Hardened Tooling Steel Selection and Injection Shrinkage Tolerances
Verify tool steel composition and heat treatment phase stability independently to ensure injection shrinkage predictions match finished part tolerances.
Electric Discharge Machining
Meaning
Thermal manufacturing process utilizing controlled electrical sparks erodes material from a conductive workpiece to create complex shapes in metal molds. Electric discharge machining is a non traditional machining method where electrical energy is used to remove material instead of mechanical cutting tools. The process takes place in a dielectric fluid that acts as an insulator until a specific voltage is reached, allowing a spark to jump between the electrode and the workpiece.
This spark creates intense local heat that melts and vaporizes a small amount of the metal surface. By repeating this process thousands of times per second, the desired shape is gradually carved into the material with high precision. This technique is especially useful for working with hardened steels and other alloys that are difficult to machine using conventional milling or turning.
Thermal Mechanism
Removal of metal occurs through a series of rapid electrical discharges that occur in the narrow gap between the tool and the part. During electric discharge machining, the dielectric fluid serves to concentrate the spark and flush away the tiny particles of eroded metal. The electrode is typically made of graphite or copper and is shaped to the inverse of the feature that needs to be created.
As the electrode moves closer to the workpiece, the intense heat of the spark creates a plasma channel that reaches temperatures of several thousand degrees. This localized heating causes a tiny crater to form on the surface of the metal as it is vaporized. The pulse duration and the intensity of the current are carefully controlled to manage the rate of material removal and the quality of the surface finish.
Surface Integrity
Interaction between the high temperature sparks and the metal leads to the formation of a distinct surface layer that must be managed in precision toolmaking. Following the electric discharge machining process, the surface of the workpiece is covered with a recast layer of re-solidified metal that can be brittle and prone to cracking. Below this layer is a heat affected zone where the microstructure of the steel has been altered by the intense thermal cycles.
For high stress applications like injection molds, these layers are often removed through polishing or secondary machining to ensure the durability of the tool. The final surface finish is determined by the size of the craters left by the sparks, with smaller sparks producing a smoother finish. Modern machines can achieve extremely tight tolerances and intricate details that would be impossible with other methods.
Application Limit
Usage of this technology is restricted to materials that are electrically conductive, such as steel, aluminum, and certain ceramics. While electric discharge machining is highly accurate, it is a relatively slow process compared to high speed milling. It is primarily used for creating deep cavities, sharp internal corners, and complex geometries that cannot be reached by a rotating cutting tool.
The process also causes wear on the electrode, which must be accounted for in the design of the manufacturing sequence. Because the material is removed without any physical force, there is no risk of distorting thin or delicate features on the workpiece. This makes it an ideal choice for the final stages of mold making where precision and detail are more important than the speed of material removal.
Consistent monitoring of the electrical parameters is necessary to ensure that the tool remains within the required specifications throughout the entire machining cycle.