Injection molding

Injection molding (injection moulding in British English, often called plastic injection molding) is a manufacturing process in which molten plastic is injected under high pressure into a metal mold, where it solidifies into its final shape. It is the dominant technology for mass-producing plastic parts, from connectors and electronics enclosures to automotive components and consumer goods.

The main advantage of injection molding is a low part price in volume production. The mold is a major investment (from a few thousand dollars or euros for a simple tool to six figures for a large, complex one), but when thousands or millions of parts are produced, the cost per part is very low. The process offers good dimensional accuracy and repeatability and can produce complex geometry.

DFM for injection molding places specific demands on the design: uniform wall thickness (to prevent sink marks and warping), sufficient draft angles to eject the part from the mold, ribs instead of thick walls to achieve stiffness, and well-placed gates. A part designed without regard to these rules leads to molding defects or costly mold modifications.

To validate the design before investing in a mold, it is worth building a 3D-printed prototype (SLA, SLS or FDM) to check shape, assembly and function. The cost of a prototype and a DFM review is usually small compared with the cost of modifying the mold afterward.

Injection molding is the right choice for plastic parts produced in quantities of thousands and up: device housings, enclosures, clips, connectors, caps, gears or technical parts made of filled plastics. It also suits applications that require consistent quality and part-to-part repeatability.

For smaller or validation runs, there are less expensive aluminum or prototype molds and interchangeable inserts for standard master unit die (MUD) frames. The break-even point at which a mold pays off compared with 3D printing or machining depends on the size and complexity of the part and should be calculated for the specific case.

The most expensive mistakes originate in part design: uneven wall thickness, missing draft, undercuts that require side actions, sharp corners and poorly placed gates. Have the design reviewed by the toolmaker or molder before you order the mold, and consider a mold filling simulation (mold flow analysis) for more complex parts.

In a mold quote, check the mold material and its expected service life (number of cycles), the number of cavities, the type of runner system, who owns the mold and where it will be stored, what sampling and modifications cost, and whether the price includes first samples with an inspection report. A design change after the mold is built is always significantly more expensive than before.