US2935234A - Powder cloud generating apparatus - Google Patents

Description

May 3, 1960 c. L. HUBER POWDER CLOUD GENERATING APPARATUS 2 Sheets-Sheet 1 Filed May 26, 1958 DEVELOPMENT ZONE AEROSOL HANDLJNG POWDER CLOUD GENERATOR REGULATING VALVE COMPRESSOR F/GZ INVENTOR. Charles L. Huber I ATTORNEY y 1960 c. L. HUBER 2,935,234

POWDER CLOUD GENERATING APPARATUS Filed May 26, 1958 2 Sheets-Sheet 2 66 6; 3 67 V f 1 WHIHHIIII l H 254/ A FTTI [[V l t f 1 I 42 49 46 45 a5 40 k 8 26 1%: 39 37 20 a! 6 A n 32 a 29 3/ 2/ n u 1/ 7/ 26 f 27 77 I 25 i i I 1!! INVENTOR.

Charles L. Huber BYWVK/ZZA/ A T TORNE V United States Patent F POWDER CLOUD GENERATING APPARATUS Charles L. Huber, Byron, N.Y., assignor to Haloid Xerox Ind, Rochester, N.Y., a corporation of New York Application May 26, 1958, Serial No. 737,584

5 Claims. (Cl. 222-193) This invention relates to the field of xerography and, particularly, to an improved powder cloud generator metering apparatus for powder cloud development of xerographically produced images. More specifically, the invention relates to improvements in powder cloud generators of the type disclosed in copending Hayford et a1. application Serial No. 489,257, filed February 18, 1955, now Patent No. 2,862,646, issued December 2, 1958, and in copending Hayford et a1. application Serial No. 737,- 939, filed concurrently herewith on May 26, 1958.

In the process of Xerography, for example, as disclosed in Carlson Patent 2,297,691, issued October 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then we posed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the light intensity that reaches them, and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material, such as an electroscopic powder, which is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.

Two methods of image development are in common use. One is described in Walkup U.S. Patent 2,618,551 and is known as cascade development, and is in general use for line copy development. In this technique, the powder is mixed with a granular material, and this twocomponent developer is poured or cascaded over the plate surface. The function of the granular material is to improve the flow characteristics of the powder and to produce, on the powder, by triboelectrification, the proper electrical charge so that the powder will be attracted to the image. More exactly, the function of the granular material is to provide the mechanical control to the powder, or to carry the powder to an image surface and, simultaneously, to provide almost complete homogeneity of charge polarity. The other form of development is known as powder cloud development, and is in general use for continuous tone development. In this technique of development, a dispersion of electrically charged powder particles in an aeriform fluid is passed to the surface bearing the electrical image and particles are drawn from the aeriform fluid dispersion to form a powder image on the plate. 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.

Generally in powder cloud creating apparatus there is included a powder source, means to create a cloud of 2,935,234 Patented May 3, 1960 powder in aeriform fluid, means to convey the cloud to a surface carrying an electrostatic image, and means to electrostically charge the powder in the cloud before it reaches the surface. Such devices, which include one or a number of the above elements, and which are used to take powder from a source whether it be a mound of powder or whether it be in other shapes or forms and convert the powder to an aerosol of powder in aeriform fluid is herein, and generally in the art, referred to as a powder cloud generator or as a cloud generator, as described in Hayford U.S. Patent 2,812,883 and in the above referred to copending Hayford et a1. application, Serial No. 489,257.

An object in the art of xerography, as in any art concerned with image reproduction, is that of uniformly developing high quality copy. Means of obtaining this objective, while using powder cloud development, is through the uniform and constant presentation to the electrostatic latent image on a surface of a powder cloud of fine developer powder particles uniformly and densely dispersed throughout.

This invention is concerned with uniformly loading a powder support and uniformly dispensing the powder particles loaded onto the support in the form of a powder cloud. The support of this invention is capable of delivering uniform dispersions of great quantities of powder in an aeriform fluid. Further, the device of this invention is able to operate over long periods of time and can produce continuous outputs which are sufficient for rapid processing in Xerography.

The support is enclosed in a housing and rotated or moved in a substantially horizontal plane. At one point on the surface of the support is positioned raw or bulk developer powder particles. These particles are held in position by a first scraper or meter blade, which also acts to meter out a layer of powder to the surface of the support as it moves or rotates beneath the powder supply and first scraper. A first brush then flufis up the powder which is then again metered by a second scraper, excess powder being returned to the pile in front of the first scraper. The particular surface of the support, described in detail hereinafter, is one which loads uniformly as it passes beneath the powder supply. An aeriform fluid is flowed into the housing at one point and powder in the aeriform fluid flows out of an output tube. An output orifice is connected within the housing to the output tube and is positioned at a distance above the surface of the support. Particles on the surface of the support are entrained in the flow of the aeriform fluid traveling from the housing out the output orifice and through the output tube, thereby creating at the output end of the output tube an aerosol of powder particles.

It is, therefore, the object of this invention to improve upon powder cloud generators so that a uniform and dense dispersion of powder particles in an aeriform fluid is created.

A further object of this invention is to provide a new powder cloud generator in which powder is uniformly loaded to a surface and then entrained in the flow of an aeriform fluid out an output tube.

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 accompanying drawings, and the scope of the invention will be pointed out in the appended claims.

Fig. l is a block diagram of elements which would generally appear in xerographic cloud creating apparatus for use in developing electrostatic images.

Fig. 2 is a top view of the powder cloud generator with parts broken away to show the internal structural elements of the device.

' 3 Fig. 3 is a cross-sectional view of the powder cloud generator taken along line 33 of Fig. 2, and

Fig. 4 is a detail sectional view of the mounting means for the meter blades taken along line 44 of Fig. 2.

eferring now with more particularity to the drawings, in Fig. l is shown a block diagram of elements which compose cloud creating apparatus for development of electrostatic images. As is indicated in this diagram, compressed aeriform fluid is fed from compressor 11 to a powder cloud generator 13 through a regulating valve 12 and the output of the powder cloud generator is fed through aerosol handling means 15 and then to the development Zone 16 whereat developer particles are passed for development purposes to a surface carrying an electrostatic latent image.

The source of compressed or pressurized aeriform fluid may be any suitable source, such as, for example an air pump or like'pressure generating member or a suitable pressurizedgas container. Such containers are readily available on thecornmercial market in the form of gas capsules of'carbon dioxide or the like under pressure, in the form of bombs or the like of gas such as fluoro chloroalkanes, which are available under the general family name of Freon. Similarly, a suitable system may comprise a pump or generating means optionally in combination with a pressure chamber whereby fluctu ations in pressure may be limited or avoided.

Regulating valve 12 is used to control the rate of flow of aeriform fluid from compressor 11 to powder cloud generator 13 and also to control the pressure of gas supplied to the powder cloud generator. The powder cloud generator, which is the next block in this diagram following regulating valve 12, is used to create an aerosol of powder in air. It may be supplied with powder in what may be termed the raw or bulk form, that is, powder taken directly from a container and directly supplied in that form without treatment. with powder which is first treated and then placed in position in the generator. The particular powder used is dependent on a number of factors such as other elements used in the cloud creating apparatus, the form of xerographic development, the desired quality of final copy,.,and the like. A more detailed discussion of powders will appear below.

'The aerosol handling block 15 of the diagram appearing in Fig. 1 may represent any number of means and apparatus for imparting an electrostatic charge or deagglomerating. the individual powder particles in the I aerosol supplied from the powder cloud generator. Charging and deagglomeration of particles may be accomplished by turbulently flowing them through fine capillary tubes, such as disclosed in copending Ricker application Serial No. 353,520, filed May 7, 1953. Charging may be accomplished by passing the aerosol of powder in air through a corona discharge zone, or the like. r

The aerosol composed of charged particles in gas is next supplied, as indicated by the block diagram, to development zone 16. Generally, this zone includes a means for expanding the aerosol to a cloud, and optionally this may be done by leading the air from tubes or the like to a larger area where the aerosol expands, creating the cloud of charged developer particles in gas. It is also feasible and sometimes desirable to use the particles in aerosol form without expansion.

In xerography in order to develop a true copy of the original image, it is generally desirable to develop against gravitational pull in that the electrostatic charges on the plate surface truly represent the pattern of the image projected to the plate surface, and allowing gravitational forces to operate in causing deposition of powder particles may result in a distorted reproduction. Also in causing the particles to deposit against the pull of gravity, deposition on the image bearing surface of agglomerates is reduced. This may be accomplished by positioningthe plate It may also be supplied aoaaasa with the-image bearing surface facing downward and creating a cloud beneath it. In some instances particles deposited because of other forces may be removed during the development process through the use of such techniques as directing slight air currents or winds to the plate surface. Such winds or currents should be sufiicient to remove particles not held in place due to electrostatic forces, but should be limited so that particles electrostatically held in place are not affected.

Reference is now had to Figs. 2 to 4, inclusive, wherein is shown a preferred embodiment of a powder cloud generator 13 according to this invention. The flanged, cupshaped body or casing 20 has mounting lugs 21 formed integrally therewith for supporting the unit on a structural element (not shown) of the xerographic machine and an air inlet'opening 55 for connection to a source of aeriform fluid under pressure' The shaft 22, which may be driven by any suitable power means (not shown), is journaled in double sealed bearings 23 and 24 mounted in the hub 25 of the body or casing 20. A groove 26 extends along the bored wall of the hub 25 to permit equalization of pressure on both sides of the bearing 23, while thebore opening in the hub 25 is sealed by oil seals 27 positioned between the bearings 23 and 24.

A cup-shaped powder carrier element 28- is rotatably positioned in the casing 20 and is secured against a shoulder of. the shaft 22 by means of the washer 29 and a screw fastener 30. The powder carrier element 28 consists of a backing rigid plate31, the upper surface of which is faced with a skin or support 32 having numerous interstices in the surface thereof for carrying powder particles, suitable materials, such as cotton flannel, for the skin or support 32 being more fully described in above referred to copending Hayford et al. application Serial No. 489,257. A shell 33 is fastened by screws 34 to backing plate 31 to retain powder on the backing plate.

First and second meter blades or scrapers 35 and 36, respectively, are adjustably positioned over the skin or support 32 in the powder carrier element 28, for metering powder across the surface of the powder carrier element 28, and first and second brushes 37 and 38, respectively, are spring mounted against the surface of the skin or support 32 to agitate powder particles deposited on the surface thereof. To accomplish this, bearing blocks 39, which rota-tably carry adjustment screws 40 positioned by retaining rings 41 are fastened as by soldering to meter blade 35. Internally threaded blocks 42 and 43 are secured by soldering to the plate 44, the block 42 also supporting on a shoulder thereof a brush carrier 45 to which brush 37 is rotatably secured. The meter blade 35 is secured to the plate 44 by threading the adjustment screws 40 into the blocks 42 and 43. v

The assembly of the meter blade 36 to the plate 44 is similar to the assembly of meter blade 35 to the plate 44, except that the brush carrier 46 is supported by block 42, the brush carrier 46 rotatably supporting brush 38. By'

means 'of the adjustment screws '40, the meter blades 35 and 36 can be adjustably positioned relative to the surface of the skin or support 32,, and the position can be maintained by locking the adjustment screws 40 against further movement by nuts 47.

The plate 44, carrying the meter blades 35' and 36' and the brushes 37 and 38, is fastened by machine screw 49 and lockwasher 50, to yoke 48 secured byfasteners 53 to the shoulder 51 formed in the flange 52 of the body or casing 20. Apertures 54 in the yoke 48 permit access from the top'to'the nuts 47 and adjustment screws 40.

Theopen end of the casing or body 20 is closed by the cover plate 60 held in position by lockwashers 61, and

bolts- 62, and sealed by O-rings gasket 64 held in an annular recess in the upper face of the flange 52. The cover plate 60 secured to the body or casing 20 forms a housingJgenerally designated 77, which surrounds and en compasses the internal elements: The cover plate 60 is provided withanopening 65'located. so that powder ma terial may be supplied to the powder carrier element 28 in front of the meter blade 35. A closure plate 66 for the opening 65 seats at its periphery on a gasket 67 held in an annular recess in the upper surface of the cover plate 60, and is secured by lockwashers 61 and bolts 62 and 63.

The cover plate 60 is also provided with a threaded opening 68 receiving an externally threaded hollow cylinder 70 adjustably secured by locking nut 71 and sealed by gaskets 72 and 73 held in annular recesses in the locking nut 71 and cover plate 60, respectively. A bored output tube 74 adapted to be secured to a discharge conduit (not shown) is fastened in the upper portion of the hollow cylinder 70, while a pick-up tube 75, in which a relatively large number of output orifices 76 are formed, is secured in the bottom portion of the hollow cylinder 70.

As has been previously pointed out, an object of this invention is to meter controlled amounts of developer powder particles to a surface and to pass this powder from the surface in an aerosol of powder in air to create a uniformly dense and constant powder cloud output. To accomplish this, a reserve or supply of developer powder is placed on the skin or support 32 of the powder carrier element 28 through the opening 65 and after the closure plate 66 is secured, air inlet opening 55 is connected to a source of high pressure air. The surface of the skin or support 32 is one that tends to draw powder with it and thereby becomes coated with powder when it moves beneath the powder supply or powder reserve. As the powder carrier element 28 is rotated by means of shaft 22, a small quantity of powder carried by the support is permitted to pass under the meter blade 35 while the remainder of the supply of powder is retained against the face of the meter blade.

Generally the meter blade or scraper 35 is curved to best retain powder; that is, it is desirable to form this blade to avoid movement of powder past the edges of the blade. It is also desirable to form the blade curved in shape so that the larger mass of powder congregates at that area over the flat surface of the powder carrier element 28, which will pass under output orifices 76 where powder will be removed. Whether the meter blade or scraper 35 is curved, as shown, or straight, the portion of the meter blade positioned over the annular surface portion of powder carrier element 28 which passes under the center of pick-up tube 75 should be tangent to a radius of the powder carrier element 28 so that the powder carried on the above described annular surface portion will strike the blade at right angles causing powder to remain positioned adjacent to the blade over this annular surface portion of the powder carrier element. The lower edge of retaining scraper or meter blade 35, which is similar to the lower edge of meter blade 36, is preferably straight, that is, it is not indented, and it is preferably spaced apart from the flat bottom surface of powder carrier element 28.

. Allowing too little powder to pass beneath the meter blade creates too thin an aerosol of powder particles, whereas allowing too much powder to pass under the meter blade will create too dense an aerosol of powder particles. Denseness is a desirable feature of an aerosol; yet, when an aerosol of powder in gas is too dense, fine grain development is detrimentally affected in that the aerosol tends to be a presentation of bulk powder including agglomerated powder to the output zone rather than a compressed cloud of powder particles in gas. The density of powder in gas or of the aerosol is a factor Which relates to the particular use the aerosol is to be put to. For example, in the development of line copy of normal printed pages, it is quite proper and not unpleasing to the eye to develop with larger sized particles or particles of smaller sizes which are less deagglomerated than in the case of the development of continuous tone images wherein it is desirable to develop with deagglomerated and individual particles and smaller sized particles. In line copy work, then, the meter blades or scrapers may beadjusted to allow passage of a greater quantity of particles to the surface being loaded, whereas when developing fine grain continuous tones, it is desirable to pass a lesser quantity of powder particles beneath the meter blades or scrapers. Spacing of the meter blades or scrapers from the surface being loaded relates to the particular surface material and to the particular powder particles being used. Adjustment of both meter blades 35 and 36 with respect to the membrane 32 is done by means of the adjustment screws 40 and nuts 47.

It has been found that, for a fixed setting of the meter blade 35 with respect to the skin or support 32, the amount of powder metered by the blade will vary. To obtain satisfactory coverage of powder particles over the surface of the covering or membrane 32, a relatively large supply of powder must be retained in front of the meter blade 35. As the support passes under the supply of powder, the powder particles will pack tightly and clog the interstices of the support. Clogging will depend generally on the particular powder being used, the amount of powder, and the particular material of the support. Clogging presents a real problem, in that once an area becomes clogged with powder, in effect the powder becomes a portion of that particular part of the support, and when that area is presented to the powder supply for replenishment, it will pick up less powder than an area which is not clogged. Packing will depend on the quantity of powder retained in front of meter blade 35, the quantity varying as the device is operated.

To obtain fine control of the powder metered, the brush 37, mounted next in line to the meter blade 35 in the direction of rotation of the powder carrier element, agitates the powder sufliciently to fluff up the powder particles and thereby prevent caking and also provides a load of uniform texture in front of meter blade 36. The loosely packed powder is then re-metered by meter blade 36. If a relatively large supply of powder is permitted to build up in front of meter blade 36, the efiiciency of this blade will decrease. To prevent a build-up of powder in front of meter blade 36, the blade is mounted so that powder removed by this blade is returned to the powder supply in front of meter blade 35. This is accomplished by mounting the meter blade 36, whether a straight blade or curved, as shown, so that the angle of incidence A, of the moving powder particles striking the blade, is an acute angle as indicated in Fig. 2. The angle of incidence A is defined as the angle formed by the line T tangent to the path of movement of a powder particle on the surface of powder carrier element 26 as it is rotated, and a line N perpendicular (normal) to the meter blade 36 at the point M of incidence of the moving powder particle on the meter blade 36. the blade, is composed of a vector force normal to the blade and a vector force tangent to the blade. The latter force will cause the excess powder particles, removed by the meter blade 36, to pass off the inner edge of the blade from where it will be returned by movement of the powder carrier element 28 to the front of the meter blade 35. Whether the meter blade or scraper 36 is curved, as shown, or straight, the meter blade or scraper must be positioned at an angle to the radius of the powder carrier element 28, and specifically in regard to a curved blade, it is preferred that no portion of the blade facing meter blade 35 should be tangent to a radius of the powder carrier element so that all powder particles striking this blade strike it at an angle other than normal. In this respect it is noted that the powder carrier element is rotated at a low speed so that the centrifugal force acting on the powder particles is insignificant.

Thus it is apparent that meter blade 36 must be positioned to perform two functions, that is, the meter blade 36 must be positioned behind the first meter blade 35 to meter the desired amount of powder particles to the an- The resultant force, which is at an angle to p 7 nular surface portion of powder carrier element 28 which passes directly under pick-up tube 75, and. the meter blade 36 must be positioned so that all excess powder particles. are projected onto a second annular portion of powder carrier element 28 to be returned to the powder supply in front of meter blade 35. If the meterv blade 36 is not positioned to perform this latter function, the excess powder particles would pile up in front of this blade to the extent that after a short period of operation the entire powder particle supply would be retained by meter blade 36 in lieu of meter blade 35. If this is permitted to occur, meter blade 36 in effect becomes the sole meter-. ing blade in the system since it would then perform the same function as meter blade 35, while meter-blade35 would become useless. It is apparent then, that, in an apparatus constructed in accordance with the instant -in-- vention, meter blade 36 is nota mere duplication of meter blade 35, since each meter different functions. a

However, it is brush 37, placed between; meter blades 35 and 36, that permits very fine control of the amount of powder metered. With a relatively large'supply of powder particles retained in front of meter blade 35, the layer of powder particles adjacent to covering 32 will be more tightly packed than when the supply of powderis nearly exhausted and, therefore, during the operation of the device the actual quantity of powder advanced to meter blade 36 will vary because of the variation in density of the layer of powder particles. Brush 37 fluffs up this layer of powder particles so that a uniform textured layer of powder particles is presented to meter blade 36. Thus it is apparent that while meter blade 35', for example, will scrape ofi powder from a pile of powder varying in height from say three inches down to a quar- 7 ter of an inch with corresponding variations in density throughout the pile, the layer of powder reaching meter blade 36 will vary in height by only a few thousandths of an inch, since the powder particles have been previously partially metered by meter blade 35 and then fluffed up by brush 37 to provide a uniform textured layer of powder particles.

-The brush 38 causes agitation ofpowd'er particles deblad'e must perform 7 posited on the covering 32 which have passed by meter 7 carriers and 46, respectively, so that a slight tension exists between the brush fibers and the surface of the skin or support 32 causing the brushes to rotate as a result of the movement of the powder carrier element 28. Rotation of the brushes 37 and 38 causes the brush bristles to rotate against and flick away powder particles from the surface of the support. The brush 38 should be so positioned and should be of such'size so as to stir up any powder particles on that portion of the covering which passes beneath the output orifices 76. The layer of metered powder on the covering is then picked up by the output orifices and discharged through the outlet plug 7 4.

The pick-up tube 75 is positioned in the casing so that the openings of the output orifices are adjacent but spaced an appreciable distance from the surface of the skin or support 32. With this arrangement there 'is established a zone of pressure, between the output orifices and the such as the particular surface material being loaded, the

particular developer powders'being used, the gas flow and gas pressure supplied to the device, the use desired 7 of the output, and the like. For a generator constructed 15 along lines of the embodiment shown in Figs; 2 and 3, it has been found that revolving the powder carrier element at a rate of 1 to 5 revolutions per minute and preferably at 3 revolutions per minute tends to give optimum outputs. It is to be understood, of course, that the output of powder in gas tends to increase as the revolutions per minute of the powder carrier element increases. If it is, therefore, desired to continue receiving a dense cloud with more gas pressures and lower gas flows, a dense cloud may be obtained with more revolutions per minute.

The principle of operation of the powder cloud generators being described herein is one of flowing gas into an enclosure having an output tube. Since the output tube is the only exit for the gas flow, the gas being flowed into the device leaves through the output tube. The output orifice attached to the output tube is closely spaced to the loaded surface and the gas rushing out the output orifice entrains into its stream 'the powder particles carried by the loaded surface positioned beneath or substantially beneath the output orifice. The operation of the device when operating properly removes completely all loaded powder from the loaded surface. As an aid to the complete removal, the brush stirs up the powder particles so that they'may more easily become entrained in the gas flow rushing out the output orifice.

, Thus it is clear that increasing the speed of rotation of the ness of cloud, in that all particles are entrained in lower.

gas flow rates and the same amount of particles for a larger flow of gas will create a thinner aerosol of powder particles.

Certain features relating to the developer material are pertinent in operating the powder cloud generator'and pertinent in developing electrostatic images. In general, particles should be grossly smaller than the output tube diameter, and it may be stated that finer sized particles in the absence of undue agglomeration may reveal a print or picture more pleasing to the eve'than larger sized particles. Thus, a convenient particle size which results in extremely high quality copy contemplates particles of sub-micron size having average diameters in the order of 0.1 micron. From the point of view of composition of the developer particles, prints or pictures may be produced with a variety'of types of finely divided electroscopic powders as disclosed in the Carlson patent. However, as the art of xerography has progressed, it has been found preferable to develop images with a powder formed of any of a variety of pigmented thermoplastic resins that have been specifically developed for the purpose. A number of such developing materials are, manufactured and marketed by Haloid Xerox Inc., of Rochester, New York,

and are specifically compounded for producing dense images of high resolution and to have characteristics to permit convenient storage and handling. 7

The amount of developer particles placed in front of meter blade or scraper 35 is dependent on the amount determines how much this meter blade'will retain.

Preferably, in a powder cloud generator, constructed in accordance with'the invention, for use on a xerographic machine for producing continuous tone reproductions developed with sub-micron sized powder particles, botlr meter blades 35and 36 are spaced .005" above a cotton flannel skin or covering 32 while pick-up tube is'spaced .100 above the skin or covering 32. Obviously, the spacing of meter blades 35 and 36 and of the pick-up'tube 75 with respect to the skin or covering 32 may be varied, for example, according to the size of the powder particles used, the particular surface material being loaded, and the like.

While the present invention as to its objects and ad- 9 vantages, as has been described herein, has been carried out in specific embodiments thereof, it is not desired to be limited thereby, but is intended to cover the inven tion broadly within the spirit and scope of the appended claims.

What is claimed is:

l. A powder cloud generator including an enclosure having an inlet opening for compressed fluid, an inlet means for powder material and an output means for the powder cloud; a powder carrier means, including a support for powder particles, rotatably journaled and sealed in said enclosure, metering means including first and second meter blades connected to said enclosure, said first and second meter blades being positioned in closely spaced relationship above said support, and said second meter blade being further positioned so that the angle of incidence of the powder particles carried by said support with said second meter blade is an acute angle, whereby excess powder particles are deflected to be returned by said support to said first meter blade, and a first brush and a second brush rotatably connected to said enclosure and positioned in contact with said support, said first brush being located between said first meter blade and said second meter blade, and said second brush being located between said second meter blade and said output means, said output means including a pick-up tube in said enclosure and which has at least one output orifice adjacent to but spaced from said support.

2. A powder cloud generator including an enclosure having an inlet opening for compressed aeriform fluid, an inlet means for powder material and an output means; a powder carrier means, including a support, rotatably journaled in said enclosure, said support being arranged to receive a supply of powder particles from said inlet means, metering means, including at least a first meter blade and a second meter blade, mounted in said enclosure so that said first meter blade and said second meter blade are positioned a slight distance above said support and said second meter blade is also positioned so that excess powder deflected by said second meter blade is returned to said first meter blade, and a brush means mounted to said enclosure in contact with said support and positioned between said first meter blade and said second meter blade in the direction of rotation of said support to agitate powder particles on said support.

3. A powder cloud generator including an enclosure having an inlet opening for compressed aeriform fluid, an inlet means for powder material and powder output means for a suspension of powder material in an aeriform fluid; powder support means rotatably mounted in the enclosure, said powder output means including at least one outlet orifice positioned in closely spaced relation above said support means, and powder metering means secured to said enclosure and arranged in the path of movement of said support means for determining the quantity of powder supplied to the output means, said last recited means comprising a first metering blade arranged in the path of movement of said support means and in spaced relation above said support means for limiting the supply of powder on said support means passing therebeneath to an amount slightly in excess of that to be delivered to the output means, a brush means positioned next adjacent to said first metering blade and in contact with said support means, a second metering blade arranged intermediate said first metering blade and said output means and in the path of movement of said support means, said second metering blade being positioned in closely spaced relation above said support means to limit the supply of powder on said support means passing therebeneath to an amount equal to that to be delivered to the output means, and being positioned at an angle relative to the path of movement of the powder to deflect excess powder from the output means.

4. A powder cloud generator including an enclosure having an inlet opening for compressed aeriform fluid, an inlet means for powder material and powder output means for a suspension of powder material in an aerifrom fluid; powder support means rotatably mounted in said enclosure, said powder output means positioned in closely spaced relation to said powder support means for withdrawing powder from the enclosure, and powder metering means connected to said enclosure and arranged in the path of movement of powder on said support means for determining the quantity of powder supplied to said output means, said last recited means comprising a first metering blade arranged in the path of movement of said support means and in spaced relation to the support means for limiting the supply of powder to an amount slightly in excess of that to be delivered to the output means,'a second metering blade arranged intermediate the firstmetering blade and the output means and in the path of movement of said support means, said second metering blade being positioned in closely spaced relation to said support means to limit the supply of powder to an amount equal to that to be delivered to said powder output means, and being positioned relative to the path of movement of the powder to deflect excess powder from the. output means into a position on said powder supporting means to be metered by said first metering blade, and a first brush and a second brush rotatably positioned in contact with said support means, said first brush being located between said first metering blade and said second metering blade, and said second brush being located between said second metering blade and said powder output means.

5. A powder cloud generator including an enclosure having an inlet for compressed aeriform fluid, an inlet means for powder material and an output means for a powder cloud; a powder carrier means, including a circular support for powder particles, rotatably journaled and sealed in said enclosure, said output means including a pick-up tube having at least one output orifice therein adjacent to but spaced from said support, support means connected to said enclosure and positioned over said support, a first meter blade connected to said support means and positioned in closely spaced relation above said support to retain a supply of powder particles on said support, a first brush rotatably connected to said support means between said first meter blade and said pickup tube in the path of rotation of said powder carrier means, said first brush contacting said support, a second meter blade connected to said support means and positioned in closely spaced relation above said support between said first brush and said pick-up tube, said second meter blade being further positioned at an angle to the radius of said powder carrier means, and a second brush connected to said support means in contact with said support and positioned between said second meter blade and said pick-up tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,213,508 Wheldon Sept. 3, 1940 FOREIGN PATENTS 833,320 Germany Mar. 6, 1952

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