Injection Molding

Injection molding produces high-volume, repeatable plastic and elastomer parts by injecting molten material into hardened molds for consistent, detailed geometries at low unit cost.

Overview

Injection molding forces molten plastic or elastomer into a hardened mold cavity to create repeatable, detailed parts at scale. After cooling, the mold opens and ejects the part, ready for the next cycle. Tooling costs are significant, but once the mold is built, per-part cost is very low, making this ideal for medium to very high volumes.

Engineers use injection molding for parts that need consistent dimensions, good surface finish, and integrated features like snaps, ribs, bosses, and living hinges. Sub-processes expand capability: standard molding for most thermoplastics, overmolding and insert molding for combining materials or encapsulating hardware, thin wall molding for lightweight housings, compression and transfer molding for thermosets, LSR molding for flexible silicone parts, multi-shot/co-injection for multi-material components, MIM for small metal parts, and blow or rotational molding for hollow bodies. Tradeoffs include long lead time and cost for tooling, tighter constraints on wall thickness and draft, and more complex design-for-manufacture requirements.

Common Materials

  • ABS
  • Polycarbonate
  • Nylon 6/6
  • Polypropylene
  • PEEK
  • Liquid silicone rubber

Tolerances

±0.002" to ±0.005" on critical features, looser on large dimensions

Applications

  • Consumer product housings and enclosures
  • Automotive interior and under-hood components
  • Medical device housings and disposables
  • Electrical connectors and cable overmolds
  • Gears, levers, and functional plastic mechanisms
  • Caps, closures, and packaging components

When to Choose Injection Molding

Choose injection molding for plastic or elastomer parts when you have medium to high production volumes and need consistent, repeatable dimensions. It suits parts with reasonably uniform wall thickness, molded-in features, and where investing in tooling is justified by volume and part cost targets.

vs CNC machining

Choose injection molding over CNC machining when part volumes are high enough that tooling cost is offset by very low per-part cost. It is better for complex plastic geometries with ribs, bosses, snaps, and textures that would be slow or impossible to machine efficiently.

vs 3D printing

Choose injection molding over 3D printing when you need production-scale quantities, consistent material properties, and smooth, production-grade surfaces. It excels once the design is stable and you need reliable, repeatable parts at unit costs far below additive methods.

vs Urethane casting (vacuum casting)

Choose injection molding over urethane casting when you move beyond low-volume prototyping into thousands or more parts. It delivers higher repeatability, broader material options for engineering-grade plastics, and far lower part cost once tooling is in place.

vs Die casting

Choose injection molding over die casting when the part can be plastic or elastomer instead of metal, especially where weight reduction, design freedom, and lower tooling wear are priorities. It suits non-structural or moderately loaded parts that benefit from molded-in features and reduced assembly steps.

vs Sheet metal fabrication

Choose injection molding over sheet metal when you need 3D features, complex organic shapes, or integrated snaps, bosses, and internal structure. It is better for sealed housings, ergonomic shapes, and parts where multiple sheet metal components would otherwise need to be assembled.

Design Considerations

  • Keep wall thickness as uniform as possible to minimize warpage, sink, and cycle time
  • Add proper draft (typically 1–2° or more) on all faces normal to mold opening to aid ejection and reduce scuffing
  • Use ribs and gussets instead of thick sections to add stiffness while controlling shrink and sink marks
  • Plan gate locations, parting line, and ejector pin marks early so they avoid cosmetic and critical functional areas
  • Specify only truly critical tolerances and datums; tight, unnecessary tolerances can drive up tooling and processing cost
  • Avoid sharp internal corners; use generous radii to improve flow, reduce stress concentrations, and extend tool life