1. Field to Which the Invention Relates
The invention relates to a method for the production of large-size densely sintered ceramic plates with a low wall thickness in the case of which a ceramic composition is first extruded as billets by means of an extruder to form plates which, after they have been cut to the desired format, are dried and fired.
In connection with the present invention the term ceramic plate is to be taken more particularly to mean a ceramic clay plate.
2. The Prior Art
As is well-known it is extremely difficult to produce ceramic clay sintered plates with a size of 0.5 .times. 0.5 meters to 2 .times. 2 meters with a wall thickness of 7 to 12 mm without warping.
The difficlties occuring in the production of thin-walled, large format plates and sheet structures are due substantially to the so-called "memory" or "recollection" of the compositions used. These phenomena occur owing to the loading of the composition during shaping and/or drying and make themselves felt more especially during firing. Thus the ceramic composition "remembers" the forced alignment of the platlet-like particles of clay during shaping and the forced or strained movements of the individual particles and has the tendency to continue this movement or to cancel it out. More particularly in the case of densely sintered thin-walled ceramic clay plates of large format this leads to undesired warping or so-called "winging."
In accordance with the German patent specification no. 1,143,429 an attempt is made to solve this problem in the case of a method for the continuous production of large area cladding or facing plates of ceramic material, which are provided with undercut members on the side opposite to the front side, using an extruder by extruding from the latter a strip having a somewhat lesser breadth and greater thickness than the cross-section of the eventual plate. This strip is subjected to a rolling operation while being continuously moved forward so as to be rolled to the prescribed breadth and thickness. Then on one of its side surfaces it is moved past cutting machining tools which undercut it and it is then subjected to a further rolling operation to shape it. The material is then cut to plates and the latter, after being dried, are fired while suspended.
Since this method did, however, not lead to complete success an attempt was made to improve it. In the German patent specification no. 1,708,877 there is the description of a method for the production of large area thin plates of ceramic material by extruding of a plastic ceramic composition from the die of an extruder. The strand is then rolled out to the desired final thickness. The strand, after leaving the die, is cut up in plates or moldings with a length corresponding to the breadth of the eventual product. These plates or moldings are rolled out in a direction perpendicular to the direction of emergence of the strand to the eventual or final thickness. It has been possible with this method to cancel out the orientation of the clay particles, which occurs on extrusion of the strand from the die of the extruder, partly by the step of rolling out the plates or moldings, produced from the strand after it has emerged from the die, in a direction perpendicular to the direction of emergence from the die of the strand, such rolling being to the final thickness.
In this case as well the plates are fired while hanging, something which is referred to more particularly in the German patent specification no. 1,571,475, in the case of which in a method for avoiding warping due to shaping and/or drying, on firing thin ceramic plates, which are dried after shaping and then fired, two respective plates are fired while suspended with surfaces adjacent to each other which are the same as regards the drying and/or shaping operation and, in the case of a preferred form of the method, the plates are mutually sealed or locked at the upper and lower ends in relation to each other.
Although the above mentioned methods of the prior art made it possible to come close to the aim of producing large area ceramic plates which are comparatively free of signs of warp, it was nevertheless found that a high percentage of the plates still suffered from slight warp despite taking all precautions. This warping made the plates unsuitable for cladding walls or floors so that they were rejected by customers owing to their excessive dimensional tolerances as regards flatness and owing to "winging." It has been found that these shortcomings made the profitability of production questionable. The previously described methods of attaining the basic aim, that is to say producing clay ceramic plates in large formats without warp are neither completely satisfactory in themselves nor do they make it possible, even when combined with each other, to avoid a comparatively high quota of useless plates.