EDM vs CNC Milling for Mold Making: Which Process to Choose?
Overview of EDM and CNC Milling
Electrical Discharge Machining (EDM) and Computer Numerical Control (CNC) milling are two fundamentally different approaches to mold cavity manufacturing. While CNC milling removes material mechanically with rotating cutting tools, EDM uses controlled electrical sparks to erode conductive materials.
Modern mold shops typically invest in both technologies, with the choice depending on part geometry, material hardness, surface finish requirements, tolerances, and production volume.
Process Comparison
| Parameter | CNC Milling | Sinker EDM | Wire EDM |
|---|---|---|---|
| Material Hardness | Up to HRC 50-55 | Any conductive (HRC 60+) | Any conductive (HRC 60+) |
| Surface Finish (Ra) | 0.4-3.2 um | 0.1-0.8 um | 0.2-1.6 um |
| Tolerance | +/-0.025mm | +/-0.005-0.013mm | +/-0.0025-0.013mm |
| Max Depth-to-Width | 4:1 | 20:1 or more | N/A |
| Setup Time | 30-90 min | 1-4 hours | 30-60 min |
| Best For | 3D cavities, cores | Deep ribs, sharp corners | Through cuts, dies |
When to Choose CNC Milling
- Large cavities with shallow depth-to-width ratios (< 4:1)
- Multi-axis 3D contours (ball-end mill finishing)
- Prototype and low-volume molds where speed matters
- Soft materials (pre-hardened steels, aluminum, copper alloys)
- Mold bases, ejector pin holes, cooling channel drilling
When to Choose EDM
- Deep ribs and narrow slots (depth-to-width > 5:1)
- Sharp internal corners (0.1mm radius or less)
- Fine texturing and surface patterning
- Hardened steel cavities (HRC 55+)
- Complex 3D shapes with undercuts
Cost Analysis
| Factor | CNC Milling | EDM (Sinker) |
|---|---|---|
| Machine Hour Rate | 0-120/hr | 0-80/hr |
| Machining Time | 4-8 hours | 8-20 hours |
| Tool Cost | 0-200 | 00-800 |
| Total Cost Per Cavity | 50-1,200 | 00-2,400 |
Hybrid Approach
Most efficient mold shops use a combined strategy: CNC mill the bulk cavity (70-80% removal), then use EDM for fine details and deep ribs. This reduces total cost by 30-40% compared to EDM-only while achieving better feature quality than milling-only.