The manufacturing of plastic components, like a majority of other mass produced discrete parts, require well designed dies and moulds. Complex geometries and increasing demands on final surface appearance, which is strongly connected to the quality impression, are pushing for high demands on mould-makers and polishers as well as steel producers. Moulds for production of shiny plastic components require tool steels with low defect levels to achieve highly gloss and very smooth mould surfaces (roughness levels in the nm-range).

It is the ability to achieve those mirror-like surfaces, the polishability of tool steels, that is the central part in this study. To increase the understanding of how material properties and different surface preparation techniques impact the polishability, a selection of high gloss polished tool steel qualities were characterised using non-contact 3D-surface texture analysis.

A first step towards a grading system of the polishablility was made based on a classification of surface defects detected on included steel samples. 3D surface parameters based on interferometer measurements seemed to be useful for a characterisation, even though further studies (involving different filters and segmentation methods) are needed to find less and more precise parameter values to grade tool steel qualities.

Future work will include analysis of surface measurements of test moulds and plastic parts, as well as studies of how quantitative parameters can be linked to qualitative estimations in order to better understand how surface features on the mould are transferred into the surface of plastic components.

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.