Graphite Electrode for EDM – Suitable for High Precision Processing

  • High electrical resistivity and thermal conductivity.
  • High purity and low coefficient of linear expansion.
  • Strong resistance to corrosion and oxidation.
  • Good mechanical strength and easy to process.
Complex-shaped graphite electrode for electrical discharge machining
Description

In the metal processing industry, metal electrical processing methods are divided into two categories: electric thermal methods based on current heat effects and electrochemical methods based on current chemical effects. Electrical discharge machining is a kind of electrical thermal method.

Electrical discharge machining (EDM) is also known as electrical spark discharge machining. Its principle is to take advantage of the electrocorrosion phenomenon between the graphite electrode for EDM and the workpiece during pulse spark discharge, to remove excess metal and meet the machining requirements of corresponding parts on size, shape, and surface quality.

Graphite electrode for EDM can not only machine general materials workpieces, but also various metal materials with high melting point, high hardness, high toughness and workpieces with high precision requirements that are difficult to machine by using traditional cutting methods. Therefore, graphite electrodes for EDM are especially suitable for machining mold parts.

Features
  • High electrical resistivity. The electrical resistivity of graphite electrodes is tens to hundreds of times greater than that of metal materials like copper, and exhibits obvious anisotropy.
  • Good mechanical strength. Compared with other materials, the mechanical strength of graphite electrodes (tensile strength, compressive strength and bending strength) increases as temperature rises.
  • High thermal conductivity. The thermal conductivity of graphite electrodes is relatively high, being approximately half that of brass and similar to that of aluminum. It is approximately 91.96–137.94 W/(m·K).
  • Low coefficient of linear expansion. The linear expansion coefficient of graphite is smaller than that of metal materials, and it has good resistance to thermal shock. The unidirectional coefficient of linear expansion is about 2.0 × 10-6 to 2.0 × 10-6 °C-1.
  • High purity. Generally, the purity of artificial graphite electrodes for electrical processing is above 99%.
Specifications
Table 1: Specifications of Graphite Electrode for EDM
Model Density (g/cm3) Particle Size (μm) Specific Resistance (μΩ.m) Porosity Shore Hardness Compressive Strength (MPa) Flexural Strength (MPa) CTE (× 10-6-1) Application
ED-1(Isostatic) 1.83 9 12 12% 65 116 51 5.8 EDM, semi finishing/finishing
ED-2(Isostatic) 1.81 7 12 12% 69 135 62 6.8 EDM, semi finishing/finishing
ED-3(Isostatic) 1.90 5 12 12% 69 135 62 6.8 EDM finishing, ultra fine grain for low electrode wear
ED-4(Isostatic) 1.92 3 11 11% 72 160 69 6.9 EDM finishing, lowest electrode wear
Notes:
  • 1 MPa = 10.2 kgf/cm2; 1 W/m.k = 0.86 kcal/cm.h.°C
  • These properties are typical values and not guaranteed.
Process Introduction: Something About EDM

EDM is conducted in a processing system as shown in the figure below.

There are many forms of EDM, of which electrical discharge forming and wire cutting are the most widely used. Electrical discharge forming is mainly used for the machining of complex-shaped cavities, convex dies and concave dies, whereas wire cutting is mainly used for the machining of punching dies and extrusion dies.

During electrical discharge machining, one pole of the pulse power source is connected to the tool electrode (graphite electrode for EDM), and the other pole is connected to the workpiece electrode. Both poles are immersed in a liquid dielectric medium with a certain degree of insulation (commonly using kerosene, mineral oil, or deionized water).

The graphite electrode of EDM is controlled by an automatic feed adjustment device to maintain a very small discharge gap (0.01–0.05 mm) between the tool and the workpiece during normal operations. When the pulse voltage is applied between the two poles, the liquid dielectric closest to the poles is broken down, forming a discharge channel. Due to the small cross-sectional area of the channel and the extremely short discharge time, the energy is highly concentrated (106–107 W/mm2). The instantaneous high temperature generated in the discharge zone is sufficient to melt or even vaporize the material, forming a small pit. When the first pulse discharge is finished, after a short interval, the second pulse is discharged in the other pole between recent click-through. This process continues to cycle at a high frequency, and the graphite electrode for EDM continuously feeds into the workpiece. Its shape is eventually replicated on the workpiece surface to create the required machining surface.

EDM is used for precision machining of conductive materials. This process is most commonly used in the production and manufacturing of tools, die-casting molds, injection molds, and forging molds.

Graphite electrode is performing electrical discharge machining.