Development apparatus having a developer feeder roll

Abstract: A development apparatus (40) includes a housing (48) defining a chamber (46) holding a supply of two-component developer material consisting of toner, and magnetizable carrier beads. The development apparatus also includes an extended life, increased reliability magnetic roll assembly within the housing for repeatably transporting a desired quantity of attracted developer material, fed from the chamber, for movement through the development zone. The improved magnetic roll assembly (50; Fig.2) includes a rotatable non-conductive shell or substrate (90; Fig.2) surrounding a magnetic member (M1-M4; Fig.2) so as to prevent the creation of eddy currents in the substrate during its rotation. The substrate has an elastomeric coating layer formed (100; Fig.2) over it. The elastomeric coating layer has a mechanically deformable smooth surface (102; Fig.2) for effectively holding, even at relatively high rates of speed, the quantity of attracted developer material thereon being transported. The smooth surface is deformable by magnetized carrier beads acting under the influence of the magnetic member.

Description: This invention relates generally to electrostatographic reproduction machines, and more particularly concerns a development apparatus having an improved developer feeder roll. Generally, the process of electrostatographic reproduction includes uniformly charging a photoconductive member, or photoreceptor, to a substantially uniform potential, and imagewise discharging it or imagewise exposing it to light reflected from an original image being reproduced. The result is an electrostatically formed latent image on the photoconductive member. The latent image so formed is developed by bringing a charged developer material into contact therewith. Two-component and single-component developer materials are commonly used. A typical two-component developer material comprises magnetic carrier particles, also known as "carrier beads," having charged toner particles adhering triboelectrically thereto. A single component developer material typically comprises charged toner particles only. In either case, the charged toner particles when brought into contact with the latent image, are attracted to such image, thus forming a toner image on the photoconductive member. The toner image is subsequently transferred to a receiver sheet which is then passed through a fuser apparatus where the toner image is heated and permanently fused to the sheet forming a copy of the original image. To develop a latent image in an electrostatographic reproduction machine, charged toner particles are brought, by a development apparatus, into contact with the latent image formed as described above. For such development using two-component developer material, the development apparatus typically includes a housing defining a chamber within which the developer material is mixed and charged. Moving and mixing two-component developer material triboelectrically and oppositely charges the "carrier beads" and the toner particles causing the toner particles to adhere to the carrier beads. As disclosed for example in US-A-5,245,392, one type of a two-component development apparatus includes a housing, a mixing chamber, a development zone, and a donor member for transporting charged toner particles from the mixing chamber to the development zone. A plurality of electrode wires are closely spaced relative to the donor member within the development zone. An AC voltage is applied to the electrode wires for forming a toner cloud in the development zone. Electrostatic fields generated by an adjacent latent image serve to attract charged toner particles from the toner cloud, thus developing the latent image. As also disclosed, it is conventional to provide in such an apparatus, a conductive, usually metallic magnetic roll for transporting developer material from the mixing chamber to the donor member. The magnetic roll is mounted rotatably between the mixing chamber and the donor member, and serves to magnetically attract and hold magnetizable carrier beads (which have charged toner particles triboelectrically adhering thereto) onto its roughened or knurled surface. The charged toner particles are then electrostatically attracted from the carrier beads on the roughened or knurled surface of the magnetic roll onto the donor member for transporting to the development zone. The uniformity and quality of latent images developed in the development zone depend significantly on the quantity and uniformity of developer material repeatably transported by the magnetic roll to the donor member. As disclosed for example in each of the following references, the quantity and uniformity of developer material transported by such a magnetic roll are determined primarily by the surface roughness of the magnetic roll. For example, in this regard US-A-4,034,709 discusses the importance of, and several ways of, roughening the surfaces of magnetic developer rolls. In particular, it discloses such a magnetic developer roll that includes a rough styrene-butadiene surface-coating for holding and directly transporting developer material through a development zone. Xerox Disclosure Journal (Vol. 4, No. 3 May/June 1979) discloses a magnetic roll in which desired surface roughness is obtained by covering the roll with a netting material such as nylon stockings. Xerox Disclosure Journal (Vol. 4, No. 4 July/August 1979) on the other hand discloses a similar magnetic roll that is roughened by forming a multiplicity of small, shallow depressions in its surface. As a further example, US-A-4,558,943 discloses a similar magnetic roll that is roughened by forming valleys in its surface which are then filled with a polymeric material. As can be expected, when such rolls are used to transport two-component developer material containing carrier beads which can be abrasive, the carrier beads tend to wear out the desired roughness of their surfaces over time. Such wearing out of the surface roughness of a roll disadvantageously and eventually reduces the frictional characteristics of the surface, and hence its ability to repeatably transport desired quantities of developer material. This particular disadvantage is further aggravated in development apparatus that are required to operate at substantially high rates of speed. In such an apparatus, the magnetic roll is accordingly required to rotate at a substantially high number of revolutions per unit time. As can be expected, at such high rates of rotation, centrifugal forces, for example, make it increasingly difficult for the rotating roll to hold onto developer material on its worn out surface. There is therefore a need for an improved magnetic roll with a surface that substantially resists wear and tear from carrier beads in two-component developer, and that continues to exhibit acceptable holding ability at high speeds on developer material being transported thereby. Conventionally too, such magnetic rolls typically include a conductive substrate or shell, such as an aluminum shell, that is coated variously. It has been found that the rotation of the conductive shell of such a roll through a magnetic field, of the magnet within its core induces eddy currents through out the conductive shell. Such eddy currents as is well known, result in power losses as well as in reductions in the magnetic flux of the magnetic field. Such power losses for a high speed roll have been found to be as much as 19% of the free-space power required for driving the roll. Such losses also undesirably cause eddy current heating within the housing of the loss development apparatus. There is therefore also a need for an improved magnetic roll that overcomes such eddy current related disadvantages. In accordance with one aspect of the present invention, there is provided a magnetic feeder assembly according to claim 1. In accordance with another aspect of the present invention, there is provided a development apparatus according to claim 7. Other features of the present invention will become apparent as the following description precedes and upon reference to the drawings, in which: Figure 1A is a simplified elevational view of one embodiment of a development apparatus according to the present invention; Figure 1B is a simplified elevational view of a second embodiment of a development apparatus according to the present invention; Figure 2 is a perspective view of a cutout section of the magnetic developer feeder roll of the present invention; Figure 3 is a detailed sectional view of a portion of the magnetic roll of FIG. 2 showing magnetic decompression of the smooth surface thereof within a transport magnetic field; Figure 4 is a graphical illustration of eddy current power gains expected from use of the magnetic developer feeder roll of the present invention; and Figure 5 is a schematic elevational view of an illustrative electrostatographic reproduction machine incorporating development apparatus of the present invention. As electrostatographic reproduction machines are well known in the art, a detailed description thereof has been omitted from the present disclosure. Such a machine, in which the present invention may be employed, is described in US-A-5,416,566 (with reference to Fig.5). It is believed that the above mentioned description is sufficient for purposes of the present application to illustrate the general operation of an electrostatographic reproduction machine incorporating the development apparatus of the present invention. Typically, the speed of such electrostatographic reproduction machines is measured in terms of a number of sheet copies produced per unit time. Among different families of such machines, speed therefore varies significantly from a low between 10 and 20 copies per minute to a high of greater than 100 copies per minute. For such machines to produce high quality copies or reproductions of original images, the processing stations (including the development station SC), must be designed so as to function effectively at a desired speed of the machine. For example, the development station SC therefore must be capable of functioning as such, even at substantially high machine speeds, to repeatably deliver a uniform, desired quantity of toner particles to the development zone for latent image development. Referring now to Figure 1A, there is shown one embodiment of the development apparatus 40 of the present invention. The development apparatus 40 includes improved elements that enable an extended life, and the repeatable delivery of a uniform, desired quantity of toner for high speed latent image development. As shown, development apparatus 40 includes a movable donor member shown as a roll 42 that is mounted, at least partially, within a mixing chamber 46. Mixing chamber 46 is defined by housing 48, and holds a supply QS of developer material consisting of toner particles and carrier beads. The donor member 42 is moved to transport toner particles fed from the chamber 46 into contact with cloud producing electrode wires 44 within a development zone DZ for latent image development. The developer material QS typically is a two-component developer material comprising at least magnetizable carrier beads and the toner particles. As is well known, the developer material is moved and mixed within the mixing chamber 46 by a mixing device 49 in order to oppositely and triboelectrically charge such 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. Importantly, the development apparatus 40 includes the developer feeder assembly or magnetic roll 50 of the present invention (to be described in detail below). As shown, the feeder assembly 50 is shown disposed interiorly of the chamber 46 for feeding a quantity QF of developer material from the chamber 46 to the donor member 42. The magnetic roller 50 and the donor member 42 are electrically biased relative to each other so that charged toner particles within the quantity QF of developer material fed to the donor member 42 are attracted from the magnetic roll 50 to such donor member 42. Positioned within the mixing chamber 46 in the bottom of housing 48 is the mixing device 49, such as a horizontal auger, which distributes developer material uniformly along the length of magnetic roll 50, so that the lowermost part of magnetic roll 50 is always substantially immersed in a body of developer material QS. As further shown in FIG. 1A, the donor member 42 is biased to a specific voltage, by a DC power supply 80 in order to enable the donor member 42 to attract charged toner particles off of magnetic roll 50 in a nip 82. To enhance the attraction of charged toner particles from the chamber 46, magnetic roll 50 is also biased by a DC voltage source 84. It is also biased by a AC voltage source 86 that functions to temporarily loosen the charged toner particles thereon from their adhesive and triboelectric bonds to the charged, magnetized carrier beads. Loosened as such, they can be attracted more easily to the donor member 42. AC voltage source 86 can be applied either to a conductive layer of the magnetic roll 50 as shown in Figure 1, or directly to the donor roll in series with the DC supply 80. Similarly as shown, an AC bias is also applied to the electrode wires 44 by an AC voltage source 88 and serves to loosen charged toner particles from the donor member 42, as well as to form a toner cloud within the development zone DZ. Referring now to Figure 1B, there is shown another embodiment 40A of the development apparatus of the present invention. In this embodiment, like elements as in FIG. 1A are shown with like reference numerals. Importantly, the development apparatus 40A also includes the improved elements that enable an extended life, as well as the repeatable delivery of a uniform, desired quantity of toner for high speed latent image development. In FIG. 1B, because of its longer life and increased developer material moving capability, the feeder assembly 50 is being used effectively, in a magnetic brush development apparatus housing 48A as a magnetic brush developer roll. As is well known, a magnetic brush developer roll as such receives developer material from the mixing chamber 46 and transports it directly into and through the development zone DZ for image development. It should be noted that although the improved developer feeder assembly 50 of the present invention (FIGS. 1A, 1B) is shown as a roll, the improvement design concepts therein are equally applicable to a belt type feeder assembly. Referring now to FIGS. 2 and 3, details of the magnetic roll or developer feeder assembly 50 of the present invention are illustrated. In accordance with the objectives of the present invention, the developer feeder assembly or roll 50 is designed so as to have a substantially longer life relative to conventional roughened surface metallic rolls. It is also designed so as to be able to maintain a high level of high speed developer feeding reliability over such life. It is further designed so as to significantly reduce or eliminate the occurrence of eddy currents and eddy current related disadvantages such as power losses and undesirable eddy current heating. Returning now to FIGS. 2 and 3, the developer feeder assembly or roll 50 of the present invention includes a movable substrate or shell 90 that has a first surface 92, a second surface 94 and a path of movement or rotation 96 (shown by the arrow 98) defined substantially by the second surface 94. The feeder assembly 50 also includes at least a magnetic member such as the magnetic members M1, M2, M3, M4 positioned interiorly of the first surface 92 of the substrate or shell 90, as well as adjacent the path of movement 96 thereof. The path of movement 96 is continuous and surrounds the position of the magnetic members. As positioned, each magnetic member M1 to M4 generates a strong magnetic field about a point along the path of movement 96, for example, about a first point P1 which is the pick up or loading point for the feeder assembly 50 within the chamber 46. As also shown, the feeder assembly 50 further includes a thin elastomeric coating or layer 100 that is formed onto the second surface 94 of the movable shell 90. The elastomeric coating 100 importantly has a mechanically deformable smooth surface 102 for holding, during transportation, a quantity QF of magnetic or magnetized developer material (FIG. 3) that is attracted thereonto by the magnetic members M1 to M4 as positioned on the opposite side of the movable substrate or shell 90. Referring in particular to FIG. 2, magnetic members M1 to M4 are stationary permanent magnets, for example, that are each coextensive in length with, and are positioned closely spaced from, the first or interior surface 92 of the movable shell 90. The shell 90 preferably is non-magnetic (to be described further below), and is designed to be rotated about the magnetic members M1 to M4 in a direction indicated by the arrow 98. Because the two-component developer material QS in the chamber 46 includes magnetic or magnetizable carrier beads, the effect of the shell 90 rotating through the strong stationary magnetic fields of M1 to M4 is to cause the quantity QF of such developer material to be attracted to the exterior of the shell 90. As also shown, a doctor blade 104 may be used to limit the radial depth of developer on the surface 102 as it rotates through a toner-transfer nip 82 (FIG. 1A) with the donor member 42. The field strength of the peak radial field of the magnetic members around the pick up or loading point P1 of the feeder assembly or magnetic roll 50 preferably is about 600 gauss. As also shown, other points, for example point P2, along the path of movement of the movable shell 90 include no, or at best a weak magnetic field. The surface of the magnetic roll 50 should be about 2.5mm away from the top of the magnetic members. The radial field of each magnetic member is commonly the component of the magnetic field thereof that is directed radially outward relative to the axis of the magnetic member. The tangential component of each magnetic field is tangent to the circumference of the roll 50, and will often be defined when all the radial peaks are defined and specified. The polarity of each magnetic field can be North or South, but preferably should be alternating from magnetic member to magnetic member as shown. The magnetic force generated by the gradient of the resulting magnetic field together with the inventive characteristics of the elastomeric coating 72 (to be detailed below) are particularly important for the functioning of the present invention. This magnetic force, which acts on magnetized developer material QF on the surface of the elastomeric coating, effectively enables reliable high speed, and extended life, feeding of developer material QF from the chamber 46 of the apparatus 10. In accordance with the present invention, the permeability (µ) of the developer material is preferably within a range of 4 to 6, and the magnetic force acting on a mass of such developer material is typically 40 - 80 times larger than a force that would be generated by gravity on the same mass of developer material. Still referring to FIGS. 2 and 3, when the feeder assembly 50 is a roll as shown, its cylindrical substrate or shell 90, for example, is usually about 475mm long in the development apparatus 10. At such a length, the beam strength requirements are such that the Modulus of Elasticity (E) thereof should be at least 300,000 pounds per square inch (2070 MPa). Furthermore, because the thickness of the shell affects the magnetic field strength therethrough, the shell 90 preferably has an inner diameter of 38.6mm, and an outside diameter of 42.0mm. As such, the roll 50 advantageously should have an allowable center deflection of about 0.0016 inch (41 µm) when the roll is fully loaded with developer material QF. In order to meet these various requirements, the substrate or shell 90, for example, can be made out of a general purpose polycarbonate. An important consideration for selecting shell material is that the selected material has to be compatible with elastomeric coating 100. Compatibility results in good adhesion which is important for the desired long, or extended service life of the feeder assembly. As such, other materials that can be used are, for example, phenolics having a Modulus of elasticity (E) of about 900,000 psi (6210 MPa), and polyesters having a Modulus of elasticity (E) of about 400,000 psi (2760 MPa). Thermo plastics materials as well as thermo setting materials of the sort that are compatible with the elastomeric coating 100 can be used. It has been found that the higher the (E) value of the material used for the shell, the thinner the walls of the shell can be made. Thinner walls can advantageously lead to increased magnetic field strength at the outside surface of the shell. They can also simplify manufacturing by allowing for larger clearances between the magnetic members and the inside surface 92 of the shell. More importantly, the shell 90 can be made out of a high strength urethane based polyester which would eliminate the need for an elastomeric coating 100 thereover. The modulus of elasticity E for such a urethane based polyester shell is about 300,000 psi (2070 MPa) with a durometer value of about 70d, which would result in a roll shell capable of receiving small magnetic force induced decompressions 110. The roll 50 as a whole when made as such would have a volume resistivity of 10 8 ohm-cm throughout. In accordance with another objective of the present invention, the movable substrate or shell 90 is advantageously made electrically non-conductive in order to eliminate or prevent the occurrence of eddy currents that would be generated within a movable electrically conductive shell due to the presence of the magnetic members M1 to M4 within its core. An eddy current is an electric current that is induced within the body of a conductor when that conductor either moves through a non-uniform magnetic field, or is in a region where there is a changing magnetic field. The movement of the shell 90 about magnetic members M1 to M4 represents such a set up if the shell 90 is conductive. Referring to FIG. 4 for example, a chart showing eddy current losses is illustrated. The chart shows two lines 106, and 108, each representing the total power needed to drive the development apparatus 10 under eddy current, and non-eddy current conditions, respectively. To generate the top line 106, a test was conducted using a conventional feeder assembly 50A (not shown) that included a conventional conductive aluminum shell, along with magnetic members positioned within its core. The conductive shell was driven, and its surface speed was measurably varied from 40 to 65 inches per second (IPS) (102 to 165 cm/sec). This test represented eddy current generating conditions. The total power required to run the apparatus 10 was recorded at the various speeds (see top line 106). For non-eddy current conditions, a second test was conducted under conditions where the magnetic members were removed from the core of the conductive shell. This was equivalent to removing the conductive shell instead, and mounting an electrically non-conductive shell around the magnetic members. In either case, no eddy currents (generated or not) would flow in the shell under these conditions. The shell and the rest of the apparatus 10 were again driven, and the surface speed thereof was measurably varied the same as above. The total power required to run the apparatus 10 was again recorded at such speeds (see the lower line 108). It is quite clear from these plots of the lines 106, 108 that more total power was required under the first test (eddy current) conditions than under the second (non-eddy current) conditions to run the apparatus 10. At each plotted point, the difference in required total power amounts to an eddy current power loss at that speed for the apparatus 10. As measured and plotted, these losses for example were 2.7watts at 40 ips (102 cm/sec), and 6.4watts at 65ips (165 cm/sec), which are losses ranging from 15% to 19%. The use of an electrically non-conductive shell around the magnetic members M1 to M4, (a condition under which no eddy currents will flow through the shell), would therefore be significantly advantageous. Referring still to FIGS. 2 and 3, the elastomeric coating 100 is a thin coating made for example from electrically conductive urethane material that has a thickness within a range of 0.040 to 0.060 inch (1 to 1.5mm). The smooth surface 102 thereof has a preferred durometer hardness within a range of 60a to 70d. As such, the smooth surface 102 can be magnetically deformed temporarily and easily, by a magnetic force (not shown) of any of the strong magnetic fields of M1 to M4 acting on magnetized carrier beads of the developer material QF on such smooth surface. Such deformation results in decompressions 110 (FIG. 3) that reliably provide a frictional structure for holding the attracted developer material QF onto the surface of the rotating shell, even when the shell is being rotated at substantially high rates of speed. Given such impact of the magnetic forces on the smooth surface 102, it is clear that when being rotated with the shell 90, the smooth surface 102 will be temporarily decompressed for example when at the first point P1 along the path of movement, but would resiliently reform and become smooth again when at the no, or weak, magnetic field point P2. Unlike conventional developer transporting surfaces with roughness formations that wear out over time, the smooth surface 102 advantageously results in a substantially improved extended life for the developer feeder assembly or roll 50 of the present invention. As can be seen, an improved development apparatus 10 for developing a latent image recorded on an image bearing surface 14, has been provided and includes a housing 48 defining a chamber 46 that holds a supply of two-component developer material QS. The two-component developer material consists of toner and magnetizable carrier beads. The improved development apparatus 10 also includes a development zone DZ adjacent the image bearing surface 14, and a movable donor member 42 for moving toner fed from the chamber 46 through the development zone for image development. The improvement comprises a magnetic roll assembly 50 for moving a quantity QF of magnetically attracted developer material from the chamber into toner-transfer relation 54 with the donor member. The magnetic roll assembly 50 includes a rotatable non-conductive shell 90 surrounding the magnetic members M1 to M4 so as to prevent the creation of eddy currents during rotation of the shell about the magnetic members. An elastomeric coating layer 100 having a desired durometer hardness within a range of 60a to 70d is formed over the non-conductive shell and has a smooth surface 102 for holding the quantity of attracted developer material thereon. The smooth surface 102 as such, is magnetically deformable temporarily by magnetic forces of the strong magnetic fields of the magnetic members acting on magnetized carrier beads on the smooth surface 102. The magnetic roll assembly as disclosed is also suitable for use generally in a development apparatus for moving magnetic or magnetizable developer material including toner particles and magnetizable carrier beads, along a desired path, for example, along a direct image development path between the mixing chamber and the development zone for image development. As illustrated, it is also particularly suitable for use in a donor type apparatus as a feeder assembly for feeding developer material from a mixing chamber to a donor member.

Claims: A magnetic feeder assembly (50) for use in a development apparatus to move magnetizable developer material (QS) including toner particles and magnetizable carrier beads, the magnetic feeder assembly (50) comprising a moveable substrate (90) having a first surface (92); a second surface (94) and a path of movement (98) at least one magnetic member (M1;M2;M3;M4) positioned adjacent said path of movement (98) and said first surface (92) of said movable substrate (90), said magnetic member (M1;M2;M3;M4) generating a strong magnetic field about a first point along said path of movement (98); characterised in that the feeder assembly further includes an elastomeric coating (100) formed on said second surface (94) of said movable substrate (90) and having a smooth surface (102) for holding a quantity of magnetized developer material (QF) attracted thereon at said first point, said smooth surface (102) being mechanically deformable by magnetized carrier beads acting under said strong magnetic field to form decompressions (110) for frictionally holding the quantity of magnetized developer material (QF) during transportation. The magnetic feeder assembly of claim 1, wherein said path of movement (98) of said movable substrate (90) is continuous and surrounds the position of said magnetic member (M1;M2;M3;M4) and/or includes at least a second point therealong that is being subject to weak magnetic field. The magnetic feeder assembly of claim 1 or 2, wherein said movable substrate (90) comprises electrically non-conductive material. The magnetic feeder assembly of any of claims 1 to 3, wherein said elastomeric coating (100) comprises electrically conductive material or a urethane material, and/or is mechanically deformable by a magnetic force of said magnetic field acting on magnetized carrier beads, on said smooth surface (102) of said coating (100). The magnetic feeder assembly as claimed in any one of the preceding claims, wherein said movable substrate (90) is a rigid shell rotatable about said magnetic member (M1;M2;M3;M4). A magnetic feeder assembly as claimed in any one of the preceding claims, wherein the smooth surface (102) of the elastomeric coating (100) has a durometer hardness within a range of 60d and 70d, thereby allowing magnetic carrier beads in said quantity (QF) of developer material being held thereon to form temporary decompressions (110) into said smooth surface (102) when moving in said strong magnetic field. A development apparatus for developing a latent image recorded on an image bearing member (12) the development apparatus comprising: a housing (48) defining a mixing chamber (46) containing a supply of magnetizable two-component developer material (QS) including toner particles and magnetizable carrier beads; and a magnetic feeder as claimed in any one of the preceding claims. The development apparatus of claim 7, including a movable donor member (42), preferably a rotatable roll, mounted for receiving charged toner particles from said quantity of magnetized developer material (QF) on said smooth surface (102) of said elastomeric coating (100) and for moving said toner particles through a development zone for developing the latent image.

Inventors: ALVAREZ JORGE A, DE YOUNG VICTORIA F, EDMUNDS CYRIL G, ALVAREZ, JORGE A., DE YOUNG, VICTORIA F., EDMUNDS, CYRIL G.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: G03G15
IPC codes: G03G15