cavity

In injection molding and die casting, a cavity is the recessed pocket or enclosed chamber cut into a mold tool where molten material flows and solidifies to form the finished part. Each cavity produces one piece per cycle, so a four-cavity mold makes four identical parts every time the press closes and ejects. The cavity's geometry is a negative image of the part you want to manufacture, precise to tolerances typically between 0.005 and 0.002 inches depending on material and application.

The number of cavities in a mold directly affects production speed and unit cost. A single-cavity mold runs slower but suits small batches, prototyping, or parts with complex undercuts that are difficult to eject cleanly. Multi-cavity molds, especially those with 8, 16, or 32 cavities for small consumer goods, dramatically increase output per cycle but require larger presses, more cooling channels, and higher tooling investment upfront. Unbalanced cavity designs, where cavities are unequal in size or poorly fed, cause uneven filling, pressure drop, and dimensional variation across the part population.

Design and maintenance

Cavity walls must be polished and finished to a specific surface roughness, measured in microinches (Ra), to ensure resin or metal flows smoothly and parts release without sticking. Thermoplastic molds typically target Ra 8 to 16 for exterior surfaces; aluminum die-cast cavities may be rougher. Gate placement, where material enters the cavity, is critical; poor gate design creates flow lines, weld marks, voids, or trapped air that compromise part strength and appearance.

In service, cavities degrade from thermal cycling, chemical attack by process material, and mechanical wear from ejector pins or undercut cores. Cavity erosion in zinc-alloy die-cast molds is common around thin sections and sharp corners where metal velocity is highest. Periodic cavity inspection using replica tape or CMM coordinate measurement reveals sink marks, flash lines, and dimensional creep; damaged cavities are often hand-polished or locally welded and reprinted to extend tool life.

The term cavity applies across injection molding, compression molding, transfer molding, blow molding, and die casting. A core, by contrast, is a male element that creates internal passages or undercuts. Together, cavities and cores form the complete mold set that defines your part's final form, surface finish, and tolerances.