In any standard plastic injection mold, the tool is split into two primary halves: the fixed half (cavity side) and the moving half (core side). The fixed half is mounted to the injection machine’s stationary platen and houses the sprue bushing, locating ring, and often the main runner system. Its job is to receive the molten polymer directly from the nozzle and guide it into the cavity. The moving half, mounted to the movable platen, carries the ejector system, core pins, and often the side-action mechanisms. The key structural difference is not just physical location—it’s functional. The fixed half remains stationary during the entire cycle, while the moving half travels with the platen to open and close the mold. This separation dictates where cooling channels, venting, and ejection are placed.

During injection, the mold is clamped shut under tonnage—typically 2 to 5 tons per square inch of projected area, depending on resin viscosity. The molten plastic enters through the sprue, flows through runners and gates, and fills the cavity. As the polymer cools, it shrinks around the core side. This is why the moving half almost always carries the ejector pins: the part naturally adheres to the core. The fixed half, by contrast, sees the highest wear from melt flow and pressure, so hardened tool steels like P20 or H13 are common there, while the moving half may use pre-hardened steel or even beryllium copper for localized cooling. A practical rule: if you need to add a lifter or slider, it usually goes on the moving half because that’s where the part stays after opening.

Understanding this split saves real time in troubleshooting. For example, if you see short shots, check the fixed half’s gate and runner balance first. If you see ejection marks or part sticking, look at the moving half’s ejector layout and draft angles. Also, remember that cooling efficiency differs: the fixed half often has less space for water lines due to the sprue and runner, so conformal cooling is more critical there. For any mold buyer or tooling engineer, specifying the correct moving/fixed half design is the first step to a stable process. For more detailed mold sourcing and design guidelines, visit MoldWorld at www.moldw.com—a solid resource for comparing mold suppliers and technical specs.