New approaches to thermal tool performance, cooling and machining strategy: the strongly correlated triple that determines the cost effectiveness of the process

Abstract

The mechanism of the cooling, including its intensity and distribution, of hot stamped components is the most important factor that determines not only the cost effectiveness of the process, but also the formability of the new ultra-high strength sheets and of more complex component geometries. The parameters that determine the cooling mechanism are: contact condition between the tool and stamped parts, initial tool temperature, thermal properties of the tool material and hence the tool, and the cooling system configuration and performance. Once the blank-tool contact conditions have been optimized in terms of applied pressures, contact surface adjustments, roughness and eventually interstitial material, the remaining most important parameters to be optimized are the tool material’s thermal properties, cooling strategy and initial tool temperature. These parameters are strongly related as the heat must be extracted through the tool material before it is evacuated through the cooling channels, and because the cooling rate determines the initial tool temperature. The distance and the diameters of the cooling lines from the surface also depend on the tool material strength. The thermal properties of tool steels may decrease when the strength of the same increase. However, the cooling channels cannot be brought too close to the active tool surface because of the risk of premature tool cracking, nor can the cooling lines be adapted to conform with the tool cavity due to conventional machining limitations. This work explains the increase of the cooling capacity of tools by using the new cost effective high thermal conductivity tool materials FASTCOOL-50 and HTCS-230, which differ from other high thermal conductivity grades by easy machining and hardening processes, as well as a homogeneous distribution of material properties along the different tool sections. Also, the usage of the high thermal conductivity tool steel powder for cavity conformal cooling channels, especially for complex geometries, will be presented by semi industrial pilot tests.