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EDM (Electrical Discharge Machining) in Mold Making

China Manufacturing Guide

Last updated: 15 June 2026

EDM in Mold Making

Electrical Discharge Machining (EDM) is a critical process in Chinese mold making for creating complex cavities, sharp internal corners, and fine details that cannot be machined with conventional cutting tools.

Types of EDM Used in Chinese Mold Shops

Wire EDM (WEDM): uses a thin wire electrode to cut through hardened steel. Accuracy: +/- 0.002-0.005mm. Used for: punch dies, blanking dies, ejector pin holes, and slotted details. Sinker EDM (Ram EDM): uses a shaped electrode to erode cavity details. Accuracy: +/- 0.005-0.01mm. Used for: complex cavity details, ribs, logos, and textured surfaces. Hole Drilling EDM: fast drilling of small holes for wire threading or cooling channels.

Surface Finish from EDM

EDM surface finish is measured in Ra (micrometers). Chinese mold shops typically offer: Rough cut: Ra 3.2-6.3 um (for non-cosmetic surfaces), Semi-finish: Ra 1.6-3.2 um, Fine finish: Ra 0.4-0.8 um, and Ultra-fine: Ra 0.1-0.2 um (requires multiple finishing passes). Each step improvement in finish doubles the EDM cycle time. Specify the required finish on your mold drawing.

Buyer's Tip: A common issue with Chinese EDM: the recast layer (white layer) left after EDM is too thick, typically 0.01-0.05mm instead of the recommended 0.002-0.005mm. A thick recast layer can crack during molding or reduce corrosion resistance. Request recast layer measurement (cross-section analysis) for critical mold cavities and specify maximum recast layer thickness in your mold specifications.
What This Means for Your Project: For most applications, EDM surface finish of Ra 1.6 um is adequate and cost-effective. For textured surfaces or areas requiring polish, request Ra 0.4 um or better. For high-gloss parts, avoid EDM altogether and use machining + polishing instead, as EDM creates a micro-cracked surface that is difficult to polish to mirror finish.

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