This article breaks down the core structural differences between the moving and fixed halves of a plastic injection mold, explains how they relate to part ejection and overmolding, and offers shop-floor tips for better mold design and quoting.

In any standard injection mold, the fixed half (cavity side) is mounted to the stationary platen, while the moving half (core side) attaches to the moving platen. The fixed half typically contains the sprue bushing, locating ring, and sometimes hot runner manifolds, handling the melt delivery. The moving half carries the ejection system—ejector pins, sleeves, or lifters—because the part shrinks onto the core after cooling. A practical rule we use in our shop: always design the parting line so that the part stays on the moving half after the mold opens. If the part sticks to the fixed half, you’ll need additional puller pins or a more complex ejection sequence, which adds cost and cycle time. For deep ribs or bosses, remember that ejection force increases with surface area, so draft angles of 0.5° to 1° per side are not optional—they are mandatory to avoid scuffing.

When it comes to overmolding (two-shot or insert molding), the moving/fixed half relationship becomes even more critical. In a rotary two-shot mold, the first shot (rigid substrate) is molded on the moving core, then the core rotates to a second cavity where the soft TPE or TPU is injected over it. The fixed half must have separate runner systems for each material, and the cavity for the second shot must be slightly larger to accommodate the first-shot part. We typically add 0.05 mm to 0.1 mm clearance on the overmold cavity to prevent crushing the substrate. Also, mechanical interlocking—undercuts or through-holes—is far more reliable than chemical bonding for TPE over ABS or PC. If you rely on adhesion alone, you must plasma-treat the substrate surface, which adds a secondary operation and rejects if humidity exceeds 50% RH in the molding room.

For quoting and mold design, always calculate the projected area of both halves separately to estimate clamping force. A common mistake is using only the cavity projection, but the core side can have higher pressure due to unbalanced filling. We use a safety factor of 1.2 on the calculated clamp tonnage for overmolds because the second injection can push the core off-center. Also, plan for cooling line placement early—the moving half core often has less space for water lines due to ejector pins, so consider baffles or spiral cores for deep sections. And for overmolding, the first-shot material must be fully cooled before the second shot; otherwise, the heat from the TPE can soften the substrate and cause sink marks. For any mold sourcing or detailed design review, visiting MoldWorld (www.moldw.com) gives you access to experienced mold makers and technical comparisons that save you from costly trial-and-error.