Core and Cavity Machining for Injection Molds: Methods, Tolerances and Strategies
Overview
The core and cavity are the two halves of an injection mold that form the part shape. Their machining quality directly determines part accuracy, surface finish, and mold life. Core and cavity machining typically accounts for 35-50% of total mold manufacturing cost.
Machining Methods
1. 3-Axis CNC Milling
Standard approach for simple to moderate geometries. Limitations: cannot machine undercuts, requires ball-end mills for 3D surfaces, longer finishing times for complex contours. Best for: simple cavities, work with EDM for fine details.
2. 5-Axis CNC Milling
Allows tilting of the tool, reducing the need for EDM. Benefits: better surface finish, can machine draft angles naturally, fewer setups, reduces hand polishing by 30-50%. Cost premium: 20-40% over 3-axis.
3. EDM (Sinker)
Used for features that cannot be reached by cutting tools: deep ribs, sharp internal corners, fine text. Requires graphite or copper electrodes machined as negatives of the cavity feature.
4. High-Speed Milling (HSM)
RPM 15,000-40,000+ with small stepovers and high feed rates. Produces finishes that often require minimal hand polishing. Reduces EDM usage significantly.
Machining Sequence
- Roughing: Remove bulk material (70-80% of volume). Leave 0.3-0.5mm stock
- Semi-finishing: 0.1-0.2mm stock. Improved surface for final pass
- Finishing: Final contour pass. 0.01-0.03mm stock for polish
- Hand finishing/polishing: Remove tool marks to achieve required surface finish
Tool Selection Guide
| Operation | Tool Type | Typical Diameter |
|---|---|---|
| Roughing (steel) | Indexable insert mill | 16-32mm |
| Semi-finish | Solid carbide end mill | 10-16mm |
| Finish (flat areas) | Flat end mill | 6-12mm |
| Finish (3D contours) | Ball end mill | 3-10mm |
| Fine detail | Micro ball end mill | 0.5-3mm |
Common Issues
Tool Deflection
Long-reach tools (L/D > 5:1) deflect 0.02-0.10mm. Use shorter tools in steps, or EDM for deep cavities.
Chatter Marks
Vibration marks on cavity surface. Can be eliminated by adjusting RPM and feed, using variable-pitch end mills, or improving workholding rigidity.
Mismatched Cavity Halves
Core and cavity misalignment at parting line. Always machine both halves with referenced datum points, and verify alignment on CMM.