Lost foam casting offers advantages such as excellent surface finish, dimensional accuracy, and high yield rates for producing ductile iron components like QT450-10 gearbox housings. However, during trial production, surface wrinkling and shrinkage porosity defects were observed at geometric hot spots. This study analyzes defect formation mechanisms and proposes optimized solutions through process modifications.
Defect Formation Mechanisms
Wrinkling Defects
In lost foam casting, wrinkling occurs due to incomplete decomposition of copolymer patterns (EPS/EPMMA) and carbon deposition at final filling regions. The original top-gating system caused turbulent flow, creating cold zones at thick-wall sections. The critical factors include:
$$
t = S \sqrt{\frac{G}{L}} \quad (1)
$$
Where \( t \) represents pouring time (s), \( G \) is casting weight (kg), \( L \) flow channel length (mm), and \( S \) empirical coefficient (1.1–1.3).
| Element | C | Si | Mn | P | S | Mg | RE |
|---|---|---|---|---|---|---|---|
| Content (%) | 3.5–4.0 | 2.0–3.0 | ≤0.45 | ≤0.05 | ≤0.025 | 0.02–0.06 | 0.015–0.04 |
Shrinkage Porosity
Shrinkage defects at bolt holes (54mm thickness) resulted from insufficient feeding during solidification. The modulus (V/A ratio) calculation explains this phenomenon:
$$
M = \frac{V}{A} \quad (2)
$$
Where \( M \) is modulus (cm), \( V \) volume (cm³), and \( A \) cooling surface area (cm²). Thick sections with \( M > 2.5 \) cm showed higher shrinkage tendency.
Process Optimization Strategies
Gating System Redesign
Modified from top-gating to bottom-gating with four ingates (70mm×40mm each) achieved laminar flow. Key parameters:
$$
H_P = H_0 – \frac{C}{2} = 34\ \text{cm} \quad (3)
$$
$$
A_g = \frac{G}{0.31t\sqrt{H_P}} = 3.46\ \text{cm²} \quad (4)
$$
Actual total ingate area: 11.2–12.8 cm², ensuring 24s pouring time at 1,370–1,440°C.
Thermal Management Innovation
A novel heat dissipation method using 50mm×30mm×7mm foam inserts reduced local modulus by 38%:
$$
\Delta M = \frac{A_{\text{added}}}{V_{\text{hot spot}}} \quad (5)
$$
Twelve inserts created forced convective cooling through (-0.06 MPa) vacuum extraction, achieving directional solidification without affecting yield rate (maintained at 78%).
Validation Results
| Parameter | Original | Optimized |
|---|---|---|
| Wrinkling Rate | 23% | 0% |
| Shrinkage Defects | 17% | 0.8% |
| Yield Rate | 68% | 78% |
The lost foam casting process demonstrates remarkable adaptability for complex ductile iron components when combining proper gating design with innovative thermal management. These improvements have been validated through 2,000+ production units, establishing reliable solutions for automotive transmission applications.
Industrial Implementation
The successful implementation in mass production confirms that lost foam casting can achieve:
- Surface roughness reduction from Ra 25μm to Ra 12.5μm
- Dimensional accuracy improvement to CT8–CT9
- Energy savings of 15–20% compared to traditional sand casting
Future research will focus on intelligent vacuum control systems and biodegradable foam materials to further enhance the sustainability of lost foam casting processes.