Recording With Donor Transfer of Magnetic Toner

1. Field of the Invention

The invention generally pertains to process and apparatus for recording information on a copy sheet by magnetic imaging procedures and is more particularly directed to developing a magnetic latent image with a magnetic toner.

2. Prior Art

The processes for providing latent images on a substrate or surface and then decorating them by a fine pigmented particulate (usually called a toner) to produce a visual image or one that is transferable to a copy sheet are well known in the art.

Generally, in the past, a number of development systems have been used to tone either an electrostatic latent image (zones having electric potential differences between image and non-image areas) or a magnetic latent image (zones having magnetic potential differences between image and non-image areas).

Normally, electrostatic and magnetic toners are not compatible. Electrostatic toners typically do not exhibit marked attraction to magnetic field forces because they are not ferromagnetic while magnetic toners are usually heavy and fairly conductive and are therefore not favored electrostatic charge carriers.

However, some ferromagnetic materials have been used in electrostatic development systems as carriers. These ferromagnetic carrier particles which are relatively large exhibit triboelectric attractions for smaller toner particles and are useful in transferring the toner to an electrostatic image. The toner particles are separated from the carrier by the stronger electrostatic forces on the latent images than the triboelectric forces between carrier and toner.

One example of a cascade development system employing ferromagnetic carrier to transfer a toner to an electrostatic latent image is U.S. Pat. No. 3,545,968 issued to Sato.

Another example of a development apparatus using a ferromagnetic carrier is U.S. Pat. No. 3,437,074 issued to Hagopian et.al. This reference describes a "magnetic brush" development system where the ferromagnetic carrier particles are formed into streamers or bristles and form a brush like mass.

A donor belt utilizing ferromagnetic carrier for toner tranfer in an electrostatic apparatus is disclosed in a U.S. Pat. No. 3,741,790 issued to Wu.

Such development systems rely on the electrostatic forces generated by potential differences in image areas to be stronger than the forces holding the toner particles to the carriers. The electrostatic forces generated by an electrostatic latent image are in fact much stronger than those which can be produced from a magnetic latent image and thus other methods had to be initiated to tone these magnetically.

This has led to the development of using ferromagnetic particles in some form that are not just carriers but actual toners for developing a magnetic latent image. As in the electrostatic development area, there have been a plurality of methods proposed for the decoration of latent magnetic images by magnetic toner.

A cascade development system for magnetic images is illustrated in U.S. Pat. No. 3,250,636 issued to Wilferth. In this reference, magnetic particulate is poured or flooded over a surface containing a magnetic latent image. The toner adheres to the image areas and excess toner flows by gravity from the surface into a reservoir.

Another magnetic toner development system, in which the toner is caused to impinge on a magnetic latent image by flicking the toner from the bristle ends of a wire brush, is illustrated in U.S. Pat. No. 3,825,936 issued to Ott et.al.

Immersion techniques are also known in the art where a tape has a recorded image thereon immersed in a reservoir within a volatile fluid medium. Upon circulation of the fluid medium around the image, toner is attracted to magnetized areas of the image. An example disclosing such a technique is found in U.S. Pat. No. 3,740,265 issued to Springer.

All the aforementioned latent magnetic imaging development apparatus have the problem of contacting toner not only within imaged areas but also within non-imaged areas and thereby producing substantial background. (Toner adhered to non-image areas.)

This is detrimental to a magnetic imaging process as the forces holding the magnetizable particles to the image areas are not as great as those found in electrostatic system and hence background is more difficult to clean from an area after the toner deposition thereon.

A non-contact magnetic imaging system is illustrated in U.S. Pat. No. 3,849,161 issued to Klaenhammer. The system provides alternate magnetizations for image areas in relation to non-image areas. However, such a system is devoid of a process to produce the resolution needed by modern imaging applications in the commercial sector.

Therefore, it would be advantageous to have a clean development system that would also be capable of uniform image development and high resolution.

Accordingly, it is an object of the invention to provide an improved process and apparatus for toning or decorating a magnetic latent image with ferromagnetic particulate.

It is another object of the invention to provide a contactless toning system where only the image areas have particulate adhered thereto.

It is still another object of this invention to reduce background in the development of a latent magnetic image for a magnetic imaging system with high resolution capabilities.

It is a further object of the invention to provide a uniform supply of toner to a latent magnetic image to produce even image development.

These objects and others are accomplished in accordance with the invention by providing a donor surface which is magnetizable in an alternating pattern of magnetizations of a specified spatial wavelength. The donor surface is toned and then, with magnetic particulate adhering to the microfield pattern, transported into non-contacting proximity with a magnetizable copy surface. The copy surface has a latent magnetic image recorded thereon which has a stronger magnetic force than that of the doner surface. The toner will be transferred by the differential in magnetic forces due to the fields produced between the copy and donor surfaces. Only the image areas of the copy surface will be toned as there are no magnetic force gradients in the non-image areas.

The toner transfer effect is magnified if the copy surface has a higher coercivity than the donor surface according to one aspect of the invention. Another feature of the invention provides for the enhancement of the transfer process by the erasure or neutralization of the doner surface microfields prior to the toner transfer with a remagnetization of the donor surface subsequent to transfer.

These and other objects, features and aspects of the invention will become clearer and more fully apparent from the following detailed description when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic system diagram of a magnetic imaging apparatus employing a magnetic doner development process and apparatus in accordance with the present invention;

FIGS. 2A and B are representative pictorials of sections of the copy web and donor web of the apparatus of FIG. 1 illustrating image domain magnetizations and pre-recorded microfields, respectively; and

FIGS. 3A, B and C are alternative embodiments of the apparatus and method for preforming a toner transfer from the donor web to the copy web of the imaging system illustrated in FIG. 1.

With reference now to FIG. 1 there is shown a magnetic imaging system incorporating the present invention. The magnetic imaging system includes a recording station 10 which produces a magnetic latent image on a copy web 2 in some manner.

There are a number of methods known in the art for accomplishing this process. Some examples are direct recording with a magnetic recording head, theremoremanent or anhysteretic copying from a recorded master tape, Curie point writing or erasure with masks or a laser, etc. The magnetic latent image that is formed by one of the above-described processes will be an alternating pattern of magnetizations in imagewise configuration. The non-imaged areas are not polarized in any magnetization direction and the magnetic material in them will produce no magnetic field gradients. The magnetic forces from the fringing fields of the magnetization pattern of the image are thus substantially the only attractive forces on the copy web surface.

The copy web 2 is generally a magnetic tape with a magnetizable surface area that has a coercivity and paramagnetic state that allow it to be magnetized in a magnetic image configuration as mentioned before.

Preferred choices for the copy web 2 would be CrO.sub.2 tape sold under the tradename Crolyn by the DuPont corporation (Br = 1600 Gauss, Hc = 5400e, squareness = 0.9) or .gamma.Fe.sub.2 O.sub.3 tape sold as 3M777 by the 3M Corporation (Br = 1400 Gauss, Hc - 312 Oe, Squareness - 0.8).

The copy web 2 is entrained about copy web rollers 4, 6 and 8 in an endless belt fashion where at least one of the copy web rollers 4, 6 and 8 may be driven by conventional motors or other means (not illustrated). Once the copy web has a latent magnetic image produced thereon the image is transferred by the rotation of the rollers into a toner transfer area 12 where it is decorated with a ferromagnetic particulate toner 24 from a reservoir 26. The toners that may be used in the practice of the invention are ones loaded with soft magnetic material or those loaded with unpoled hard magnetic materials. Toners such as these are described in U.S. Pat. Nos. 3,639,245; 2,932,278; 3,052,564 and 3,250,636 the disclosure of which is herein incorporated by reference.

The magnetic toner 24 is transported to the toner transfer area 12 by means of a doner web 16 entrained about three doner web rollers 18, 20 and 22. The doner web 16 forms an endless belt around the doner web rollers 16, 18 and 20. At least one of the doner web rollers drives the doner web 16 and is able to transport the magnetic toner into the toner transfer area 12.

The doner web 16 is generally a magnetic tape with a magnetizable surface area that has a coercivity and paramagnetic state that allow it to be magnetized in a microfield pattern of alternating magnetization. Preferred choices are those that have been mentioned above for the copy web 2 and include Crolyn and 3M777. Additionally, one could use a .gamma.Fe.sub.2 O.sub.3 tape sold as 3M206 by the 3M Corporation (Br = 1303 Gauss, Hc = 3320e, squareness = 0.8). This tape has a thicker magnetizable layer (14.mu.) than the others mentioned and can be used when longer doner web wavelengths are chosen.

The transportation of toner takes place from the reservoir 26 to the transfer area 12 because the doner web has a magnetizable surface containing a magnetic pattern which attracts the toner while it is driven over the doner web roller 22.

The toner transfer mechanism for the donor web 16 illustrated in FIG. 1 is an immersion technique where the doner web is pulled into a pile or bath of magnetic toner 12 on one side of donor web roller 22 and out of the toner on the other. This type of toner development is acceptable for the doner web 16 as what is needed is a uniform coverage of the entire doner web.

Other techniques for toning magnetic patterns, such as those discussed in the prior art sections, could be used for this step of the process as there is no background problem at this point in the process and the necessity is to transport as much toner in as uniform fashion as possible.

The magnetization pattern of the doner web 16 causes the toner particulate 24 to adhere thereto and to remain on the web until it is transported into the transfer area 12. The magnetization pattern of the doner web is similar to that of the imaged areas of the copy web and comprises alternating magnetizations of a specific spatial wavelength and frequency. However, the doner web magnetizations are of a different wavelength to produce a weaker magnetic force than that of the copy web 2.

In a preferred form the alternations are formed widthwise across the doner web and parallel to the magnetization patterns in the copy web, although this is not a necessity for the operability of the invention. It should be apparent that the alternating magnetic microfields in the doner web may, as was explained previously in relation to the copy web pattern, be formed in numerous ways.

The numerous microfields in the doner web provide fringing fields for the toner 24 to adhere to and produce a substantially uniform toner coverage on the doner web 16. This is an important aspect of the invention as the doner web 16 constantly transports an endless supply of toner in even quantities into the toner transfer area 12. This allows uniform development of a latent magnetic image without under toning or over toning the copy web 2.

Once in the transfer area 12, the stronger magnetic forces of the copy web 2 transfers the toner particulate in imagewise configuration onto the copy web 2 while excess particulate remains on the doner web 16, in non-imagewise configuration. As erase head 14 is used to enhance this transfer process by demagnetizing the doner web 16 as it passes into the transfer area 12 before the toner transfer takes place. Alternatively, heating means 40 can be used to demagnetize doner web 16 by raising the temperature of web 16 above its Curie point. When the erase head 14 is used, a rewrite head 28 is provided to remagnetize the doner web 16 before it enters the toner reservoir 26.

As the latent magnetic image having the toner 24 adhered thereto moves out of the transfer area 12 it comes into contact with a copy sheet 23 which is held against the copy web 2 by a pair of pressure rollers 25 and 27. The magnetic toner transfers in imagewise configuration from the copy web 2 to the copy sheet 23 under the influence of this pressure. The copy sheet is moved from the supply reel 31 to a takeup reel 29 in a continuous fashion to provide either multiple copies or individual images as the copy web 2 continues its endless path.

The copy sheet 23 may then have the toner 24 fixed to this surface in some manner, many of which are known in the art. Subsequently, the copy web 2 enters the recording station 10 once again where it can be erased and rewritten with another magnetic image or pass through to pick up or renew the toner supply from the toner transfer area 12 and continue on to the copy sheet 23 again for multiple copy capabilities.

While the imaging processes and apparatus illustrated in FIG. 1 are preferred for practicing the invention numerous adaptations are available. For example, both the copy web 2 and the doner web 16 could be cylindrical drums with a magnetic surface or a non-magnetic drum with an overlayer of magnetizable tape. Combinations are possible where the copy web 2 can be a drum and a donor web is used or where a copy web 2 is provided, a donor drum replaces the donor web for the transfer. The required structure is that a magnetizable surface capable of holding the latent magnetic image pattern and a magnetizable surface capable of holding the donor pattern be provided.

A portion of copy web 2 is illustrated in FIG. 2A where an image area 30 is shown with a magnetic recording pattern 32. The recording pattern 32 is actually a series of alternating magnetizations of a certain spatial wavelength and frequency. Where the magnetization sections oppose, fringing fields will be developed to attract the magnetic particulate 24 thereto. The spatial frequency for the image magnetization reversals would be typically on the order of 25 - 100.mu.. Likewise, in FIG. 2B there is shown a portion of the donor web 16 having a second magnetization pattern 34 recorded thereon. It is seen that the magnetization pattern 34 is comprised similarly of alternating domains having a certain spatial wavelength and frequency to produce fringing fields for the attraction of toner where the domains are in opposite. It should be particularly noted that the donor web 16 magnetization pattern 34 is of a different wavelength than the magnetization pattern 32 of the copy web 2. Also it is preferred that the copy web 2 have a different coercivity and Curie Point temperature than the donor web 16 as will be more fully discussed herein below.

With reference now to FIG. 3 where is shown an enlargement of the toner transfer area 12 including the copy web 2 and the donor web 16. The alternations in the magnetization domains of the pattern 32 are shown as the image area 30 and illustrate the field lines producing the fringing fields and the magnetic force lines that will cause the toner transfer. It is seen that the spatial wavelength of the image pattern 32 is different than that of magnetization pattern 34 to represent the greater attraction power. The field on the copy web is constructed to produce a differential in force to accelerate the particles adhering to the donor web onto the copy web in the image areas. Preferably, the copy web 2 as mentioned before has a greater coercivity than the donor web 16 to insure this effect.

In the uniform development of the copy web 2, the spacing separating the copy web 2 and the donor web 16 are important to the transfer process. For different spacings, the wavelength of the donor web will change as it will for different wavelengths of the image recorded on copy web 2. Table 1 is illustrative of the donor wavelength that must be recorded on the donor web 16 in relation to the copy web 2 spacing and image wavelength.

It is understood that because there are no magnetization domains recorded in the non-imaged areas of the copy tape there are no force fields to accelerate the particles to the non-imaged areas. Thus, this non-contacting toning system solves many problems found in the art. A low background is maintained, therefore, by not depositing toner into these areas and consequently a much simpler clean up process than usual is permissible for the present system.

Also, the back of the copy tape 2 remains untoned so cleaning there is also unnecessary. Further, since the recorded pattern is weaker than that on the copy web 26 an amount of toner 24 corresponding to full development of the magnetic latent image may be loaded on it and transferred nearly uniformly onto the copy web 2.

For further enhancing the toner transfer to the copy web 2, a transfer head 14 may be used to erase the donor web microfields or to provide a neutralizing field opposite to the microfields recorded therein. The transfer head 14 then neutralizes the forces holding the toner 24 to the donor web 16 just prior to the transfer to the copy web 2. This assures there will be a larger net force produced by the gradient of the magnetization patterns 32 recorded on the copy web 2. The larger coercive force of the copy web 2 prevents erasure of the latent magnetic image during this process as only a magnetic field less than or equal to the coercivity of the donor web 16 is required by neutralization.

It should be realized that the erasures of the microfields in the doner web could also be accomplished by heating the donor web above its Curie point temperature thereby erasing the magnetization patterns 34 as does the transfer head 14. In such a manner though the Curie temperature of the copy web 2 should be above the Curie temperature of the donor web to prevent erasure of the imaged area. In both such cases a re-recording or write head 28 is positioned in proximity to the tape to allow for the re-recording of the microfields on the magnetizable surface of the donor web 16 in the aforementioned pattern.

Turning now to FIG. 3B another embodiment of the present invention is illustrated where the copy web 2 and the donor web 16 perform their ferromagnetic transfer in the presence of a coil 36. The coil again acts to selectively erase or neutralize the microfields of the donor web 16 while not effecting the field gradients of the copy web 2. It is understood that either an a.c. or a d.c. current may be used to provide a magnetization in the correct direction to perform this neutralization. If an a.c. field is used, the reversal of the fields may be helpful in that they to some extent strobe the particles in the transfer back and forth between the two surfaces.

Another alternative embodiment of the imaging transfer system is illustrated in FIG. 3C where the copy drum 41 and the donor web 16 are brought into proximity in the transfer area 12. In this embodiment it is seen that the copy drum 41 is made of the non-magnetizable surface material 43 and that a magnetic or magnetizable 42 material is set thereon in a relief configuration forming an image 38. The image 38 in relief has been recorded with the first spatial wavelength of the image magnetization pattern and therefore will perform a similar transfer when brought into proximity with the toner laden microfields of the donor web 16. The toner 24 transfers into the field gradients formed by the magnetic domain opposites of the relief image 38. Again the effect can be enhanced by the transfer head 14 and the donor web 16 may be re-recorded by the rewrite head 28.

While the invention has been described in detail in relation to a number of preferred embodiments those skilled in the art will understand that other changes in form and detail may be made therein without departing from the spirit and scope of the invention wherein all such changes obvious to one skilled in the art are encompassed in the following claims.