Abstract:
A gas-assisted flow and filling device for transporting particulate material and filling die cavities includes a source of low-pressure gas for delivering gas to a chamber that is separated from a fluidizing chamber by a porous distributor plate which permits the gas to flow through the particulate material and out a vent screen so that the particulate material is permitted to flow out of the fluidizing chamber essentially free of gas and wherein the pressure of the gas delivered to the fluidizing chamber is sufficient to provide a gas bearing to said particulate material adjacent the distributor plate, but not high enough to cause turbulence in said fluidizing chamber.

Description:
This application is a divisional application of U.S. patent application Ser. No. 09/688,168, filed Oct. 16, 2000 now U.S. Pat. No. 6,485,284, which is a continuation-in-part of application Ser. No. 09/418,502, filed on Oct. 15, 1999 now U.S. Pat. No. 6,402,500; which is a continuation-in-part application of U.S. patent application Ser. No. 08/964,128, filed Nov. 6, 1997, now abandoned. Applicants claim priority pursuant to 35 U.S.C. §120; and, the subject matter of those applications is incorporated herein in its entirety. 
    
    
     BACKGROUND 
     The invention relates in general to techniques and apparatus for consistently and uniformly transporting and delivering particulate material such as powder. Such transport and delivery systems are used for filling cavities such as in die-casting machines prior to powder compaction in processes for fabricating consolidated parts for automotive, aerospace, micro-electronics, vitamins, pharmaceuticals, and the like. 
     Particulates such as powder are typically fed from a main hopper and transferred through a tube to a feed or fill shoe which deposits the particulates into the die cavity by gravity or pressure. There are several problems, however, associated with prior art processes for powder delivery and filling of die cavities especially die cavities for high precision, small parts. One such problem is a variation or inconsistency in powder flow in the flexible tube connecting a main hopper or powder supply to a feed shoe on a die surface of a die casting machine. Clumping and surge of the particulates within the flexible tubing and/or the feed shoe also contribute to the non-uniform filling of die cavities. Mechanical shaking of the feed structure above the die cavity can reduce clumping in the powder and improve fill uniformity, but such shaking is not necessarily consistent during successive filling operations. Moreover, such shaking causes segregation of fine materials from coarse materials which results in a loss of uniformity in particle-size distribution and chemical composition. 
     SUMMARY 
     Preferred embodiments of the invention provide improved particulate flow and transport during delivery of the particulate materials to structures such as the die cavities of die-casting machines. In one embodiment, the invention includes an improved transport and filling system which employs a gas-control unit. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a fluidizing portion of a delivery tube. 
     FIG. 2 is a sectional view of FIG. 1 taken along the lines  2 — 2  and includes additional detail. 
     FIG. 3 is an exploded view of portions of the FIG.  1  and FIG. 2 structure. 
     FIG. 4 is a schematic view of a gas-control unit that is useful in the practice of the invention. 
     FIG. 5 is a pictorial view of an alternative embodiment of the invention. 
     FIG. 6 is a schematic side view of the embodiment of FIG.  5 . 
     FIG. 7 is a pictorial view illustrating a pre-assembly relationship between selected elements of the FIG. 6 structure. 
     FIG. 8 is a pictorial view, a portion of the FIG. 7 structure. 
     FIG. 9 is a schematic, cross-sectional view taken along the lines  9 — 9  of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Generally speaking, a typical die-cavity is fed by a system comprising a main hopper which is connected to a fill shoe by a flexible or semi-flexible delivery tube or the like. It is then the fill shoe itself which interfaces with the die cavity as the fill shoe is moved back and forth above the cavity to deposit the particulate material into the cavity. In this respect, FIG. 1 illustrates a fluidizing portion  10  of a delivery tube. The fluidizing portion  10  may be inserted, for example, between two portions of the feed tube or between the feed tube and the fill shoe as the case may be. The fluidizing portion  10  has an inlet port  12  which receives the particulates such as powder into a first chamber  16  located between a porous distributor plate  18  and a vent screen  20  (FIGS. 2 and 3.) 
     A solid semi-cylindrical gas-chamber plate  22  is located adjacent, but spaced from the porous distributor plate  18  to form a second chamber  24  between the porous distributor plate  18  and the plate  22 . In this respect, the porous plate  18  has pores that are between about 0.1 micron and about 50 microns (or even 100 microns depending upon the powder or other particulate-material size that is being fluidized)—the particle sizes being larger than the pores of the porous plate  18 —the pores preferably being on the order of about ½ micron. In any event, the pore size of the porous distributor plate  18  depends on the particle-size distribution of the particulate material and is suitably chosen to prevent clogging by entrapment of small particles within the pores. 
     The porous distributor plate  18  can have flanges  26  and  28  and can be conveniently semi-cylindrical as at  30 , and shaped to form one side of each of the first and second chambers  16  and  24  respectively. The porous material is preferably sintered stainless steel to provide high strength, good wear properties, good weldability, and, corrosion resistance, but other similarly suitable materials may be used as well. 
     The vent screen  20  may also be flanged, and semi-cylindrical. The vent screen  20  is supported by a similarly shaped vent screen support  34 ; and, the vent screen  20  is covered by a vent screen protective cover  36 . The vent screen  20 , the vent screen support  34 , and a protective cover  36  are inverted over the porous plate  18  to form an overall tubular shape as shown to form a particulate flow channel  38  (FIG.  2 )—the parts being affixed by suitable fasteners such as  40 . 
     A gas-inlet port  42  is affixed, such as by welding, to the solid plate  22 . The port  42 , in turn, is connected to a pressure regulator  44 . In this respect, the gas pressure is carefully regulated only to loosen and fluidize the particulate material in contact with the porous distributor plate  18 . The gas pressure and gas flow should be kept sufficiently low that the powder does not bubble. In this regard, it is the particulate material that is to be transported—not the gas. That is, the particulate material on the porous plate  18  is simply loosened by migration of the low-pressure gas through the porous distributor plate  18 . The bottom surface of the particulate that is in contact with the porous distributor plate  18  is loosened first and as the gas flow is increased, the particulates are fluidized—the amount of fluidization being controlled by regulating the gas flow to the inlet gas port  42 . 
     Proper regulation of the gas flow is important to the proper performance of the system. In the most preferred embodiment of the invention, the powder layer in contact with the porous distributor plate is merely loosened to provide a “gas bearing” which reduces friction and increases the powder flow rate so that, as will be discussed in more detail shortly, the gas flows through the vent screen and out of the top of the transport tube  36  and essentially it is only a solid stream of powder that is transported through the flow channel rather than the gas. 
     Metal powders typically used in the powder metallurgy industry often include lubricants and have relatively good flow characteristics. For these powders, the gas pressure applied to the second chamber  24  is typically between about 1 to 2 psi and rarely greater than about 5 psi. The pressure that is employed in any given instance depends upon not only the nature of the particulate material, but the diameter of the flow tube, the mesh size of the porous distributor plate and its thickness. In any event, the pressure is adjusted so that the gas leaves through the vent screen  20  rather than flowing with the particulate material. 
     Ceramic powders typically used in the ceramics industry are spray-dried and can also be characterized as having good flow characteristics. For these powders, the applied gas pressure used in the chamber  24  is usually less than 3 psi and typically closer to 1 psi. 
     For finer powders with poor flow characteristics, the magnitude of the applied gas pressure may be increased depending upon the powder characteristics, but again, care should be exercised to keep the applied gas pressure to the minimum that is necessary to accomplish the functions of loosening the lowermost layer of powder; permitting the gas to escape through the vent screen; and, limiting the amount of gas that is permitted to flow through the transport tube with the loosened particulate material. 
     The above-discussed pressures are those applied to the upstream side of the porous distributor plate  18 . Because of the pressure drop across the porous distributor plate, the pressure applied to the powder itself is proportionately less. Additionally, the semi-cylindrical shape of the porous plate has no corners and reduces dead space so that essentially all of the particulates in contact with the porous distributor plate  18  are fluidized at very low gas flow rates. In this manner, the powder in contact with the porous distributor plate  18  is fluidized while the remaining powder is merely loosened. The use of such low gas pressure also prevents powder segregation in the transport tube when using a powder mixture or alloy with a wide powder-size distribution; prevents the dusting of fine particles; and, discharges the particulates in a solid stream. The use of too high a gas pressure, on the other hand, results in turbulence in the entire powder mass which causes powder segregation, dusting of fine particles, and a resulting low discharge volume of powder which can cause a malfunctioning of the delivery system. 
     As noted above, the fluidizing portion  10  is covered with vent screen  20  which allows the fluidizing gas to escape from the transport to prevent the build-up of pressure within the transport while entraining particles. The mesh size of the venting screen  20  depends, again, on the particle-size distribution of the particulate material that is being used, but a preferred embodiment of the vent screen  20  can be made out of woven stainless steel screen having fine mesh openings of about 25-100 microns with about 40 microns being preferred. The woven stainless steel mesh is flexible but supported by the vent screen support  34  on the underside and protected by the vent-screen protective cover  36  on its upper side. The vent screen support  34  is made out of a perforated material and controls the shape of the top half of the fluidizing portion  10  of the transport tube. While many shapes can be used, the preferred embodiment is a semi-circular shape, but the woven stainless steel vent screen  20  takes its shape from the vent screen support  34 . The vent-screen protective cover  36  is made out of a perforated material that is preferably about the same perforation pattern and size as the vent screen support  34 ; and, care is taken during assembly to align the holes in the vent screen support  34  with the vent screen protective cover  36  to maximize the venting area and insure that the fluidizing gas escapes through the top of the fluidizing portion  10  rather than being permitted to cause mischief in the flow channel  38 . Depending upon the application, a structure such as the porous distributor plate  18  having a coarse mesh size such as about 100 microns can be used as a substitute for the vent screen support  34 , the vent screen  20 , and the vent screen protective cover  36 . 
     The Fasteners  40  permit the vent screen protective cover  36  to be easily removed for cleaning or replacement of the vent screen  20  without disassembling the transport device in its entirety. 
     In a preferred embodiment an insert tube  46  (FIG. 1) connects the fluidizing portion  10  to the remainder of the feed tube. The insert tube  46  can have a uniform inside diameter and a variable outside diameter so that the end of the insert tube  46  which connects to the fluidizing portion  10  is essentially equal to the inside diameter of the fluidizing portion  10 ; and, the other end of the insert tube has at least a portion of its outside diameter that is essentially equal to the inside diameter of the overall transport tube. A suitable collar such as  48  can be used to surround the insert tube  46  and suitably affix it to the overall transport tube. 
     A typical embodiment of the gas control unit  44  is illustrated in FIG.  4  and can be located in a separate housing. It is used to control the moisture content of the gas and to regulate the gas pressure and gas flow rate to both the gas-inlet port  42  and a corresponding port in a fill-shoe portion to be discussed shortly. A gas supply  50  to the regulator  44  passes it through an in-line dryer  52  which removes moisture from the supply gas and an in-line Filter  54  which removes solid impurities from the supply gas. The dried, filtered gas is delivered to one or more pressure regulators such as  56  and  58  from which lines  60  and  62 , respectively, deliver gas to inlet ports  42  and  78 . The line  60  is equipped with a pressure gauge  64  and a flow meter  66 ; and, the line  62  is equipped with a pressure gauge  68  and a flow meter  70 . Similar regulators and associated equipment can be added for additional gas inlet ports corresponding to  42  and  78 . Such additions, however, will be apparent to those skilled in the art and will not be further discussed. 
     In the FIGS. 5 and 6 embodiment, a fluidizing portion  10 ′ corresponds to fluidizing portion  10  in the earlier embodiment, but is at a downstream location and connected directly to a fill-shoe  76  which has a gas inlet port  78 . A sintered metal vent screen  80  is attached, such as by welding, to a porous fluidizer plate  18 ′ (FIG.  7 )—the remainder of the fluidizing portion  10 ′ corresponds to that of the FIGS. 1-3 embodiment and will not be further discussed. 
     The fluidizing aspects of the fill-shoe  76  correspond functionally to the fluidizing portion  10 , but are structured differently as will now be described. 
     The fill-shoe structure  76  is comprised of the fluidizing portion  10 ′ which acts, in this instance, as a particulate transport mechanism to a delivery chute  82  of the fill-shoe  76  (FIG.  6 ). The fluidizer  10 ′ receives gas from the regulator  44  at gas inlet port  42 ′ and passes the particulates through an aperture  83  in the vent screen  80 . The screen  80  is, in turn, attached to the delivery chute  82  which, if desired, is attached to a bottom plate  84  fabricated from a slidable material such as Delrin, polycarbonate, Teflon, or the like. 
     The delivery chute  82  includes a conically shaped segment  86  (FIGS. 7 and 9) which functions as a porous distributor plate corresponding to porous plate  18  in the earlier embodiment. That is, except for the shape, it is structured and pore-sized in the same manner as the porous distributor plate  18 . A delivery chute housing  88  (FIG. 9) includes an aperture  90  which communicates to a conventional die cavity such as  100  in FIG.  6 . 
     Gas from the inlet port  78  is delivered to a chamber  94  formed between the ring-shaped porous distributor plate  86  and the housing  88  of the fill-shoe  76  to provide fluidizing gas at roughly the same low pressures as in the first embodiment to loosen and fluidize the particulate layer that is in contact with the segment  86 . The low-pressure gas is then vented through the sintered vent screen  80  which is affixed to the delivery chute  82  by suitable fasteners (not shown) in apertures  92  of the vent screen  80 . The inlet-gas port  78 , however, functions as one of the fasteners and delivers the low pressure gas from the regulator  44  to the chamber  94  located between the porous distributor ring  86  and the housing  88  and corresponding to the second chamber  24  in the first embodiment. The inlet gas port  78  is located so that it is directly aligned with the weld seam that joins the two ends of the porous distributor ring  86 . In this manner, the weld seam serves as a baffle to distribute the gas around the ring  86 . The space between the porous distributor ring  86  and the vent screen  80  forms a first chamber  96  corresponding to the first chamber  16  and the first embodiment. 
     In a preferred embodiment, the fluidizing portion  10 ′ intersects the vent screen  80  at an angle of about 45 degrees to the horizontal. This angle can be adjusted, however, to meet the clearance and space requirements of a given die-casting machine; and, the length of the fluidizing portion  10 ′ can also be adjusted to meet the space requirements of the die-casting machine. In the illustrated embodiments, the length of the fluidizing portion  10 ′ is about 5 inches, but it can be made shorter or longer as necessary; and, can be separated from the vent screen  80  by a length of conventional transport material if desired—its location depending upon where it is that fluidization is required. 
     In the illustrated embodiment, the fluidizing portion  10 ′ serves an additional function of isolating the delivery chute  82  from the affects of variable head pressures. It also conditions the particulate material before it is transferred to the delivery chute  82  to ensure uniform and consistent flow of particulate material into the delivery chute  82 . 
     The inclination of the porous distributor ring or plate  86  to the horizontal is preferably about 45 degrees, but can be adjusted depending upon the powder-flow characteristics, the cavity size, and the available space. If desired, the slope of the porous distributor ring  86  can be as steep as vertical to maximize the opening size on the bottom of the delivery chute  82 . 
     Fluidization within the delivery chute  82  is controlled by regulating the gas pressure to inlet port  78  depending on the characteristics of the particulate material that is being delivered, but again, the gas pressure should be kept to a minimum. The use of low gas pressure ensures that essentially only the layer of particulate material near the porous distributor ring  86  is fluidized while the rest of the powder within the delivery chute  82  is merely loosened, uniform, and consistent. The use of low pressure gas and low gas-flow rates prevent turbulence in the particulate material within the delivery chute  82  and prevents powder segregation when using a powder mixture with a wide particle-size distribution. It also prevents the dusting of fine particles. The vent screen  80  allows the gas to escape and prevents gas-pressure build up within the delivery chute  82  so that the particulate material is delivered to the die cavity by gravity rather than pressure. The venting screen  80  is preferably made of sintered stainless steel, but so long as it is porous, other materials can be used. Advantages of the sintered stainless steel screen are that it provides structural strength; permits the porous fluidizer  18 ′ to be welded to it; and, provides structural integrity to the apparatus. 
     The mesh size of the vent Screen  80 , as in the first embodiment, depends on the particle size distribution of the particulate material that is being employed, but it should be proximate the die-cavity  100  to ensure that gas escapes from the area of the delivery chute and avoids variations in filling, and partial filling, especially for particulates with poor flow characteristics. In this regard, the size of the pores in the vent screen should be maximized to insure that fluidizing gas escapes without interfering with the gravity flow of the particulate material into the die cavity. For metal powders with admixed lubricants typically used in powder metallurgy, the applied gas to the second chamber  94  should at least be less than about 5 psi so that, after the pressure drop across the porous distribution ring  86 , there is no turbulence in the particulate material; the fluidizing gas easily escapes through the vent screen  80 ; and, the particulate material is permitted to drop into the die cavity by gravity. For ceramic powders typically used in the ceramic industry, the gas pressure applied at the inlet port  78  should be less than about 3 psi, but for fine powders with poor flow characteristics, the magnitude of the applied gas might be increased slightly with care being taken to keep the applied gas pressure at the minimum necessary to merely loosen the lower-most layer of particulate material. 
     In some embodiments, the gas pressure may be selectively turned on and off, but in the preferred embodiments the gas pressure is turned on and left on continuously. It is monitored, however, and adjusted to provide the various advantages of the invention. In this respect, low-tonnage die-casting machines are operated at speeds of up to about 120 strokes per minutes and more. Given the very low gas-flow rates and the high press feeds, therefore, leaving the gas on at all times helps to stabilize the gravity-filling operation. 
     The fill shoe of a typical die-casting machine is customarily oscillated back and forth by a slide finger (not shown) which is part of the press mechanism. The slide fingers holds the bottom plate to the die surface so that excess powder is swept away from the die cavity. In this regard, the quantity measurement of the invention is volumetric in that the die cavity is filled only by gravity and not pressurized. That is, the system of the invention completely fills the die cavity to obtain an exact volume rather than attempting to force a measured weight of material into the die cavity by means of pressure. 
     The foregoing and other advantages and features of the invention will be apparent to those skilled in the art; and, certain modifications and departures can be made without departing from the spirit and scope of the invention which is defined by the following claims.