US3369917A

US3369917A - Magnetic brush development of electrostatic images utilizing a high voltage corona

Info

Publication number: US3369917A
Application number: US307887A
Authority: US
Inventors: Daniel B Granzow; John L Tregay
Original assignee: Individual
Current assignee: JOHN L TREGAY
Priority date: 1963-09-10
Filing date: 1963-09-10
Publication date: 1968-02-20
Anticipated expiration: 1985-02-20
Also published as: DE1472904B1; GB1076595A

Description

I Feb. 20, 1968 GRANZQW ET AL I 3,369,917

MAGNETIC BRUSH DEVELOPMENT OF ELECTROSTATIC IMAGES UTILIZING A HIGH VOLTAGE CORONA Filed Sept. 10, 1965 3 Sheets-SheetZ 5 L/OA/A/ L. TEE-GAY Arrow/5Y5 DAA/IEL 5. GEAA/ZOIA/AA/D 1 777mm, mam, fizf/zwmmd @u. v

Feb. 20, 1968 GRANZQW ET AL 3,369,917 V MAGNETIC BRUSH DEVELOPMENT OF ELECTROSTATIC IMAGES UTILIZING A HIGH VOLTAGE CORONA Filed Sept. 10, 1965 -3 Sheets-Sheet 5 MMm'LXMMmMMe m (a MflzM rt, Md W 44 United States This invention relates to an electrostatic copying apparatus, and more particularly, to such an apparatus including new and improved means for developing an electrostatic image.

One type of electrostatic printing or copying equipment now in extensive use includes a copying sheet having a substrate of paper to one surface of which a layer of photoconductive material, such as zinc oxide, in a resin binder is applied. The copying machine includes means for applying a uniform electrostatic charge to the coating surface of the sheet and then for selectively illuminating the coated surface in accordance with the original to be reproduced to provide an electrostatic image or pattern of charged and substantially charge-free areas corresponding to the original. A developer material such as an electroscopic powder is then applied to the electrostatic image by such diverse means as a magnetic brush or a cascade to be selectively held on the copy sheet in accordance with the charged and charge-free areas to form a powder image loosely held on the sheet. This powder or visible image is placed in a permanent form by heating it in a fuser oven or by contact with a fixing agent.

One of the problems commonly encountered in developing electrostatic images is the tendency of the developer or electroscopic powder to cling to the substantially charge free-areas during the production of positive prints. It is believed that this retention of the developer powder in the background or nonimage area results from a residual charge on these areas that is not dissipated during the selective illumination or a charge that is developed by the brushing or cascading action of the developer. This adherence of the electroscopic powder or developer material, commonly a thermoplastic resin, produces streaks, dark deposits, or blotches in the white backgorund area of the finished copy. Several different approaches to preventing this retention of the developer powder on the background areas have been tried in the past in different types of equipment.

As an example, Walkup et al. Patent No. 2,573,881 discloses an electrostatic printing apparatus in which layers of dielectric material are fastened to the outer surface of a highly conductive metal drum in accordance with the material to be repetitively printed, and the outer periphery of the drum is then charged with the charge being retained only on the insulated layer to provide an electrostatic image. This image is then moved into a cascade developer unit in which a quantity of developer powder and carrier particles is continuously poured over the outer surface of the metal drum to be electrostatically attracted to and held by only the charged dielectric layers. To prevent or at least reduce the undesired retention of the developer powder on the background or nonimage areas of the metal drum, a wide area electrode is spaced outwardly from the drum so that the electroscopic powder pours down between the drum and the developing electrode. This electrode is connected to a potential source of the same polarity as the image charge on the drum so as to induce an opposite charge in the exposed portions of the metal drum. This induced charge is of the same polarity as the electroscopic powder and tends to repel the powder from the bare metal or background area of the drum.

atcnt Another approach to providing clear background areas in machines of the type using photoconductively coated sheets of copy paper is illustrated in Sugarman et al. Patent No. 3,058,444. In the apparatus shown in this patent, the electrostatic image on the coated surface of the copy sheet is contacted by a magnetic brush comprising a mixture of iron particles and toner or developer material in the presence of a magnetic field to selectively deposit the electroscopic powder on the image. To prevent the deposition of powder on the nonimage or background areas, a wide area electrode, either bare or covered by a dielectric sheet, is disposed adjacent the back of the sheet in the area in which the coated surface carrying the electrostatic image is engaged by the magnetic brush. This electrode is connected to a potential of the same polarity as the developer powder to develop a charge, by contact or induction, on the back of the sheet that is of the same polarity as the charge on the electroscopic powder. This charge is stated as having the effect of neutralizing the undesired charge on the background areas on the other or coated surface of the sheet to prevent the adherence of the electroscopic powder in these areas.

However, both of these techniques are subject to a number of deficiencies in addition to the somewhat unpredictable cleaning of the background areas that make them unsatisfactory for use in commercial machines of the type using photo-conductively coated sheets. The apparatus shown in the Walkup et al. patent requires a large area electrode spaced from the charged surface to induce an opposite polarity charge in the background area. This electrode must be relatively close to the charged surface to effectively induce a uniform charge in the meal drum. It must also be spaced away from the drum sufficiently to permit the flow of the electroscopic powder therebetween and sufficiently to prevent the attraction of the electroscopic powder to the developing electrode. This means that this charge induction method and apparatus cannot be used with machines in which the electroscopic powder is applied with a magnetic brush. The other technique suffers from the basic fault that a large area electrode must overlie and be disposed immediately adjacent the developing area to contact the back of the copy sheet during developing so as to induce a charge thereon. This large area electrode is contacted by and attracts particles from the magnetic brush during the not inconsiderable periods of time during which a copy sheet is not passing through the developer unit between the brush and electrode. This deposits iron and electroscopic powder on the electrode or the dielectric covering sheet therefor which is transferred to the back of the copy sheet subsequently fed into the developer unit with the result that the backs of the sheet become gritty and streaked with fused electroscopic powder.

Accordingly, one object of the present invention is to provide a new and improved apparatus for and method of developing electrostatic images.

Another object is to provide an electrostatic copying machine including means for applying a corona discharge to one side of the copy sheet while contacting the opposite side with a magnetic brush containing an electroscopic powder.

Another object is to provide an electrostatic copying or printing apparatus utilizing a radically new concept of developing electrostatic images in which a current is forced through a photoconductively coated copy sheet simultaneously with the application of an electroscopic powder.

A further object is to provide a developer for an electrostatic copying machine of the type using a magnetic brush in which all structures engaging a copy sheet or web are spaced from contact with the magnetic brush to avoid 3 smearing copy sheets with developer powder collected on the structures.

A further object is to provide a developer unit for an electrostatic copying apparatus including adjustable means for feeding a copy sheet into engagement with the magnetic brush.

Another object is to provide a method of developing electrostatic images on photoconductively coated paper sheets with a magnetic brush in which a current is continuously passed through the sheet and the particles of the brush to insure a potential difference betwen the brush and the sheet for controlling the deposition of electrostatic powder on the nonimage or background areas of the sheet.

In accordance with these and many other objects, an embodiment of the present invention comprises a developer unit including a rotatable magnetic brush arranged so that the bristle portions are formed in an upper portion of its cycle of rotation. The developer unit includes a developer guide assembly adjustably mounted on the housing and disposed above the path of movement of the outer extremities of the bristles of the magnetic brush. Surfaces on the guide assembly are adapted to engage the back of a copy sheet supplied from an exposure station or unit with the electrostatic image facing downwardly. The guide assembly directs the exposed surface of the sheet against the magnetic brush so that the latent or electrostatic image is developed by the selective application of the developer powder.

To provide means for controlling the deposition of the developer powder in the nonimage or background areas, a corona discharge wire is mounted on the guide assembly overlying the portion of the path of rotary movement of the magnetic brush in which the magnetic brush contacts the coated or image bearing surface of the copy sheet. The application of the corona discharge to the reverse of the back surface of the copy sheet in the area in which it is engaged by the magnetic brush not only prevents the adherence of the developer powder to the nonimage areas of the sheet when positive printing is being performed but also intensifies the image when reversal printing is being performed. The copy sheet guide means and the corona discharge means are spaced from the outer ends of the bristles of the magnetic brush so that none of the structures in the developer unit are engaged by the brush to pick up electroscopic powder or iron particles that can be subsequently deposited on another sheet. Further, even though the developer unit does not require any contact between the copy sheets and the developer electrode in the area in which the coated surface of the sheet is engaged by the magnetic brush, the biasing arrangement made possible by the corona discharge wire insures perfectly clear background areas during positive printing and a uniform intensity of the image areas during reversal printing.

Many other objects and advantages of the present invention will become apparent from considering the following detailed description in conjunction with the drawings in which:

FIG. 1 is a series of five schematic diagrams illustrating the steps performed in making a copy of an original by an electrostatic process in accordance with the present invention;

FIG. 2 is a sectional view of a developer unit included in an electrostatic copying machine;

FIG. 3 is a fragmentary end elevational view of the developer unit;

FIG. 4 is a fragmentary perspective view of a combined paper guide and bias assembly included in the developer unit; and

FIG. 5 is a schematic diagram illustrating the biased developing method embodying the present invention.

Referring now more specifically to the drawings, therein is illustrated a developer unit (FIGS. 2 and 3) which embodies the present invention and which is adapted for use in an electrostatic copying or printing machine. The developer unit 10 develops an electrostatic image on a copy sheet 12 (FIG. 2) by the selective application of a developer powder to the image areas and includes means for preventing the deposition of the developer powder on the nonimage or the background areas. The construction of the unit 10 is such that the deposition of the developer material on the back surface of the copy sheet 12 is prevented to provide a completed copy having not only clear background or nonimage areas but also a clean back surface.

Electrostatic printing machines of the type in which the developer unit 10 can be used commonly perform the sequence of operational or functional steps illustrated schematically in FIG. 1 to produce a copy of an original on the copy sheet or web 12. The sheet 12 comprises (FIG. 1) a paper layer or substrate 14 having an electrical conductivity on the order of from 10 mhos/ centimeter to 10* mhos/ centimeter. A photo-conductive layer 16 is coated on one surface of the substrate 14 and can comprise zinc oxide or selenium in a resin binder of the silicone-alkyd type. The layer of photoconductive coating 16 has a conductivity on the order of from 10- to 10- mhos/ centimeter and exhibits the capacity of accepting a charge of between and 300 volts.

\Vhen the copy sheet 12 is fed into electrostatic printing or copying machine, it is fed between a positive corona discharge assembly 18 (FIG. 1) and a negative corona discharge means 20 so that positive and negative charges are applied to the opposite surfaces of the photoconductive layer 16. In FIG. 1, the positive and negative charges enclosed in circles represent the charges applied by the

assemblies

18 and 20, respectively, with the negative charges enclosed within rectangles representing the net negative charge on the outer surface of the photoconductive layer 16.

The charged sheet 12 is then transported to an exposing unit or station in which light from a source 22 is selectively applied to the photoconductive layer 16 of the copy sheet 12 under the control of an original 23 to be copied. The selective illumination of the areas or segments of the photoconductive layer 16 places these areas in a conductive state so that the positive and negative charges applied by the

corona discharge assemblies

18 and 20 are dissipated in the illuminated areas and are retained in the areas that are not exposed to illumination. As illustrated in FIG. 1, it appears that the illuminated areas of the layer 16 do not become fully discharged and thus retain a residual or remnant charge. This may be due to the failure of the incident light energy to raise all of the electrons to the conduction band. The exposed copy sheet 12 is then transported to the developer unit 10 with the electrostatic image comprising charged and charge-free areas disposed on its lower surface or the outer surface of the photoconductive layer 16.

In the developer station, the unit 10 selectively applies a developer or toner powder, such as a thermoplastic resin, to the electrostatic image to provide a powder or a visible image corresponding to the original 23 to be copied. In general, the developer unit includes a magnetic brush means 24 including a rotating cylinder disposed at least partially in a mixture of magnetic material, such as iron filings, and the toner material. During a portion of its path of revolution, the cylinder in the magnetic brush assembly 24 is subjected to the influence of a magnetic field which forms the iron filings into a bristle-like configuration carrying the toner material. The toner material and the iron filings interact so that the toner particles receive a positive charge by the triboelectric effect to condition them to be attracted to the negatively charged areas forming the image.

As the copy sheet 12 is fed into the developer unit 10 with electrostatic image on the lower surface of the photoconductive layer 16 facing downwardly toward the magr nctic brush 24, the bristles of the magnetic brush sweep over the lower surface of the photoconductive layer 16 including both the charged and the non-charged areas to permit the positively charged toner or developer particles to be attracted to and loosely held on the layer 16 by the negative charges carried thereon. It is believed that this brushing action between the bristles of the magnetic brush means 24 and the photoconductive layer 16 generates additional negative charges which are shown in FIG. 1 as enclosed in triangles. The electrical charges resulting from the brushing action of the magnetic brush means 24 are deposited on the substantially charge-free areas of the background and could combine with any residual negative charges to cause the loose adherence of the toner or developer particles, shown as black dots in FIG. 1, to the background areas.

The developer unit 11 includes a developer biasing assembly 2-6 for overcoming the effect of these charges in the background or nonimage areas to prevent the retention of the developer powder. The developer biasing means 26 includes a corona discharge Wire 28 disposed in a housing 30 affording a ground plane and connected to a potential source 32 that supplies the corona wire 28 with a positive potential on the order of from 4,000 to 6,000 volts. The corona discharge wire 28 applies a positive corona discharge to the back or upper surface of the paper layer 14 in the copy sheet 12 in the area in which the lower surface of the photoconductive layer 16- is engaged by the bristles of the magnetic brush means 24. The positive corona discharge provided by the means 26 prevents the retention of the toner particles on the background or nonimage areas of the copy sheet 12 without changing the density or clarity of the image areas in which the positively charged particles are deposited on the copy sheet 12. When the copying machine of which the developer unit forms a part is used for reversal printing, the developer bias assembly 26 has the effect of permitting and of providing a uniform density deposition of the toner powder on the noncharged image areas and of preventing the undesired retention of developer iron in the charged nonimage areas. In this application, the toner powder can be of any suitable type but preferably is of the reversal type disclosed in a copending application of Harmon et al., Ser. No. 221,888, filed Sept. 6, 1962, now abandoned.

When the copy sheet 12 is discharged from the developer unit 10, it is forwarded by suitable conveying means to a fusing or fixing station in which a heating means 34 applies heat to the thermoplastic resin forming the toner particles and cause them to fuse to the exposed or outer surface of the photoconductive layer 16 on the copy sheet 12. Thus, the loosely adhered powder image provided in the developer station 10 is now placed in permanent form. Although FIG. 1 schematically illustrates the use of a heating means 34, other fixing means such as a liquid contacting means can be used to fix the powder image in a permanent form.

Referring now more specifically to FIGS. 24 of the drawings, therein is illustrated the mechanical construction of the developer unit 10 embodying the present invention. In general, the unit 10includes a supporting or frame structure having a pair of opposed side or end walls 36 on which the remaining components of the unit 10 are mounted. The unit 10 is supported on the main frame of the electrostatic copying machine interposed between the exposing apparatus and the fusing or fixing unit. The magnetic brush assembly 24 and the bias assembly 26 as well as a copy sheet guiding assembly are mounted on and disposed between the spaced sidewalls 36. These components are supported on the end plates 36 so that the magnetic brush assembly 24 is disposed substantially below the path of movement of the copy sheet 12 through the unit 11 in a position to apply the developer powder to the electrostatic image on the lower surface of the copy sheet 12, and the developer bias means 26 together with the sheet guiding means are disposed above the path of movement of this sheet.

The magnetic brush assembly 24 can be of any suitable construction but preferably comprises a construction of the general type shown and described in detail in Streich, Sr. Patent No. 3,003,462. The magnetic brush means or assembly 24 includes a somewhat trough-like member 38 of dielectric material disposed between the end walls 36 and defining a group of communicating troughs or channels in which the mixture of magnetic material and toner or developer powder material is disposed. Two of the troughs defined by the member 38 receive feeding and distributing

elements

40 and 42 for uniformly distributing the developer mixture along the length of a rotatable cylinder or drum 44 in which a permanet magnet 46 is disposed. The cylinder 44 is rotated in a clockwise direction (FIG. 2) so that bristles of magnetic material mixed with toner particles formed on its outer periphery in the portions thereof in which the magnetic field of the permanent magnet 46 is effective are rotated upwardly to an area in which they are brushed against the lower surface of the copy sheet 12. A doctor blade or scraper 48 engages the periphery of the cylinder 44 during a portion of its path of travel following the point of contact with the copy sheet 12 to remove the bristles from the drum.

The exposed sheet 12 is fed into the developer unit 10 from the exposing station by feeding means so that the leading edge of the copy sheet 12 engages a dielectric paper guide element 50 (FIG. 2) disposed at the inlet to the developer unit 10. This leading portion of the copy sheet 12 slides over the guide 50 into proximity to the rotating drum or cylinder 44 in the magnetic brush assembly and is held in engagement with the bristles on the magnetic brush by a guide assembly indicated generally as 52. The sheet 12 is advanced during contact With the bristles of the magnetic brush by the feed means in the developing station until the leading edge of the sheet 12 is received in the nip between a pair of

feed rollers

54 and 56. Since the lower surface of the copy sheet 12 now carries the loosely adhered powder image, the

feed rollers

54 and 56 preferably are of the construction disclosed and described in detail in Schulze Patent No. 2,894,744. These feed rollers include an outer surface of nap or pile material that does not disturb the loosely adhered powder held on the lower surface of the photoconductive layer 16 by the charges thereon. The

feed rollers

54 and 56 advance the sheet 12 through the developer unit 10 following its discharge from the feeding means in the exposing station and serve to advance this sheet into a transporting assembly which advances the sheet to the heating means 34.

The copy sheet guiding assembly 52 (FIGS. 2-4) is mounted on the sidewalls 36 of the developer unit 10 and includes means for engaging the back surface of the copy sheet 10 to hold this sheet in engagement with the bristles of the magnetic brush means 24. However, the guide assembly 52 is supported so that all portions thereof are out of contact with the magnetic bristles and do not receive toner or developer particles or iron filings during the period in which a sheet 12 is not passing through the developer unit 10, which particles or filings could be subsequently deposited on the back of the next sheet. This prevents smearing, smudging, and discoloration of the backs of the copy sheets 12.

The assembly 52 includes a generally L-shaped bracket or angle iron 58 having a lower arm 58a secured by a plurality of fasteners59 to a dielectric guide member 60 having an inclined front wall to which a generally U-shaped paper guide 61 is secured. The lower edges of the arms of the paper guide 61 and the lower edge of the member 60 engage the back surface of the copy sheet 12 to deflect this sheet into engagement with the magnetic bristles on the rotaing brush assembly 24. However, these edges are disposed above the bristles to avoid contact therewith.

To provide means for mounting the paper guide assembly 52 on the developer assembly 10, the opposite outer ends of the arm 58a of the angle iron 58 are provided with downwardly depending portions or flanges. A bracket 62 is secured to one of the depending flanges by a screw 64 and an additional screw 66 having an outwardly extending stud portion. A similar screw 68 having a stud portion is secured to the depending flange at the other end of the arm 58a. These two studs are received Within aligned notches 70 (FIG. 3) in the end walls 36 to pro vide means for pivotally mounting the paper guide assembly 52 on these walls. The bracket 62 includes an upwardly extending arm portion 62a having an outwardly extending flange to which a nut 72 is secured. An adjusting screw 74 is received within the nut 72 and includes a lower end bearing on an upper edge of the wall 36 (FIG. 3). A tension spring 76 connected at one end to the flanged arm 62a and at its other end to a bracket 73 secured to the side wall 36 by a screw 80 resiliently biases the guide assembly 52 for pivotal movement about the studs 66 and 68 in a clockwise direction so that the lower end of the screw 74 bears against the upper edge of the side wall 36.

By adjusting the position of the screw 74, the position of the paper guide elements 60 and 61 in the assembly 52 relative to the periphery of the drum or cylinder 44 is adjusted, and the path of movement of the copy sheet 12 can be controlled. In this manner, the point and degree of contact of the lower surface of the copy sheet 12 with the bristles on the magnetic brush assembly can be controlled and, similarly, the location of the paper engaging surfaces of the parts in the assembly 52 relative to the bristles can be controlled so that they are not contaminated with iron filings and toner particles during the intervals in which a copy sheet 12 is not passing through the developer unit 10.

The bias assembly 26 for applying a positive corona discharge to the back of the sheet 12 in the area in which the front or lower surface of the sheet is engaged by the bristles of the magnetic brush assembly 24 is carried on the adjustable paper guide assembly 52. More specifically, the paper guide assembly 26 includes a pair of dielectric or ceramic stand-ofl insulators 82 secured in spaced positions to two parallel legs of two

angle iron elements

84 and 86 by two machine screws 88. The corona discharge wire 28 is secured to the lower depending ends of the insulators 82 by a pair of dielectric screws 92. One end of the corona discharge wire 28 is connected to the positive potential supply 32 through suitable control means, such as a variable resistor 94 (FIGS. 1 and The other two legs 84a and 86a of the

angle irons

84 and 86 are spaced from each other to define a slot in which the upper end of the other leg 58b of the angle iron 58 is slidably received so that the bias assembly 26 is mounted on the paper guide assembly 52 with the corona discharge wire 28 disposed above the area in which the copy sheet 12 is engaged by the bristles of the magnetic brush 24. The

angle irons

84, 86, and 58 are connected to ground through the frame of the machine to establish a ground plane for the corona discharge element 90. Since the developer bias assembly 26 is mounted on the paper guide assembly 52, the adjustment of the position of the assembly 52 relative to the drum 44 in the magnetic brush assembly 24 concurrently adjusts the spacing of the corona discharge wire 28 from the back of the sheet 12 Without varying the spacing between the sheet engaging portions of the assembly 52 and the corona element 28.

To provide means for removing any iron filings that inadvertently may have become deposited on the back of the sheet 12 during its passage through the developer unit 10, a bracket 95 (FIG. 2) is secured between the end walls 36 and carries a housing 96 having a lower edge 96:: adapted to engage the entire transverse dimension of the back surface of the copy sheet 12 immediately before it passes into the nip of the

feed rollers

54 and 56. A permanent magnet 08 disposed Within the housing 96 continuously applies a magnetic field to the elements and 96 to remove any magnetic particles that are inadvertently deposited on the back surface of the copy sheet 12 before it is discharged from the developer unit 10 and conveyed to the fuser unit. Thus, the magnetic cleaning afforded by the permanent magnet 98 positively assures a uniform clean condition of the backs of the exposed and developed copy sheets 12.

The application of the positive polarized corona discharge to the back of the copy sheet 12 in the area in which the photoconductive layer 16 is engaged by the bristles of the magnetic brush means 24 has the result of preventing the retention of the developer or electroscopic powder on the non-images areas of the sheet 12, but the bias assembly 26 is of such a construction that it permits the area above the magnetic brush means 24 to remain entirely open to prevent the collection and subsequent deposition of iron filings and electroscopic powder on the back surface of the sheet 12. One possible explanation for the greatly improved results obtained with the bias assembly 26 in the developer unit 10 is set forth below with reference to FIG. 5 of the drawings. FIG. 5 is a schematic diagram of electrical circuit including the corona discharge element 28, the copy sheet 12, and the drum 44in the magnetic brush assembly 24.

When the electrostatic printing machine is adjusted incident to being placed in operation, the potentiometer or variable resistor 94 is adjusted to supply a sufficiently high potential on the order of from 4,000 to 6,000 volts poled positively with respect to ground to the corona element 28. By virtue of the ground plane established by the grounded supports for the corona wire 28, ionization takes place, and a resistance element 100 represents the resistance of the ionized atmosphere between the corona wire 28 and the back surface of the copy sheet 12. An additional resistance element 102 represents the resistance of the paper substrate 14 of the copy sheet 12, and a resistance element 104 represents the resistance of the photoconductive layer 16. A resistance element 106 represents the resistance of the magnetic fiilings bridged between the grounded and highly conductive drum 44 and the engaged lower surface of the photoconductive layer 16. It has been experimentally determined that a minute current on the order of microamperes flows from the grounded drum 44 to the corona discharge wire 28 over the path including the schematically illustrated

resistance elements

100, 102, 104, and 106.

The continuous flow of the small amount of direct current between the corona discharge Wire 28 and the drum 44 through the bristles of the magnetic brush produces a variety of effects. In the first place, the magnitude of the flow of current is rather small because of the high impedance of the circuit between the corona wire 28 and the drum 44. However, this current is great enough to carry off the spurious charges in the nonimage or background areas so that the electroscopic powder is not attracted to and held in the background areas. Quite surprisingly, the flow of current which also passes through the charged image areas does not dissipate this charge sufliciently to have any effect on the clarity or density of the image areas. This is probably due to the relative quantities of the charges on the image and background areas. In addition, the flow of current through the developer mix including both the iron filings and the electroscopic powder insures that the surface of the photoconductive layer 16 is more positive than the electrically conductive iron filings in the developer mix, and this appears to reduce the tendency of the electroscopic material to deposit on the background or nonimage areas of the copy sheet 12. Further, even though the iron particles are positively biased by the current llow, the electroscopic material on the biased particles become more positive by the triboelectric effect. This means that the more positively charged toner particles are always transferred to the image areas prior to the magnetic filings and will satisfy the charge demand before any iron is deposited. This phenomena is assisted by the force of magnetic attraction acting on the iron particles which must be overcome prior to deposition of the iron in the image areas.

Although the present invention has been described with reference to a single illustrative embodiment thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A method of developing the image bearing area on one surface of a copy sheet containing an electrostatic charge without developing nonimage areas on the same surface of the sheet having no more than a small amount of undesired charge, which method comprises the steps of applying a magnetic brush formed by a mixture of magnetic particles and developer powder to the image and nonimage areas on the one surface of the copy sheet to permit the retention of the developer powder by the charge in the image areas, and concurrently applying a voltage in the range from four thousand to six thousand volts to electrode means to apply a corona discharge to the opposite surface of the copy sheet to produce a direct current flow through the copy sheet of such proportions as to carry off the undesired charge in the nonimage areas without alfecting the ability of the charge in the image areas to retain the applied developer powder.

2. A method of developing the image bearing area on one surface of a copy sheet containing an electrostatic charge without developing nonimage areas on the same surface of the sheet having no more than a small amount of undesired charge by the use of a magnetic brush, which method comprises the steps of applying a developer powder to the image and nonimage areas on the one surface of the copy sheet through the magnetic brush to permit the retention of the developer powder by the charge in the image areas, connecting the magnetic brush to a reference potential, applying a voltage in the range of four thousand to six thousand volts to electrode means to apply a corona discharge to the other surface of the copy sheet in the area oppoiste to that engaged by the magnetic brush of suflicient magnitude to produce a direct current flow through the copy sheet of such proportions as to carry off the undesired charge in the nonimage areas without affecting the ability of the charge in the image areas to retain the applied developer powder.

3. A method of making a copy of an original on a paper sheet coated on a front surface with a photoconductive material, which comprises the steps of producing a pattern of charged and substantially charge-free areas on the coated surface of the sheet in accordance with the original to be copied, applying a magnetic brush formed of a mixture of magnetic patricles and developer powder to successive segments of the coated surface embracing both charged and charge-free areas during relative movement between the magnetic brush and the paper sheet so that the developer powder is selectively retained on the coated surface in accordance with the charged and chargefree areas, establishing a corona discharge by connecting a potential in the range of from four thousand to six thousand volts to an electrode means, and applying the corona discharge to the other surface of the sheet in the area in which the front surface of the sheet is contacted by the magnetic brush to produce a direct current flow through the paper sheet in the segments thereof successively contacted by the magnetic brush of a great enough magnitude to remove spurious charges from the substantially charge-free areas without effecting any substantial change in the charged areas.

4. A method of making a copy of an original on a paper sheet coated with a photoconductive material, which comprises the steps of producing a pattern of charged and substantially charge-free areas on the coated surface of the sheet in accordance with the original to be copied, moving the sheet along a path with the coated pattern bearing surface of the sheet facing downwardly, applying a magnetic brush formed of a mixture of magnetic particles and developer powder to successive segments of the coated surface embracing both charged and charge-free areas during relative movement between the magnetic brush and the paper sheet so that the developer powder is selectively retained on the coated surface in accordance with the charged and charge-free areas, establishing a corona discharge by connecting a potential in the range from four thousand to six thousand volts to an electrode means, and applying the corona discharge to the upper surface of the sheet in the area in which the downwardly facing surface is engaged by the magnetic brush to produce a direct current flow through the paper sheet in the segments thereof successively contacted by the magnetic brush of a great enough magnitude to remove spurious charges from the substantially charge-free areas without effecting any substantial change in the charged areas.

5. An apparatus for developing an electrostatic image on a copy sheet comprising a magnetic brush assembly for applying a developer powder to the electrostatic image, guide means spaced from the magnetic brush assembly a sufficient distance to avoid contact with the developer powder for guiding movement of the copy sheet over the magnetic brush assembly with the electrostatic image facing downwardly to be engaged by the magnetic brush so that the developer powder is selectively retained on the surface of the copy sheet in a pattern determined by the electrostatic image, a corona discharge wire extending transverse to the path of movement of the copy sheet and disposed above the back of the copy sheet in substantial alignment with the area of contact of the magnetic brush with the image bearing surface of the copy sheet, and a potential source connected to the corona discharge wire and adapted to supply a voltage to the wire in the range from four thousand to six thousand volts.

6. An apparatus for developing an electrostatic image on a copy sheet comprising a magnetic brush assembly for applying a developer powder to the electrostatic image, guide means for guiding the copy sheet into a position with the image bearing surface of the sheet engaging the magnetic brush, said guide means including a transverse- 1y extending dielectric member having a sheet engaging portion disposed generally to one side of the magnetic brush and spaced from the magnetic brush a distance sufiicient to prevent contact of the member by the brush, said copy sheet being fed from said one side into engagement with the dielectric member and said member directing the copy sheet toward the magnetic brush, corona discharge means spaced from the magnetic brush assembly for applying a corona discharge to the back of the copy sheet in proximity to the area of contact between the magnetic brush and the image bearing surface of the copy sheet, and a potential source connected to the corona discharge means and adapted to supply a voltage to the corona discharge means in the range from four thousand to six thousand volts, the dielectric member being mounted generally between the magnetic brush and the corona discharge means so that the back of the copy sheet is shielded from the corona discharge by the dielectric member until the corresponding image bearing surface of the copy sheet is contacted by the magnetic brush.

7. The apparatus set forth in claim 6 including means for adjusting the position of the guide means and the corona discharge means relative to the magnetic brush assembly.

8. A developer unit for use with a copy sheet having an electrostatic image on one surface thereof comprising a support, a magnetic brush assembly carried on the support and including means providing a moving bristle-like formation, first and second sheet engaging means mounted on the support and disposed on opposite sides of the bristle-like formation along a path of sheet movement through the developer unit, the bristle-like formation being disposed on one side of the path to contact the image bearing surface of the sheet and the sheet engaging means being disposed on the opposite side of the path to engage the back of the sheet, the first and second sheet engaging means being spaced from contact with the bristle-like formation to avoid contact with particles therefrom and the area between the first and second sheet engaging means on said opposite side of the path being open and free of any structure that could be contacted by the bristlelike formation, the first sheet engaging means having a guiding suface for engaging the back of the sheet as it is fed into the developer unit along the path and for moving the image bearing surface into engagement with the bristle-like formation, a corona discharge unit disposed on said opposite side of the path and in the open area between the first and second sheet engaging means for applying a corona discharge to the back of the sheet in the area of contact between the bristle-like formation and the image bearing surface of the sheet, a potential source connected to the corona discharge unit and adapted to supply voltage to the corona discharge unit in the range from four thousand to six thousand volts, and a structure on the support located on said opposite side of the path and interposed between the corona discharge unit and the back of the sheet to prevent the application of the corona discharge to the back of the sheet before the corresponding area of the image bearing surface of the sheet is contacted by the bristle-like formation.

9. The unit set forth in claim 8 in which the structure forms a part of the first sheet engaging means.

10. The unit set forth in claim 9 including means mounting the corona discharge means on the first sheet engaging means.

11. The unit set forth in claim 10 including means for movably mounting the first sheet engaging means on the support to permit the position of the first sheet engaging means and the corona discharge means to be concurrently adjusted relative to the bristle-like formation without changing their positions relative to each other.

12. In an apparatus for making electrostatic reproductions of a graphic original on an electrophotographic member having an insulating photoconductive layer on a paper substrate, the photoconductive layer having a latent electrostatic image of one polarity, the combination comprising developer means having a magnetic brush made up of magnetic particles and electroscopic powder including magnetic means to form said magnetic brush for applying said electroscopic powder to the latent image, said magnetic brush being connected to ground, a corona charging means for applying a corona discharge of a second polarity opposite to the one polarity to the back of said member opposite the magnetic brush whereby said member is driven to a potential removed from ground potential by conduction, and a potential source connected to the second corona charging means and adapted to supply a voltage to the second corona charging means in the range from four thousand to six thousand volts.

References Cited UNITED STATES PATENTS 2,815,449 12/1957 Giaimo 117--17.5 X 2,854,947 10/1958 Giaimo 1.7 X 2,890,968 6/1959 Giaimo 11717.5 2,904,000 9/1959 Fisher et al 95-17 X 2,951,443 9/1960 Byrne 11717.5 X 2,952,241 9/1960 Clark et al 11717.5 X 2,956,487 10/1960 Giaimo 951.7 2,965,481 12/1960 Gundlach 117-17.5 X 3,003,462 10/1961 Streich 11717.5 X 3,037,478 6/1962 Lace 11717.5 X 3,058,444 10/1962 Sugarman et al. 117--17.5 X 3,088,386 5/1963 Sugarman 95-1.7 3,117,884 1/1964 Greig 117-17.5 3,145,122 8/1964 Streich 118-637 3,150,003 9/1964 Murray et al 11717.5 X 3,232,190 2/1966 Willmott 11717.5 X 2,297,691 10/1942 Carlson 11717.5 X 2,965,573 12/1960 Gundlach 11717.5 X 3,011,473 12/1961 Gundlach 11717.5 X 3,094,429 6/1963 Howell 11717.5

WILLIAM D, MARTIN, Primary Examiner.

EDWARD J. CABIC, Assistant Examiner.

Claims

1. A METHOD OF DEVELOPING THE IMAGE BEARING AREA ON ONE SURFACE OF A COPY SHEET CONTAINING AN ELECTROSTATIC CHARGE WITHOUT DEVELOPING NONIMAGE AREAS ON THE SAME SURFACE OF THE SHEET HAVING NO MORE THAN A SMALL AMOUNT OF UNDERSIRED CHARGE, WHICH METHOD COMPRISES THE STEPS OF APPLYING A MAGNETIC BRUSH FORMED BY A MIXTURE OF MAGNETIC PARTICLES AND DEVELOPER POWDER TO THE IMAGE AND NONIMAGE AREAS ON THE ONE SURFACE OF THE COPY SHEET TO PERMIT THE RETENTION OF THE DEVELOPER POWDER BY THE CHARGE IN THE IMAGE AREAS, AND CONCURRENTLY APPLYING A VOLTAGE IN THE RANGE FROMFOUR THOUSAND TO SIX THOUSAND VOLTS TO ELECTRODE MEANS TO APPLY A CORONA DISCHARGE TO THE OPPOSITE SURFACE OF THE COPY SHEET TO PRODUCE A DIRECT CURRENT FLOW THROUGH THE COPY SHEET OF SUCH PROPORTIONS AS TO CARRY OFF THE UNDESIRED CHARGE IN THE NONIMAGE AREAS WITHOUT AFFECTING THE ABILITY OF THE CHARGE IN THE IMAGE AREAS TO RETAIN THE APPLIED DEVELOPER POWDER.