In everyday plastic injection molding, the dynamic half (core) and fixed half (cavity) define the part’s internal and external geometry, respectively. The core side typically carries the ejection system, while the cavity side houses the sprue bushing and runner. A common practical rule: place the parting line at the largest cross‑section of the part, but always verify draft angles—0.5° to 1° per side for textured surfaces, 1° to 2° for polished steel. For deep ribs or bosses, increase draft to 2°–3° to avoid drag marks. Also, balance the projected area between core and cavity; if the difference exceeds 20%, consider adding interlock or taper locks to prevent flash due to lateral deflection during clamping.

When designing for overmolding (two‑shot or insert molding), the substrate material’s shrinkage and the overmold layer’s adhesion must be evaluated together. For example, with a TPE overmolded onto a rigid PP substrate, the melt temperature of the TPE should be kept 20–30°C below the substrate’s heat deflection temperature to prevent collapse. Mechanical interlocking—undercuts, through‑holes, or roughened surfaces (Ra 1.6–3.2 µm)—is often more reliable than chemical bonding, especially for high‑cycle production. Also, ensure the core side of the first shot is designed with recessed areas to locate the second shot’s cavity, and add shut‑offs with 0.05–0.1 mm clearance for venting, not for steel contact. In practice, run mold flow analysis to check weld lines and air traps at the interface; a 0.02 mm vent depth on the parting line is typical for TPE, but for high‑viscosity materials like PC, reduce to 0.015 mm.

From a maintenance standpoint, always specify hardened tool steel (e.g., S136 or 1.2344) for core inserts subjected to high ejection forces, and use beryllium‑copper for localized cooling near the overmold area to shorten cycle time. For overmolding, verify the substrate’s surface cleanliness—mold release residue is a common cause of delamination; use a solvent wipe or plasma treatment if needed. Finally, document the exact molding window (melt temp, mold temp, injection speed, and hold pressure) for each shot, because even a 5°C change in mold temperature can shift the adhesion strength by 15%. For more detailed mold sourcing, tooling standards, or supplier comparisons, visit MoldWorld (www.moldw.com) for a comprehensive directory and technical resources.