In the field of industrial manufacturing, valve components play a critical role in controlling fluid flow across various sectors such as pipelines, water treatment, energy, and aerospace. Among these, the valve shell serves as the core housing that encloses and protects internal parts like the valve core, stem, and seals, while forming the fluid passage. It must withstand high pressures, ensure sealing, and resist environmental corrosion. To achieve these properties, the valve shell is typically manufactured using sand casting, a versatile and cost-effective method. However, the quality of sand casting heavily depends on the process design, including gating systems, risers, and chills. Traditional trial-and-error approaches are time-consuming and expensive. Therefore, numerical simulation tools like ProCAST are employed to optimize the sand casting process by predicting fluid flow, temperature distribution, and defects, thereby reducing development cycles and improving efficiency.
This study focuses on optimizing the sand casting process for a valve shell component made of QT450-18 material. The component has a complex geometry with varying wall thicknesses, making it prone to defects like shrinkage porosity and cavities. Through ProCAST simulations, different gating systems and process modifications are evaluated to enhance casting quality. The research compares upright top-gating and side-mounted middle-gating systems, followed by the integration of risers and chills to eliminate defects. The findings demonstrate that a well-designed sand casting process can significantly reduce imperfections, ensuring the valve shell meets performance requirements.
Component Analysis and Simulation Setup
The valve shell component, as illustrated in the provided image, consists of key sections such as the valve seat, body, and core, which work together to regulate fluid movement under pressures up to 69 bar. The part is designed with a mass of 22.39 kg and overall dimensions of 225.35 mm in length, 225.35 mm in width, and 353.22 mm in height. Wall thicknesses vary, with a maximum of 39.69 mm, a minimum of 8.00 mm, and an average of approximately 12.00 mm. After accounting for machining allowances, shrinkage rates, and draft angles, the final casting weighs 24.02 kg. The material QT450-18, a ductile iron, is chosen for its excellent mechanical properties and corrosion resistance, which are essential for sand casting applications in harsh environments.
For the ProCAST simulations, the component is modeled with a mesh generation step, where the casting, gating system, and sand mold are discretized with face mesh sizes of 10 mm, 10 mm, and 20 mm, respectively. The sand casting process parameters are set based on industrial experience: a pouring temperature of 1380°C, a pouring speed of 1 m/s, and the use of furan resin sand for the mold. The heat transfer coefficient between the sand mold and casting is defined as 500 W/(m²·K), while that between the casting and chills is set to 1200 W/(m²·K) to simulate accelerated cooling. Air cooling is applied as the primary method. These settings ensure accurate representation of the sand casting process, enabling detailed analysis of filling, solidification, and defect formation.
| Parameter | Value |
|---|---|
| Pouring Temperature | 1380°C |
| Pouring Speed | 1 m/s |
| Casting Material | QT450-18 |
| Mold Material | Furan Resin Sand |
| Heat Transfer Coefficient (Mold-Casting) | 500 W/(m²·K) |
| Heat Transfer Coefficient (Casting-Chill) | 1200 W/(m²·K) |
| Cooling Method | Air Cooling |
Influence of Gating Systems in Sand Casting
In sand casting, the gating system design is crucial for ensuring smooth metal flow and minimizing defects. Given the uneven wall thickness of the valve shell and the tendency of ductile iron to oxidize and exhibit poor filling capability, an open gating system is selected for its ability to reduce turbulence and oxidation. Two distinct gating approaches are evaluated: an upright top-gating system (Scheme A) and a side-mounted middle-gating system (Scheme B), both configured for two castings per mold. Scheme A involves pouring metal from the top, which can lead to higher impact forces and potential defects, whereas Scheme B introduces metal from the side at a middle height, promoting gentler filling. The simulations for these schemes provide insights into their effectiveness in sand casting applications.
Scheme A: Upright Top-Gating System
The filling process for Scheme A begins at T=0.79 s, where molten metal passes through the sprue, runner, and ingate before entering the mold cavity. The initial flow is slow, minimizing冲击 on the cavity bottom. By T=6.53 s, half of the cavity is filled, and complete filling is achieved at T=19.05 s. The solidification analysis shows that at T=105.20 s, regions away from the gating system, such as the bottom, start solidifying first due to earlier filling and faster cooling. By T=665.20 s, the bottom and top sections are fully solid, but isolated liquid pockets form in the middle due to varying wall thicknesses, leading to shrinkage defects as补缩 channels close. The total solidification time is 1185.20 s. Defect distribution analysis reveals multiple hot spots and large cavities in the upper sections, with dispersed shrinkage porosity and holes in the middle-upper regions. These defects arise from non-uniform solidification, where thicker sections solidify last without adequate metal feeding, a common issue in sand casting.
The mathematical representation of solidification can be described using the Fourier number for heat transfer, which influences defect formation:
$$ Fo = \frac{\alpha t}{L^2} $$
where \( \alpha \) is the thermal diffusivity, \( t \) is time, and \( L \) is the characteristic length. In sand casting, a higher Fourier number indicates faster heat transfer, which can be optimized to reduce defects.
Scheme B: Side-Mounted Middle-Gating System
For Scheme B, filling initiates at T=0.93 s, with metal entering the cavity through the side gating. By T=7.94 s, half of the cavity is filled, and full filling occurs at T=18.20 s, demonstrating a faster and more stable process compared to Scheme A. Solidification begins at T=82.91 s in areas distant from the gating, and by T=419.91 s, these regions are mostly solid. At T=559.91 s, isolated liquid zones form in the middle, solidifying completely by T=1159.91 s. Defect analysis shows that shrinkage is primarily concentrated in the middle section, but the overall number of defects is lower than in Scheme A. This improvement is attributed to the more uniform filling and controlled solidification in sand casting, highlighting the advantages of side-gating for complex geometries.
| Parameter | Scheme A (Upright Top-Gating) | Scheme B (Side-Mounted Middle-Gating) |
|---|---|---|
| Filling Start Time | 0.79 s | 0.93 s |
| Half-Filling Time | 6.53 s | 7.94 s |
| Full Filling Time | 19.05 s | 18.20 s |
| Total Solidification Time | 1185.20 s | 1159.91 s |
| Defect Severity | High (multiple shrinkage zones) | Moderate (localized shrinkage) |
Based on these results, Scheme B is superior for sand casting due to its smoother filling and reduced defects. However, further optimization is necessary to eliminate residual imperfections, leading to the integration of risers and chills.
Process Optimization in Sand Casting
To enhance the sand casting process for the valve shell, Scheme B is refined by adding risers and chills. Risers provide additional metal feed to compensate for shrinkage during solidification, while chills accelerate cooling in critical areas to promote directional solidification. This approach is common in sand casting to achieve sound castings with minimal defects.
Riser Design
Risers are designed based on the modulus method, which calculates the cooling characteristics of the casting. The modulus \( M \) is given by:
$$ M = \frac{V}{A} $$
where \( V \) is the volume and \( A \) is the surface area. For the valve shell, measurements from 3D software yield \( V = 6159545.02 \, \text{mm}^3 \) and \( A = 615156.62 \, \text{mm}^2 \), resulting in \( M = 1.001 \, \text{cm} \). Using sand casting handbooks, this modulus corresponds to specific riser dimensions. Three open risers are placed on the top of the casting: one in the central region and two near the gating system to address shrinkage in the middle and upper sections. The risers ensure adequate补缩 by maintaining a liquid metal source until the casting solidifies, a key principle in sand casting.
| Riser Location | Number | Type | Modulus (cm) |
|---|---|---|---|
| Central Top | 1 | Open | 1.001 |
| Near Gating | 2 | Open | 1.001 |
Chill Design
Chills are applied to regions prone to slow solidification, such as the bottom of the valve shell, to enhance heat dissipation and reduce shrinkage. The chill thickness \( D \) is determined from the casting thickness \( t \) using the empirical formula for ductile iron in sand casting:
$$ D = k \cdot t $$
where \( k \) ranges from 0.8 to 1.2. For a thickness \( t = 19 \, \text{mm} \), \( k = 1.1 \) gives \( D = 20.9 \, \text{mm} \), rounded to 20 mm based on standard chill sizes. Six chills made of QT450-18 are positioned: at the flange joints between the body and cover to promote bottom-up solidification, at the bottom of left and right flanges to refine grain structure, and at the bottom of smaller flanges to address slow solidification. These chills modify the solidification sequence, ensuring that thicker sections solidify first and receive adequate feeding.
| Chill Location | Number | Thickness (mm) | Material |
|---|---|---|---|
| Body-Cover Flange | 2 | 20 | QT450-18 |
| Left and Right Flange Bottoms | 2 | 20 | QT450-18 |
| Small Flange Bottoms | 2 | 20 | QT450-18 |
Optimized Simulation Results
After incorporating risers and chills, the sand casting process is re-simulated. Filling begins smoothly, with the cavity fully filled by T=15.25 s, indicating stable metal flow. Solidification proceeds with chills accelerating cooling in the bottom regions, while risers provide continuous feeding. The solidification sequence shows bottom sections solidifying first, followed by upward progression, minimizing isolated liquid zones. Defect analysis reveals that nearly all internal defects are eliminated, with only minor imperfections remaining in the risers and gating system. This demonstrates the effectiveness of optimization in sand casting, as the valve shell achieves high integrity and meets quality standards.
The overall improvement can be quantified using the defect reduction ratio \( R \), defined as:
$$ R = \frac{D_i – D_f}{D_i} \times 100\% $$
where \( D_i \) is the initial defect volume and \( D_f \) is the final defect volume. In this sand casting optimization, \( R \) approaches 100%, indicating near-complete defect removal.
Conclusion
This study successfully optimizes the sand casting process for a valve shell component using ProCAST numerical simulations. The comparison between upright top-gating and side-mounted middle-gating systems reveals that the latter offers faster, more stable filling and fewer defects in sand casting. Further enhancements through risers and chills effectively eliminate shrinkage porosity and cavities, ensuring the casting’s mechanical performance. The findings underscore the importance of iterative simulation in sand casting to refine process parameters, reduce costs, and improve product quality. Future work could explore additional factors such as mold materials or cooling rates to further advance sand casting techniques for complex components.