In any plastic injection mold, the split between the moving half (core side) and the fixed half (cavity side) is not just a mechanical convenience—it defines the entire ejection and cooling strategy. The fixed half typically carries the sprue bushing, locating ring, and the main cavity surface, which determines the part’s external geometry. The moving half holds the core, ejector pins, and often the side-action mechanisms. For parts with deep ribs or internal undercuts, the core side must be designed with sufficient draft (usually 0.5° to 1.5° per side) to avoid drag marks and to allow clean ejection. A common mistake is placing excessive surface finish on the core side, which increases demolding force and can cause stress whitening on the part. In production, the fixed half also houses the hot runner manifold when used, and its thermal expansion must be calculated to maintain shutoff alignment with the core at operating temperatures (typically 80°C–120°C for ABS/PC blends).

Overmolding, or two-shot molding, adds another layer of complexity to the core/cavity relationship. The first shot (substrate) is molded in the cavity, then either rotated or transferred to a second cavity where the second material encapsulates it. Key process points include: the substrate must be fully cooled below its heat deflection temperature before the second shot, otherwise the second material’s heat will cause sink marks or warpage. For TPE over PP, the recommended substrate temperature at the interface is 40°C–60°C, and the second shot melt temperature should be controlled within ±5°C to ensure chemical bonding. Mechanical interlocking (undercuts, through-holes, or roughened surfaces) is still required when the two materials are incompatible—for example, TPE over POM or nylon. The mold designer must also account for the differential shrinkage: a typical PP substrate shrinks 1.5%–2.0%, while a TPE skin shrinks 0.8%–1.2%, so the core for the second shot must be oversized by that difference to prevent the skin from cracking.

Practical shop-floor advice: always verify the parting line seal on the moving half before running overmolding trials, because any flash on the first shot will act as a stress riser and cause delamination at the interface. Also, use thermal pins or conformal cooling lines in the core to keep the substrate’s surface temperature uniform—a 10°C gradient across the core can lead to inconsistent bond strength. Finally, document the exact overmolding cycle parameters (injection speed, hold pressure, and mold open time) for each material pair, as these are the first variables to adjust when defects like peeling or edge tearing appear. For more detailed mold sourcing and process troubleshooting, visit MoldWorld at www.moldw.com—a practical resource for mold builders and injection molders alike.