Injection Molding

Injection molding produces high-volume plastic parts by injecting molten polymer into precision tooling, delivering repeatable geometry, good surface finish, and low per-part cost.

Overview

Injection molding forms thermoplastic (and some thermoset) parts by injecting molten material into a cooled mold, then ejecting the solidified part. It excels at repeatability, cosmetic surfaces, and integrating features like ribs, bosses, snaps, threads, and seals in a single cycle.

Choose it when you have stable geometry and need medium-to-high volumes where tooling cost can be amortized. Tradeoffs: higher upfront mold cost and lead time, DFM constraints (draft, uniform walls, ejection), and material shrink/warp risk that must be designed and tooled around. Sub-process options cover more complex needs: overmolding and insert molding for multi-material/metal-plastic assemblies, thin-wall molding for fast cycles, compression/transfer for certain thermosets, LSR molding for elastomers, multi-shot/co-injection for multi-material aesthetics or properties, MIM for small complex metal parts, and related molding families like blow and rotational molding for hollow or large parts.

Common Materials

  • ABS
  • Polypropylene (PP)
  • Polycarbonate (PC)
  • Nylon 6/6
  • POM (Acetal)
  • Liquid Silicone Rubber (LSR)

Tolerances

±0.005 in

Applications

  • Enclosures and housings
  • Snap-fit clips and latches
  • Medical device disposable components
  • Electrical connectors and covers
  • Consumer product bezels and buttons
  • Automotive interior trim parts

When to Choose Injection Molding

Injection molding fits parts that will run in repeat production and benefit from low per-unit cost with consistent dimensions and cosmetics. It’s a strong choice for complex plastic geometries with integrated features, especially once volumes justify dedicated tooling and process validation.

vs CNC machining

Choose injection molding when you need hundreds to millions of parts with consistent cost and cycle-time-driven throughput. Molding also enables molded-in features (ribs, snaps, living hinges) that would be slow or expensive to machine repeatedly.

vs 3D printing (additive manufacturing)

Choose injection molding when the design is frozen and repeatability, surface finish, and unit cost matter more than iteration speed. Molding also supports production materials and appearance standards that are hard to match at scale with additive.

vs Urethane casting (vacuum casting)

Choose injection molding when you need tighter part-to-part consistency and long-run economics. Injection molds support higher volumes, faster cycles, and more robust process control than silicone-tool casting.

vs Thermoforming

Choose injection molding when you need detailed features, tight fit with mating parts, and consistent wall thickness control in small geometries. Molding supports complex undercuts via tooling actions and integrates functional details that thermoforming can’t form cleanly.

vs Die casting

Choose injection molding when the part is plastic, needs electrical insulation, low weight, corrosion resistance, or living hinges/snaps. Molding also offers a broader set of colors, textures, and polymer property options without post-finishing.

Design Considerations

  • Hold nominal wall thickness consistent; use ribs instead of thick sections to avoid sink and long cycle times
  • Add draft on all pull-direction surfaces (often 1–2° minimum; more for textured surfaces)
  • Place parting lines and gate locations early; they drive cosmetics, weld lines, and dimensional control
  • Design for ejection with flat ejector-pad areas and avoid deep, un-drafted pockets that stick in the tool
  • Use generous radii at internal corners; sharp corners concentrate stress and hinder flow
  • Call out critical-to-function dimensions and acceptable cosmetic areas so the tool and process can be optimized for what matters