Injection molding

Overview

Injection molding injects molten thermoplastic into a hardened steel mold cavity at high pressure. After cooling and ejection, parts repeat within tight dimensional bands and excellent surface definition—ideal when design is stable and annual quantity amortizes tooling.

When to choose

• Stable CAD and validated fit—tooling changes after steel cutting are expensive.
• Thousands to millions of units where cycle time and cavity count drive unit cost down.
• Cosmetic Class-A surfaces, textures, multi-material strategies (overmolding) once process is locked.
• Need for commodity engineering plastics (ABS, PC, PP, PA, POM) with supplier-grade traceability.

When not to choose

• Early concept validation—use CNC or 3D printing until geometry and material freeze.
• Parts without adequate draft, impossible undercuts without slides, or uncontrolled thick sections.
• Ultra-low volumes unless bridge tooling or soft tooling explicitly fits the schedule.

Design rules

• Maintain uniform wall thickness to reduce sink, voids, and warpage.
• Add draft on vertical walls; define parting line, gate, and visible surfaces early.
• Use ribs and bosses instead of bulky solids; specify radii at rib junctions.
• Plan for weld lines, air traps, and knit lines when flow fronts meet.

Typical plastics

Common families include ABS and PC for housings, PP and PA for structural clips and gears, POM for wear surfaces, and PEEK for aggressive environments. Glass-filled variants trade stiffness for tool wear and anisotropic shrink.

Tolerance notes

Shrinkage is material- and geometry-dependent. Critical fits often require tolerance stacks reviewed against mold shrink data and may need secondary machining or gauging.