On Polishability of Tool Steels

Doctoral thesis, 2013

One of the world’s fastest growing industries is the plastic industry. Today’s ever increasing demands of high quality products, shorter lead times and reduced costs push development and research forwards. Moulds for plastic injection moulding need to have a functional surface to meet demands on demoulding and wear properties, but also to produce the required final surface quality, which for ‘standard mould qualities’ of high gloss applications means nearly defect free, shiny and smooth mould surfaces with roughness levels in the nm-range. The aim of this thesis was to develop a metrology framework to quantitatively characterise these mould surfaces in order to gain better understanding of which defect structures are critical at injection moulding, and how these are correlated to material properties and operating conditions in surface preparation of tool steels. In practice this means to capture surface features of some few nm in height/depth up to some hundreds of microns in lateral dimension within insert areas of cm2 and larger. Experiments combining polishers’ experience with steel producers’ as well as non-contact areal texture examinations of surface topography were performed to overcome and link practical skills to academic ones. Based on areal surface metrology, defect classification and image analysis based surface characterisation, an evaluation procedure for polished tool steel surfaces was developed, initially tested and verified. The suggested method involves descriptions of relevant defect structures and acceptance levels for high gloss polished tool steels in the form of numerical parameter values based on interferometric measurements. It was also concluded that the cleanness of the steels was less important as long as it was kept within reasonable levels; the surface preparation strategy is a major factor influencing the mould surface quality e.g. it was found that a ‘several-step-strategy’ was favourable to avoid defect structures; not all ‘mirror-like’ mould surfaces had desirable topographies for injection moulding, therefore a well-defined mould surface assessment with numerical values describing mould surface quality is necessary to secure effective mould surfaces.