INFLUENCE OF OPERATING TEMPERATURE ON SHRINK FITTING PRESSURE OF PM DIES

The reasons for shrink fitting and the procedure of assembling liners and rings for PM dies are surveyed. Some basic principles for calculat ions are presented and the importance of checking heating temperature for rings, to avoid dangerous decrements of yield strength, is emphasi sed. Available methods of calculation or the judgement of experienced personnel in PM and toolmaking companies have proved to be reliable fo r normal compaction conditions. However, the significant innovation re presented by the new method of warm compaction requires that safe runn ing conditions are ensured at temperatures that could exceed 150 degre es C. A critical analysis of the changes from room to warm compaction temperature of elastic parameters, such as elasticity moduli and Poiss on's ratio, of the materials involved has been made. It has been shown that the requirements of warm compaction do not modify the basic cons iderations and the corresponding evaluation of the heating limit for t he rings. On the basis of an assumed ratio of radial to axial pressure during warm compaction, design data for dies are established, enablin g the shrink fitting pressure required to avoid ring failure or dramat ic sliding of inserts from their shrink fitting rings during ejection to be evaluated. Using the finite element method a parameterised set o f cases has been analysed, taking into account the ratio of compact to die height. Numerical results are reported in tabular and graphical f orm to emphasise the influence of these parameters on the main variabl es involved. High speed steel, hipped PM high speed steel, and cemente d carbide insert materials have been investigated. It is found that th e distribution of time spent under maximum and minimum stress differs radically between room temperature and warm compaction, particularly f or carbide dies used for warm compaction, which are always under highe r stresses in the rest condition than when under load. Such considerat ions need to be incorporated into the design process. (C) 1996 The Ins titute of Materials.