US3301179A

US3301179A - Electrostatic printing with density control provided by charge measuring means

Info

Publication number: US3301179A
Application number: US439799A
Authority: US
Inventors: William E Johnson
Original assignee: Owens Illinois Inc
Current assignee: OI Glass Inc
Priority date: 1965-03-15
Filing date: 1965-03-15
Publication date: 1967-01-31
Anticipated expiration: 1984-01-31

Description

Jan. 31, 1967 w. E. JOHNSON 301,179

ELECTROSTATIC PRINTING WITH DENSITY CONTROL PROVIDED BY CHARGE MEASURING MEANS Filed March 15, 1965 2 Sheets-Sheet 1 RI/ Fe. 2

'5 II II HZ j QTTORNEKS 3,301,179 IDED Jan. 31, 1967 w. E. JOHNSON ELECTROSTATIC PRINTING WITH DENSITY CONTROL PROV BY CHARGE MEASURING MEANS 2 Sheets-Sheet 2 Filed March 15, 1965 FORMERG REcT/F/EIZ FfluuTza Djc/LLHTO HMPLlF/E Z Firm/ lak Scum/Ale 1| IIII United atent Ofiice 3,301,179 ELECTROSTATIC PRINTING WITH DENSITY CONTRGL PROVIDED BY CHARGE MEAS- URING MEANS William E. Johnson, Temperance, Mich, assignor t Owens-Illinois Inc, a corporation of Ohio Filed Mar. 15, 1965, Ser. No. 439,799 Claims. (Cl. 101114) This invention relates to electrical printing methods and apparatus, and more particularly to improvements in methods and apparatus for electrically transferring printing powder particles from a supply source or bed through an image-defining aperture in a stencil screen to an article surface by the application of an electric field.

A typical decorating process to which the present invention has application is that disclosed in my copending application Serial No. 393,817, filed August 31, 1964, and assigned to the assignee of the present application.

In one exemplary process of that type, a bed of printing powder particles is supported upon an electrically conductive plate in spaced relationship beneath a stencil screen having an image defining aperture or opening in the screen. An article to be decorated is placed in registry with the screen apertures in spaced relationship above the screen. Electric potential source means are connected between the conductive plate, screen and article to establish an electric field operable to electrically charge the particles in the supply and impel the particles from the supply upwardly through the screen aperture to the surface of the article.

Experience in the practice of the process outlined above has shown that where several articles in succession are decorated from the same powder bed, the density or thickness of the image layer applied to the article decreases as the number of articles increase even though an adequate supply of powder particles remains in the bed. The bed is initially prepared by placing a loose, unpacked bed of particles in a layer of uniform thickness upon a fiat conductive plate. After several articles have been decorated from the same bed, it has been observed that the particles in the bed tend to become packed to some extent and it is believed that this pack ing action makes it more difficult for particles to be transferred from the bed.

Accordingly, it is one object of the present invention to provide methods and apparatus for achieving a uniform image density on each of a plurality of articles decorated in succession from a powder bed.

. It is another object of the present invention to provide methods and apparatus for automatically controlling the application of the electric field in a decorating process of the type referred to above to assure that a uniform quantity of printing powder particles is applied to the article in each decorating operation, regardless of variations in the decorating conditions.

The foregoing objects and features of the invention will become apparent by reference to the following specification and to the drawings.

In the drawings:

FIGURE 1 is a schematic diagram of one form of the invention;

FIGURE 2 is a schematic diagram of another form of the invention;

FIGURE 3 is a block diagram of one form of electric control circuit embodying the invention; and

FIGURE 4 is a wave diagram representing signals generated at various points in the circuit of FIGURE 3.

In FIGURE 1 there is schematically disclosed an arrangement operable to apply an image-shaped layer of printing powder particles to the lower surface of an article A supported as by supports schematically illustrated at 10. A bed 12 of printing powder particles is supported upon an electrically conductive support member 14 and a stencil screen 16 having image defining apertures such as 18 is interposed between the powder bed 12 and the article A.

Screen 16 is preferably constructed of a relatively fine wire mesh which is coated with a coating which fills the openings in the screen. Image apertures 18 are formed in the screen by removing the coating from selected portions of the screen, the openings being crisscrossed by the wire mesh. For details of suitable materials and methods for preparing such screens, reference may be had to United States Patent No. 3,100,150.

These printing powder particles in bed 12 may take the form of finely powdered particles of a glass frit, many suitable frits being commercially available.

A voltage source V1, grounded at one output terminal, is connected by an output lead 26 to powder bed support 14, while a second voltage source V2 is electrically connected through one output lead 22 to the article A. The other output lead 24 of voltage source V2 is connected to electrical ground through a black box 26 whose function will be described in greater detail below. Screen 16 is also connected to electrical ground.

Voltage sources V1 and V2 are operable, when energized, to establish an electric field which extends from powder bed support 14 to the surface of article A, by electrically charging support member 14, screen 16 and article A to different electric potentials. The electric field strength is chosen to be high enough to electrically charge the particles in bed 12 and to electrically impel them upwardly through the apertures of screen 16 to the surface of article A. Further details of electric field strength, particle size, screen mesh and the spacing between the bed, screen and article surface are set forth in my co-pending application Serial No. 393,817 referred to above.

In the normal operation of electrically transferring particles from the bed 12 to the surface of article A, many particles rebound from the screen back to the bed. This action results in a packing of the powder bed which becomes more and more pronounced as successive articles are decorated from the same powder bed. Because of this packing of the powder bed, it becomes more and more difficult to transfer particles from the bed to the article surface, as the number of articles in succession decorated from the same bed increases.

This effect becomes apparent when an apparatus such as that disclosed in FIGURE 1 is operated to decorate a number of articles in succession by energizing the electric field for a given amount of time for each article. The first articles decorated under these circumstances are decorated with images of satisfactory density, but as the number of successive articles increases, the density of the image applied decreases. In order to achieve a uniform density in images applied to several successive articles from the same powder bed, it is necessary to increase the time period of application of the electric field as the number of articles decorated increases. Unfortunately, the packing effect on the powder bed, while having a general trend to increase the difiiculty in transferring particles with the increase in number of transfers, does not increase in a predictable manner.

In order to achieve uniform image density over a series of particles decorated successively from the same powder bed, it is necessary to regulate the time period of energization of the electric field in accordance with the number of particles which have been accumulated on the article surface.

The charging of a particle in powder bed 12 and the subsequent transfer of the charged particle from the bed to the surface of article A can be considered 'as a flow of electric current in that a unit of electric charge is physically transferred from support 14 to article A.

The function of black box 26 which is connected in series between screen 16 and article A is to measure the flow of current through this circuit. Because current flow in this circuit, after the transient charging current, can be induced only by the transfer of a particle from screen 18 to the surface of article A, current flow through this circuit is a measurement of the number of unit electric charges transferred from screen 16 to the surface of article A and hence a measurement of the number of charged particles which reach article A. By integrating the current, a measurement proportional to the total number of particles accumulated at any given time upon article A is obtained. By employing the integrated current -to generate a signal when the number of particles on the surface of article A reaches a predetermined number, corresponding to the desired image density, the signal thus generated may be applied, through an appropriate switching device 28, to automatically shut off the voltage sources to discontinue the electric field.

In this manner, the electric field is automatically maintained until a predetermined number of particles have been transferred to the article surface, regardless of the amount of time required to effect the transfer.

In FIGURE 3, an electrical block diagram of 'a suitable electrical arrangement for performing the functions of the FIGURE 1 embodiment is disclosed, the various units being enclosed in broken line enclosures designated V1, V2, 26 and 28 to indicate the approximate correspondence of the various elements to those more generally described in the description of FIGURE 1 above. In FIGURE 3, the article A, stencil screen 16, powder bed 12 and its support 14 appear in the upper right hand portion of the figure.

A decorating operation is initiated in the circuit of FIGURE 3 by manually switching a pulse switch 30 in the upper left hand corner of the diagram from a normal or inactive contact 32 to a pulse contact 34. When switch 30 is shifted from contact 32 to contact 34, one level of a voltage source 36 is grounded at the juncture of resistors R4 and R5, and this voltage change is applied by capacitor C1 across resistor R1 as 'a negative peak indicated at T=O on curve I of FIGURE 4. This negative pulse is applied to a monostable multivibrator of the one shot type indicated at 38 which shifts the multivi-brator from one of its two possible conductive conditions to the other. Multivibrator 38 may taken the form of a standard circuit module sold by Engineering Electronics Company of Santa Ana, California, under Catalog No. Z8889. This action of the multivibrator activates squaring circuit 40 (Engineering Electronics Company Catalog No. 290001) whose output is fed through two cathode follower buffer amplifiers 42 (Engineering Electronics Company Catalog No. Z8309) and is applied as a constant potential input to a gate circuit 44, one of the cathode follower amplifier output waves being indicated at curve II of FIGURE 4. Gate 44 may take the form of a more or less conventional diode gate circuit such as those explained in Pulse and Digital Circuits, by Millman and Taub, 1956 Ed., section 147.

When these outputs are applied to gate 44, gate 44 is operable to conduct the output of a conventional 2 tube RC feedback oscillator 46 to a power amplifier 48 which amplifies the A.C. wave form oscillator output indicated at curve III in FIGURE 4. Amplifier 48 may be any suitable industrial type amplifier such as the Mark III 60 watt Dynakit amplifier sold by Dynaco, Inc. of Philadelphia, Pennsylvania. The amplified A.C. wave is then passed through 'a high voltage transformer and rectifier 50 whose constant voltage output, curve IV of FIGURE 4, is applied directly to powder support 14.

Low voltage source V2 is normally maintained ener- 4 gized at all times and hence, upon the. application of the high voltage output of transformer and rectifier 50 to powder support base 14, the particles in bed 12 become electrically charged and attracted to the article surface A as described in more detail above.

As the charged particles arrive at the surface of article A, they lose or give up their charge to the article and the charge thus imparted to the article is accumulated upon an integrating capacitor C3, one side of which became isolated from ground by the original shifting of pulse switch 30 which initiated the powder transfer. As the charge is accumulated on capacitor C3, the voltage begins to accumulate as indicated by curve V of FIGURE 4. The initial step on this curve is due to the low voltage power supply and the fact that there is a finite electrical resistance between article A and stencil 16.

During the powder transfer, capacitor C3 is connected to an amplifier circuit 52 which is normally held non-conductive by a bias voltage as from a bias voltage source 54. When the voltage across capacitor C3 reaches a pre-selected value determined by the bias voltage applied to amplifier 52, the bias is overcome and the amplifier becomes conductive. A small portion of the output of oscillator 46 is fed into amplifier 52, and when the amplifier becomes conductive, upon the reaching of a predetermined voltage drop across capacitor C3, this portion of the oscillator output is passed through the amplifier and fed into a squaring circuit 56 (Engineering Electronic Company Catalog No. Z9000l). The amplifier output curve is indicated in FIGURE 4 at VI, and the resultant output of squaring circuit 56 is indicated at curve VII. The output of the squaring circuit is applied to multivibr'ator 38 to shift it back to its original conductive condition, thereby breaking the circuit to the elements designated generally as V1 in FIGURE 3. As will be noted from FIGURE 4, this action terminates the application of high voltage to powder base 14 as indicated in curve IV.

The waves indicated in curves V, VI, and VII of FIG- URE 4 do not return to zero at this time, because a voltage drop still exists across capacitor C3 and is maintained until pulse switch 30 is manually returned to its original position in contact with contact 32. This action grounds both sides of capacitor C3, thereby dissipating the voltage .drop across the capacitor.

Normally, under conditions such as those described in my co-pending application Serial No. 393,817, images of desired density can be formed with the high voltage power supply energized for time periods in the neighborhood of milliseconds. As stated above, because of the effect of successive transfers upon the condition of the powder bed, the time necessary to obtain an image of desired density upon the article increases with successive transfers. By the arrangement disclosed above, the time during which the high voltage is applied to powder base 14 is automatically determined by the accumulation of an electric charge upon the capacitor C3 of the FIGURE 3 circuit, this accumulation of charge being directly proportional to the number of particles which have been accumulated on the article surface.

In the event a satisfactory image has not been accumulated on the article within a given amount of time, the cycle may be automatically terminated by connecting a second capacitor C2 to multivibrator 38 to perform the reversing function after a predetermined interval determined by the time constant of the capacitor.

A second arrangement is disclosed in FIGURE 2 in which the potential applied to the article, screen and powder bed support 14 is supplied from a single voltage source V3 as shown. In the FIGURE 2 arrangement, voltage source V3 has its opposite output terminals connected to powder bed support 14 as by conductor 60' and to a common point 62, which is grounded, by a second conductor 64. Screen 16 is connected to common points 62 by a circuit which includes a variable resistor R10 and a Zener diode Z connected in parallel with each other.

Common point 62 is connected to the article A through a second circuit 66 which includes a resistance R11 and a capacitor C5 connected in parallel with each other. A conductor 68 also connects article A to an on-ofi control device 70, which may be similar in general to the corresponding portions of the circuit of FIGURE 3. Element 70 is employed to automatically energize and de-energize voltage source V3.

The parallel circuit containing resistor R and Zener diode Z functions to maintain screen 16 at a precisely regulated potential determined by the characteristics of the Zener diode and the adjusted resistance of variable resistor R19.

While various embodiments of the invention have been disclosed and described in detail above, it will be apparent to those skilled in the art that these embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting, and the true scope of the invention is that defined in the following claims.

I claim:

1. The method of applying a layer of powder particles to an article surface in a predetermined pattern comprising the steps of interposing a stencil screen having a pattern defining aperture therethrough between a supply of powder particles and the surface of an article, connecting electric potential source means to said supply, screen and article operable when energized to electrically charge particles in said supply and impel charged particles from the supply through the aperture of said screen to the article surface, energizing said electric potential source means, and de-energizing the potential source means upon the accumulation of a predetermined electrical charge on the article surface due to the accumulation of charged particles upon the article surface representative of a desired thickness of the layer of particles on the article.

2. The method of applying a layer of powder particles to an article surface in a predetermined pattern comprising the steps of interposing a stencil screen having a pattern defining aperture therethrough between a supply of powder particles and the surface of an article, connecting electric potential source means to said supply, screen and article operable when energized to establish an electric field capable of electrically charging particles in said sup ply and impelling the charged particles from the supply through the aperture of said screen to the article surface, energizing said electric potential source means, continuously measuring the accumulation of electrical charge upon the article surface while said potential means is energized, and de-energizing the potential source means in response to the accumulation of a pre-determined electrical charge upon the article surface representative of a preselected thickness of the layer of particles on the article surface.

3. The method of applying powder particles in an image-shaped layer of predetermined thickness to an article surface comprising the steps of interposing a stencil screen having an image defining aperture therethrough between a supply of printing powder particles and the surface of an article, connecting electric potential source means to said supply screen and article operable when en ergized to establish a first electric potential difference between said supply and said screen and to establish a second electric potential dilference between said screen and said article, said potential differences being of a magnitude sufiicient to electrically charge particles in said supply and impel charged particles from the supply through the aperture of said screen to the article surface, energizing said electric potential source means, measuring the variation in said second potential difference due to the transfer of charged particles from said screen to the article surface, and de energizing the potential source means when said second potential difference reaches a value representative of the accumulation of a predetermined thickness of the image-shaped layer of particles accumulated upon the article surface.

4. The method of applying a layer of powder particles to an article surface in a predetermined pattern comprising the steps of interposing a stencil screen having a pattern defining aperture therethrough between a supply of powder particles, and the surface of an article applying electric potential pulses to said supply, screen and article to electrically charge particles in said supply and impel charged particles from the supply through the aperture of said screen to the article surface, and regulating the time duration of the application of said electric pulses to terminate upon the accumulation of a predetermined electrical charge on said article surface due to the transfer of charged particles from said supply to said article surface.

5. The method of applying an image-shaped layer of printing powder particles to an article surface comprising the steps of interposing a stencil screen having an image defining aperture therethrough between a supply of printing powder particles and the surface of an article, connecting electrical potential source means to said supply screen and article operable when energized to establish an electric field capable of electrically charging particles in said supply and impelling the charged particles from the supply through the aperture of said screen to the article surface, energizing said electric potential source means, measuring the electric charge transferred to said article surface. by particles accumulating upon the article surface while said potential means is energized, and de-energizing the potential source means when the total electric charge transferred to said article surface by the particles accumulated thereon reaches a preselected minimum magnitude.

6. In an apparatus for applying an image-shaped layer of printing powder particles to an article surface including a stencil screen having an image defining aperture therethrough, means for supporting a supply of printing powder particles at one side of said screen, means for supporting an article to which the particles are to be supplied at the opposite side of said screen, electric potential source means operable when energized to establish an electric field between said supply and article capable of electrically charging powder particles in said supply and impelling the charged particles from said supply through the aperture of said screen to the surface of the article, and means for energizing said electric potential source means; the improvement comprising means for measuring the variation of electric potential between said screen and said article due to the accumulation of charged particles on said article while said potential source means .is energized, and means operable by said measuring means for de-energizing said potential source means when the potential difference between said screen and article reaches a preselected magnitude.

7. In an apparatus for applying an image-shaped layer of printing powder particles to an article surface including a stencil screen having an image defining aperture therethrough, means for supporting a supply of printing powder particles at one side of said screen, means for supporting an article to which the particles are to be applied at the opposite side of said screen, electric potential source means operable when energized to establish an electric field between said supply and article capable of electrically charging powder particles in said supply and impelling the charged particles from said supply through the aperture of said screen to the surface of the article, and means for energizing said electric potential source means; the improvement wherein said potential source means comprises a first voltage source means for applying a first potential difference between said supply and said screen, second voltage source means for applying a second potential difference between said screen and said article, and means for measuring variations in said second potential difference due to the passage of charged particles from said screen to said article.

8. In an apparatus for applying an image-shaped layer of printing powder particles to an article surface including a stencil screen having an image defining aperture therethrough, means for supporting a supply of printing powder particles at one side of said screen, means for supporting an article to which the particles are to be applied at the opposite side of said screen, electric potential source means operable when energized to establish an electric field between said supply and article capable of electrically charging powder particles in said supply and impelling the charged particles from said supply through the aperture of said screen to the surface of the article, and means for energizing said electric potential source means; the improvement wherein said potential source means comprises a voltage source having two output terminals, means connecting one output terminal to said supply, a first electric circuit connecting the other output terminal of said source to said article, and a second electric circuit connecting said other output terminal to said screen, said second circuit including an electrical resistance and a Zener diode connected in parallel with each other between said other output terminal and said screen.

9. In an apparatus as defined in claim 8; the further improvement wherein said first circuit comprises a resistor and a capacitor connected in parallel with each other between said other output terminal and said article, and switching means responsive to the accumulation of a predetermined charge on said capacitor for de-energizing said electric potential source means.

10. In an apparatus for applying an image-shaped layer of printing powder particles to an article. surface including a stencil screen having an image defining aperture therethrough, means for supporting a supply of printing powder 8 particles at one side of said screen, means for supporting an article to which the particles are to be applied at the opposite side of said screen, electric potential source means operable when energized to establish an electric field between said supply and article capable of electrically charging powder particles in said supply and impelling the charged particles from said supply through the aperture of said screen to the surface of the article and means for energizing said electric potential source means; the improvement wherein said electric potential source means comprises first means for applying a potential pulse of predetermined magnitude to said supply, second means for applying a potential pulse of a predetermined magnitude to said article, and means for de-energizing said potential source means when a sufiicient number of electrically charged particles have accumulated upon said article to change the electric potential on said surface to a predetermined potential.

References Cited by the Examiner UNITED STATES PATENTS 2,987,037 6/1961 Bolton 118637 2,996,575 8/1961 Sims. 3,081,698 3/1963 Childress et al. 10l129 3,218,967 11/1965 Childress 1011 14 3,218,968 11/1965 Childress et al. 101-115 FOREIGN PATENTS 81,920 9/ 1956 Denmark.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner.

Claims

6. IN AN APPARATUS FOR APPLYING AN IMAGE-SHAPED LAYER OF PRINTING POWDER PARTICLES TO AN ARTICLE SURFACE INCLUDING A STENCIL SCREEN HAVING AN IMAGE DEFINING APERTURE THERETHROUGH, MEANS FOR SUPPORTING A SUPPLY OF PRINTING POWDER PARTICLES AT ONE SIDE OF SAID SCREEN, MEANS FOR SUPPORTING AN ARTICLE TO WHICH THE PARTICLES ARE TO BE SUPPLIED AT THE OPPOSITE SIDE OF SAID SCREEN, ELECTRIC POTENTIAL SOURCE MEANS OPERABLE WHEN ENERGIZED TO ESTABLISH AN ELECTRIC FIELD BETWEEN SAID SUPPLY AND ARTICLE CAPABLE OF ELECTRICALLY CHARGING POWDER PARTICLES IN SAID SUPPLY AND IMPELLING THE CHARGED PARTICLES FROM SAID SUPPLY THROUGH THE APERTURE OF SAID SCREEN TO THE SURFACE OF THE ARTICLE, AND MEANS FOR ENERGIZING SAID ELECTRIC POTENTIAL SOURCE MEANS; THE IMPROVEMENT COMPRISING MEANS FOR MEASURING THE VARIATION OF ELECTRIC POTENTIAL BETWEEN SAID SCREEN AND SAID ARTICLE DUE TO THE ACCUMULATION OF CHARGED PARTICLES ON SAID ARTICLE WHILE SAID POTENTIAL SOURCE MEANS IS ENERGIZED, AND MEANS OPERABLE BY SAID MEASURING MEANS FOR DE-ENERGIZING SAID POTENTIAL SOURCE MEANS WHEN THE POTENTIAL DIFFERENCE BETWEEN SAID SCREEN AND ARTICLE REACHES A PRESELECTED MAGNITUDE.