Electrical discharge machining (EDM) machine tools: The "core essential" and technological iteration in mold manufacturing
I. EDM: The Irreplaceable Equipment in Mold Manufacturing
In the entire process of mold manufacturing, electrical discharge machining (EDM) machines have long been upgraded from auxiliary tools to core support. Data clearly outlines its status: Approximately 80% of wire electrical discharge machining (WEDM) machines and 75% of sinker electrical discharge machining (SEDM) machines in China are directly used for mold processing. Nowadays, it is normal for mold enterprises above a designated size to own dozens of EDM devices. From simple stamping dies to complex progressive dies, and from plastic molds to cemented carbide molds, the versatility + high precision of EDM makes it the basic chassis of mold manufacturing. With technological iteration, the importance of EDM in the mold industry continues to rise. Especially in the field of precision, complex, and long - life molds, its irreplaceability becomes increasingly prominent.
II. Upgrade of the mold industry: The growth catalyst of LSWEDM
The rapid growth of China's mold industry is essentially an "upgrade of the demand structure": In 2005, the output value of molds is expected to reach 57 - 60 billion yuan, and it will double to 100 billion yuan by the end of the "11th Five - Year Plan" in 2010. More importantly, the proportion of technical molds has increased sharply. Large - scale, precise, complex, and long - life molds (such as multi - station progressive dies and cemented carbide molds) have become the mainstream of demand. The core requirements for this type of mold are "high precision (micron - level), high consistency (no attenuation in the service life of one million times), and high complexity (thousands of stations/hole types)". Traditional processing technologies (such as milling and grinding) cannot meet these requirements comprehensively, while the low - speed wire electrical discharge machining (LSWEDM) has become the "lone ranger" in stamping die processing thanks to its features of "non - contact machining + micron - level precision + cemented carbide processing ability".
III. LSWEDM: The "equipment with technological barriers" for precision stamping dies
The core value of LSWEDM lies in solving the processing pain points of high-difficulty stamping dies, especially the multi-position progressive dies that represent the advanced level of stamping dies. The technical indicators of this type of die can be described as "stringent":
Progressive die for automatic valve plates of motor iron cores: Precision of 2μm, pitch of 3μm, precision of assembled blocks of 1μm, double rotation precision of 1′, surface roughness Ra of 0.1 - 0.4μm, and service life exceeding 100 million cycles;
Air conditioner fin progressive die: There are nearly a thousand working surfaces of the male and female dies. More than 300 punching clearances are only 0.01mm. The same type can be interchanged, and the service life reaches 200 million times.
G5 bottom electron gun progressive die: Stamping non-magnetic stainless steel with a thickness of 0.245 mm, the precision of the products is ±5 μm, and the service life is 30 million times.
50-station progressive die for mobile phone connectors: Precision is 2μm, pitch is 3μm, punching speed exceeds 400 times per minute, and service life is 200 million times.
For the machining of the key parts (such as cemented carbide die holes) of these molds, only LSWEDM can achieve the triple balance of "precision, efficiency, and consistency". It is not only the "admission ticket" for this type of high - value molds, but also the "technological moat" for mold enterprises. In turn, the explosive demand for this type of molds has become the "primary driving force" for the technological breakthrough of LSWEDM.
IV. Breakthrough in LSWEDM Efficiency: From "Scale" to "Precision"
Efficiency is the "lifeline" of mold processing. The efficiency upgrade of LSWEDM is essentially "the precise matching of technology to scenarios":
1. Maximum processing efficiency: Breakthrough from "300" to "500"
Previously, the maximum machining efficiency of foreign LSWEDM had remained at 300 mm²/min for a long time. In recent years, a breakthrough has been achieved through the combined strategy of "narrow pulse width + high peak current" - the narrow pulse width (shortening the discharge time) reduces the negative impact of melting and erosion, and the high peak current (≥1200A) ensures the erosion efficiency. Coupled with the optimization of the control algorithm, the liquid supply system, and the composite electrode wire, the maximum efficiency has jumped to 400 - 500 mm²/min.
- Swiss AGIE CUT PROGESS: Equipped with a digital IPG intelligent power supply. Using Φ0.33mm electrode wire, the efficiency exceeds 500mm²/min.
- Japanese Mitsubishi FA-V: Using a wire electrode with a diameter of Φ0.36mm, the efficiency reaches 500mm²/min.
- Swiss Agie e - cut power supply: More revolutionary. While achieving an efficiency of 350 - 500 mm²/min with standard electrode wires, the surface roughness Ra is only 0.8 μm, directly bringing the quality of "rough machining" to the level of "finish machining". The machining time is reduced by 50%, and the consumption of filters/resins/wires is reduced by 40% - 60%.
2. Average processing efficiency: Twin-wire cutting breaks through the "efficiency-accuracy" barrier
Although the maximum efficiency is high, fine machining requires thin wires (e.g., Φ0.10mm), while thick wires (Φ0.33 - 0.36mm) offer high efficiency but insufficient precision. The double-wire cutting technology is precisely the key to resolving this contradiction. The Swiss Charmilles ROBOFIL 2050TW/6050TW and Agie double-wire systems can automatically exchange electrode wires of different diameters: thick wires are used for rough machining to pursue efficiency, and thin wires are used for finishing to ensure precision. The overall machining time is shortened by 30% - 50%, and at the same time, the cost of expensive thin wires is saved. More importantly, there is "no loss of precision" during the wire-changing process. The wire-changing time is only 15 seconds, and the reliability is 100%, completely breaking through the barrier between "high efficiency" and "precision".
3. Machining efficiency of variable cross - section: Intelligent control bids farewell to "passive adjustment"
In mold processing, parts with variable cross - sections (such as stepped, hollow, and thin parts) are often encountered. The change in cross - section will lead to a mismatch of processing energy. In the mild case, the wire breaks, and in the severe case, the efficiency drops sharply. The core of improving the efficiency of variable - cross - section processing is "intelligent energy control". Real - time data is collected through the workpiece thickness detector and the processing state detector, and the pulse energy is automatically adjusted to maintain the optimal efficiency. For example, the "Maximum Energy Control Expert System" of the FA series of Mitsubishi in Japan only requires inputting the diameter of the electrode wire and the workpiece material. It can automatically increase or decrease the energy according to the thickness change, and the efficiency is increased by 30%. This kind of system not only prevents wire breakage but also greatly improves the efficiency stability of variable - cross - section processing, which is especially suitable for the integrated processing of complex molds.
V. Breakthrough in surface quality: From "tolerance" to "control"
The surface quality of LSWEDM used to be its "fatal weakness" - the "altered layer" (changes in the surface metallographic structure, uneven microhardness, residual stress, and cracks) and the "softened layer" (formed by the electrochemical reaction of the water-based working fluid) after processing directly affect the service life of the mold. The breakthrough in recent years stems from a profound understanding of the "essence of discharge energy":
There are two modes of removing metal by electric spark erosion:
Melting discharge: Wide pulse width (long discharge time). The metal is eroded in a melting form, resulting in poor surface morphology, thick metamorphic layer, large stress, and prone to cracks.
Vaporization discharge: Narrow pulse width (short discharge time). The metal is eroded in the form of vaporization, with a thin metamorphic layer, small stress, no cracks, and good surface quality.
Therefore, "under the premise of keeping the total discharge energy constant, compressing the discharge time and increasing the peak current" and shifting the discharge mode from "melting-dominated" to "vaporization-dominated" can not only maintain the efficiency but also significantly improve the surface quality. For example, the optimized pulse power supply can reduce the surface roughness Ra from the traditional 1.6μm to below 0.8μm and decrease the thickness of the metamorphic layer by more than 50%, completely solving the problem of "incompatibility between efficiency and quality" - today's LSWEDM can not only "cut fast" but also "cut well".
From "auxiliary equipment" to "core essential requirement", and from "efficiency first" to "balancing efficiency and quality", the technological iteration of LSWEDM is essentially the synchronized resonance between the "demand upgrade" of the mold industry and the "capability upgrade" of equipment technology. In the future, as molds develop towards the direction of "higher precision, greater complexity, and longer service life", the "process barrier" of LSWEDM will continue to strengthen, making it an "irreplaceable technological high - ground" in the field of mold manufacturing.
Anti-electrolysis (AE) pulse power supply: Solving the problem of electrochemical corrosion in water-based processing
The core advantages of LSWEDM using water-based working fluid are environmental protection and cooling efficiency. However, deionized water is not absolutely "ion-free" - the dissociation equilibrium of water (H₂O⇌H⁺ + OH⁻) always exists, and the erosion of the electrode wire and the workpiece during processing will release trace amounts of metal ions (such as Fe²⁺, Co²⁺). When the workpiece is connected to the positive electrode, the electric field will drive the OH⁻ negative ions to migrate directionally towards the workpiece surface, and after continuous deposition, an electrochemical reaction will be triggered:
- Iron-based workpieces: OH⁻ reacts with Fe to form Fe(OH)₂, which is further oxidized to Fe₂O₃ (rust), forming a loose "softening layer".
- Cemented carbide workpieces: Cobalt (Co), which acts as a binder, will dissociate into Co²⁺ under the action of an electric field and dissolve in water, resulting in a decrease in the hardness and strength of the cemented carbide.
Traditional pulse power supplies use unidirectional DC pulses and cannot block this process - OH⁻ ions will continuously migrate to and deposit on the workpiece. The core design of the AE pulse power supply is an "alternating pulse waveform": by alternately outputting positive and negative pulses, the average voltage of the pulse sequence is made zero. The key function of this design is to interfere with the directional migration of ions - when a positive pulse is output, OH⁻ ions move towards the workpiece; immediately afterwards, the negative pulse reverses the direction of the electric field, and the OH⁻ ions are "pulled back" into the working fluid by the reverse electric field just as they are about to reach the workpiece surface. This process repeats, and the OH⁻ ions are always in an "oscillating" state, unable to continuously deposit, thus completely blocking the electrochemical reaction.
The effect of the AE power supply can be maximized only when it is combined with "optimized discharge energy". Optimized discharge energy reduces the heat - affected zone by adjusting pulse parameters (such as narrow pulse width and low peak current), and the AE power supply suppresses electro - corrosion. The combination of the two can control the thickness of the "metamorphic layer" on the workpiece surface to less than 1μm (the metamorphic layer of traditional power supplies usually exceeds 5μm). Taking the machining of IC lead frame molds as an example, the life test results of four schemes directly verify the value of the AE power supply:
- Traditional power supply + deionized water: The service life of the mold is only 60% of that of mechanical grinding.
- Traditional power supply + Oil immersion processing: The service life is increased to 80%, but oil immersion will increase pollution and cleaning costs.
- AE power supply + deionized water: The service life reaches 100% of that of mechanical grinding. When approaching the wear limit (e.g., the cutting edge wear is 0.02mm), the mold processed by AE can still maintain accuracy, and its performance is superior to that of grinding.
- Mechanical grinding: Based on service life, but it cannot process complex narrow slits (such as the 0.08mm pitch of IC leads).
This result has made "cutting instead of grinding" a reality— the service life of cemented carbide dies processed by LSWEDM has matched and even exceeded that of those processed by mechanical grinding, and it is more suitable for processing complex shapes.
Foundation of precision machining: Machine tool structure with high rigidity and constant temperature
The machining accuracy of LSWEDM is a "systems engineering", and the machine tool structure is the foundation of all accuracies. Only when the accuracy of the machine tool itself is stable can the performance of the pulse power supply and the numerical control system be fully utilized. Taking the AGIE CUT VERTEX machine tool of the Swiss company Agie as an example, its structural design focuses on "eliminating error sources":
1. Three-point vibration absorption support: Avoid interference between shafts
A statically determinate three-point support similar to that of a coordinate measuring machine is adopted (the support points are distributed in the triangular area of the bed). The core advantage of this design is "independent movement of each axis" - the movement of the X/Y/U/V axes will not transfer forces to other axes, avoiding stress accumulation and deformation in traditional multi-support structures. For example, when the X-axis moves, the support points only bear the force in the X direction and will not affect the positioning accuracy of the Y-axis.
2. Dual measurement feedback: Achieve a positioning accuracy of 0.1μm
Each motion axis is equipped with a dual detection system of linear grating scale + encoder:
- The linear grating scale directly measures the actual displacement of the axis (with an accuracy of 0.1μm) and corrects the error of the encoder in real - time (the encoder indirectly measures the displacement through the motor rotation angle, and there is a pitch error).
- The encoder provides a fast response, and the optical grating ruler ensures the final accuracy. The combination of the two enables the positioning control accuracy to reach 0.1μm.
3. Full-range constant temperature control: Suppress thermal deformation
Temperature sensors are installed on all heat sources (such as servo motors and spindles) of the machine tool, in conjunction with three temperature control schemes:
Circulating gas cooling: Deliver cooling gas to the heat-generating parts through pipes to carry away heat (suitable for components with a large heat dissipation area such as motors).
Water cooling system: Install a circulating water jacket outside the spindle and lead screw to maintain a stable temperature (water temperature fluctuation ≤ ±0.5°C).
Thermal isolation design: Use ceramic fibers to separate the heat source from the machine bed and guide rails to avoid thermal deformation caused by heat transfer.
In addition, the bed is made of high-mass cast iron (with a large heat capacity). Its rigidity reaches 1.8×10⁵ MPa. When the load changes (for example, the workpiece weight changes by 100 kg), the deformation is less than 0.2 μm, ensuring the processing stability.
4. Special design for filament processing
For the cutting of 0.02mm thin wires, the machine tool is equipped with a double - wire automatic exchange system: it can quickly switch between thick wires (e.g., 0.2mm) and thin wires (0.02mm), and the automatic wire threading accuracy of thin wires is extremely high - the diameter of the threading hole only needs to be 5μm larger than the electrode wire (i.e., 0.025mm), which is achieved through "visual positioning + high - pressure micro - water flow assistance": the visual system first locates the position of the threading hole, and then uses a high - pressure water flow of 0.1MPa to "push" the thin wire through the hole to avoid breakage.
Multi-cutting process: A breakthrough from "multiple-tool finishing" to "less but more efficient"
Multiple cutting is the core process for LSWEDM to achieve precision machining. Through the combination of "rough cutting (removing the allowance) + fine cutting (finishing the surface)", high precision and low surface roughness can be achieved. However, the problem with traditional multiple cutting is "a large number of cuts and low efficiency". In the past, 7 - 9 cuts were required to reach Ra0.4μm, but now only 3 - 4 cuts are needed. The key reason is the technological progress of the rough machining pulse power supply.
In traditional rough machining, to pursue efficiency (e.g., 200 mm²/min), wide pulse width and high peak current are used, resulting in poor surface roughness (Ra ≥ 1.6 μm), and multiple fine cutting and finishing operations are required. However, the new - generation high - efficiency rough machining power supply optimizes the waveform with "narrow pulse width + high peak current". While maintaining high cutting efficiency (350 - 500 mm²/min), it improves the surface roughness after rough cutting to Ra 0.8 μm, which is equivalent to the effect of one - time traditional fine cutting. For example:
- First rough cutting: Remove 80% of the allowance, with a surface roughness of Ra0.8μm.
- Second fine cutting: Adjust the pulse parameters to a small current and short pulse width, and improve the surface roughness to Ra0.4μm.
- Third precision cutting: Further optimize the parameters to achieve a mirror finish of Ra0.2μm.
The technological indicators of multiple cutting are essentially a comprehensive manifestation of the overall machine technology.
- Machine tool accuracy: Repeatability positioning accuracy is 0.4μm, ensuring the consistency of each cutting path.
- CNC system: It has "adaptive path planning" and can automatically adjust the fine cutting compensation amount (e.g., adjust the path according to the rough cutting error).
- Pulse power supply: Pulse width error of fine cutting pulses<1 μs to avoid excessive erosion;
- Working fluid system: Reduce the flushing fluid pressure during fine cutting (from 0.5 MPa to 0.1 MPa) to avoid micro - cracks on the surface caused by flushing.
Filament cutting: The "micro-machining scalpel" for precision parts
Fine wire cutting (wire diameter ≤ 0.05mm) is the key technology for LSWEDM to achieve "micro-nano precision". Its core value lies in machining extremely small-sized features, such as internal sharp corners, narrow slits, and micro holes, which cannot be accomplished by mechanical grinding or milling.
1. Technological breakthrough in filament cutting
Material improvement: High-strength tungsten wire (tensile strength ≥ 3000 MPa) is used to avoid wire breakage during cutting.
Automatic wire threading: With the assistance of visual positioning (accuracy ±2μm) and high-pressure micro water flow (0.1MPa), automatic threading of 0.02mm fine wires is achieved, with a success rate of ≥95%.
Tension control: Use a piezoelectric ceramic sensor to detect the tension of the wire in real time (with an accuracy of ±1 cN). Adjust the tension wheel through a servo motor to keep the tension stable (the tension of the 0.02 mm wire is controlled at 5 - 8 N), and avoid machining errors caused by vibration.
2. Application scenarios of filament cutting
Internal clear corners of precision stamping dies: The requirement for the inner corner radius of the mobile phone SIM card mold is ≤15μm. A 0.02mm fine wire (with a discharge gap of about 5μm) can just machine an inner corner of 15μm (wire diameter/2 + discharge gap = 0.01mm + 0.005mm = 0.015mm).
Narrow slits in IC lead frames: The total width of a frame with 100 leads is only 10 mm. A 0.02 mm fine wire can machine a 0.08 mm narrow slit (wire diameter + discharge gap = 0.025 mm), ensuring the conductivity and strength of the leads.
Micro part processing: The slots of the micro motor core (slot width: 0.05mm) and the pins of the micro connector (diameter: 0.03mm) require a dimensional accuracy of ±0.005mm, which can only be achieved by wire cutting.
Domestic market of LSWEDM: Driving logic for high-speed growth
Since 2001, the domestic LSWEDM market has witnessed explosive growth. In 2001, the output was 150 units. In 2005, it increased to 2,400 units (with an annual compound growth rate of 120%). It is estimated that the output will reach 6,000 to 8,000 units by the end of the "Eleventh - Five - Year Plan". The core driving factors for its growth can be summarized into four points:
1. Demand upgrade in application fields
Mold industry: In 2005, there were more than 20,000 domestic mold enterprises. The molds are developing towards "precision, complexity, and long service life" (for example, the precision of automobile panel molds is ±0.01mm, and the surface roughness Ra of mobile phone molds is 0.2μm). LSWEDM is the only equipment that can meet these requirements.
Aerospace: Cooling holes in engine blades (diameter: 0.5mm, depth: 10mm) and high-precision parts of satellite antennas require the "non-contact machining" of LSWEDM (to avoid deformation caused by mechanical force).
Military industry: Micro parts (such as gyroscope rotors) for missile guidance systems, with a dimensional accuracy of ±0.002mm, can only be processed by LSWEDM.
2. Breakthrough in surface quality: "Replacing grinding with cutting" becomes possible
In the past, the fatal flaw of LSWEDM was the "recast layer". However, with the application of AE power supplies and optimized discharge energy, the thickness of the recast layer can be controlled below 1μm, and the surface quality is comparable to, or even better than, mechanical grinding (without grinding scratches). This has made LSWEDM the "final processing method" for precision molds. There is no need for subsequent polishing (polishing can cause micro - cracks and reduce the service life), and it has been widely accepted by mold enterprises.
3. The irreplaceability of processing performance
The "non-contact, electrical discharge erosion" characteristics of LSWEDM enable it to process any conductive materials (carbide, titanium alloy, superalloy) without being affected by hardness, which is unparalleled by mechanical grinding. For example, grinding carbide (with a hardness above HRC65) requires a diamond grinding wheel and is prone to cracks, while LSWEDM uses electrical discharge erosion without mechanical stress, resulting in more stable quality. In addition, the cost of LSWEDM is already lower than that of grinding: when processing carbide molds, the cost is about 80% of that of grinding, and the efficiency is 30% higher.
4. Substitute demand for HSWEDM
The domestic WEDM market is divided into two categories: LSWEDM (low-speed wire-cut, high-end) and HSWEDM (high-speed wire-cut, low-end). The advantage of HSWEDM is its low price (50,000 - 100,000 yuan per unit), but it has obvious defects: the high wire speed (8 - 12 m/s) leads to large vibrations, poor surface roughness (Ra ≥ 1.6 μm), and low precision (above ±0.01 mm), which cannot meet the precision requirements. With the improvement of the performance-to-price ratio of LSWEDM (the price was about 300,000 - 500,000 yuan per unit in 2005, a 40% decrease compared to 2001), the 40,000-unit market of HSWEDM is being squeezed by LSWEDM - more and more enterprises are replacing HSWEDM with LSWEDM to meet the needs of technological upgrading.
The core competitiveness and future trends of LSWEDM
The core competitiveness of LSWEDM lies in "precision + high efficiency + complex shape processing": The surface quality problem is solved through the AE power supply, high precision is achieved through high-rigidity machine tools and multiple cuts, and the micro-processing field is expanded through thin wire cutting. The rapid growth of the domestic market is essentially the result of the combined effect of technological progress (solving surface quality and precision) and demand upgrading (precision requirements in molds, aerospace). In the future, the development direction of LSWEDM will be "more precise (0.001mm precision), more efficient (1000mm²/min efficiency), and more intelligent (adaptive processing system)", further consolidating its position in the field of precision machining.