This article breaks down the functional differences between the moving and fixed mold halves in plastic injection molds, and outlines the critical process parameters for successful overmolding (two-shot) applications.

In any standard plastic injection mold, the fixed half (cavity side) is mounted to the stationary platen and houses the sprue bushing, locating ring, and often the hot runner manifold. Its primary job is to receive the molten polymer and distribute it evenly into the cavity. The moving half (core side), on the other hand, is attached to the moving platen and carries the ejection system, side actions, and lifters. The parting line—where these two halves meet—must be precisely matched, typically within a 0.02–0.05 mm tolerance, to prevent flash and ensure consistent wall thickness. For molds with deep ribs or bosses, the core side often requires additional cooling channels (e.g., 6–8 mm diameter baffles or spiral cores) to reduce cycle time by 15–20% compared to a poorly cooled design.

When it comes to overmolding (also called two-shot or multi-material molding), the dynamic/fixed half relationship becomes even more critical. The first shot (substrate) is molded in the fixed half, then the mold opens and the rotating core plate—usually mounted on the moving half—transfers the substrate to a second cavity where the overmold material (e.g., TPE or TPU) is injected. Key parameters to control include melt temperature differential: for a PP substrate with a TPE overmold, the TPE melt should be 20–30°C lower than the substrate’s melting point to avoid melting the first shot’s surface. Also, the interface bond strength depends on the substrate’s surface roughness (Ra 0.4–0.8 µm is optimal) and the use of adhesion promoters like maleic anhydride-grafted PP. Injection speed for the second shot should be slower (typically 30–50% of the first shot) to prevent washing away the substrate surface.

Another practical point is venting at the overmold interface. Because the second shot flows over a pre-formed geometry, trapped air can cause burn marks or incomplete filling. Mold designers should add 0.02–0.03 mm deep vents along the parting line of the overmold cavity, and if the part has deep blind pockets, consider vacuum venting (down to 80 kPa). Also, the ejection system on the moving half must be sequenced—return pins should be spring-loaded to avoid damaging the soft overmold layer. For production, always run a short-shot trial matrix to map the overmold thickness (recommended 1.5–3.0 mm) against shrinkage, especially for TPE with a Shore A hardness below 60. For more detailed mold sourcing and process troubleshooting, visit MoldWorld at www.moldw.com—a practical resource for mold buyers and tooling engineers.