IL28362A - Electrostatic printing machine and method of electrostatic printing - Google Patents

Description

ELECTROSTATIC PRINTING MACHINE AND IMPROVED METHOD OF ELECTROSTATIC PRINTING noainV n^3is?& ΓΙ«ΡΪ?Ι n rDo io R oTsn naiaa This invention relates to an electrostatic printing machine and to an improved method of electrostatic printing, and has for one of its objects the provision of improved means providing for consistently uniform printing in an electrostatic printing system.

Heretofore in a printing machine and method for electrostatic printing, one of the principal problems has been the uniform feeding of the printing powder onto a stencilled screen, for passage of the powder through unobstructed mesh openings of the screen and into an electrostatic field established in the air gap between the screen and the article onto which the powder is to be finally deposited in a design corresponding to the design of the plurality of unobstructed mesh openings.

One object of the invention is the provision of improved powder feeding means in an electrostatic printing machine of the type in which the printing is effected by a dry powder being conducted across an air gap, and in an electrostatic field and deposited onto the article to receive the powder so conducted and in which the powder is uniformly fed onto the screen from a substantially stationary bed thereof.

A still further object of the invention is the provision of a structure in a printing machine of the general character described for upward feeding of the powder through the screen openings into an electrostatic field for overhead printing.

An added object is the provision of a structure that enables quick replenishment of printing powder and quick access to parts for maintenance and Other objects and advantages will appear in the description and drawings.

In the drawings: Fig. 1 is a top plan view of the machine.

Fig. 2 is a part-sectional part-elevational view of the wheel-like drum of the machine in which the front wall thereof is omitted and a portion of the powder container is shown in elevation with the remainder thereof broken away to show structure.

Fig. 3 is a vertical sectional view through a fully assembled machine taken generally along line 3-3 of Fig. 2. Certain parts within the machine are shown in elevation for clarity.

Fig. 4 is an enlarged fragmentary cross-sectional view of a portion of the machine shown in Fig. 3 to more clearly show the separate parts.

Fig. 5 is a fragmentary elevational view of a modification in the part of the powder container shown in elevation in Fig. 2.

Fig. 6 is a reduced size front elevational view of the wheel-like drum of the machine showing the structure and arrangement of Fig. 2 and 3 modified and rearranged for printing on articles passing below the machine instead of above it.

Fig. 7 is a fragmentary vertical cross-sectional view of the part shown in Figs. 6.

In detail the machine incorporating the invention comprises a cylindrical drum generally designated 1 that is supported for rotation about a horizontal axis, and the outer wall of which drum may perforations or mesh openings stopped out by any suitable stopping out medium except the portions designated 2:' (Fig. 1) that define the design to be printed. The design may be continuous around the drum or in separate parts either for repetition of the same design, or it may be any other printable matter . · The machine, as illustrated, including the drum is adapted to be supported on any suitable support 3 (Fig. 3) by a standard 4 or any other suitable support that may have a base 5 that, in turn, is adapted to be bolted by bolts 6 to said support, and which standard includes a clamp 7 at its upper end for gripping one end of a horizontally disposed shaft 8 for holding said shaft stationary, but permitting adjustment of the latter relative to the clamp.

The shaft 8 projects laterally from the clamp 7 and the drum 1 is supported on the projecting end of shaft 8, coaxial therewith. A stud bolt 9 may be provided on clamp 7 for tightening and loosening the grip of the clamp on shaft 8 for adjustment or replacement purposes.

Said cylindrical screen 2 is held between a pair of circular, horizontally spaced, opposed end walls 10, 11, the former being at the front end of; the drum 1 and wall 11 being at the rear end (Fig. 3) and which walls close the screen at its front and rear ends. Walls 10, 11 are coaxial with shaft 8 and the projecting end of the latter extends through said walls.

Disposed between the clamp 7 and the rear wall 11 of the drum 1 and ad acent to said clam 7 a driving pulley or to any suitable power means (not shown) for driving pulley 14. A tubular sleeve 16, coaxial with and around shaft 8 extends between pulley 14 and wall 11, and bolts 17 extend through the pulley 14 arid into threaded holes in the sleeve 16 rigidly but releasably secure the pulley to said sleeve, while a ball bearing 18 supports the pulley 14 on shaft 8 coaxial therewith and for rotation of the pulley 14 and sleeve 16 relative to shaft 8.

A second ball bearing 19, coaxial with bear ing 18, rotatably supports the end of sleeve 16 that is opposite to pulley 14 on shaft 8, and said bearing also supports the inner wall 11 of drum 1 on said shaft.

A timing gear 20, (Fig. 4) coaxial with shaft 8, is held against the inner side of end wall 11 by screws 23 that extend through registering holes in a clamping ring 24 and in inner wall 11 and into threaded engagement with threads within an axial : extension of the openings in sleeve 16 for bolts 15. The clamping ring is formed with an annular lip 25 that extends over a portion of the inner periphery of the gear 20 for clamping the pulley 14, sleeve 16, wall 11, and gear 20 together as a unit and to permit rotary adjustment of gear 20 relative to wheel 1. The heads of screws 23 are accessible from the side of ring 24 that faces toward the frorit end of the machine.

Supported within the upper portion of the drum 1 is a powder moving roller 26, secured on a ft 27 for rotation therewith. A timin ear 28 spaced above, and in the same vertical plane as timing gear 20, and a timing belt 29 connects gear 20 with the gear 28.

A sleeve 30, coaxial with shaft 21 is spaced around the latter and a pair of ball bearings 33 within the sleeve 30 support shaft 27, the latter being rotatable within sleeve 30.

A vertically disposed plate 34 of electrical insulating material is releasably clamped at its lower end to stationary shaft 8 by a screw 35 and the upper end of the plate 34 is clamped to sleeve 30 by a screw 36 (Fig. 4) . A conventional spring clip 37 on the end of shaft 27, adjacent to wall 10, releasably retains the roller 26 against axial movement off the adjacent end of shaft 27. 28 The roller %1 includes a hub having an annular radially outwardly projecting web 38. One side of said web faces sleeve 30, and said side is formed with a recess extending radially oppositely and outwardly of shaft 27. A pin 40 extends through an opening in shaft 27 and projects from said shaft into the portions of recess 39 at opposite sides of said shaft to secure the roller 26 and shaft 27 for rotation together.

Said roller 26 has a relatively wide annular outer rim 43 that supports thereon a layer 44 of porous resilient filter material commonly known under the trade name of SCOTT FOAM. The width of rim 43 and the layer 44 thereon are such that the layer will extend across and engage all of the inner surface of the cylinder 2 in which any design 21 is From the description to this point, it is apparent that, upon removal of the front wall 10 of the drum 1, the wheel 26 may be readily removed from the drum independently of any other elements, upon removal of the spring clip 37.

A powder container or hopper generally designated 45, comprises a vertical front wall 46 and a similar rear wall 47 in horizontally spaced opposed relation, and through which shaft 8 extends.

The use of the words "front", "rear", "forward", and "rearwardly" and the words of similar connotation are used in the specification with respect to the standard 4 which is the rearmost element.

Thus walls 10 , 11 are forwardly of standard 4 and wall 11 is rearwardly of wall 10, with wall 10 being the front wall of the drum 1 and wall 11 being the rear wall.

The walls 46, 47 of the powder containing hopper 45 are formed with coaxial central openings into which the ends of a central sleeve 48 extend, and to which the front wall 46 is removably secured by screws 49.. The rear end of sleeve 48 is formed with a radially outwardly projecting annular lip 50 (Fig. 4) that is seated in a complementarily formed rabbet around the rear corner of the opening into which the rear end of sleeve 48 extends.

The bore of sleeve 48 is greater in diameter than the external diameter of shaft 8, and substantially the rear half of said bore is counterbored at 52, to provide an axially rearwardly facing shoulder 53 at a oint intermediate the ends of the bore in between said ends. An' annular strike plate 54 or anvil element is secured against said shoulder.

A lock member 56 (Figs. 2, 4) is secured on shaft 8 by pin 57 at a point spaced rearwardly of the strike plate 54 and adjacent to, but spaced forwardly of the plate 34, which lock member 56 is formed with an annular row of equally spaced outwardly opening recesses 58 (Fig. 2) around its outer periphery.

Sleeve 48, in turn, is provided at its rear end with an annular row of radially inwardly extending projections 59 (Fig. 4) that are adapted to be moved through recesses 58, axially of sleeve 48 into a space between the lock member 56, rearwardly thereof, and the forward side of plate 34. When the projections 59 are in the aforesaid space, a fractional rotation of the sleeve and hopper 45 will move the projections to positions behind the portions 60 that are between recesses 58 to retain the hopper against forward movement toward the front wall 10. In this connection a vertically extending leaf spring 63 (Fig. 4) is pinned or otherwise suitably secured at its lower end to the front face of rear wall 47 of the hopper 45 and its upper end projects above said rear wall and is formed with an opening into which the forwardly projecting end of a pin 64 extends when said opening and pin 64 are in registration. Pin 64 is secured in a recess formed in the plate 34. This pin releasably holds the hopper locked to stationary plate 34 against rotation relative thereto and when the in 64 is in the o enin in s rin 63 the radial the lock member 56. Upon pulling the upper end of spring 63 forwardly, the spring is disengaged from the pin and the hopper and sleeve 48 may be rotated so that said projections 59 (Fig. 4) will register with the recesses 58 for withdrawal of the hopper axially thereof, assuming, of course, the front wall 10 is removed, as will later appear.

The shaft 8 is formed with an open ended slot 66 extending transversely through said shaft at a point adjacent to, but spaced forwardly of the locking member 56, and a pin 67 extends through said slot, projecting at its opposite ends radially outwardly of the shaft. The forward sides of the projecting ends of pin 67 engage the rearwardly facing end of a tubular sleeve 68, on the forward end of shaft 8, which sleeve, in turn, is reciprocable on 8 shaft HI forwardly of the slot 66.

The rear end of sleeve 68 is formed with a radially outwardly projecting flange or striker 69, which striker has an axially forwardly facing surface adapted to engage the rearwardly facing surface of the annular strike plate or anvil 54. Said rear end of sleeve 68 is also formed with rearwardly opening recesses in which the opposite ends of pin 67 are positioned to prevent relative rotary movement of pin 67 and sleeve 68 about the axis of the latter.

The forward end of sleeve 68 is formed with a pair of circumferentially extending inclined. cam surfaces 70 terminating in forwardly projecting risers 71, each similar to a ratchet tooth.

Forwardl of the sleeve 68 and on the that is formed on its rear end with cam surfaces and risers that are complementary to the cam surfaces 70 a,nd risers 71 on the forward end of sleeve 68. The flange or striker 69 and anvil 54 are so positioned that when the striker and anvil are in engagement, the risers on the sleeves 68 , 74 extend axially past each other (Fig. 4) , with the inclined cam surfaces facing each other but spaced apart.

The forward end of shaft 8, fprwardly of slot 66 is hollow, providing a bore 75 that is open at the forward end of shaft 8, and a coil spring 76 is within the rear end of bore 75, which spring is connected at its rear end with pin 67, while its forward end is connected to the rear end of an externally threaded rod 77. Said rod 77, in turn, is formed with a slot 78 extending longitudinally thereof, and a pin 79 secured at its ends in openings in the opposite sides of the bore 75 extends through said slot to prevent rotation of the rod 77.

A collar 80 is around rod 77 at the forward end of bore 75, and partially extends into said bore. This collar also extends partially over the forward end of shaft 8 and is formed with a shoulder abutting the forward end of shaft 8, and rearwardly opening recesses 83 in collar 80 are adapted to receive the ends of pin 79.

The forward end of rod 77 is adapted to project from the forward end of bore 75, and a finger actuatable nut 84 threaded onto the projecting forward end of rod 77 enables adjusting the tension of spring 76. A lock nut 85 on said rod enables releasably The forward end of shaft 8 projects a substantial distance forwardly of the front wall 10 of the drum 1, and sleeve 74 terminates at the forward end in a radially outwardly projecting flange 86 (Fig. 4) that abuts and is secured to the front wall 10 by screws 87. A ball bearing 88 rotatably supports the front wall 10 on the forwardly projecting end of shaft 8, and a disc 89 at the forward side of the front end wall 10 is secured to said end wall 10 by screws 90, which disc engages the outer race of ball bearing 88, the inner race being engaged by an axially rear-wardly projecting annular flange 93 on a lock nut 94 that, in turn, is threaded on the forward end of shaft 8 forwardly of disc 89, and is releasably clamped onto said outer end by a finger engageable head on a screw 95. The nut 94 is split transversely thereof with screw 95 extending across the split to clamp the nut to the threads upon tightening screw 95. The nuts 84 and screw 95, as well as nut 94, preferably have knurled heads permitting rapid and easy turning by the fingers of a hand.

Hopper 45 within the drum 1 has vertically extending side walls 96 (Fig. 2) equally spaced at opposite sides of sleeve 48, and convergently downwardly inclined lower portions 97. A thin horizontally disposed perforated screen or plate 98 provides a powder supporting bottom wall, below which is a layer 99 of resilient, porous; filter material of substantially the same character as that of layer 44 on the powder moving roller 26. The marginal portion of layer 99 is compressed and secured against the in turn, is secured against the compressed electrical insulating material of layer 99 by screws 103 that extend into the lower edges of the walls of the hopper .

The plate 98 may be the equivalent of an approximately 20 mesh screen, while the layer 99 may have approximately sixty tortuous passageways to the inch therethrough while the openings in the area 2 ' in the screen 2 may be the equivalent of a three hundred mesh screen, the openings in the latter being larger than the discrete particles of powder held in the hopper 45, so as to freely pass therethrough.

A bed 104 of powder (Fig. 3) is within hopper 45 and a horizontal row of horizontally extending parallel, cylindrical bars 105 (Figs. 2,3) in spaced side-by-side relation are secured at their ends in walls 46, 47 adjacent to but spaced above the plate 98. These bars are preferably spaced apart a distance less than the diameter of a bar, and where the diameter of each bar is approximately 0.4 inches, the spacing may be 0.1 inches. In, some instances it is desirable to provide a similar second row of bars or rods 106 (Fig. 5) below the single row of bars 105 with the bars 106 in staggered relation relative to the row of bars 105 to further contribute to support the weight of the portion of powder within hopper 45 above said rows of bars, and said bars also function to uniformly distribute the powder onto the inner sides of the cylindrical screen 2. The adjacent convex sides of adjacent pairs of bars facilitate passage of the owder between each adjacent pair. acid etched to provide the openings, and the remainder of the cylinder may be imperforate, but partially etched to provide a roughness sufficient to carry powder passing through the perforations in plate 98 and through layer 99 around with the cylinder 2 during rotation of the latter, as also occurs at the perforations, instead of any surplus powder tending to slide to the lowest point as the drum rotates. The etching is on the inner side of the cylinder.

A belt tightener 107 (Fig. 2 ) is on an arm 108 that, in turn, is pivotally supported at 109 on a bracket carried by plate 34 for engagement of the tightener with belt 29 to maintain the latter in taut relation to the timing gears 20 , 28 .

To load the hopper, it is merely necessary to remove the front wall ID of the drum-like wheel 1 and the open upper end of the hopper is accessible for filling the hopper to the desired degree with the dry printing powder. The disc 89 is dished forwardly around its outer periphery for ready grasping by the finger of a hand to withdraw and to hold the wall 10 . When the clamping screw 95 is loosened and nut 94 is removed the front wall 10 may be removed for access to hopper 45 for filling the latter.

The hopper 45 may be readily withdrawn after, removal of wall 10 by merely retracting spring 63 from pin 64 and then fractionally rotating the hopper until the projections 59 register with recesses 58 whereupon the hopper may be withdrawn to expose the plate 34 The roller 26 may be removed by removal of spring clip 37 . However, for filling the hopper.45 , it is only After the hopper is filled and the front wall 10 replaced, the motor actuated timing belt 15 is actuated to rotate the drum 1 for moving the cylindrical screen 2 to carry the design or designs 2' past the rotary powder moving roller 26 and below the article to be printed upon. Such article is indicated as being a sheet of material 110, such as plywood, for example, (Fig. 2), supported between drive rollers 111, actuatable for driving said material past drum 1 at a substantially uniform distance above the cylinder or screen 2 at the same rate of speed as the surface speed of the screen 2 and in the same direction as the direction in which the latter is moved.

Simultaneously with rotation of the drum 1, the powder moving roller 26 will be rotated at a substantially higher rate than screen 2, and the outer periphery of layer 44 where it contacts the inner surface of screen 2, will be moved in the same direction as the direction of rotation of the screen 2. This point of contact between the roller 44 and the inner surface of screen 2 is preferably at the point where the screen 2 is closest to the sheet 110. The roller 44 is electrically insulated from the rest of the printing machine by reason of the plate 34 that supports it, being of electrical insulating material, and the screen 2 may be connected with a D.C. source of power 113 while the sheet 108 may be grounded at 114, to establish an. electrostatic field between the screen 2 and the plywood or material 110 for conducting powder moved by the layer 44 of roller 26 through the material. Walls 10, 11 of the drum 1 are of electrical insulation material and wall 11 carries a conductor ring 114 contacted by a stationary wiper element 115. A conductor element 116 (Fig. 4) connects ring 114 with screen 2 (Fig. 4).

The rollers 111 may each be formed with annular ribs to span the design imprinted on the sheets 110 before such design is fixed by any suitable means (not shown) such as a spray, heat, etc. An air jet 117 (Fig. 1) directed across the outer surface of the screen 2 adjacent to but spaced from the point where the powder is conducted onto the sheet, may serve to clean the outer surface of the screen 2 of any powder that may tend to cling thereto, so that only a clear imprint of the design 2' will be deposited on the material 110.

Referring to Figs. 6, 7, an arrangement is shown in which the powder hopper 45 and the powder moving roller 26 of Figs. 2, 3, 4 are in inverted positions. The powder hopper in Figs. 6, 7 is generally designated 118 and the roller 119.

A conventional conveyor 120, indicated in dot-dash lines in Figs. 7 is adapted to carry rods 121 past the lowermost surface of the cylindrical screen indicated at 122 for being printed upon.

In the present invention, the lower side of the hopper 118 comprises a horizontal screen 123 and a layer of filter material 124 that corresponds structurally to the screen 98 and filter material 99 of Figs. 3, 4.

Plate 125 performs the same function as to support the roller .119 below the powder hopper, while the cylindrical bars 127 perform the function of rods 105 being in a horizontal row spaced above screen 121.

The remaining elements, such as the central assembly for supporting the hopper and shock members, are the same as in Fig. 3, and function in the same manner. In Pig. 7, it is apparent that the width of the outer drum 128 and its screen is wider than the outer drum 1 of Fig. 3 and the powder moving roller 119 of Fig. 7 is longer than roller 26, but the structure is otherwise substantially the same.

The drums 1 and 128 may be called printing wheels and rollers 26, 119 will be called powder moving rollers, since the latter perform the function of positively moving the powder deposited on the inner surfaces of the screens 2, 122 through the unobstructed openings in said screens that form the designs to be printed. While the discrete powder particles are of a size to freely pass through the openings in the screen, it is essential that they be moved through the openings and into the electrostatic field estab-, lished between the article to be printed upon and the screen 2 or 120 to be properly conducted onto said article.

The problem of providing a consistently uniform layer of powder onto the screen of the printing wheel is important in most instances, particularly over a constant and lengthy period of time .

Upon rotation of the printing wheel (Fig. 4) , the flange 69 on the end of the hollow sleeve 68 comprises an anvil to receive the blow from the striker upon each revolution of the sleeve 74, which sleeve is connected with the printing wheel 1. The anvil is rigid with both ends of the powder hopper 45 and the force of each blow from the striker is transmitted to the sleeve 48 that, in turn, connects the end walls 46, 47. Since the point of impact is approximately centrally between walls 1, 2, the impact is substantially equally transmitted, and the intensity of each blow against the anvil 55 is regulated by the tension of spring 76, which tension can be quickly regulated by rotation of nut 84. The number of impacts or blows against the anvil during each revolution of the drum 1 is according to the number of risers 71 on the cam surfaces of sleeves 68, 74.

Each blow of striker 69 is transmitted substantially uniformly to the powder within the hopper 45 through the walls of the latter, including the screen 98 and filter layer 99 and bars 105, resulting in an accurately metered amount of printing powder being uniformly deposited onto the inner surface of screen 2 (Fig. 2) or onto the outer layer of filter material 129 on roller 119 (Fig. 7).

The filter material 99 and 124 substantially providesasubstantially uniform number of adjoining tortuous open ended passageways, many being in communication with each other at points intermediate their ends whereby the movement of the powder particles through the layers 99, 124 will be checked, but will require vibration or shocks transmitted to the powder and layers for progressive passage through the pasr The powder will also be distributed uniformly through the layers 99, 124.

The provision of cylindrical bars 105, 106 and 127 in relation to the screen 98 or 123 also materially contributes to the desired uniformity in the feed of the powder onto the screen 2 or 122.

The use of the word "screen" is not intended to be restricted to a woven screen or to a perforated plate, although the latter is normally preferred.

It is to be understood that the detailed description is not to be considered as limiting the scope of the claims, it being obvious that modifications and changes may be made that come within said scope and within the spirit of the invention.

Claims (14)

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is :- - I

1. A method of electrostatically printing with a dry printing powder that includes the steps of : positioning the article to be printed upon in spaced opposed relation to a screen having a group of unobstructed openings corresponding in outline to the design to be printed; moving discrete particles of said powder downwardly along a plurality of closed adjacent generally vertically extending differently tortuous paths that have upper and lower open ends, which open ends and paths are of sufficient size to freely pass the particles of powder therethrough and which paths are defined by walls positioned to define the sides of said paths and to obstruct direct vertical downward movement of said particles under the influence of gravity and to cause said particles to follow said paths by administering shocks of predetermined force and regularity to said walls and particles for shifting said particles relative to said walls until the particles reach and pass through said lower open ends for movement therefrom to said openings; and thereafter moving said particles through said unobstructed openings in said screen by application of physical force against said particles, and establishing an electrical voltage potential between said screen and the said article to provide an electrostatic field between said screen and said article for conducting the particles moved through said screen and onto said article.

2. An electrostatic printing apparatus including: a printing screen having a perforated portion adapted to receive thereon printing powder for movement through the openings in said portion into an electrostatic field established in an air gap between said screen and an article to be printed upon for conduction of the powder so moved across said air gap; a horizontally disposed, stationary layer of material over which a bed of printing powder is adapted to be supported, said material being formed with substantially uniformly distributed tortuous adjacent passageways extending therethrough from top to bottom of sufficient size to pass the particles of said powder downwardly therethrough from said bed for substantially uniform discharge from the underside of said layer upon administration of shocks to said layer and body; said layer having outer edges, and a rigid layer-support connected with said edges supporting said layer with the area thereof inwardly of its edges spaced over said portion of said screen for passage of said powder through said passageways and for discharge and deposit on said portion; a powder supporting screen disposed between said bed of printing powder and said layer supporting said bed of printing powder over said layer and through the openings of which screen powder passes from said bed to said layer, said screen being inelastic to stretching and held against distortion under the weight of said bed of powder; and movable shock-administering means operatively connected with said layer for administering shocks thereto and to such bed for causing downward movement of powder of said bed through said passageways and said discharge therefrom under the combined influence of said shocks and gravity.

3. An apparatus according to claim 2, wherein said layer includes a relatively thick sheet of resilient, flexible material in which said passageways are formed, said powder supporting screen being a perforated sheet of material over said layer secured at its edges to said layer support for transmission of said shocks from said shock administering means to the bed of powder adapted to be supported thereon, said sheet being quite thin compared to the thickness of said sheet of flexible material.

4. An apparatus according to claim 2 or 3 , wherein said rigid layer-support includes spaced opposed pairs of stationary, rigid walls connected with and extending upwardly from the edges of said layer providing the sides of a hopper for said powder with said layer defining the bottom of such hopper.

5. An apparatus according to claim 2, 3 or 4, wherein said shock-administering means comprises a striker element and an anvil member and means supporting said striker element for intermittent movement into sharp impacting engagement with said anvil member, and means connected with said striker element for so moving it, said anvil member being rigidly connected with said rigid layer-support for intermittently transmitting a shock administered to the latter and to said layer upon movement of said striker element into said impacting engagement with said anvil member.

6. An apparatus according to claims 4 and 5, including means supporting said shock-administering means within said hopper and between said pair of opposed walls, and connected with said pair of walls.

7. An apparatus according to claim 4, 5 or 6, wherein said printing screen is cylindrical, and has imperforate end walls, screen-supporting means supporting said printing screen for rotation about a horizontal axis, means supporting said layer-support stationary within said printing screen with said layer below said axis of rotation of said printing screen and said portion of said screen being the portion spaced below said layer when said printing screen is rotated, and a horizontally extending row of parallel, horizontally spaced bars rigidly secured to and extending between said opposed pair of walls at a level spaced above said layer for distributing powder of said bed adapted to be positioned above said layer to opposite sides of the rods of adjacent pairs thereof as powder from said body is discharged from said printing screen.

8. An apparatus according to claim 4, 5 or 6, wherein said printing screen is cylindrical, screen-supporting means supporting said printing screen for rotation about a horizontal axis around said layer with said portion of said printing screen being the portion below said layer when said printing screen is rotated, the upper side of said hopper being open and facing the upper side of said printing screen, a roller of resilient material supported over said hopper for rotation in engagement with the inner surface of said printing screen for forcing powder discharged onto said lower portion from said layer and carried upwardly by said printing screen through the openings in said printing screen, means for establishing an electrostatic field between the upper side of said printing screen and an article thereover for conducting the powder moved through the openings in said printing screen onto said article during rotation of said printing screen, and means for supporting such article over said printing screen and spaced thereabove for receiving said powder.

9. An apparatus according to claim 8, wherein the inner surface of said printing screen is roughened for carrying particles of powder deposited thereon to said roller.

10. An electrostatic printing apparatus including: a drum-like printing wheel having a pair of horizontally spaced, opposed, imperforate vertically disposed end walls and an outer peripheral wall extending between said end walls at their outer peripheral edges and secured to the latter, which outer peripheral wall includes a perforated portion adapted to receive thereon printing powder for movement through the perforations of said portion into an electrostatic field to be established in an air gap 1 between said portion and an article to be printed upon for conduction of the powder so moved; a .stationary central shaft extending through said wheel having bearings supporting said wheel thereon for rotation about its horizontal axis; a powder holding hopper within said wheel for holding a suppl of printing powder, said hopper having a pair of spaced opposed side walls through which said shaft extends; a horizontal bottom wall on the lower end of said hopper and side walls for supporting said supply of powder, said bottom wall including a layer of porous filter material having tortuous substantially uniformly distributed passage* ways therethrough for passage of said powder therethrough substantially only upon administering shocks to said layer, and said passageways being of a size to freely pass the discrete particles of said powder upon administration of said shocks; a tubular member coaxial with and around said shaft extending between and rigid with said pair of side walls of said hopper, said tubular member including an axially facing annular surface therewithin and coaxial with said shaft in a position intermediate said pair of opposed end walls of said housing for receiving impacts from a shock administering element; an annular shock administering element reciprocable on said shaft intermediate said opposed side walls of said hopper and within said tubular member having an axially facing annular surface facing adapted to intermittently engage said axially annular facing surface on said tubular member upon said reciprocation of said element for administering shocks to said tubular member for transmission to said pair of opposed side walls of said, hopper and to said layer of porous material; means for reciprocating said shock administering element upon rotation of said wheel.

11. An apparatus according to claim 10, wherein, said means for reciprocating said shock administering element includes a spring enclosed within said shaft in engagement with said shock administering element for yieldably urging the latter toward said axially facing surface on said tubular member at all times and complementary cam surfaces on said shock administering element and on said tubular member engageable during rotation of said wheel for intermittently moving said shock administering element in a direction away from said tubular member against the resistance of said springs and for releasing said shock administering element at the end of each movement thereof away from said tubular member for ≤aid movement of said element toward and into engagement with said member under the force of said spring; means on said shaft outside said wheel connected with said spring manually actuatable for regulating the tension of said spring.

12. An apparatus according to claim 10 or 11, wherein one of the end walls of said wheel is removable from said outer peripheral wall for access to said hopper, finger engageable means on said shaft outside said wheel for releasably securing said one of said end walls in supporting relation to said outer peripheral wall and finger actuatable for release and removal of said one end wall from said wheel, a tubular member coaxial with and around said shaft extending between and rigid with said pair of side walls of said hopper, locking means respectively on said member and on said shaft in releasable locking relation for releasably locking said hopper to said shaft against movement of said hopper axially off said shaft when said one of said end walls is removed, said locking means being releasable from each other for withdrawal of said hopper off said shaft when said one end wall is removed upon a predetermined rotation of said hopper in one direction about the axis of said wheel, and release means accessible from the open side of said wheel when said one wall is removed for releasably securing said hopper against said rotation thereof when said interlocking means are in locking relation, said release means being manually movable for releasing it to enable said rotation of said hopper.

13. A method of electrostatically printing with a dry printing powder substantially as herein described with reference to the accompanying drawings.

14. An electrostatic printing apparatus constructed substantially as herein described with reference to the accompanying drawings. DATED the 18th July, 1967