US 3392432 A
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Description (OCR text may contain errors)
July 16, 1968 G. NAUMANN 3,392,432
MAGNETIC ROLLER FOR ELECTROPHOTOGRAPHIC DEVELOPMENT Filed Dec. 16, 1964 INVENTOR.
' GERHARD NAUMANN FIG 5 r J 0, BY ATTORNEY.
United States Patent Oflice 3,392,432 Patented July 16, 1968 3,392,432 MAGNETIC ROLLER FOR ELECTRO- PHOTOGRAPHIC DEVELOPMENT Gerhard Naumann, Wiesbaden, Germany, assignor, by mesne assignments, to Azoplate Corporation, Murray Hill, NJ.
Filed Dec. 16, 1964, Ser. No. 418,606 Claims priority, application Germany, Dec. 18, 1963, K 51,671 9 Claims. (Cl. 29-110) The invention relates to a device for the development of latent electrostatic images, in which a mixture of magnetizable carrier particles and toner powder is applied to a surface carrying a latent image by means of a roller comprising a permanent magnet exciter system and a roller shell surrounding the exciter system. The magnetic excitation of the roller shell can, as desired, be fixed or variable between zero and a maximum value.
Magnetic rollers are known which have, parallel to their axes, a series of permanent magnet or electricallyexcited magnetic poles and which dip into a storage container containing developer mixture and attract the latter magnetically. By rotation of the roller, the mixture taken up by it is applied to the surface to be developed. In the case of such known magnetic rollers, it is diflicult to compensate for the impoverishment in toner powder of the developer mixture during the developing process, since without complicated additional devices susceptible to mechanical trouble, e.g. metal scrapers, the same magnetized carrier particles always adhere to the magnetic roller, as a result of which no sufficient mixture of the entire developer supply and enrichment of the carrier particles with toner can be effected.
This disadvantage can be avoided with electrically excited magnetic rollers since, in the case of such rollers, there exists the possibility to periodically switch off the total magnetization during or between two developing processes, or periodically to switch off the magnetization of individual magnetic poles during a part of their rotation. The use of electrically magnetized rollers is, however, also accompanied by disadvantages. The manufacture of the rollers is complicated and expensive which, especially in the case of the usual long magnetic rollers, is important. The magnetizing current must be supplied through slip rings or a collector; this can very easily give rise to trouble. The ohmic losses in the magnet windings cause, in the case of operation over lengthy periods, a sometimes very detrimental heating resulting in agglomeration of the developer mixture. Finally, for the operation of electrically magnetized rollers, they must have a power supply unit of their own, the space occupied by which and the heat developed by which are obstacles to the desirable compact construction of developing devices.
The present invention provides a magnetic roller which combines the above-mentioned advantages of permanent and electrical magnetization without the disadvantages associated therewith.
The present invention more specifically relates to a magnetic roller for the development of latent electrostatic images with a developer mixture which contains toner powder and magnetizable particles, comprising a roller shell having substantially axially parallel so'ft magnetic pole shoes distributed over the circumference thereof and being separated from one another by non-magnetic separatin g strips: a magnetizing system inside the roller shell comprising substantially axially parallel permanent magnet poles distributed over the circumference of the roller,
the magnetizing system and the roller shell being capable of rotating round a common axis.
The device according to the invention provides in an advantageous manner the possibility to switch off the magnetization of the developer roller. Since the magnetization takes place by means of a magnetizing system containing permanent magnets, all the electrical devices which arerequired in the operation of a conventional electromagnetic roller capable of being switched oil are thus unnecessary. This results in a substantially reduced susceptibility to trouble and a simple device requiring a relatively small amount of space.
Compared with the known, permanent magnet rollers which cannot be switched off there is achieved in desirable manner the possibility to switch off over the entire circumference of the roller or over any desired portion thereof. In addition, there is provided a variation of the strength of the magnetic field and thus an influencing of the brush hardness of the developer powder to be taken up, i.e., practically an influencing of the amount of powder attracted.
The magnetic roller according to the invention comprises two parts, a roller shell and a magnetizing system. The roller shell is tubular and possesses a common axis with the cylindrical magnetizing system, around which common axis the two systems are capable of rotating. The roller shell and magnetizing system together form the developer roller of the invention.
The roller shell is divided into individual pole shoes which are distributed in a substantially axially parallel manner over the entire circumference of the shell. The number of the pole shoes is not critical but an even number is preferable. The pole shoes are separated from one another by non-magnetic separating strips so that the tube-shaped roller shell is alternately made up of pole shoe-separating strip-pole shoe-separating strip, etc. The extension of the pole shoes in a radial direction is also not critical. This will be dependent on the design of the magnetizing system which is inside the roller shell and on the roller diameter required.
The pole shoes consist of soft magnetic material which is magnetized only for the duration of the eliect of a magnetic field and which, after the collapse of the magnetic field, retains only a small residual magnetism. Materials which are suitable are, for example, soft iron, cobalt, nickel or the known Heusler alloys.
The separating strips preferably are made of a material having a low magnetic permeability, suitable examples being numerous metals, such as aluminum, brass and others, as well as thermoplastic and thermosetting plastics. The separating strips may, however, also be air slits. In this case, the penetrations of magnetizable particles of the developer into the slits must, of course, be prevented, since otherwise these particles would nullify the separating effect of the strips.
The combination of the magnetic separating strips and the pole shoes into a rigid complete tube can be alfected by mechanical connecting members at the roller ends such as tension hoops etc., or by bonding or by end flanges. These constructional elements preferably consist of non-magnetic material so that they do not influence the field distribution of the magnetic roller at the roller ends. Because of the residual magnetism of the pole shoe material, which magnetism is practically always present, it has proved advantageous to provide the outer surface of the roller shell with a thin, smooth covering of a plastic, for example a known thermosetting molding composition, or with a thin non-magnetizable metal foil, in order to promote the disengagement of the developer powder during the demagnetized state of the roller shell.
The cylindrical magnetizing system inside the tubular roller shell includes one or several permanent magnets which are so magnetized that, distributed over the cirmumference, axially parallel magnetic poles point outwardly to the roller shell. When several permanent magnets are used, they may be mounted on a soft magnetic core. Suitable metallic permanent magnet materials are, for example, alloys of iron-cobalt vanadium, coppernickel-iron, and copper-nickel-cobalt. Exemplary of oxidic permanent magnet materials are barium and cobalt ferrites.
The number of magnet poles facing the roller shell should at the most be equal to the number of the pole shoes; an equal number is preferred. Between the individual magnet poles there are intermediate spaces which possess a high magnetic resistance, i.e., they consist of a non-magnetizable material.
The magnetizing system and the surrounding shell rotate around a common axis. Magnetizing system and shell can be adjusted relative to each other in such a manner that the magnetizing system can be rotated around the common axis to take different positions with regard to the shell. Preferably, the magnetizing system is automatically displaced with regard to the shell at the beginning of the common revolution, and turned back to its original position at the end of said common revolution, this being effected, e.g., by providing a drive for only one of these parts, whereas the other is a braked part and entrained by the driven part by means of an adjustable catch against the action of a spring, the angle of rotation or shift of the magnetizing system relative to the shell being defined by stationary or adjustably fixed stops. If the drive is switched on, the driven part begins to revolve, and only when the catch reaches the stop mounted in front thereof, the braked part is taken along. When the drive motor is switched off, the rotational shift of the two parts relative to each other is revoked because the spring displaces the two parts in relation to each other until the catch reaches the second stop mounted behind the catch. Alternatively, the two parts may be driven separately. This may be achieved by known mechanical means, e.g., by a combination of separately driven knockout spindles. If, as a result of a particular adjustment of the magnetizing system in relation to the roller shell, one magnet pole of the magnetized system lies exactly underneath one pole shoe of the roller shell, this magnet pole produces a magnetic induction in the pole shoe whereby a magnetic field forms which extends beyond the roller surface. This renders it possible to attract the magnetizable particles of the developer mixture. These form a so-called magnetic brush by which the developer powder can, in conventional manner, be brought to the surface of the latent electrostatic image to be developed. If the magnetizing system is now rotated in relation to the roller shell, and, in an extreme case, so far that the magnet poles of the magnetizing system are exactly opposite the non-magnetic separating strips of the roller shell, there occurs only an infinitesimal magnetic field outside the roller shell, since practically all field lines going out from the magnet poles go inside the pole shoes to the adjacent magnet poles. In this position of the magnetizing system with respect to the roller shell, no developer mixture is therefore attracted.
Between the two extreme cases mentioned, namely the maximum formation and the practically complete disappearance of the outer magnetic field, all intermediate strengths are realizable. This can take place by rotation of the magnetizing system with respect to the roller shell by angles which are smaller than the angle between the two extreme positions. The strength of the magnetic field, and thus the strength of the attraction of the developer mixure and the hardness of the magnetic brush, can
therefore be adjusted continuously or discontinuously up to a maximum.
The width of the non-magnetic separating strips between the pole shoes of the roller shell may vary within wide limits. By the width, the amount of attracted developer mixture, its distribution around the circumference of the roller shell, and thus the hardness of the magnetic brush, can be influenced. The width of the separating strips must, however, be large enough for their magnetic resistance to be greater than the magnetic resistance of the developer mixture held fast between two pole shoes. Since the mixture contains a high proportion of a ferromagnetic material, this condition is fulfilled where the width of the separating strips is only a little greater than the diameter of the ferromagnetic particles contained in the developer mixture.
When the width of the separating strips is relatively small, the magnetic lines of force emerge from the pole shoes primarily in the areas adjacent to the separating strips, so that the field strength in these areas is substantially greater than in the middle of the pole shoe. In this case, therefore, there is formed a relatively hard magnetic brush whose density varies somewhat around the circumference of the roller. If, on the other hand, the width of the separating strips is relatively large, the field strength differences between the middle of the pole shoes and the areas of the pole shoes adjoining the separating strips are very much smaller so that a softer magnetic brush with practically uniform density around the roller circumference is formed.
The separating strips between the pole shoes may extend in a radial direction or in a direction inclined thereto. In the first case, the magnetic brush formed by the roller is in mirror symmetry with reference to every plane containing the rotational axis of the magnetic roller and the middle of the pole shoes or the middle of the separating strips. In the second case, this symmetry is not present, since then the field strength of the pole shoe at the two separating strips adjacent to each pole shoe is different. As a result, the stiffness or hardness of the magnetic brush held by such a magnetic roller is unsymmetrical, for example greater in the direction of rotation than in the opposite direction. This fact can be favorable for special developing problems.
The above-described effect which the separating strips have on the magnetic field is governed primarily by the Width and form of the separating strips at the outer surface of the roller shell, whereas the width and form of the separating strips at the inner surface of the shell have a substantially smaller effect on the form and hardness of the magnetic brush. The cross-section of the separating strips can, therefore, be chosen substantially according to manufacturing considerations. The sole requirement is that the magnetic resistance of the separating strips over their entire radial extension should be appreciably greater than the magnetic resistance of the part of the magnetic circuit-magnetizing system-pole shoe-developer mixturepole shoe-magnetizing system consisting of two adjacent pole shoes and the developer mixture held fast thereby.
Referring to the accompanying drawings:
FIGURES 1 and 2 show a cross-section of one example of an embodiment of the magnetic roller of the invention in which the magnetization as a whole can be switched on and off,
FIGURE 3 is a further embodiment of the magnetic roller of the invention in which the magnetization can be switched on and off and, in addition, the magnetization of the pole shoes can be interrupted periodically in a fixed angular region, and
FIGURES 4 and 5 show particular examples for the formation of the magnetizing system.
In FIGURE 1, the magnetic roller comprises an even number of pole shoes 1-10 of soft iron which, in each case, are separated from one another by the separating strips 2-2c of non-magnetic material and which are united into a tube by conventional means not shown, e.g. end
flanges. The magnetizing system inside the tube com prises a magnetizable core 3, on which the same number of permanent magnets 4-4c as the number of pole shoes are mounted. The permanent magnets are so magnetized that the N and S poles face the pole shoes alternately and that adjacent poles possess different polarity. The intermediate spaces 5-50 between the permanent magnets should have a very high magnetic resistance, i.e., they consist of non-magnetic material. The magnetizing system can be rotated independently of or in dependence upon the rotation of the pole shoes around the common axis 311. The means for the positioning of the magnetizing system and for the rotation thereof relative to the pole shoes, and the driving means are conventional, so they are not shown in FIGURE 1 and the following figures.
In FIGURE 1 there is a permanent magnet under each pole shoe. Since there is a high magnetic resistance between each two pole shoes in the form of the non-magnetic separating strip 2, the field lines close through the space outside the magnetic roller so that developer mixture is attracted. For illustration, in FIGURE 1 the direction followed is shown by the field lines 6.
In FIGURE 2 the same magnetic roller is shown as in FIGURE 1 with the difference that each permanent magnet is opposite a separating strip 2. The field lines 6a emerging at the north poles of the permanent magnets 4 run inside the pole shoe 1 to the adjacent south poles so that the pole shoes do not produce a field outside the magnetic roller and thus do not attract any developer mixture.
The change from the position of the magnetic system and pole shoes shown in FIGURE 1 to that shown in FIGURE 2 can be controlled automatically, e.g. by movement of the recording means carrying the latent image with conventional mechanical, hydraulic, pneumatic or electromagnetic means being used to effect the necessary rotation. Preferably, e.g. in the case of recording means in sheet form, during the developing process the position shown in FIGURE 1 is maintained, whereas the renewal of developer between developing operations is effected by temporarily adopting the position shown in FIGURE 2.
The magnetic roller shown in FIGURE 3 makes possible, in addition to the renewal of all of the attracted developer mixture on the roller by switching off the magnetization, the continuous renewal of the developer mixture during the developing process. This magnetic roller is therefore practicularly suitable for the developing of latent electrostatic images on record-carrying materials in the form of Webs of indefinite length. As in FIGURES 1 and 2, the magnetic roller of FIGURE 3 comprises soft iron pole shoes 7, between which there are separating strips 8 of non-magnetic material. The permanent magnets 9 have rectangular cross-sections so they are simple and inexpensive to manufacture. The intermediate spaces 10 are not magnetizable. The permanent magnets 9 magnetized parallel to one edge of their cross-section are mounted on a magnetizable core 11 of polygonal cross-section in a manner such that the magnet poles point outwardly and, progressively around the circumference of the roller, north and south poles alternate.
As in the case of the magnetic roller in FIGURES 1 and 2, in the magnetic roller shown in FIGURE 3, the magnetizing system comprising the core 11 and the permanent magnets 9 are rotatable with respect to the pole shoes, so that during the developing process the permanent magnets can be positioned opposite the pole shoes 7 and, for the purpose of renewing the developer, opposite the separating strips 8. On one part of the circumference there is incorporated, between the permanent magnets 9 and the pole shoes 7, a magnetic short circuit in the form of a cylindrically curved metal sheet 12 of highly permeable material which does not rotate with the pole shoes and the magnetizing system but is mounted in fixed position. The
magnetic field lines 15a, which emerge from the magnet poles in the area of the metal sheet 12, go through the metal sheet 12 to the adajcent poles, so that the pole shoes in the area of the metal sheet 12 are not magnetized.
Consequently, there occurs during the rotation of the magnetic roller a constant renewal of the developer mixture held by the magnetic roller in that the pole shoes, which are moved past the short circuiting metal sheet 12, let the developer mixture fall at 13 and take up new developer mixture at 14 when they leave the area of the short circuiting metal sheet. In FIGURE 3, to indicate the field distribution in the area in which attraction of the developer mixture occurs, the magnetic field lines 15 are shown schematically.
When the magnetizing system comprises several permanent magnets of suitable dimensions which, in relation to the cores 3 or 11 in FIGURES l or 3, can be moved backwardly and forwardly individually in the direction of rotation of the roller, it is also possible to keep a spatially fixed sector of the roller circumference constantly demagnetized during operation of the magnetic roller without the short circuiting metal sheet 12. This can for example be elfected by bringing each permanent magnet, when entering the sector, into position in relation to the magnetic roller corresponding to the position shown in FIGURE 2 by moving each permanent magnet in a direction opposite to the direction of rotation of the roller or in the direction of rotation. Each permanent magnet remains in the position shown in FIGURE 2 until in the course of the rotation of the roller it leaves the sector when it is moved back into the position corresponding to FIGURE 1. The necessary movements can be effected easily with conventional mechanical devices.
The magnetizing system shown in FIGURES 1 and 3 can be simplified and the cost thereof reduced by replacing every second permanent magnet by a soft magnetic body of the same shape.
In FIGURE 4 a further specific embodiment of the magnetizing system of a magnetic roller according to the invention is shown. It comprises a magnetizable core 16, over which is fitted a tube 17 of permanent magnet material, magnetized in the radial direction, so there are an even number of magnet poles of alternating polarity uniformly distributed around the circumference. The tube 17 may also be replaced by a suitable number of permanent magnet rings which are held together and are of corresponding magnetism.
FIGURE 5 shows another embodiment of the magnetizing system. In this case, the magnetizing system comprises a rod 18 of a permanent magnet material of suitable length which is magnetized at the circumference thereof in the manner shown. The magnetic field lines inside the magnetic rod run substantially in the direction of the circumference. The field lines 19 are shown for illustration.
It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.
What is claimed is:
1. A magnetic roller for developing latent electrostatic images comprising a roller shell having a plurality of substantially axially parallel soft magnetic pole shoes on the circumference thereof, said shoes being separated by non-magnetic strips having relatively high magnetic resistance, and magnetizing means inside the roller shell including a plurality of substantially axially parallel permanent magnet magnetic poles, the roller shell and the magnetizing means having a common axis and being relatively rotatable to positions whereby a magnetic field of sufficient strength to hold magnetic developer material is produced on said roller and to positions where said magnetic poles are opposite said non-magnetic strips.
2. A magnetic roller according to claim 1 in which the number of pole shoes is even and equal to the number of magnetic poles.
3. A magnetic roller according to claim 1 in which the magnetic poles are carried by permanent bar magnets on the surface of a soft magnetic core.
4. A magnetic roller according to claim 3 in which the bar magnets are movable relative to the core.
5. A magnetic roller according to claim 3 in which the number of permanent bar magnets is less than the number of. pole shoes.
6. A magnetic roller according to claim 1 in which the magnetic poles are carried by a magnetized permanent magnet hollow cylinder on a soft magnetic core.
7. A magnetic roller according to claim 1 in which the magnetic poles are carried by a cylinder magnetized substantially in a circumferential direction.
8. A magnetic roller according to claim 1 in which the pole shoes are covered with a thin layer of non-magnetic material.
9. A magnetic roller according to claim 1 in which a highly permeable material is fixedly mounted between the roller shell and the magnetizing means over only a portion of the surface of the latter.
References Cited UNITED STATES PATENTS 2,177,809 10/1939 Queneau 209-219 2,717,080 9/1955 Anderson 210222 2,786,440 3/1957 Giamio 11717.5 2,968,402 1/1961 Spodig 209-219 3,040,704 6/1962 Bliss 11717.5 3,098,765 7/1963 Keller et al 117-17.5 3,152,924 10/1964 Wanielista et al 11717.5
FOREIGN PATENTS 507,101 5/1954 Canada.
191,-492 11/1907 Germany. 1,865,366 1/1963 Germany.
OTHER REFERENCES Indiana Steel Products Co., Indiana Permanent Magnet Manual, No. 3, Permanent Magnets May do it Better,
WILLIAM D. MARTIN, Primary Examiner.
G. L. HUBBARD, E. J. CABIC, Assistant Examiners.