Sintered Filter Having Straight Holes Therethrough

Abstract

A sintered metal, or ceramic filter having parallel, straight holes of uniform diameter and with smooth inner surfaces passing therethrough is formed by sintering a green compact about a large number of straight wires arranged parallel to one another. Upon application of heat at a sintering temperature above the melting point of the wire material and less than the melting point of the sintering powders, the wires are absorbed into the pores of the sintering powders and leave holes of the same shape and size as the original wires in the resulting sintered filter.

Description

BACKGROUND OF THE INVENTION

For a long time attempts have been made to apply powder metallurgical techniques to the making of filters so as to obtain the advantage of the porosity of the sintered material. Filters have been produced by sintering spheroidal shaped metal powders of nearly uniform size at a relatively low temperature for a long period of time under no pressure, or very low pressure. The sintered filters are superior to those made of an organic material, or metal screen, evidencing better mechanical properties, such as ease of processing and durability. Also, it is easy to control the porosity, or the size and distribution of holes, in a sintered filter by selecting powders of suitable size and shape. Since in sintering the filter holes are formed as pores between the sintered powders, such holes have complicated shapes and are not oriented directionally, and the volume of the holes is very small compared with the volume of the whole filter. Sintering powders do not have definite sizes and shapes. Therefore, the shapes of the holes and the porosity respectively differ in the resulting sintered filters. Thus, sintered filters made by known processes lack homogeneity.

Numerous attempts have been made to increase the porosity of sintered filters. One such attempt has been to sinter powders under no pressure, or very little pressure, but this causes loss of strength and homogeneity of the resulting filters, and requires careful fabrication treatment. Another attempt has been made to add spacer powders to form holes during sintering. This, too, however, yields filters with the same defects mentioned above because of the difficulty of uniformly dispersing the spacer powders, and the difficulty of controlling the shapes and sizes of the holes formed by the spacer powders.

SUMMARY OF THE INVENTION

This invention relates to filters formed of sintered material having uniform, straight holes therethrough, and made by sintering metal, or ceramic powders. The invention aims to overcome the above-mentioned defects of conventional techniques and forms linear, parallel holes in the sintered filters by placing parallel wires in a green compact and melting the wires during sintering to form holes having the same shape as the wires and located in the same places. The filtration ability of the finished product can be controlled by suitably selecting the diameter and number of wires used.

A primary object of the present invention is to produce a sintered filter having uniform and straight holes of the same diameter and lying in the same direction.

Another object of the invention is to produce a sintered filter having improved strength.

A further object of the invention is to provide an easy and inexpensive method for mass production of sintered filters.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention, itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, wherein like reference characters indicate like parts throughout the several FIGS., and in which:

FIG. 1 is a schematic diagram illustrating an apparatus for coating a wire with sintering powder;

FIG. 2 is a schematic diagram illustrating apparatus for forming filters by a slip casting method;

FIG. 3 is a perspective view of the dies of FIG. 2 shown spread apart;

FIG. 4 is a diagram illustrating apparatus using a rubber press for forming filters;

FIG. 5 is a diagram illustrating apparatus using rolls for forming filters;

FIG. 6 is a plan view of the rolls in FIG. 5;

FIG. 7 is a diagram illustrating apparatus for forming filters by a loose sintering method; and

FIG. 8 is a perspective view of a sintered filter made in accordance with the present invention.

DETAILED EXPLANATION OF THE INVENTION

The filter product, of the invention is made by sintering metal, or ceramic powders in which wires are positioned, the wires being of a material which melts, burns, or vaporizes at a lower temperature than the melting point of the sintering powders. The wires are placed in rows in the powder, then the powders are compacted and heated to a sintering temperature which must be higher than the melting, point of the wires. As a result, the wires disappear, leaving holes of the same shape as the original wires. The distribution, number and diameter of the holes in the sintered filter can be easily controlled by selecting suitable wires of a predetermined diameter, and by changing the number and arrangement of the wires.

The sintering powders used, according to this invention, are metal powders, such as iron, iron base alloys, tungsten, tungsten base alloys, and chromium. The usable ceramic powders include such materials as chromium carbide, titanium carbide, alumina and glass.

The invention utilizes wires made of metal, such as copper, steel, iron, nickel, and nichrome. The wires may be organic fibers such as natural and synthetic fibers, and inorganic fibers such as glass fibers.

When metal wires are used, it is preferable that they be formed of a metal which melts and which is capable of being absorbed in the pores of the sintered body to form the holes during sintering. The molten metal of the wires should wet the sintering powder in such manner that the latter will absorb the former and thereby infiltrate the wire metal into the pores of the sintering powder. The combinations of the materials of the sintering powders and the metal wires which tend to wet the powders and to be absorbed into the powders are shown in the following Table I: ##SPC1##

When ceramic sintering powders are used, they are preferably used with metal wires, or glass fibers. It is desirable to use wires which will wet the sintering ceramic powders and which will be absorbed into the sintered body. When glass fibers are used, the process can easily be controlled by selecting fibers having suitable softening points and wetting properties.

The metal wires and glass fibers work not only to form holes in the sintered filters, but also to smooth the walls of the holes and to strengthen the filters mechanically. Especially, a sintered filter made of metal powders and wires listed in Table I has remarable strength and toughness and will be durable when cut, or machined to suitable size. Organic fibers such as natural and synthetic fibers having relatively low melting points are burnt away at relatively low temperatures. Therefore, they are suitable to be used as the wires according to this invention. Natural fibers including rayon, polypropylene, and acetate fibers, or the like, are burnt away without leaving ashes, but fibers such as polyamid, polyester and polyvinyl alcohol are difficult to burnaway without leaving pyrolytic residue so that these fibers have to be heated for a relatively long time to burn away the residue.

The organic and glass fibers mentioned above may be used as the wires with sintering powders whether metal, or ceramic.

As to the structure of the fiber wires, when used, monofilaments are favored for better control of the shape and diameter of the holes to be formed in the sintered filter. However, fibers having a plurality of thread plies can be used, In such instance, the thread plies are soaked with a solution containing a binder and dried to smooth their surfaces. When metal wires are employed, the metal wires obtainable as sold commercially in the market can be used without any special treatment. If, however, it is necessary to make purchased wires thinner, this may be done by rolling, swaging, or drawing. Experimentally it has been determined that wires having a diameter as small as 0.05 mm. form holes in the sintered body of the filter having almost the same diameter as that of the wires. However, wires having diameters ranging between 0.1 and 2.0 mm. may be more practical to use in view of ease of production and economy. When a filter having holes of more than 2.0 mm. diameter is required, it can be made by mechanically drilling a sintered block rather than by using this invention. There is no limit to the number of wires which should be used in a sintered filter according to this invention, but when only a few wires are employed, the cross-section of the filter will not yield uniform filtration furnace over its entire area. The filtration ability is increased when at least ten wires are used, and technically the number of wires may be increased to as many as 5,000 and in a filter unit. But, there are some troublesome8c problems when more than 1,000 wires are necessary. In the latter instance, ordinarily, it is possible to use several small filters instead of one large filter. The number of wires which may be practicably employed in a sintered filter range between 10 and 1,000.

EXAMPLE 1 3

In this example an embodiment of the invention is described which involves coating the hole forming wires with sintering powders, bundling the coated wires to form a compact, and sintering said bundle.

The wires are coated with a slurry composed of sintering powders, a binder and a solvent for the binder. After drying the wires as thus coated, they are cut to a predtermined length, and the short wires are bundled and sintered. This method is explained in more detail with reference to FIG. 1. A copper wire 2 wound on the bobbin 1 is pulled up through an annealing furnace 3 and enters a hopper 4 through a small hole 5 at the lower end of the hopper. Passing out of the top of the hopper, the wire enters and passes through a drying furnace 6 and is then wound on a drum 7. The annealing performed in furnace 3 is required not only for metallic fibers, but also for synthetic fibers. The bobbin 1 gives the wire 2 a slight tension by the exercise of small friction force between the bobbin 1 and its supporting device, or axle (not shown). The hopper 4 contains a slurry 8 composed of sintering iron powders and binders such as cellulose acetate, and acetone as the binder solvent. The wire 2 as it passes through the hopper 4 is coated with the slurry 8, the thickness of the coating depending on the viscosity of the slurry. The slurry coating on the wire is dried in furnace 5. The binders retain their elasticity even after drying, and such elasticity prevents the coating from peeling off the wire surface. The hole 5 at the bottom of hopper 4 is small and only slightly larger than the diameter of the wire 2. Usually, there is no leakage of slurry from the hole 5, even without a seal, because of the high viscosity of the slurry. When a seal is necessary, because of the use of a low viscosity slurry, or because a larger hole 5 is used than the diameter of the wire 2, an elastic seal having a hole of only slightly smaller diameter than the diameter of the wire 2 is provided in or about the hole 5.

After the wire 2 is coated with the sintering powders and is wound on the drum 7, it is cut to a predetermined length and several tens of the cut lengths (the exact number, as previously described being selected in accordance with required filtration ability) are arranged parallel to one another and secured together using another slurry as a binder to form a compact of columnar shape. The viscosity of this slurry is greater than that of the slurry 8 in the hopper 4. The formed compact thus has passing therethrough many wires of nearly uniform distribution and lying parallel to one another in the direction of the compact, or column axis, the sintering powders contained in the slurry being packed between the wires.

Next, the compact containing the wires is put into a furnace which is kept at a sintering temperature higher than the melting point of said wires. When iron powders have been employed as the sintering powders and copper wires as the wire 2, the sintering temperatures of the iron utilized is about 1,120.degree. C, while the melting point of the copper is 1,083.degree. C, so that the copper wires are melted at the same time the iron powders are sintered. The molten copper wets the sintered iron very well and is absorbed into the pores of the sintered iron. Therefore, as shown in FIG. 8, straight holes 9 are left in the compact where the wires originally pass through. These holes have the same diameter as the wires 2, and produce a filter 10. When the obtained filter is used, fluid passes through the holes 9, and solid materials contained in the fluid are filtered. According to necessity, the filter 10 is cut in a direction perpendicular to the compact, or column axis so as to give it the thickness, or height required for the specific filter.

When natural fibers are employed instead of metal wires, the annealing furnace 3 in the system illustrated in FIG. 1 is not necessary and can be omitted.

EXAMPLE 2

An embodiment of the invention wherein the compact is formed by slip casting will now be explained with reference to FIGS. 2 and 3 which illustrate apparatus for forming the compact. Wires 11 of metal, organic material, or glass, are fed vertically downwardly from many bobbins 12 which are rotatably supported. The bobbins are so arranged that the wires are spaced apart equally and this spacing is ensured by passing the wires through holes 13' of a wire mesh guide 13. The lower ends of wires 11 are fixed to a member 15 after passing through the central discharge aperture 14' of a hopper 14. The member 15 also supports a wire mesh, or screen 15', and the lower ends of the wires 11 are secured to the intersections of the screen by welding, or other means. In this manner the wires 11 are distributed with uniform spacing in horizontal cross-section and are parallel to each other lying in vertical, or longitudinal directions. The bobbins 12 are fixed to supporting axles (not shown) through which slight tension forces are applied to the wires after they are pulled out to the desired length. The bobbin axles may be engaged frictionally by means of a clutch mechanism, or the like. If necessary, a wire mesh guide 19, which is movable parallel to the wires 11 guided by rails 19', is placed between the fixed guide 13 in the hopper 14 so that the wires 11, passing through openings in screen 19, are retained spaced and guided over their full lengths.

The group of parallel wires 11 above the fixed lower guide 15 is encircled by a pair of moulds 16 and 17 (best shown in FIG. 3). The moulds are made of a hygroscopic, or porous material such as plaster, and are symmetrical semicylinders fitted about the discharge opening 14' of the hopper below the hopper and above the member 15. Both of the semicylinders 16, 17 are thus fitted at their upper ends about the cylindrical downward projection surrounding the discharge aperture 14'. The fixed guide 15 may be equipped with a stopper such as 24' (FIG. 4) below the bottom of the moulds. Slurry 18 composed of sintering powders, such as iron base alloy powders, a binder such as ammonium alginate, and water, as a solvent, is poured into the interior of moulds 16 and 17 through the hopper 14, and the slurry is entirely filled into the interior of the moulds 16 and 17 and the gaps between the wires 11. The water, as solvent of the slurry, is absorbed into the moulds, and the slurry thus dries, or stiffens to form a compact in which the wires 11 are uniformly arranged. When the slurry 18 is almost dried, or solid, and the compact is formed, the wet moulds 16, 17 are separated transversely of their axes and removed. In the next step, the movable guide 19 is moved upwardly and other dry moulds 16, 17 are applied about the upper end of the compact. More slurry is then poured into the dry moulds. In this way the compact can be lengthened into a column of any desired length. The wet moulds 16, 17 can be re-used after being dried. When the cylindrical compact, containing the wires 11 parallel to its axis, is formed to the desired length, the projecting parts of the wires 11 are cut and the compact is moved to a furnace in which it is sintered at the sintering temperature of the sintering powders forming the dry slurry. During sintering the wires 11 are burnt, melted, or vaporized, and may be infiltrated into the sintered material, leaving holes having diameters equal to the wire diameters. A filter of desired thickness can then be made by cutting the cylindrical compact transversely to the longitudinal axis of the cylinder.

After obtaining the compact by cutting the wires 11, as described above, the bobbins 12 are rotated to feed additional lengths of wires 11 downwardly, and these are secured to the intersections of the wire net 15' passing through the mesh holes of both the fixed guide 13 and the movable guide 19. The above process for making the compact is then repeated to make another compact.

In the apparatus of FIG. 2, the fixed member 15 may be arranged to be movable vertically downwardly with respect to its support. In such instance the member 15 and the formed compact will move downwardly when the frictional engagement of the moulds 16 and 17 with the compact and the bobbins 12 are released after one forming step is finished. The moulds 16, 17 may then be closed after the member 15 has been moved, and additional slurry 18 can be packed above the initially completed portion of the compact.

EXAMPLE 3

An embodiment of the invention utilizing a rubber press as part of the forming apparatus is now described with particuler reference to FIG. 4. Wires 11, similar to those shown in FIG. 2, are arranged parallel to pass through wire screen holes 21' of a fixed guide 21, then through the central discharge aperture 22' of a hopper 22 and through the central bore 23' of a cylindrical rubber press 23, being secured at their bottom ends to intersections 25' of a wire screen 25 which is bridged with tension across the upper surface of a frame 24. The screen 25 is supported to be slidable vertically along a suitable support member by means of guide 30.

Seated against the bottom of the hopper discharge spout 22' is the cover 26 of a press chamber 29 defined by a cylindrical sidewall 28 and a bottom plate 27. The cover and bottom plate are formed with complementary shoulders 26', 27' of circular form, and surrounded by outwardly spaced unnumbered shoulders which seat the sidewall 28. A cylindrical rubber press 23 is inserted to seat between the shoulders 26', 27' and has a central cylindrical bore 23' which aligns coaxially with the discharge spout of the hopper and corresponding openings in the cover and bottom plates of the press chamber 29. The press chamber 29 is connected to a suitable source of fluid pressure, not shown, by way of a switching valve, also not shown, through port 28' penetrating the sidewall 28.

As shown in broken lines, a stopper 24' made of an elastic material, such as rubber, is provided at the lower surface of the frame 24 to prevent the apparatus from leaking powders, and the wire net 25 is moved to contact the underside of the bottom plate 27 of the pressing chamber by movement of frame 24 in guiderail 30. The sintering powders 20 are fed from the hopper 22 and then pressure fluid is introduced into the chamber 29 through port 28' by operating the mentioned switching valve. This pressure causes the diameter of the central bore 23' of the rubber press to be reduced and the sintering powders encircling the wires 11 arranged parallel in the bore 23' are compacted by the rubber press 23. Afterwards, the pressure fluid in chamber 29 is returned to a reservoir (not shown) by operating the switching valve mentioned. The rubber press 23 then expands to its original size by its inherent elasticity. The compact is then moved downwardly together with the wires 11 as the frame 24 is moved downwardly along the guide 30. Since the pressure was not applied to the sintering powders near the wire net 25, the powders in this area fall away leaving the wires 11 exposed, but the portion of the powders to which pressing force of the rubber press was applied is compacted and retains an unbroken cylindrical shape. After positioning this compacted cylinder at the lower half of the central bore 23' of the rubber press, additional sintering powders 20 are introduced into the emptied portion of the bore 23', the pressure fluid is again introduced into the chamber 29 to form a new portion of the compact above and connected to the part of the compact previously formed. Thus, by repetition of the cycle of exhausting the pressure fluid, feeding the sintering powders 20, moving the frame 24 and introducing the pressure fluid, a long and column-shaped compact of sintering powders containing wires 11 arranged parallel to its axis, can be obtained.

Next, the resulting compact is put into a sintering furnace and heated to a sintering temperature higher than the burning, vaporizing, or melting temperature of the wires 11 in order to eliminate the wires and obtain the sintered filter with holes of the shape and size of the wires. A filter of the desired height, or thickness is then obtained by cutting the sintered compact transversely of its axis.

EXAMPLE 4

An embodiment of the invention is now described wherein the forming apparatus utilizes rollers as shown in FIGS. 5 and 6. Wires 11, as in FIG. 2, are arranged in the same manner so that the wires pass from bobbins to lie parallel to one another and are retained properly spaced by the openings 31' of wire net 31. The wires pass through a central discharge opening 32' of the hopper 32. Then, the wires pass through and between two forming rollers 34 arranged on parallel axles 37 lying in a plane perpendicular to wires 11. Each roller has a ring-shaped forming groove 34' which meet to form a cylindrical forming mould diverging at top and bottom. The wires are secured at their lower ends to the intersections of the wire screen 35', part of the guide member 35. Member 35 is slidable along the guide 38 with respect to a suitable support. Thus, the wires are arranged in parallel and kept in suitable tension in the same manner as explained for previous examples.

As shown in FIG. 6, the forming rollers 34 are mutually in contact along their cylindrical surface parts 36 at both sides of the forming grooves 34', and they are driven to rotate in opposite directions at the same velocity by drive shafts 37. The discharge spout 33 which forms the central hole 32' of the hopper 32 is so shaped and positioned as to feed sintering powders 40 from the hopper to the forming grooves 34' without dropping onto the cylindrical surfaces 36 of the rollers. When sintering powders 40 are fed into the hopper while the forming rollers 34 are driven to rotate, the powders will discharge through spout 33 and will be packed in the forming grooves 34', being pushed downwardly and out after forming. At the same time, the member 35 is moved downwardly along the guide 38, while the rollers 34 turn and a long, column-shaped, continuous compact is obtained. After sintering the obtained compact in the same way as described in Examples 1 and 2, the resulting sintered material has continuous holes of diameters equal to the wires at the portions where the wires were inserted. A filter of desired size is then made by cutting the sintered material transversely to the described column so as to obtain a filter of necessary thickness.

EXAMPLE 5

An embodiment of the invention utilizing apparatus for forming a compact by loose sintering is hereinafter described with particular reference to FIG. 7. In this embodiment the compact is formed by sintering material about the wires at a temperature lower than the melting point of the wires and higher than the initial sintering temperature of the sintering powders. When iron metal powders are employed as the sintering powders, the initial sintering temperature is about 700.degree. C. Therefore, for example, copper wires are utilized whose melting point is 1,083.degree. C. and the wires have a melting point above the 700.degree. C. which is the sintering temperature. Organic fibers cannot be used as the wires because their melting, or burning points are too low.

Referring now to FIG. 7, as in the example shown in FIG. 2, the wires 11 are pulled out from each bobbin and passed through wire screen openings 41' of the fixed guide 41. The wires are secured to member 44 at their bottoms after passing through the central discharge hole 43' of the hopper 43 which is fixed at the upper end of the forming cylinder 42 through which the wires also pass. The wires 11 are kept in suitable tension between the bobbins and the wire net 44'. The member 44 is vertically movable along guide 45 with respect to a suitable support. Also, a stopper 49 of elastic material, such as rubber, is removably positioned at the lower surface of the wire net 44'.

A cooling device 46 surrounds the forming cylinder 42 for introducing a cooling medium such as water around the surface of the cylinder 42. Another cooling device 48, the same or similar to the cooling device 46, surrounds the forming cylinder 42 below the heater 47. The heating device 47 is operated to heat the sintering powders 50 in the cylinder 42 at a temperature somewhat higher than its initial sintering temperature and lower than the melting temperature of the wires 11. Thus, the wires 11 are arranged in parallel between the guide 41 and the member 44 without being melted. The sintering powders 50 are introduced into the interior of the forming cylinder 42 from the hopper 43, and upon application of heat from member 47 the powders, as individual particles, are bound together without compression being mildly sintered by the heat to form a unitary network. In the network the connections between individual particles is in the form of necks and the bound particles contain and hold the wires 11 passing through the network.

The sintering powders 50, as the mildly sintered material forming the network, move downardly along the inside of the forming cylinder 42 as a result of movement of the member 44 in guide 45 and the tensioning force of the wires 11 regulated to a velocity which enables said mild sintering to occur, the sintering powders 50 are constantly fed from the hopper 43 so that the formed compact is of long-column shape penetrated by the parallel wires 11. After the compact has been thus formed and mildly sintered, the compact is sintered at a temperature above the melting point of the wires 11 and below the melting point of the powders 50. This melts the wires and strengthens the sintered network of the powders simultaneously. The melted metal of the wires infiltrates into the pores in the network of powders to increase the strength of the sintered material. Thus, a sintered product is obtained in which continuous and straight holes are formed at the positions where the wires originally were arranged in the compact. The diameter of the holes is equal to that of the wires. During the second sintering step the network of the sintered powders already formed in the first sintering step is not deformed, but merely strengthened. A filter of the desired thickness can be obtained then by cutting the sintered compact column transversely to its axis.

In each of the previous examples, when the amount of the metal of the wires which will be infiltrated into the sintered material is determined to be greater than the volume of the pores in the sintered material, additional sintering powders can be added so as to enlarge, or add to the compact sintered bodies on the side surfaces, the upper surface, or the lower surface of the compact, and the excessive metal melted during the sintering step will be absorbed into the added portions of the compact.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof are possible. The invention, therefore, is not intended to be restricted to the exact showing of the drawings and description thereof, but is considered to include reasonable and obvious equivalents.

Claims

We claim:

1. A filter having a sintered body and a plurality of uniform and straight holes passing through said sintered body, wherein said holes have a diameter ranging from 0.1 mm. to 2.0 mm. and are parallel to each other, and said sintered body consists essentially of a sintered matrix of a material having a network structure and a different infiltrated material within said sintered matrix reinforcing said sintered matrix and smoothening the wall surfaces of said holes, said sintered matrix being made of one powder selected from the group consisting of iron, iron base alloys, tungsten, tungsten base alloys, molybdenum, stainless steel, chromium, chromium carbide, titanium carbide, and alumina, and said infiltrated material being a material selected from the group consisting of copper, iron, nickel, nickel chromium alloys, and glass.

2. A filter as claimed in claim 1, wherein said sintered matrix is made of one powder selected from the group consisting of iron, iron base alloys, tungsten, tungsten base alloys, molybdenum and stainless steel, and said infiltrated material being copper.

3. A filter as claimed in claim 1, wherein said sintered matrix is made of chromium and said infiltrated material is iron.

4. A filter as claimed in claim 1, wherein said sintered matrix is made of one powder selected from the group consisting of chromium carbide and titanium carbide, and said infiltrated material is one selected from the group consisting of iron, nickel and nickel chromium alloys.

5. A filter as claimed in claim 1, wherein said sintered matrix is made of alumina and said infiltrated material is glass.

6. A sintered filter having uniform and straight, parallel holes with smooth inner surfaces, comprising a sintered body and a plurality of holes therethrough which are straight and parallel to each other, said sintered body consisting essentially of a sintered iron matrix having a network structure and copper infiltrated within said sintered iron matrix for reinforcing said sintered iron matrix and smoothening the wall surfaces of said holes.