for training a portion of said belt donor member across a width of said development nip; is characterised in that said second back up roller is movable relative to said first back up roller to vary said width of said development nip. Preferably, the toner transport assembly includes at least a third back up roller mounted within said housing for training said belt donor member through a toner-transfer relation with said developer material moving means. Preferably, the toner transport assembly further includes a fourth back up roller mounted spaced from said at least third back up roller for training said belt type donor member through said toner-transfer relation with developer material moving means. Preferably, the at least third back up roller is mounted spaced adjustably from a line connecting said first and said second back up rollers. Preferably, the developer material moving means within said housing includes first and second magnetic rollers biased appropriately for loading and for removing respectively, charged toner particles relative to said belt donor member. Preferably, the first and second belt donor members each include embedded biasable conductive strips, and a non-conductive backing formed underneath said conductive strips for directing substantially all development biasing electric field lines from said biasable conductive strips into the development nip thereof. Preferably, the second width of said second development nip is greater than said first width of said first development nip. Preferably, the first type of toner in said first development unit comprises yellow toner. Preferably, the first type of toner in said first development unit comprises dry toner particles having an average particle size greater than that of dry toner particles of said second type of toner. Other features of the present invention will become apparent as the following description precedes and upon reference to the drawings, in which: Figure 1 is a schematic elevational view (partly in section) showing the development unit of the present invention; Figure 2A is an in-track sectional view of a segment of the electroded donor belt showing a commutator brush in the development nip region of the development unit of FIG. 1; Figure 2B is a cross-track sectional view of the segment of FIG.2A; Figure 3 is a schematic elevational view of a part of the development system of a color electrostatographic reproduction machine, showing two development units according to the present invention having two adjusted, different and unequal development nip widths; and Figure 4 is a schematic elevational view of an exemplary color electrostatographic reproduction machine incorporating a plurality of development units according to the present invention. Since electrostatographic reproduction machines are well known in the art, a detailed description thereof has been omitted from the present disclosure. An exemplary electrostatographic reproduction machine in which the present invention may be used is described in detail (with reference to Fig.4) in U.S.- A - 5491538. Although the development units of the present invention are described with reference to toner particles, and hence dry developer materials, it is to be understood that the development apparatus concepts of the present invention are equally applicable to development units employing liquid developer materials, and hence liquid toner. As such, reference in the description to toner particles equally applies to liquid toners, and references in the claims to toner, include both dry and liquid toners. Referring briefly to Fig.4, as can be appreciated by those skilled in the art, the four color component toner images may be formed image-on-image in superimposed registration with each other on a single imaging frame or portion of the surface 14 (FIG. 3). The particular electrostatographic process for such image-on-image formation, of course, requires an IPS such as 42, and an upstream RIS and ROS assembly such as 180A, 180B (FIG. 3) for each development unit. Referring still to FIG. 4, each of the development units 52, 54, 56 and 58 is mounted so as to be movable into and out of an operative development position with a latent image on the surface 14 of the image bearing member 12. In a non-operative position, each unit is spaced from the photoconductive surface 14 of the image bearing member. During development of each electrostatic latent image, only one development unit is in the operative position, the remaining development units are in the non-operative position. This insures that each electrostatic latent image is developed only with toner particles of the appropriate color, without co-mingling. In Figure 4, development unit 52 for example is shown in the operative position with development units 54, 56 and 58 being in the non-operative position. Alternatively too, all four units may each have an operative mode as well as an inoperative mode depending on whether or not a development bias source (to be described below) is turned on or off. In this alternative, all four stations instead of being moved towards and away, may actually be spaced the same distance, from the image bearing surface 14. Referring now to FIG. 1, an improved scavengeless non-interactive development unit, such as unit 52 (FIGS. 4), according to the present invention is illustrated. Since the features of the development units 52, 54, 56 and 58 (FIG. 4) and 52A, 54B (FIG. 3) of the present invention are the same for each unit, only one such unit, 52, will be described in detail. As shown, development unit 52 includes a housing 100 defining a mixing chamber 102 for mixing and charging a supply of magnetizable two-component developer material. The developer material typically is a two-component developer material comprising at least ferrous or magnetizable carrier beads and pigmented polymer toner particles. The developer material is moved and mixed within the mixing chamber 102 by developer material means mounted within the mixing chamber. The developer material moving means for example include mixing devices such as augers 104, 106. Such moving and mixing of the developer material oppositely and triboelectrically charges the carrier beads and toner particles respectively. As a consequence of such charging, the oppositely charged toner particles adhere triboelectrically to the charged magnetizable carrier beads. The development unit 52 also includes a spent toner particles removing or cleaning roll 108 for removing spent toner particles from a toner transport assembly 110 which includes the belt donor member 112 of the present invention (to be described in detail below). Cleaning roll 108, for example, is a magnetic roll that includes a stationary multi-polar magnetic core inside a rotating aluminum shell 116 cleaning roll 108 is mounted within the chamber 102 so as to form a cleaning nip 114 with the belt donor member 112. As shown, the conductive shell 116 cleaning roll 108 is grounded or biased by a DC source 118. In the cleaning nip 114, a portion of the belt donor member 112 is biased appropriately by a source 120, through a commutator device 122. The bias source 120 includes an AC bias 120A and a DC bias 120D, and is such that, relative to the bias source 118 of the shell 116, toner particles on the belt donor member 112 will be attracted onto the shell 116, thereby cleaning a portion of the belt donor member. The development unit 52 further includes a developer material feeder or magnetic roll 124 that is disposed interiorly of the mixing chamber 102 for feeding a quantity of developer material from the chamber to the belt donor member 112. The magnetic roll 124, for example, includes a stationary multi-polar magnetic core inside a rotating aluminum shell 126. The shell 126 also is biased by a DC source 118. As shown, the feeder magnetic roll 124 is mounted in close proximity to the belt donor member 112 and forms a toner-transfer or loading nip 130 therewith. In the loading nip 130, a portion of the belt donor member 112 is biased by a source 132 through a commutator device 134. The bias source 132 includes an DC bias 160 and an AC bias 162, and is such that, relative to the bias source 128 of the shell 126, toner particles on the shell 126 will be attracted onto the surface of the belt donor member 112, thereby loading a portion of the donor member with such charged toner particles. Also, mixing devices, such as the horizontal augers 104, 106, are provided within the mixing chamber 102 for moving and distributing developer material uniformly along the length of magnetic roll 124 for example. In each case, as each shell 115, 126 rotates, developer material is magnetically attracted from the augers 104, 106 onto the outer surface of such shell. The attracted developer material which preferably is trimmed by a trim bar (not shown) to a specified thickness, is carried on the shell and transported around with the shell into contact with a portion of the belt donor member 112 in the toner transfer nips 114, 130, respectively. Referring now to FIGS. 1 to 2B, the toner transport assembly 110 importantly includes the belt donor member 112 of the present invention. As illustrated, belt donor member 112 advantageously forms an adjustable width development nip 136 with the image bearing member 12 and is movable in the direction of the arrow 138, to transport toner particles into and through the development nip 136. Belt donor member 112 (FIGS. 2A and 2B) preferably includes a non-conductive backing 140, and an embedded layer of biasable conductive segments or strips 142. It also includes a dielectric surface layer 144 that is formed over the embedded conductive segments or strips 142. As shown, the conductive strips 142 as embedded, each run widthwise through the belt donor member 112, and additionally are spaced and electrically isolated along a closed loop length of the donor member. In order to allow contact by a commutator device 146 with the biasable strips 142, the dielectric layer 144 extends only a part of the way relative to one of the edges of the belt donor member 112. The non-conductive backing 140 is movable against a plurality of backing support members, such as rollers 148, 150. Additionally, the non-conductive backing 140 is comprised of mylar, and advantageously functions to direct substantially all development biasing electric field lines from the biased conductive strips 142 into the development nip 136, thereby increasing the rate and quality of development. Referring still to FIG. 1, the belt donor member 112 of transport assembly 110 is movable in a direction as shown by the arrow 138 through the cleaning nip 114 where it is cleaned. After that, belt donor member 112 receives charged toner particles fed thereto within toner-transfer nip 130, and then transports such toner particles to and through the development nip 136. The toner transport assembly 110 as shown includes a plurality of backing support members, such as a contoured back plate 152 for spacing a portion of the belt donor member 112 from the image bearing member 12 within the development nip 136. The toner transport assembly 110 also includes first and second back up rollers 148, 150 that are mounted for training a portion of the belt donor member 112 across the development nip 136. According to one advantageous aspect of the present invention, the second back up roller 150 is adjustable relative to the first back up roller 148 in order to vary or adjust the width "Wi" of the development nip 136. As such, the width "Wi" of the development nip 136 formed by the belt donor member 112 can be adjusted for optimum performance depending on the requirements of a host machine. For example, in a machine having a photoconductive belt type image bearing member, the belt donor member 112 can be held substantially parallel to the photoconductive belt within the development nip area, thus resulting in a wider and more uniform development nip 136. Such an extended nip as can be expected, produces better developed images than short development nips. Also, since the belt donor member spacing from the image bearing member 12 is defined by a backer bar or backing support plate 152, run-out of the donor member as occurs with donor rolls is not an issue. Furthermore, the capability according to the present invention to adjust the width "Wi" of the development nip 136 is valuable because it conserves image bearing member or photoreceptor real estate in the development nip area by making the width "Wi" only as wide as is necessary for perceived quality development. Depending on the color or particle size of the toner particles in the developer material being used, for example, some development units may require a wider or narrower width development nip in order to achieve a desired level of perceived quality image development. According to the present invention, given a common structure development unit having the ability for nip width adjustment, development units that require wider width development nips can be provided from such a common structure type unit by increasing the distance between the first and second back support rollers 148, 150. On the other hand, development units that do not require wide development nips can also be provided from the common structure type unit by narrowing the distance or free space between such rollers 148, 150. One of the reasons for including development units having different size nip widths in a reproduction machine, for example, involve DMA or Developed Mass/Area requirements. In color process reproduction machines for example, toner particles of some colors such as yellow toner particles may not have the same DMA requirements in order to achieve perceived uniform quality development as compared to that from toner particles of the other colors. This is because in the case of yellow toners, smaller masses of the yellow toners can be used per unit area, since the human eye is less sensitive to yellow. As such, DMA variations due to a narrow width development nip in a yellow toner development unit would likely not be perceived. Another reason for including development units having different size nip widths in a reproduction machine, involves toner particle size. In a multi-color process machine, the different color toners often do have different particle sizes. Typical toner particle sizes can range from 5 to 15 µm. Such differences can be by design, since smaller size toner particles are more expensive to produce. In such machines, since the ability of a toner to effect development is a function of its particle size, larger particle size toners will not require as wide a development nip as smaller particle size toners in order to provide a satisfactory mass of such toners per developed unit area. Thus, according to the present invention, development units having yellow toner or larger particle size toners can be provided with adjusted narrower width development nips. A final reason for including development units having different size nip widths in a reproduction machine, involves the inherent differences between developer materials in their ability to develop. By design, developer materials may contain different additives for charge control, flow control, pigment color, or other reasons. These differences ordinarily make each developer material behave slightly differently from the others, thus often requiring different development unit characteristics for meeting uniform quality development expectations. According to the present invention, such development unit characteristics include the width of the development nip. Advantageously, space on the photoreceptor saved by adjusting the development nips according to the present invention can desirably result in small and more compact machines, or in machines including more desired components. Referring still to FIG. 1, at least a third back up roller 154, and a fourth back up roller 156 of the toner transport assembly 110 are mounted adjacent the mixing chamber 102 for training the belt donor member 112 through a toner-transfer relation with the developer material moving rolls 108, 124. In order to reduce costs by standardizing a length for the belt donor member 112 in a common structure development unit of the present invention, the at least third and fourth back up rollers 154, 156 respectively are mounted therein spaced adjustably from a line (not shown) connecting the first and second back up rollers 148, 150 thereof. As further shown in FIG. 1, the belt donor member 112 is biased within the toner transfer nip 130 to a specific voltage, by the source 132 that includes a DC power supply 160 for enabling the donor member 112 to attract charged toner particles off of the magnetic roll 124. The bias source 132 also includes an AC voltage source 162 that functions to temporarily loosen the charged toner particles on the magnetic roll 124 from their adhesive and triboelectric bonds to the charged magnetized carrier beads thereon. Loosened as such, the toner particles can then be attracted more easily to the donor member 112 AC voltage source 162 can be applied either to the belt donor member 112 as shown in series with DC source 160, to the magnetic roll 124. Within the development nip 136, a commutator device, such as a brush 134 of carbon or metal fibers, selectively contacts and biases the conductive strips 142 of the belt donor member then moving through the development nip 136. The commutator brush 134 is biased appropriately by a source 163 that includes an AC source 164. AC source 164 serves to loosen charged toner particles from the surface of the belt donor member 112, as well as to form a cloud of such loosened toner particles within the development nip 136. The bias source 163 also includes a DC bias 158 for enabling the directional transfer of charged toner powder cloud form the belt donor member 112 to the charged latent image on surface 14. Referring now to FIG. 3, a section of an image-on-image color process reproduction machine is shown and includes two development units 52A, 54B mounted so as to each form a development nip 136A, 136B respectively with a photoreceptor, such as the image bearing member 12. As shown, the first development unit 52A has a first housing 100A containing toner particles of a first type or color for developing a first latent image of a first color component of an original color image on the image bearing member 12. For the reasons explained above, for example, the first type or color of toner particles can be larger particle size toners(compared to those in the second development unit 54B), or yellow toner. In accordance with the present invention, the first development unit 52A advantageously includes a belt donor member 112 that is mounted within its housing, and forms a first development nip 136A with the image bearing member 12, such that the nip 136A has a first width "W1" thereto. The second development unit 54B similarly has a second housing 100B containing toner particles of a second type or color for developing a second latent image of a second color component of an original color image on the photoreceptor 12. The second development unit 54B includes a belt donor member 112 that is mounted within its housing 100B, and forms a second development nip 136B with the image bearing member 12, such that the nip 136B has a second width "W2". According to the present invention, the second width W2 of the second development nip 136B is advantageously made greater than the first width "W1" of the first development nip 136A, in order to produce satisfactory perceived uniform quality development from the different toners of the first and second development units 52A, 54B. In such an image-on-image color process reproduction machine, electrostatographic process devices, such as charge corotrons or exposure ROSes shown as 180A, 180B, respectively, can be mounted appropriately upstream of the developer units 52A and 54B. As described above, each of the development units 52A, 54B according to the present invention includes elements for adjusting the width of its development nip. These elements for example include first and second guide or back up rollers 148, 150 that are adjustably spaced apart relative to each other, and thus define for each unit a spacing for mounting various size backer bars or plates 152. The additional back up rollers 154, 156 for training the belt donor member within the mixing chamber 102 of each unit, are also adjustable vertically relative to a line (not shown) connecting the first and second back up rollers 148, 150, thus allowing for the use of a common length belt donor member 112.

Claims: A colour reproduction machine including: (a) an image bearing member (12); (b) means (58) for electrostatically recording a first and at least a second latent images on said image bearing member (12); and, (c) a plurality of development units (52,54,56,58) each storing developer material containing a different type of toner for developing said latent images on the image bearing member (12), said plurality of development units including: (i) a first development unit (52) containing a first type of toner for developing said first latent image, said first development unit (52) having a movable first belt donor member (112) forming a first development nip (136A) with the image bearing member (12); and, (ii) a second development unit (54) containing second and different type of toner for developing said at least second latent images, said second development unit (54) having a second belt donor member (112) forming a second development nip (136B) with the image bearing member (12); characterised in that said second development nip (136B) has a second width that is unequal to a first width of said first development nip (136A). A colour electrostatographic reproduction machine according to claim 1, wherein said second development nip (136B) has a second width thereto greater than said first width of said first development nip (136A) for enabling uniform quality development from both said first and second development nips (136). A colour reproduction machine according to claim 1 or 2, further comprising a first set of back up rollers (148,150) defining said first width of said first development nip (136A) and a second set of back up rollers (148,150) defining said second width of said second development nip (136B). A colour reproduction machine according to any one of the preceding claims which includes third (56) and fourth (58) development units each having a belt type donor member (112) forming an additional development nip (136) with the image bearing member (12) of each said additional development nip having a width thereto substantially equal to said second width of said second development nip (136B). A colour reproduction machine according to any one of the preceding claims, wherein the toner particles of first colour in said first development unit (52) comprise yellow toner particles. A colour reproduction machine according to any one of the preceding claims, wherein the developer material stored in said first development unit (52) includes toner particles each having an average size substantially greater than that of toner particles includes in said developer material stored in said second development unit (54). A development unit (52,54,56,58) for developing a latent image recorded on an image bearing member (12), the development unit (52,54,56,58) comprising: (a) a housing (100) storing a supply of magnetizable two-component developer material containing toner particles; (b) means (124) mounted within said housing (100) for moving developer material thereabout; and (c) a toner transport assembly including: (i) a movable belt donor member (112) forming a development nip (136) with the image bearing member (12) and transporting toner particles into and through said development nip (136) for latent image development; (ii) a contoured back supporting plate (152) located across said development nip (136) for spacing said belt donor member (112) from the image bearing member (12); and, (iii) first and second back up rollers (148, 150) mounted for training a portion of said belt donor member (112) across a width of said development nip (136); characterised in that said second back up roller (150) is movable relative to said first back up roller (148) to vary said width of said development nip (136). A development unit according to claim 7, wherein the toner transport assembly includes at least a third back up roller (154) mounted within said housing (100) for training said belt donor member (112) through a toner-transfer relation with said developer material moving means (124), and optionally includes a fourth back up roller (156) mounted spaced from said at least third back up roller (154) for training said belt type donor member (112) through said toner-transfer relation with developer material moving means (124, 108). A development unit according to claim 7 or 8, wherein the developer material moving means within said housing includes first and second magnetic rollers (124, 108) biased appropriately for loading and for removing respectively, charged toner particles relative to said belt donor member (112). A colour reproduction machine or a development unit for it in accordance with any one of the preceding claims, wherein the or each belt donor member (112) includes embedded biasable conductive strips, and a non-conductive backing formed underneath said conductive strips for directing substantially all development biasing electric field lines from said biasable conductive strips into the development nip (136) thereof. A colour reproduction machine or a development unit according to claim 9, wherein the or each belt donor member (112) includes a dielectric surface layer formed over said embedded layer of conductive strips.

Inventors: GWALTNEY MARK A, GWALTNEY, MARK A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: G03G15
IPC codes: G03G15