Some properties of elastic press components in powder material quasiisostatic pressing

Abstract

An elastic press buffer is a basic element in quasiisostatic pressing, so a major aspect of developing such pressing is to choose a suitable constructional material for the elastic pressing elements to provide the necessary conditions for consolidating a powder and providing reliable operation of the pressing mold in the industrial production of pressed items [1, 2]. Various elastomers have been tested: gelatin, some resins, technical rubber, and natural and synthetic rubbers. The gelatin pressing elements were made by pressing a specially prepared solution in metal molds of appropriate configuration. However, the low hardness made it undesirable. Buffers have also been made of technical rubber with medium hardness, which is usually supplied as sheets of thickness 30 – 60 mm, and which was used by cementing together previously cut blanks and then processing with a cutter or abrasive wheel. The preparation labor and inhomogeneity of some compound press buffers has forced one to seek other materials. Some polyurethane elastomers have been tested in molds for pressing ceramic materials based on Al2O3, which have been found to be promising [3, 4]. Table 1 gives the basic physicomechanical characteristics of polyurethane elastomers [5]. These tests showed that the best material is SKU-7l material prepared by casting. This elastomer if prepared by other technologies such as pressing shows a deterioration in the parameters of the pressed components. The SKU-7l material provides components equally dense throughout the volume, and with more even and less rough surfaces than when other elastomers are used. The SKU-7l has exceptionally high wear resistance and capacity to work at pressures exceeding 1000 MPa. Its strength is 6 – 8 times higher than that of rubber, and the wear resistance exceeds that of St3 steel by a factor 3, and of epoxide resin by a factor 60. Most ordinary elastomers lose their properties on attaining a Shore hardness of A85. Out of all known elastomers, only polyurethane ones preserve their elasticity in a wide range of Shore hardnesses ( A95). The unique combination of physicomechanical properties means that they can be used for pressing powders. They as constructional materials not only replace metals but also are better than the latter as regards working properties. Press buffers were made of SKU-7l polyurethane by firms that specialized in making polyurethane components by pouring a solution into aluminum molds we supplied and subsequent heat treatment. The resulting buffers did not require mechanical processing, but with that technology it was difficult to provide tools for the pouring, so it was decided to be economically justified to make buffers of appropriate shape by the mechanical processing of polyurethane blanks provided by the specialized firms. Much of the research was directed to methods and modes of mechanical processing. In the mechanical treatment it must be remembered that polyurethane is an elastic material that has a tendency to deflect away from the cutting tool, so the latter should be very sharp. The elasticity means that the clamps holding an item for treatment should not distort it too much. It is found that in any mechanical treatment operations one needs a high speed for the cutting tool and a slow feed for the workpiece to avoid overheating. Water-soluble oils and emulsions can be used in cutting to prevent excessive heating and are usually recommended for the softer polyurethanes. Satisfactory results have been obtained with the following parameters: cutting tool speed Refractories and Industrial Ceramics Vol. 50, No. 3, 2009