Influence of Operating Temperature on Shrink Fitting Pressure of PM Dies

Abstract

The reasons for shrink fitting and the procedure of assembling liners and rings for PM dies are surveyed. Some basic principles for calculations are presented and the importance of checking heating temperature for rings, to avoid dangerous decrements of yield strength, is emphasised. Available methods of calculation or the judgement of experienced personnel in PM and toolmaking companies have proved to be reliable for normal compaction conditions. However, the significant innovation represented by the new method of warm compaction requires that safe running conditions are ensured at temperatures that could exceed 150°C. A critical analysis of the changes from room to warm compaction temperature of elastic parameters, such as elasticity moduli and Poisson's ratio, of the materials involved has been made. It has been shown that the requirements of warm compaction do not modify the basic considerations and the corresponding evaluation of the heating limit for the rings. On the basis of an assumed ratio of radial to axial pressure during warm compaction, design data for dies are established, enabling the shrink fitting pressure required to avoid ring failure or dramatic sliding of inserts from their shrink fitting rings during ejection to be evaluated. Using the finite element method a parameterised set of cases has been analysed, taking into account the ratio of compact to die height. Numerical results are reported in tabular and graphical form to emphasise the influence of these parameters on the main variables involved. High speed steel, hipped PM high speed steel, and cemented carbide insert materials have been investigated. It is found that the distribution of time spent under maximum and minimum stress differs radically between room temperature and warm compaction, particularly for carbide dies used for warm compaction, which are always under higher stresses in the rest condition than when under load. Such considerations need to be incorporated into the design process. PM/0738