GB2132966A - Powder feeder - Google Patents

Abstract

A powder feeder comprises a hopper (22) which includes a powder- containing chamber (28) bounded by upper and lower porous membranes (40,38) that are pervious to gas and impervious to powder. Pressurized gas is directed upwardly through the lower membrane to suspend powder particles of a desired size at the elevation of one or more discharge tubes (44), the inlet(s) (46) of which communicate with the powder chamber (28) between the membranes (40,38). The gas swirls while travelling through the powder- chamber (28) to produce a centrifugal displacement of particles toward a side wall of the powder chamber (28) where they gravitate toward the lower membrane (38). Accumulated powder is redistributed upon the lower membrane (38) by means of a multibladed rotor (60) which simultaneously compacts the powder. The inner end(s) of the discharge tube(s) (44) extend into the powder chamber and extend upwardly. Pressurized gas is delivered to the lower membrane (38) from a gas entry chamber (27) disposed therebelow. Heating members (120) heat the side wall (72) of the gas entry chamber (27) in order to dehumidify the gas before it reaches the powder chamber (28). <IMAGE>

Description

SPECIFICATION Powder feeder The present invention relates to the application of wear resistant coatings to objects.

It is common to apply wear resistant coatings to objects in orderto increase their durability and life span. Among the most wear resistant coatings are those formed of tungsten carbide, aluminium oxide and chromium oxide, for example, which are ideally applied by means of a flame spray technique. The flame spray technique is conventional and involves the feeding of the coating material in powder form to a spray gun. An electric arc generated by the spray gun melts the powder particles and the melt is sprayed onto the workpiece.

It is conventional to supply powder particles to the spray gun for a powder hopper in which the particles are suspended by means of an upward flow of gas.

Since the particles tend to hover at a given elevation, depending upon their mass, it is theoretically possible to discharge particles of a generally uniform size by positioning the discharge openings at the corresponding elevation within the hopper. For various reasons, however, such a uniformity of particle size in the discharge flow has traditionally been difficult to achieve.

In U.S. Patent No. 4,262,034 issued April 14, 1981, to the present inventor there is disclosed a powder feeding mechanism and technique which as significantly advanced the art by enabling a much more precise and uniform segregation of the particles to be achieved within the hopper. Thus, there can be obtained a discharge flow which contains a relatively uniform density of particles which are of relatively uniform size. Importantly, this result was achieved in the case of very small particles (e.g., from 1 to 8 microns diameter) and thus enables a very thin, uniform coating (e.g., from 15 to 35 microns thickness) to be applied. This is of importance to many workpieces, such as rotogravure cylinders for example, where it is necessary that the coating correspond to the contour of the workpiece with a high degree of precision.

Despite the successful results achieved by the mechanism and technique disclosed in the aforementioned patent of this inventor, room for improvement remains, especially insofar as the need for increasing the density of powder entrained in the particle/air flow from the powder hopper, whereby the thickness of the coating can be increased. In that regard, the coating of workpieces is ideally performed by applying a single layer of a desired final thickness of the workpiece. In many cases, however, the density of the particles in the discharge flow from the powder feeder is insufficient to deposit a coating of the desired final thickness in a single pass.

Thus, it may be necessary to make successive passes across the workpiece to build-up the coating to its desired final thickness. Not only does such multi-layering involve a longer operating period, but the adherence of the various layers to each other is not as strong as in the case of a single pass coating.

It is, therefore, an object of the present invention to provide a novel apparatus for coating workpieces with wear resistant substances.

It is another object to improve the apparatus disclosed in U.S. Patent 4,262,034 by increasing the density of powder in the output flow.

A further object is to promote the entry of powder particles into powder discharge tubes of the powder feeder.

Another object is to prevent moisture in the incoming pressurized gas from reaching the powder in the hopper.

These objects are achieved by embodiments of the present invention which relates to a powder feeder of the type in which a hopper includes a powdercontaining chamber bounded by upper and lower porous membranes which are pervious to gas and impervious to powder. Pressurized gas is directed upwardly through the lower membrane into the powder chamber, then outwardly through the upper membrane, and then through a gas outlet of the hopper. At least one discharge tube having an inlet communicates with the powder chamber between the membranes. Means is provided for regulating the flow of gas through the powder chamber to suspend the powder particles of a desired size at the elevation of the discharge inlet.The air swirls while traveling through the powder chamber to produce a centrifugal dispiacement of particles toward a side wall of the powder chamber where they gravitate toward the lower membrane. Means is provided for redistributing the powder particles on the lower membrane. The redistributing means comprises at least one arm traveling across the above the upper surface of the lower membrane in a manner compacting the particles upon the lower membrane.

Preferably, the redistributing means comprises a multi-blade rotor having its rotary axis aligned with the central axis of the hopper. The blades each have a leading surface facing in the direction of rotation and inclined slightly downwardly to produce the compaction ofthe powder.

Preferably, a gas entry chamber is disposed below the lower membrane. Means is provided for heating the pressurized gas within the gas entry chamber so as to dehumidify the gas before the gas passes into the powder chamber.

The discharge tube extends into the powder chamber, with the inner end of the tube extending upwardly, preferably at an angle of ninety degrees relative to horizontal.

The objects and advantages of the invention will become apparent from the following detailed description of a preferred embodiment thereof, in connection with the accompanying drawings, in which like numerals designate like elements, and in which Figure 1 is a schematic view of a system for applying a wear resistant coating to an object; and Figure 2 is a vertical cross-section through a powder feeding mechanism of the system.

A system 10 is depicted in Figure 1 for applying a coating of wear resistant material into a workpiece, such as a photo-etched roto-gravure cylinder 12. The coating system includes a plasma flame spray gun 14 of a conventional type which is mounted on a movable carrier 16. An electric suppy conduit 15 and an argon gas supply conduit 17 extend into the carrier and feed into the flame gun. A powder feed hose 18 communicates a powder feed mechanism 20 with the flame gun for conducting a powder, such as tungsten carbide, aluminium oxide, chromium oxide, etc. to the gun. The spray gun itself is well known and further details thereof are not needed.

Suffice it to say that an electric arc is established by the gun which melts the powder particles that are introduced into the gun. The melt is then sprayed from the gun by the argon gas and onto the workpiece.

As is conventional, the gravure cylinder 12 is mounted for rotation about its longitudinal axis and the gun carrier 16 is mounted for translational motion in a direction parallel to the roller axis. The rotational speed of the roller and thetranslational speed of the gun are correlated so that the entire periphery of the roller is coated.

The powder feed mechanism 20 comprises a hopper 22 having a cylindrical upper portion 24 of circular cross-section, a conical portion 26 of circular cross-section disposed below the cylindrical portion, and a cylindrical gas entry portion 27 disposed below the conical portion. The cylindrical upper portion 24 and the conical portion 26 form an internal powder chamber 28. A gas entrance 30 is provided in a side wall of the gas entry portion 27, and an air outlet 32 is provided in a cover 34 which covers the upper portion 24. A gas delivery duct 36 supplies a stream of pressurized gas such as air which enters the hopper through the inlet 30 and exits the hopper through the outlet 32.

Extending completely across the cross-section of the conical portion 26 above the air inlet 30 is a lower circular porous membrane 38, and extending completely across the cross-section of the cylindrical portion 24 below the air outlet 32 is an upper circular porous membrane 40. The porous membranes 38, 40 conduct gas flow but prevent the passage of powder particles. Preferably, the membranes each comprise microporous polyethylene with a mesh size preferably in the range of 5 to 10 microns. A membrane thickness of about 3/8" is preferred. It will be understood that the membranes 38,40 define the upper and lower limits of the powder chamber 28.

Thus, the membranes 38,40 constitute an air inlet and an air outlet relative to the powder chamber 28 of the hopper.

Disposed in the wall of the cylindrical portion 24 is a closable loading hatch 42 through which the powder can be introduced into the chamber 28.

A series of powder discharge tubes 44 are positioned within the cylindrical portion 24 and communicate with the plasma gun 14 by means of suitable tubing. The discharge tubes 44 may all converge and communicate with a single, common feed hose 45, as shown, or a separate feed hose may be provided for each discharge tube. The inner ends of the tubes 44 extend upwardly (i.e., nonhorizontally) and the inlet openings 46 of the tubes are generally disposed within a common horizontal plane at the desired discharge elevation, i.e., at the elevation where the particles of the desired mass tend to hover. Thus, those particles are impelled into the discharge tubes.

By orienting the inner ends of the tubes upwardly, the passage of particles into the inlet ends 46 is facilitated to a greater extent that if the inner ends of the tubes are directed horizontally. That is, it has been observed that the powder particles generally travel in a downward direction at the instant of entering a discharge tube. Hence, by orienting the tubes 46 upwardly, preferably by ninety deg ress relative to horizontal, the inlets 46 tend to be more closely aligned with the direction of particle travel.

Thus, any change of direction which need be imposed on the particle to feed same into the discharge tube is of a much lesser degree than would be involved if a downwardly traveling particle had to enter a purely horizontally facing inlet.

Before a feeding operation commences, the powder within the hopper 22 rests upon the lower membrane 38. By inducing an air flow upwardly through the chamber 28, the powder becomes elevated and dispersed within the chamber. At a given constant air velocity, powder particles of similar mass tend to remain suspended at a particular level within the chamber. The discharge inlets 46 are positioned at a level corresponding to a desired particle size and density. By varying the air velocity, those characteristics of the suspended powder can be controlled.

The air velocity and pressure are controlled by regulations 50,52 in the air inlet and outlet lines. In this fashion, it is possible to regulate the discharge velocity of the particles through the discharge tubes 44. For example, it is preferable to maintain a pressure of 20-25 psi through the chamber 28 when feeding tungsten carbide particles.

The pressurized air from the gas entry portion 27 acts mainly upon the powder particles located closest to the center of the lower membrane 28 and propels those center particles upwardly.

During the travel of air through the chamber 28, a swirling air stream is established due mainly to the conical configuration of the portion 26 of the hopper.

Such swirling action creates a random flow pattern of the powder particles, assuring that a uniform particle density (i.e., particle count per volume) is established within the chamber.

The height to which the particles rise in the chamber, for a given air velocity. It has been found that for a given air pressure, is a function of the size of the particles and the air velocity within the chamber. That is, heavier particles will not be lifted as high as the ligher particles per a given air pressure through the chamber, the powder particles will be suspended at a substantially uniform density, with particles of generally common size being situated at a respective level in the chamber. Thus, by suitable regulation of air pressure through the chamber, particles of a desired mass can be discharged through the discharge tubes 44. Accordingly, it will be appreciated that the feeder mechanism functions to classify particles according to size.

Moreover, the heavier, unwanted particles will remain suspended at the lower end of the chamber and may eventually be disposed of when the coating operation is finished.

The swirling air flow causes the particles to be acted upon centrifugally and thus the particles tend to move outwardly and approach the side wall of the chamber. Since the upward air flow is weakest along the side wall, the particles, especially the heavier particles, tend to gravitate back onto the lower membrane 38. The outward movement of the particles assures that the inner part of the chamber, where the upward particle flow is greatest, will not become excessively blocked by heavier, unwanted particles located at elevations below the discharge elevation. Such blockage could interfere with the upward flow of the desired-size particles and create a situation where the discharge elevation is, in effect, "starved" for particles of the desired size.

The downwardly gfavitating particles collect near the outer periphery of the lower membrane 38. It is necessary to redistribute those particles inwardly of that membrane to maintain a continuous upward flow of particles. In the afore-mentioned U.S. Patent 4,262,034 this was achieved by providing a rotary brush above the membrane, the bush being motordriven about an axis extending coplanar with the central axis of the lower membrane. Rotation of the brush with the bristles in contact with the membrane caused the powder particles to be pushed from the periphery toward the center of the membrane for entrainment within the upward air flow.Although this technique produced a density of desired-size particles at the discharge elevation which was sufficient for carrying-out an effective flame spray operation, it has now been found that the density of desired-size particles at the discharge elevation can be significantly increased by employing, in lieu of a brush-type particle distributor, a packing rotor 60 which not only redistributes the powder particles, but also tends to compact the powder particles upon the lower membrane.

The packing rotor comprises a central hub 62 and a plurality of radial blades 64 projecting therefrom.

The blades 64 are each oriented such that the leading surface 66, i.e., the surface facing in the direction of travel, is inclined relative to vertical so as to face somewhat downwardly. As the rotor rotates, the blades 64 contact the accumulated particles around the membrane periphery, the height of the accumulated particles being higherthan the spacing between the rotor and the membrane (e.g., a 1/4" spacing), and tend to sweep those particles inwardly toward the center of the lower membrane. Because of the aforedescribed inclination of the blades, the particles being swept inwardly also become compacted upon the lower membrane. This causes air pressure to build-up beneath the compacted particles which eventually produces an explosion-like which propels the particles upwardly at a greater rate.It has been found that this phenomenon establishes a much greater density of suspended particles within the hopper, including a greater density of desired-size particles at the discharge elevation.

It has been found, in practice, that the use of a packing rotor can increase the powder output density somewhere on the order of tenfold as compared with the use of a rotary brush. As a result, a much thicker layer of the wear resistant material can be deposited onto the workpiece per given pass, thereby reducing or eliminating the need for multilayering in order to achieve the desired-thickness coating.

It will be appreciated that the rotation of the rotor enhances the swirling of the air within the powder chamber and thus contributes to the advantages described earlier in connection with the swirling action.

The rotor is mounted on a drive shaft 68 which extends coaxiallythrough the hopper and through the air entry portion 27 of the hopper. In that regard, the air entry portion 27 of the hopper includes a bottom wall 70 which is sandwiched between the cylindrical skirt 72 of that portion 27 and a collar 74.

Screws 76 pass through the collar and bottom wall and secure those parts to the skirt 72. A sealing gasket 78 is disposed between the bottom wall 70 and the skirt 72 to establish an air-tight seal therebetween. The collar 74 is clamped to a base 80 by a series of conventional clamps 82.

A motor 84 is fixedly secured to a plate 86 of the base and has an output shaft 88 connected by a coupler 90 to the drive shaft 68. The latter is journaled within the bottom wall 70 by means of a rotary bearing 92. A sleeve 94 is coupled between the bottom wall 70 and the lower membrane 38 in surrounding relationship to the drive shaft. The sleeve 94 supports a bearing 96 in which the drive shaft is mounted. A seal housing 98 is secured to the upper end of the sleeve 94 and carries a conventional magnetic-type seal 100 which seals against the escape of powder along the drive shaft 68. Gaskets 102 and 104 are interposed between the lower membrane 38 on the one hand and the seal housing and a hopper flange 106 on the other hand, to seal against the escape of powder.

Mounted at an upper end of the drive shaft 68 is a rotary brush 108, the bristles 109 of which scour powder particles from the underside in the upper membrane 40 to prevent the latter from becoming clogged with powder particles to the extent that air flowtherethrough is excessively obstructed.

Gaskets 110 and 112 are interposed between the upper membrane 40 in the one hand and the hopper cover 34 and a flange 114 of the hopper on the other hand, in orderto seal against the escape of powder particles. Conventional fasteners 116 are provided for clamping the cover 34 onto the cylindrical portion 24 of the gasket.

Surrounding the air entry portion of the hopper 22 are a series of electric heating elements 120. These elements heat the air received from the delivery duct 36 sufficiently to evaporate any moisture contained therein. It will be appreciated that the presence of moisture within the powder particles will significantly alterthe behavior of the particles and diminish the performance of the mechanism. By performing the air heating step in the air entry portion 27, rather than in the powder chambers 28 as was disclosed in the afore-mentioned patent of the inventor, it is assured to a greater extent that the powder will not come into contact with excessive moisture.

A suitable pressure gauge 122 and pressure relief valve 124 for the powder chamber can be provided to enable proper pressure conditions within that chamber to be regulated.

It will be appreciated that the particle feeding mechanism is adapted to the application of a wear resistant coating or the like to all types of workpieces, with roto-gravure cylinders being merely exemplary.

In operation, the powder feeder is activated by forcing air through the air inlet 30 of the powdercontaining chamber 28. Air passing upwardly through the container entrains the powder particles and suspends them at a substantially uniform density within the chamber, with particles of common size (mass) becoming situated at respective levels within the chamber. The inlet and outlet pressure valves are regulated to achieve a desired constant air velocity and powder density within the chamber to assure that particles of a given diameter, preferably in the range of from 1 to 8 microns, are disposed at the level of the powder discharge inlets 46. Those valves are also adjusted to achieve a desired powder velocity through the powder discharge tubes 44.Due to the swirling action which is created within the powder chamber, powder particles are moved by centrifugal force to the side wall of the hopper, thereby leaving the center of the chamber below the discharge inlets 46 sufficiently clear for the desiredsize particles to ascend to the discharge inlets.

Upon reaching the side wall, the centrifugally displaced particles gravitate onto the lower membrane 38 where they are swept inwardly by the rotor blades 64. The latter simultaneously compact the particles, whereby gas pressure beneath the compacted particles builds-up and eventually "explodes" the particles upwardly. This creates a relatively high density of desired-size particles at the discharge elevation of the inlets 46. Entry of the particles into those inlets 46 is promoted by the upward orientation of the inner ends of the discharge tubes 44, whereby the inlets 46 are more closely aligned with the direction of movement of the particles adjacent the inlets.

The powder is kept virtualiy moisture-free, especially by means of the heating elements 120 which dehumidify the air before the air reaches the powder chamber.

Powder particles, preferably in the range of from 1 to 8 microns, are supplied at a uniform density to the torch, enabling a coating to be applied in a single pass with is of uniform thickness in the range of from 15 to 35 microns, a coating which was heretofore not possible to achieve with consistent uniformity. After the completion of the coating operation, the cylinder is transferred to a wet polishing machine.

The end result is a gravure printing cylinder having a much higher resistance to abrasion and erosion, and a life expectancy nearly ten times that of a conventional chromium plated cylinder.

The thin, uniform coating made possible by the present invention is ideally suited to roto-gravure printing cylinders because the definition of the printing cells is not destroyed. It will be appreciated, of course, that the present invention is also applicable to the coating of many other objects with a wear-resistant substance.

Claims (16)

1. A powder feeder on the type in which a hopper (22) includes a powder-containing chamber (28) bounded by upper and lower porous membranes (40,38) which are pervious to gas and impervious to powder; means (36, 27) for directing pressurized gas upwardly through said lower membrane into said powderchamber,then outwardly through said upper membrane, and then through a gas outlet (32) of said hopper; at least one discharge tube (44) having an inlet (46) communicating with said powder chamber between said membranes; means (50, 52) for regulating the flow of gas through said powder chambers to suspend powder particles of a desired size at the elevation of said discharge inlet, the gas swirling while traveling through said powder chamber to produce a centrifugal displacement of particles toward a side wall of said powder chamber where they gravitate toward said lower membrane; and means (60) for redistributing the powder particles disposed on said lower membrane; characterized in that said powder redistributing means comprises at least one arm (64) traveling across and above the upper surface of said lower membrane in a manner compacting the particles upon said lower membrane.

2. A powder feeder according to claim 1, wherein said powder redistributing means comprises a multibladed rotor (60) having its rotary axis aligned with the central axis of said hopper, and motor means (84) for driving said rotor.

3. A powder feeder according to claim 2, wherein each of said blades (64) has a leading surface (66) facing in the direction of rotation and inclined slightly downwardly.

4. A powder feeder according to claim 2 or claim 3 wherein said gas directing means comprises a gas entry chamber (27) disposed below said lower membrane, said motor disposed below said gas entry chamber and connected to a drive shaft (68) extending upwardly through said gas entry chamber and into said powder chamber where it is connected to said rotor.

5. A powder feeder according to claim 4 including a rotary brush (108) connected to an upper end of said drive shaft, said brush arranged to wipe powder particles from the undersurface of said upper membrane.

6. A powder feeder according to any one of the preceding claims, wherein said gas directing means comprises a gas entry chamber (27) disposed below said lower membrane, and means (120) for heating the pressurized gas within said gas entry chamber so as to dehumidify the gas before the passage thereof into said powder chamber.

7. A powder feeder according to claim 6, wherein said heating means heats the side wall (72) of said gas entry chamber (27).

8. A powder feeder according too any one of the preceding claims, wherein said discharge tube extends into said powder chamber, the inner end of said tube extending upwardly.

9. A powder feeder according to claim 8, wherein there are a plurality of said discharge tubes.

10. A powder feeder according to claim 9, wherein said discharge tubes all communicate with a single common outlet pipe (45).

11. A powder feeder according to claim 8 or claim 9, wherein said inner end of said tube extends at an angle of ninety degrees relative to horizontal.

12. A powder feeder of the type in which a hopper (22) includes a powder-containing chamber (28) bounded by upper and lower porous membranes (40,38) which are pervious to gas and impervious to powder; a gas entry chamber (27) below said lower membrane; means (36) for introducing pressurized gas into said gas entry chamber for travel upwardly through said lower membrane, into said powder chamber and then outwardly through said upper membrane and through a gas outlet (32) of said hopper; at least one discharge tube (44) having an inlet (46) communicating with said powder chamber between said membranes; means (50, 52) for regulating the flow of gas through said powder chamber to suspend powder particles of a desired size at the elevation of said discharge inlet, the gas swirling while traveling through said powder chamber to produce a centrifugal displacement toward a side wall of said powder chamber where they gravitate toward said lower membrane; and means (120) for heating the pressurized gas to dehumidify same, characterized in that said heating means (120) is arranged to heat said gas in said gas entry chamber so as to dehumidify the gas before the gas passes into said powder chamber.

13. A powder feeder according to claim 12, wherein said heating means heats the side wall (72) of said gas entry chamber (27).

14. A powder feeder of the type in which a hopper (22) includes a powder-containing chamber (28) bounded by upper and lower porous membranes (40, 38) which are pervious to gas and impervious to powder; means (36,27) for directing pressurized gas upwardly through said lower membrane, into said powder chamber and then outwardly through said upper membrane and through a gas outlet (32) of said hopper; at least one discharge tube (44) having an inlet (46) communicating with said powder chamber between said membranes; means (50, 52) for regulating the flow of gas through said powder chamber to suspend powder particles of a desired size at the elevation of said discharge inlet, the gas swirling while traveling through said powder chamber to produce a centrifugal displacement of particles toward a sidewall of said powder chamber where they gravitate toward said lower membrane; characterized in that said discharge tube (44) extends into said powder chamber, the inner end of said tube extending upwardly.

15. A powder feeder according to claim 14, wherein said inner end of tube extends ninety degrees relative to horizontal.

16. A powder feeder according to any one of the preceding claims substantially as herein described with reference to the accompanying drawings.