Centrifugal casting of ceramics. Main parameters and the regularities of the process

Abstract

Until the present ime, slip casting in gypsum molds has been the preferred method for shaping quartz ceramics [I, 2].When using highly concentrated binder suspensions (HCBS) that are based on quartz glass or fused quartz, one can attain low porosity levels in the castings [3,4]. This is extremely important from the standpoint of technology (in particular, during the production of unfired materials [4]). However, this method has significant shortcomings such as slow rate of shaping and rapid wear of the gypsum molds.In view of this, the method of centrifugal forming of castings from the same HCBS that are often used for slip casting is of considerable practical interest. This method makes it possible to eliminate the aforementioned shortcomings of slip casting.It is mainly used for shaping solids of revolution such as the nozzles of the steel casting ladles (downtubes), heat insulating pipes, and roller casings of the "Tandem" type furnaces. However, when the process is perfected, products having inserts (lateral parts) can also be molded.As concerns the shaping of ceramics and refractories, the process of centrifugal casting has been used rarely up to the present time and, therefore, virtually no studies have been carried out on these aspects. In view of this, we examined the general regularities of the process of centrifugal casting and the effect of the main technological factors on it.The highly concentrated (C V = 0.70-0.75) fused quartz suspensions that are used foe obtaining unfired quartz ceramics [4,5] formed the basic experimental materials. Experimental specimens were obtained in a metallic (porefree) mold using the previously described installation [5]. The experiments conducted using HCBS of the mullite, chamotte, and other systems showed that the main regularities established during the study of the HCBS of fused quartz are generally valid for the centrifugal tasting process of all the investigated systems.Just as in the process of centrifugal forming of ferro-concretes [6,7], when a mold containing a uniformly distributed suspension is rotated around a fixed axis at a predetermined speed, a centrifugal pressure Pc is developed in the molding system, and under the action of this compressive pressure the dispersing medium is squeezed out from the suspension along with the finest particles suspended in it. During this process, the larger particles of the solid phase are simultaneously drawn closer to each other.The dispersing medium of HCBS receives the centrifugal pressure directly. In view of this, the excess hydrostatic pressure increases and leads to filtration of the liquid and simultaneous densification of the solid phase of HCBS. The nmnber of revolutions of the mold per unit time n (or the circumferential or the linear velocity ~ derived from it) forms the most important parameter of the process.The compressive (compacting) pressure is distributed nonuniformly across the wall thickness of the product (just as the hydrostatic pressure). Its maxim~n and the minimum values are obtained at the inte...