Head Stretching Die, Head Forming Stretching Die

In wire EDM, a pulsed discharge circuit is formed between the electrode wire (usually molybdenum wire) and the workpiece. The difference in the positive and negative polarities of the power supply leads to changes in electron emission intensity and ion bombardment direction-this is known as the "polarity effect." When negative polarity is used, electrons flow from the electrode wire to the workpiece, subjecting the workpiece surface to more electron beam impact; conversely, positive polarity enhances the erosion effect on the electrode wire itself. This difference in energy distribution directly affects the size and uniformity of the discharge pits, thus altering the surface microstructure.

Studies have shown that the proper utilization of the polarity effect can significantly improve surface roughness. For example, selecting a negative polarity mode during the finishing stage can result in a finer surface texture because the weak energy discharge dominated by electrons reduces overheating and melting of the material. Conversely, improper polarity settings can lead to excessively intense localized discharges, forming large crater-like pits and causing a sharp increase in roughness.

The polarity effect does not exist in isolation; it needs to be dynamically balanced with parameters such as pulse width and current amplitude. Taking fast wire EDM as an example, when the pulse width is controlled within 20μs and matched with a low current, combined with a negative polarity connection, the accumulation of electrolytic corrosion products can be effectively suppressed, achieving a high-finish surface finish with an Ra value below 1.6μm. It is worth noting that the response of workpieces of different thicknesses to polarity also follows a pattern: thin parts are more suitable for a short pulse width + negative polarity combination, while thicker plates require a slightly longer pulse interval to avoid short-circuit risks.

The choice of working fluid also affects the effectiveness of the polarity effect. Emulsion-based cooling media can not only remove heat promptly but also optimize the stability of the discharge channel through ionic conductivity. Experimental data shows that under the same polarity conditions, using a dedicated water-based working fluid can reduce surface roughness by about 30%, thanks to its better deionization ability and chip removal effect.