Anyone who has ever purchased graphite materials knows that the physical properties of the material are important. You want to use the right product for the application. But in efforts to promote less expensive products, and compete on price, many distributors are focusing on the density of the material. They are presenting density as an indicator of enhanced quality. I would argue that particle size is by far more important than density when choosing the quality of a product.
All EDM Graphites are evaluated using the following physical properties:
• Particle Size
• Apparent Density
• Flexural and Compressive Strength
• Electrical Resistivity
EDM Graphite is made from petroleum coke, a manufactured carbon product. The petroleum coke is pulverized into small particles and mixed with a binder material and then isostaticallly pressed. A series of thermal treatments then convert the material to carbon and ultimately to graphite.
Graphites should not be compared only on density. Density is the mass per unit volume. In efforts to add density, some materials are pitch impregnated prior to the graphitizing process to produce greater density. Pitch is a complex mixture of polynuclear aromatics generally derived from the thermal treatment of coal tar or a petroleum tar. Although this adds density, it may cause erratic particle patterns within the graphite.
While the pitch adds density to EDM Graphite materials, it actually may cause issues:
• It leads to greater spark erosion, by promoting the breaking up of the particles within electrode material, causing the electrode to wear more quickly.
• It slows down the connectivity through the particles of the electrode, causing the burn to take longer, again using up the electrode causing consumption more quickly.
All materials, however, can be compared equally when it comes to particle size. Generally, the smaller the particle size, the better the mechanical properties, the finer the detail and slower the erosion. All one needs to do is have the material analyzed to see the particle size. The differences in particle size can be startling when examined at 100x magnification. Particle size is by far the best indicator of material quality. The smaller the particle size, the slower the electrode wear.
Graphite can also be compared on the uniformity of its microstructure. The more consistent the distribution of particles and porosity, the more even and predictable will be the electrode wear. Lesser graphites have inconsistent microstructure which may lead to inconsistent wear. This can occur between or even within batches. Better quality materials have more consistent and even microstructure, leading to more consistent and slower wear.
Since graphite is a porous material, density must be closely controlled. Generally, high density is preferable, but be sure to consider particle size and microstructure, when purchasing graphite for your application.