Polishability of tool steels - Characterisation of high gloss polished tool steels

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

Polishability of tool steels - Characterisation of high gloss polished tool steels

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

Evaluation of a robot assisted polishing equipment

This study is based on several tests of a robot assisted polishing equipment, the Strecon RAP-200, which was recently developed by Strecon A/S in purpose to polish punches and dies for metal forming. The main part of the study was a 4-factor 2-level design of experiment, where the influence of pressure, pulse, repetitions and linear speed on the final surfaces were studied with help of both a mechanical stylus and an interferometer. The number of repetitions seemed to be the main factor affecting the final surface quality, but also the rotation speed seemed to affect the surface roughness. Further evaluations will be performed in the forthcoming three years

High gloss polishing of tool steels – step by step

The surface preparation of moulds for high gloss applications, e.g. tools for injection moulding of plastic parts, is still a relatively unknown area. Most knowledge is accumulated by individual polishers with long experience in the field. Literature covering the polishing process/mechanisms is rare and it becomes harder and harder to recruit new skilled polishers. Today it is also a matter of time, and cost; how long time is needed to prepare a new mould? How many steps are needed? Which tools? Is it possible to replace the manual polishers by robots? This article is part of a study where equivalent steel samples were prepared in different polishing shops to investigate the surface quality achieved in every step of the process. The polishers have documented their processes and comment on the final results. The main goal was to study how different proceedings affect the final surface quality; which are the crucial factors in order to achieve a high gloss polished surface with low defect levels? The sample analysis is based on visual estimations as well as interferometer measurements and belonging surface parameters; e.g. seems the number of preparation steps be crucial to the final surface quality

A Step-by-Step Analysis of Manual Polishing Sequences

Polishing of tool steels for e.g. injection moulding of plastic parts or direct polishing of products is perhaps the most important industrial finishing process today. Polishing is often made manually to ensure the highest quality and nanometre roughness. This study compare steel samples prepared in different companies to study how different polishing sequences affect final surface qualities, i.e. trying to find crucial factors controlling high gloss appearance and defect levels. The samples were prepared in a way that every step of the polishing process could be studied quantitatively at nanometre level using phase shifting- and coherence scanning interferometry. In addition, all samples were qualitatively documented and judged visually by the polishers themselves. It could be concluded that a ‘several-step’ strategy in combination with short polishing time was advantageous. Also, guidelines for tool and abrasive selection is presented suggesting cotton as carrier and diamonds of 1 µm size for more time efficient final polishing.Funding: Uddeholms AB, The Swedish Foundation for Strategic Research Fund and ProViking as well as the European Commission Seventh Framework and the poliMATIC projectpoliMATI

Tool steel polishing and topography characterization

Manual polishing is a common method to accomplish a mirror-like mould surface, although it is a cumbersome and time-consuming process. It is, thus, of great interest within the industry to find faster and better polishing techniques to decrease the costs and lead times. A key factor towards improved polishing techniques is to find ways to characterize polished surfaces; which parameters and instruments can be used and what is the definition of polishability? This paper summarises a study where three steel grades were analysed with five different measuring devices. The results showed that mechanical stylus instrumentations are not enough to describe very smooth surfaces, but measurement devices with better resolution are needed, such as interferometers

A quantitative method to estimate high gloss polished tool steel surfaces

Visual estimations are today the most common way to assess the surface quality of moulds and dies; a method that are both subjective and, with today’s high demands on surfaces, hardly usable to distinguish between the finest surface qualities. Instead a method based on non-contact 3D-surface texture analysis is suggested. Several types of tool steel samples, manually as well as machine polished, were analysed to study different types of surface defects such as pitting, orange peel and outwardly features. The classification of the defect structures serves as a catalogue where known defects are described. Suggestions of different levels of ‘high surface quality’ defined in numerical values adapted to high gloss polished tool steel surfaces are presented. The final goal is to develop a new manual that can work as a ‘standard’ for estimations of tool steel surfaces for steel producers, mould makers, polishers etc

Polished injection moulds’ and surface defects’ influence on the quality of plastic components

The quality of injection mould surfaces is important as it has a major influence on tool performance. In the specific field of plastic moulding, many products require defect free, glossy, and very smooth mould surfaces (roughness parameters in the nm-range) to achieve a satisfactory surface appearance. The surface quality is also crucial for the mould to function properly in the actual tooling application; too rough surfaces might disturb the flow of the polymer melt and increase wear, while too smooth surfaces give rise to sticking problems.   A key factor towards improved tool performance is the development of a metrology framework to characterise the polished surfaces. This paper summarises a study based on a non-contact 3D-surface texture analysis, where tool steel moulds and injection moulded plastic components are characterised in terms of surface quality. Different types of defects, such as non-metallic inclusions and carbides, are studied in more detail to better understand their effect on final plastic component surfaces. Critical defect size?, Types of defects? and Defect distribution? are typical questions at issue. A test mould with intentional manufactured pores and scratches in different sizes and distributions will be designed and verified in order to enable injection mould industry to optimize selection of polymer and process variables with respect to plastic component surface quality and tool material selection

Tool steel polishing and topography characterization

Manual polishing is a common method to accomplish a mirror-like mould surface, although it is a cumbersome and time-consuming process. It is, thus, of great interest within the industry to find faster and better polishing techniques to decrease the costs and lead times. A key factor towards improved polishing techniques is to find ways to characterize polished surfaces; which parameters and instruments can be used and what is the definition of polishability? This paper summarises a study where three steel grades were analysed with five different measuring devices. The results showed that mechanical stylus instrumentations are not enough to describe very smooth surfaces, but measurement devices with better resolution are needed, such as interferometers