Graphite is the material of choice for the majority of EDM electrodes produced today in the Western World. Selecting the best grade of graphite for a particular application can be difficult if the differences between graphite grades are not understood. Depending on the graphite grade selected and the application, the graphite can be the limiting or the key factor in achieving the desired results from the equipment.
There is a wide range of grades from a number of manufacturers to choose from. Each manufacturer uses different processing techniques, source materials and process controls, which mean the end product, will be quite different. Each manufacturer's grades are designed for optimum performance for specific types of applications. To aid in material selection, each manufacturer publishes technical specifications on their material, but there are no standard test methods.
Since any grade of graphite looks similar to another grade, appearance is not part of the selection criteria. Each grade should be selected by its physical characteristics and properties. To make this process easier, various grades of graphite are grouped into six classifications that are segregated by average particle size. Only four of the six classifications are suitable for use as EDM electrodes. How the different grades rank within the classifications is an indicator of their performance potential.
Advances in Graphite
The graphite industry is constantly striving to produce higher quality grades. Graphite materials have continued to evolve along with the other aspects of the EDM industry, but at a less dramatic pace. Advancements have been made in the microstructure of the graphite materials, as this is the key to performance.
Coarse graphites with particle sizes over 100 microns have never been suitable as an electrode material. During the last decade, medium grades with a particles size between 21-100 microns have all but disappeared from the market as an EDM material. In the last few years many of the low-end grades within the fine classification (11-20 micron particle size materials) also have disappeared. The superfine classification (6-10 particle size) materials have remained stable. Some of the manufacturers of these materials allow their graphite grades to be sold as house brands, which can be confusing to the end user. Confusion occurs when consumable distributors change the names of their house brands, but still use the same material or change the material, but keep the same name for the house brand. The same grade may be offered under many house brand names.
The ultrafine classification (1-5 micron particle size) is where most of the real development efforts are targeted. Many of the plastic consumer products require molds with fine detail and finishes that can easily be achieved with ultrafine materials. Materials in this classification are very difficult and expensive to make and it is even more difficult to produce consistent material batch after batch and year after year.
There are very few grades in the angstrofine classification (< 1 micron particle size). The grades are available in small blocks to control the uniformity of the graphite. These grades are the most expensive to produce and have limited use. Generally, they are used for fine detailed engraving electrodes and small featured electrodes that produce very high surface finishes without the use of powder additives when polishing of cavities is not possible.
The lower end graphite grades are slowly disappearing from the market as EDM applications change. Large cavity mold work without fine detail and forging dies can easily be done by high-speed milling thus reducing the need for grades in the fine classification. At the same time intricate detailed cavity work requires graphite with small particle sizes, uniform microstructure and high strengths to produce complex, small-featured cavities.
Grades within Classifications
The physical properties of each grade of graphite determine the ranking within classifications. The properties that influence performance are particle size, flexural strength and shore hardness. These properties along with a photomicrograph of the microstructure are the best tools for predicting graphite performance.
The best graphite in any classification has tightly packed particles with little variation in size. This kind of uniform material resists wear caused by the thermal nature of the EDM process. Particle size is generally stated as an average size. When particle size spans a small range, the microstructure of the material becomes more uniform with reduced porosity. The porosity in the graphite is boundary between particles. The particles are bound together by chemical or mechanical means and the failure of this system is what releases particles into the gap when EDMing. If the material's particles are small, uniform in size and tightly packed, erosion of the electrode is minimal. Particle size has a bearing on the minimum surface finish that the material will produce. Since the electrode reproduces its structure in the cavity, fine surface finishes cannot be obtained with graphite grades that have large particle and non-uniform microstructure.
The microstructure of the graphite grade is often the limiting factor determining EDM performance. A non-uniform micro-structure with a wide range of particle and pore sizes can have soft spots that are large areas of porosity and/or hard spots which are conglomerates caused by inconsistent blending. Hard spots also can be caused by impregnating the open porosity of the material with pitch and then reprocessing the material giving the particles and pitch impregnated areas different hardness values. Since the unaided eye cannot see the microstructure, there is no way to detect these problems prior to the machining process. Identification of the cause of machining problems involves destructive testing and the examination of photomicrographs.