IMPROVEMENTS IN MAGNETOGRAPHIC RECORDING

IMPROVEMENTS IN MAGNETOGRAPHIC RECORDING

Background and Summary of the Invention The present invention relates to the creation or formation of latent images on a uniquely constructed magnetic medium through specialized dot or pixal (zones) matrix recording techniques and the development of such images with toner particles meeting certain specifications, and more particularly to such recording in high-print-quality magnetographics systems in which the toned magnetic image is precharged and thereafter transferred to a permanent medium, such as paper, with the aid of an electrostatic transfer technique. Such a system is described, for example, in U. S. Patent Application Serial No. 228,526, filed January 26, 1981, the subject matter of which, in so far as it is pertinent to the present invention, is incorporated herein by reference.

In particulate magnetic recording material where crystals or crystalites of ferromagnetic pigment are dispersed in a suitable binder and coated on a flexible substrate, it is customary to orient the particles in a preferred direction during the coating process to optimize recording properties. This is because the particles exhibit magnetic anisotrophy i.e. their coercivity and remanent induction vary with the direction of the applied field with respect to their crystalline axes. The two most commonly used magnetic pigments, iron oxide and chromium dioxide, are acicular (needle like) and exhibit highest remanent induction in the direction of the long axis of the particle. Longitudinal orientation relates to the long axes of the magnetizable crystals or particles (e.g. CrO₂ or Fe₂O₃) being arranged substantially parallel to the direction of intended movement of the medium past a recording or writing head, which direction usually is considered the "length" direction of the magnetic medium.

Longitudinal orientation is utilized predominantly in the audio, digital and helical video magnetic recording applications and constitutes the great bulk of the magnetic tape production.

Skewed orientation normally relates to the elongated crystals or particles of magnetizable material being arranged physically such that their long axes are, as in the case of longitudinal orientation, statistically parallel to one another, but at an acute angle (e.g. as much as 70°) with respect to the direction of intended movement of the tape past the record head. Skewed orientation is presently used with regard to the professional video tape recording industry. While it is noted that for quadraplex video recording (this technology is believed to be the only commercially available application of near-transverse recording) a preferred orientation would be near 90°, substantially lesser skewed angles are actually utilized because of the difficulties of orienting wide webs of magnetic material perpendicular to the machine direction during manufacture.

Perpendicular or vertical orientation refers to the magnetizable particles being physically oriented such that their long axes are perpendicular to the direction of intended tape movement but are parallel to the so-called thickness dimension of the tape. There is believed to be no presently commercially available arrangement utilizing perpendicular orientation, although commercial entries into the marketplace have been attempted.

There are inherent disadvantages to each of the above-recited magnetic particle orientations which are particularly noteworthy in applications involving toning of latent magnetic images, especially where high-quality imaging is desired. In the case of longitudinal orientation, certain categories of horizontal lines are not considered practically possible for a high-quality printing product output, which point will become more apparent hereinafter. Moreover, the potential adverse impact on latent magnetic images from the longitudinal magnetic fields associated to conventional toner developer arrangements (e.g. magnetic roll developers) dictates that such developer magnetic forces be kept low, thus limiting the developer's ability to transport toner to, and, therefore, adequately tone, the latent magnetic images.

This latter drawback also applies to a substantial degree with respect to vertically or perpendicularly oriented tapes, in that conventional developers also provide significant vertical components of magnetic field that could materially affect a recorded magnetic latent image. Also, several of the most promising magnetic materials from which magnetic tapes may be made are unsuitable from a properties standpoint for use in connection with so-called vertical recording. For example, such materials as chromium dioxide (CrO₂) and iron oxide (Fe₂O₃) having a thickness of say .3 mils may be said to possess inappropriate magnetic properties for vertical recording. It is known in vertical recording, for example, that only tape magnetic materials having extremely high coercivity (and which are relatively quite expensive) would be truly useful and yield satisfactory results, whereas materials such as those identified above have insufficient coercivity for the given geometry of a vertically oriented magnetic recording medium. This is so principally because in verticial orientation the geometry of the recording medium is such that the magnetic length-to-width ratio (Actually "length" here corresponds to the thickness dimension of the magnetic layer and "width" to the extent of the magnetized region.) of the magnetic medium is less than one, i.e. the demagnetization effects are indeed substantial. Without a very high coercivity material the magnetic layer, as a magnet, becomes weak relatively easily, i.e. with such a geometry the demagnetizing forces are relatively high. Thus vertical orientation could be considered unavailable to those who desire to make use of magnetic materials such as CrO₂ which many consider to have perhaps the best B/H (magnetic remanence and coercivity) and other characteristics of the commercially available materials. In the case of recording arrangements making use of skewed crystal orientation, recording of the latent image itself, i.e. the recording magnetic field, may or may not be aligned with the direction of the long axes of the magnetic particles. If the recording magnetic fields are actually angled with respect to the magnetic particles (e.g. in the longitudinal direction), some of the advantageous properties of the material may be lost, e.g. the so-called "squareness" property. To compensate for this, one would have to provide input recording levels that are higher to have the magnetic material react as fully as possible in a skewed system (i.e. to provide a sufficient magnetic component in the chosen skewed direction). This requires, in turn, substantially greater current levels (typically greater than fifteen amperes) in generating the latent magnetic images, which translate into a growing series of prospective problems such as greater heat accumulation in the record head (and the need for its dissipation) and more complex and expensive recording circuitry and record head design. A fourth basic system is possible, so-called "transverse" orientation, which relates to the magnetizable particles being oriented, as in the case of vertical orientation, perpendicular to the intended direction of tape movement, but parallel instead to the width dimension of the tape (and thus perpendicular to the thickness dimension). Such a system avoids the horizontal-line writing limitations of longitudinally oriented systems, and is not adversely impacted by conventional developer-station magnetic field considerations as are the horizontally and vertically oriented systems. Moreover, a transverse system is able to take advantage of the full magnetic properties and characteristics of the recording medium, and at substantially lower levels of recording current, in contrast to certain skewed systems. It is thus an object of this invention to avoid the limitations and drawbacks herein mentioned in connection with longitudinal, vertical and skewed systerns. The predominant s ignif icance of ordering the crys tals or part icles of magnetic materi al in a certain direction or orientation is that normally the magnetic part icles have their mos t des irable ( e . g . best level ) magnet ic properites such as coercivity and remanence in the direct ion of their long axes . Randomizing the direct ion of the major axes of the part icles would minimize the material ' s ef fect ivenes s as a magnetic substance. Thus it is that one would customarily desire to have the magnetic particles aligned in the direction in which the "writing" magnetic fields will appear for magnetizing preselected zones of the recording medium. That is, the best use of such a magnetic material having good magnetic squareness characteristics is to create the magnetic images in the recording medium by causing the magnetic zones of the recording medium comprising the latent recorded image to be oriented in the same direction as the physical direction of the long axes of the magnetic particles comprising the recording medium. It is another object of this invention to make full use of the magnetic characteristics of the recording medium material.

In the commercial analog audio and video magnetic tape recording arrangements, it is generally the case that recording is done on a medium that is initially erased or demagnetized. Digital applications as well do not require and thus do not employ premagnetization. It is an object of this invention to provide magnetic latent imaging and toning capable of implementation in connection with high-quality (i.e. so-called letter quality) printing, utilizing premagnetization.

A particularly desirable recording head configuration for magnetographic printing comprises a multiplicity of closely spaced write zones in a linear array. In one configuration the array is the height of a printed line. A magnetic latent image is created by current pulses applied to the elements in the array, as the recording material passes in close proximity, in a predetermined sequence to produce the desired pattern. If the field that is produced in each of the write zones is of the same polarity

(utilizing, as example, the recording head depicted in U.S. Patent 4,025,927 to Nelson), it is highly advantageous to have the recording material premagnetized in the opposite direction. When this is done each recorded pixal creates a pair of magnetic poles on each side of the write zone. This produces a magnetic field pattern with approximately twice the intensity of that which would result from a pixal recorded on a medium having an initially erased state, where only one pole would be produced on each side. The excursion of the magnetization from say +M to -M at these pole pairs defines so-called magnetic boundary regions. unfortunately those magnetic transition regions themselves amount to the effective generation of magnetic fields that tend to reverse the pixal, i.e. reverse the magnetization of the pixal back to the preorientation state or direction. Such fields, then, have to be considered additive to the potentially disruptive magnetic fields associated with the toner/developer apparatus and thus, in the case of longitudinal recording especially, they cannot be ignored in determining the maximum allowable magnetic field of the developer station that can avoid disturbance of the recorded pixals while toning or developing the latent image. The developer field in such systems thus has to remain well below the actual coercivity of the pixal elements themselves since the external magnetic fields generated by the developer are directed in major part longitudinally. It has been determined that with premagnetization of the medium, a field intensity of 100-150 Orsteads from the developer can be high enough to alter the boundary configurations of the pixals and, therefore, begin the reversal of the magnetic orientation of the pixal areas. On the other hand, 100-150 Orsteads is the upper limit range generally desired for developer levels when utilizing a magnetic medium having coercivity on the order of 500-600 Orsteads such as CrO₂.

Vertical recording is unable to escape this developer phenomenon of pixal disturbance in connection with premagnetized tapes, since conventional developers also provide a significant portion of their external magnetic fields in the vertical direction, generally as the tape approaches the developer and also as it leaves the developer station. Thus in these instances one must again be careful to take these potentially disruptive magnetic forces into consideration when determining the maximum allowable developer field. The result is that in attempting to keep the developer field sufficiently below the effective coercivity level of the pixal areas, this field is often inadequate to satisfactorily tone the latent magnetic images. This can especially be true in instances where bold print is desired. The magnitude of this problem does not arise in the case of transverse recording, since conventional developers do not provide a "transverse" magnetic component of magnetic field. Thus substantially greater developer fields may be used, thereby ensuring full toning of the magnetic latent images. It is thus another principal objective of this invention to provide transverse recording of latent magnetic images comprised of a matrix of individual pixals, in which the recording medium has its magnetization transversely preoriented and the magnetic particles thereof also transversely aligned.

In magnetographic printing an objective is to maximize toner attraction forces and in high-quality imaging applications to maximize resolution. Toner attractive forces are dependent on the magnetic pole intensity and pole spacing created on the recording medium. At long pole spacing (i.e. where the lengths between the poles of the created magnets are such that there is generated on the recording medium less than 100 pole pairs per inch) the forces at the poles can be very high, but the toner attracting force in the center region between the poles is significantly diminished over that where pole spacing is shorter (e.g. greater than 200 pole pairs per inch). As pole spacing is further shortened, however, self-demagnetizing effects eventually lower pole strength and toner attraction is again diminished. For typical recording materials such as CrO₂ the optimum spatial frequency has been found to be between 200 and 100 cycles/in. (where a cycle is a saturated zone having equal and opposite magnetization associated thereto). For iron oxide, the optimum upper frequency has been found to be somewhat lower because of its lower coercivity. Moreover, toner attraction has been found to be maximized for symmetric saturated recordings.

In horizontal recording, if pixals are recorded by locally reversing a premagnetized medium over zones of two mils at 4-mil increments for instance, line segments would have to begin and end at multiples of 4-mil increments (thus limiting resolution) or there would be asymmetric recordings and therefore diminished toner attraction.

In transverse recording, on the other hand, horizontal line segments can be overlapped to any degree desired, since the recording spatial frequency is determined solely by the element array spacing. Thus, much finer detail can be imaged in transverse premagnetized recording than in longitudinal recording. It has been found, for example, that recordings with transverse recording element spacing of 250 per inch have been substantially improved when horizontal pixal spatial frequencies were increased (pixals overlapped) from 250 per inch to 500 per inch, even when the pixal size is the same in both directions (4 mil by 4 mil).

In general terms, a particle of magnetic toner is drawn or attracted to the latent image of the recording medium on the basis of the force of attraction the latter provides, i.e. the magnetic force on the toner particle, FMT, which is directly proportional to the product of the H field associated to the pixal region and the gradients existing there or associated therewith as a function of distance (dH/dx). Thus, F_MT H • dH/dx.

The ratio of this force F_MT to the force attributed to the "magnetic noise" associated to the recording medium (F_jj) effectively defines the signal-to-noise ratio for the system, i.e. F_Mτ/F_N. It is desirable, therefore, to maximize F_MT relative to F_N. This could involve in part maximizing the product of H and dH/dx. This in turn can involve optimizing either or both the H field associated to the recording medium and in particular to the pixal zones or areas and the gradients dH/dx associated thereto. The higher the ratio of F_Mτ/F_N the higher the optical density of the image and the cleaner the background, relatively speaking. Also, one is able to maximize edge acuity (sharpness). These factors constitute the essential criteria of high-quality printing.

By the recording technique of premagnetization and imaging by locally reversing the direction of magnetization in the pixal regions or areas, sharp boundary regions are created, as to which the gradient component or factor dB/dx in the above is dramatically increased. This is because the directionality of the magnetization of the pixal opposes the direction of the magnetization of the surrounding medium, with the result that over a narrow distance (say 3/4 mil) the resultant magnetiza tion varies considerably, from a maximum within the pixal region to a null at the center of the boundary or transition region, to a maximum in the opposite direction outside the pixal region. Thus, the precise pixal region is defined by this narrow boundary region over which the gradient dB/dx is guite high relative to adjacent regions of the magnetic medium.

It will be appreciated that in a system such as that herein depicted, one realistically has to consider the properties of both the magnetic medium (tape) and the magnetic toner in combination. The tape and toner must, for example, be compatible enough to enable the mutually attracting forces to be predictably (within a high degree of accuracy) retained as to effectiveness for the time period desired, i.e. from the moment of toning of the latent image to the time of actual toned image transfer. Further, in a system where the toner is precharged for purposes of effecting desirable conditions for its transfer from the magnetic medium to a sheet of paper (e.g. lower electrostatic transfer voltage, well below any voltage breakdown level, with nevertheless high toner transfer efficiency and uniformity) the toner must, first of all, be comprised of dielectric material. The precharging requirement, on the other hand, dictates that the toner material be such that all of the toner particles should be charged and not just the top particle layer or two of the toned image on the magnetic medium. Nevertheless, precharging must occur after the latent magnetic image is toned in order to avoid a host of additional problems. Thus, in precharging the toner particles while they reside on the magnetic medium in the arrangement manifesting the latent magnetic image, one must avoid any disruption of the toner arrangement or degradation of the latent magnetic image. Also, charge accumulation must be restricted to the toned areas, as otherwise the tape medium will quickly become contaminated with stray toner and other charged particles. Were the tape permitted to otherwise accumulate charge on its surface, this would tend to cause adherance of the tape to other surfaces it contacts, such as the paper at the transfer station, which in turn could disrupt the transfer process.

These factors thus require inter alia that as to the precharging requirement, the magnetic tape medium should be conductive, at least as to or through its magnetic layer, to avoid unwanted accumulation of charge in and on the magnetic layer of the tape. This factor also allows charges to be conducted within the magnetic layer to provide the needed "image charges" for those charges present on the toner particles.

It is thus yet a further principal objective of this invention to provide a magnetographic printing system having electrostatic transfer of an image comprised of precharged magnetic toner particles attracted to and arranged according to individual magnetic zones and groups of zones, in which the tape/toner combination of materials ensures a high magnetic signal-to-noiεe ratio and high-quality printing capability. While it is desirable to provide conductivity to the magnetic layer for the reasons stated, attaining sufficient conductivity must not be permitted at the expense of the loss of a sufficient concentration of magnetic pigment. It has been found, for example, that an unacceptable dilution in the magnetic pigment can occur due to the inclusion of enough conductive particle additive (e.g. carbon black). Also, it is not recommended to have significiant current flowing in a magnetic layer having a substantial resistivity (i.e. 10⁷ ohms/sq.) for the reason that a significant potential drop would be developed along the tape whereby the surface of the tape would have a potential which could interfere with the transfer potential in the transfer operation. Thus, for purposes of the transfer operation in a system as described e.g. in the above-identified co-pending patent application, the tape should be maintained at ground potential, and provide a discharge current path to avoid unwanted build-up of charges on the front tape surface due to the toner precharging step (i.e. the tape assuming the potential of the precharger arrangement). It would, therefore, be desirable for the magnetic layer of the recording medium to have substantial conductivity in the vertical direction, i.e. through its thickness, but significantly lower conductivity in the plane of the magnetic layer, and be wedded to a highly conductive back-layer, and such is another object of this invention. The thickness dimension of the magnetic layer, as contrasted with its planar geometry, thus becomes particularly important since conductivity of a material in a given direction is a function of that material's geometry. On the other hand, to achieve this one-dimensional or directional conductivity, by making the layer sufficiently thin relative to its planar geometry, one cannot ignore the potentially conflicting criterion of having the magnetic layer sufficiently thick to take full advantage of the recording magnetic field strength. That is, for optimum efficiency of operation, for a given strength of recording field, the full thickness of the magnetic layer of the tape for each pixal should be adequately influenced or magnetically aligned. It is very important, too, for the tape medium to be durable for extended operating periods, when even operating under tension, such as could be the case with an endless tape being driven about a loop which includes the various operating stations of premagnetization, recording, developing, precharging, transfer and cleaning. This aspect of the tape properties may be satisfied by suitably securing the magnetic layer and the conductive layer associated therewith to a selected substrate having the desired properties leading to durability. Such a substrate may, for example, be Mylar. With the use of such a material, however, its insulative properties provide good opportunity for charge accumulation on the back of the tape due, for example, to triboelectrification. A conductive backing would thus be appropriate to avoid charge accumulations that could disrupt the toner particle arrangement on the latent image and/or the transfer thereof to the permanent paper medium. Alternatively, the substrate could be composed entirely of a conductive material having the desired physical properties. It is yet a further principal object of this invention to provide a magnetic medium (tape) structure which satisfactorily addresses and balances all of the above-discussed factors and more relating to the tape medium.

The combination of the within-discussed magnetic medium with magnetically attractable, chargeable, single-component toner as provided in accordance with this invention permits the achievement of crisp character imaging necessary for high quality (letter quality) printing. In addition, this combination is an indispensible part of the process and system which assures that the steps subsequent to recording, i.e. toning the latent magnetic images formed on the tape and comprised of matrices of pix als, precharging the toner images, and electrically transferring the toned images to a permanent medium, will not materially degrade the recorded image.

According to the broader aspects of this invention, there is provided in various inventive combinations the following. Images are created on a magnetizable medium comprised of a suitable substrate having a conductive surface and a magnetizable layer of e.g. chromium dioxide or iron oxide particles suspended in a carrier, wherein this layer is conductive through its thickness and is adjacent to the conductive surface of the substrate. The magnetic particles are transversely aligned (relative to the direction of movement of the medium past a recording head) and the medium premagnetized in the transverse direction. Latent magnetic images are created on the medium via a multi-element recording head which reverses the magnetization locally in pixal units selectively arranged in matrices. The direction of magnetization associated to each pixal is in all cases the same. The magnetization reversal effects boundary regions around the pixals having high spatial magnetic gradients. The latent images are developed with electrically chargeable, magnetically attractable toner and the toned images subjected to a charging process. The charged toner is electrostatically transferred to a permanent medium with the aid of the aforementioned conductive surface of the magnetic medium.

Brief Description of the Drawings The above-mentioned and other objectives and features will become better understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a combined schematic and block diagram of a system incorporating the present invention; Fig. 2 is an enlarged cross-sectional side or end view of a portion of a magnetizable medium useful in a system according to Fig. 1; Fig. 3 graphically illustrates certain magnetic characteristics of the magnetic medium of Fig. 2;

Fig. 4 graphically illustrates the relationship of magnetic remanence to concentration of magnetic pigment in the magnetizable layer of a magnetic medium such as that in Fig. 2;

Fig. 5 diagrammatically illustrates a process of alignment of the magnetic crystal pigment within the magnetic layer in the formation of a medium such as that depicted in Fig. 2;

Fig. 6 illustrates in an enlarged perspective view the physical and magnetic relationships of a magnetic tape medium such as that depicted in Fig. 2 in a premagnetized condition to several write elements of a magnetic recording head; Fig. 7A is an enlarged cross-sectional end view of the arrangement of Fig. 6 illustrating the creation of adjacent pixals in a premagnetized medium;

Fig. 7B graphically illustrates the B_τ characteristics of the magnetic medium of Fig. 7A just beneath the surface thereof, as a function of distance; Fig. 7C is a cross-sectional end view illustrating the magnetic field pattern emerging from the magnetic medium as a result of the recording condition depicted in Figs. 7A and 7B, prior to the toning step; Fig. 7D graphically illustrates by comparison the relative toner-attracting magnetic forces as a function of distance provided by identical recordings (images) on premagnetized and non-premagnetized portions of the same magnetic medium depicted respectively in Figs. 7C and 9C;

Fig. 8 graphically illustrates by way of the B/H curve of the magnetic medium of Fig. 7A, the field line relationships derived from the recording of a pixal;

Figs. 9A - 9C are respectively the same views and illustrations of Figs. 7A - 7C except for the case of a magnetic medium which is non-premagnetized; and

Fig. 10 is a schematic diagram illustrating that pixals can be overlapped in the direction of tape motion to provide finer resolution than the pixal quantitization.

Description of Preferred Embodiments In Fig. 1, there is shown a magnetographic printing apparatus or system 11 responsive, for example, to applied digital data, for recording data images on an endless magnetizable medium or web such as a magnetic tape or band 13, for toning or developing the image and for transferring the toned image to paper 15, in producing high-quality printed output at relatively high-speed operation. The apparatus includes a magnetic recording head 17 for creating a magnetic latent image on tape 13, a developer station 19 for developing the latent image by applying toner (dry magnetically attractable ink particles contained in the developer station) to the latent image, and a transfer station 21 for transferring toner from the developed image to paper 15 or some other medium. The latent image creation and development operations may be performed in much the same manner as described in U.S. Patent 4,110,758 issued to Nelson et al.

As shown in Fig. 1, tape 13 is operatively coupled to a shaft of motor 23 and is advanced by the motor as a closed loop through the various stations of the system. The tape is initially preoriented at station 85, where the magnetization associated with each area of the tape is transversely aligned in uniform manner. As the tape 13 passes recording station 18, recording head 17 records a magnetic latent image onto the tape. As such the record head is effecting localized reversals of the magnetization to the opposite transverse direction. The tape is then advanced to developer station 19 where toner applicator brush or drum 27 develops the latent image by applying magnetic toner particles to the tape. The magnetized latent image areas on the tape attract the toner, thereby developing (toning) these image areas. Excess toner is removed from the tape by a scavenger brush or drum 29 and returned to toner reservoir 31. As a further aid in cleaning the tape (i.e., removing unwanted toner from the tape), a first vacuum outlet 33a coupled to a vacuum source (not shown) provides a suction of air across the imaged (front) surface of the tape to remove background toner particles not tightly bound by the magnetic forces exerted by the image areas. A second vacuum outlet 33b provides a suction of air across the back surface of the tape to remove any toner or dust particles which may have accumulated thereon during the developing process or in the travel of the tape around the loop depicted in Fig. 1.

An example of toner material by which high-quality printed product can be obtained (i.e. uniform development, high efficiency of transfer, as well as high optical density) is the so-called single-component toner of Hitachi, product #480. Following developing and cleaning, the toner particles remaining on the tape are charged by, for example, a scorotron 35. The εcorotron provides an ionizing source 37 and a bias screen 39 proximate the tape surface to charge the toner particles on the tape prior to transfer of the toner to the paper. The screen 39 of the εcorotron 35 is maintained at a constant potential to ensure that the toner receives a uniform charge. After the recorded latent image is developed, the tape cleaned, and the toner particles remaining on the image areas charged, the tape 13 is advanced to transfer station 21. In the example embodiment here depicted, the tape 13 is comprised of a predetermined number (e.g., three) of segments for recording latent images. A hole (not shown) is formed through the tape preceding each such segment, a segment representing a length of tape used to print a line of characters (symbols). It will be appreciated that tape 13 could provide two or more lines of characters simultaneously to the transfer station. As a tape segment is moved into position in transfer station 21, light from a light source of photosensor arrangement 41 passes through the hole and is detected by a detector of photosensor 41. In response to the detection of the hole, photosensor arrangement 41 applies a signal to controller 25 which brakes and stops the motor 23 (e.g., by applying a reverse polarity signal, then no signal to the motor) to position the tape segment within transfer station 21 in preparation for image transfer.

With the tape segment having the toned image positioned thus for transfer, a moveable platen 43 of a tape positioner 49 is then actuated from the controller 25 via drive circuit 65 to press the tape segment with its toned image into gentle contact with the paper 15. A transfer voltage (pulse) is then applied to electrode 45 contained in back plate 47 located adjacent to and above paper 15 which creates an electrostatic force attracting the toner (the toned image) to the paper during the transfer period. Following image transfer, platen 43 is returned to an open (non-contact) positon.

The tape is then advanced by motor 23 as governed by controller 25 such that the tape segment corresponding to the transferred developed image paεses by another vacuum outlet 75 which removes any residual toner particles from the tape's surface. Thereafter, the tape εegment advances to station 85 where the imaged portions are magnetically reoriented, thus preparing the tape εegment for the next imaging operation.

The toner particles constituting the developed images are uniformly precharged to a preselected sufficient level inter alia for the following reasons. The transfer to paper of the charged toner particles comprising the images is accomplished by subjecting them to an electrostatic field force which forces the toner to remain with the paper when the paper and tape medium are being separated following their contact. Without uniform precharging, one could likely achieve "spotty" transfer. Without sufficient precharging, one would need much higher transfer fields, which could readily lead to possible breakdown. Either way, transfer efficiency would suffer appreciably.

Fig. 2 is an enlarged (not particularly to scale) cross-sectional view of a portion of the tape medium 13 which, in accordance with the invention, is suitable for magnetographic printing and may be utilized in an apparatus such as that depicted in Fig. 1. Tape 13 is a laminate structure comprised of a substrate 3, having conductive layers 4 and 5 on either side thereof, and a magnetizable layer 6. The substrate layer 3 is preferrably composed of a stable (i.e. non-stretchable, repeatedly-bendable, substantially unaffected by temperature with regard to size or surface consistency, etc.) readily-splicable, polymeric material or plastic such aε polyester terephethalate (e.g., Mylar) or polimide (e.g., Kapton). Substrate layer 3 preferably has a thicknesε range on the order of two to four mils, and most preferably between three and four mils for a system such as depicted in Fig. 1. Thinner materials are apt to be susceptible to damage by handling (e.g., fraying) while thicker materials give rise, inter alia, to relatively high stresses when conformed to small radius rollers and guides.

The back-coat conductive layer 4 is preferably on the order of .1-.2 mils in thickness with the resistivity preferably as low as possible and no greater than 10⁴ ohms per square. The thickneεs dimension of conductive back-coat 4 is at least in part a function of the normal wear life of such a tape laminate structure. A principal purpose of back-coat conductive layer 1 is to prevent surface charges from accumulating on or from the rear surface of the tape due , for example, to triboelectrification. Thus, provision is made, in for example the system depicted in Fig. 1, for grounding conductive surface 4 in at least one place along the tape path. In Fig. 1, this grounding is effected, as schematically indicated, at point C.

Conductive underlayer 5 essentially has the same dimensional and electrical characteristics as back conductive coating 4. A principal purpose of conductive underlayer 5 is to ensure that any charge deposited (or otherwise present) on the exposed surface of magnetic layer 6 can be conducted through to the underεide thereof, and along conductive underlayer 5 to ground. While conductive underlayer 5 would likely not be needed if the magnetic layer 6 is conductive enough, such a requirment (i.e., rendering magnetic layer 6 sufficiently conductive) might compromise the optimization of the magnetic properties of the magnetic layer 6 such that as for example by causing a reduction in the concentration of the magnetic pigment resulting from the inclusion of conductive particles. With the conductive underlayer 5 present, the effective resistivity of it in combination with magnetic layer 6 would only be slightly higher than the conductive underlayer alone even if the conductive layer resistivity would be several orders of magnitude greater than the magnetic layer. Thuε, provision is made, in the example system of Fig. 1, for grounding the surface of magnetic layer 4 in at least one place along the tape path. In Fig. 1, this is effected, as schematically indicated, at point C . This may be considered a convenient point about the tape loop, whereas one can appreciate that between points A and B (in direction of tape movement), it would be undesirable to attempt the ground contact in view of the presence of toner on the tape.

Layer 6 is comprised of an emulsion of elongated magnetizable crystal's or particles of, for example, CrO₂ or Fe₂O₃, concentrated in a carrier substance such as a mixture of the polymers of Saran and Eεtane, as more particularly described hereinafter. The magnetic layer preferably would have a thickness in the range of .-3 to .6 mils, and for a εyεtem such as that depicted in Fig. 1, it has been determined that a most preferred range would be .35-.45 mils in thickness. In considering the relevant parameters of a system such as that in Fig. 1, there should be considered especially the write head geometry, the spacing between the write head electrode elementε and the tape, and the current level in the write head conductorε. In addition, the thickness dimension of magnetizable layer 6 is determined with particular regard to the magnetic material itself, the saturation characteristics of the magnetic tape and, of course, the concentration of magnetic crystals or particles in the magnetic layer 6. As stated hereinbefore, magnetic layer 6 should have a relatively high coercivity and remanent induction. Especially preferred ranges of coercivity (H_c) and magnetic remanence or remanent induction (B_r) are respectively 500 to 600 Orsteads and 1,000 to 1,400 gauss for a system such as that depicted in Fig. 1.

The tape configuration may be supplemented by a lubricant 7 applied to the magnetic layer 6, such as by exposing magnetic layer 6 to a silicone lubricant treatment, whereby a small amount of lubricant may be retained in the magnetic layer (up to 10% by weight of the magnetic layer). This lubricant has as its principal functions to provide protection of the magnetic layer 6 and optimize the interface between the toner and other surfaces that contact the front tape surface, such as cleaner 75 in Fig. 1. Material 7 can take the form of any suitable low surface energy material.

Since it is desired to produce a magnetic latent image on the recording surface which will provide a strong, uniform attraction for magnetic toner patricles, it is appropriate to provide optimized magnetic characteristics. Thus it is that with the toner attracting force of the tape being proportional to the product of H and dH/dx, high uniform field intensity and high field gradient are needed. As indicated in the above discussion in connection with Fig. 1, the magnetic surface layer 6 of the tape 13 is premagnetized to saturation by orienting arrangement 85.

The direction of premagnetization is opposite to that of the direction of magnetization which is produced by the current in the record head elements of write head 17 in effecting the latent magnetic images on the tape 13. Using by way of example a recording element which in the recording zone is 2 mils wide by .5 mils thick by 3.5 mils long, with a peak current of say 11 to 14 amps, a pixal 4 mils-square may be achieved with a magnetic material having the within-discussed magnetic characteristics and a thickness that is in the range of 0.3 to 0.6 mils, and with the spacing between the record head 17 and the recording surface of tape 13 being on the order of 0.25 mils. The magnetic orientation of such a pixal would be transverse to the direction of the recording current in the conductor at the recording zone. The above is particularly illustrated in Fig. 6. Fig. 3 illustrates an intrinsic characteristic B/H curve of a magnetic material comprising magnetizable layer 6, such as Crθ2, which has a good squareness ratio (B_r/B_s), i.e. a -preferred squarenesε value of at leaεt .8. It is important for optimized recording capability that the recording medium have B/H curve character istics exhibiting sharp slope and as little as possible degradation in the level of B from that achieved at saturation by the H field. A major advantage of having a material which possesses so-called "square" B/H curve characteristics, such as illustrated in Fig. 3, is that a sharp, pronounced recording effect can be achieved. The achievable boundary regions between the recorded pixals and the remainder of the premagnetized tape magnetic layer in a system such as illustrated in Fig. 1 is illustrated in greater detail in connection with Figs. 7-9.

In achieving as high a value of magnetic remanence as possible for the tape and the selected recording current, consideration should be given to the volume concentration of the magnetic pigment inthe slurry formulation from which the magnetic layer 6 of the tape is formed. Fig. 4 graphically illustrates the remanent intensity B_r as a function of the concentration (p) of the magnetic pigment in the slurry as applied to the tape structure. What is most desired is to provide a slurry mixture which will achieve and exhibit the characteristics represented by point p₀, which represents maximized remanent intensity B_r, as opposed, for example, to one having a magnetic pigment concentration (i.e. an even greater concentration) represented by say point P1, which, as Fig. 4 shows, relates to a lower value of remanence (due principally to an inability to uniformly align the elongated magnetic particles at such a concentration).

In constructing the tape laminate depicted in Fig. 2, one can begin with a commercially-available Mylar substrate already having one or both surfaces coated with conductive material. The other conductive layer may be provided by conventional techniques, as needed. However, due regard should be given to the conductive underlayer 5 in terms of it providing or contributing to a good bonding of the magnetic layer 6 to the substrate 3. It is thus important that acceptable bonding exist between the substrate layer 3 and conductive layer 5 on the one hand, and between the magnetic layer 6 and the conductive underlayer on the other hand. In applying the magnetic layer 6, the slurry of magnetic pigment is coated onto the substrate laminate and allowed to dry and cure. Fig. 5 illustrates that before this drying and curing process the tape may be passed through a magnetic field provided by pole pieces 51 and 52. As the tape is drawns therethrough in its length direction, the randomly oriented crystalline magnetic pigment 53a will become oriented in the transverse direction as illustrated at 53b, such that when the magnetic layer has completely cured, these elongated particles will retain the pre-established transverse directionality.

Preparation of magnetic material layer 6 coating may be effected, by way of example, in accordance with the following. Formulation:

Ingredients A

(partε by weight) a. Estane 5703 30.9

Saran F-310 61.8

Methyl Ethyl Ketone (MEK) 168.0

Methyl Isobutyl Keton (MIBK) 92.0 b. MEK 268.2

MIBK 445.1 c. Toluene 65.5

Yelkin TS 16.9

Aerosol OT 5.4

Cr0₂ (e.g. Dupont S4 or

D-500-01) 525.3 d. Mondur CB-75 1

MEK 15.7

MIBK 30.0

Total Weight 1738.9

Dry Weight 643.4

% Solids 37%

Ball Mill Time 60 hrs

Viscosity* 1550 cps

*Brookf ield #4 spindle , 20 RPM This formulation contains approximately 10% by weight of Mondur based on the Estane content of the binder system. In place of CrO₂ in the above, there may be utilized iron oxide, for example formulations HIEN 627 or HIEN 597 of Hercules Co., or cobalt-modified iron oxide, for example Phizer formulation 2560.

Preparation:

The formulations are made up in four parts: a, b, c, and d. In part a, the resins are dissolved in the preweighed solvent blend by adding first the Estane resin followed by the Saran resin to the solvents in a closed container with rapid agitation using a high speed disperser. In part b, the solvent blend is weighed into a sufficiently large container. Approximately one-half of the solvent is set aside and retained for use in rinsing later in the preparation procedure. The other half is used for preparing part c. For part c, the ingredients are added in the order liεted to the above solvents in the container. The CrO₂ material is added with stirring until all the pigment is thoroughly wetted. To a porcelain ball mill roughly half filled with grinding media is charged part a first, followed by part c. The solvent blend retained from part b is then used to rinse the container into the ball mill jar. The mill is rolled for about sixty hours. Upon removal from the ball mill, the coating slurry is filtered through several layers of cheesecloth into a tared one-gallon metal can. The percent yield is calculated prior to the addition of part d.

Part d of the formulation is prepared last by mixing all ingredients. The appropriate amount of part d is added to the slurry to obtain the 10% Mondur crosslinker to the Estane ratio. A Premier dispersator may be used to thoroughly dzsperse the crosslinker solution in the oxide slurry by mixing for say ten minutes.

Application of Magnetic Oxide Coating: Once the crosslinker has beer dispersed in the oxide coating formulation, the slurry may be poured into a pressure bomb and filtered using nitrogen for pressure. This filtration procedure may be repeated and the filtrate collected n a gallon metal can which has been specially fittal for dispensing slurry directly into the coster pond. This container is fitted with a cover and a stirring paddle. The paddle is attached to an air motor and the formulation is kept rapidly agitated during the duration of the coating run. This prevents coagulation and/or settling of the oxide pigment. A Beta gauge may be used to obtain reproducible wet film thickness to yield the desired dry coating thickness with satisfactory magnetic output. The coating can be accomplished by reverse roll technique.

For transverse orientation, the electromagnet (Fig. 5) is placed in the coating line say thirty inches downstream from the coeting head. A magnetic field of 1500 gauss has been used yielding good squareness (0.86 _+ 0.01).

All tapes can be redried by passing through the coater oven a second time. The redry conditions would not necessarily be changed. It is mentioned hereinabove that the tape may be subjected to a lubricant treatment. As example, such a surface treatment may employ Silane A-187 of Union Carbide Corp., applied as a solution (or neat) to the wet surface of the magnet layer and rubbed in. Alternatively, silicone fluid or other lubricant could be added to the magnetic pigment slurry itself, for example, Dow Coming's formulation 200 or Butoxy ethel stearate. An acceptable range for this lubricant additive is 1/100%-1% by weight. Preparation of conductive underlayer 5 coating maybe effected, as example, in accordance with the following.

Formulation and Preparation of Conductive Coating:

Ingredients Parts by Weight Methyl Ethyl Ketone 141.6

Cellosolve Acetate 141.6

Toluene 424.9

Xylene 708.2

Vitel PE-200 399.5 Vul'Can XC-72 140.0

Yelkin TS 4.08

Aerosol OT-100 1.36

Total Weight 1961.24 Total Solids 27.8% Viscosity* 400 cps

Application Of Conductive Underlayer: By way of example, the conductive coating can be applied to both sides of a Mylar substrate. The one side would become the underlayer 5 to the magnetic oxide coating 6 and the other side would become the conductive back coat 4 (e.g. 4 x 10³ ohms per square). In any event the desired resistivity is provided to the magnetic coated side of the tape. Upon removal from the ball mill the conductive coating slurry is filtered through several layers of cheesecloth into a pressure bomb. Nitrogen pressure is used to push the slurry through a filter cartridge. Only a small amount of material is added at a time to the pond to prevent coagulation. Both

*Brookfield #4 spindle, 20 RPM sides of the Mylar are cleaned with a flannel cloth wetted with MEK immediately prior to application of the coating.

A Beta gauge can be used as a means for controlling thickness. The Beta gauge allows the measurement of the wet film thickness. It has been determined that at a Beta setting difference of 32 (Beta setting for the substrate minus the Beta setting for the wet coating) the desired dry thickness and resistivity are obtained. The Beta gauge signals any changes occurring in the wet film thickness so that corrective measures (e.g. changes in nip clearance) can be taken immediately.

Fig. 6 illustrates in perspective view a portion of a tape magnetic medium and its relation to several of the recording elements of a write head constructed, for example, in accordance with the teachings of U. S. Patent 4,025,927.

Fig. 6 specifically shows in an enlarged view cut-away portions of three recording elements 57a, 57b and 57c of write head 17 which portions include the respective write zones Z as positioned atop the magnetic tape 13, the latter having a direction of movement as indicated by arrow 59. Recording zone Z is shown in its entirity for the write element 57a. The other two elements are sectioned at the recording zone to more clearly demonstrate the write element geometry and the interaction of the induced magnetic field 55 due to the current I with the magnetic tape medium 13. The tape is premagnetized in the transverse direction as indicated by arrows 16. As shown, the induced magnetic field in the tape is opposite in direction to the premagnetization direction of the medium, and thereby locally reverses the direction of the premagnetization, thus creating a recorded pixal under the write zone of say write element 57c. The width (d₁) and thickness (d₂) dimensions of the condudtive write elements are specifically indicated in connection with write element 57b. Typically d₁ is on the order of 1.9 to 2.1 mils and d₂ is on the prder of .2 to .5 mils.

Fig. 7A depicts an enlarged cross-sectional end view of the arrangement of Fig. 6 taken through the write zones of the three recording elements 57. Fig. 7A iε roughly drawn to scale. In Fig. 7A, it is assumed that the recording elements 57a and 57b are excited by the recording current I with the direction of the current being into the page. Specifically, Fig. 7A demonstrates in greater detail the interaction between the induced magnetic field due to current flowing in conductors 57a and 57b as represented by field lines 55 and the premagnetization of the medium as represented by arrows 16. As shown, the adjacent recorded pixals created by write elements 57a and 57b are separated by a retained premagnetized region 64. Region 64 has remained unswitched by the induced magnetic fields becauεe such fieldε within this region are smaller in magnitude than H_c (shown in Fig. 8 as the coercive magnetic force of the tape medium, herein indicated by way of example as 550 Orst., for say CrO₂ ) and thus are insufficient in strength to reverse the direction of premagnetization.

The write element geometry (substantially as shown in Figs. 6 and 7A), the spaced separation of adjacent write elements (Fig. 7A), the write current I therein (in the range of εay 10-13 amps) and the separation (typically .25 mils) of the write elements from the tape medium, all contribute to ensuring that when adjacent write elements (say 57a and 57b) are simultaneously energized, region 64 remains unaltered between the resulting pixals created remains substnatially unaffected. This ensures in turn that there is no overlapping of pixals (a condition which could tend to allow the pixals to cancel one another out in the overlapped regions), resulting in relatively poor toner-attracting force in the overlapped regions. The field lines depicted in Fig. 7A demonstrate the result of sequential excitation of the adjacent elements 57a and 57b.

The magnetic field intensity associated with the creation of a pixal by the corresponding write element (say 57b) is strongest within the dimension d₁ (Fig. 6) and proximate to the surface of the medium (represented by say field line H2 in Fig. 7A) and is weaker with distance as represented by field lines H3 and H4. For the purposes of this discussion it is assumed that the induced tangential components of magnetic fields H₁, H2. H3 and H4, at the surface of the magnetic medium, have the relative strengths as indicated by Fig. 8 (e.g. for H₁ : 1250 Orst.; for H₂: 880 Orst.; for H3 : 540 Orst.; and for H4: 360 Orst.) and the coercive force of the tape, H_c, being on the order of 550 Orst. Thus, Hi surely, and H2 also, are strong enough, for the given conditions, to reverse the direction of the premagnetization, causing saturation. Fig. 8 demonstrates further that the field repreεented by the H3 line in Fig. 7A has reduced the net magnetization in the magnetic medium to near zero. From the recording pattern created by the excitation of write elements 57a and 57b, it can be seen from Figs. 7A-7C that boundary zoneε or regions are created around each pixal between the premagnetized area illustraed by arrows 16 and the opposing magnetization. For example, in the case of the pixal created by write element 57b, the right-side boundary (in the view presented by Fig. 7A) iε defined between points bi and b2 and the left-side boundary is defined between points b3 and b4. Points bi and b2 (and b3 and b4) represent the effective end points of the region of high magnetization gradient dB/dx, as shown more particularly in Fig. 7B.

In accordance with the asεociated curve depicted in Fig. 7B, the tangential magnetization By just below the surface of the magnetic medium generated by element 57b varies from approximately -B_R at the point that the field line H2 enters the magnetic medium to approximately BR at the point that field line H2 enters the magnetic medium to approximately B_R just after the point where G4 intersects the surface. Fig. 7B thus depicts Brp aε a function of distance x along and just beneath the surface of the magnetic medium in the transverse direction. It is of importance to observe the dBrp/dx is defined by the slope of the curve between the levels -B_R and B_R. Fig. 7B demonstrates that premagnetization of the magnetic medium results in an increase of the magnitude of dBrp/dx by at least a factor of two as compared with the results of a recording condition which employs employs a non-premagnetized tape. The latter situation is depicted in Figs. 9A-9C, corresponding in their views with Fig. 7A-7C respectively. While it is to be noted that the shape of and the area under the portion of the curve in Fig. 7B associated to region 64 is a reflection of the example field lines illustrated in Fig. 7A, in the ideal case this portion of the Fig. 7B curve would be substantially the same as that illustrated between points b2 and b3 corresponding to the recorded pixal region associated to pixal 71 of element 57b. That is, the length of the region of saturation and the high level of d_BT/dx it contributes to the boundary region of pixal 71 approach that as achieved in connection with the pixals 70 and 71 surrounding it. Figs. 9A-9C illustrate in comparison to Fig. 7A-7C the substantially different results obtained for the same recording geometry and recording element excitation, when utilizing a magnetic- medium which has not been premagnetized (i.e. curve portion 82 of Fig. 8).

Fig. 7C shows, again in cross-sectional and view, the portion of the magnetic medium of Fig. 7A with the created magnetic field pattern of the pixals 70 and 71 (relating to recording elements 67a and 57b respectively) and the retained premagnetized region 64 therebetween at a post-recording position of the tape prior to the toning step. Also illustrated is a portion of the unrecorded region of the tape to the right of pixal 71. The field lines in Fig. 7C demonstrate magnetically speaking the pole interactions of theεe adjacent magnet regions, indicated in conventional terms of N's and S's. As indicated by Figs. 7B and 7C the discontinuities in the spacial variations of the magnetization in the tape medium give rise to the existence of magnetic fields H on and above the surface 14 of the medium. It will be appreciated that the toner-attracting magnetic forces above and near the surface 14 are proportional to the product of dH/ds and H, where dH/ds is the spacial variation of H. Fig. 7C (and also Fig. 7B) demonstrates further that the premagnetized region 64 situated between the pixals generated by write elements 57a and 57b is very significant in size and pole strength relative to the adjacent pixals, and as such region 64 contributes substantially to the toner attracting forces of the field pattern. It is this contribution which is perhaps most greatly missing from a corresponding field pattern developed in connection with a non-premagnetied tape medium, illustrated in Fig. 9C. From the standpoint of toner particles having a high permeability property, the boundary areas between the competing magnetic regions represent a high contributing component of the force of attraction. Thus, in the case of say pixal 71, the toner particles are initially most attracted to points where the magnetic poles appear. The toner particles are able by their presence at these locations to alter the local fields and field gradients to allow their accumulations to grow outward from these points until the distances therebetween fill-in, resulting in the entire pixal region S1-S2 (Figs. 7A and 7B) being completely toned with satisfactory optical density, i.e. approximately a 4-mil square pixal created for a say 2 x 3.5 mil write zone. There is provided a resultant toner-holding force more than sufficient to permit the toner particles to survive a cleaning process intended to remove excess toner and background.

It can be appreciated particularly from Figs. 9B and 9C that the recorded pixals from write elements 57a and 57b for the non-premagnetized tape noticeably lack the relatively high spatial magnetic gradients of the pixals generated in connection with the premagnetized tape (Fig. 7A-7c), which would translate to correspondingly weaker toner attracting forces. As a result, the toning of such an image will bear the marks of substantial loss of edge acquity an appreciably weakened optical density as well (or alternatively a corresponding increase in background) which are indeed the essential parameters of a high-quality printed product.

Finally, Fig. 7D illustratres the concept of the comparative composite attracting forces experienced by the toner particles applied to the magnetic field patterns illustrated in Figs. 7C and 9C. It can readily be appreciated from Fig. 7D that the curve 72 associated to the premagnetized medium demonstrates that the composite attracting and holding force on the toner particles needed for high-quality product is substantially greater than that (curve 73) provided by the non-premagnetized tape situation.

Referring to Fig. 10, pixal or dot length is established primarily by the geometry of the current path. Dots in any channel can be overlapped horizontally in the direction of the tape motion without limit and thus horizontal edge quantization can be started and stopped at any fractional spatial interval (i.e. spatial interval eqauls one dot length ) with a minimum length of one dot length.

For example, consider a dot length of four mils. It is desired to record a line length in a given channel that extends from say nine mils to thirty one mils (from a reference edge). A clock is generated every one mil of recording surface movement. At clock number eleven the record head channel is pulsed (pulse duration of approximately one micro-second with correεponding tape motion of << one mil) creating a magnetized area extending from nine to thirteen mils from the reference. At clock twelve it is pulsed again magnetizing the area from thirteen to fourteen mils (the area from ten to thirteen mils is unaffected by the record magnetization). Thereafter, it is pulsed every fourth clock through clock twenty-eight and again at clock twenty nine. The resulting magnetized region from this "partial step" recording technique extends continuously from position nine to thirty-one as desired.

The advantage of this method is that much finer quantitization can be achieved. For example, in recording a capital "K" the sloped, but nonvertical outside strokes will tend to appear jagged even at pixal densities of 250 per inch if the offset approximating the sloped stroke is at full pixal intervals. By making the offset intervals at a half or a quarter of a pixal interval, the much smaller steps more precisely approximate the slope and are not detectable to the unaided eye. The recording duty cycle is only slightly increased so that power and heating are not significantly affected. Also a wider range of stroke widths can be acommodated and therefore a more precise rendition of type styles produced.