In any plastic injection mold, the fundamental split between the fixed half (cavity side) and the moving half (core side) dictates everything from part ejection to cooling efficiency. The fixed half is bolted to the injection machine’s platen and houses the sprue bushing, runner system, and often the gate. The moving half, attached to the ejector platen, carries the core, ejector pins, and return pins. This separation is not arbitrary—it ensures that when the mold opens, the part stays on the core side, allowing the ejector system to push it out cleanly. A typical 2-plate mold runs with a parting line at the cavity/core interface, and for parts with deep undercuts, side actions or lifters are added on the moving half to avoid premature ejection damage.

During injection, melt enters through the sprue, flows across the runner, and passes the gate into the cavity. The gate location and type—whether edge, submarine, or fan—directly influence weld lines, air traps, and residual stress. For example, a submarine gate is often used on the moving side to allow automatic degating, but it requires a sharp shear edge to avoid stringing. Cooling channels are then laid out symmetrically around both halves, with baffles or bubblers used near deep cores to prevent hot spots. Real-world data shows that a 3-mm wall thickness part with a 20-second cycle can reduce its cooling time by 15% if the core-side cooling lines are placed within 8–10 mm of the cavity surface, versus 15 mm away. That is why mold designers prioritize conformal cooling on the moving half, where most heat accumulates.

Ejection is another area where the moving side takes the lead. Standard ejector pins, sleeve ejectors, or stripper plates are mounted on the ejector plate, and their return is governed by return pins that contact the fixed half before the mold closes. For parts with fragile ribs, a common practice is to add a small taper (0.5°–1°) on the core side to reduce stripping force, and to use a double-ejection sequence if the part sticks to the cavity. On the fixed side, the only moving elements are usually the sprue puller and, in some three-plate molds, the runner plate. Understanding this balance—where the cavity defines the outer geometry and the core defines the inner geometry—is the first step in troubleshooting warpage or sink marks. For more detailed mold sourcing and design references, visit MoldWorld (www.moldw.com) for supplier directories and technical guides.