US3732809A

US3732809A - Electrostatic printing apparatus for printing on curved surfaces

Info

Publication number: US3732809A
Application number: US00197211A
Authority: US
Inventors: H Sawada; S Shimazawa
Original assignee: NIPPON DENSHI INSATSUKI KK
Current assignee: NIPPON DENSHI INSATSUKI KK
Priority date: 1970-11-13
Filing date: 1971-11-10
Publication date: 1973-05-15
Anticipated expiration: 1990-05-15
Also published as: IT940535B; DE2156298A1; FR2114626A5; GB1314929A

Abstract

An apparatus for electrostatically printing on the curved surface of an article, in which the article securely held on a supporting member is set in such a position that its surface to be printed is located in close proximity of an image forming screen in evenly spaced opposed relation thereto thereby defining a predetermined space therebetween, and a powder feeding box containing a finely divided electroscopic ink powder therein and having a rotatable cylindrical brush mounted therein and further carrying a rubbing member rotated about a vertical shaft with said rubbing member in contact with said screen, while establishing an electrostatic field in said space, whereby the ink powder flows from said feeding box onto the surface of the rubbing member through a slit formed at the bottom of said feeding box by the action of said rotating cylindrical is then rubbed through the screen into said space by the action of said rubbing member, is urged electrostatically toward the surface of the article and comes to rest thereon in a pattern defined by the image forming openings of said screen.

Description

United States Patent 11 1 Sawada et a1.

[ ELECTROSTATIC PRINTING APPARATUS FOR PRINTING 0N CURVED SURFACES [75] Inventors: Hiroomi Sawada; Shigekazu Shimazawa, both of Tokyo, Japan [73] Assignee: Nippon Denshi lnsatsuki Kabushiki Kaisha, Chuo-ky, Tokyo, Japan [22] Filed: Nov. 10, 1971 [21] Appl. No.: 197,211

[30] Foreign Application Priority Data 111 3,732,809 1 May 15, 1973 3,413,654 11/1968 Strong ....101/1)1o. 13 3,665,851 5 1972 Edwardsetal. ..101 114 Primary ExaminerEdgar S. Burr Attorney- Eric H. Waters [5 7] ABSTRACT An apparatus for electrostatically printing on the curved surface of an article, in which the article securely held on a supporting member is set in such a position that its surface to be printed is located in close proximity of an image forming screen in evenly spaced opposed relation thereto thereby defining a predetermined space therebetween, and a powder feeding box containing a finely divided electroscopic ink powder therein and having a rotatable cylindrical brush mounted therein and further carrying a rubbing member rotated about a vertical shaft with said rubbing member in contact with said screen, while establishing an electrostatic field in said space, whereby the ink powder flows from said feeding box onto the surface of the rubbing member through a slit formed at the bottom of said feeding box by the action of said rotating cylindrical is then rubbed through the screen into said space by the action of said rubbing member, is urged electrostatically toward the surface of the article and comes to rest thereon in a pattern defined by the image forming openings of said screen.

7 Claims, 16 Drawing Figures PATENTEDMAY 1 51m SHEU 1 [1F 9 Ill/ PATENTU] MAY 1 51375 3 732,809

SHEET 3 OF 9 PAIENIEUHAYI SIEHS 3 7 2, 09

sum 5 OF 9 PATENTEUHAY 1 Elm 8 ,732,809

SHEET 7 UF 9 I ELECTROSTATIC PRINTING APPARATUS FOR PRINTING ON CURVED SURFACES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrostatic prining apparatus comprising a supporting member for holding an article upon which it is desired to print, an image forming screen, a powder feeding box containing an ink powder, a rubbing member for printing and means for applying an electrostatic field between said article and said screen; and adapted for use in the printing on the surfaces of three-dimensional articles, particularly concave surfaces of ceramic and porcelain articles.

2. Description of the Prior Art A method of electrostatically printing flat surfaces is well known, for example, from US. Pat. No. 3,081,698. This method comprises arranging a first conductive flat screen with a plurality of apertures over a portion thereof defining areas desired to be printed and a second conductive flat member in evenly spaced opposed relation, establishing a dc. electric potential difference between said conductive member and applying a finely divided pigment powder having a particle size small enough to freely pass through the apertured portion of the first conductive screen to the outer surface of said screen, whereby the pigment powder will transfer toward and come to rest on the surface of said conductive member in a pattern defined by the apertured printing areas of said screen.

ln another method, a conductive screen having apertures therethrough arranged in the form of a desired image is moved from a screen loading location to a screen unloading location, and at the screen loading location a powder is electrostatically adhered to one surface of the screen from a powder source, while at the screen unloading location the powder thus adhered to the screen is wiped by a wiper and transferred through the apertures of the screen onto the surface ofa cylindrical object positioned on the opposite side of said wiper with respect to said screen and rotating at a peripheral speed equal to the moving speed of the screen.

However, these prior art methods are intended for either the electrostatic printing on fiat surfaces by holding a flat screen stationary and transferring a pigment powder through said screen or the electrostatic printing on cylindrical surfaces by moving a flat screen while rotating a cylindrical surface to be printed at a peripheral speed equal to the moving speed of said screen and transferring a pigment powder through said screen onto said cylindrical surface, thereby to print said cylindrical surface successively at a portion in closest proximity of said screen; and are not intended for the electrostatic printing on curved surfaces, such as those of ceramic and porcelain plates or bowls, in one step.

SUMMARY OF THE INVENTION The present invention is an improvement of the above-mentioned apparatus and makes it possible to carry out not only unicolor electrostatic printing but also multicolor electrostatic printing economically by a cycle of printing operations.

It is, therefore, an object of the present invention to provide an apparatus for carrying out electrostatically unicolor and multicolor printings.

Still another object of the invention is to provide an apparatus for electrostatically printing on the inner bottom surfaces of ceramic and porcelain bowls.

A further object of the invention is to provide an apparatus for electrostatically printing on the inner inclined surfaces of the aforesaid bowls.

In order to achieve the objects set forth above, an apparatus for electrostatically printing on the curved surface of an article comprises a supporting member for securely holding said article, a screen being in a shape complementary to the shape of the surface of said article to be printed upon and having a plurality of apertures over a portion thereof defining areas to be printed, means for causing a relative movement of said supporting member and said screen while maintaining a predetermined space between the confronting surfaces of said screen and said article held on said supporting member, means for establishing and maintaining an electrostatic field at least in the space between said confronting surfaces, a powder feeding box containing therein an electroscopic ink powder having a particle size small enough to freely pass through the apertures of said screen, said powder feeding box being provided with a slit at the bottom thereof and having disposed therein a rotatable cylindrical brush which in a stationary state keeps said slit closed to prevent the ink powder from flowing through said slit from said feeding box and in a rotating state enables the ink powder to flow through said slit, and a rotatable rubbing member for receiving the ink powder from said feeding box through said slit on the surface thereof and rubbing the ink powder from one side of the screen to pass it through the apertures into said electrostatic field, whereby the ink powder will be urged electrostatically toward and come to rest on the surface of said article in a pattern defined by said apertured printing area of said screen.

With the electrostatic printing apparatus of the invention described above, it is possible to print on an article from either side. Where the printing is effected on the upper side of an article, the rubbing member is disposed beneath the slit of the powder feeding box and rotatably supported by the downwardly extending opposite side walls of the box. The feeding box is suspended from a vertical shaft for rotation about said shaft. When the feeding box rotates about the vertical shaft in the printing operation, the rubbing member rotates together with the feeding box, thereby rubbing the ink powder through the screen disposed therebeneath.

On the other hand, where the printing is effected on the underside of an article, the rubbing member is arranged so as to be movable upwardly toward the lower surface of the screen which is spaced a predetermined distance from the lower surface of the article to be printed upon, while the powder feeding box is arranged so as to be movable horizontally, so that when the rubbing member is in its lowered position, the feeding box is located immediately above the upper surface of said rubbing member to feed the ink powder to the surface of the rubbing member.

Further, where the printing is effected on an inclined surface of an article, it is proposed to form the rubbing member into a shape complementaryto the shape of said inclined surface, e.g., into a frustoconical shape. In this case, the ink powder may be fed to the rubbing member from the feeding box either directly or through the intermediary of an intermediate roller.

The electrostatic printing apparatus of the invention when combined with ink powder fixing means is capable of multicolor printing as well as unicolor printing. For this purpose, the present invention proposes an electrostatic printing apparatus comprising at least tables or supporting members for holding articles to be printed mounted thereon, and means for circulatorily intermittently shifting said tables to a loading station, a printing station, a fixing station and an unloading station one after another, said printing station being provided with a screen, a powder feeding box having a rotatable cylindrical brush therein, a rotatable rubbing member, means for causing a relative movement of the supporting member and the screen, and means for establishing and maintaining an electrostatic field between the confronting surfaces of the screen and the article held on the supporting member, of the type described above; and said fixing station being provided with means for applying a fixing fluid onto the printed surface of the article on the table then located at said station. The ink powder used in the present invention is a toner of non-conductive particles, which may be, for example, pigmented or dyed particles of a mixture of ceramic type ink powders and resin powders soluble by a vapor of an appropriate solvent.

Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an article supporting table in the inventive apparatus;

FIG. 2 is a vertical sectional view of an ink powder feeding hopper or box, a rubbing member and portions of a screen and an article to be printed;

FIG. 3 is a perspective view, partially broken away, of an embodiment of the apparatus according to the invention;

FIG. 4 is a sectional view of another apparatus embodiment;

FIG. 5 is a perspective view of the screen frame used in the apparatus shown in FIG. 4;

FIG. 6 is a plan view of a turret type multicolor electrostatic printing apparatus embodying the present invention;

FIG. 7 is a side elevational view of the apparatus shown in FIG. 6;

FIGS. 8 and 9 are respectively side elevational views, partially in section, of an electrostatic printing apparatus embodying the present invention which is adapted for printing on the underside of an article;

FIG. 8a is a perspective view of the rubbing member;

FIG. 8b is a plan view of the slit formed in the ink powder feeding hopper or box;

FIG. 10 is a side elevational view, partially in section, of fixing means;

FIG. 11 is a side elevational view, partially in section, of still another embodiment of the invention;

FIG. Ila is a perspective view of the rubbing member used in the apparatus of FIG. 11;

FIG. 12 is a side elevational view, partially in section, of a modification of the apparatus shown in FIG. 11; and

FIG. 12a is a plan view of the slit formed in the ink powder feeding hopper or box of the embodiments shown in FIGS. 11 and I2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIGS. 1, 2 and 3a, a bottom plate 11 of a hexahedral feeding box or feeder case 10 is provided with a slit l2 and an ink powder supply brush 14 is rotatably mounted on a shaft 15 within said feeding box, blocking off said slit 12 through the intermediary of a screen 13 which is in contact with the bottom plate 11. A frame 16 is attached to the outer surface of the bottom plate 11, with an opening 18 formed in the web 17 thereof immediatly below the slit 12. Thus, it will be understood that an ink powder to be described later discharged from the slit [2 moves downward through the opening 18. The frame 16 has a flange 19 extending along the extensions of the side walls of the feeding box 10 parallel to the shaft 15 and perpendicular to the bottom plate 13, and a rubbing roller 20 is disposed beneath the frame 16 with the upper portion thereof surrounded by the frange 19, said rubbing roller 20 being fixedly mounted on a shaft 21 which is parallel to the shaft 15 and journaled in extentions 22 (FIG. 3) of the side walls of the feeding box 10 perpendicular to the shaft 15.

The shaft 15 is also journaled in the side walls of the feeding box 10. The

shafts

15 and 21 extend in the same direction and respectively have pulleys 23, 24 mounted on one ends thereof projecting outwardly of the feeding box. The pulleys 23, 24 are operatively coupled with pulleys 27, 28 mounted on the driving shafts of motors 25, 26 fixed on the outer wall of the feeding box 10, by means of endless belts or chains 29, 30 respectively so as to be driven from said motors (FIG. 3).

A vertical shaft 32 is provided on the top wall 31 of the feeding box 10, extending upright from a point offset from the center of said top wall, and is rotatably supported by a bearing 34 provided in a cantilever 33 projecting from a printing head. A pulley 35 is mounted on the upper end portion of the shaft 32, while a pulley 37 is mounted on the driving shaft ofa motor 36 fixedly mounted in the cantilever 33, and these pulleys 35, 37 are operatively connected to each other by means of a belt or chain 38. Thus, it will be seen that when the motor 36 is set in motion, the shaft 32- is driven rotating the feeding box 10.

Beneath the feeding box 10 is disposed an electrically conductive table 40 which is mounted on the upper end of a supporting shaft 41 in opposed relation to said feeding box. The table 40 has a central hole 42 which is incommunication with a vacuum source not shown through an axial bore of a cylindrical member 43 fixed to the underside of said table 40 and a flexible tube 44 connected to said cylindrical member 43. An electrically insulating material 45 is preferably provided concentrically on the table 40 around the central hole 42 (FIG. I). The supporting shaft 41 extends upwardly through a sleeve 47 fixedly mounted in a frame 46 by which the table 40 is shifted laterally or horizontally, and is secured to the bottom of the cylindrical member 43. The shaft 41 is moved upwardly by means not shown to lift the table 40 from the position in abutting engagement with the sleeve 47.

Between the table 40 and the feeding box 10 is interposed a screen frame 50. In FIG. 3, the screen frame 50 is shown in the form ofa dish and an image forming screen 51 is extended over the bottom of the screen frame 50. The openings of the image forming screen 51 are masked except for those which form a desired image which it is desired to print. In the embodiment shown, both the screen frame 50 and the screen 51 are made of a conductive material, and, as will be described later, an electrostatic field, i.e., a d.c. voltage is applied between the screen 51 and the conductive table 40. The screen 51 may, for example, be a stainless steel screen having the openings thereof blocked off, with only those being left open which form an image which it is desired to print, or a metallic plate having a number of apertures formed therein by etching to form an image to be printed.

In printing the image on an article M to be printed, the article M is mounted on the table 40 and, after correcting the position of the article, the flexible tube 44 is connected with the vacuum source, whereupon the article is secured in its position by the vacuum pressure acting in the hole 42. In this state, the supporting shaft 41 is lifted as by a solenoid, an air cylinder or other means not shown, toward the feeding box 10, whereby the article M to be printed is brought to the proximity of the underside of the screen 51 in opposed relation thereto. The screen 51 is preferably provided with short projections extending downwardly from the blocked off part or masked part, so as to space the article M to be printed and the screen 51 from each other. Under such condition, positive and negative voltage is applied or, in other words, an electrostatic field is applied between the screen 51 and the table 40, and the motors 25, 26, 36 are set in motion while maintaining said electrostatic field.

The finely divided powdered ink P contained in the feeding box does not flow through the slit 12 while the brush 14 is held stationary, because the slit 12 is closed by said brush 14, but when the brush 14 rotates together with the shaft 15, the powdered ink P is caused to flow through the slit 12 by the rotating brush 14 and, in the embodiment shown, drops onto the peripheral surface of the rubbing roller 20. On the other hand, the feeding box 10 rotates eccentrically around the shaft 32, with the rubbing roller 20 rotating on the surface of the screen 51 while rubbing said surface. Thus, the ink powder carried on the peripheral surface of the rubbing roller 20 is supplied on the screen surface and rubbed therein. The ink powder rubbed into the screen 51 has an electric charge of a definite polarity induced therein mainly by frictional electrification, so that said ink powder transfers rectilinearly toward the article to be printed through the image-forming openings of the screen under the influence of said electrostatic field and attaches onto the surface of said article. Thus, the image defined by the openings of the screen is printed on the article.

The brush [4 within the feeding box 10 and the rubbing roller 20 start rotating and hence the printing is commenced, concurrently with the rotation ofthe shaft 32. However, the brush 14 need be rotated only for a period of time necessary for feeding a predetermined quantity of the ink powder, and generally it is preferable that this period of time is set to be shorter than the printing period. Therefore, it is advantageous to arrange that the rubbing roller and the shaft 32 only continue to rotate after the brush 14 stops rotating, so as to print the powder ink remaining on said roller and the screen.

If the width of the slit 12 in the feeding box 10 is uniform in the longitudinal direction of the brush 14, the

quantity of the ink powder supplied through the slit 12 in the radial direction of the screen 51 from the center of said screen will become uniform and hence the thickness of the ink powder stretched over the entire surface of the screen will become smaller at the peripheral portion than at the central portion of the screen. It is, therefore, preferable that the slit 12 has a substantially triangular or sectoral shape, with its width progressively increasing from the central portion toward the peripheral portion of the screen 51 or, in other words, the width of the slit is progressively increased from the end adjacent the vertical shaft 32 toward the other end longitudinally of the brush 14, as shown in FIG. 8b or 12a. By so doing, it becomes possible to keep the density of the ink powder uniform over the entire surface of the screen 51. However, it will be obvious that the width of the slit 12 can be varied locally, based upon the above concept, according to the quantity of the ink powder required which is variable at portions of the desired image.

Although in the embodiment described above the rubbing roller has been employed for rubbing the ink powder through the image-forming openings of the screen, it should be understood that means employed for this purpose is not restricted only to the rubbing roller mentioned but any other means may be employed provided that the ink powder can be transferred uniformly toward the article to be printed. For instance, it may be a rubbing member which slides on the screen in contact therewith incident to the rotation of the feeding box. It is also possible to design such that the printing is carried out under the influence of an electrostatic field which is established between an electrode embedded in the rubbing roller 20 and the table 40.

The above-described embodiment, wherein use is made of the dish-shaped screen frame 50, is the case when it is desired to print a pattern on the inner bottom surface of an article, e.g., a dish, through the screen stretched over the bottom opening of said screen frame. However, when it is desired to print a pattern on an inclined surface of an article, e.g., the inner surface of the inclined rim portion of a soup plate, the printing can be achieved in the following manner: Namely, first of all, a frustoconical rubbing roller 20a is connected to a pulley 24 by means of a flexible rotary shaft 21a as shown in FIG. 4. On the other hand, image bearing screens 53 are attached to a screen frame 50a of the type shown in FIG. 5, which has a shape complementary to the shape of the inclined inner surface of plate N upon which it is desired to print. Thereafter, the flexible rotary shaft 21a is secured to a frame 16 with the conical surface of the rubbing roller 20a in contact with the inclined inner surface 52 of the screens 53, and at the same time, the screen frame 50a is fixed in a predetermined position relative to the feeding box 10.

The plate N upon which it is desired to print is mounted on the table 40 in the manner described previously and after getting the table close to the screen frame, an electrostatic field is established between the screens 53 and the table 40. The brush l4 and the rubbing roller 20a are rotated while rotating the feeding box 10 about the eccentric shaft 32, whereby the ink powder is printed on the inner surface of the inclined rim portion of the plate through the screens 53 in the same manner as in the preceding embodiment.

The printing may also be achieved by establishing an electrostatic field between the rubbing roller and the plate. If, in this case, the screens are of conductive material, a bias potential may be applied on the screens.

Now, an embodiment in which a multicolor printing, inter alia a four-color printing is carried out using the apparatus described above, will be described with reference to FIGS. 6 and 7.

l2 cantilevers

62a, 62b, 62c, 62d, 62e, 62f, 62g, 62h, 62i, 62j, 62k, 621 of equal length are provided radially in circumferentialy equally spaced relation on a disc 61 rotatable about a shaft 60. These cantilevers 62a 62! are respectively provided at their outer ends with collars 47a 47l Supporting shafts 41a 41! extending extend from tables 400 40] extend through the respective collars 47a 471 respectively have tables 40a 401 on their top ends. These tables are mounted on cylindrical members 43a 431 respectively in the manner shown in FIG. 1. Further, the cylindrical members are connected with a vacuum source not shown through flexible tubes 44a 441 respectively which extend through the disc 61 and communicating with the axial bore of the shaft 60. The disc 61 is fixedly mounted at its center on a rotary shaft of a cam table 65 which is mounted on a mount 64 mounted on a base plate 63, and is rotated intermittently (through an angle of 30 at each movement) through a reduction gear 67 from a motor 66 for driving the cam table 65.

Pressure cylinders 68 are arranged concentrically around the cam table 65 at locations corresponding to the stopping positions of the tables 40a 4401 The pressure cylinders 68 are respectively erected upright between a frame 69, concentric with the cam table 65, and the base plate 63. The arrangement is such that when each pressure cylinder is actuated, its piston moves upward and a cross bar 70 provided at the top end of said piston abuts against a corresponding cylindrical stopper 48a 481 provided at the lower end of respective supporting shaft 41a of the table 40a causing said cylindrical stopper to move upward and thereby lifting said table 40a The cylinder 68 is omitted at positions A, G to be described later.

Large characters A, B L shown in FIG. 6 and arranged in a counterclockwise direction designate treating stations indicated by the arrows respectively. Of these characters, C, E, I, K each designate a printing station; D, F, .l, L each designate a fixing station; A designates a and unloading station; B designates a registration station where an article to be printed is positioned on the table; and G, H each designate an intermediate inspection station. In the event when the apparatus shown in FIG. 6 is used for the simultaneous two-color printing, the station G is used as a loading and unloading station as well as the station A, and the station H is used as a registration station as well as the station B. At each printing station are provided the feeding box 10 and the screen frame 50 shown in FIG. 3. Besides these elements, means for clearing the ink powder remaining on the screen 51 and accumulated on the underside of the screen under suction is provided but such means is not apparent in the Figure.

Each fixing station is provided with a bell-jar 72. The bell-jar 72 has its open side facing downwardly, and an ink powder fixing gas is introduced thereinto from the top end thereof through a tube 74.

The registration station B is a station where an article to be printed is fixed centrally of the table 40a by means, for example, of a plurality of positioning legs which move toward the center of the table from around the article concurrently with the upward movement of said table, thereto to fix said article. This station may be omitted if the article can be exactly positioned at the station A. The positioning method will not be described in detail because any known means may be used for this purpose.

The operations of the cam table 65 and the respective cylinders 68, and the supply of vacuum pressure through the respective flexible tubes 44a 441 are actuated according to predetermined programs.

These programs are so composed that the respective elements are actuated according to a predetermined sequence, for example, by a circuit incorporating relays and limit switches. The practical composition of this type of program will not be described herein as it can be obtained without requiring a particular inventive ability.

In operating the apparatus described above, an article such as the plate M is first of all mounted on the table 40a at the station A, whereupon said plate M is securely held in its position under vacuum pressure supplied from the vacuum source through the flexible tube 44a. After the lapse of a predetermined period of time, the motor 66 is set in motion and the cam table 65 is rotated 30 in a counterclockwise direction by the rotating force of the motor 66 transmitted thereto through the reduction gear 67. Thus, the table 40a is shifted to and held stationary at the station B.

At the station B, the pressure cylinder 68 thereat is actuated and the cross bar 70 carried on the piston rod thereof abuts against the underside of the cylindrical stopper 48a, causing the shaft 41a and hence the table 40a to move upward.

The suction through the flexible tube 44a is stopped at this point and at the same time, the plate M on the table is positioned in a predermined position by the positioning legs. Immediately after the positioning, vacuum pressure is again supplied through the flexible tube 44a, thereby to secure the plate M in its position on the table under the sucking force. Then, the pressure cylinder is released and the table is lowered to the original position with the plate M securely held thereon. The vacuum pressure is continuously supplied through the flexible tube 44a until the printing operation is completed, to hold the plate M securely on the table. After passage of a predetermined period of time from the time when the table 40a has arrived at the station B, including the time required for the positioning and the time required for the table 40a to return to its original position, the cam table 65 is again rotated 30 in a counterclockwise direction and the table 40a is thus shifted to the printing station C.

At the printing station C, the table 40a is lifted by the action of the corresponding pressure cylinder 68 in the manner as at the station B, but in this case, the plate M is brought to a position in the proximity of the screen 51 which is provided in the manner shown in FIG. 3. The ink powder is transferred from the feeder box 10 onto the plate and printed thereon. Upon completion of the printing, the table 40a is lowered to the original position and then shifted to the station D in the manner described above. At the station D, the plate M is lifted again and moved into the ball-jar 72 from the lower opening thereof. Then, a fixing gas supplied through the tube 74 is applied onto the printed surface of the plate M to fix the printing. As the fixing gas, a gassified organic solvent capable of dissolving a toner to be described later, e.g., trichloroethylene or methylene chloride, is used.

After the toner is fixed on the printed surface of the plate by means of the fixing gas at the station D, the table 40a is lowered to the original position and shifted to the station E after passage of the predertermined period of time. The station E is a printing station similar to the station C but here an ink powder of a different color is printed on the plate in the same manner as at the station C. The plate thus printed is lowered again and shifted to the station F where the newly printed toner is fixed. Thereafter, the plate is subjected to inspections at the stations G and H, the third printing and fixing at the stations I and .l, and the fourth printing and fixing at the stations K and L, and returned back to the station A. At the station A, the vacuum pressure is released and the printed plate M is unloaded from the table.

The above description has been made with reference to one plate M mounted on the table 40a and processed from the station A through the stations B, C, L back to the station A for printing. In practice, however, the table 401 is located at the station A when the table 40a has been shifted from the station A to B. Therefore, by loading new plates M each on each of the tables 400 401 at the station A, said plates are printed and returned to the station A one after another in a predetermined period of time by the above-described cycle of printing operation.

When it is desired to achieve two-color printing by using the turret type printing apparatus described above, a plate loaded on the table at the station A can be unloaded at the station G upon completion of the printing and fixing at the intermediate stations. If, in this case, a positioning device similar to that at the station B is provided at the station H and the flexible tubes 44g, 44h are connected with the vacuum source in the similar manner as the flexible tubes 44a, 44b, a plate loaded at the station A can be unloaded at the station G and a plate loaded at the station G can be unloaded at the station A upon completion of the printing and fixing. Thus, it becomes possible to obtain in the same cycle of operation two-color printed plates of the same or different patterns of printing.

In the apparatus described above, the operating period of the pressure cylinder at each station, the rotating periods of the brush 14, the rubbing roller and the shaft 32 at each printing station, and the feeding period of the fixing fluid at each fixing station can be optionally designed as required.

Now, another embodiment of the invention in which an article is printed from the underside thereof, will be described hereunder with reference to FlGS. 8 to 10. in this embodiment, an article M to be printed is fixedly mounted in an inverted position on a table 80 in a mannor to cover an opening 84 formed in said table, and the table is shifted to the printing stations and then the fixing stations one after another. With reference to a printing device 81 provided at each printing station, the screen frame 50 is provided projecting upwardly from an upper opening of a supporting frame 85 with a plurality of tension springs 86 interposed therebetween as shown in FlG. 8. The screen frame 50 is movable vertically and it moves upwardly when pushed from the underside, whereas it moves downwardly when the pushing force is released, until it abuts against an inward flange 87 formed at the lower portion of the supporting frame under the biasing forces of the springs 86.

A rubbing member or rubbing brush 82 is in the shape of a disc as shown in FIG. 8a and consists of a sponge or tufted piles. The rubbing member 82 is carried on a rotatable shaft 88 extending vertically from a bracket 89 and provided with a pulley 90 at the lower end thereof within said bracket. On the other hand, a motor 91 is mounted on the bracket 89 and a pulley 92 is mounted on the driving shaft of said motor in horizontal alignment with the pulley 90. The

pulleys

90, 92 are operatively connected by means of an endless belt. The bracket 89 is vertically slidable along a guide rail 93 which is fixedly mounted in and supported by a frame 81 of the apparatus, and is carried on a piston rod 95 of an elevating device 94 provided below the guide rail 93 and comprising a pressure cylinder or solenoid means. Thus, it will be seen that the rubbing brush 82 is moved up and down along with the bracket 89 by the piston rod 95.

For applying an ink powder to the rubbing brush, an ink powder feeding device 83 is provided. The ink powder feeding device 83 in this embodiment is so designed as to be shiftable horizontally, so that it is retracted to a position not to interfere with the upward movement of the rubbing brush 82 when said rubbing brush is moved upwardly (FIG. 8), while it is advanced to a position immediately above the rubbing brush 82 when said rubbing brush is in its lowermost position (FIG. 9). The feeding device 83 includes an ink containing box 97 having a slit 96 formed through the bottom wall thereof. Similar to the feeding box 10 in the preceding embodiment, an ink feeding brush 98 is rotatably mounted in the ink container box 97 in such a manner as to close said slit 96. Preferably, a screen 13 is stretched over the upper opening of the slit 96 or the bottom surface of the box 97. The brush 98 is rotated from a motor 100 mounted in the box 97, by way of endless belt or chain which is engaged around a pulley 99, fixedly mounted on the shaft of said brush 98, and a pulley 101 fixedly mounted on the driving shaft of said motor 100. It will be appreciated that when the brush 98 is rotated, the surface of said brush is rubbed and the ink powder in the box 97 flows through the slit 96 onto the upper surface of the rubbing member 82. The entire ink feeding device 83 is horizontally slidably supported on a pair of horizontal guide rails 102 and connected with a piston shaft 104 of piston means 103 similar to the above-described elevating device 94, to be advanced and retracted thereby.

The printing apparatus constructed as described above operates in the following manner: Initially, the rubbing brush 82 is located in its lowered position to receive the supply of the ink powder. When the table 80 is rotatably shifted and the article to be printed mounted thereon is set in a printing position, the rubbing member 82 starts to move upwardly along with the bracket 89 by the action ofthe elevating device 94. The screen 51 is also located in its lowered position at the outset, but is moved upwardly to its highest position by the upwardly moving rubbing brush 82. Thus, the screen 51 is set in the proximity of the region to be printed of the article M in opposed relation thereto. The distance between the article M and the screen 51 is preferably in the range of 0.5 mm. 3 mm. When the screen 51 has thus been pushed up to its highest position, a dc. potential is applied from a power source not shown to establish an electrostatic field between the screen 51 and the article M. The application of the potential can be easily attained by a contactor, a limit switch, a timer or thelike means provided at a suitable location.

After the rubbing brush 82 and the screen 51 have been set in their highest positions and the potential is applied on the screen, the motor 91 on the bracket 89 is set in motion. The rubbing brush 82 rotates about the shaft 88 and the ink powder carried on said brush 82 transfers linearly onto the surface of the article under the influence of the electrostatic field in the manner described previously. The ink powder thus transferred is held intact on the surface of the article by its own electric charge, and thus the printing of the article is achieved. Therefore, it will be obvious that if the ink powder is previously uniformly attached to the rubbing brush 82, a printing of excellent uniformity can be obtained. However, the degree of rubbing between the brush and the screen is relatively slight at the central portion of the screen around the axis of the vertical rotatable shaft 88, and therefore, if an image is desired to be printed at this portion, the resulting printing will tend to become somewhat thin in thickness. This can be avoided by supporting the rotatable shaft 88 on the bracket 89 such that said shaft will make a slight planetary motion.

After the printing has been carried out for a predetermined period of time, the motor 91 is stopped. Therefore, the rubbing brush 82 stops rubbing and at the same time the elevating device 94 is actuated to lower the brush. in this case, the screen 51 is also moved down under the biasing forces of the tension springs 86, until it abuts against the flange 87. Then, the application of the potential is interrupted and successively thereafter, the rubbing brush 82 is parted from the screen 51 and moved down to its lowest position. Then, the piston means 103 is actuated to advance the ink feeding device 83. When the slit 96 is located immediately above the rubbing brush 82, the motor 100 is set in motion, so that the feeding brush 98 is rotated and the ink powder is dropped onto the rubbing brush through the slit 96. The motor 91 is also in motion while the ink powder is being supplied through the slit 96. Therefore, the rubbing brush 82 is rotating horizontally and the ink powder'is sprayed evenly over the entire surface of said rubbing brush. However, as stated previously, if the width of the slit 96 is uniform over the length thereof, the density of the sprayed ink powder will become higher at the central portion than at the peripheral portion of the rubbing brush.

Therefore, in order to spray the ink powder evenly over the entire surface of the rubbing brush 82, it should be considered to make the width of the slit 96 narrowest at the portion in the closest proximity of the center of rotation of said rubbing brush and increase the width progressively toward the outer ends above the peripheral portion of said brush, as shown in FIG. 8b. After the ink powder has been fed for a certain period of time, the

motors

91 and 100 are stopped, and the ink feeding device 83 is retracted by the action of the piston means 103, providing for the next cycle of operation.

When the screen 51 has been lowered upon completion of the printing, the table 80 is rotatably shifted to a fixing station with the printed article thereon. In this embodiment, a cleaning device is provided above the table 80 between the adjacent loading stations. The cleaning device 110, as shown in FIG. 9, includes a cleaning brush 111 projecting downwardly from the table 80, by which the upper surface of the screen 51 is wiped during rotation of the table 80. The cleaning brush 111 is connected to a suction device not shown, through a flexible tube 112 extending from the center of the table, and the ink powder remaining on the upper surface of the screen 51, which will provide a cause of contamination of the printing in the next printing operation, is wiped off therefrom and removed under vacuum pressure.

When the printed article M has been position in the fixing station, a fixing device is actuated and the ink powder of the printing, which is retained on the surface of the article merely eiectrostatically by its own electric charge, is fixed on said surface. The fixing device 120, as shown in FIG. 10 by way of example, includes gasifying means 121 for evaporating an ink powder fixing solvent, a nozzle 122 for discharging the solvent vapor and a bell-jar 123 surrounding said nozzle. The nozzle 122 is connected to the gasifying means 121 through an electromagnetic valve 124 and a flexible tube 125. The nozzle I22 and the bell-jar 123 are fixed to a bracket 126 which in turn is fixed to a piston rod 128 of an elevating device 127. The gasifying device 121 includes a tank containing a solvent, such as trichloroethylene or methylene chloride, which is capable of dissolving aresin used in the ink powder. The tank is heated and the solvent vapor generated therein is led to the nozzle 122 through the flexible tube 125.

The nozzle 122 and the bell-jar 123 are held in their lowered positions during rotation of the table 80. However, when the printed article M has been located in the fixing position and the table 80 has stopped rotating, the elevating device 127 is actuated and the bell-jar 123 is caused to move upward until it abuts against the table 80 and the space defined by said belljar and the printed surface of the article M is completely sealed. Then, the electromagnetic valve 124 is opened, whereupon the solvent vapor from the tank is injected into the sealed space from the nozzle 122 to fix the printed ink powder. The electromagnetic valve 124 need not be held open for an extended period of time and is closed when a predetermined quantity of the solvent vapor has been injected into the space. Upon completion of the fixing, the elevating device 127 is again actuated, and the nozzle 122 and the bell-jar 123 are lowered. The table 80 is rotated to shift the fixed printed article M to an unloading station or the next printing station, and at the same time, the following article to be fixed is located in the fixing position.

Referring to FIGS. 11 and 12 there is shown still another embodiment of the electrostatic printing apparatus of the invention, which is used for printing the inclined inner surface ofa cup or the like. This apparatus is essentially the same as that shown in FIGS. 8 and 9, but is so designed as to be adapted for the printing on an inclined surface such as the inner (or outer) surface of a cup. Accordingly, conveyor means 131 and a cleaning device used therein are of different constructions from those used in the preceding embodiment.

The conveyor means 131 for the articles M to be printed is, for instance, a turret having a number of radial brackets 132 of the same length, like that shown in FIGS. 6 and 7. A flexible tube 133 connected with a vacuum source not shown is extended through the outer end of each bracket 132, and a disc-shaped cusion member 134 is attached to the lower surface of the outer end of each bracket. An article M to be printed is securely held at the outer end of each bracket and suspended therefrom in an inverted position, with the surface upon which it is desired to print facing downwardly, under the vacuum pressure supplied through the flexible tube 133. Of course, the turret is rotated intermittently, and incident to the intermittent rotation of the turret, the article M suspended from each bracket 132 is shifted to printing stations and fixing stations sequentially.

A printing device provided at each printing station includes a rubbing brush 82a which is frustoconical in shape as shown in FIG. 11a. A screen 53 has a shape complementary to the shape of an inclined surface, such as the inner surface ofa cup, upon which it is desired to print, and has apertures formed therein defining the desired pattern of printing. Therefore, the inclined surface of the frusto-conical rubbing brush 32a rubs the imaged part of the screen 53. On the other hand, an ink feeding device 83 includes an ink containing box 97 of the type described previously having a slit 96 formed therein, and a frustoconical auxiliary feeding brush 135 disposed immediately below said slit 96. The auxiliary feeding brush 135 is driven from a motor 136. The constructions of the other portions of the ink feeding device are identical with those shown in FIGS. 8 and 9.

When the rubbing brush 82a is to receive the supply of the ink powder from the ink feeding device 83, it is located in its lowered position as in the case of FIGS. 8 and 9, and the ink feeding device 83 is advanced toward the rubbing brush 820 by the action of piston means 103. The auxiliary feeding brush 135 used in this embodiment is so designed that its inclined surface element facing the surface of the brush 82a is intimately contactable with the surface of the rubbing brush 82a in its advanced position and the surface element facing the surface of the slit 96 is in parallel relation to the slit. Therefore, when the ink powder need be fed to the rubbing brush 82a, the ink feeding device 83 is advanced and stopped in the position in which the surface of the auxiliary feeding brush 135 is held in abutting engagement with the surface of said rubbing brush 82a, and then the ink feeding brush 98, the auxiliary feeding brush I35 and the rubbing brush 82a are driven.

Upon rotation of the ink feeding brush 98, the ink powder flows from the slit 96 and drops onto the surface of the auxiliary brush 135. In order that the ink powder is uniformly attached to the inclined surface of the frustoconical auxiliary brush 135 at this time, the slit 96 is shaped preferably such that its width progressively becomes larger toward the bottom of the auxiliary brush 135 as shown in FIG. 12a. The ink powder fed to the auxiliary feeding brush 135 is successively transferred onto the surface of the rubbing brush 820. It is preferable that the auxiliary brush I35 and the rubbing brush 82a are rotated at different peripheral speeds from each other while being held in contact with each other.

Similar to the preceding embodiment, the surface of the screen 53 is cleaned by a cleaning device while the ink powder is being fed to the rubbing brush. In this embodiment, however, the cleaning device is horizontally retractably mounted on the frame of the apparatus. The cleaning device includes a brush 113 of a configuration adapted to sweep along the outer surface of the screen 53. This brush 113 is rotated from a motor 115, mounted on a cantilever 114, through

pulleys

116 and 117. The cleaning device is connected to a piston rod 119 of piston means 118 as a whole, and it is held in its retracted position during the printing operation but is advanced during the period from the time when the'printed article has been carried away by conveyor means to the time when the next article is located in the printing position, to effect the cleaning of the screen 53.

The piston means 118 is actuated at the same time when the printing operation has been completed and the cleaning device is advanced thereby, and the cleaning brush 113 is brought into contact with the screen as shown in FIG. 12 when said screen 53 is set in its lowered position. Then, the brush 113 is driven from the motor 1 15 to sweep the entire peripheral surface of the screen 53, thereby removing the ink powder which may possibly remain on said surface. In this case, it will be advantageous to apply vacuum pressure to the inner surface of the brush 113 as in the case of FIG. 9. In the meantime, the brush I13 stops rotating and the entire cleaning apparatus is retracted by the action of the piston means 118.

The electrostatic printing apparatus of the invention has been described and illustrated herein in terms of an embodiment which is designed so as to effect printing and fixing from the upper side of an article upon which it is desired to print, and an embodiment which is designed so as to effect printing and fixing from the underside of an article upon which it is desired to print. However, in either embodiment, there is employed a method comprising setting a screen close to the article with a predetermined gap therebetween, applying an electrostatic field between said screen and said article while maintaining the gap and rubbing an ink powder through said screen.

Therefore, by employing the apparatus of the invention the operation of printing a unicolor or multicolor printing of a desired pattern, even on fragile articles such as ceramic and porcelain articles which are susceptible to breakage under printing pressure, can be carried out automatically and with high accuracy and high speed without requiring much labor.

In the first embodiment described herein which is adapted to effect printing and fixing on the upper side of an article, the finished printed surface after the fixing tends to become blurred unless the ink powder remaining on the printed surface is wiped off immediately after the printing. However, the printing apparatus is so simple as shown in FIG. 3 that said apparatus is adapted for the mass production of general printed articles. In the embodiments of the second group, an ink powder is applied on the underside of an article and therefore, the ink powder does not remain on the printed surface of the article immediately after the printing. This is advantageous in that the finished printing after the fixing is very sharp and precise. On the other hand, however, the ink feeding device used is more complicated as shown in FIGS. 8 and 9 than that shown in FIGS. 3, 4, 6 and 70. Therefore, these apparatus are selectively used according to the type of the article upon which it is desired to print and the degree of accuracy of the desired printing.

We claim:

1. An apparatus for electrostatically printing on a curved surface of an article, comprising a supporting member for securely holding the article, a screen having a shape complementary to the shape of the surface of the article, to be placed thereon, and having a plurality of apertures over a portion thereof defining areas to be printed, means for causing a relative movement of said supporting member and said screen while maintaining a predetermined space between the confronting surfaces of said screen and the article held on said supporting member, means for establishing and maintaining an electrostatic field at least in said space between the confronting surfaces, a powder feeding box fixedly and eccentrically supported by a vertical shaft by which it is rotated, said box containing an electroscopic ink powder having a particle size small enough to freely pass through said apertures of the screen, said box being provided with a slit at the bottom thereof and having disposed therein a rotatable, cylindrical brush which in a stationary position keeps said slit closed to prevent the ink powder from flowing therethrough, and in a rotating state enables the ink powder to flow through said slit, and a rotatable rubbing member for receiving the ink powder from said box through said slit on the surface thereof to carry the ink powder thereon and rubbing the same from one side of said screen to pass it through said apertures into said electrostatic field, whereby the ink powder is urged electrostatically toward and comes to rest on the surface of the article in a pattern defined by the apertured printing area of said screen.

2. The apparatus as defined in claim 1, wherein said vertical shaft is vertically slidingly movable with said rubbing member along a guide from a position remote downwardly from said screen to a position adjacent the same.

3. The apparatus as defined in claim 2, wherein said rubbing member is a flat cylindrical body fixedly supported on the top of said vertical shaft.

4. The apparatus as defined in claim 3, wherein said box is horizontally shiftable from a position away from the path of the vertical movement of said rubbing member to a position just above said surface of the rubbing member when the latter is in its lowered position.

5. The apparatus as defined in claim 2, wherein said rubbing member is fixedly supported on the top of said vertical shaft and has a frustoconical surface on which the ink powder is received.

6. The apparatus as defined in claim 5, wherein said box is horizontally shiftable from a position away from the path of the vertical movement of said rubbing member to a position just above said surface of the rubbing member when the latter is in its lowered position.

7. The apparatus as defined in claim 6, wherein said box carries an auxiliary brush having a frustoconical surface the shape of which is complementary to that of said rubbing member and the ink powder transfers from said box onto said surface of the rubbing member through said surface of the auxiliary brush.