CA1107342A - Electrostatic transfer of magnetically held toner images - Google Patents

Abstract

ABSTRACT
A process for reproducing graphic information wherein a magnetic image is formed in a premagnetized layer of acicular chromium dioxide by heating the chromium dioxide selectively to above its Curie point. Unchanged ferromagnetic toner particles are then applied uniformly to the chromium dioxide layer, but adhere only in the magnetized areas. The toner particles are then transferred electrostatically to a substrate.

Description

Background of the Invention Field of the Inventlon: The present invention relates to a process for dry printing of information. The process involves f.'orming a magnetic image on a master followed by decorating the magnetic image with ferromagnetic toner particles which are then electrostatically transferred to a substrate capable of maintaining a charge and fixed in place.
Description_of the P'rior Art: Both xerography and magnetography are known. Xerography involves: forming an electrostatic charge on a photoconductive material such as selenium; imagewise exposing the photoconductive material to light whereby the exposed areas lose their charge; and applying a pigmented, finely divided, electrically charged powder which is attracted to and held on the electrostatic image. The charged toner image is then transferred to copy paper either with an opposite electrostakic charge or by means of pressure.
In magnetography a magnetic image is formed, and ferromagnetic particles applied thereto which adhere to khe ' ' magnetized areas of the image. The particles are then trans-ferred to copy paper either by pressure or magnetically. The pressure technique causes objectionable wear to the imaging member and can also cause bu~ldup of a film on the imaging ~ member which.causes smudgin~. ' :~ In magnetic transfer it has been ~ound di~ficult to effect trans~er of toner without erasing the latent magnet ~ image on the im~ging member : Summ~ry o'f''t'he'Invent'ion The~present invention reIat.es to ~orming a latent .; .

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magnetic image, clecorating the latent magnetic image with a uncharged ferromagnetic toner, and then transferriny the toner to a substrate electrostatically whereby the problems of pressure or maynetic transfer are overcome. By uncharged toner we mean toner which has not purposely been charged by means such as corona or triboelectric means but which may contain small triboelectric charges of either polarity.
Description of the Drawings Fig. 1 is a schematic side view of a printer used to perform the process of the present invention.
Fig. 2 is a side vie~ of a printer equipped with a magnetic printing head used to perform the process of the present invention.
Fig. 3 is a diagram of the exposure of the magnetic master by radiant energy.
-~ Fig. 4 is a diagram of the latent ma~netic ima~e.
Fig. 5 is a diagram of the toned magnetic ~mage superposed adjacent the copy paper.
Fig~ 6 is a diagra~ of the copy paper decorated with the transferred image.
Fig. 7 is a diagram of the final copy decorated with ` the fused image.
~-~ Fig. 3-7 show the stepwise formation of ~he latent ma~netic imaye, the decoration thereof with toner, the transfer of the toner to the copy paper, and the fusion of the toner to the copy paper. `
~ n aluminized polyester film having a layer of periodically ma~netized chromium dioxide particles in a binder adhered to the surface thereof whlch is to be used as- a copying -~' `: : `:

device is exposed to uniform illumination as shown in Fiy. 3.
As can be seen from Fig. 3 the printing on the document pre-~ents the illumination from reaching the magnetized chromium dioxide particles, thus, leaving them magnetized in the areas under the printing. On the other hand, those areas of the document being copied which contain no printing do not prevent the illumination from reaching the magnetized chromium dioxide particles, thus heating them to above their Curie point of about 116C, thus demagnetizing them. In this way the latent magnetic image shown in Fig. 4 is prepared. Ferromagnetic toner particles are appliecl to the latent magnetic image to form a developed magnetic image. Copy paper is brought into superposition with the magnetic image as shown in Fig. 5. A
corona discharge device then electrostatically charges the back of the paper. Upon separation of the paper from the grounded drum an electrostatic force sufficient to o~ercome the magnetic attraction between the previously uncharged toner particles and the latent magneti,c image is generated, thereby cau$i,n~ the toner particles to transfer to the copy paper and be adhered thereto wi,th surprisingly high effic~ency as shown in Fi~. 6. Groundin~ is a means of preventing the accumulation of eIectrostatic char~es on the surface c)~ th,e ~; drum, which may interfere ~ith the' printi,ng process~ This electrostatic transfer has no ef~ect on the'laten~ ~agnetic image which ~ay be reused many times~ The transferre~ tonex ' , particles- are then fused to the copy paper as shbwn in ~ig. 7 by heat.
~ he~toner particles prefe~ably are magnetic pigments encapsulated in a suitable binder. Generally the toner : ~ :

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particles have an average size ranging from 10 to 30 microns with a preferred average size ranging from 15 to 20 microns.
Spherical particles such as prepared by spray drying are preferred because of their super:ior flow properties which can be enhanced by the addition of minute amounts of a flow additive such as fumed silica. A further description of the preparation of toner particles may be found in U.S. Patent 3,627,682. When using the apparatus disclosed herein the toner particles should have a low electrical conductivity.
If the particles have high conductivity, they will be passed back and forth between the drum and the paper causing a diffuse image and low transfer efficiency. Generally the toner powder electrical conductivity is less than 1 x 10 13 mho/cm. The ferromagnetic component can consist of hard magnetic particles or a binary mixture of hard and soft mag-netic particles. The magnetically soft particles can be iron or another high-permeable, low-remanence material, such as certain ferrites, for example, (Zn, Mn) Fe2O4, or permalloys.
The magnetically hard particles can be an iron oxide, prefer-ably Fe3O4, ~-Fe2O3, other ferrites, for example, BaFel2Olg, chi-iron carbide, chromium dloxide or alloys of Fe3O4 and nickel or cobalt. A magnetically hard substance has a high-intrinsic coercivity, ranging generally from about 40 to about 40,000 oersteds and a high remanence (20 percent or more o~
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~- the satuXation magnetization~) ~hen removed from the magnetic f;eId. Such substances are o~ low permeability and re~ui:re high f~elds for magnetic saturation. A~magnetioally so~t substance~has low coerci~ity, for exampler one oersted or less, high permeab~lityr permitting saturatlon to be obtained :: y ~ ~

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with a small applied ~ield, and exhibits a remanence of less than 5 percent of the saturation ma~netization. A particularly preferred toner has an average particle size of 20 microns and contains ~0 weight percent thermoplastic binder 30 weight percent Fe3O4 (ma~netite) and 30 weight percent soft iron (carbonyl iron).
Referring to Fig. l, the document which is to be copied is placed on shelf ll and urged against gate 12. The copier is then activated to lift gate 12 and lower feed roll 13 into contact with the document. Feed roll 13 feeds the document into the nip between endless belt l~ and drum 15.
Endless belt 14 is made of a transparent film such as poly (ethylene terephthalate) about 2-7 mils in thickness. Rollers 16, 17, and 18 serve to drive and guide endless belt 14. The surface of drum 15 is preferably a poly(ethylene terephthalate) film about 5 mils in thickness. The convex sur~ace of this film is coated with a conAuctive layer such as by bein~ alumi-nized with a layer of aluminum to a surface resistivity of l to 1,000 ohms per square. The aluminum layer is ~rounded.
The conductive support may also ~e a plastic such as polyoxy-methylene sleeve coated with aluminum, nickel, copper or other conducti~e metal. The support may also be the conductive metal itself. The surface o~ the aluminum is coated with a layer of ferroma~netic material such as ac~cular chromium dioxide i,n an al~yd or other suitable binder. Generally the acicular chromium dioxide layer is from 0.001 to 0.012 mm in thickness and con-tains from 40 to 85 ~ei~ht percent acicular chromium dioxide and from 15 to 60 ~ei~ht percent alkyd or other su,itable res~n binder. Suitable acicular chromium dloxide can be Prepare~d in ,' .

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accordance with the teachings of U.S. Patent 2J956~955~ issued October 18, 1960, to Paul Arthur, Jr. However, the preferred acicular chromium dioxide particl.es are produced by the tech-niques disclosed in U.S. Patents 2,923,683 and 3,512,930.
Generally the chromium dioxide produced as disclosed in these patents consists essentially of uniform small acicular parti-cles whose average length is 1 with aspect ratios of 6:1, the said oxides containing from 58.9 to 61.9% chromium, and exhibiting an X-ray diffraction pattern which analysis sho-ws to correspond in its entirety to a tetragonal structure having cell constants of aO = 4.~1 + 0.10A and cO = 2.90 ~ 0.10~.
The acicular chromium dioxide layer should ha~e a resistivity of from about 1 x 10 1 to 1 x 10+9 ohm - cm which insures that any electrostatic charge imposed thereon will dissipate in less than a millisecond. The coating of the conductive support may be accomplished in a variety of ways, e.g., by gravure coating a slurry of CrO2 and resin in tetrahydrofurancyclohexanone on a web of aluminized polyethylene terephthalate or by spray-coating a conductive metal sleeve, etc. It is preferred to orient the CrO2 by passing the wet coated conductive support between the pole pieces o~ two bar magnets (approximately 1500 ; gauss average ~ield strength) aligned with the same poles facin~
:.; one another. The magnetic flux lines orient the acicular CrO2 all in the same di.rection. Ratios of magnetic remanence to magnetic saturation (Br/Bs) of up to 0.80 with an intrinsic coercivity (iHc) of 510 to 550 oersteds have been achie~ed by ~; this method.
.~ Drum 15 rotates in a counterclock.wise direction. The ferromagnetic coatin~ on~the drum is magnetized by premagnetizer -:
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19, which records a spatial periodic pattern. We ~ind 300 to 1000 magnetic reversals per inch on the magnetizable surface to be a working range and prefer about ~00-600 magnetic reversals per inch. The technique of roll-in magnetization can be used to structure the CrO2 surface, wherein a high permeability material such as nickel, which has been surface structured to the desired groove width is placed in contact with the DC magnetized CrO2 sur~ace. A permanent magnet or an electromagnet is placed on the backside o~ the permeable material. As the structured high permeability material is brought in contact with the CrO2 surface, the nickel concen-trates the magnetic flux lines at the points of contact resulting in the magnetization of the CrO2 coating. The CrO2 surface can also be thermoremanently structured by placing the continuously coated CrO2 surface on top of a magnetic master recording of the desired periodic pattern.
An external energy source then heats the CrO2 surface above the Curie temperature. As the surface cools below the Curie point the periodic magnetic signal from the master film thermoremanently magnetized it. As little as 20 oersteds can be used to structure the CrO2 in this way, whereas over 200 oersteds are needed to apply detectable magnetism to the CrO2 at room temperature. Also, a scanning laser ~eam may be used to structure a uniformly magnetized CrO2 surface.
Alternatively, a film structured by grooves containing acicular chromium dioxide can be used for the surface of dru~ 15 in which case a simple DC magnet can be used as prema~netizer 19. Generally from 200 to 300 grooves per i~ch across the drum will be used giving 4no to 600 , ~
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magnetic reversals per inch. Then the magnetized drum surface in contact with the document is moved past exposure station indicated generally at 20. The exposure station consists of lamp 21 and reflector 22. A suitable lamp 21 is a xenon flashl which has a color temperature equivalent to 6,000C.
The surface of drum 15 is exposed stepwise until the entire document has been recorded as a latent magnetic image on the surface of drum 15. The chromium dioxide as used herein has a Curie temperature of about 116C. The printing of the document being copied shades the areas of the chromium dioxide over which such printing is situated during exposure thereby preventing their reaching the Curie point. Thus, after exposure, the surface of drum will have magnetized areas of chromium dioxide correspanding to the printed areas of the document being copied.
After exposure, the document bein~ copied is dropped into tray 23.
The imagewise magnetized drum 15 is rotated past a toner decorator. The toner decorator comprises a trough.
24 fitted with rapidly rotatin~ roll 25, and bar 26, The toner is a fine powder of a magnetic mater~al such as i.ron oxide encapsul.ated in a thermoplastic resin having a relatively lo~ so~tening point of from 75 to 120C. The toner ~enerally will have an average parti.cle size of ~rom lQ to 3~ microns. ~ vacuum knlfe 31 is used to remove ~hatever toner particles may have adventitiously become :.
attached to the dema~net~zed:areas o~ the chromium dioxide on the surface:of drum 15. :~

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X :' ' The paper 32 on which the copy is to be made is Eed from roll 33 arouncl idler rolls 34, 35, and 36 to feed rolls 37 and 38. If desired, other substrates such as fabrics and films may be used rat:her than paper. Backing roll 39 cooperates with roll 40 equipped with cutting edges 41. Rolls 39 and 40 are activated by means not shown to cut the paper 32 to the same length as the length of the document being copied. The paper is then fed by feed rolls 42 and 43 into physical contact with the surface of drum 15. The paper 32 in contact with the surface of drum 15 is fed past corona discharge device 44. Corona discharge device 44 preferably is of the type known as a COROTRON* which comprises a corona wire spaced about 11/16" (17.5 mm) from the paper and a metal shield around about 75 percent of the corona wire leaving an opening of about 90 around the corona wire exposed facing the paper 32. The metal shield is insulated from the corona wire. The metal shield is maintained at ground potential.
~ Generally the corona wire will be from 0.025 to 0.25 mm. in ; diameter and will be maintained at from 3,000 to 10,000 volts~
The corona wire may be at either a negative, or a positive potential with negative potential being preferred. The COROTRON 44 electrostatically charges the back side of paper 32. This lightly pins the paper to the drum, and upon sepa- -ration of the paper from the drum causes image-wise transfer of toner particles to paper 32. At the region in which paper . ~ .
32 separates from the surface of drum 15 under the action of -endless vacuum b~elt 50, the toner particles remain heId in image-wise fashion to paper 32. We have observed that .:
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COROTRON ~ should be placed over the arc of intimate contact between the paper and the drum for best results. If COROTRON
44 is not so located or if there are forces present preventing the paper 32 from forming an arc of intimate contact, the resultant image becomes fuzzy. There is only a light amount of pressure between paper 32 and the surface of drum 15 (i.e., merely enough to hold them adjacent each other). The pressure between paper 32 and drum 15 is essentially entirely generated by the electrostatic attraction generated by corona discharge device or COROTRON ~. Nevertheless transfer efficiency is surprisingly high and approaches 100% for toners with nontacky surface characteristics and low conductivity. The paper 32 is then removed from the surface of drum 15 by the action of the vacuum belt 50 in conjunction with the action of puffer 45 that forces it onto the surface of endless vacuum belt 50 driven by rollers 51 and 52. Endless vacuum belt 50 trans-ports paper 32 past infrared lamps 53, 54, and 55 which heat the thermoplastic resin encapsulating the ferromagnetic material in the toner particles causing them to melt and fuse to the paper 32. The decorated paper 32 is then fed into ~
; hopper 56. ~`
When multiple copies of the same document are to be made, a control meansr not shown, is so actuated that drum 15 is continuously rotated without activating demagne-tizer 60, vacuum box 61, magnetizer 1~ or lamp 21 because the electrostatic transfer of the toner particles does not ~ .
affect the magnetic state in the chromium dioxide layer on the surface of drum 15. Many copies can be printed from a ;
single exposure at speeds of up to 300 feet/minute. Over ~`
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10,000 copies from a slngle image have been demons-trated.
Toner particles which do not transfer, may themselves become electrostatically charged in the transfer zone adjacent to the COROTRON 44. Subsequently, these particles will pick up other particles electrostatically and ultimately transfer these to produce unwanted markings. To prevent this, a static eliminator 62 is used. Conveniently, this i.5 an AC
corona discharge bar.
When it is desired to prepare copies from a different document image eraser 60, which conveniently can be a DC magnetic head in the case of continuously coated film, is activated and the chromium dioxide is uniformly magnetized. Whatever toner particles may be remaining on the previously magnetized areas of chromium dioxide, are removed by vacuum box 61 which preferably acts in conjunc-tion with brushes. The chromium dioxide is then magnetiz d by magnetizer 19 to provide a periodic magnetic structure and the process described above repeated.
It is to be understood that substrates other than paper, such as cloth and dielectric films, can be used.
Fig. 2 shows an alternate form of printer using a magnetic printing head such as have been reviewed by W. H. Meiklejohn in A.I.P. Conference, Proc. (Pt. 2) 10, (1973) pages 1102 to 1114. In the example of Fig. 2 mag-~- netic printing head 71 is used to form the latent image on the magnetic surface of drum 72 which has the same structure as drum 15 described above. Magnetic printing head 71 is a ~
multitrack prlnting head such as have been developed for ~ -' ~ '; ', ~; , .
~ ~ -12-~ixed head per track discs. Preferably track density will be about 200 magnets per inch which is adequate to print with good resolution. Generally the multitrack write head will be activated by head drivers which can be activated ' by a read-only memory character generator. The read-only memory character generator can respond to an in~ormation storage device such as a magnetic tape which may be part of the printer or remote therefrom. Alternatively, a keyboard can activate the multitrack write head r wherein magnetic structuring is accomplished with the magnetic write head. Toner particles which do not trans~er may themselves become elec-trostatically charged in the transfer zone adjacent by the COROTRON 96. Subsequently, these particles will pick up other particles electrostatically and ultimately transfer these to produce unwanted markings~
To prevent this a static eliminator lQ7 is used. Conven- - -iently, this is an AC corona discharge bar. The thus ~` magnetized drum 72 is rotated counterclockwise past a toner slinger which comprises a trough 73 fitted with rapidly rotating rolls 74, and stationary bar 75. A vacuum knife 81 is used to remove whatever toner particles may have :, . .
adventitiously become attached to the demagnetized areas ~-of the chromium dioxide on the surface of drum 72.
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The paper 82 to which the toner pattern is to be applied is fed from roll;83 around idler rolls 84, 85, and ; 86 to feed rolls 87 and 88. Backing roll 91 cooperates with roll 92 e~ulpped with cutting edges 93~ Rolls 91 and 92 are ~; activated by means not shown to cut the paper 82 to the , :

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desired length. The paper is then fed by feed rolls into physical contact with the surface of drum 72. The paper 82 in contact with the surface of drum 72 is fed past corona discharge device or COROTRON 96 which electrostatically charges the back of the paper. Upon separation of the paper from the grounded drum an electrostatic forc~e sufficient to overcome the magnetic attraction between the previously uncharged toner particles and the latent magnetic image is generated, thereby causing the toner particles to transfer to the copy paper and be adhered thereto. The paper 82 is then removed from the surface of drum 72 by the action of puffer 97 that forces it onto the surface of endless vacuum belt 98 driven by rollers 99 and 100. Endless vacuum belt 98 transports paper 82 past infrared lamps 101, 102, and 103 which heat the thermoplastic resin encapsulating the ferromagnetic material in the toner particles causing them to melt and fuse to the paper 82. The decorated paper 82 is then fed into hopper 104. The drum can be continuously rotated to make a plurality of copies.
When it is desired to make a different print, image eraser 105 is actuated to erase the latent magnetic image and vacuum box 106 is used to remove any toner particles remaining on the old latent magnetic image.
The process can also be operated using either a thermal stylus or an electrical stylus to create the latent magnetic image, the former by conductive heating and the latter by electrical resistance heating of the imaging layer. Either stylus can demagnetize selected areas by '~

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73~;2 hea-ting previously magnetizecl mat.erial above the Curie point or it can magnetize selected areas thermoremanently by allowing the heated imaging material to cool through its Curie point in the presence of a magnetic field. A field of 20 to 200 Oe adjacent to the stylus has been found to be sufficient for thermoremanent magnetization, while a much stronger field of at least 800 Oe is necessary to magnetize unheated chromium dioxide sufficiently. It is recognized, of course, that imaging with electromagnetic or thermal transducers onto a continuous coating with its surface magnetized with a DC
magnet will require modulation consistent with establishing magnetic gradients for adequate toner attraction in magnetized image areas.

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Claims (18)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process comprising bringing a substrate capable of maintaining an electrostatic charge into superposed posit on and intimate contact with an image of uncharged toner particles which have an electrical conductivity of less than about 1 x 10-13 mho/cm magnetically adhered to an electrically conductive magnetic imaging member having a resistivity of less than about 1 x 10+9 ohm/cm which is adapted to dissipate an electric charge and applying an electric field at this position whereby said toner particles adhere to said substrate upon separation of said substrate from said electrically conductive magnetic imaging member.

2. The process of Claim 1 wherein the imaging member is grounded.

3. The process of Claim 2 wherein the electric field is generated by applying an electrostatic charge to the side of the substrate away from the toner particles.

4. The process of Claim 3 wherein the electric charge field is generated by spraying ions on the side of said substrate away from the toner particles.

5. The process of Claim 4 wherein the toner particles are fused to the substrate by heat.

6. The process of Claim 4 wherein the image of uncharged toner particles is held by magnetized acicular chromium dioxide.

7. A process comprising spatially periodically magnetizing a layer of acicular chromium dioxide particles in a binder which particles comprise from 40 to 85 weight percent of a layer from 0.001 to 0.015 mm thick which layer has a resistivity of less than about 1 x 10+9 ohm/cm adhered to a grounded electrically conductive layer, bringing said layer of acicular chromium dioxide into superposed position with a document containing thereon indicia which are to be copied, uniformly illuminating said document so that radiant energy is transmitted through said document in the areas of said document not covered by indicia whereby the acicular chromium dioxide in the areas where it is illuminated is heated to above its Curie point and demagnetized while the areas of acicular chromium dioxide covered by the indicia contained on said document are not heated above their Curie point, applying uncharged toner particles which have a low electrical conductivity of less than about 1 x 10-13 mho/cm and which comprise ferromagnetic material and a fixable material uniformly to the chromium dioxide layer whereby said toner particles adhere only to the magnetized areas of the chromium dioxide, maintaining said toner particles in the uncharged condition, superposing a dielectric substrate capable of maintaining an electrostatic charge in intimate contact with said acicular chromium dioxide layer, and applying an electric field while said substrate is positioned adjacent said layer of acicular chromium dioxide whereby said toner particles adhere to said substrate upon separation of said substrate from said layer of chromium dioxide.

8. The process of Claim 7 wherein the electric field is generated by applying an electrostatic charge to the side of the substrate away from the toner particles.

9. The process of Claim 8 wherein the electric charge is generated by spraying ions on the side of said substrate away from the toner particles.

10. The process of Claim 9 wherein the substrate is paper.

11. The process of Claim 9 wherein the substrate is a fabric.

12. The process of Claim 9 wherein the substrate is a dielectric film.

13. The process of Claim 9 wherein the toner particles are fused to the substrate by heat.

14. A magnetic printing apparatus for applying non-conductive ferromagnetic particles to selected areas of a substrate capable of maintaining an electrostatic charge comprising a movable electrically grounded magnetic imaging member containing selectively magnetized areas of ferromagnetic particles and non-magnetized background areas, which magnetic imaging member has a resistivity of less than about 1 x 10+9 ohm/cm in both background areas and selectively magnetized areas, drive means to advance said imaging member, means adapted to apply ferromagnetic toner particles in the uncharged condition to said magnetic imaging member, means to bring said substrate into superposed position with and in intimate contact with said magnetic imaging member, means for generating an electric charge on the side of the substrate away from the toner particles there said magnetic imaging member is super-positioned against said substrate and means for removing said substrate from said magnetic imaging member.

15. The apparatus of Claim 14 wherein said means for generating an electrical charge comprises means for spraying ions.

16. The apparatus of Claim 14 wherein magnetic means are provided to selectively magnetize the movable magnetic imaging member.

17. The apparatus of Claim 14 wherein electrical or thermal stylus means are provided to selectively magnetize at the points of contact of said stylus the movable magnetic imaging member.

18. The apparatus of Claim 14 wherein electrical or thermal stylus means are provided to selectively demagnetize at the points of contact of said stylus areas of the movable magnetic imaging member.