Stress Analysis at Step Transitions in Stepped Core Pins
The Stress Concentration Problem
During each injection cycle, the pin tip experiences lateral bending force from the injected resin. In a uniform-diameter pin, this stress distributes evenly along the shaft. But at a step transition, the sudden diameter change creates a geometric notch that amplifies the stress locally:
| Transition Geometry | Stress Concentration Factor (Kt) | Relative Pin Life |
|---|---|---|
| Sharp 90° shoulder (R = 0) | 3.0–3.5 | 1× (baseline — shortest) |
| Small radius (R = 0.2 mm) | 2.0–2.5 | 2–3× |
| Medium radius (R = 0.3 mm) | 1.5–1.8 | 3–4× |
| Large radius (R = 0.5 mm) | 1.3–1.5 | 4–5× |
| No step (uniform diameter) | 1.0 | Maximum |
According to ScienceDirect's stress concentration reference, the stress concentration factor for a stepped shaft under bending depends on the ratio of diameters (D/d) and the ratio of fillet radius to smaller diameter (r/d). Peterson's Stress Concentration Factors provides the authoritative charts for these calculations.
Fatigue Failure Mechanism
Core pin failure at step transitions follows the classic fatigue crack initiation and propagation sequence:
- Stage 1: Crack initiation — Micro-cracks form at the surface of the step shoulder where stress is highest. This can occur after 50,000–200,000 cycles depending on stress level.
- Stage 2: Crack propagation — Each injection cycle opens and closes the crack, causing it to grow incrementally. The crack growth rate accelerates as the remaining cross-section decreases.
- Stage 3: Final fracture — When the remaining cross-section can no longer support the injection load, the pin breaks suddenly. The fracture surface typically shows a smooth fatigue zone (Stages 1–2) and a rough final-fracture zone.
Material Selection for High-Stress Steps
| Material | Fatigue Strength (MPa) | Toughness | When to Use |
|---|---|---|---|
| SKH51 (M2) — HRC 60–62 | ~650 | Moderate | Standard stepped pins, step ø ≥ 2 mm, ratio ≤ 1.5:1 |
| SKD61 (H13) — HRC 48–52 | ~500 | Excellent | Thin steps (< ø2 mm), high ratio (> 1.5:1), breakage-prone |
| STAVAX ESR — HRC 48–52 | ~480 | Good | Corrosive resins + stepped geometry |
While SKH51 has higher fatigue strength, its lower toughness means cracks propagate faster once initiated. SKD61's superior toughness slows crack propagation significantly, making it the better choice when the step geometry creates unavoidable stress concentration.
Design Recommendations
- Maximize transition radius — Use the largest R that the part's internal feature geometry allows
- Avoid step ratios > 2:1 — If a 2:1+ ratio is unavoidable, switch to SKD61 and specify R ≥ 0.5 mm
- Consider two one-step pins vs one two-step pin — If concentricity allows, using two separate one-step pins may be more reliable than a single two-step pin with high stress at both transitions
- Specify nitriding — Surface nitriding increases the fatigue limit of the step surface by 20–30% by creating compressive residual stress that resists crack initiation
Frequently Asked Questions
Why do stepped core pins break at the step transition?+
What transition radius prevents step breakage?+
Should I use SKD61 instead of SKH51 for stepped core pins?+
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