Hot pressing

Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat and pressure. Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat and pressure. Hot pressing is mainly used to fabricate hard and brittle materials. One large use is in the consolidation of diamond-metal composite cutting tools and technical ceramics. The densification works through particle rearrangement and plastic flow at the particle contacts. The loose powder or the pre-compacted part is in most of the cases filled to a graphite mould that allows induction or resistance heating up to temperatures of typically 2,400 °C (4,350 °F). Pressures of up to 50 MPa (7,300 psi) can be applied. Other great use is in the pressing of different types of polymers. Within hot pressing technology, three distinctly different types of heating can be found in use: induction heating, indirect resistance heating and field assisted sintering technique (FAST) / direct hot pressing. In this process heat is produced within the mould when it is subjected to a high frequency electromagnetic field, generated by using an induction coil coupled to an electronic generator. The mold is made out of graphite or steel, and pressure is applied by one or two cylinders onto the punches. The mold is positioned within the induction coil. The advantage here is that the pressure and the inductive power are completely independent. Even powders with a liquid phase are amenable to this process and low pressures are possible, too. Among the disadvantages are the expense of a high-frequency generator and the need for proper alignment. If the mold is placed off centre, the heat distribution is uneven. But the main disadvantage is the dependence of the process on good inductive coupling and thermal conductivity of the mold. The magnetic field can penetrate the mold only 0.5mm to 3mm. From there on, the heat has to be 'transported' into the mold by the thermal conductivity of the mould material. Uniform heating is much more difficult if the air gap between the mold and the inductive coil is not the same all along the mould profile. Another potential problem is heating rate. Too high a heat up rate will result in high temperature differences between the surface and core that can destroy the mold. With indirect resistance heating technology, the mold is placed in a heating chamber. The chamber is heated by graphite heating elements. These elements are heated by electric current. The heat is then transferred into the mold by convection. As the electrical energy heats the heating elements that then heat the mold in a secondary manner, the process is called indirect resistance heating.