In connection with isostatic pressing into a fully dense body starting from a powdered material, a porous body is preformed by injection moulding, die pressing, slip casting or some other suitable forming method. Alternatively, the powder is filled into a container which, after the isostatic pressing, will give a body of the desired shape and size. However, the present invention relates to a method in the isostatic pressing of a preformed porous body. The above-mentioned preforming methods may be used in connection with pseudo-isostatic pressing.
To obtain an essentially dense body during isostatic pressing of a porous body, the body is enclosed in a coherent dense casing or enclosure to prevent the pressure medium used from penetrating into the pore system of the body. Usually, a glass or a material forming glass is used, which is applied in powdered state. Prior to compaction by isostatic pressing, the porous body and the glass powder are heated, the powdered glass or glass-forming material thus forming a dense coherent casing around the porous body. Problems will arise if glass from the casing penetrates, to varying extent, into the porous body and/or reacts with superficially located portions of the porous body in connection with the porous body being compacted or while the porous body is compacted.
During pseudo-isostatic pressing, a mechanical pressure is applied by means of one or more punches onto a powdered or liquid medium which transfers the press force from the punches into an essentially isostatic pressure acting on the preformed body.
From Swedish patent specification SE-B-456 651, it is known to apply one or more layers comprising powdered material onto the porous body. The layers are applied to the external surfaces of the porous body that are to be exposed to glass. In the following these surfaces are referred to as external surfaces. External surfaces in this context are all surfaces that define the shape of the body, i.e. also surfaces within open cavities, depressions and larger holes but not internal surfaces such as surfaces within the pore system. The layers are primarily applied in order to prevent the penetration of glass into the pores and/or prevent reactions between the glass and the porous body, intermediate layers. But layers are also applied for other purposes, e.g. to simplify the removal of the glass casing from the compacted, sintered, essentially fully dense body, supplementary layers.
It is important that the powder in an intermediate layer, possibly after certain reaction with the surrounding glass, at the pressing temperature functions as a dense, fully-covering barrier against the penetration of glass into the preformed body.
The application of an intermediate layer by spraying may be made to function; however, the material losses are considerable and therefore the method is too expensive in production.
However, the application of an intermediate layer by dipping, which is a preferred method of application from the point of view of production, often results in problems with bubbling and blistering, which entails holes in the intermediate layer. The bubbling or blistering is due to the pore system and is accentuated by coarse or unevenly distributed pores. Such a pore system is obtained, among other things, when using spray-dried powder. A particularly unfavourable pore system is obtained in material comprising whiskers, other needle- or disc-shaped single crystals or fibres. Attempts to eliminate these problems by conventional measures, such as closing surface pores with organic material, dipping under vacuum or modifications of the dipping process, such as cooled liquid and/or oscillating dipping, have only resulted in marginal improvements.