Tool and mould making Recycled graphite dust creates sustainable tool electrodes

Von E. Uhlmann, M. Polte, J. Streckenbach, N. C. Dinh und W. Penske

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Researchers at the IWF of TU Berlin and specialists at Gezea GmbH can recycle environmentally harmful graphite dust. The electrodes can be finished in a matter of days! This is how they do it.

(Source: Phoenix-Contact)

EDM is an important thermal erosion process that is used primarily in the field of tool and mould making for the manufacture of products with demanding specifications. One process variant of electrical discharge machining is die-sinking EDM, in which the geometry of a tool electrode is transferred equidistantly into a workpiece. Graphite is a frequently used material for tool electrodes due to its high sublimation temperature of about ϑS = 3,700 °C and the associated wear resistance. However, graphite dust is produced during the mechanical processing of these electrodes. In general, graphite is not hazardous to the environment because it is comprised of carbon. Nevertheless, it can lead to technical and health complications in the form of dust. For example, the dust can enter the respiratory tract and the eyes, causing severe irritation. Particularly fine particles can also cause considerable lung damage [GIS22].

Furthermore, there are few to no uses for this residual product. Last but not least, the disposal of these graphite residues involves a great deal of effort and is very expensive. Consequently, reprocessing the waste product graphite dust into new tool electrodes for electrical discharge machining is a particularly sustainable and economically lucrative approach.

A hybrid graphite-copper electrode for EDM

During the EDM process, electrically conductive, eroded particles collect in the working gap s, which can lead to short circuits and arc discharges. These effects have a negative influence on process target variables such as the removal rate V̇W and the wear rate V̇E. To counteract this, active and passive flushing methods are integrated into the process with the aim of removing the particles.

In the case of a low insertion depth et, external flushing is particularly suitable for this purpose. Pressure or suction flushing is very effective for greater sink depths. Pressure or suction flushing can be carried out via flushing channels that are integrated into the tool electrode.

Gezea GmbH, a service provider for tool and mould making, and the Institute for Machine Tools and Factory Management (IWF) at the Technical University of Berlin are now working together on the development of a hybrid graphite-copper electrode based on graphite dust. It is to be used as a new type of tool electrode for spark-erosive countersinking. The goals of this project are to protect both people and the environment. The environmentally friendly preparation of graphite dust also improves the process results in spark erosion by means of inserted internal channels and the use of alternative materials. As part of the BMWi's "Central Innovation Programme for SMEs" (ZIM) funding programme, this cooperation project is financially supported and supervised by AIF Projekt GmbH.

A fully functional graphite electrode in record time

Usually, the process of manufacturing tool electrodes from graphite takes over four months [MER22]. By using graphite dust, this process can be reduced to less than five days in the proposed project. This would result in a runtime that is at least 96 per cent shorter. The concrete difference to the state of the art lies in the fact that numerous steps, such as crushing the base material, the time-consuming impregnation with pitch and the graphitisation for the transformation from the amorphous to the crystalline structure of the graphite are no longer required, because the graphite dust to be used is already in the crystalline state. The reprocessing technology to be developed from graphite dust to tool electrodes then only consists of the process steps of sieving, grinding and mixing with metal powder. This is followed by pressing, sintering and post-processing by milling (Fig. 2).

Figure 2: a) Comparison of the usual production of graphite electrodes with all process steps [GAB22, GRA22]. And b) the environmentally friendly, new method to be developed during the project using graphite dust.
Figure 2: a) Comparison of the usual production of graphite electrodes with all process steps [GAB22, GRA22]. And b) the environmentally friendly, new method to be developed during the project using graphite dust.
(Source: IWF)


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