US3218968A - Multicolor electrostatic printing - Google Patents

Multicolor electrostatic printing Download PDF

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Publication number
US3218968A
US3218968A US245321A US24532162A US3218968A US 3218968 A US3218968 A US 3218968A US 245321 A US245321 A US 245321A US 24532162 A US24532162 A US 24532162A US 3218968 A US3218968 A US 3218968A
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US
United States
Prior art keywords
screen
printing
polarity
powder
electric field
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US245321A
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English (en)
Inventor
Clyde O Childress
Samuel E Graf
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Electrostatic Printing Corp of America
Original Assignee
Electrostatic Printing Corp of America
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Electrostatic Printing Corp of America filed Critical Electrostatic Printing Corp of America
Priority to US245321A priority Critical patent/US3218968A/en
Priority to GB44989/63A priority patent/GB978408A/en
Priority to FR954992A priority patent/FR1383205A/fr
Priority to DEE25952A priority patent/DE1277875B/de
Priority to CH1491563A priority patent/CH435333A/de
Application granted granted Critical
Publication of US3218968A publication Critical patent/US3218968A/en
Anticipated expiration legal-status Critical
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G17/00—Electrographic processes using patterns other than charge patterns, e.g. an electric conductivity pattern; Processes involving a migration, e.g. photoelectrophoresis, photoelectrosolography; Processes involving a selective transfer, e.g. electrophoto-adhesive processes; Apparatus essentially involving a single such process
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41M—PRINTING, DUPLICATING, MARKING, OR COPYING PROCESSES; COLOUR PRINTING
    • B41M1/00—Inking and printing with a printer's forme
    • B41M1/12—Stencil printing; Silk-screen printing
    • B41M1/125—Stencil printing; Silk-screen printing using a field of force, e.g. an electrostatic field, or an electric current
    • G—PHYSICS
    • G06—COMPUTING OR CALCULATING; COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K15/00—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers
    • G06K15/02—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers using printers
    • G06K15/14—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers using printers by electrographic printing, e.g. xerography; by magnetographic printing
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S101/00—Printing
    • Y10S101/37—Printing employing electrostatic force

Definitions

  • the material to be printed upon is inserted into the field whereupon it intercepts the powder which is introduced into the field, and which maintains the shape of the open aperture in the screen.
  • An object of this invention is the provision of a novel arrangement for printing more than one color using electrostatic printing techniques.
  • Another object of this invention is the provision of an arrangement for printing a plurality of colors with electrostatic techniques which is more rapid than previously known systems.
  • Yet another object of the present invention is the provision of an arrangement for multi-color printing using electrostatic techniques which is simpler than previous known arrangements.
  • a dry ink consisting of a mixture of two differently colored finely ground or powdered resins. These are selected so that the triboelectric inter-actions between the two resins, the brush which is employed for introducing them into the electric field, and the metal screen, all coact 3,218,968 Patented Nov. 23, 1965 ice to impart a charge of one sign exclusively to one of the colored resins and a charge of the opposite sign to the other resin.
  • the other powder component can pass through the screen into the electric field to be deposited on the surface on which printing is to occur, while the first powder is rejected by the electric field.
  • FIGURE 1 is a diagram of an embodiment of this invention
  • FIGURE 2 is a diagram of another embodiment of this invention.
  • FIGURES 3 through 6 show various other possible arrangements for this invention.
  • FIGURE 1 there may be seen a diagram of an embodiment of this invention in its simplest form.
  • This comprises a conductive backing plate 10, and a printing element 12, which may comprise a fine mesh open screen of conductive material or material which is rendered conductive, in the printing areas and the non-printing areas being suitably masked.
  • the printing area is represented by the image of a flask 12A. Spaced from the screen is a sheet of paper 14, on which the printing is to occur.
  • a source of direct current potential 16 is connected to a double pole double throw reversing switch 18.
  • the reversing switch is connected to the conductive backing plate 10, and the screen 12, ina manner so that, when the switch is in one position the plate and screen are connected to opposite terminals of the potential source 16, and when the switch is in the other of its two operative positions, the polarity of the connections of the plate and screen to the source of potential 16, is reversed.
  • an electric field is established between the screen and stencil which has a polarity as determined by the position of the reversing switch.
  • a pigment powder of a suitable type and color is applied to the open mesh region 12A of the stencil screen, and under the influence of the electric field the powder is drawn toward the conductive backing plate. However, it is intercepted by the paper 14 to which it adheres in the shape of the open mesh region 12A of the screen 12. The paper can then be withdrawn from between the screen and backplate and the pigment may be fixed thereon by the application of heat, or by spraying a transparent fixative over the powder image.
  • the manner of introducing the pigment powder into the electric field may be for example, by employing a brush, here designated as a rotating brush 20 of natural animal hair or of a suitable synthetic fiber, which in rotating passes through suitable toner powder particles which are held in the container 22.
  • the powder particles may consist of a mixture of two differently colored finely ground resins. These are selected so that the triboelectric interactions between the two resins, the brush and the metal screen impart a charge of one sign exclusively to one of the colored resins and a charge of the opposite sign to the other resin.
  • suitable powders to form a mixture of the type described may comprise a red toner, sold by the Switzer Company of Cleveland, Ohio, under the commercial designation of Rocket A-13 Red Toner, sold by Radiant Color Corp., of Oakland, California, a blue toner, under the commercial designation Velva-Glo Blue R-l03.
  • the paper 14 was substantially in contact with a metal backplate and was spaced away from the screen by a distance on the order of .025 inch.
  • a brush of natural animal hair or of a suitable synthetic fiber was employed to pick up the powder from the container 22 and brush it against the screen printing element.
  • FIGURE 2 wherein there may be seen an arrangement of an embodiment of the invention for employing the concept described above in an automatic fashion.
  • Screen printing stencils 30, 32, 34, and others, not shown, are mounted in a belt arrangement 36.
  • This end-less belt is rotatably moved by means of two rollers 38, 40, driven by a motor 42.
  • the belt 36, holding the stencil screens, is mounted over the two rollers to be rotatably driven as the rollers are rotated.
  • the web 46 Spaced from the belt arrangement of the screens is the backing plate 44.
  • the web 46 on which writing is to take place, is positioned between the backing plate 44 and the belt stencil screen arrangements. In order to preserve clarity in the invention however, it is here fragmented. It will be understood however, that in the normal course of writing, the web 46 is interposed between the backing plate 44 and the stencil screen belt 36.
  • the figures or pattern of openings in the stencil screen may be complementary in the sense that for example, the stencil screen 32 may have the central pattern desired to be printed and the stencil screen 30 may have the frame around the central pattern. It is desired to print the central pattern in one color and the frame in the second color.
  • a two color pigment powder and applicator 48 of the type described in connection with FIGURE 1, is positioned so that the pigment powders are applied to the stencil screen which is in the writing position opposite the backing plate 44.
  • an electromechanical pulse generator 50 This can constitute any known arrangement for generating pulses when driven by means of a shaft 52.
  • This can be for example, a wheel having magnets in the periphery which pass by a pickup coil as the wheel rotates to induce voltages in the pickup coil.
  • These may be subsequently amplified and shaped for application to a cyclic counter 54, to advance the count thereof.
  • Each output of the counter 54 is connected to a separate switch respectively 56, 58, 60, 62, 64. These switches are individually settable to be open or closed as desired, in order to program the polarity of the potential to be applied between the stencil screens on the belt 36 and the back plate 44.
  • All of these switches in turn are connected to a relay coil 66, which has a pair of double pole double throw contacts 68.
  • the contacts 68 have a normally closed position, wherein they connect the positive terminal of a source of operating potential 70 to a contact brush 72 and the negative source of operating potential is connected to the backing plate 44.
  • the relay 66 is operated by reason of one of these switches 56 through 64 being closed, and by reason of the counter 54 having the count condition which can apply its output through the closed switch to the relay 66, then the double pole double throw contact 68 reverses the polarity of the voltage being applied to the backplate 44 and contact brush 72.
  • the contact brush 72 makes contact with conductive extension tabs for example, 32A, which connect to each one of the stencils 30, 32, 34, on the belt. Accordingly, the brush 72 and contact tab 32A, serve not only to connect operating potential to the screen, but also to apply the potential only when the screen is located at the proper writing position, with respect to the backplate 44. Proper powder transfer occurs only upon the application of potential between screen and backplate. This is triggered and maintained when, and over the interval brush 72 and tab 32A are in contact with each other.
  • the stencil 32 contains the central pattern of the figures desired to be printed, and the counter is placed in its first count condition, the switch 56 being open, the relay 66 is not energized and therefore, the central pattern is printed upon the web 46 with a colored powder which has a negative triboelectric charge.
  • the stencil 30 is brought into posi tion. This stencil contains openings which define a border pattern.
  • the pulse generator 50 applies its second pulse to the counter 54.
  • the second switch 58 is closed and therefore relay 66 is operated. This enables the relay contacts 68 to reverse the polarity of the voltages applied between the stencil 30 and the backplate 44. Accordingly, the other colored powder will pass through the openings in the stencil and be deposited upon the web 46. A two color figure has now been produced.
  • the web advances to enable the next pattern to be printed.
  • the program for the polarity to be applied between screen and backing plate is established by means of a counter 54, the switches 56, and the relay 66.
  • FIGURE 2 shows an arrangement for printing, using a mixture of two pigment powders, each having opposite polarity triboelectric properties, it is possible to simultaneously print with the scheme shown in FIGURE 3, more than one image.
  • the belt 36 In order to operate in this manner, the belt 36, must carry more than one stencil screen respectively 74A, 74B, 76A, 76B, 78A, 78B, 80A, 80B, in a direction transverse to the direction of rotation. Two of these stencils are shown by way of example, although the scheme shown can be extended to include any desired number.
  • the web 46 upon which printing is to occur, as described before,'is shown fragmented in order to enable a better presentation in the drawing.
  • a backing plate respectively 82A, 82B is provided for each one of the screens at the writing position.
  • Each one of the screens 74A through 80A, and 74B through 80B has its own associated conductive tab respectively 84A, 84B, 86A, 86B, 88A, 88B, 90A, 90B, which extend from the stencil screen toward the edge of the belt to a location where the respective A and B tabs can respectively contact brushes 92A, 92B.
  • Each one of the brushes 92A, 92B is respectively con nected to a separate polarity programing circuit respectively 93A, 94B.
  • Each one of the polarity programing circuits constitutes a separate array of settable switches, one switch in each array being connected to the same output of a common cyclic counter 96.
  • each one of the polarity program circuits also includes, besides a set of switches 56 through 64, such as is shown in FIGURE 2, a relay 66, 68, connected to a source of potential 70.
  • the structure of the polarity programing circuits respectively 94A, 94B are identical with that shown and described in FIGURE 2 for the purpose of programing the polarity of the respective potentials being applied between the backplate 82A and whichover one of the A stencil screens attains the writing position, and between the backplate 82B and whichever one of the B stencil screens reaches the writing position.
  • Proper powder transfer is triggered and maintained when and over theinterval of contact of brushes 92A, 92B with the respective tabs 84A through 90A, 84B through 90B.
  • FIGURE 4 a web 98, representative of material upon which electrostatic printing is to occur, is made of conductive material. Accordingly, contact is made directly thereto by means of a brush 100, which is connected. to a polarity programing circuit 102.
  • the structure represented by the rectangle 102, is the same as is shown in FIGURE 2, comprising switches 56, through 64, and the reversible relay 66, 68, and the source of potential 70.
  • the remaining structure shown in FIGURE 4 is given the same reference numerals as are applied in FIGURE 2, since their functions are substantially identical.
  • each one of the screen stencils 30, 32, 34 reaches the printing position, its conductive tab respectively 30A, 32A, 34A, contacts the brush 72 and thus an electric field is established between the screen stencil at the printing position and the conductive web 98, and more specifically with the portion of the conductive web which is opposite the screen stencil at the time.
  • the setting of the switches in the polarity programing circuits determines the polarity established between screen and web and therefore the color of the triboelectric powder which is attracted toward the web which therefore is printed out in the pattern established by the opened aperture portion of the screen.
  • the operation of the embodiment of the invention, shown in FIGURE 4 accordingly is substantially identical with the operation of the embodiment of the invention shown in FIGURE 2, except that the web 46 and the conductive backing plate 44 have their functions combined in the material 98 upon which it is desired to print.
  • FIGURE 3 It was shown in FIGURE 3 that it is possible to simultaneously print from two screen stencils using a single counter which instructs two polarity programing circuits.
  • This nmber shown by way of illustration, is not to be construed as a limitation, since it is possible, of course, to drive more than two.
  • FIGURE 5 shows an arrangement for printing simultaneously with two colors using the principles of the invention which have been described.
  • a center tapped power supply has the center tap connected to the conductive backing plate 112 (or to the material upon which printing is desired if such material can serve as an electrode in establishing the electric field).
  • the backing plate can be a single conductive sheet, or to eliminate possible effects of a fringing field caused by the split construction of the screen 114, can consist of two conductive sections 112A, 112B separated by an insulating section 113. Connection is made to both sections 112A, 112B from the center tap of the power supply 110.
  • the stencil screen 114 consists of two conductive portions respectively 114A and 114B, which are separated from one another by an insulating portion 116.
  • the conductive portion 114B is connected to the negative terminal of the center tap source of potential and the conductive portion 114A is connected to the positive terminal of the center tap source of potential.
  • the two sections 112A, 1123, are preferably disposed opposite the portions 114A, 114B.
  • the powder in the container 118 is applied over the entire screen by an applicator brush 120, which is moved in a manner to pick up this powder and apply it to the screen.
  • the web 121 is positioned between the backplate and the screen 114.
  • the conductive backing plate 112 is negative with respect to the outer portion, 114A, of the screen 114 and therefore powder particles having positive triboelectric charges will be drawn through the screen apertures in the outer portion to be deposited upon the web 121.
  • the conductive backing plate is positive with respect to the central portion, 114B, of the stencil screen 114 and therefore, those powder particles which have a charge such as to be attracted toward the relatively positive conductive backing plate will be drawn through the apertures at the center of the screen toward the web 121.
  • FIGURE 6 shows an arrangement for electrostatic printing on opposite sides of a web 122, using two polarity screens respectively 124, 126, which are positioned on opposite sides of the web.
  • Each of the two screens has an outer conductive section, respectively 124A, 126A, and an inner conductive screen section respectively 124B, 126B. These are contacted by brushes which connect by leads to a polarity programing circuit 128.
  • the polarity programing circuit may be of the type described previously, which consists of a relay, switches and a counter for establishing desired voltage relationships between opposite screens 124, 126, as well as between the inner and outer conductive sections of the screens themselves.
  • the two color powder containers 130, 132 are positioned in the manner previously described, to apply to the respective stencil screens triboelectric powder particles having mutually opposite polarity charges.
  • An electric field is established between the screen section 12413 and the screen section 126B and the web which is interposed therebetween will be printed on both sides with a powder pattern having the configuration of the open or unmasked portions of the screen. There will be a different colored powder deposited on opposite sides of the paper. Similarly, printing will occur through the open portions of the screens 124A and 126A with opposite color toner powders.
  • the two resins will print selectively when no brush is used for pushing the resin toner particles through a screen but instead they are deposited upon a vibration agitated screen directly from a grounded metal container and an electric field of one or the other relative polarity is established.
  • the two powders triboelectrically react with one another so as to acquire charges of the opposite sign, whereby they may be printed selectively as a function of a polarity of a field created between the charged printing element and a conductive backing plate, which may be the material on which the printing is to occur if it is conductive.
  • An electrostatic printing system for printing with a mixture of at least two powders having different physical properties and having relatively opposite polarity triboelectric properties, said system comprising means for establishing an electric field including two opposite electrodes, one of said opposite electrodes comprising a screen having two apertured portions therein in a pattern defining the desired printing patterns, said two apertured portions being conductive and being separated from one another by an insulating region, means for applying said powder mixture to the two apertured regions of said screen, the apertures in said screen being larger than particles of said powder, a source of operating potential having two opposite polarity output terminals, means for connecting one of said two terminals to one of said apertured regions and the other of said two terminals to said opposite electrode, whereby only one of said two powders can pass through said apertures into said electric field to be carried thereby away from said screen to said other of said opposite electrodes, and means for connecting said one of said two terminals to said opposite electrode and the other of said two terminals to said other apertured region of said screen whereby only the other of said two
  • An electrostatic printing system for printing with a powder mixture including at least two powders having different physical properties and having opposite .polarity triboelectric properties, said system comprising a pair of oppositely spaced electrodes one of said electrodes being a screen having two conductive regions spaced apart by an insulating region, each of said conductive regions including a plurality of apertures in a pattern desired to be printed, each of said apertures being larger than the particles of said powder mixture, means for applying a first potential to the first conductive region of said screen, means for applying a second potential having a polarity opposite to that of said first potential to the second conductive region of said screen, means for applying a third potential intermediate said first and second potential to said other electrode whereby a first electric field is established between said first conductive region and said other electrode, and a second electric field is established between said second conductive region and said other electrode, and means for applying said powder mixture to said first and second regions of said screen whereby only one of said powders will pass through one of said regions into said electric field to be carried by
  • a system for electrostatic printing with a powder mixture including at least two powders having different physical properties and having relatively opposite polarity triboelectric properties comprising means establishing a printing station including a plurality of screens each of which has a conductive region therein including apertures in a pattern defining the desired form of said printing, the particles of powder in said powder mixture being smaller than the apertures of said screen, electrode means, means for moving each one of said plurality of screens in sequence to a position spaced opposite said electrode means to thereby establish said printing position, means for applying said powder mixture to said screen when in said printing position, a source of potental, means for predetermining with what polarity connection is made from said source of potential to said electrode means and a screen at said printing position,

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • General Engineering & Computer Science (AREA)
  • Theoretical Computer Science (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Molecular Biology (AREA)
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US245321A 1962-12-17 1962-12-17 Multicolor electrostatic printing Expired - Lifetime US3218968A (en)

Priority Applications (5)

Application Number Priority Date Filing Date Title
US245321A US3218968A (en) 1962-12-17 1962-12-17 Multicolor electrostatic printing
GB44989/63A GB978408A (en) 1962-12-17 1963-11-14 Arrangement and process for electrostatic printing
FR954992A FR1383205A (fr) 1962-12-17 1963-11-26 Procédé et appareil pour l'impression électrostatique
DEE25952A DE1277875B (de) 1962-12-17 1963-12-03 Elektrostatisches Druckverfahren
CH1491563A CH435333A (de) 1962-12-17 1963-12-05 Verfahren zum elektrostatischen Drucken auf nichttextiles Material und elektrostatische Druckvorrichtung zur Durchführung des Verfahrens

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Application Number Priority Date Filing Date Title
US245321A US3218968A (en) 1962-12-17 1962-12-17 Multicolor electrostatic printing

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US3218968A true US3218968A (en) 1965-11-23

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US245321A Expired - Lifetime US3218968A (en) 1962-12-17 1962-12-17 Multicolor electrostatic printing

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CH (1) CH435333A (de)
DE (1) DE1277875B (de)
GB (1) GB978408A (de)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3301179A (en) * 1965-03-15 1967-01-31 Owens Illinois Inc Electrostatic printing with density control provided by charge measuring means
US3302560A (en) * 1965-06-11 1967-02-07 Mousanto Company Semi-automatic electrostatic printing system having moving screen
US3302579A (en) * 1965-08-25 1967-02-07 Monsanto Co Electrostatic printing with oscillating screen frame and dual printing at a single station
US3302580A (en) * 1965-08-25 1967-02-07 Monsanto Co Electrostatic printing with rotating screen frame and plural print stations
US3343483A (en) * 1965-03-15 1967-09-26 Unimark Corp Electrostatic printing with stencils mounted on a drum
US3392667A (en) * 1965-06-07 1968-07-16 Interchem Corp Multicolor electrostatic printing
US3402659A (en) * 1966-08-29 1968-09-24 Owens Illinois Inc Electrical printing processes employing two fields of different strengths
US3589283A (en) * 1969-02-19 1971-06-29 Electrostatic Printing Corp Multicolor electrostatic printing system
US3665856A (en) * 1970-02-24 1972-05-30 Heller William C Jun Printing method using electric through-field to indelibly lodge particles

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2890968A (en) * 1955-06-02 1959-06-16 Rca Corp Electrostatic printing process and developer composition therefor
US3081698A (en) * 1960-03-04 1963-03-19 Electrostatic Printing Corp Electrostatic printing system
US3103445A (en) * 1963-09-10 Method of developing an electrostatic

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3103445A (en) * 1963-09-10 Method of developing an electrostatic
US2890968A (en) * 1955-06-02 1959-06-16 Rca Corp Electrostatic printing process and developer composition therefor
US3081698A (en) * 1960-03-04 1963-03-19 Electrostatic Printing Corp Electrostatic printing system

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3301179A (en) * 1965-03-15 1967-01-31 Owens Illinois Inc Electrostatic printing with density control provided by charge measuring means
US3343483A (en) * 1965-03-15 1967-09-26 Unimark Corp Electrostatic printing with stencils mounted on a drum
US3392667A (en) * 1965-06-07 1968-07-16 Interchem Corp Multicolor electrostatic printing
US3302560A (en) * 1965-06-11 1967-02-07 Mousanto Company Semi-automatic electrostatic printing system having moving screen
US3302579A (en) * 1965-08-25 1967-02-07 Monsanto Co Electrostatic printing with oscillating screen frame and dual printing at a single station
US3302580A (en) * 1965-08-25 1967-02-07 Monsanto Co Electrostatic printing with rotating screen frame and plural print stations
US3402659A (en) * 1966-08-29 1968-09-24 Owens Illinois Inc Electrical printing processes employing two fields of different strengths
US3589283A (en) * 1969-02-19 1971-06-29 Electrostatic Printing Corp Multicolor electrostatic printing system
US3665856A (en) * 1970-02-24 1972-05-30 Heller William C Jun Printing method using electric through-field to indelibly lodge particles

Also Published As

Publication number Publication date
GB978408A (en) 1964-12-23
DE1277875B (de) 1968-09-19
CH435333A (de) 1967-05-15

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