GB2078551A - Method and Apparatus for Spraying Powder - Google Patents

Abstract

In the spraying of dry powder from a fluidized bed powder reservoir 10 through an injector pump 12 having a chamber which communicates with the reservoir through valve-controlled openings 34, 36 to admit fluidized powders and in which air is blown through said pump to flow said fluidized powders through a barrel assembly 16, a more concentrated quantity of powder can be sprayed at high velocity for a greater distance by opening the valve to admit a charge of powder then closing the valve, thereby effectively sealing the chamber, with a fixed quantity of powder therein, from the reservoir atmosphere before air is blown through the chamber to discharge the fixed quantity of powder therefrom. <IMAGE>

Description

SPECIFICATION Method and Apparatus for Spraying Powder The present invention relates to a method and apparatus for spraying powder. During various steel finishing processes wherein steel is shaped or formed while it is hot, it is often desirable to mark the hot steel for, for example, identification or marking defects. Since the steel is hot, marking materials such as paint cannot be used as these would quickly burn off. Hence the practice has been to apply dry powder pigments or metallic powders onto the hot steel surface, preferably by pneumatically spraying such powders.

Although there is a selection of commercially available equipment for spraying dry powders, such prior art equipment has not been completely suitable for spraying the types of powders used to mark hot steel. That is to say, the powders most commonly preferred for marking hot steel, for example, titanium dioxide or zinc oxide, tend to agglomerate when pneumatically pumped, and thus frequently plug and clog the powder spraying equipment.

United States Patent Specification No.

3,899,131 discloses a method and apparatus for spraying agglomerating powders such as titanium dioxide wherein the agglomeration problems are somewhat reduced. Although that method and apparatus are currently being used successfully in some commercial operations, they have certain limitations which render the practice unsuitable under certain conditions such as the marking of such hot products as bars, billets or slabs moving at high line speeds, that is, greater than 1000 feet per minute. Specifically, the impact of the marking powder on fast moving products has not been sufficient to ensure permanent adhesion to the product, and/or the volume of the marking powder reaching the product does not provide a marking of adequate contrast for visibility at remote distances.Indeed, the system requires close proximity of the marking apparatus to a stationary, or slow moving product in order to assure adequate contrast, but this is not always possible on some production lines as the spraying apparatus soon becomes damaged, or it is not possible to stop or slow the production line.

According to the present invention, there is provided a method of pneumatically spraying powder through an apparatus comprising a fluidized bed powder reservoir, a pump housing attached within said reservoir and having a chamber therein communicating with a barrel assembly and at least one opening in a wall thereof affording communication between the chamber and the interior of said reservoir, a valve for selectively opening and closing the or each said opening, a gas-tight seal between said valve and said housing for sealing said chamber with respect to the interior of said reservoir when the valve is closed, and means for blowing a gas through said housing and said barrel assembly: the method comprising activating said fluidized bed powder reservoir to fluidize powder therein, opening said valve to admit a quantity of fluidized powder through the or each opening into said chamber, closing said valve to isolate said quantity of fluidized powder within said chamber, and activating said blowing means to blow said quantity of fluidized powder out through said barrel assembly.

The invention also provides an apparatus, as specified above, for carrying out the method.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an elevation view in partial section showing an apparatus for spraying dry powder in accordance with this invention, Figure 2 is a plan view in partial section of the apparatus shown in Figure 1 taken on line Il-Il of Figure 1, Figure 3 is a schematic diagram of the pneumatic control circuit for operating the apparatus shown in Figures 1 and 2, Figure 4 is a sectional view of an improved nozzle design pursuant to one embodiment of this invention, and Figure 5 is a sectional view of another embodiment of the improved nozzle design.

With reference to the drawings, the apparatus comprises a fluidized bed powder reservoir 10, a pump assembly 12, an air cylinder 14 and a barrel assembly 16. The pump 12 is supported as a single unit by a framework consisting of two side bars 20, a front plate 22 and a rear plate 24. The framework is attached within reservoir 10 by bolts 26 and 28. Such an arrangement isolates the pump housing from the outside atmosphere.

The pump itself consists of a tubular housing 30 welded to front plate 22, and is provided with a pair of elongate, opposed openings 34 and 36. A pump cylinder 38, consisting of a hollow metal barrel coated with polytetrafluoroethylene is slidably mounted within a polytetrafluoroethylene sleeve 40 which is retained within housing 30 by a keeper plate 42. Cylinder 38 thus acts as a valve closure member for selectively opening and closing openings 34 and 36. The forward end of cylinder 38 is fitted with a replaceable nozzle 44.

In the closed position as shown, nozzle 44 seats against a hollow cylindrical muzzle 46 which is fitted into the forward end of housing 30. Muzzle 46 is also made of polytetrafluoroethylene.

Attached to the forward end of the housing 30 by means of threaded nut 48, is the replaceable barrel assembly 16, which consists of a metal tube 50 having an adapter ring 52 soldered to its inside end. Tube 50 contains a polytetrafluoroethylene sleeve 54 retained therein by a metal cap 56 soldered to the forward end of tube 50 and having an open discharge bore therethrough.

The rearward end of cylinder 38 is fitted with an angled adapter 60 for accommodating an air inlet supply hose 62, and for attachment of a push rod 64. Push rod 64 extends through the rear wall of reservoir 10 with its outer end connected to air cylinder rod 66 of the air cylinder 14 by coupling 68. A bellows 70 is supported on rod 64 and attached to the rear plate 24. Accordingly, reciprocal activation of air cylinder 14 will cause cylinder 38 to reciprocate between the closed position as shown, and an open position wherein the inside of-housing 30 is exposed to the atmosphere within reservoir 10 through openings 34 and 36.

For operation, cylinder 14 is actuated to the closed position and the powder to be sprayed through barrel assembly 1 6 is placed into reservoir 10 and fluidized by air passing through a porous membrane 72. Thereafter, purging air or other gas is admitted at low velocity through hose 62. This low velocity purging gas flows through cylinder 38, muzzle 46 and barrel assembly 16.

The apparatus is thus activated and ready to spray powders on demand. To spray powders pursuant to prior art practices, the velocity of air or gas admitted through hose 62 would be increased as cylinder 38 is opened by air cylinder 14 to admit fluidized powder within housing 30. Thus admitted, the powder would be pneumatically ejected through muzzle 46 and barrel assembly 16. When it is desired that powder spraying be discontinued, air cylinder 14 would be activated to close cylinder 38 as the air admitted therethrough is reduced back to a low velocity purging blow to keep the passageway clear of accumulated powders.

Pursuant to the prior art practice therefore, cylinder 38 would remain in the open position throughout the entire period that of spraying. In contrast thereto, the present invention requires that cylinder 38 be closed when spraying. That is, cylinder 38 is opened just prior to the blow in order to admit the desired quantity of fluidized powder within housing 30. Thereafter, cylinder 38 is closed, trapping a small quantity of fluidized powder within cylinder 38. When cylinder 38 is completely closed, the small quantity of powder within cylinder 38 is blown through barrel assembly 1 6 using a relatively low pressure, for example, 40 psig, to effect a substantially higher rate of speed than is possible with an open cylinder 38.Although a significantly smaller volume of powder is so blown, its greater speed provides a greater impact force on a more concentrated area of impact. The powder as so projected has a stronger adhesion on the target area and its more concentrated nature renders a more visible area. In addition, the powder can be sprayed over a greater distance and still leave a readily visible mark, even on products traveling at line speeds in excess of 1000 feet per minute.

In order to achieve a high velocity of powder through barrel assembly 16, it is necessary that the seal between nozzle 44 and muzzle 46 be very effective. Any leakage therebetween will cause turbulence in muzzle 46 and reduce velocity. Referring to Figure 4, the sealing face of nozzle 44 is tapered to provide a contact angle a of from 400 to 500. In the alternative, a spherical contact surface may be provided as shown in Figure 5 where the tangent to radius R at the contact point with nozzle 44 is between 400 and 500.

The interior surfaces of muzzle 46 and barrel assembly 16 should be streamlined to minimize turbulence, that is, sharp corners on any orifices should be removed to avoid turbulence which would tend to reduce air velocity. The ideal velocity should approach but should not exceed sonic velocity.

The pneumatic control circuitry of the apparatus is shown in Figure 3. A main air supply line 76 from a manifold 74 connects to a fourway valve 78 which supplies air to the air cylinder 14 either at its forward end 80 or rearward end 82 depending on the position of valve 78.

Solenoid 84, which activates valve 78, is energized by any suitable power supply 86 through a spring return switch 88. A second solenoid valve 90 is provided on air line 76 which supplies air supply to pump inlet supply hose 62.

Solenoid 90 is energized by any suitable power supply 92 through a spring return switch 94. Air is continuously supplied to the fluidized bed reservoir 10 via line 96 through regulator 98.

In operation, when reservoir 10 is fluidized, the operator may depress switch 88 which sends air to inlet 80 on air cylinder 14. This causes cylinder 38 to be retracted, thereby admitting fluidized powder into housing 30. Release of switch 88 causes air cylinder to reverse its direction, thus closing cylinder 38 trapping a metered or fixed quantity of powder in housing 30. Switch 94 is then depressed which opens solenoid valve 90 and permits full air supply pressure to suddenly surge through hose 62 and cylinder 38 projecting the fixed quantity of powder through barrel assembly 1 6. In an actual production line application, switches 88 and 94 are usually operated automatically in sequence in response to a signal for spraying powder.

Claims (6)

Claims

1. A method of pneumatically spraying powder through an apparatus comprising a fluidized bed powder reservoir, a pump housing attached within said reservoir and having a chamber therein communicating with a barrel assembly and at least one opening in a wall thereof affording communication between the chamber and the interior of said reservoir, a valve for selectively opening and closing the or each said opening, a gas-tight seal between said valve and said housing for sealing said chamber with respect to the interior of said reservoir when the valve is closed, and means for blowing a gas through said housing and said barrel assembly; the method comprising activating said fluidized bed powder reservoir to fluidize powder therein, opening said valve to admit a quantity of fluidized powder through the or each opening into said chamber, closing said valve to isolate said quantity of fluidized powder within said chamber, and activating said blowing means to blow said quantity of fluidized powder out through said barrel assembly.

2. An apparatus for pneumatically spraying powder, comprising a fluidized bed powder reservoir, a pump housing attached within said reservoir and having a chamber therein scommunicating with a barrel assembly and at least one opening in a wall thereof affording communication between the chamber and the interior of said reservoir, a valve for selectively opening and closing the or each said opening, a gas-tight seal between said valve and said housing for sealing said chamber with respect to the interior of said reservoir when the valve is closed, and means for blowing a gas through said housing and said barrel assembly.

3. An apparatus as claimed in claim 2, in which said valve is a nozzle slidably mounted in said housing and said gas-tight seal comprises a polytetrafluoroethylene muzzle secured within the forward end of said chamber and contacted by said nozzle on closure of the valve at a mating surface tapered at 400 to 500 to the direction of sliding of the nozzle.

4. An apparatus as claimed in claim 3, in which said nozzle and muzzle contact along mating partspherical surfaces, a tangent to the part-spherical surfaces at the contact area being inclined at 400 to 500 to the direction of sliding of the nozzle.

5. A method of pneumatically spraying powder substantially as hereinbefore described with reference to the accompanying drawings.

6. An apparatus for pneumatically spraying powder, constructed and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.