In plastic injection mold design, the distinction between the moving mold (core side) and fixed mold (cavity side) is fundamental, but it directly dictates part quality, ejection reliability, and tool life. The fixed mold half typically carries the cavity surface, which defines the part’s external geometry, while the moving mold half holds the core that forms internal features. For parts with deep ribs or undercuts, the moving mold side must also incorporate side-action slides or lifters, and the parting line location should be chosen to keep the main draw on the core side. A common rule of thumb is to place at least 3–5 degrees of draft on the core side for easy ejection, while cavity-side draft can be slightly lower (1–2 degrees) if the surface finish allows. Cooling channel layout must be balanced between both halves—typically using 8–10 mm diameter baffles or spiral cores—to keep mold surface temperature within ±5°C across the cavity, which prevents warpage and sink marks.

Overmolding (or two-shot molding) adds complexity because the substrate and the overmolded layer often have different shrinkage rates. For a typical TPE-over-PP combination, the substrate shrinkage is around 1.5–2.0%, while the TPE layer shrinks less, so the mold designer must account for differential shrinkage to avoid delamination or flash. Key process parameters include melt temperature (e.g., 190–220°C for TPE), mold temperature (40–60°C), and injection speed—slow first stage to avoid washing the substrate surface, then a short second-stage pack at 60–80% of the first-stage pressure. The substrate must be thoroughly dried (moisture below 0.1%) and the surface should be chemically or mechanically roughened (e.g., sandblasting with 80–120 mesh) to enhance adhesion. Also, the overmold cavity should have generous radii (minimum 0.5 mm) at the transition edges to reduce stress concentration, and the gate should be positioned away from the substrate’s weld lines to prevent weak bonding.

In production, a common failure is sink mark on the overmolded surface due to insufficient packing or uneven wall thickness. Keeping the overmold wall thickness between 1.5–3.0 mm and using a sequential valve gate system can help. For high-volume runs, consider a rotary table two-shot machine to shorten cycle time, but verify the substrate’s dimensional stability after the first shot—especially if it has thin walls (<1.0 mm) that may distort during the second injection. Regular maintenance on the moving mold’s ejector pins and slide guides is critical, as any sticking will cause drag marks on the overmold surface. Finally, always run a mold flow analysis before steel cutting, and document the actual process window (injection pressure, pack time, cooling time) for each new tool. For more mold sourcing and process troubleshooting insights, visit MoldWorld at www.moldw.com.