1. Introduction
In modern foundry practices, both lost foam casting and sand casting are widely utilized for producing complex geometries with high dimensional accuracy. While sand casting remains a traditional method due to its cost-effectiveness and versatility, lost foam casting offers superior surface finish and reduced post-processing requirements. However, defects such as wrinkling and shrinkage holes often plague these processes, particularly in thick-section components like ductile iron reducer housings.
Our research focuses on addressing these defects in lost foam casting by optimizing the pouring system and introducing a novel heat dissipation technology. This study also draws parallels with sand casting methodologies to highlight process-specific challenges and solutions.
2. Original Process and Defect Analysis
The reducer housing, weighing 112 kg with wall thickness ranging from 14 mm to 54 mm, was initially produced using a top-gating system (Figure 1). Key parameters of the original process are summarized in Table 1.
Table 1: Original Process Parameters
| Parameter | Value |
|---|---|
| Pouring Temperature | 1370–1440°C |
| Mold Negative Pressure | -0.06–0.04 MPa |
| Holding Time | 900 s |
| Chemical Composition (QT450-10) | C: 3.5–4.0%, Si: 2.0–3.0%, Mn ≤0.45% |
Despite meeting mechanical property requirements (e.g., QT450-10 grade), defects persisted:
- Wrinkling: Observed on the top surface and vertical walls.
- Shrinkage Holes: Concentrated in geometric hot spots (e.g., bolt holes).
3. Causes of Wrinkling and Shrinkage Holes
3.1 Wrinkling Mechanism
In lost foam casting, polystyrene (EPS) decomposition generates gaseous and solid residues. Turbulent flow during top-gating caused incomplete residue evacuation, leading to carbon deposition and surface wrinkling. This contrasts with sand casting, where mold permeability inherently reduces gas entrapment.
3.2 Shrinkage Hole Formation
Shrinkage in ductile iron arises from inadequate liquid metal feeding during solidification. Geometric hot spots (e.g., thick sections) exacerbated this issue. Unlike sand casting, which employs risers and chills, lost foam casting required innovative solutions due to limitations in riser placement and cold iron stability.
4. Optimization Measures and Validation
4.1 Pouring System Redesign
The top-gating system was replaced with a bottom-gating design (Figure 2) to ensure laminar flow. Key calculations included:
- Pouring Time: t=SG−24 st=SG−24s
- Average Pressure Head: Hp=H0−C2=34 cmHp=H0−2C=34cm
- Ingate Cross-Section: As=G0.31×Hp=3.46 cm2As=0.31×HpG=3.46cm2
Four ingates (7 mm × 40 mm each) were implemented, increasing total cross-sectional area to 11.2–12.8 cm². Post-optimization, wrinkling defects were eliminated in 2,000 trial castings.
Table 2: Comparison of Original vs. Optimized Pouring Systems
| Parameter | Original System | Optimized System |
|---|---|---|
| Gating Type | Top-Gating | Bottom-Gating |
| Ingate Cross-Section | 3.5 cm² | 11.2–12.8 cm² |
| Defect Rate (Wrinkling) | 15% | 0% |
4.2 Heat Dissipation Technology
To address shrinkage, heat dissipation fins (50 mm × 30 mm × 7 mm EPS sheets) were bonded to hot spots (Figure 4). These fins enhanced localized cooling by:
- Increasing surface area for heat exchange.
- Facilitating airflow under negative pressure (-0.04 MPa).
Table 3: Shrinkage Defect Reduction
| Process | Defect Rate (Shrinkage) |
|---|---|
| Original | 12% |
| With Heat Dissipation | 0.5% |
5. Comparative Analysis with Sand Casting
While sand casting relies on risers and chills to manage shrinkage, lost foam casting’s unique challenges demand tailored solutions:
Table 4: Lost Foam vs. Sand Casting
| Aspect | Lost Foam Casting | Sand Casting |
|---|---|---|
| Surface Finish | Superior | Moderate |
| Defect Mitigation | Heat dissipation fins | Risers, chills |
| Complexity | High (foam preparation) | Moderate |
| Cost | Higher | Lower |
Notably, sand casting offers better adaptability for thick sections but struggles with surface defects like sand inclusion—a non-issue in lost foam processes.
6. Conclusion
Through systematic optimization, we resolved wrinkling and shrinkage defects in lost foam-cast reducer housings. Key outcomes include:
- Bottom-gating systems ensure laminar flow, eliminating wrinkling.
- Heat dissipation fins mimic the role of chills in sand casting, effectively managing shrinkage.
This study underscores the importance of process-specific innovations while drawing inspiration from established methods like sand casting. Future work will explore hybrid techniques combining the strengths of both casting methodologies.