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Publication numberUS3561356 A
Publication typeGrant
Publication dateFeb 9, 1971
Filing dateFeb 24, 1967
Priority dateFeb 24, 1967
Also published asDE1671527A1, DE1671527B2, DE1671527C3
Publication numberUS 3561356 A, US 3561356A, US-A-3561356, US3561356 A, US3561356A
InventorsBuck James G, Higgins Edward D, Javorik Laszlo J, Kennedy John B Jr, Rarey Kenneth W
Original AssigneeContinental Can Co
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Precharging of substrate for electrostatic printing
US 3561356 A
Images(3)
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Description  (OCR text may contain errors)

Kenneth W. Rarey South Holland;

James G. Buck, Western Springs; John B.

Kennedy, Jr., Western Springs; Laszlo J.

Javorik, Chicago; Edward D. Higgins,

Palos Heights, 111.

[21] Appl. No. 618,395

[22] Filed Feb. 24, 1967 [45] Patented Feb. 9, 1971 [73] Assignee Continental Can Company New York, N.Y.

a corporation of New York Continuation-impart of application Ser. No. 396,060, Sept. 14, 1964, now Patent No. 3,306,193, which is a Continuation-'in-part of application Ser. No. 599,822, Dec. 7, 1966, which is a division of application Ser. No. 396,060 which is a Continuation-impart of application Ser. No. 409,213, Nov. 5, 1964, now abandoned and a continuation-in-part of 609,275, Jan. 10, 1967, which is a continuation of application Ser. No.

[72] Inventors [54] PRECHARGING OF SUBSTRATE FOR ELECTROSTATIC PRINTING [51] Int. Cl 1341f 15/14 [50] Field ofSearch 101/114,

(ESD), 129, 426; 317/2; 313/325 [56] References Cited UNITED STATES PATENTS 2,940,864 6/1960 Watson 10l/ESD 3,241,483 3/1966 Duff l0l/ESD 1,653,599 12/1927 Chapman 317/2 1,678,869 7/1928 Morrison..... 317/2 1,903,840 4/1933 Simons 317/2 2,087,915 7/1937 Kimball.... 317/2 2,163,294 6/1939 Simons 317/2 2,394,656 2/1946 Beregh 317/2X 2,944,147 7/1960 Bolton lOI/ESD 2,965,481 12/1960 Gundlach IOI/ESD 3,081,698 3/1963 Childress et a1. l01/ESD 3,102,045 8/1963 Metcalfe et al.-.. 101/ESD 3,273,496 9/1966 Melmon l01/ESD 3,295,440 l/1967 Rareyetal. 101/114 3,296,963 1/1967 Rarey et a1. 101/114 3,306,193 2/1967 Rarey et a1. 101/114 3,306,198 2/1967 Rarey lOI/ESD Primary Examiner-Edgar S. Burr Attorney-Diller, Brown, Romik & Holt ABSTRACT: Various methods and apparatus are disclosed for the treatment of poorly conducting substrates with ions so that the substrate is more receptive to printing material and the printing material is more adherent to the substrate prior to being fused thereon.

ATENTED FEB 9 I97! INVENTORS KENNETH w. EAQEY JOHN aueuuem, 1k,

JAMES G. BUCK,

LASZLO JJAVORW 8r" w FEM Q ERB'P flfi'ij ATTORNIZYfi PRECHARGING OF SUBSTRATE FOR ELECTROSTATIC PRINTING CROSS-REFERENCE TO RELATED APPLICATIONS 409,2l3, filed on Nov. 5, 1964, now abandoned; and a continuation-in-part of application Ser. No. 609,275 filed Jan. 10, 1967 as a continuation of application Ser. No. 409,213.

BACKGROUND OF THE INVENTION The invention relates to the field of corona phenomena and particularly to the application of such phenomena to the field of electrostatic printing.

Prior art electrostatic printing methods and apparatus utilize toner for producing the desired image upon a substrate. The toner is generally provided with an electrostatic charge of a single polarity and, by electrical means, is deposited upon a substrate. Consequently, initially deposited toner creates an electric field which tends to repel deposition'of additional toner having the same polarity. Additional toner can be deposited if an electric field is utilized to effect the deposition; however, during transfer of the printed substrate to a fixing or fusing station, the substrate is removed from the influence of the electric field and part of the deposited toner is repelled from the substrate prior to the fixing or fusing operation. As a result, top-quality printing cannot be achieved by prior art devices.

SUMMARY OF THE INVENTION The present invention provides improved methods and apparatus for depositing toner upon a substrate and for holding the toner upon the substrate until it is fixed or fused thereon.

In addition, the invention provides improved apparatus for creating and maintaining corona discharges and control means for governing the quantitative and qualitative distribution of ions upon an article to be charged.

Printing rates for poorly conducting materials such as paper, cardboard, paperboard and the like have heretofore been relatively slow, as compared to the printing rates of the present invention, and have been considerably slow under conditions of low relative humidity. With the ambient atmosphere having a relative humidity of 20 percent, or less, printing rates greater than 100 impressions per minute could not be attained; the present invention provides for improved printing rates, and rates in. excess of I impressions per minute have been attained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic elevational view illustrating one embodiment of the present invention;

FIG. 2 is a diagrammatic elevational view, partly in section, showing one operative embodiment of a printing machine embodying the present invention;

FIG. 3 is a diagrammatic elevational view illustrating a second embodiment of the invention wherein the substrate is precharged at a precharging location outside of the printing zone;

FIG. 4 is a diagrammatic elevational view illustrating another embodiment of the invention wherein the substrate is precharged outside of the printing zone but on the side of the substrate upon which printing will occur;

FIG. 5 is a diagrammatic view of a second form of printing machine embodying the present invention;

' FIG. 6 is a diagrammatic perspective view illustrating one form of the invention wherein a flow of air is used to move ions into contact with a substrate to be charged;

FIG. 7 is a diagrammatic elevational view, similar to FIG. 6, but showing the substrate as being charged on the surface upon which printing is to be performed;

FIG. 8 is a graphical representation qualitatively illustrating the potential of a card, or substrate, with respect to time before printing, during printing, after printing and after removal of the card from the printing location during a process which includes charging of the card, or substrate, both prior to and during printing;

FIG. 8-A is a graphical representation qualitatively illustrating the net charge on a card, with respect to time, during a printing process performed as in FIG. 8;

FIG. 9 is a graphical representation, similar to FIG. 8, but for a printing process wherein the cards, or substrates. are charged only prior to printing;

FIG. 9-A is a graphical representation similar to FIG. 8-A but for a process as in FIG. 9 wherein charging occurs only prior to printing;

FIG. 10 is a graphical representation similar to FIGS. 8 and 9 except that the card, or substrate, is simply interposed between a stencil and backing electrode and only toner deposition during the printing process adds charge to the card;

FIG. 10-A is a graphical representation similar to FIGS. 8-A and 9-A except that the printing process is as described with respect to FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates one embodiment of apparatus for practicing the present invention. A corona source, generally indicated by the numeral 10, is comprised of a conductive plate member 11 having a plurality of corona needles 12 secured thereto. A conductive base electrode 14 is disposed in spaced parallel relation to the conductive plate member 11. Interposed between the conductive plate member 11 and the conductive base electrode 14 are a stencil screen 16, a substrate 18 which is to be printed upon and a conductive control grid 20.

The material with which printing isto be accomplished is illustrated as being toner 22 which is initially deposited upon the base electrode 14. The toner 22 may be fine granulated particles of conductive material, such as charcoal, or may be comprised of conductive carrier particles 23 and nonconductive toner 24 (FIG. 2) which triboelectrically adheres to the conductive carrier particles 23.

The stencil screen 16 is illustrated as being comprised of a conductive, fine wire-mesh screen 26 which has appropriate portions thereof blocked out by coating material 28 so as to provide openings 30 which permit passage of toner 22, or 24, so that the toner can impinge upon substrate 18 to print the desired pattern or image thereon. The substrate 18 is comprised of relatively nonconductive material, e.g., as compared to metal, and is generally comprised of paper, cardboard, paperboard, plastics, or similar materials.

The conductive control grid 20 is illustrated as being a perforated electrode which can be generally described as a woven screen made up of conductive wires such as might be used in conventional copper screen.

A power source 32 of direct current is provided and has connection means 33 and 34 for connecting the power source 32 across the conductive plate member 11 and the conductive base electrode 14. Switch means 36 is provided between the power source 32 and the conductive plate member 11, while switch means 38 is provided between the power source 32 and the base electrode 14. Connection means 34 may also contain a secondary power source 40 which can be connected in series with the power source 32 by the switch means 38. A voltage divider 42 is connected across the power source 32 and has an adjustable tap 44 which provides an electrical connection to the conductive control grid 20.

OPERATION OF FIRST EMBODIMENT A substantially nonconductive card or substrate 18 is placed into the printing location in spaced relation with the stencil screen 16 and the conductive control grid 20. When switch means 36 is closed an electric field is established between the corona source 10 and the conductive stencil screen 16. An

3 electricifield 'is-also established between the conductive controlt grid 20"and the corona source 10, as well as an electric field being established between the conductive control grid'20 and; the conductive stencil screen 16. The electric field between the corona source and the perforated control grid produces. a corona dischargeatthe corona needles 12. If the needles 12 are positive, they collect the negative ions while the perforatedcontrol grid 20 collects some positive ions. The electric field-between thecontrol grid 20 and the conductive stencil screen 16 causes positive ions to be deposited upon the substrate 18 and such deposition of positive ions occurs until the potential of thesubstrate 18 reaches that of the control grid 20.. After the substrate 18-is uniformly charged, closingof the switch means 38 will cause toner 22 to be charged inegatively andobe repelled by the baseelectrode 14. Some of thetoner 22passes through; openings inthe stencil screen 16 and is deposited upon the substrate l8. As the-buildupof'toner 22 onth'e substrate-18 continues, the accumulation of negatively charged'toner tendsto repel deposition of additional tonerghowever, the original positive charge on the substrate l8-is-reduced becauseof therneutralizing effect of the negatively charged toner and, since the potentialof the substrate 18 is no longer the same as-the potentialof'the control grid20, additional positive ions are deposited upon the substrate 18 from the corona source 10.

As a result, therepellingetfect of the negatively charged toner onthe substrate 18 is diminished and additional toner 22 passes through the opening 30 and becomes deposited upon the substrate 18. A printing operation as just described can be carriedout in something on the order ofone-twentieth vof-a second. After the toner 22is deposited uponthe-substrate l8, the substrate 18 is removed-from the printing location and the toner is fused-thereto by conventional processes-using heator solventvapor. It-shouldbenoted that as the substrate '18-is, removed from the electric 'fieIdat the printing station, the toner 22 will remain on the substrate l8because the accumulation of negative charges resultingfrom deposition of charged toner 22 has beeneffectively neutralized by the addition of positive ionsunder control of-the conductive control grid or perforated electrode 20.

THE EMBODIMENT OFFIG. 2

FIG. 2isa diagrammatic illustration of a printing machine which utilizes the present invention. A more complete disclosure of this printing machine is set out in application Ser. No. 396,060, filed on Sept. 14, 1964, and now US. Pat. No. 3,-306il93; A corona source, generally indicated bythe numeral48, is similar-to thecorona source 10 but may be comprised of corona-wires50 insteadof corona needles 12. A conductive control grid or perforated electrode 52 is interposed in spaced relationbetween the corona source 48and a substrate 54: A conductive stencil screen 56, which isthe same as stencil screen 16, is-disposed-in spaced relation between the substrate 54 and a conductive base electrode 581 The conductive particles 23 are preferably comprised of magnetic material and may-be uniformly-deposited upon the'base electrode58- by a magnetic drum'60 and-a magnetic roller 62. A conductive doctor blade 64uniformlyspreads the carrier particles 23,- and nonconductive-toner24 up'onsthe baseelectrode SS'and also providesfor an electricalconnection for establishing an elec- I trie field between thebaseelectrode 58'; the stencil screen 56; the control grid 52 and thecorona source 48. Suitable switch means 66, '67, 68-and 69 are utilized for connecting the various'elements-tosuitable power sources-70, 71-, 72-and73. If desired, a.mechanical-connection, indicated by the broken line 74,- can couple thexsw-itch means 66, 67 and 68 for simul ta'neous-operation; The jusbdescribedcircuitry is more particularly disclosedin the-aforementioned application Ser. No. l

4 OPERATIONOF rne smaomtisnr on-Ftoz:

At the beginning of 'the pr-inting;operation, it is preferable that a corona discharge occur above the-substrate S t-(Corona wires 50 are provided forproducing the' desired corona I discharge and an electricfield is established betweenthe stencil screen '56 and control grid- 52'to, cause positive ions formed, by the-corona discharge to be carriedto thesubs'trate. 54:. This establishes a high intensity electricjfield betweenthe substrate 54'and the stencil screen 5 6. In addition, thecharge deposited upon the substrate 54 tends to neutraliz'ethe field associated witha charged tonerimage-which is caused by accumulation of negatively charged-toner 24- upon the substrate 54. Thus, accumulation of toner 24.on. the substrate; 54:has little influence on subsequently arriving toner 24s Durin'gthis printing operation,v the carrierparticles 23 are charged by contact with the base electrode- 58]: and are repelledtherefrom' toward the stencil screen 56. The mesh size of the stencil 'screen'56' is such that carrier particles 23 cannot pass therethrough but toneri24'passesthrough the openings in the stencil screen 56 and become deposited upon the substrate 54.- The carrierparticles which are stopped by contact with the stencil'screen 56 becomereversely charged and are repelled back toward base electrode 58so that'th'e carrier particles oscillate between the base electrode. 58' andthe stencil screen 56 throughoutthe printing operation:

, After an image'of the desired optical'density'isdeposited A upon substrate 54; the sub'strate-54-is movedfrom thepriri -ti'ng zone to aconventionalfixing station where heater-solvent EMBODIMENT OE-FIG.

A potentially adverse characteristicrof the: apparatus and processes of FIGS-Q Land =2=has to do with the timelrequired forchargeto-attain a properly uniform.- distribution on=the surfaces of thesubstrates 18 and I54-which are to be printed; lf'the substrate to .be printed is 'sufficiently' conductive so "that 1 the required time forthis uniform-.dis'tributionvto occurjisvery small, as compared to the-times required to position thesubstrate, perforrnthe-print-ingoperatiomandtransfer the substrate out of'the printing-zone, then there is noproblemi However, if a relatively long timeis required; it'imposesa severe} limit on the printingrate- It hasbeen found that under condi-., tions of low relative humidity, such as up to 20 percent; printing rates greater than: l00impressions,penminute,could not be attained.

This time. problem can be. solved w -ma apparatus 1 and method illustrated diagrammaticallyin"FIG. 3; 'Thismethod has been practicedby'simply usingtwo of the unitsipreviously. disclosed and connecting the-units electrically a's shown FIG; 3. A firstcorona source-'islprovided attheprechargin location and is electricallyJconnected bywjre 8lto a-secondt corona source 82 which' is located attheprintinglocation?A? first control 'grid-84 is situated atthe -precharging location'ands is connected bywire. 8-5*to a secondcontrolgrid 8,6disposed at;

the printing location. A stencil screen-88 mayz'conveniently ex: tend between .the precharging location and the printin'g loca-w tion; ofcourse; the portion-ofjstencil screerii 88m thepri'ntin location is provided-with openings 90 in the formofth desiredrimage. The corona sources 80 -and-82 are connected t the oppositeside of a power, source 92 "from theiconductivei'i stencil screen '88. Control grids 84* and {86 are connected I" across-the power source 92by a voltage. divider 94*and-an ad justable tap 96'.-Suitable switch means-98*is provided-for clo's z ing the circuit.

OPERATION OF THE EMBODIMENT OF FIG. 3

A first substrate 100 is shown as being disposed at the printing location above that portion of the stencil 88 which contains an opening 90 for defining the desired image to be printed. Of course, it is understood that a base electrode, such as 14 or 58, is located beneath the stencil 88 in the same manner as previously described with respect to FIGS. 1 and 2. A second substrate 102 is located at a precharging location prior to entry into the printing location. The substrates 100 and 102 are shown as being comprised of cardboard stock having spaced sheets of paper material 104 and 105 separated by an intermediate sheet of fluted paper material 106. The first substrate 100 has been previously charged so that, for example, positive ions are uniformly distributed on the side of substrate 100 adjacent to the stencil screen 88. Upon closing the switch 98, toner will pass through the image-defining opening 90, from the base electrode (not shown), and become deposited upon the lowermost surface of substrate 100. Simultaneously, ions will be emitted from the corona source 82 such that control grid 86 will allow additional ions to be deposited upon the substrate 100 in order to neutralize the charge buildup due to the negatively charged toner being deposited upon the substrate. Also simultaneously, positive ions will be emitted from the corona source 80 and, under the influence of control grid 84, will become deposited upon the uppermost surface of substrate 102. After the printing of substrate 100 has been completed, it is moved out of the printing location to a fixing station (not shown) and the substrate 102 is moved into the printing location. The time required for the first printing operation and the time required for movement of the substrates 100 and 102 provide sufficient time for the ions on sheet 105 to become uniformly distributed upon sheet 104. If the time lag is too small, the printed image will have a striped effect along those portions of the flutes 106 which engage the sheet 104. With a proper time sequence, it is possible to improve the printing rate and obtain in excess of 100 impressions per minute. It has been found that charging the substrate at the precharging location is sufficient for some toners. With this arrangement, the only function the corona source 82 has is to maintain the substrate at the potential of the perforated electrode 86 while toner is being deposited. For some toners, the total amount of charge borne by the toner is sufficiently small that the potential of the substrate is not significantly altered. An example of such toner is that known as [PI 9431; for other toners, such as Xerox 914, the corona source 82 is desirable when extended solid areas are printed to a high optical density. In this case, a considerable change in potential of the substrate can occur because of the relatively large amount of charge associated with the toner.

DESCRIPTION OF THE EMBODIMENT OF IG. 4

The method and apparatus illustrated in FIG. 3 has been demonstrated as being superior to that shown in FIGS. 1 and 2. However, the situation still exists where charges are not being deposited directly onto the surface that is to receive the toner. Thus, it is still required that the charge flow through a material that can have a low electrical conductivity. It is possible to acquire independence of conductivity by a direct application of ions onto the surface to be printed. Such a method would work on material of any conductivity and at a rate limited only by the speed with which the substrates can be handled and printed. The time required for charge already deposited on the substrate to flow to, and become uniformly distributed on, the print receiving surface can be eliminated as a rate-limiting factor. Such a method and apparatus has been successfully tested and illustrated in FIG. 4.

Charging of the substrate is done by the same basic method as described in the previously discussed approaches. A power source 110 and switch means 112 are connected across a back electrode 114 and a corona source 116. A voltage divider 118 is connected by an adjustable tap 120 to a perforated electrode or control grid 122. A substrate 124 is disposed in spaced relation between the control grid 122 and the buck electrode 114. Closing the switch 112 will cause positive ions to be emitted from corona source 116 and be deposited upon the lowermost surface of substrate 124 under the influence of control grid 122. The substrate 124 and back electrode 114 combine to form a parallel-plate capacitor. The capacitance of this system is much greater than that of an isolated substrate. Thus, for any given quantity of charge required on the substrate, the voltage to which it must be charged is less when used with a back electrode than it is without one. If the capacitance of the substrate-back electrode system is C, and the potential to which it is charged is V, then these are related to the quantity of charge Q stored in the capacitor by the expression Q= CV. If the capacitance of the isolated substrate is Cc, and the potential to which it must be charged to store the same quantity of charge as that given above is Vc, then these variables are related to those of the substrate-back electrode system by the expression CV= Cc Vc.

Now, since Cc is much smaller than C, Vc must be much greater than V. As a practical matter, higher voltages aremore difficult to attain and maintain. Attainment requires a power supply of higher voltage; maintenance may require a power supply of higher current capability. This latter point may not be immediately obvious. The upper limit on the potential to which an object can be maintained in air depends to a large extent on the rate of leakage of charge from the object due to breakdown of the air. Now this breakdown, and subsequent leakage of charge, is primarily associated with discrete point of small radii on the object surface. Around these points the field intensity has the maximum value that it experiences near the object. These points are the deviations from an ideally perfectly smooth surface that any practical object must have. These flaws are so tiny that, from a practical point of view. they are always isolated from nearby surfaces, such as back electrodes or charging sources. Thus, the electrical field about a given point is a function of only its radius and electrical potential. It is not a function of the separation of such flaws from nearby objects. These flaws can produce discharges which may be continuous and may be maintained sometimes by a power supply with an adequate current capability. But any attempt to raise the voltage beyond this range can result in a catastrophic discharge to a nearby object. Thus, the highest charging of an object can generally be accomplished by use of a high-capacitance system and a relatively low voltage. Voltages such as 3-5 kilovolts are common OPERATION OF THE EMBODIMENT OF FIG. 4

Positive ions are deposited upon the lowermost surface of substrate 124 upon closing the switch means 112. The substrate 124 can then immediately be transferred by known conveying means to a printing location above stencil while the previously printed substrate 132 is being removed. With the apparatus located as disclosed, improved printing rates can be attained, as compared with those of FIGS. 1, 2 and 3.

EMBODIMENT OF FIG. 5

The printing machine, generally indicated by the numeral 140, is described in detail in application Ser. No. 409,2l3, filed on Nov. 5, 1964. Essentially, toner is charged by corona means 142 and expelled from tubes 144 so as to pass through openings 146 in a continuous belt-type stencil screen 148 so as to be deposited on substrate 150 to produce the desired image. Substrate 150 is carried by a conductive conveyor means 152 which is electrically connected to a corona source 154 by a power source 156 and switch means 158. The electrical contacts 160 cause the conveyor means 162 to function as a back electrode in a manner similar to that of back electrode 114 in FIG. 4. Positive ions are emitted from corona source 154 and become deposited upon that surface of substrate 150 upon which toner will be deposited by the printing machine 140. It will be apparent that a voltage divider means and perforated electrode or control grid may also be utilized in the same manner as described with respect to FIG. 4.

DESCRIPTION OF THE EMBODIMENT OF FIG. 6

The previously discussed charging methods have all involved electrostatic deposition of ions generated in a corona discharge. The electric field between the control grid of the charge source and either the back electrode of conductive stencil screen deposits charge generated in the corona discharge onto the substrate to be printed. However, the deposition need not be accomplished by electrostatic means. Charge can be conveyed from a corona discharge to an object to be printed by a moving airstream. The method illustrated in FIG. 6 is quite similar to the first method described; the substrate is charged in the printing location; it is fully charged prior to printing; charge should flow to and become uniform on the surface to be printed, prior to printing; ions are deposited on the substrate during printing to compensate for the toner-borne charge and maintain the card at a constant potential; after printing, the card retains a net charging that maintains the toner image in location until the toner is permanently fixed to the substrate. This form of deposition can be substituted for electrostatic deposition in each of the previously discussed charging methods.

One or more corona chambers, generally indicated by the nt metal 170 are constructed of first conductive end members 172 and have at least one opening 174 herein. The second conductive end members 176 are spaced from the end members 172 and an insulative sleeve 178 is connected between each of the end members-l72 and 176 for defining a hollow chamber 180. A power source l82'and switch means 184 are connected between the first and second conductive end members 172 and 176. A corona discharge means, such as needles 186, are provided upon the end members 176 for providing a source of ions. It is to be understood that end members 176 and needles 186 could be replaced by other types of corona discharge means such as fine wires. A flow of air is introduced into hollow chambers 180 through hollow tube means 190 which is connected to a source of pressurized air or other means for producing a flow of air such as fans, or the like.

OPERATION OF THE EMBODIMENT OF FIG. 6

A substrate 192 is located above a conductive stencil screen 194 which has openings 196 for producing the desired image. Upon closing switch 184, ions are emitted by the corona needles 186 while air flowing through the hollow chamber 180 carries the ions outwardly through the openings 174 in the end members 172 such that the ions become deposited upon the upper surface of substrate 192. This continues until the substrate becomes charged sufficiently to repel newly arriving ions. With a suitable time pause to allow the ions on substrate 192 to become uniformly distributed, toner is subsequently passed through openings 196 in the stencil 194 and becomes deposited in the desired image upon the bottom surface of substrate 192. As illustrated, the corona means 186 emits positive ions; with such an arrangement, negatively charged toner is used to produce the desired image. As the charged toner becomes attracted to and deposited upon the substrate 192, additional positive ions become deposited upon the substrate 192 and, after the printing operation, the substrate 192 is moved to a fixing station and the toner image does not deteriorate prior to fixing.

DESCRIPTION OF THE EMBODIMENT OF FIG. 7

The embodiment of FIG. 7 is somewhat similar to that in FIG. 3 in that a substrate 200 is charged at a precharging location, prior to being moved into the printing location above stencil screen 202. Likewise, charges are deposited upon that surface of substrate 200 upon which the toner image is to be deposited. Accordingly, faster printing rates are attained because there is no need to hesitate in order to permit the charges to become uniformly distributed on the substrate. Also, low conductivity of the substrate is no longer a factor.

A corona chamber, generally indicated by the numeral 204. is comprised of conductive members 206 and 208 which are separated by an insulative sleeve 210 in order to provide a hollow chamber 212. Cdrona means 214 are carried by end member 208 for emitting ions within the hollow chamber 212. Power source 216 and switch means 218 are connected across end members 206 and 208 such that, upon closing switch 218, a corona discharge is caused within the hollow chamber 212. Air flowing through tube 220 then carries the ions outwardly from the hollow chamber 212 through an opening 224 in the end member 206. i

With this embodiment, it is preferable to provide a back electrode 230 which may be grounded as at 232. The function of the back electrode 230 is to reduce the repulsive field that charges already accumulated on the substrate 200 exhibit to subsequently arriving ions. In FIG. 6, the conductive stencil screen 194 satisfies this same function.

During the time that a toner image is being deposited upon a substrate 232, the substrate 200 is being charged Subsequently, the substrate 232 is moved out of the printing location to a fixing station while substrate 200 is being moved into position adjacent the stencil screen 202. The potential of the substrate in the printing .zone, relative to the conductive stencil screen 202, is the same as that of the substrate relative to the back electrode 230 if the substrate is separated from each of these the same amount at the appropriate location.

EXPLANATION OF FIGS. 8-- 10 FIG. 8 is a graphical, qualitative representation of the potential on the substrate, or card, during the intervals of I before printing, (2) during printing, (3) after printing and (4) after removal of the substrate,or card, from the printing location. The potential in FIG. 8 occurs during that printing method where the card, or substrate, is charged from a corona source both prior to and during printing. It will be seen that once a substrate is charged to a given potential (illustrated as being positive) the potential does not vary because during the actual printing, i.e., while toner is being deposited upon the substrate, negative charges associated with the toner are balanced out by the addition of positive charges from the.

corona discharge. After printing, and after removal of the substrate from the printing location, the potential remains the same because of the balance maintained between the positive charges from the corona discharge and the negative charges associated with the toner.

FIG. 8-A graphically illustrates the net charge on the card, or substrate, for the same printing process as in FIG. 8. Before printing, the positive charges from the corona discharge produce a net charge which remains constant throughout the actual printing, after the printing, and after removal of the substrate from the printing location.

FIGS. 9 and 9-A graphically illustrate the potential and net charge on a substrate during the same time intervals as in FIGS. 8 and 8-A except that the substrate, or card, are charged only prior to printing and charge is not added during the printing except by the charged toner. It will be apparent that the potential and the net charge becomes less positive during the printing but then remains constant upon completion of the printing and after removal of the substrate from the printing location. The charge associated with IPI 943l toner is less negative than that associated with Xerox 9l4 toner, but the potential and net charge are both sensitive to toner characteristics.

FIGS. 10 and I0-A graphically illustrate that method in which the card, or substrate, is simply interposed between a stencil screen and a backing electrode without the addition of any charge being applied to the substrate from a corona discharge. The substrate is held at a potential determined by the electrostatic field associated with the printing machines.

As charged toner is applied during the printing, the potential.

of the substrate and the net charge on the substrate decrease. The potential and net charge remain constant after printing but the potential immediately decreases upon removal of the substrate from the associated electric fields while the net charge remains constant at its lowermost values.

It is readily apparent that the substrate, desirably, should be fully charged prior to printing. In addition, during printing, additional charge is to be deposited on the substrate. Part of this charge is that borne by the toner while the rest is that delivered by the corona to maintain the card at the same potential as the perforated electrode or control grid in spite of the toner depositions. If there is no leakage of charge from the card, the net charge addition during printing is near zero; after printing, the substrate is removed from the printing location and, without leakage, the accumulated charge should remain constant. This charge aids in maintaining the toner image on the substrate until permanent adhesion is produced by a fusing operation. After fusing, this charge can be neutralized by known means if desired. The electrical potential of the substrate before, during and after printing should be the same as that of the perforated electrode if leakage is avoided.

While preferred forms and arrangement of parts have been shown in illustrating the invention, and preferred methods have been described, it is to be clearly understood that various changes in details and arrangement of parts and method steps may be made without departing from the spirit and scope of the invention as defined in the appended claimed subject matter.

We claim:

1. A method of electrostatic printing comprising the steps of providing a substantially electrically nonconductive substrate to be printed upon, a first step of treating an area of said substrate with electrical charges having a first polarity for providing said substrate with an electrical potential, said area being greater than the area to be printed, applying electrically charged toner having a different polarity to less than the treated area of said substrate, maintaining said substrate at substantially said electrical potential by a second step of treating said substrate for providing additional electrical charges to compensate for the charge of the applied toner, and fusing said toner to said substrate.

2. A method as defined in claim 1, wherein said first step is comprised of treating said substrate with ions on the side thereof which is opposite to the side upon which toner is to be applied.

3. A method as defined in claim 1 wherein said first step is comprised of treating said substrate with ions on the side thereof upon which toner is to be applied.

4. A method as defined in claim 1, wherein said first step of treating and said toner applying step are performed with the substrate at the same location.

5. A method as defined in claim 1, wherein said first step of treating is performed while said substrate is at a precharging location, and said toner applying step is performed while said substrate is at a printing location.

6. A method as defined in claim 1, wherein said first step of treating is performed prior to the step of applying toner to said substrate.

7. A method as defined in claim 6 wherein said second step of treating said substrate with electrical charges is performed simultaneously with the step of applying toner to said substrate.

8. in an electrostatic printing apparatus for printing upon a substrate, electric field means for selectively depositing electrically charged toner on a substrate to form an image upon said substrate, said toner tending to migrate on said substrate in the absence of said electric field prior to fixing said toner to said substrate, means for preventing said migration of said toner prior to fixing including separate means for depositing an electrical charge upon said substrate for neutralizing the electric field associated with the charged toner which defines said image.

9. In an electrostatic printing apparatus as defined in claim 8, said separate means for depositing an electric charge being comprised of corona electrode means disposed in spaced relation to said substrate, and a control grid disposed in spaced relation from and between said corona electrode means and said substrate for controlling the flow of ions from said corona electrode means to said substrate.

10. in an electrostatic printing apparatus as defined in claim 8, said means for selectively depositing charged toner being comprised of a base electrode, a conductive stencil screen disposed in spaced relation between said substrate and said base electrode, and means for establishing an electric field between said stencil screen and said base electrode.

11. In an electrostatic printing apparatus as defined in claim 10, said separate means for depositing an electric charge being comprised of corona electrode means disposed in spaced relation to said substrate on a side thereof opposite from said stencil screen, and a control grid disposed in spaced relation from and between said corona electrode means and said substrate to control the flow of ions from said corona electrode means to said substrate.

12. In an electrostatic printing apparatus as defined in claim 8, said means for selectively depositing charged toner being comprised of a stencil screen having a pattern of openings formed therein, means for positioning said substrate adjacent to said stencil screen, and means for causing toner particles to pass through said openings and be deposited upon said substrate in a pattern determined by said pattern of openings; said separate means being comprised of means for substantially uniformly coating" said substrate with ions prior to printing upon said substrate.

13. in an electrostatic printing apparatus as defined in claim 12, said means for substantially uniformly coating said substrate including a corona discharge means for coating said substrate with ions on a side of said substrate upon which said charged toner is to be deposited.

14. In an electrostatic printing apparatus as defined in claim 8, said separate means being comprised of corona means for emitting ions, electrode means disposed in spaced relation from said corona means, and means for connecting a source of potential between said electrode means and said corona means.

15. In an electrostatic printing apparatus as defined in claim 14, grid means located in spaced relation from and between said electrode means and said corona means, and means for applying a potential to said grid means.

16. in an electrostatic printing apparatus as defined in claim 15, wherein said electrode means comprises a conductive stencil screen having openings therein defining an image to be printed upon said substrate.

17. In an electrostatic printing apparatus as defined in claim 8, wherein said separate means comprises a corona chamber including a first conductive end member having at least one opening therein, a second conductive end member spaced from said first conductive end member, an insulative sleeve connected between said first and second end members for defining a hollow chamber, means for connecting a source of potential between said first and second end members, corona discharge means on said second conductive end member for providing a source of ions, and means for providing a flow of air through said hollow chamber whereby ions can be caused to flow through said opening in said first conductive end member.

18. In an electrostatic printing apparatus as defined in claim 17,.an electrode spaced from said corona chamber a distance just large enough for allowing a substrate to be interposed in slightly spaced relation therebetween, and means for creating a potential difference between said electrode and said corona discharge means.

19. In an electrostatic printing apparatus as defined in claim 8, said printing apparatus including a printing station and a precharging station, said separate means being located at said precharging station, and being comprised of corona electrode means, a back electrode located at said precharging station and spaced from said corona electrode means, means for connecting a source of potential between said back electrode and said corona electrode means, and means for moving a substrate through said precharging station and into said printing station, said corona electrode means being located so that ions can be deposited on the side of said substrate upon which toner is to be deposited at said printing station.

20. In an electrostatic printing apparatus as defined in claim 8, said separate means comprising a first conductive member and a second conductive member disposed in spaced relation from each other, an insulative sleeve connected between said first and second conductive members for defining a hollow chamber, means for connecting a source of potential between said first and second conductive members, said first conductive member having at least one opening therein, said second conductive member comprising a source of corona discharge, and means for providing a flow of air through said hollow chamber whereby ions can be caused to flow through said opening in said first conductive member.

21-. In an electrostatic printing apparatus as defined in claim 8, said separate means comprising a first conductive member and a second conductive member disposed in spaced relation from each other, means for connecting a source of potential between said first and second conductive members, needle means connected to one of said conductive members, control means disposed in spaced relation from and located between said first and second conductive members, and means for applying a potential to said control means.

22. Apparatus as defined in claim 21, wherein said control means comprises a conductive mesh screen.

23. Apparatus as defined in claim 21, wherein said means for applying a potential to said control means comprises a voltage divider means connected across said means for connecting a source of potential between said first and second conductive members.

24. Apparatus as defined in claim 23, wherein said control means comprises a conductive mesh screen, and adjustable tap means connected between said conductive mesh screen and said voltage divider means.

25. In an electrostatic printing apparatus for printing upon a substrate, electric field means for selectively depositing electrically charged toner on a substrate to form an image upon said substrate, said toner tending to migrate on said substrate in the absence of said electric field prior to fixing said toner to. said substrate, means for preventing said migration of said toner prior to fixing including means for neutralizing the electric field associated with the charged toner which defines said image, said means for neutralizing including separate means for depositing an electrical charge upon said substrate prior to and simultaneously with the depositing of charged toner upon said substrate.

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Referenced by
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Classifications
U.S. Classification101/129, 361/229, 313/325, 101/114
International ClassificationG03G17/00, B41M1/12
Cooperative ClassificationB41M1/125, G03G17/00
European ClassificationG03G17/00, B41M1/12B