In recent years, the rapid development of the automotive industry has intensified market competition. This has led to significant price reductions for vehicles, compelling component suppliers to reduce manufacturing costs to adapt. For foundry production, optimizing processes to improve yield is an effective method for cost reduction. Transitioning from traditional single-layer molding schemes to double-layer schemes presents a significant opportunity. However, double-layer casting of spheroidal graphite cast iron components is fraught with technical challenges. The primary difficulty lies in the complex geometries of such castings, which necessitate intricate feeding systems, making the design and production of a double-layer layout seem unfeasible. This article details the thought process and implementation of a successful double-layer process for a specific spheroidal graphite cast iron component.
The production environment utilized an ACE-5 horizontal flaskless molding line and an automatic pouring system. The flask dimensions were 900 mm x 800 mm x (250/230) mm. The component in question was a bracket made of QT450-10. Its key parameters are summarized below:
| Parameter | Value/Specification | ||
|---|---|---|---|
| Material | QT450-10 (Spheroidal Graphite Iron) | ||
| Dimensions (L x W x H) | 243 mm x 118 mm x 50 mm | ||
| Single Casting Weight | 2.3 kg | ||
| Tensile Strength | ≥ 450 MPa | ||
| Yield Strength | ≥ 310 MPa | Elongation | ≥ 10% |
| Internal Defects (ASTM E446) | ≤ Level 2 |
Initial analysis using Magma simulation software for a single casting revealed significant challenges. The thermal and shrinkage analysis indicated multiple dispersed hot spots and a high risk of shrinkage porosity in several areas. The simulation predicted that at least two feeders would be required to adequately feed these isolated hot spots, as illustrated in the results. Based on this, an initial single-layer layout was designed with 8 cavities per mold. The total casting weight was 18.4 kg, and the estimated total poured weight was approximately 65 kg, resulting in a casting yield calculated as:
$$ Yield = \frac{W_c}{W_t} \times 100\% $$
where $W_c$ is the total casting weight and $W_t$ is the total poured weight. For the initial scheme:
$$ Yield_{initial} = \frac{2.3 \times 8}{65} \times 100\% \approx 22\% $$
This initial design had two major pain points: firstly, the complex backside geometry of the part posed a risk of poor sand mold integrity, leading to potential sand erosion defects; secondly, the 22% yield was unacceptably low, leading to high production costs and low efficiency.
A thorough re-examination of the component geometry provided the breakthrough. The casting had a relatively flat, plate-like structure with one side being simple and the other highly complex. The proposed solution was to stack two castings together, separating them with a core that would form the complex features of both. This created a “sandwich” structure: Casting A – Core – Casting B. This ingenious approach solved both problems simultaneously: the core ensured robust formation of the complex features without relying on fragile mold walls, and it allowed for a drastic increase in mold cavity density within the same flask footprint.
The new double-layer scheme accommodated 16 cavities per mold. The total casting weight became 36.8 kg. The introduction of the core (weighing 2.5 kg per unit) and the necessary modifications to the gating and feeding system increased the total poured weight to approximately 100 kg. The yield for the double-layer scheme was:
$$ Yield_{double} = \frac{2.3 \times 16}{100} \times 100\% = 36.8\% $$
This represented a substantial yield improvement $\Delta Y$:
$$ \Delta Y = Y_{double} – Y_{initial} = 36.8\% – 22\% = 14.8\% $$
Furthermore, the production output rate per molding cycle was doubled, significantly enhancing productivity and reducing unit cost.
The feasibility of this double-layer design was rigorously validated through comprehensive mold filling and solidification simulation. The analysis covered several critical aspects:
| Analysis Type | Key Criteria & Results |
|---|---|
| Filling Pattern & Velocity | The metal front progressed smoothly without excessive turbulence or air entrapment. Velocity vectors remained within acceptable limits to prevent mold erosion. |
| Temperature Distribution during Filling | No premature cooling or excessive temperature loss was observed in critical sections like the gates and runners, ensuring proper feed metal liquidity. |
| Solidification Sequence (Liquid Fraction) | The solidification fronts progressed systematically from the casting extremities towards the feeders. The critical junctions and hot spots remained liquid longest, confirming the feeders were effective as thermal and material reservoirs. |
| Shrinkage Porosity Prediction | The Niyama criterion and shrinkage potential models showed no significant risk of macro- or micro-shrinkage in the final casting bodies. The predicted areas of high risk were confined to the feeders themselves. |
| Pressure & Porosity | The analysis of pressure development during solidification confirmed that adequate metallostatic pressure was maintained in the feeding paths to counteract shrinkage. |
The successful simulation paved the way for tooling manufacturing and trial production. The castings were produced, and samples underwent rigorous testing. The mechanical properties far exceeded the specification requirements, as shown in the average results from 10 batches:
| Property | Specification | Average Measured Value | Judgment |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 450 | 527 | OK |
| Yield Strength (MPa) | ≥ 310 | 334 | OK |
| Hardness (HBW) | 160 – 210 | 171 | OK |
| Nodularity (%) | ≥ 80 | 94 | OK |
| Elongation (%) | ≥ 10 | 14 | OK |
Non-destructive testing via X-ray radiography was performed according to ASTM E446. The internal soundness of the castings was excellent, with no detectable shrinkage cavities or porosity exceeding Level 2, thus meeting the stringent quality standard.
In conclusion, this study demonstrates that a double-layer casting process is a viable and highly advantageous strategy for specific geometries of spheroidal graphite cast iron components. The key innovation lies in utilizing an intermediary sand core to both form complex features and enable part stacking. This method directly addresses the traditional limitations associated with feeding complex spheroidal graphite cast iron castings in multi-level molds. The process led to a dramatic increase in casting yield from 22% to 36.8%, effectively lowering the cost per part. It also doubled productivity per mold cycle. The implementation was validated through advanced simulation tools, ensuring the design’s integrity before physical trials, and the final castings met all mechanical and quality specifications. This double-layer development provides a significant competitive edge in the cost-sensitive automotive components market and opens a new avenue for optimizing the production of flat or compact spheroidal graphite cast iron parts. The success of this project underscores the importance of innovative process design combined with simulation-led validation in modern foundry engineering.