Aerosol bomb development

Nov. 24, 1959 j. f. rosenthal 2,914,221

AEROSOL BOMB DEVELOPMENT Filed Aug. 16, 1955

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ATTORNEY

2,914,221

Patented Nov. 24, 1959

United States Patent Office

2,914,221

AEROSOL BOMB DEVELOPMENT

Joseph F. Rosenthal, Rochester, N.Y., assignor to Haloid Xerox Inc., Rochester, N.Y., a corporation of New York

Application August 16, 1955, Serial No. 528,761 ;

Claims. (Cl. 222—146)

This invention relates in general to xerography and, in particular, to development of xerographic images.

In xerography, plates, composed of photoconductive insulating material overlying support members, are made sensitive to light or other radiation by placing a uniform electrostatic charge on the surface of the photoconductive insulating layer while the plate is kept in darkness. Following charging, an electrostatic image composed' of electrostatic charges in image formation is formed bn the surface of the insulating layer by exposing this layer to a light or other activating pattern. Areas of the photoconductive insulating layer exposed become conductive and charges on those areas of the surface are rapidly dissipated, whereas charges on the surface of areas not exposed substantially remain in place. Areas of intermediate charge will be formed on the surface of the photoconductive insulating layer which correspond to areas of intermediate activation of the activation pattern of the image to which the sensitive plate was exposed.

The electrostatic image or electrostatic charge pattern thus formed may be allowed to remain on the plate or may be transferred. When allowed to remain on the plate, scanning systems or the like may be used to make further use of the electrostatic charge pattern, or the charge pattern may be developed. Development of the charge pattern as is generally carried out in the art,, is the deposition of finely divided powder particles carrying electrostatic charges on the plate surface in conformity with the electrostatic charge pattern. A developed powder image on the plate surface may be transferred and affixed to another surface, such as a sheet of paper, and the plate may be cleaned for reuse, or the powder image may be allowed to remain on the plate for photographing, viewing, or the like.

One method of development in use in the art of xerography is generally known as “powder cloud development.” In this technique of development a dispersion of electrically charged powder particles in gas is presented to the surface bearing the electrical image and particles are drawn from the gas dispersion to forrn a powder image on the plate surface in conformity with the electric charge pattern. This form of development is disclosed and described in Carlson U.S. Patent 2,221,776, wherein a rotating vane wheel or propeller is used to stir up powder in a chamber, thereby creating a cloud , of particles for presentation to the electrostatic image. The vane or propeller in that patent may be connected to a terminal of a battery to impart charge to the powder particles.

Various other techniques of powder cloud development have been utilized in the art. However, all generally involve rather complex mechanism which in order to function properly must be fixed in position. Typically these prior art generators require a compressed air or gas source which, in addition to the other elements, create large and cumbersome development units.

This invention improves the art of xerography by devising an aerosol bomb to generate powder clouds for development in xerography. This novel device overcomes the various problems which existed with the prior art generators. An aerosol bomb, for example, can be conveniently handled, produces an aerosol of particles for development purposes immediately, overcomes the clumsiness generally present because of the need for compressors and the like in the prior art generators, and generally improves development in the art of xerography.

According, it is an object of this invention to improve upon means, methods, and apparatus for the development of electrostatic images.

It is another object of this invention to improve upon apparatus to generate an aerosol of powder particles in 16 gas.

It is a further object of this invention to improve upon apparatus for image development in xerography.

It is a further object of this invention to improve upon methods of forming an aerosol of powder particles in 20 gas.

It is a still further object of this invention to devise an improved aerosol bomb for the development of electrostatic charge patterns.

It is a still further object of this invention to improve 25 upon methods of developing an electrostatic charge pattern.

For a better understanding of this invention, together with other further objects thereof, reference is now had to the following description taken in connection with the 30 accompanying drawings, and the scope of the invention will be pointed out in the appended claims.

Fig. 1, A, B, and C , illustrates an embodiment of image formation in xerography.

Fig. 2 is a diagrammatic view of an embodiment of 35 developing apparatus according to this invention.

Fig. 3 is a pictorial representation of an embodiment of a powder cloud generator according to this invention and,

Fig. 4 is an enlarged view partially diagrammatic and ⁴θ partially in cross section of an embodiment of the output mechanism of the generator shown in Fig. 3.

For a better understanding of this invention reference is now had to Fig. 1 wherein there is illustrated an embodiment of xerographic image formation. The steps ⁴³ illustrated in this figure are: Fig. 1-A, charging or plate sensitization; Fig. 1-B, exposure of a plate previously sensitized; and, Fig. 1-C, development of the plate following exposure. The plate which is used in xerography, and Which in this figure is generally designated 15, comprises a photoconductive insulating layer 1 overlying a conductive backing member 2. The photoconductive insulating layer may be any of a number of materials as, for example, sulphur, vitreous or amorphous selenium, zinc oxide in a resin binder, or the like. Functionally, ⁶³ it may be described as a material able to retain an electrostatic charge for sufficiently long periods to allow ex¹ posure and development or other utilization of the electrostatic charge pattern on its surface and as a material which, on exposure to activating radiation, rapidly dissipates charge.

In Fig. 1-A, the charging or sensitizing step is illustrated using a corona discharge electrode generally designated 3 comprising a grounded shield 4 and corona discharge wires 5. The corona discharge wires 5 are con⁸⁵ nected to high voltage source 6 which supplies a corona generating potential to wires 5, the potential being in the order of from 6,000 to 10,000 volts. The voltage may be direct current, positive or negative, or alternating current. . Generally, the surface of the photoconductive insulating layer 1 to be charged is charged from 100 to 800 volts either positive or negative. The corona elec2,914,£S1 trade 3 is driven by motor 7 along screw 8 connected to motor 7 and the corona discharge electrode 3. Relative motion between the corona discharge electrode and the surface of the plate to be charged is desirable when the grid or discharge electrode only covers a small -portion of the plate surface. In this figure, movement of tne electrode takes place whereas it is possible to use a stationary electrode with a moving plate, or a stationary plate and stationary electrode when the electrode covers and is able to sensitize substantially the entire image bearing area.

During charging, the backing member 2 of plate 15 is maintained at a; grounded potential and, generally, charging is carried out in darkness.

Fifi·. 1-B represents and illustrates the-exposure step. In this figure, copy 9 to be reproduced is projected through lens 10 to the charged surface of photoconductive insulating layer 1 overlying backing member 2 of plate 15. The backing member 2 of plate 15’ need not be grounded during exposure but may, if desired, be held at ground potential. Exposure causes dissipation of charge in those areas struck by light-or other activating radiation resulting in a charge pattern of electrostatic charges on the surface of the photoconductive insulating layer 1.

Fig. 1-C illustrates the technique of powder cloud development which is one possible development technique which may be used to make visible an electrostatic charge pattern on the surface of the photoconductive insulating layer 1 overlying backing member 2 of the xero- 30 graphic plate generally designated 15.

In powder cloud development, a cloud of developer __________{_}__________ ?^a5„“ ^{ln gaaI}f ^{1S pres}f^{nted t0 the ima}ge bearing sur- and then into the photoconductive insulating layer do nt f particles are electrostatically charged through not develop. Development of such a plate often results me use or corona charging, frictional charging, or the 35 in hollow characters and in developed areas surrounded like, and the electrostatic fields of force which exist be- ^K----~ ' ' ' surrounded tween the charges on the particles and the charges on formity with the electrostatic charge pattern on the image steps in Figs. 1-A, B, and C, a powder image reproduction of the original copy.

There is no desire to limit this invention to the particular steps of image formation illustrated in Fig. 1. For example, although corona charging is filu_8ualed in Fig. 1-A, there is also intended to be included herein radioactive charging, frictional charging, or the like. Ais°&gt; although in Fig. 1-B projection exposure is illustrated, there is intended to be included herein contact exposure, X-ray exposure, reflex exposure, or the like Various other modifications which will readily occur to those skilled in the art are also intended to be included herein.

Reference is now had to Fig. 2 wherein is shown partially an cross section and partially diagrammatically 55 an embodiment of a developing device according to this invention. A developing box 11 has positioned extending into one wall thereof a nozzle 12 which is connected to a powder cloud generator 13. Positioned along anfilter¹ iT ¹ tn aⁿ-^SUiK® ^{and sup}P°^rt means 51 is a. 60 fiiter 16. Illustrated m this figure positioned along yet another wafi of developing box 11 is a plate 15 com^a P.^h°t°^ponductive insulating layer 1 overlying a base 2 which is positioned in guide and support means ^poe^rt “^eanL51 prefcabiy are of _{a prO}peTsize _to⁶° ^⁴^.⁸^ ⁵⁰·,. Switch 55 may be ^s ‘¹⁰n of the piate and filter to be interchanged.

Optionally, the developing device illustrated in this figure may be used to develop two plates simultaneously by positioning a plate on guide and support means 50 and ro mr.mi.pr -&gt; J—j’ “““ “·“» piacc uacxing by positioning another plate in guide and support means ™ ^{2 and} development electrode 52 at the same

51. In such an instance it is desirable to provide an escape outlet to relieve the pressure created within the development box by the propellant gas.

When a plate is in position in developing box 11, the 75 box is light tight to allow development without affecting the charge pattern on the plate surface.

Positioned beneath the opening defined by guide and support means 56 is a development electrode 52. The development electrode 52 is mounted in a plane which is parallel to the plane of the image bearing surface of the photoconductive insulating layer 1 of plate 15. Preferably, development electrode 52 is a porous conductive layer such as a fine mesh screen or the like. Optionally, the electrode may be formed of a solid conductive material. When formed of porous material a cloud produced by powder cloud generator 13 and fed into developing box 11 through nozzle 12 will penetrate through the screen and around the edges of the screen to develop a plate positioned in guide and support means 50. When development electrode 52 is a non-porous material, the cloud will seep around the edges of electrode 52 to develop the image bearing surface of the photoconductive insulating layer 1.

Development electrodes as used in xerography serve the function of drawing outward from the plate surface the electrostatic lines of force by creating the electrostatic field between the charge carrying surface and the development electrode. Plates carrying electrostatic charge pattern have lines of force extending inward through the photoconductive insulating layer and outward into air and then into the photoconductive insulating layer. When such a charge pattern is developed without the Use of a development electrode, particles tend not to deposit in those areas in which the lines of force extend directly inward to the backing member of the plate. Those lines of force extending outward into air noT develop/ Development of such a plate often results by a slight halo. This type of development which is considered particularly valuable in the field of xcroradiof Fig.^2 by positioning a plate in guide and support To avoid hollow images and to do away with the halo surrounding developed image areas, a development electrode may be positioned in close proximity to the sur^^ace Be developed. The development electrode when is illustrated in 45 properly biased causes the lines of force of the charge u pattern to extend to the development electrode instead of to the backing member of the plate and instead of to areas on the plate which are at a different potential. Development in xerography generally results in deposi50 tion in accordance with the electrostatic fields or electrostatic lines of force. Thus, development while using a development electrode biased to cause the lines of force to extend between the development electrode and the nnage bearing surface results in deposition of particles m conformity with the charge pattern existing on the image bearing layer and results in high quality reproductions. . Typically, continuous tone prints are made xerographically using the development electrode during the development step.

^fi^^Ure development electrode 52 is connected to lead 53 which is connected to one end of switch 55. Connected to the other end of switch 55 is lead 56 which extends to a tension spring 57 mounted to contact backing member 2 of plate 15 when plate 15 is in position thrown between terminals 58 and 60. Terminate 58 apply a battery bias between conductive backing member 2 ot plate 15 and development electrode 52, whereas terminals 6(1 apply a short circuit and thus place backing potential. ^oame

Various biases may be applied to the development electrode to accomplish various purposes. For . example, if after exposure of a sensitive plate, the background areas 5 are found to carry 10 volts negative potential, the develop- . Τ' --- €*Χ4,\Λ LXXv* vixaigca UH ¹ _ _ _ formiiv ι^Ρ₊^ate ,^{cause particle} deposition in con- - ography may be accomplished with the’ developing ‘devtee iormity with the electrostatic charge pattern on the image cf Fig. 2 — ·- · bearing surface. There is thus formed, following the 40 means 51.- *

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2,014,221 θ

switch 30 is provided to break the battery circuit by breaking wire 28. The switch is shown in this figure in an open position. When switch 30 is closed, the battery supplies voltage to the opposite ends of nozzle 12.

Housing 18 may be composed of various substances, as, for example, thin or heavy metals, plastics, compositions of metal or the like. Output valve 23 is generally a plastic member, but may be composed of any number of various materials as, for example, rubber, metal, compositions of 10 metal, or the like. Preferably output valve 23 is composed of an electrically and heat insulating material to protect the operator from any heat and/or electricity applied to nozzle 12.

Cap 25 is for purposes of protecting output valve 23 15 from accidental depression. Depression of output valve 23 will act to bring about an output aerosol from the output end of tube 12. Thus, cap 25 when in position on ridge 26 of housing 18 is spaced above output valve 23, and any downward pressure brought to bear near the top 20 of powder cloud generator 13 is stopped from reaching the top of output valve 23 by protective cap 25.

The application of electricity from battery 20 to nozzle 12 is for purposes of heating nozzle 12 and the aerosol passing therethrough. In this embodiment, heating of the 25 nozzle is accomplished by using a conductive material or a low resistant material, such as metal or the like, as the nozzle material, and by positioning the nozzle or a portion thereof in an electrical circuit. The use of metal as the nozzle material generally creates an additional advantage 30 of good thermal conductivity and thus efficient aerosol heating as the aerosol travels through the length of nozzle 12. Although in this figure tube or nozzle 12 is heated using a battery, it is to be realized that other sources of electricity may be used as, for example, feeding 35 A.C. voltage through a transformer and tapping off the desired amount of heating voltage from the secondary. It is also to be realized that other means of applying heat to tube or nozzle 12 may be used and are intended to be included in this invention. For example, heating of tube 40 12 may be accomplished by positioning a coil of wire around nozzle or tube 12 to electrically inductively heat the tubular member. It is also possible to position a Bunsen burner or the like beneath nozzle 12 for heating purposes, or the tube may be jacketed in a heating ele45 ment. Various other means of heating nozzle 12 generally known to those skilled in the art may also be used and are intended to be included in this invention.

In Fig. 4 there is shown in greater detail an embodiment of the output mechanism of powder cloud generators in50 eluding the nozzle or tube according to this invention.

The elements in this figure are illustrated partially in cross section and partially diagrammatically. As shown more . clearly in this figure output tube 12 extends into output valve 23 which is positioned above the upper portion of 55 housing 18 and above ridge 26 of the powder generator. A support jacket 29 surrounds tube 12 and acts to support the tube. Support jacket 29 may be formed of a material which is heat insulating and thus the jacket will protect operators from burns while the tube is in a heated con60 dition as well as efficiently keeping the heat at the tube.

As in Fig. 3 a battery 20 is connected through lead 27 to one end of output tube 12. The battery is connected to the other end of output tube 12 through a wire 28. A switch 30 shown in an open position is provided in lead 65 28 to break the circuit. A variable resistor 31 is included in the electrical heating circuit to allow greater control of the amount of heat generated in tube 12 by controlling the amount of resistance in the circuit and thus controlling the current flowing through the cir70 cuit. Since the amount of heat generated is equal to current squared times the resistance, controlling the current flow by using variable resistor 31 in the system results in controlling the amount of heat developed in the substantially fixed resistance of tube 12.

Positioned within output valve 23 are the components inent electrode could be biased to a negative 10 volts through the use of a battery or the like and this will prevent deposition of particles during development in the background areas on the plate surface. Also, for example, if it is desired to make a photographic positive from a plate exposed to a photographic negative or if it is desired to make a photographic negative from a plate exposed to a photographic positive, applying a bias to the development electrode which raises the potential on the development electrode to a point which is equal to the high point of charge on the surface of the xerographic plate (whether negative or positive) will result in the type of reproduction desired. Various other uses of the development electrode generally known to the art are encompassed herein and may be carried out in the device illustrated in this figure.

It is desirable to avoid positioning a plate to be developed in the lower or base wall of developing box 11. During development a cloud of particles in gas is created within developing box 11 and the larger particles or the agglomerated particles tend to settle downward. Deposition of large particles and agglomerated particles tends to create crude reproductions and somewhat distort image development. For higher quality developed images it is preferred that the plate be positioned either in the side walls or in the top wall of the developing box. It is to be realized, however, that in some applications of xerography fine grain development is not necessary and is sometimes purposely avoided. In such instances, the plate may be positioned in the base of the developing box. However, somewhat higher quality images would be produced if, for example, the plate were positioned in guide and support means 51 which are opposite from nozzle 12 and thus would tend to receive some of the agglomerated and heavier particles while avoiding many of the settling agglomerates and larger particles.

Reference is now had to Fig. 2, wherein is shown partially in cross section and partially diagrammatically an embodiment of a developing device according to this invention. A developing box 11 has positioned extending into one wall thereof a nozzle 12 which is connected to powder generator 13. Positioned along another wall thereof is a filter 16 and along yet another wall a plate to be developed 15 comprising a photoconductive insulating layer 1 overlying a base 2 positioned within support means 50. Filter 61 is positioned in guide means 51 and, preferably, guide means 51 are of a proper size to accept and hold in position a plate 15 in place of filter 16. It is also desirable that support means 50 be adaptable to carry filter 13.

In the device illustrated in Fig. 2, a compressed aerosol of powder in gas issues forth from the output end of nozzle 12 into the development chamber of the inner area of developing box 11. This inner area or development chamber is a substantially larger area and the compressed gas carrying particles of powder expands substantially. Expansion of the aerosol creates a slow moving floating cloud of particles for development purposes. Development takes place as the electrostatic charge pattern draws particles from this cloud for deposition in image configuration on the surface of the plate.

The developing box may be composed of any number of materials as, for example, wood, plastics, metals, various compositions, and the like.

In Fig. 3 there is illustrated pictorially an embodiment of a powder cloud generator including the nozzle according to this invention. Powder cloud generator 13 is basically an aerosol bomb for the dispersion of powder in gas. It comprises a housing 18, a battery 20 shown fitted into housing 18, a tube or nozzle 12, output control valve 23, and a protective cap 25. The cap is shown removed from powder cloud generator 13. It fits, as in normal aerosol cans or aerosol bombs, over output valve *23 and onto ridge 25 of housing 18. Battery 20 is connected through wires 27 and 28 to tube or nozzle 12. A £,914,222 of the output system. At the top of the output system there is a hollow chamber 32 having a hole 33 which connects the hollow chamber 32 to tube 22. A hole 35 at the base of chamber 32 connects hollow chamber 32 to conduit 36. Pressing against the base of conduit 36 is a cuplike member 37. As illustrated, portions of conduit 36 positioned within cuplike member 37 are spaced apart from the walls of cuplike member 37. A groove 38, formed in the base of cuplike member 37, extends horizontally across and is positioned centrally in the base to mesh with the opening of conduit 36. A spring 40 is positioned in housing 41 and surrounds cuplike member 37. The spring is shown in this figure in a depressed condition. To depress spring 40, downward pressure is brought to bear against output valve 23. When pressure on output valve 23 is released spring 40 drives cuplike member 37 against stop washer 42. The movement of cuplike member 37 upward and against stop washer 42 causes movement of output valve 23 upward due to pressure against conduit 36 by that area of the base of cuplike member 37 extending beneath or defining groove 38. Housing 41 is extended at its base by extension tube 43. Connected to extension tube 43 is hose 45.

As should be apparent the various elements described which make up the output system are internal elements. For example, chamber 32 is an element surrounded by output valve 23, and stop washer 42 and the elements included within housing 41 are all included within housing 18 of the powder cloud generator. Hose 45 extends withm housing 18 to a point near the base and acts to supply material from within housing IS to the output system of the powder cloud generator. Material supplied through hose 45 is carried into extension tube 43 and into housing 41. If the output valve 22 is depressed then the material carried to housing 41 can move upward along the walls of housing 41 and over the upper edge of cuplike member 37 and into cuplike member 37 then through groove 38 into conduit 36 then through hole 35 through chamber 32 and then through hole 33 into output tube 12 and out the output end of output tube 12. However, when output valve 23 is not depressed, material reaching housing 41 is prevented from moving into cuplike member 37 due to the seal created by cuplike member 37 pressing against stop washer 42. Cuplike member 37 is pressed against washer 42 due to the action of spring 48. Spring 40 also acts to push the output valve upward and a space is thereby created between the lower edge of output valve 23 and the top portion of housing 18. A connection, however, is maintained between output valve 23 and housing 18 by conduit 36.

Positioned within housing 18 of powder cloud generator 12 are powder particles which will be utilized to make an electrostatic image visible and a liquid propellant to carry the particles out of the generator. Preferably the liquid propellant employed should have a pressure at room temperature to forceably expel a liquid powder mixture from within the container and out the end of the output tube. Such low boiling point propellants include, but are in now way limited to, various halogenated lower hydrocarbons having at least one fluorine atom per molecule commercially available under trade , names such as the Freons, Genetrons and the like, propane, and the like as is well known to those skilled in the art. The Freons and Genetrons are fluorinated hydrocarbon derivatives of the short chain and small ring aliphatic series of organic compounds, characterized by their extreme stability. Those of commercial importance today are from the methane and ethane series, although future developments will undoubtedly encompass members of both the propane and cyclobutane series. The propellant used should be chosen because of its boiling point and because of its relation to the particular powder material included in the housing.

Preferably the liquid propellant used should not chemically react with the developer material. To date best results have been had using the various Freons which have the additional advantages of being non-toxic and non-inflammable.

It is also possible to use a liquid propellant in this invention which has a vapor pressure near atmospheric at room temperature. In such an instance prior to using or during usage the container or housing 18 or its contents are heated. Heating the propellant raises the internal pressure in the container so that a liquid powder mixture will be forcefully supplied from the output tube when the output valve is opened. Heating could be brought about by positioning housing 18 in a heated jacket prior to or during use or heating may be brought about by providing means within the walls or base of housing 18 to internally heat its contents. An advantage of using as the liquid propellant liquid having a vapor pressure near atmospheric at room temperature is that housing 18 can be filled and refilled without special equipment. It is also possible that when using this type of liquid propellant the powder cloud generator according to this invention could be stored for a greater period of time. To date, however, no problems have been encountered when using low boiling propellants.

Liquids having a vapor pressure lower than atmospheric at room temperature may also be used as long as the liquid is volatile. In such an instance the pressure to expel the liquid powder mixture may be generated by forcing compressed gas such as compressed air or the like into the container carrying the liquid powder mixture. Desirably the gas should be inert with respect to the container and the contents.

Generally the powder cloud generators of this invention are loaded by first positioning within the housing a suitable developing powder. Then the liquid propellant is either fed or forced into the container. One technique for forcing the propellant in its liquid form, into the container is to assemble the container, depress the output valve and feed the liquid in liquid form directly into hole 33 illustrated in Fig. 4 within the output valve. When sufficient pressure is applied to the supply of liquid propellant, it will be forced inward into housing 18 of powder cloud generator 13. After the container is filled it is usual to shake the container to thereby more uniformly disburse the powder particles through the liquid propellant, or it is at least desirable to so stir up the powder particles prior to use. When the output valve is depressed the pressure within container 18 forces the mixture, out the output tube. The nozzle or tube 12 is heated in this invention to boil off the liquid propellant. Attempts to use aerosol bombs for development without the heated output tube results in the deposition of liquid droplets on the electrostatic image bearing surface. Liquid droplets are undesirable in the xerographic process in that droplets distort the copy produced and may be detrimental to the xerographic plate. Also, a droplet will continue to hold within itself powder particles thereby taking from the efficiency of the cloud. Further, when the liquid of the droplet evaporates the particles will tend to remain in place. Although initially there is fed a liquid particle mixture into output tube 12, the heated nozzles evaporate the liquid away and thus a particle gas aerosol issues from the output end of the tube.

The static tube temperature for complete vaporization should be in the order of from 600’ to 1200° F. When using a stainless steel tube it has been found that heating the tube until it has a dull red appearance produces best results. Heating beyond that point, which is believed to be in the temperature range of about 800° to 1,000° F., produces bending, breaking, or clogging of the tubular material when using a tube having an internal diameter of 0.023 inch. The particular tube used in this instance had a 0.035 inch outside diameter and was a

2,914,221 on plates. The powder employed for developing purposes was micron sized calcium carbonate with a 1% stearate coating. This resulted in a high quality white powder image on the plate surface. Other powders which 6 might be used for viewing or the like on the plate surface include, but are in no way limited to, coated or uncoated calcium carbonates, finely divided zinc oxide, titanium dioxide, or the like. Again, it is noted that the particular liquid propellant used should be one in which 10 the finely divided powder is dispersible but substantially insoluble.

Charging and deagglomeration of the particles is accomplished in this invention by turbulently flowing them through the output tube. The high degree of turbulence 15 created by the propelling mechanism of this invention causes shearing forces to be more effective in bringing about deagglomeration than the effect of contact forces which are acting to cause agglomeration. The striking of the particles against one another and against the walls 20 of the output tube causes electrostatic charging. Since electrostatically charged particles are desired for development of the electrostatically charged pattern on the surface of the plate, electrostatic charging of the particles is brought about by turbulently flowing the particles 25 through a tubular material, the output tube, which will react on the particles to charge them in the direction desired. For example, if it is desired to charge charcoal particles positively, output tubes composed of magnesium silicate, stainless steel and the like may be used. 30 If it is desired to charge charcoal particles negatively, output tubes composed of sandstone, nickel, and the like may be used When using calcium carbonate particles they may be charged positively by using an output tube composed of stainless steel, Inconel and the like, and 35 negative charging of particles will be brought about by nickel or the like. To determine the type of charging that will be imparted to the particles, it is only necessary to examine the position of the particulate material being used as it relates to the position of the tubular material 40 being used in the triboelectric series Thus, if the position of the particulate material is below that of the output tube material in the triboelectric series, negatively charged particles will be produced

When using the aerosol bomb or powder cloud gener45 ator of this invention for developing an electrostatic charge pattern on an image bearing surface, various techniques may be used. The technique which is preferred when developing without a development electrode is to position the aerosol bomb at a sufficient distance from 50 the image bearing surface to allow formation of a cloud of particles in gas which when directed perpendicular to the image bearing surface will substantially cover the image area of the image bearing surface. In using, for example, a plate having an image bearing layer equalling 55 11 inches by 14 inches, it has been found that spacing the output end of the output tube from about 15 to 30 inches from the surface of the image bearing layer will produce a cloud large enough to encompass the plate. Wholly encompassing the image with the cloud results in even 50 development throughout. However, it is to be realized that spacing between the output tube and the plate may vary depending on such factors as the particular propellant being used, the force with which the powder in gas is expelled from the output tube and the like. When de55 veloping with a development electrode the technique to be used is somewhat dependent on the particular electrode being used.

Another technique which may be used for developing purposes is to position the aerosol bomb close to the sur7Q face bearing the image and to move the bomb sidewise across the image bearing surface so that all areas of the image are supplied with the cloud of particles in gas created. Other techniques which will occur to those skilled in the art are intended to be included herein.

In spraying a particle aerosol from the powder cloud standard type 304 stainless steel tube. Heat was applied using 6.3 volt filament transformer which was connected to the power line through an adjustable transformer. The tube length was 6 inches and 3 volts was applied across the tube. A static temperature in the tube below red heat was produced and complete vaporization of the propellant took place when using Freon 12 as the liquid propellant.

The length of tube 12 is determined by a number of factors as, for example, the diameter of the tube, the pressure generated on opening valve 23, the particular material being propelled and the like. A short tube, it has been found, is cooled immediately by the liquid propellant and liquid droplets deposit during development. On the other hand, too long a tube causes too much pressure drop through the length of the tube and a point is eventually reached beyond which the flow out the output end of output tube 22 is too slow and too weak to create a cloud valuable for development purposes. When using a type 304 stainless steel .tubing having an internal diameter of 0.023 inch, tube lengths varying from 4.5 inches to 12 inches have given optimum results.

Although a particular nozzle or tube has been discussed, there is no intent to limit this invention to any particular tubular structure. The tube or nozzle need not have a circular section and it need not be cylindrical in shape. It can, for example, be square or rectangular in shape or it may vary stepwise in its internal diameter whether cylindrical or of a different shape.

Although heating of the nozzle is illustrated as heating of the entire nozzle, it should be realized that portions of the nozzle, such as the exit or entrance half, or the like, may be used to accomplish the same end purpose of creating an output aerosol in which the liquid propellant has been completely boiled off. It is also to be realized that by heating portions of the nozzle various controls may be exercised over the triboelectric charging of the powder particles. Thus, if the particular particles tend to charge in a desired direction using a nozzle of a particular material at room temperature, heating the first portion of the nozzle and keeping the last portion of the nozzle at room temperature will allow charging of the particles as they pass through, first, a heated portion and then the portion at room temperature. Various other modifications in the heating arrangement of the nozzle may be made along lines which will occur to those generally skilled in the art, and such modifications are intended to be included herein.

As to the powder particles inserted in container or housing 18, in general, they should be grossly smaller than the output tube 22, and it may be stated that finely sized particles in the absence of undue agglomeration produce a print or picture more pleasing to the eye than larger size particles. Yet particles in a size range of about 5 to 10 microns are substantially undetectable as fine particles with the aid of a magnifying glass and would result in high quality reproductions.

From the point of view of composition of the developer particles, prints or pictures may be produced with charcoal, carbon blacks, or carbonaceous pigments or other pigment or pigmented material. Under proper conditions, any of a number of various carbon or lamp black materials may be employed, including such material as furnace blacks, channel blacks, and the like. In addition, there may be used such material as milled charcoals and similar materials, or, if desired, finely divided materials having added pigment matter. In the latter category are materials such as finely divided resins containing pigments or dyes such as carbonaceous pigments or various coloring pigments and the like, compositions of this type being preferred where the print or picture ultimately is to be made permanent by a fusing process including heat or vapor fusing.

In one of the numerous experiments carried out in connection with this invention X-ray images were formed

2,914,221 . Μ generator or bomb of this invention it is sometimes desired to pulse the output valve and thus pulse the. particle aerosol output. Pulsing, the output aerosol in this manner aids in creating a cloud of particles in gas in which particles tend to constantly move and float. This tech- 5 njque of cloud formation has particular value when using a development chamber similar to. the one illustrated in Fig. 1.

While the present invention as to its objects and ad-, vantages, as has been described herein, has been carried 10 out in specific embodiments thereof, it is not desired to be limited thereby, but is intended to cover the invention broadly within the spirit and scope of the appended claims.