Machining of micro/miniature dies and moulds by electrical discharge machining—Recent development

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Abstract

Mass production of micro or miniature parts is increasingly based on replication technologies, such as hot embossing, micro-injection moulding, and bulk forming. These technologies rely on the applications of high thermal and mechanical loads on the forming tools and especially on the integrated microstructures. To achieve sufficient tool life, functional tool materials like hardened steel or cemented carbide are used. The mechanical properties of these materials are a limitation to the variety of structuring technologies which can be applied. Due to their non-contact thermal material removal mechanism electrical discharge machining technologies offer a suitable alternative, in terms of obtainable structure dimensions and accuracy. This paper presents an overview on the state of the art of applicable micro-electrical discharge machining technologies for applications in the field of micro/miniature die and mould making.

Introduction

Based on a study of the European NEXUS organisation, the world market for products based on MEMS shows a yearly increase of about 20% from approximately US$ 30 billion in the year 2000 up to US$ 68 billion in the year 2005. In various industrial sectors a number of micro-technical components with a part volume of less than 1 mm3 or structure dimensions in the range of micrometers come into application. The production of these components is usually done by manufacturing technologies originated in semiconductor processing. To avoid the technological and economical limitations of these processes, cutting and non-conventional processes well known from the macroscopic world are increasingly applied in micro-technology. Next to micro-cutting, laser machining, and LIGA technology electrical discharge machining (EDM) is a manufacturing technology, which is applied in many micro-technological applications. It is especially suitable due to its thermal material removal mechanism, allowing an almost process force free machining independently from the mechanical properties of the processed material. In combination with a wide freedom of geometry and high accuracy electrical discharge machining can process functional materials like hardened steel, cemented carbide and electrically conductive ceramics with sub-micron precision. Since electrical discharge machining can be economically used especially in single and small batch production, next to primary structuring of micro parts it is predestined for the production of micro dies and moulds [1], [2], [3].

Section snippets

Technology of micro-electrical discharge machining

Micro-electrical discharge machining (μ-EDM) is the application of electrical discharge machining technologies for the manufacture of micro and miniature parts and structures. It means the adoption of process technologies and machine tools to the requirements of micro-production [4], [5], [6]. Next to the miniaturization of the tool electrodes, compared to conventional EDM discharge energy is reduced to less than We = 100 μJ per single discharge. Static impulse generators, which are commonly used

Micro-wire electrical discharge machining

Characteristic for micro-wire electrical discharge machining (μ-WEDM) is the application of smallest wire electrodes with wire diameters of dw = 0.1 mm down to dw = 0.02 mm, in order to machine smallest cutting widths or inner corners. It is most commonly used for the direct production of micro-mechanical devices (Fig. 2) and for micro-stamping tools. Usually, wire electrodes from tungsten or coated steel with tensile strengths of more than 2000 N/mm2 are used, to guarantee sufficient thermal and

Micro die sinking

Micro die sinking using microstructured form electrodes is mainly used for the manufacture of replication tools for micro-injection moulding or hot embossing, where micro-mechanical parts can be produced in large numbers (Fig. 3).

Obtainable structure sizes are not only limited by the erosion process but moreover by the design and manufacture of the form electrodes. Today different microstructuring technologies like micro-wire EDM, micro-milling, electro-forming, or even LIGA technique are used

Micro-electrical discharge drilling

A special case of micro die sinking EDM is the use of rotation pin electrodes in order to manufacture rotationally symmetrical bores and through holes. This process variant called micro-electrical discharge drilling is industrially used in the manufacture of injection nozzles and starting holes for wire EDM. Since pin electrodes can show minimum electrode diameters of less than 25 μm, special demands are made on the electrode production, electrode handling, and electrode positioning. To ensure

Micro-electrical discharge contouring

Large moulds with widely spread microstructures for instance needed in glass embossing processes for flat panel displays can often not be structured by wire EDM or die sinking EDM due to their dimensions. Either the work piece is too high for the limited thermal and mechanical stability of the electrode in micro-wire EDM or overall dimensions permit the use of shaped die sinking electrodes because of process instabilities caused by poor flushing and large active electrode surfaces. As an

Micro-electrical discharge dressing

Prerequisite for micro-electrical discharge drilling and micro-electrical discharge milling is the availability of geometrically defined electrodes. Due to the filigree form of the electrodes and the electrode clamping deviations regarding true running accuracy occur. This can be avoided by the direct manufacture of the electrodes using electrical discharge dressing technologies (Fig. 7).

Electrical discharge dressing can be done using a block of cemented carbide, causing over proportional wear

Conclusions

In this paper, an overview on the technology and the different process variants of micro-electrical discharge machining is given. The paper focuses on the practicable application aspects, rather than theoretical research. It is shown that micro-electrical discharge machining is of high potentials to be used as structuring technology, especially for the manufacture of micro/miniaturized dies and moulds for micro-replication processes. Table 1 summarises a survey carried out on the different

Acknowledgements

The work was carried out as part of the European Integrated Project “Integration of Manufacturing Systems for Mass-Manufacture of Miniature/Micro-Products-MASMICRO”, the DFG-Research Project (German Research Foundation) UH 100/40-1 “Electrical Discharge Microstructuring of Rotating Components of Hard Materials”, and the DFG-Research Project (German Research Foundation) UH 100/9 “Development of an EDM Machining Centre for Micro-Technology”.

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