Knowledge of EDM in mould manufacturing

      EDM is a new process that uses electrical and thermal energy for machining, also known as electrical discharge machining. When EDM is used, there is a gap between the tool electrode and the workpiece. Unlike cutting, the gap is used to discharge the metal by using the pulse voltage generated all the time, and the high temperature etches away the metal material. EDM is named because sparks can be observed during the discharge.

     The following conditions must be present for EDM machining. A reasonable gap between the tool electrode and the workpiece. This gap needs to meet two points: (i) the pulse voltage can continuously penetrate the working medium and discharge. (ii) When the spark discharge is completed, the working medium can be eliminated by ionisation and the removal of the ionisation products. The gap between the tool electrode and the workpiece cannot be too large or too small. If it is too large, the pulse voltage cannot penetrate the working medium and spark discharge cannot be carried out; if it is too small, a short circuit will occur and there will be no pulse voltage between the tool electrode and the workpiece and no electrical processing will occur. The working medium exists between the tool electrode and the workpiece. After the spark channel has been formed, the pulse voltage between the tool electrode and the workpiece must not change significantly, so the output pulse energy must be large enough. The energy density in the spark channel can be expressed in terms of current density. When the energy density meets the requirements, the material will only melt or vaporise, thus allowing spark discharge machining to take place. The pulse discharge time must be short. When discharging, the heat energy caused by its high temperature cannot be dissipated within the machined material because of the short discharge time, which is the cold polarity that EDM has. The pulsed discharge occurs uninterruptedly and is dispersed in both time and space. This can be explained: (i) two similar pulses at the same time cannot form a discharge channel at the same point; (ii) if there are two different fixed time periods, then the pulsed discharge cannot both be in the same area, which prevents carbon build-up in the discharge or even causing a more serious situation. The etchings produced after the spark discharge can be quickly discharged from the gap between the tool electrode and the workpiece, thus allowing a smooth uninterrupted discharge.

      According to the form of the electric shock of the tool during the process and the characteristics of its relative movement to the workpiece, it can be divided into EDM forming, EDM wire-cutting, EDM grinding, EDM spreading, non-metallic EDM and EDM surface strengthening.

       EDM has the following characteristics: ① any conductive material can be processed; ② suitable for processing low stiffness workpieces and microfabrication; ③ by EDM processing the surface hardness is harder compared to mechanical processing, and can protect the lubricant; ④ direct use of electricity for processing, easy to achieve automation, etc. According to the above characteristics, the main uses of EDM are as follows: ① manufacturing of various types of hot forging dies, die-casting dies, extrusion dies and plastic dies, etc.; ② processing of shaped holes (round, polygonal, square and shaped holes), curved holes (curved holes, spiral holes), small holes and micro holes; ③ cutting out parts on metal plates; ④ processing of sample plates, various shapes of complex shaped holes, shaped surfaces and narrow slits, etc.

       The following factors affect the amount of material discharge corrosion (galvanic corrosion): ① electrical parameters. Through the adjustment of various electrical parameters (pulse width, pulse frequency and pulse energy, etc.), the amount of galvanic corrosion can be changed. If you increase the current, you can increase the amount of galvanic corrosion. ②The polarity effect. When the tool electrode and the workpiece use the same material for electrical discharge processing, one of them must be corroded more than the other, which is the polarity effect. When the tool electrode and the workpiece use different materials, the polarity effect is greater. ③ Metal materials. When the electrical parameters are the same, the thermal properties of the workpiece material determine the amount of galvanic corrosion. If the specific heat capacity, melting point and thermal stability of the material are greater, the smaller the amount of galvanic corrosion. ④Working fluid. In the EDM working fluid is used as a discharge medium, the main role is cooling, chip removal. Commonly used working fluids have low viscosity, stable performance and other characteristics, such as paraffin, deionized water and emulsion, etc.

      The following two factors affect machining accuracy: ① Machining clearance. There is a reasonable clearance between the tool electrode and the workpiece, too large or too small machining clearance will directly affect the machining accuracy. ② Machining slope. In addition to the slope required by the electrode design, the slope caused by technical reasons, uneven electrode loss, multiple discharges and machining depth during the manufacturing process will reduce the machining accuracy.

      Depending on the processing needs, some of the mould workpieces and mould cavities require the selection of high strength materials, which are heat treated to the required hardness and cannot use normal machining methods. Mould cavities are processed by EDM for it is an inseparable part of mould machining. However, EDM has drawbacks like slow processing and unstable quality, from this point of view EDM can not reach mass production. Nowadays, the development and manufacturing of moulds play a key role in the speed of new product development, and improving the development and manufacturing speed of moulds has become a top priority. The emergence of high-speed machining technology, for mold manufacturing technology to open a new door, electric machining technology is replaced by high-speed machining technology is to speed up the development of mold speed and improve the quality of mold manufacturing must go through. However, at this stage, EDM still occupies a very important position in the mould processing, and cannot be replaced.