Characterisation of Functional Pressing Die Surfaces

Doctoral thesis, 2011

The manufacture of dies and moulds is a critical aspect of many production systems since the manufacturing and try-out of new dies and moulds often is essential in determining the lead-time and quality of a new production system. In the automotive industry, many new car models are introduced each year and for each of these models, a new set of pressing dies has to be designed and manufactured. The manufacturing of pressing dies consists of several different process steps of which machining and manual polishing contribute largely to the time and cost. To be able to improve the manufacturing processes rationally, for example by optimising the machining to reduce or eliminate the subsequent manual work, an appropriate specification of the required surface quality, using a relevant parametric description of the surface, is needed. In pressing dies, the effects of manufacturing processes on functional performance are not fully understood. One of the reasons for this is the lack of effective methods for characterisation. In the work described in this thesis, research is conducted to evaluate and establish such methods. It was found that surface roughness measurement of dies, with the purpose of manufacturing process development, requires 3D data. Replication often needs to be used in these cases since dies usually are too large to bring into a lab measurement equipment. The replication techniques tested in this thesis work adequately. For quality control in production 2D measurements from a handheld instrument are good enough if an appropriate measuring strategy is used and limits for the evaluated parameters are defined. Using a multi-scale approach when analysing roughness data it may be possible to find so called functional bandwidths. With the analysis focused on the functional bandwidth the characterisation is more effective and it is easier to identify roughness parameters which correlate to the functional property or the process parameter of interest. Such a method for functional filtering of roughness data is developed and presented in the thesis. Surface texture anisotropy has been found to be important for the function of a die surface. It has also been observed that texture anisotropy can vary depending on the scale of observation. The method developed in this work to analyse and visualise texture anisotropy as a function of scale can be a helpful tool when evaluating die surfaces, especially when analysing surfaces produced with different manufacturing methods to make sure that the manufactured surface has the required texture properties in the relevant scales.