Quasi-isostatic pressing?a promising technology in the production of refractory components

Abstract

Benefits of the quasi-static pressing technique in the production of ceramic and refractory components are discussed. This technique, in which the pressure-transmitting medium is a solid elastic material, is a modified version of the isostatic pressing and is believed to be the most efficient molding method for ceramics technology.The quasi-isostatic pressing (QISP) technique has been developed as an updating of the dry hydrostatic pressing method which, as is known, just provides a biaxial pressing effect. In sufficient detail, the hydrostatic pressing method has been dealt with in the ceramic insulator industry 2 . A pressing technology for a range of insulators was developed; a system of movable limit stops was designed, which made it possible to fabricate ribbed insulators using a hydrostatic pressing technique.Hydrostatic pressing technique provides a means of making high-strength engineering components from plastic, lowplastic, and nonplastic powdered materials, which, because of their shape and size, cannot be fabricated by other technologies. In principle, this pressing technique makes it possible to manufacture components of any size. Still, our 15-years expertise in the field has shown that this method, for all its benefits, is not devoid of grave drawbacks that pose problems for its large-scale application, such as: it requires significant capital costs and large working areas, expensive hydrostats (available only from specialized manufacturers), and high energy supply. The productivity of the process is rather low. Furthermore, durability of the hydrostat's rubber shells is likewise low. Hydrostats because of their massiveness provide no means of using vibratory techniques to properly mold the powdered material, whereas the use of a vibratory process outside the isostat sharply reduces productivity. Furthermore, pressed components with high vacuum density or thick-walled components with a low density may require additional mechanical finish (trimming at the edges), which will necessitate the use of additional facilities, working areas, and skilled personnel.Therefore an alternative production process -quasi-isostatic -has been developed in which the pressure-transmitting medium was an elastic polyurethane material which simultaneously served as a part of the molding die for shaping the inner or outer cavity of the molded component. Thickwalled components can be treated by a compression technique in which pressure can be applied on the inside and the outside simultaneously. The molding process does not require the use of costly isostats equipped with hydraulic or mechanical presses. The QISP provides volumetric (three-dimensional) compression and thus combines the advantages of static (one-dimensional) and hydrostatic (two-dimensional) compression techniques. Volumetric compression method provides a uniform density in the pressed component and, consequently, its superior physicomechanical properties. Furthermore, the uniform distribution of internal stresses provides a uniform shrinkage during sinterin...

Design specifics of quasi-isostatic molds

“…In developing quasi-isostatic molding, molds have been designed for a wide range of ceramic products: saggers (round, rectangular, or square), shells made of chamotte and alundum mixtures, smooth rings of diameter 86, 180, 190, 250, and 370 mm, 2 and up to 250 mm high, finned rings of diameter 156 mm from a corundum material, milling balls of diameters 20, 30, 40, 50, and 60 mm, pot condensers of height 170 and 120 mm, several sizes of tube of length 1.5 mm, including tubes with wall thickness 1.5 mm, several types of plates (up to 370´370´65 mm), crucibles [1,2], cylindrical and rectangular briquettes, rods, etc. The molds have been designed for domestic hydraulic and mechanical presses and for automatic molding machines.…”