GB2089711A - Manufacturing ceramic tubes by isostatic moulding - Google Patents

Abstract

For forming a green shape of compacted powder material for subsequent firing to form a sintered ceramic tube having a closed end, an isostatic pressing apparatus has a flexible tubular envelope 24 closed at its top end by a bung 30, the envelope lying around but spaced from a mandrel 10 to form a chamber 22 defining the required shape. The material to be pressed is put into chamber 22 through an aperture 31 in the bung 30, the aperture is closed and an end closure plate 43 is put on an external chamber 12 around the envelope 24 and then hydraulic pressure is applied to the envelope through an inlet 13. Subsequent removal of the end closure plate 43 and bung 30 permits the green shape to be removed without taking out the tube 24. <IMAGE>

Description

SPECIFICATION Apparatus for manufacturing sintered ceramic tubular articles This invention relates to apparatus for manufacturing sintered ceramic tubular articles.

It is known to make sintered ceramic articles of tubular form by pressing a suitable powder material, with a binder if necessary, into the required shape so as to form what is known as a green body. This green body has then to be sintered by firing in a furnace. The present invention is concerned more particularly with the formation of a green body for a long tubular article having a closed end. It is well-known to form such articles by packing the powder material between a metal mandrel and a surrounding envelope of flexible or elastomeric material, typically a polymeric material, and then to apply pressure to the outer surface of the surrounding envelope so as thereby to compress the powdered material against the mandrel. Isostatic pressing in this way is generally desirable to obtain a good green shape of uniform characteristics.

In utilising these techniques however for repetitive quantity production of articles, the problem arises of repetitively filling the region between the envelope and mandrel with powder, isostatically pressing it and then removing the green shape. Heretofore the practice has been to use an envelope with a closable end, through which the powder can be inserted. The envelope can then be put in a chamber where it can be pressurised e.g. with hydraulic fluid, and then removed from the chamber. The end is then opened and the green shape removed. Such a technique is referred to as a wet bag process because the bag has to be removed from the hydraulic pressure fluid before the green body can be removed from inside the bag.

It is an object of the present invention to provide an improved form of apparatus for forming a green body for subsequent sintering and which facilitates repetitive production of the green shapes.

According to this invention apparatus for forming a green shape of compacted powder material for subsequent firing to form a sintered ceramic tube having a closed end comprises a mandrel which tapers from near a first end to the second end, which second end is shaped with the required internal shape of the closed end of the tube to be formed, a flexible or elastomeric tubular envelope of impermeable material fitting over the mandrel but dimensioned to be spaced away from the mandrel by an amount corresponding to the required thickness of the tube to be formed, said tubular envelope being open at the shaped end of the mandrel, a pressure chamber surrounding the mandrel and envelope, said mandrel at itsfirst end being located in said pressure chamber and the envelope, at that end, being sealed to the mandrel and to the pressure chamber, the envelope, at its other end, being arranged to seal against an internal wall of the pressure chamber, an end element having at least a portion of resilient impermeable material adapted sealingly to engage in the end of the envelope at the shaped end of the mandrel, said portion of said element having an internal shape corresponding to the required form of the green shape and said element furthermore having a filling aperture axially aligned with the axis of the mandrel, said end element furthermore being adapted sealingly to engage the end of the envelope, a closure for said aperture, and means for pressurising hydraulic fluid in said chamber so as to apply pressure isostatically to the envelope and to the part of the resilient end element within said envelope and thus to the powder.

The envelope is preferably provided with flexible end flanges for sealing engagement with the pressure chamber.

With this construction, the resilient portion of said end element is put into the envelope and the powder to be compressed is then fed into the region around the mandrel btween the mandrel and envelope and mandrel and the end element. Conveniently the mandrel is arranged vertically with said end element at the upper end and feed means are provided for feeding in by gravity. An approximate quantity may be metered into the assembly, slightly in excess of the amount required, and the excess removed by a suction device, operative to suck away any excess powder above a predetermined level in the aforementioned aperture. This aperture is then closed and pressure applied to form the green shape. The closure and the aforesaid end element may then be removed and the green shape drawn off the mandrel.Conveniently, with a vertical mandrel, the green shape is lifted by a suction gripping device applied to its upper end.

Hydraulic means may be provided for holding said closure to seal the pressure chamber. The closure may be held in sealing engagement with the pressure chamber for example by a hydraulic ram.

This construction permits the green shape to be formed without having to remove the envelope from the pressure chamber. The apparatus may readily be employed in an automated production unit in which means are provided for automatically effecting a cycle of operations including filling, pressing and removing the tube. Forthis purpose, means may be provided for moving the pressure chamber cyclically between a filling station where the top end of the assembly is below a filler, and a tube removal station. The hydraulic pressure may be applied at one or other of these stations or at an intermediate station. With such an arrangement, a plurality of pressure chambers with associated mandrels and envelopes may be arranged for passing cyclically through the various stations.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagrammaic sectional elevation of one form of apparatus for forming a tubular green shape; and Figure 2 shows, to a larger scale, the closure arrangement at the top end of the envelope and pressure vessel of Figure 1.

Referring to the drawings, there is shown apparatus for forming a green shape in the form of a tube closed at one end and which, after sintering, will form a polycrystalline ceramic electrolyte element of beta-alumina. The green shape is made by pressing a mixture of the appropriate powdered precursor materials, e.g. sodium aluminate, alumina and magnesium and/or lithium oxide, into the required shape. The green shape is formed from this mixed powder material by isostatic pressing using the apparatus shown in the accompanying drawings.

in the drawings there is shown a steel mandrel 10 which is of substantially cylindrical form but tapering slightly along the whole of its length. The mandrel is upright and, at its lower end it is located in a cylindrical element 11 which in turn forms the lower closure of a pressure chamber constituted by a cylindrical pressure vessel 12 with the lower closure 11 and an upper closure to be described later. The tapering of the mandrel is such that the diameter at the upper end is slightly less than the diameter at the lower end. The tapering is to enable the green shape, after being formed, to be removable from the mandrel by lifting off the upper end of the mandrel.

Afluid inlet 13 for hydraulic fluid extends through the wall of the vessel 12 for admission of hydraulic fluid from a pressure-fluid source 15 having control means 16 for controlling the application and release of pressure. There is a further passage 18 through the wall of the vessel 12 to form an air bleed outlet, which outlet is normally closed by means shown diagrammatically as valve 19.

The mandrel 10 has a domed upper end 20 and this mandrel defines the required shape of the inner surface of the green body to be formed. The cavity in which this green body is formed is shown at 22. The outer wall of this cavity 22 is constituted, along the greater part of its length, by a polyurethane tube 24 which surrounds the mandrel leaving the required gap for the cavity 22. At its lower end the tube 24 has a flange 25 fitting closely against the mandrel 10, the pressure vessel 12 and closure 11.

At the upper end of the assembly, the required dome-shaped outer surface of the green body is formed by an internal surface 29 of a polyurethane bung 30 having an aperture 31 extending in the axial directon. This polyurethane bung 30 extends into an enlarged diameter portion 33 of the envelope 24. The polyurethane bung 30 is secured to one end of a metal element 40 of cylindrical form having, at its other end, a radially extending flange 41 seating in sealing engagement on a seat portion 42 on the upper end of the vessel 12. An end closure plate 43 with a spigot 44 fitting within a bore 45 through the element 40 carries, on the end of the spigot, a bung 46 fitting within the aforementioned aperture 31 in the bung 30 for closing the pressure chamber.

Closure is maintained during an operating cycle by hydraulic ram means indicated diagrammatically at 47,48. The upper ram 48 carries a platen (not shown) which, during normal operation of the device in producing green bodies, remains stationary, applying the required resistive force to the upper end of the closure for the pressure chamber. The lower ram 47 applies the required loading to the bottom end of the assembly. A further ram (not shown) is provided for raising and ejecting the mandrel for maintenance purposes. At the upper end of the envelope 24, the enlarged diameter portion 33 constitutes a flange with a shaped lower surface; this flange is supported by a back-up ring 49 and locating bush 50 and seals against the internal surface of the pressure vessel 12.

At the start of moulding operation, the closure plate 43 and its attached bung 46 is removed and a metered quantity of powder is fed from a hopper 51 through the aforementioned aperture 31 via metering means 52 and a pivoted vibrating feed chute 53 which can be swung in position over the aperture.

The powder is free-flowing and fills the cavity between the polyurethane bag 24 and the mandrel 10 and between the bung 30 and the mandrel 10 with a slight excess in the aperture 31. This excess is removed by means of a movable suction tube 58 which can be swung over and lowered into the aperture 31 and which removes the excess powder down to the level of the bottom of the suction tube.

The mechanism for lowering the tube 58 is arranged so that the lower end comes to the correct level. The top closure plate 43 is put in position and pressure applied by ram 48, to hold the pressure chamber shut. Hydraulic pressure is applied through the fluid inlet 13 and this is applied isostaticallyto the powder via the polyurethane tube 24 which applies pressure directly onto the powder onto the greater part of the length of the mandrel 10 and applies pressure also onto the polyurethane bung 30 and thus onto the powder around the domed end 20 ofthe mandrel 10.

After the green shape has thus been formed, the top closure plate 43 and the element 40 with the bung 30 are removed and the green body is lifted out by means of a lifting device 60 having a suction cup 61 adapted to grip the upper end of the green shape.

Alternatively it would be possible to use a gripping device which extends around the top end of the tube with an inflatable element which can be inflated to bear against the periphery of the green shape and thereby enabling this shape to be lifted.

It is convenient to make the pressure chamber 12 movable between a first filling station, a second station where the chamber is ciamped shut and a station where the green shape is removed. For this purpose the pressure chamber may be moved around a cyclic track (not shown) and it would be possible to have a number of pressure chambers moved in succession around the same track. Using such a cyclic track, the station where the green shape is removed may be the first station, i.e. the filling station.

Claims (14)

1. Apparatus for forming a green shape of compacted powder material for subsequent firing to form a sintered ceramic tube having a closed end, which apparatus comprises a mandrel which tapers from near a first end to the second end, which second end is shaped with the required internal shape of the closed end of the tube to be formed, a flexible or elastomeric tubular envelope of impermeable material fitting over the mandrel but dimensioned to be spaced away from the mandrel by an amount corresponding to the required thickness of the tube to be formed, said tubular envelope being open at the shaped end of the mandrel, a pressure chamber surrounding the mandrel and envelope, said mandrel at its first end being located in said pressure chamber and the envelope, at that end, being sealed to the mandrel and to the pressure chamber, said envelope, at its other end, being arranged to seal against an internal wall of the pressure chamber, an end element having at least a portion of resilient impermeable material adapted sealingly to engage in the end of the envelope atthe shaped end of the mandrel, said portion of said element having an internal shape corresponding to the required form of the green shape and said element furthermore having a filling aperture axially aligned with the axis of the mandrel, said end element furthermore being adapted sealinglyto engage the end of the envelope, a closure for said aperture, and means for pressurising hydraulic fluid in said chamber so as to apply pressure isostatically to the envelope and to the part of the resilient end element within said envelope and thus to the powder.

2. Apparatus as claimed in claim 1 wherein the envelope has flexible end flanges for sealing en gagement with the pressure chamber.

3. Apparatus as claimed in either claim 1 or claim 2 wherein the mandrel is arranged vertically with the closure at the upper end and wherein feed means are provided for feeding the powder in by gravity.

4. Apparatus as claimed in any of the preceding claims wherein there is provided a suction device, operative to suck away any excess powder above a predetermined level in said aperture.

5. Apparatus as claimed in any of the preceding claims and having a vertical mandrel, wherein a suction gripping device is provided for lifting the green shape off the mandrel.

6. Apparatus as claimed in any of the preceding claims and having hydraulic means for holding said closure in engagement with the pressure chamber.

7. Apparatus as claimed in claim 6 wherein the pressure chamber is a vessel of substantially cylindrical form and wherein said end closure seats in sealing engagement with one end of said vessel.

8. Apparatus as claimed in any of the preceding claims wherein said end element has a metal closure portion for engaging the pressure chamber, the metal closure portion carrying a bung constituting said portion of resilient material which sealingly engages said envelope.

9. Apparatus as claimed in any of the preceding claims wherein means are provided for automatically effecting a cycle of operations including filling, pressing and removing the tube.

10. Apparatus as claimed in claim 9 wherein means are provided for moving the pressure chamber cyclically between a filling station where the top end of the assembly is below a filler, and a tube removal station.

11. Apparatus as claimed in claim 10 wherein means are provided for applying the hydraulic pressure at the filling station or at the tube removal station.

12. Apparatus as claimed in claim 10 wherein means are provided for applying the hydraulic pressure at an intermediate station between the filling station and the tube removal station.

13. Apparatus as claimed in any of claims 9 to 12 and having a plurality of pressure chambers with associated mandrels and envelopes arranged for passing cyclically through the various stations.

14. Apparatus for forming a green shape of compacted powder material substantially as hereinbefore described with reference to the accompanying drawings.