Synergy in tool and mold making
A drastic change has taken place in tool and mold making. In practically no other area are the traces of globalization as obvious as in this branch of industry.
Many wage-intensive jobs have been transferred to Eastern Europe and Asia and new technologies have been introduced. Purposefully utilizing the key technologies, electrical discharge machining and milling, is therefore equally relevant for manufacturers and users of such machines.
EDM and high-speed cutting (HSC) are among today’s key technologies for classic tool and mold machining. While the electrical discharge machining method works by thermal removal, high-speed cutting is a method of cutting with a geometrically defined cutting edge.
Even from this simple and hardly noteworthy comparison it can be seen that there are characteristics, which determine the basic suitability of the method for certain applications.
The term high-speed cutting typically means machining with a geometrically defi ned cutting edge, which enables the maximum possible cutting speed of modern tool materials and coatings in the various workpiece materials. In relation to tool and mold making, the main focus is on the machining of easily machinable materials for model and electrode production (cutting speeds 800 – 2000 m/min-1) as well as the machining of tempered and hardened tool steels (cutting speeds 100 to 500 m/min-1). Modern fi nest carbide grade hard metals with coatings suited to the workpiece material
are used as tools. Typical tool diameters in tool and mold making range from D = 0.2 mm to D = 16 mm, whereby a maximum ratio of cutting length to diameter of L/D = 10 to L/D = 15 is possible.
With the EDM method, two charged electrodes in an electrically insulated fluid (dielectric) approach each other, until sparkover occurs between the tips, and, as a result, a plasma channel forms between the electrodes. The charge carriers, which are impinging on the electrode surfaces at high speed, convert their kinetic energy into heat, causing local melting of the electrode material on the contact surfaces between the plasma channel and the electrodes.
Due to the impinging of the charge carriers, a part of the melted electrode material is already vaporized or flung out of the existing crater. If, subsequently, the power supply is interrupted, the plasma channel collapses at high speed. The resulting low pressure increases the efficiency of the process, in which additional melted electrode material is removed from the crater by the high/low pressure effect.
By sequential repetition of this process, considerable material volumes can be removed from almost all electrically conductive electrode materials. The processes of die sinking EDM and wire EDM are differentiated. In the first case, one of the electrodes corresponding to the required end geometry of the typically three-dimensional cavity, is sunk into the workpiece material, and in the second case a maximum two-dimensional geometry is cut out of a workpiece with a retracting wire electrode.
In principle, almost all electrically conductive materials can be machined by the EDM process. On the basis of the underlying functional principle, the EDM process is independent of material hardness and, to a large extent, is independent of the material composition of the workpiece. In combination with negligible machining power, this results in particular characteristics, which have predestined the process for tool and mold making in the past and for which up until now no alternative method has been found.
The methods that really compete with each other in tool and mold making are die sinking EDM and high-speed milling. Both methods can be used primarily to produce complex three-dimensional cavities and mold cores for injection or die casting molds as well as for the manufacture of forging punches and dies, compression molds, forming punches, sintering press molds and blow molds.
High-speed machining has become particularly established in the production of graphite and copper electrodes and is, from this point of view, also a complementary method to die sinking EDM.
The operating costs of die sinking EDM machines are largely the same as those of high-speed milling machines, provided that machines of the same category are compared with each other.
An important factor in this comparison is taking into consideration the costs of tools (milling tools, electrode production), auxiliary materials (dielectric, oil for minimum quantity lubrication) replacement and exchange parts (spindles), waste disposal (dielectric, chips, sludge, etc.).
While wire cutting EDM with executed cutting electrodes is now to a great extent unrivalled in the production of punching and cutting dies, die sinking EDM and high-speed milling meet each other directly as competing production methods in many areas of application.
Nevertheless, the question of advantages and disadvantages of die sinking and high-speed milling in tool and mold making can not be answered conclusively. Despite the massive advantages of highspeed milling, special geometry elements remain, which limit this method. In this respect, both state-of-the-art methods should be regarded as complementing each other.