In any plastic injection mold, the dynamic half (moving side) and fixed half (cavity side) must work as one rigid system under clamp pressure, yet separate cleanly at ejection. The core fit logic revolves around the parting line, guide pins, and interlocks. Guide pins are typically hardened to HRC 58–62 and ground to a sliding fit of 0.01–0.02 mm clearance, while interlocks (side locks) take up shear forces to prevent mismatch on deep-draw parts. A common mistake is relying only on guide pins for heavy side loads—this leads to flashing or uneven wear after 50,000 cycles. For production molds, always add taper locks or flat locks on the parting line, especially when the projected area exceeds 200 cm², to maintain consistent shut-off pressure.

Overmolding, or two-shot molding, adds another layer of complexity because the first shot (substrate) and second shot (overmold) must bond mechanically or chemically. The moving half usually carries the first-shot part through rotation or sliding cores, while the fixed half holds the cavity for the second material. Key parameters are melt temperature differential—often 20–40°C between the two materials—and dwell time at the interface. For TPE over PP, for example, the substrate surface should be roughened to Ra 0.8–1.6 µm to improve adhesion, and the mold must include shut-off edges that prevent flash from the second shot bleeding onto the first. Also, the clamping force should be calculated based on the combined projected area of both shots, not just the largest one, or you risk opening the parting line during the second injection.

From a practical standpoint, always verify the dynamic/fixed half fit before running overmolding trials. Use Prussian blue to check contact patterns on the parting line—target 80% or higher uniform contact. If you see low spots, re-fit the mold before adjusting process parameters. Also, confirm that the ejector return springs are strong enough to handle the added weight of rotating plates or sliding cores in the moving half. A weak return can cause the first shot to shift, leading to uneven wall thickness in the second shot. For sourcing molds with proven overmolding capability, visit MoldWorld (www.moldw.com) for verified suppliers and technical case studies.