Summary: A practical guide to distinguishing interference fit (碰穿) and insertion fit (插穿) in mold construction, with real-world implications for part quality, tool life, and quoting accuracy.

In everyday mold design reviews, the distinction between 碰穿 (direct face-to-face contact) and 插穿 (sliding insertion contact) is often glossed over, but getting it wrong can lead to flash, premature wear, or even mold seizure. For 碰穿, the two mold halves close along a flat parting plane, typically with a zero to 0.02 mm clearance under clamping force. This is the simplest and most cost-effective approach, but it demands that the steel surfaces are perfectly parallel and that the press tonnage is sufficient to hold the shutoff. In practice, we recommend a minimum land width of 5–8 mm for direct contact areas, and if the projected area exceeds 150 cm², you must verify that the clamp force can resist the injection pressure—otherwise, you will see edge flash that is almost impossible to polish out.

插穿, on the other hand, involves a male core entering a female cavity with a controlled interference, usually 0.5–1.5° of draft angle on the sliding surfaces. This is common for side actions, lifters, and angled ejectors. The critical parameter is the insertion depth: for every 10 mm of engagement, we add 0.02–0.03 mm of radial clearance to avoid galling on hardened steel (e.g., P20 or H13). A typical mistake is to use the same clearance for both vertical and horizontal insertion—but gravity and thermal expansion behave differently. For horizontal 插穿, always add 0.05 mm extra clearance on the upper half to accommodate mold deflection under tonnage. We also specify a minimum of 3 mm of pre-load on the insert base, so the core does not wobble during the injection phase.

From a quoting perspective, these two choices directly affect machining time and steel cost. A 碰穿 design can be cut with standard 3-axis milling and requires only a surface grind, often saving 15–20% of cavity machining hours. An 插穿 design, however, needs wire EDM for the angled surfaces and a lapping step to achieve the required finish (Ra 0.4 or better), adding roughly 10–15% to tooling cost. When you receive a part drawing, always ask: is the shutoff purely vertical, or does it require a sliding action? If the latter, check for undercuts that force additional side cores—this can double your lead time. For more practical mold sourcing and quoting benchmarks, visit MoldWorld at www.moldw.com.