Injection mold

An injection mold (injection mould in British English, in the shop often just the tool) is a precision metal tool with a shaped cavity into which molten plastic is injected during injection molding and where it solidifies into the finished part. The mold defines the shape, accuracy and surface of the molded part and is usually the largest upfront investment.

A mold has two halves that meet at the parting line: the stationary half (A side) at the injection unit and the moving half (B side), which holds the part when the mold opens and whose ejector system pushes it out. The part geometry is formed by the core and cavity inserts. Plastic enters through the feed system (sprue, runners and gate), and air escapes through vents. Undercuts, features that prevent the part from releasing in the opening direction, require slides or lifters and add cost.

Temperature control channels hold the mold at the temperature the plastic requires: typically tens of degrees Celsius for common plastics, over 100 °C for some engineering plastics. Cooling usually takes up most of the cycle. Molds also differ in cavity count (single-cavity or multi-cavity, with dozens of cavities for small parts), runner system (a cold runner, whose runner is scrapped or reground, or a hot runner, which reduces waste and shortens the cycle) and construction (two-plate, three-plate and stack molds). A family mold makes several different parts of an assembly in one cycle, and a two-shot mold combines a rigid and a soft plastic in one process.

Production molds are made of tool steel, pre-hardened for small and medium runs or hardened for large runs and abrasive glass-fiber-filled plastics. Hardened molds are usually designed for hundreds of thousands to millions of cycles. Aluminum molds are cheaper and quicker to make but wear out sooner and serve mainly as prototype molds for smaller quantities. A production mold typically takes several weeks to months to build and costs from a few thousand dollars or euros for a simple tool to six figures for a large, complex one.

An injection mold is worth the investment once the part design is validated and expected volume reaches thousands of parts or more, or when accuracy, surface or material requirements rule out other processes. Cavity count, mold material and runner type follow mainly from annual volume: a single-cavity cold runner mold suits smaller runs, while hundreds of thousands of parts per year usually justify a multi-cavity hot runner mold with hardened inserts.

To validate the design or for smaller quantities, first consider a cheaper aluminum mold, interchangeable inserts in a master unit die (MUD) frame, or vacuum casting.

The most expensive mistakes originate in the part design, not in the mold: missing draft angles, uneven wall thickness, unnecessary undercuts and sharp corners. Changes to a finished mold are not symmetrical either. Removing steel (and thus adding material to the part) is relatively easy, while the opposite requires welding or a new insert. Critical dimensions are therefore often designed so they can be fine-tuned by removing steel, an approach known as "steel safe".

Before ordering, have the toolmaker do a DFM analysis of the part (plus a mold flow simulation for complex parts) and agree on gate, parting line and ejector pin locations relative to cosmetic surfaces. The contract should state the required mold life and material (in the US, mold life is often specified as an SPI mold class), cavity count, mold ownership and storage location, how many sampling and modification rounds are included, the inspection report for first samples, and who pays for maintenance and repairs.