IE912062A1

IE912062A1 - Ceramic moulding with hollow chambers

Info

Publication number: IE912062A1
Application number: IE206291A
Authority: IE
Original assignee: Hoechst Ceram Tec Ag
Priority date: 1990-06-18
Filing date: 1991-06-17
Publication date: 1991-12-18
Also published as: DE4029651A1; PT98005A; DK0462473T3; ES2047362T3; JPH06270114A; EP0462473B1; EP0462473A3; EP0462473A2; DE59100525D1; DE4029651C2

Abstract

To produce a hollow ceramic product, a green ceramic mass is compressed to form a green moulded product and is sintered. In this process, at least two green moulded products are produced from the same ceramic mass, preferably rotationally symmetrically, with projections. After fitting together, the projections of the one moulded product enclose the projections of the other moulded product. The pressing density of the enclosing moulded product is less than that of the enclosed moulded product.

Description

The present process relates to the production of ceramic 5 hollow bodies of complicated shape from a green molding.

According to known fabrication processes, ceramic hollow bodies are produced by slip-casting in gypsum molds, and these are fired after drying. Although a defined external shape can be produced in this way, the internal shape is largely fixed because of the almost constant wall thickness .

It was therefore the object to provide a process for producing ceramic hollow bodies, in which the internal surface and the external surface can be fixed almost independently of one another.

A process for producing a ceramic hollow body from a green ceramic mass by pressing to give a green molding and by sintering has now been found, wherein at least two green ceramic moldings, both of which have projections, are produced from the same ceramic mass, the projections of one molding enclosing the projections of the other molding after the parts have been fitted together, a pressed density less than that of the enclosed molding is selected for the enclosing molding, and the green parts are fitted into each other and further processed.

The term further processing comprises drying and sintering.

The process according to the invention is applicable to all ceramic masses having contents of ceramic binders which are susceptible to a shrinkage during the sintering step. Parts composed of alumina are preferred. - 2 The differences in the green density of the green parts fitted into each other depend on the apparent density and on the shrinkage behavior of the ceramic mass used. These differences must be defined from case to case, specific to the mass. In the process according to the invention, the pressed moldings, which are placed on top of each other or fitted into each other, sinter together in the firing process to give a monolithic body.

Because of the low green density of the enclosing outer 10 shape, a defined pretension, which compresses the parts fitted into each other and leads to the formation of a monolithic body, is generated in the fired sintered product.

The individual parts to be assembled are fitted into each 15 other without using a sinter aid, for example a frit. The bodies themselves can, however, contain conventional sinter aids, for example green alumina bodies can contain magnesium oxide and/or zirconium oxide and green silicon nitride bodies can also contain Y2O3.

According to an embodiment of the invention, the green moldings produced and fitted into each other have a rotationally symmetrical shape. According to another embodiment, the green moldings have the shape of a polygon, in particular that of a regular polygon. In this respect it has been found that the parts are all the more easily bonded to give monoliths, the more the assembled shape resembles a circle. Preferably, the projections of the mutually enclosing bodies are free of sections of concave shape.

Closed cavities can also be formed by the process according to the invention. The gases escape when the sinter firing starts. In this case, slow heating up to the sintering temperature and/or the provision of a small orifice are advantageous. - 3 The constructional elaboration of the invention is explained in more detail by the figures. Figure la shows green compacts 1 and 3. The part 1 is in the form of a can and has the rim 2. The part 3 is in the form of a lid and has the internal projection 4. After the parts 1 and have been fitted together, the internal lid part 4 is enclosed by the external rim 2. The green density of the part 1 is lower than that of part 3. After the parts have been fitted together and sintered, the fired molding 5 with the hollow chamber 6 is produced (Figure lb).

In Figures 1 and 3 to 7, the enclosing moldings have an annular projection. In Figure 2, the projections of the two moldings fit together like cones and hollow cones.

If more than two parts are fitted successively into each other via projections to form the hollow green compact, as shown in Figures 3 to 5, it is then necessary for at least one part to have projections on two sides. It is advantageous to design these projections in such a way that they both enclose the projections of their neighbor20 ing parts or that both are enclosed by these. This applies, for example, to the middle part when the green compact of Figure 4 is built up.

In the process according to the invention, the internal and external shapes of the individual parts can be produced within narrow tolerances, corresponding to the rules of pressing technology in accordance with the particular functions of the parts. The individual parts can also be further machined in the green state.

EP 232,048 has already disclosed a process for fitting a closing plug to a translucent ceramic pipe. For this purpose, a green pipe compact is first produced from ceramic material, a closing plug of ceramic material of the same type is then produced, both parts having a cylindrical surface, and both parts are fitted into each other and fired. The sintering shrinkage of the material - 4 in the form of a pipe should be greater than that of the material for the end closure plug. It is not stated, however, how this different sintering shrinkage is obtained.

Example A pulverulent alumina-based ceramic pressing mass gives, at a defined ram pressure, a green centering part (3) according to Figure 1, having a diameter of 24 mm at the narrowest point and a green density of 2.4755 g/cm3. After sintering, the diameter is 20.00 mm, i.e. the shrinkage is 20% (relative to the sintered body).

To ensure that the parts 1 and 3 weld in sinter firing, part 3 must - if fired in isolation - lead to a smaller internal diameter. A difference of 0.01 mm is sufficient.

Such a part 3 must have a shrinkage of (24 - 19.99):19.99 = 0.2006 or 20.06%.

In the range of 19 - 21% shrinkage, it was observed on compacts pressed from the same pressing mass that the density of the green compact must decrease by 0.07 g/cm3 for each additional percent of shrinkage. It follows that the green density of one part must be 2.4755 - (0.06 x 0.07) = 2.4713 (g/cm3) to ensure that the two parts will weld.

This means that, with a difference of 0.0042 g/cm3 in the green density, the desired pretension of 0.01 mm can be achieved at a nominal dimension of 20 mm.

To accomplish the lower density, either the pressing force can be reduced in a controlled manner, which is expensive in engineering terms, or the quantity of pressing mass filled into the hollow mold can be reduced correspondingly at constant piston stroke. An additional check of the density by measuring the raw dimensions (length of the cylinder) and the weight is possible here. - 5 Analogously thereto, for other ceramic masses which have a known green density and a known dependence of the shrinkage on the green density, the pretension achieved on sinter firing can be calculated and adjusted in accordance with the specific requirements.

Claims

1. it Claims: A process for producing a ceramic hollow body from a green ceramic mass by pressing to give a green molding and by sintering, wherein at least two green ceramic moldings, both of which have projections, are produced from the same ceramic mass, and the projections of one molding enclose the projections of the other molding after they have been fitted together, the pressed density of the enclosing molding is less than that of the enclosed molding, and the green parts are fitted into each other and further processed. The process as claimed in claim 1, wherein the green moldings produced are rotationally symmetrical. The process as claimed in claim 1 or 2, wherein the projections of the two moldings enclose one another like cones and hollow cones. The process as claimed in claim 2, wherein the enclosing molding has an annular projection. A process according to claim 1 for producing a ceramic hollow body, substantially as hereinbefore described and exemplified. A ceramic hollow body whenever produced by a process claimed in a preceding claim.