In mold industry, especially in plastic moulding industry, EDM is a very important process, which requires design of a large number of electrodes (electrode design) for EDM.
1. Part of mold where electrode needs to be designed
There are many deep groove narrow slits and complex cavities on mold surface, which is difficult to be machined by milling machine. This requires EDM, and other parts with high processing precision and surface roughness are also commonly used for EDM.
There are right or sharp corners in mold
Rounded corner is too deep and position is narrow
An angular position consisting of a curved surface and a straight wall or a slanted wall
There is a deep and narrow part in mold structure
2. Structure of electrode
There are two types of structures: integral electrode and insert electrode. Entire electrode is machined from a single piece of material and is the most commonly used form of construction. When it is difficult to process electrode with complicated shape, it can be divided into several pieces, which are separately processed and then integrated into a whole. Insert electrode can save material, but precision between electrode blocks should be ensured.
Integral electrode and insert electrode
3. Experience in designing electrodes
(1) Before designing electrode, fully understand mold structure. It should be clear that mold’s injection position, kiss-off, shut-off, etc., to confirm which parts need electric discharge machining, whether mold core and insert should be assembled and discharged.
(2) Design electrodes in a certain order to prevent leakage of electrodes. This is very important for electrode design of complex molds.
(3) Design electrodes should consider fabrication of electrodes. Designed electrode should be easy to fabricate, preferably using only one processing method. It is very convenient to use CNC milling to make complex electrodes, and it is easy to ensure electrode accuracy.
(4) For molds with appearance and ridge requirements, it is preferable to design electrode as a structure that can process entire cavity at one time; sometimes whole electrode processing is difficult, there is a dead angle that cannot be processed or it is not easy to process. If required tool is too long or too small, you can consider dividing one electrode and sometimes partially need a electrode for cleaning corner .
(5) Lost of raised corners, edges and other raised parts of electrode are faster than flat part in electrical discharge machining. In order to improve accuracy of EDM, when designing electrode, it can be decomposed into main electrode and sub-electrode. Main electrode is used to process main part of cavity or hole, and then sub-electrode is used to process sharp corner and narrow slit.
(6) For some electrodes with small thickness, high and low drop, it is very easy to be deformed in CNC milling and EDM. When designing electrode, structure of reinforcing electrode should be adopted.
(7) Direction in which electrode is opened in processing part must be extended to a certain size to ensure that there is no raised rib at sprue after process finished.
(8) Consider avoidance of some electrodes to avoid undesired discharges other than processed parts in EDM.
(9) When designing electrode, consider reducing number of electrodes. It is reasonable to combine some different machining parts on workpiece as a whole machining or to realize machining at multiple positions by moving coordinates. As shown in following figure, multiple identical machining parts on workpiece are processed by electrode movement coordinates.
Electrodes with different processing parts combined
(10) When designing electrode, parts with different processing requirements should be designed separately to meet respective processing requirements. For example, surface roughness requirements and dimensional accuracy of assembly parts, molding parts in mold parts are different, so electrodes of these parts cannot be mixed and designed together.
(11) Design a suitable base for electrode. Base is reference for correction electrode and positioning in EDM, and it is also processing reference for multiple steps of electrode. For example, in processing of corner portion of tool on wire cutting electrode, base needs to be positioned. In addition, base is preferably designed to facilitate reference angle of direction in which electrode is mounted.
(12) EDM process should be considered when designing electrode. If Z-axis servo machining, lateral machining or multi-axis linkage machining is adopted; electrode should be easy to be clamped and positioned; chip removal and exhaust holes are opened according to specific conditions.
(13) Determination of number of electrodes. Number of electrodes is determined mainly by shape and number of workpiece. Secondly, material of workpiece, depth and area of processing to be processed.
(14) There are two methods for designing base of electrode. One method is to uniformly enlarge base on the basis of maximum shape of electrode processing part. As a result, X, Y, and Z coordinate values based on base are often decimal. Second method is to first determine an integer for X, Y, and Z coordinate values of base reference. Obviously second method can avoid situation where operator misreads complicated decimals in EDM process.
(15) After designing all electrodes of a set of molds, fill in preparation list (confirm electrode base length, width, height and number of electrodes according to electrode requirements, material), arrange electrode fabrication, and design EDM drawing (Discharge coordinates, processing requirements and details remarks).
4. Determine amount of electrode scaling
Main factors to consider amount of electrode scaling are: machining shape, size, allowance, accuracy requirements, surface roughness requirements, electrode and workpiece material.
(1) Single-side zooming amount of rough, medium and fine machining electrodes of numerical control electric spark is generally 0.30-0.15mm, 0.2-0.15mm, 0.15-0.05mm. For example, electrode size of most mold factories is 0.2mm for rough side and 0.1mm for fine side.
(2) For application of EDM where processing area is relatively small, amount of electrode scaling should be smaller; for application of EDM where processing area is relatively large, amount of electrode scaling should be larger.
(3) In the case of EDM with large depth value, amount of electrode scaling should be larger to avoid roughing efficiency and secondary discharge causing size of station mouth to be out of tolerance.
(4) When workpiece material is cemented carbide, discharge gap in actual machining is only about half of that of steel material, so determined amount of electrode scaling is also smaller.
(5) Amount of electrode scaling largely determines processing speed. If discharge energy is large, discharge gap will be larger. Opposite is true. Processing speed of larger discharge energy will also be faster. If amount of electrode scaling is increased, processing speed will be doubled.
5. Select electrode material
(1) Copper electrode
Copper is the most widely used electrode material in EDM. Because most of electrodes are processed by copper, electrodes for EDM are called copper in coastal areas. It is easy to obtain a stable machining state, and a low-cut gauge can be used in finishing process to obtain a clear-cut cavity, which can be mirror-finished. Disadvantage is that it is not suitable to withstand a large current density. In processing of deep and narrow ribs, local high temperature is easy to deform electrode. Burr handling of small electrode is difficult.
(2) Graphite electrode
Graphite is easy to make and has no processing burrs. In rough machining of large currents, processing speed is fast. Density is small, only 1/5 of copper material, making large electrode fabrication and preparation easier. Ultra-thin electrodes are not easily deformed during electrical discharge machining. Shortage of graphite electrodes is that they cannot process very fine surfaces. In fine processing, electrode loss is larger than that of copper. In addition, graphite electrode is produced by a special graphite processing machine.
(3) Copper tungsten alloy
Copper tungsten alloy electrode materials are rarely used in EDM, and are often used only in high-precision molds and some special superhard alloys for EDM. Copper tungsten alloy electrode material can effectively resist loss during EDM, and can ensure extremely low electrode loss. Disadvantage is that material is expensive.