Mirror EDM Parameter Specifications for Injection Molds
Mirror EDM for injection molds is mainly applied to mold cavities of quenched or pre-hardened plastic mold steels including S136, STAVAX and NAK80. It handles visible-surface ribs, deep cavities and sharp corners inaccessible to manual polishing. Targets include uniform mirror surface quality, reduced subsequent polishing workload and elimination of defects such as pits, carbon deposition, orange peel texture and microcracks. Mirror electrical discharge machining must follow progressive multi-stage processing: roughing → semi-finishing → finishing → mirror micro-finishing. Discharge energy decreases stepwise to remove deteriorated white layers generated from previous processes layer by layer. Combined with workshop mass production experience, unified parameter specifications and process control standards are summarized below.
Ⅰ. Specifications for Electrode Material Selection, Spark Gap and Machining Allowance
Oxygen-free copper electrodes are uniformly adopted for mirror finishing. Ordinary electrolytic copper is prohibited. No tool marks shall remain on electrodes after CNC machining, and pre-grinding can be performed when necessary. Separate rough electrodes and finish electrodes are manufactured for roughing using high-density graphite to prevent electrode defects from roughing from copying onto mirror surfaces. Negative polarity machining (electrode connected to negative pole) is adopted to lower electrode loss.
Standard unilateral spark gap reference: Roughing 0.12～0.18mm, semi-finishing 0.04～0.08mm, finishing 0.015～0.03mm, mirror micro-finishing 0.005～0.01mm.
Machining allowance control: After roughing, reserve 0.08～0.12mm for semi-finishing. Semi-finishing leaves 0.025～0.04mm for finishing. Only 0.008～0.015mm remains after finishing for mirror micro-finishing. Excessive allowance leads to difficult chip removal and carbon deposition during mirror processing. Insufficient allowance fails to completely remove deteriorated discharge layers from prior procedures, resulting in dark streaks and color difference on surfaces. Appropriately enlarge allowances for large-area cavities, and slightly reduce allowances for narrow deep ribs.
Ⅱ. Standard Discharge Parameters by Processing Stage (Copper Electrode Machining Quenched Mold Steel)
Roughing Stage
Objective: Rapid material removal, guarantee contour shape and control excessive electrode loss without pursuing surface quality. Peak current:6～12A, Pulse-on time Ton:40～80μs, Pulse-off time Toff:30～60μs, Open-circuit voltage:90～110V. Medium servo speed, automatic lifting enabled, medium-pressure side flushing with dielectric oil. Uniform orange sparks indicate stable processing. Persistent red sparks require extended pulse-off time. Clean cavity residues after roughing and inspect for local burns and short-circuit pits. Defects must be eliminated before semi-finishing.
Semi-finishing Stage
As a critical transitional procedure for mirror processing, it removes coarse discharge pits and thick deteriorated layers from roughing. Peak current:2.5～5A, Ton:15～30μs, Toff:20～40μs, Open-circuit voltage:100～120V. Reduce electrode lifting frequency and stroke, and lower flushing pressure. Surface roughness reaches Ra0.8～1.6μm after this stage with no obvious rough spark marks visible to naked eyes. Extend processing duration or slightly reduce current for partial pits.
Finishing Stage
Formal transition toward mirror surfaces with strictly controlled single discharge energy. Peak current:1～2.5A, Ton:5～12μs, Toff:12～25μs, Open-circuit voltage:110～130V. Reduce external flushing and prioritize internal electrode flushing. High-speed oil flow disturbs discharge gaps and generates surface ripples. Slow planar orbiting can be enabled for large flat surfaces to repair side and bottom surfaces. Surface roughness reaches Ra0.25～0.6μm after processing with uniform texture and no visible light-dark dividing lines.
Mirror Micro-finishing Stage (Core Mirror Procedure)
Target: Achieve mirror effect with Ra≤0.1～0.2μm. Peak current:0.5～1.2A, Ton:1～5μs, Toff:10～20μs, Open-circuit voltage:120～140V. Forced flushing is preferably disabled and immersion-type processing adopted. Reduce electrode lifting stroke to minimum, and continuous lifting can be disabled on some machine tools to avoid periodic tool marks on surfaces. Orbiting amplitude narrows to 0.003～0.008mm with low-speed planar motion. Mirror processing proceeds slowly. Randomly increasing current to shorten processing time easily causes permanent orange peel texture from abrupt energy change. Pulse-off time can be moderately extended for deep narrow cavities to prevent carbon deposition black spots.
Ⅲ. Control Specifications for Dielectric Oil, Chip Removal and Machine Environment
Mirror EDM imposes strict requirements on dielectric oil cleanliness. Special mirror spark oil must be used with filter element precision ≤5μm. Filters are replaced regularly to avoid impurity and water contamination. Oil temperature stabilizes at 22～28℃. Excessive temperature difference triggers workpiece thermal deformation and partial mirror ripples.
Follow the principle vigorous flushing for roughing, weak or no flushing for finishing. Sufficient flushing ensures chip removal in roughing and semi-finishing. Low-pressure immersion mode is adopted for finishing and mirror micro-finishing. Powerful flushing disrupts stable discharge gaps and induces airflow texture on surfaces. Intermittent short low-pressure auxiliary flushing can be applied for deep rib cavities with poor chip removal.
Machine worktables, fixtures and electrode clamping surfaces must stay clean with repeated positioning precision ≤0.003mm. Fully exhaust air bubbles before processing. Oil levels must completely submerge processing zones. Air bubbles attached to workpiece surfaces form white spot defects.
Ⅳ. Defect Adjustment Guidelines for Common Surface Problems
Carbon deposition black spots on surfaces: Prioritize extending Toff pulse-off time, lower peak current, inspect flushing smoothness and clear cavity residues.
Hazy uneven orange peel mirror surfaces: Excessively long Ton pulse width. Reduce pulse width and current, and confirm complete removal of deteriorated layers from previous processing.
Horizontal ripples and stripes: Excessive lifting stroke, overhigh flushing pressure or fast orbiting speed. Reduce lifting height, weaken flushing pressure and lower orbiting rotation speed.
Severe electrode corner loss and incomplete sharp-corner machining: Moderately extend pulse width and reduce current to avoid concentrated long-duration discharge. Reserve extra finishing time for sharp corners.
Surface microcracks: Excess discharge energy. Reduce current and pulse width gradually. Avoid jumping to large parameters when machining quenched steel.
Conclusion
Mirror EDM for injection molds cannot rely on fixed parameters applied universally for all cavities. The core process logic involves stepwise reduction of discharge energy through multi-stage processing, reasonable distribution of machining allowances for each procedure and stable chip removal environments to achieve uniform mirror surfaces. During actual production, select slightly lower current and extend processing duration for large visible cavities. For narrow deep ribs, fine-tune pulse-off time to balance chip removal under conditions preventing carbon deposition. Operators apply standard parameters as benchmarks and adjust in real time according to cavity area, depth and steel hardness while observing spark status. Strictly implement four-stage progressive processing flow and standardize electrode quality and dielectric filtration conditions. Stable mirror surface appearance can be achieved to reduce subsequent polishing hours, avoid mold rework caused by EDM surface defects, and guarantee molding appearance quality of plastic products.
