US2876737A

US2876737A - Apparatus for developing electrostatic images on sheet material

Info

Publication number: US2876737A
Application number: US385314A
Authority: US
Inventors: Chester F Carlson
Original assignee: Battelle Development Corp
Current assignee: Battelle Development Corp
Priority date: 1953-10-12
Filing date: 1953-10-12
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES ON SHEET MATERIAL Filed Oct. 12, 1953 4 Sheets-Sheet l FlG.l

INVENTOR.

Mairch 10, 1959 c, E. CARLSON 2,876,737

APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES ON SHEET MATERIAL Filed Oct. 12, 1953 v 4 Sheets-Sheet '2 FIG. 2

lilllllllmlwlll INVEN TOR.

March 10, 1959 c. F. CARLSON 7 APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES 0N SHEET MATERIAL Filed Oct. 12. 1953 I I 4 Sheets-heet 3 FIGS INVEN TOR.

C. F. CARLSON March 10, 1959 APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES ON SHEET MATERIAL Filed Oct. 12, 1953 4 Sheets-Sheet 4 FIGA INVENTOR.

United States Patent APPARATUS FOR DEVELGPING ELECTROSTATIC IMAGES 0N SHEET MATERIAL Chester F. Carlson, Fairport, N. Y., assignor to The Battelle Development Corporation, Columbus, Ohio, a corporation of Delaware Application October 12, 1953, Serial No. 385,314

14 Claims. (Cl. 118-637) This-invention relates to apparatus for the development of electrostatic images.

In electrostatic image processes, such as Electron Photography disclosed in Carlson Patent 2,221,776, issued November 19, 1940, and Electrophotography disclosed in Carlson Patents 2,297,691, issued October 6, 1942, and 2,357,809, issued September 12, 1944, the electrostatic latent image which has been formed on a surface is developed by bringing a finely-divided material, such as powder, or a liquid mist, adjacent to the image to produce a deposition of material on the surface or on a closely-adjacent surface to form a visible image. In some instances the powder may be sprinkled or tumbled over the surface to be developed and excellent results have been obtained by a cascading method using a two component powder-carrier mixture as described, for example in Wise Patent 2,618,552, issued November 18,

1952. Such methods have been most successful where contrasting images are to be developed, such as black lines or letters on white background. Where continuous tone images are to be developed, and also where large dark areas are present and even in some cases where high definition of line images is required it has been found that the most sensitive development method is one in which an air-suspension of finely-divided 'material, such as powder or liquid mist, is blown past the image surface. An important further measure of control of development can be obtained in such cloud development apparatus by providing a development electrode facing the image surface, as disclosed in application for U. S. Letters Patent, Serial No. 500,207, filed August 27, .1943, by Chester F. Carlson, now U. S. Patent 2,5.51,582,-issued May 8, 1951, for Printing and Developing Solvent Images, and in divisional application Serial No. 166,411, filed June 6, 1950, for Electrophotography, now Patent 2,690,394. The development electrode can be electrically biased to a desired potential or series of potentials during development in order to control the deposition of material.

With liquid developers, such electrodes may be substantially self-cleaning or self-draining. However, with powder clouds it has been found that powder accumulates quite rapidly on the development electrode necessitating frequent shut-downs for cleaning, or requiring awkward wiping mechanisms.

One feature of the present invention comprises an improved developing mechanism for developing electrostatic images on flexible sheet material by means of powder clouds and development electrodes. and in which convenient cleaning means are provided for the develop ment electrode. The development mechanism preferably comprises cylindrical surfaces around'which a web of sheet material can be drawn and a cylindrical development electrode which can be rotated inside the channel formed in the web, and cleaning means co-operating with the development electrode.

Another feature of the present invention comprises the powder cloud generating and transporting means whereby a powder cloud is continuously generated by air or other gas-currents at low pressure. A further feature includes separatory mechanisms and devices in the powder cloud transporting system by which large particles, conglomerates and agglomerates are separated and diverted from the'main developer stream of the powder cloud.

Other features include means to spread a powder cloud stream into a layer wide enough for development of the entire Width of a web and means to prevent loss or leakage of powder cloud.

While the invention is especially applicable to powder cloud development certain parts of the apparatus .and certain features thereof are also applicable to other cloud development methods, such as liquid mist development.

Further aspects and novel features of the invention will be apparent from the following detailed description of certain embodiments thereof, and by reference to the appended drawings in which:

Figure 1 is an elevation, partly in section, ofa powder cloud development apparatus for developing electrostatic images on flexible sheets or webs;

Figure 2 is a vertical section through the development head;

Figure 3 is a section on the line 33 of Figure 2;

Figure 4 is a section on the line 44 of Figure 2;

Figure 5 is a detail of a modified guide for the sheets or websto be developed; and

Figure 6 isa section on the line 6-6 of Figure 5.

Referring to Figure 1 the apparatus comprises a blower 10 for producing a low pressure air or gas stream, an air vent cylinder 11, and a powder cloud generator 12 for introducing a powder suspension into the air stream. A powder cloud conduit 13 leads from generator 12 to development head 14 through whicha web or sheet 15 carrying an electrostatic image is drawn for development by the powder cloud. The air stream and residual powder remaining in the stream after passing through the developing head is returned to the intake part of blower 10 by conduit 16.

Blower 10 may comprise an'electric motor-driven rotary vane-type blower such as is used in household' vacuumcleaners of various sizes. In one embodiment a 200 watt'blower generated a powder cloud stream adequate for developing an 8% inch width of image surface at web speeds of 20 to 60 feet per minute.

The air stream from blower 10 passes through conduit 17 into the base of air vent chamber 11 which consists of an upright cylinder having a closed bottom and a porous filter cloth or other air filter layer 18 secured over the open top of the chamber. A plate or disk 19 is pivotally mounted at one edge of the top of cylinder 11 to be swung over the top by handle 20 to close a portion of the filter surface and increase the air resistance of the vent. It will be apparent that such powder as collects on the inside of filter 18 can fall by gravity to the bottom of cylinder 11 where it can again be picked up by the air stream. Except for the small proportion of air which passes out through the filter the remainder of the air stream and returning powder are carried through cylinder 11 to conduit 21 which leads into the base of powder cloud generator 12.

Powder cloud generator 12 comprises an upright cylinder 22 which is closed at the bottom except for'air inlet conduit 21 which enters at the base of the side wall. The top of the cylinder 22 is closed by a top wall 23 and powder cloud conduit 13 passes vertically through the center of top wall 23. A powder filling passage 24 fitted with a screw cap seal 25 is also provided in. the top wall 23 beside conduit 13.

Inside the base of generator 12 inlet conduit 21 extends to a point near the center where it turns upward to provide a central vertical standpipe portion 26. A wall comprising an inverted cone 27 is secured over the top of standpipe portion 26 and extends out to the side Wall 22 to which it is secured. The apex of the cone is at the axis of cylinder 22 and since the wall of the cone slopes downward and outward to cylindrical wall 22 it forms an annular and substantially V-shaped trough 30 in which powder may settle from the upper portions of the generator. It is preferred that the cone wall make an angle with the central axis of 45 degrees or less to facilitate sliding of powder toward the outer edges of the trough.

Several tubes 28 are secured in the cone near its apex and extend horizontally and radially to positions spaced circumferentially inside wall 22 where they are provided with constricted nozzles 29 pointed slantingly downward to eject air into V-shaped powder channel 30. The nozzles are all pointed in a clockwise direction as viewed from above so as to produce a circular drifting or swirling of the powder 31 in channel 30 as it is picked up by the air streams ejected from nozzles 29. From two to eight or more nozzle tubes 28 are mounted in cone 27 to conduct the air and any entrained powder from standpipe 26 into the powder cloud generating section of the generator. The total cross section of the tubes 28 may be about equal to that of each of the

conduits

13, 16, 17 and 21 but it is preferred that the nozzles 29 be constricted slightly at least so that the air or gas streams issuing from the nozzles are accelerated and able to raise a substantial amount of powder 31 resting in channel 30 into the air stream and create a powder cloud. Such powder as falls back into the channel drifts around in a clockwise direction and soon comes under the stream of the next succeeding nozzle around the channel. The nozzles are spaced closely enough so that the powder drift from one nozzle is brought under the next one and hence no chance is provided for drifts of powder to form and lie dormant.

Within generator wall 22 there is provided a vertical cylindrical bin wall 32 which is open at the top and bottom. The lower edge of wall 32 is slightly spaced above the wall of cone 27 so that powder can slide down the cone into the channel 30, and the upper edge of wall 32 is preferably at or slightly below the level of the lower end of conduit 13 which extends down a short distance below top wall 23 of the generator. Wall 32 is spaced from wall 22 sufiiciently to make the cross section of the annular space 33 greater than that of any of

conduits

13, 16, 17 and 21 and preferably twice as great or more. Nozzles 29 are located in this annular space and as the powder cloud is generated it rises in this space with a clockwise swirling motion.

Heavy powder particles and agglomerates fall back into the channel partly because of some centrifugal separation which takes place and partly because of the fact that the cross section of space 33 is larger than the conduits and hence the slower speed of the rising air stream is insufiicient to support heavier particles. Only the desirable finer particles are carried by the air stream up over the top of wall 32 and out through conduit 13. Any further agglomerates which may form after the stream passes over wall 32 have a further opportunity to settle out of the air stream into the central bin provided by wall 32.

The generator may be replenished with powder from time to time by removing screw cap 25 and introducing powder into the central bin through tube 24. The powder used may be any of those commonly used for powder cloud development, such as a pigmented resin powder, carbon or pigment powder or mixture. In some cases a small proportion of larger so-called carrier particles may be maintained in the generator. These particles are preferably triboelectrically related to the developer powder 31 in such a manner as to impart electrostatic charges of the desired polarity to the powder particles to enable 4 the powder particles to be attracted to the electrostatic images to be developed on web 15. Thus, if the images are positively charged the powder is preferably negatively charged, and vice versa. In some cases, however, it may be desirable to develop the images with powder of the same charge polarity to obtain a reversal print as will be described later. The larger and heavier carrier particles, when used, continually fall back into the channel from space 33, and hence remain in the powder cloud generator and are not carried through the developing head 14. Examples of some powder-carrier mixtures are described in U. S. Patent 2,618,551 granted November 18, 1952 to Lewis E. Walkup and in U. S. Patent 2,638,416 granted May 12, 1953 to Lewis E. Walkup and Edward N. Wise but for the purposes of the present invention the proportions of carrier are much less, the carrier amounting to only about 5% to 25% by weight. Also slightly smaller carrier particles are preferred, such as particles which pass through 60 mesh and are retained on mesh, or in some cases even including particles between 100 mesh and 200 mesh, so that they may be. readily agitated and moved by the high speed air jets issuing from nozzles 29.

The interior walls of generator 12 and of conduit 13 and development head 14 may be coated, if desired, with films of material having the desired triboelectric properties to impart the correct polarity of charge to the powder particles. In many cases the structural material itself will have the desired effect and no coating will be necessary. Thus many resinous powders become negatively charged by contact or rubbing against many common metals.

Conduit 13, also return conduit 16 and

conduits

17 and 21, may be partly or entirely formed of flexible material, such as rubber or plastic hose to enable convenient mounting of the developing head 14 in relation to generator 12 and blower 10. The flexible conduit also helps to isolate vibrations generated by blower 10 from the rest of the system.

Development head 14 is shown in side view in Figure 1 and in section in Figures 2, 3 and 4. The powder cloud rising through conduit 13 first enters a distributor section 34 in which the stream is spread or distributed in a series of steps into a lamina the width of the image area on sheet or web 15 to be developed, and preferably slightly wider than the image area. The distributor is made of a stack of plates 35 tightly clamped together by

bolts

36, 37 and 38 and provided with cut-out apertures which connect with those in adjacent plates to provide a zig-zag path 39 for the powder cloud, as seen most clearly in Figure 2. The path gradually widens as it passes upward as seen from Figures 3 and 4. The lowermost plates 35, where the powder cloud path is still narrow, may be thicker than those above to preserve a relatively constant cross section for the passage as it follows its zig-zag upward course. The average cross section of the passage in distributor 34 may be substantially equal to the cross section of the passage in conduit 13 or it may be slightly less in order to accelerate the stream slightly and reduce the possibility of clogging in the distributor section.

The shape of the passage 39 may be further understood by reference to Figure 4 which is a section taken horizontally through the lowermost cut-out plate looking upward as indicated on the section line in Figure 2.

The passage 39 starts at the outlet of conduit 13 and flares outward at a uniform rate until it reaches an arcuate aperture 40 in adjacent plate 235 after which it reverses its direction but continues in a flaring path cut in plate 335 as indicated by'the dashed lines 41 until it reaches an elongated aperture 42 in the next plate 435, after which it again reverses direction and so forth. Apertures 40 and 42 are shaped to contribute to the uniform distribu tion of the powder cloud stream across the width of the path. Thus, aperture 40 comprises an arc having its center at the intersection of the extension of the lines of the sides of the passage cut in plate 135. Aperture-42 may also have an outward curvature centered at the intersection of the lines of sides 41 of the passage in plate 335, or it may have a lesser curvature or be straight. This and subsequent apertures may also be varied in width along their length to aid the spreading of the powder cloud. Thus, they may be constricted slightly in the middle, as shown for aperture 42, to deflect the stream outward toward the ends of the slot. After the stream has passed through the first two or three apertures, such as 40 and 42 it will have reached the full width of the image area but it is desirable to convey the stream through two or more further reversals in its zig-zag path to permit further equalization to take place in the speed and density of the powder stream across its width. Thewalls of the passage in distributor 34 also contribute to the breaking up of powder agglomerates due to turbulent regions, and to the charging of the powder particles due to contact with the walls of the passage. Plates 35 may preferably be made of conductive material, such as aluminum or other metal. Insulating films, if present, are preferably of negligible thickness to prevent the building up of high electric charges on the inside walls. In some cases, however, the walls may be treated with thin insulating coatings of the desired triboelectric properties to impart charges to the powder, or with conductive coating for the same purpose.

The flattened powder cloud stream passes from the top of distributor 34 to a by-pass valve chamber 43 (Figure 2) where the stream is divided into two flat streams consisting of a developing stream, which passes upward in a fiat passage 47 between vertical walls 48, and a by-pass stream which flows through a passage 49 toward the right, as viewed in Figure 2, and joins the returning spent developing stream 50 returning from the image developing zone. An adjustable control valve 44 comprising a wedge-shaped vane on a circular shaft 45 is located in valve chamber 43. The shaft can be rotated a few degrees by manual control handle 46 (Figure 3) to bring valve vane 44 against either of the flaring walls of chamber 43 to divert the stream entirely to either

passage

47 or 49 or the vane may be set at any position between these limits to divert a greater or lesser proportion of the powder cloud stream into by-pass passage 49.

The slope of wall 91 which co-operates with vane 44 to form the entrance to by-pass passage 49 is substantially parallel to the direction of the powder cloud stream as it enters valve chamber 43 while the development passage 47 turns upward within the valve chamber. Heavier particles and agglomerates, which may be present in the stream at this point are carried by their inertia into bypass passage 49 whereas the smaller and lighter particles, which are most desirable for development, are readily carried along with the air stream and are divided between the two passages substantially in the same proportion as the air itself is divided. This provides a developing stream containing mainly or exclusively the finer developer particles, while the larger particles and agglomerates are by-passed to the return passage.

Developer passage 47 extends vertically to its outlet 51 which comprises a horizontal slot from which the developing stream of the powder cloud emerges in an upward direction. A cylindrical development electrode .52 having integral web guide flanges 53 at its ends is mounted for rotation on a shaft 54 supported in bearings in end walls 55 of development head 14.

The axis of shaft 54 and cylinder 52 is parallel to the outlet slot 51 for the powder cloud and is to the right and slightly above the slot outlet, as viewed in Figure 2. Walls 48 of the developer passage 47 are tapered or curved inward near the outlet and the outlet slot is positioned close to the surface of cylinder 52 so that the powder cloud is ejected tangent to the cylinder surface and substantially in contact with it.

A web guide roller 56 mounted on free-turning shaft 57 has its axis parallel to cylinder 52 in the same horizontal plane and the surface of roller 56 is substantially tangent with the flanges 53 at the left side of the cylinder as seen in Figure 2.

Web 15, carrying an electrostatic image to be developed, passes down around roller 56 and up between the roller and flanges 53, then around the top of cylinder 52 resting on flanges 53 so that the image area of the web is spaced from the surface of cylinder 52 providing a developing space 58 of approximately the same thickness as that of passage 47 which conveys the powder cloud to the developing space. For greatest effectiveness the developing space is made as thin as possible to bring development electrode 52 as close as possible to the Sheet surface to be developed While still permitting passage of the powder cloud between the surfaces. In most cases the spacing is between 5 and mils, which means that flanges 53 will have a diameter of 0.01 to 0.2 inch greater than the diameter of cylinder 52. Roller 56 may be formed of slightly yielding material such as rubber to permit web 15 to be wedged between the roller and flanges 53 at the point of contact or a rigid roller can be used and shaft 57 be mounted on a yielding bearing support.

The electrostatic image on web 15 is located on the surface which faces the development space 58. The web may comprise a flexible electrophotographic plate such as is described, for example in aforementioned Patents 2,297,691 and 2,551,582. The electrostatic images, of course, are produced by electrophotographic techniques prior to entering the development apparatus. The web may also comprise a conductive foil carrying an electrically charged insulating image, as described, for example, in connection with Figure 11 in aforementioned Patent 2,357,809. In other instances the web may consist of insulating sheet material, such as cellulose acetate, polystyrene, polyethylene or other plastic sheeting, or thoroughly dry paper with or without a plastic coating, on which an electrostatic image has been formed as disclosed in aforementioned Patent 2,221,776 or to which an electrostatic image has been transferred or applied by other methods.

As the powder cloud passes around developing space 58 between the surface of cylinder 52 and the imagecarrying face of web 15 the powder is attracted to the electrostatic image and is deposited upon the web to form a powder image. Due to the presence of closely-spaced conductive cylinder 52 the lines of force extending from the electrostatic image are largely aligned normal to the image surface and the electric field is concentrated in the developing space where it is effective to attract powder.

In some instances further control of development can be imposed by applying a suitable potential to a curved electrode 59 which follows the curvature of the web as it passes around the development space. Electrode 59 is spaced close to the back of the web and is connected through commutator 65 to the sliding contact 63 of a potentiometer 60. For the present we shall consider the commutator 65 as stationary so that electrode 59 is permanently connected to contact 63. The resistance element of potentiometer 60 is grounded at its mid-point and cylinder 52 is also connected to ground. The ends of the resistance element are connected to the terminals of a battery 61 of several hundred volts potential. By adjusting the position of contact 63 on the resistance element, an electric field is applied between electrode 59 and cylinder 52, and its strength and polarity are readily controlled by the operator.

In most instances a triboelectric charge is applied to the powder cloud during its generation which is opposite in polarity to the charge of the electrostatic image so that the powder is readily attracted to the image without the aid of an externally applied field. However, frequently the web may carry a small charge in the background areas which will give rise to undesirable background powder in the developed image. By applying a smallfield from electrode 59 to cylinder 52 which opposes the field of the background charge the resultant field in background areas is cancelled or even reversed in direction so that powder is not attracted to these areas. Powder is still deposited in the image areas, however, as these carry higher charges and their field is reduced only slightly by the superimposed field.

In cases where the electrostatic image is weak the potentiometer can be adjusted to provide a field aiding the image field. This results in some background powder but it produces a heavier deposit on the image areas which is often helpful in making a weak image more legible.

Reversal development may also be effected by use of electrode 59. A powder cloud is produced having the same polarity of triboelectric charge as the image areas. Hence the powder is repelled from the image. When a field is applied by electrode 59 which opposes the image field powder is attracted to the uncharged background areas to develop a reversal print.

The quality and character of continuous tone images can also be controlled by placing potentials on electrode 59. It is often desirable to vary the maximum and minimum density of a print, to control the contrast, and in some cases to accentuate development of areas having a given charge density. To increase the brightness of highlights contact 63 may be set to a point which will apply a field which cancels the field from the highlight areas. To increase the overall density a field may be applied to aid the deposition of powder over the entire web area.

To emphasize the darker areas a different technique may be employed, involving the use of commutator 65. The commutator has a rotary contact 66 connected to electrode 59 and normally at rest on a stationary commutator segment 67 which is connected to potentiometer contact 63. The commutator is provided with a series of short contact segments similar to 67 and rotary contact 66 can be driven by motor 68, when energized, to sweep across the stationary contacts.

Potentiometer contacts

62 and 64 are connected to other segments and movable conductive bridging pieces 69 are used to connect adjacent segments where required to produce larger segments connected to any of the potentiometer contacts. By setting the positions of

contacts

62, 63 and 64 and placing bridges 69 to connect the segments together in groups it will be apparent that the potential of electrode 59 is cycled through a series of values as contact 66 is rotated and that the percentage of time at any potential is determined by the number of segments joined to a given potentiometer contact by the bridges 69.

To emphasize the darker areas of the image a potential is applied to electrode 59, for say 50% of the time, which is opposite in polarity to the image and sufiicient to produce a field strength which will cancel or reverse the field from all except the most highly charged image areas. During this period of each cycle of commutator 65 only the highly charged areas are developed. For another period, say 40% of the cycle, the potential is lowered to permit some development of intermediate tones along with further development of dark areas and finally, for the last 10% of the cycle the potential may be lowered still further to allow a slight fill-in in the lighter areas. The commutator may complete a cycle in one second or less and pass through several cycles during the travel of a given point of the web through the development zone. Other forms of development can also be achieved with the cycling system including reversal development.

Electrode 59 is most suitably used with webs consisting entirely of insulating material. Where a conductive backing is provided on the web electrode 59 may be brought into direct sliding contact with the backing to apply its potential directly to the conductive backing. Where this is not feasible cylinder 52 may be electrically insulated from the head 14 and various potentials applied 8 to it while the conductive web backing is held at ground potential, or if no conductive backing is present electrode 59 may be used at ground potential.

The web 15 is drawn out of developing head 14 and downward and outward to the right (Figure 2) by a pair of

co-acting rubber rollers

70, 71 driven at the desired developing speed. In place of

rollers

70, 71 the web may be wound up on a motor-driven take-up reel. In event the powder image is to be transferred to another surface the web may be carried through a transfer mechanism where a web of sheet transfer material is brought against it, or in some instances transfer can be efiected by feeding a sheet or web of adhesive-coated material between roller 71 and web 15. Between head 14 and

rollers

70, 71 the web presses through a fuser 72 which may be energized when it is desired to afiix the powder images directly to web 15. The fuser may comprise an electric oven having entrance and exit slots for the web, or it may be a chamber containing the vapor of a solvent for the powder image or for a plastic coating on the web.

As Web 15 is drawn away from flanges 53 it passes over the edge of enclosing wall 73 of head 14 and is spaced slightly from it in the image area to avoid contact with the powder image. The spent powder cloud continues down around cylinder 52 and then straight down through return passage 50 where it joins the bypass stream from passage 49 and the combined stream returns to the blower 10 through conduit 16. Distributor plates 35 are provided with aligned apertures 74 (Figures 2 and 4) of decreasing length from top to bottom to converge the return stream into circular conduit 16.

While cylinder 52 may be rotated by the drag of the web as it passes around flanges 53, it is often desirable to provide a positive drive for the cylinder to enable accurate control of the web speed in the development zone and to avoid imposing a drag on other parts of the equipment. As seen in Figure 3, shaft 54 on which cylinder 52 is mounted, can be driven by motor 75 through speed reducer 76, which may include a one-way clutch 92 to enable the web to be pulled forward independently of motor 75 when the motor is not needed, and also during initial threading of the web.

As previously stated one of the principal drawbacks of development electrodes in the past has been the need for frequent shutdowns for cleaning. The development electrode cylinder 52 here provided is cleaned during each cycle of rotation. A thin blade 77 is supported on frame member 78 to slide lightly against the surface of cylinder 52 where it descends away from the developing space. Any large accumulations of powder are scraped lightly from the cylinder and fall into the returning spent powder cloud stream. Any thin powder deposit still remaining on the cylinder is then brushed off the cylinder by a brush 79 supported on spring arms 80 inside a chamber 81 below the cylinder. A clean-out hole 82 is provided in the end wall of the head at the bottom of chamber 81 for occasional removal of accumulated powder.

Blade 17 may be formed of thin steel or Phosphor bronze shim stock. Brush 79 may be a fine bristle brush, plush, or a soft felt pad.

By virtue of the expulsion of a small amount of air through filter 18 there is a slight suction produced at the places where web 15 enters and leaves developing head 14 so that a slight intake of air is produced and loss of powder cloud is prevented or minimized.

Figures 5 and 6 illustrate a modification of the development head to provide a more positive support for web 15 and independent rotation of the development electrode. Web 15 passes around the development zone on a pair of rollers 83 which are free to rotate on shaft 84 of developing electrode cylinder 85. A pair of endless rubber or fabric belts 86 pass around guide pulleys 87 then down aroundpulleys 88 and up between pulleys 88 and rollers 83 where web 15 is brought against the rollers. The belts then pass around the, rollers 83 resting on the edges of web 15 and securely .holding it against the rollers. The belts thenpass around rollers 89-at the point where web 15 leaves rollers 83,,and the belts return over pulleys 87 to complete their cycle. The web may be advanced atany desiredspeed by takeup rollers 70, 71 (Figure 1) independently of the motion of cylinder '85. Cylinder 85-is driven by gear 90 on shaft 84 from a motor and speed reducer drive, as previously described. This .makes it possible to adjust the :web speed and electrode speed independently for most elficient operation.

A powder cloud development mechanism has been described which provides for efficient and continuous generation of a powder cloud, triboelectric charging of theparticles thereofto a desired polarity by contact with carrier particles or with .the walls of the generator, or both, separation of large particles and agglomerates, and breaking up of agglomerates by turbulence. The

system also provides for spreading of the powdercloud intoauniform lamina the width of the area to bedeveloped, .the controlled by-passing of any proportionof the powder cloud to vary the intensity of the developingstream and also to remove further large particles and agglomerates from thedeveloping stream, and the return of spent developer and by-passed material to the powder cloud .generator.

Provision is also made .for control of development by a conductive development electrode and by potentials applied tosaid electrode ortoa control electrode spaced from. the development electrode. vThe mechanism-provides for continuous development of web or sheet material and for continuous cleaning of the development electrode.

Means are also provided to maintain a net suction on the development mechanism to prevent escape of powder cloud out of the system.

Other features include independent control of web speed and speed of cycling of the development electrode. Further features are apparent in the preceding specification and in the appended drawings.

It is apparent from the principles set forth in this invention that certain modifications can be made without departing from the spirit of the invention. For example, it is contemplated that cloud return passage 50 in the developing head can be provided with a zig-zag path similar to entering passage 39 to converge the stream uniformly into return conduit 16.

Although the system has been described as an air system it is apparent that other gases can be circulated and where the word air is used in the claims, gases in general are intended.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. Developing mechanism for developing electrostatic images on sheet material with a finely-divided material, comprising, in combination, a rotatable cylindrical development electrode having a substantially smooth surface, said electrode being rotatable on its longitudinal axis in a path parallel to its surface, means for moving a surface carrying an electrostatic image in a path closely-spaced and substantially parallel to part of said electrode surface to form a developing space, means for generating an air suspension of finely-divided powder and a conduit fed thereby and having an outlet leading into the developing space between said surfaces to develop said electrostatic image under control of said development electrode.

2. Developing mechanism as claimed in claim 1, in

which said development electrode rotates in a closed path to bring each part of its surface repeatedly through said developing space, and electrode cleaning ,meansare provided adjacent to the path of said electrode surface outside of said developing space to remove residual powder from said surface on each rotation thereof.

3. Developingmechanism for developing electrostatic images on sheet material with ,a finely-divided material, comprising, in combination, an electrically conductive cylinder, means mounting said cylinder for rotation on itsaxis, means for guiding sheet material carrying electrostatic images in a path which makes a cylindrical are around part of the surface of said cylinder and spaced fromnsaid surface, means for generating an air .suspen:

sion of finely-divided powder anda conduit fed thereby and having an outlet leading into the space between said sheet material and said surface.

4. Developing mechanism as claimed in claim 3, in which a cleaning member to remove deposited powder from the surface of said cylinder is mounted adjacent to the surface of said .cylinder to clean said cylinder surface during rotation thereof.

.5. Developing mechanism for developing electrostatic images on a sheet surface with a finely-divided material, comprising, in combination, a cylinder mounted for rotation, a pair of circular flanges of slightly larger diameter than said cylinder at the ends thereof, feed guides for feeding flexible sheet material around a part of the circumference of said cylinder with the edges of said sheet material resting on said flanges, and a passage terminating in a slot having itsopening directed into the spaceformed betweensaid sheet material and said cylinder for introducing an air suspension of finely-divided material in said space.

6. Developing mechanism as claimed in claim 5, in which said flanges are mounted for rotation independently of said cylinder.

7. Developing mechanism as claimed in claim 5, in which said flanges are mounted for rotation independently of said cylinder and a drive mechanism is coupled to said cylinder for rotating said cylinder.

8. Developing mechanism as claimed in claim 5 in which said feed guides include belts riding on said flanges for clamping the edges of said sheet material against said flanges during its travel around part of the circumference of said cylinder.

9. A powder cloud development apparatus for developing an electrostatic image with an air suspension of fine powder, comprising a powder cloud generator and a blower, a powder cloud main conduit fed by said generator and blower, said main conduit being branched at its end into a developer conduit and a by-pass conduit, said developer conduit making a substantially greater angle with said main conduit than said by-pass conduit makes with said main conduit, whereby a first part of said powder cloud leaving said main conduit is deflected at a substantial angle and enters said developer conduit and the remainder of said powder cloud enters said by-pass conduit, the deflection of said powder cloud causing inertial removal of at least part of the heavier particles from the part of the powder cloud entering said developer conduit, and developing means fed by said developer conduit to apply the powder cloud from said conduit to an electrostatic image.

10. A powder cloud development apparatus as claimed in claim 9 in which valve means are provided in said conduits to vary the proportion of the air stream which is diverted to said by-pass conduit.

11. Developing apparatus for: electrostatic images comprising, in combination, means for generating and blowing a rapidly moving air stream containing suspended finelydivided developer particles, a common conduit fed therby, and a pair of branch conduits fed by said common conduit, one of said branch conduits comprising a development conduit for conveying air-suspended developer particles to a developing space, the other of said branch couduits comprising a by-pass conduit for diverting air and particles away from the development conduit, the entrance to said by-pass .conduit being substantially in alignment with the output air stream issuing from said common conduit and the entrance to said developmentconduit being substantially out of alignment with said output air stream, whereby heavier particles issuing from said common conduit are carriedby their inertia toward said by-pass conduit, and the air stream entering said development conduit contains proportionally less of said heavier particles than are present in the air stream issuing from said common conduit.

12. Developing apparatus as claimed in claim 11 in which a common return conduit is provided, said return conduit connecting the output of said developing space and of said by-pass conduit to the input of said generating and blowing means.

13. A developing system for developing a surface carrying an electrostatic image with finely-divided material comprising means for producing and blowing an air suspension of finely-divided powder, a conduit fed thereby having a cross-section in which any diameter is not substantially greater than any other diameter, a lamellar distributor fed by said conduit, the individual plates of said distributor being positioned and adapted so as to define a folded path tapered outward to a Width at least equal to that of the surface carrying said electrostatic image at an outlet slot terminating said path whereby said air suspension is converted into laminar flow during its transit through said distributor and means for supporting the surface to be developed adjacent to said outlet slot.

14. Apparatus according to claim 13 wherein said 12 lamellar distributor reverses the direction ofsaid air suspension during its transit therethrough at least twice after reaching the full width of the surface to be, developed whereby said air suspension is evenly dispersed through out the entire width of said outlet slot.

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