Magnet knife assembly for a toner developing device

A magnet knife assembly for a toner developing device including a ferromagnetic strip held between like poles (N) of two permanent magnets such that a knife edge portion of the strip projects outwardly beyond the two magnets and is subject to a magnet force that tends to urge the strip in a direction in which the knife edge projects further out of the magnets, wherein the length of the strip is selected such that a portion of the strip opposite to the knife edge is subject to a magnetic force that at least counterbalances said magnetic force on the knife edge portion.

This non-provisional application claims priority under 35 U.S.C. §119(a) on European Patent Application No. 071122611.2 filed in the European Patent Office on Dec. 7, 2007, which is herein incorporated by reference

BACKGROUND OF THE INVENTION

The present invention relates to a magnet knife assembly for a toner developing device, comprising a support body and a ferromagnetic strip that is held between like poles of two permanent magnets and extends between these poles from a knife edge that faces outwardly of the support body to an inner edge facing inwardly of the support body, the strip being held such that an outer knife edge portion of the strip projects outwardly beyond the two magnets and is subject to a magnetic force that tends to urge the strip in a direction in which the knife edge portion projects further out of the magnets.

A magnet knife assembly of this type is used in toner developing devices for printers, copiers and the like for creating, along the knife edge, a localized strong and strongly divergent magnetic field, so that, when magnetically attractable toner particles are supplied into that field, they will form a magnetic brush extending along the knife edge and across an image forming medium so as to assist in the transfer of the toner onto the image forming medium.

Typically, the magnet knife assembly is held stationary relative to the path along which the image forming medium is moved, and is surrounded by a thin sleeve, so that the knife edge faces the internal surface of the sleeve and the magnetic field penetrates through the wall of the sleeve towards the image forming medium. Toner particles may then be supplied to the magnetic field by distributing the toner on the surface of the sleeve and rotating the sleeve so that the toner approaches the magnet field created by the knife edge.

In order to obtain a high and constant quality of the developed image, certain parameters of the magnet field created at the knife edge must fulfil a number of criteria. For example, the absolute strength of the magnetic field directly above the knife edge should be relatively high, and the field should further be highly inhomogeneous, i.e., the gradient of the radial component of the magnetic field above the knife edge should also be high. Moreover, the angle a which the magnetic field vector forms with the surface of the sleeve (the tangent plane thereof at the position above the knife edge) should be relatively high and should be larger than 45° over a certain distance in the circumferential direction of the sleeve.

Magnet knife assemblies of the type indicated above are disclosed in EP 0310209A, EP 0298532A and EP 0773484A.

EP 0304983A discloses another magnet knife assembly of this type that was optimised in view of the above requirements. In this magnet knife assembly, the two permanent magnets have rectangular cross-sections that may be chamfered on the sides facing away from the ferromagnetic strip interposed therebetween. The plane of the strip is inclined at an angle of about 15° relative to the radial direction of the sleeve. It has been found that, for this configuration, the absolute strength and the inhomogenity of the magnetic field above the knife edge increases when the length of the strip (essentially in the radial direction of the sleeve) is reduced. For that reason, the length of the strip is shorter than the length of the two magnets. This has the consequence that the magnetic force tends to push the knife edge portion of the strip away from the magnets, i.e., tends to cause the strip to project further from the magnets.

For this reason, it is necessary in the known assembly that the strip is mechanically fixed at a support structure that carries the two magnets, e.g., by gluing the strip and the magnets to the support structure with an adhesive, by clamping the strip and/or the magnets with fastening screws, and the like. However, the necessity to fix the strip and the magnets in their desired positions requires cumbersome procedures and therefore increases the production costs for the magnet knife assembly as a whole. Moreover, differential thermal expansion of the magnet knife assembly and the support structure may lead to undesired mechanical strains and distortions.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a magnet knife assembly which can be produced at reduced costs without substantial sacrifices in the quality of the magnetic field.

According to the present invention, this object is achieved by a magnet knife assembly of the type indicated above, wherein a length L0of the strip from said knife edge to the inner edge is selected such that a portion of the strip opposite to the knife edge and closer to the inner edge is subject to a magnetic force that is larger than the force acting upon the knife edge portion, and the strip is held in position relative to the support body in the direction from the knife edge to the inner edge only by magnetic forces of the magnets that urge the inner edge of the strip against the support body.

The assembly according to the invention has the advantage that the additional magnetic forces on the strip, which tend to counterbalance the forces exerted onto the knife edge portion of the strip, can be utilized for a self-aligning effect which significantly reduces or completely eliminates the need for additional fastening means for fastening the strip relative to the magnets.

It could be expected that the increased length of the strip according to the present invention would tend to reduce the strength of the magnetic field at the knife edge. It has been found, however, that, in spite of the increased length of the strip, it is still possible, by appropriately selecting the shape and arrangement and the direction of magnetization of the magnets, to achieve an absolute strength and inhomogenity of the magnetic field at the knife edge that is comparable to that of the conventional magnet knife assembly, without having to use magnets with a larger overall size.

Preferably, the magnets of the knife assembly are held in position relative to the support structure only by the magnetic forces, so that these components are free to move relative to one another in the width direction of the assembly and differential thermal expansions will not lead to any strains or distortions.

In this embodiment, the length of the strip is selected such that a resultant magnetic force on the strip has the tendency to withdraw the knife edge portion of the strip into the gap between the two magnets and, conversely, to cause the opposite edge portion of the strip to project further from the magnets. Then, the support structure for the strip and the magnets may be formed by a substrate body having an outer surface and an internal cavity that communicates with the outer surface only through a narrow gap for accommodating the knife edge portion of the strip. The magnets are then accommodated in the cavity on either side of the strip, and the magnetic forces will tend to urge the strip against the bottom of the cavity, while the reaction forces acting upon the magnets tend to urge the magnets against walls of the substrate body that separate the cavity from the outer surface. In this way, the entire magnet knife assembly is clampingly held in position only by its own magnetic forces.

In a particularly preferred embodiment, the bottom of the cavity is formed with a step that is engaged by the edge portion of the strip opposite to the knife edge portion, and the magnets have cross-sectional shapes that assure that the magnetic forces of the magnets, that are supported by the substrate body create a torque acting on the strip so as to hold the same in engagement with the step in the bottom wall of the cavity.

DETAILED DESCRIPTION OF THE INVENTION

As is shown inFIG. 1A, a first embodiment of a toner developing device comprises a thin-walled cylindrical sleeve10that surrounds a cylindrical, non-magnetic substrate body12in which a magnet knife assembly14is embedded.

The knife assembly14comprises two permanent magnets16,18and a ferromagnetic strip20interposed therebetween. The strip20forms a knife edge22that is flush with the outer peripheral surface of the substrate body12and faces the internal surface of the sleeve10.

The substrate body12is held stationary on a stationary shaft24, whereas the sleeve10, in operation, rotates in the direction of an arrow A (the drive mechanism is not shown).

As is generally known in the art of toner developing devices, in the first embodiment, a toner powder with magnetically attractable toner particles is uniformly applied to the surface of the rotating sleeve10so as to form a toner layer26that is then conveyed towards the knife edge22of the stationary magnetic knife with the rotation of the sleeve10. A strong inhomogeneous magnetic field created by the magnets16,18above the knife edge22causes the toner particles to form a toner brush28extending away from the outer surface of the sleeve10. When an image forming medium (not shown) which may, for example, have a latent electrostatic charged image formed thereon, is moved past the magnetic brush28, and a suitable voltage is applied between the image forming medium and the sleeve10, a part of the toner particles will be attracted to the image forming medium so as to form thereon a toner image that corresponds to the charged image.

As is shown inFIG. 1B, a second embodiment of a toner developing device comprises a thin-walled cylindrical sleeve10that surrounds a cylindrical, non-magnetic substrate body12in which a magnet knife assembly14is embedded.

The knife assembly14comprises two permanent magnets16,18and a ferromagnetic strip20interposed therebetween. The strip20forms a knife edge22that is flush with the outer peripheral surface of the substrate body12and faces the internal surface of the sleeve10.

The substrate body12is held stationary on a stationary shaft24, whereas the sleeve10, in operation, rotates in the direction of an arrow A (the drive mechanism is not shown).

FIG. 1Bfurther shows an image forming member50. The image forming member50is rotatable in a direction Z.

In the second embodiment, as is known from the prior art, a toner powder with magnetically attractable toner particles is uniformly applied to the surface of the image forming member50so as to form a toner layer26that is then conveyed with the rotation of the image forming member50in the direction Z towards the knife edge22of the stationary magnetic knife. A strong inhomogeneous magnetic field created by the magnets16,18above the knife edge22urges the toner particles towards the sleeve10. Then, the sleeve10conveys the toner particles in the direction A.

When the image forming member50which may, for example, have an Electrical charge or voltage for attracting toner particles, is moved past the knife edge22, and a suitable voltage is applied between the image forming medium and the sleeve10, a part of the toner particles will be attracted to the image forming member50at each location where the electrical charge or voltage is provided on the image forming member50. As a result particles attracted to the image forming member50will remain on the image forming member50, while other toner particles will be moved to the sleeve10due to the presence of the magnetic field originating from the knife edge22. Thus, a toner image52is formed at the outer surface of the image forming member50.

It will be understood that the toner brush28and, consequently, also the sleeve10, the substrate body12and the entire magnet knife assembly including the magnets16,18and the strip20will extend over the entire width of the image forming medium in a direction normal to the plane of the drawing inFIGS. 1A and 1B. The magnets16and18are prismatic bodies which have the cross-sectional shape shown inFIGS. 1A and 1B. These magnets16,18may be made of an NeFeB-alloy, for example, and are magnetized such that like magnetic poles, e.g., the N-poles, of the respective magnets are facing the strip20. Although the magnets16,18tend to repel one another, the presence of the strip20between them has the effect that both magnets are attracted by the strip and cling to the opposite sides of the strip.

In addition, as will be explained in detail as the description proceeds, the magnets16,18and the strip20are subject to mutual magnetic forces that act in the direction of the length of the strip20, i.e., the direction from the internal edge to the external knife edge22of the strip20. These forces are indicated by arrows inFIGS. 1A and 1B.

As can be seen inFIGS. 1A and 1B, the magnets16,18and the strip20are accommodated in a cavity30of the substrate body12. This cavity30communicates with the outer peripheral surface of the body12only through a narrow gap which accommodates and is filled by the knife edge22. As is indicated by the arrows inFIGS. 1A and 1B, the magnetic forces tend to draw the strip20back into the interior of the body12and urge the internal edge of the strip, i.e., the edge opposite to the knife edge22, against a bottom surface32of the cavity30.

Consequently, the reaction forces acting upon the magnets16,18tend to urge these magnets outwardly against flange portions34of the body12which separate the cavity30from the external surface of the body12on either side of the strip20. Due to the specific cross-sectional shape of the magnets16,18, these magnets are supported at the flange portions34at support points36and38(or rather support lines extending in the direction normal to the plane of the drawing inFIGS. 1A and 1B).

The strip20is inclined relative to the radial direction of the body12and the sleeve10by an angle of 15°, in this example. As a consequence, the strip20is supported at the bottom surface32of the cavity30only at a single support point40. While in the illustrated embodiment, the single support point40coincides with a corner of the strip20, the single support point40does not necessarily coincides with such a corner of the strip20, which may depend on a shape of the strip20and a shape of the cavity30.

Since, as is shown inFIGS. 1A and 1B, the support point36of the magnet16are located in close proximity to the strip20, whereas the support point38of the other magnet18is located at the edge of this magnet facing away from the strip20, and both magnets are urged upwardly against the flange portions34, the whole magnet knife assembly14will be subject to a torque that tends to rotate the assembly clock-wise inFIGS. 1A and 1B. As a consequence, the outer portion of the strip20, i.e., the portion forming the knife edge22, is urged against a support point42at the tip end of one of the flange portions34, and the opposite (internal) edge portion of the strip20is urged against a support point44at a step46formed in the bottom surface32of the cavity30.

In the plane of the drawing ofFIGS. 1A and 1B, the strip20has one rotational and two translational degrees of freedom, i.e., three degrees of freedom in total. The position of the strip20in each of these degrees of freedom is entirely determined by the three support points40,42and44. Since the magnets16and18are attracted by the strip20, they may only slide along the length of the strip20, i.e., each of them has only a Single degree of freedom, and this is determined by the support point36and38, respectively.

Thus, the positions of all three components of the magnet knife assembly are entirely and uniquely determined, and the magnets16,18and the strip20are held in their positions only by the magnetic forces acting therebetween and by the forces acting between these members and the substrate body12. It will therefore be understood that the magnet knife assembly according to the invention can be assembled very easily just by thrusting the magnets16,18(which may also be segmented over the width of the image forming medium), and the strip20into the cavity32, so that they will automatically align themselves in the manner illustrated inFIGS. 1A and 1B.

InFIG. 2, the geometry of the magnetic field created by the magnets16and18in and around the strip20is indicated by magnetic field lines48. The two permanent magnets16,18are magnetized in a direction essentially (but not necessarily exactly) normal to the strip20, such that their north poles N are facing the strip20. It can be seen that the magnetic field lines are “repelling” each other in a central portion of the strip20, whereas they converge inside of the ferromagnetic strip20towards the knife edge22. As is generally known, a non-magnetized ferromagnetic body that is brought into an inhomogeneous magnetic field experiences a resulting force in the direction in which the field becomes stronger. Thus, the outer portion of the strip20adjacent to the knife edge22experiences a force that tends to push the knife edge22away from the two magnets, so that the strip would tend to project further from the magnets.

However, in the shown embodiment, the length of the strip22is so large that a similar effect occurs in the internal edge portion of the strip. Here, the magnetic force tends to push the strip into the opposite direction (towards the bottom of the cavity30inFIGS. 1A and 1B). When the strip20is intended to assume a position in which its knife edge22projects a certain amount beyond the outer surfaces of the magnets16,18, the force that tends to push the strip20against the bottom of the cavity will increase with increasing length of the strip. Here, the length has been selected such that the force acting towards the bottom surface32of the cavity dominates the force that tends to push the knife edge22away from the magnets, as has been explained in conjunction withFIGS. 1A and 1B.

InFIGS. 3 and 4, F1is a vector of the resultant magnetic force that the magnet16experiences from the strip20and the magnet18; F2is the vector of the magnetic force that the magnet18experiences from the magnet16and the strip20; and F0is the vector of the resultant magnetic force that the strip20experiences from the magnets16and18. As is shown inFIG. 4, these three force vectors sum up to zero. The components of the force vectors directed normal to the plane of the strip20will only have the effect to urge the magnets16against the opposite faces of the strip20, whereas the components of these forces in parallel with the strip20(the forces shown inFIGS. 1A and 1B) provide the desired self-aligning effect.

FIG. 5illustrates the general shape of the magnets16,18and the strip20and indicates the relevant dimensions. L0is the total length of the strip20. L1and L2are the corresponding lengths of the magnets16and18, respectively, and B0, B1and B2are the thicknesses of the strip20and the magnets16,18, respectively.

The basic shape of the magnets16and18is rectangular (with length L1or L2and width B1or B2). In the shown embodiment, the magnets16and18are provided with a full-width chamfer with a height E1and E2, respectively, at their bottom side (facing the bottom surface32of the cavity) and chamfers with a height C1, C2and width D1, D2, respectively, on their top sides facing the flange portions34. H1and H2are the distances which the knife edge22projects beyond the magnets16and18, respectively, on either side of the strip20. The angle a is the angle which the lengthwise direction of the strip20forms with the radial direction of the substrate body12.

Other parameters that may be varied in order to optimize the magnetic field at the knife edge22are the angles that the directions of magnetisation of the magnets16,18form with the strip20.