Mold Core and Cavity Design Guide

Principles of core and cavity design for injection molds. Split lines, undercuts, side actions, cooling, and ejection.

Core and Cavity Fundamentals

Every injection mold has a cavity (A-side, outer surface) and core (B-side, internal features). The parting line is where they meet.

Parting Line Selection

• Flat parting line — Simplest and cheapest
• Stepped parting line — Accommodates features at different heights
• Contoured parting line — Follows part surface. Most expensive

Undercuts

• Side Cores / Slides: Move perpendicular to draw. Most common.
• Lifters: Rise at angle during ejection. For internal undercuts.
• Collapsible Cores: For internal threads.

Cooling Integration

Cooling accounts for ~70% of cycle time. Baffles and bubblers for deep cores. Conformal cooling via 3D-printed inserts for complex geometry. Channels should be 2-3x the channel diameter from cavity surface.

Ejection System

• Ejector Pins: Simplest. Place on strong features.
• Ejector Sleeves: Surround cylindrical cores.
• Stripper Plate: Ejects at parting line. Best for thin-wall parts.

Venting

Trapped air causes burn marks. Vent 0.02-0.05 mm deep at last fill points. At least 20-30% of cavity perimeter should have venting.