In the valve manufacturing industry, gate valves are widely used due to their low fluid resistance, ease of operation, and bidirectional flow capability. A key trend to reduce production costs and enhance market competitiveness is the integral design of the valve cover and support as a single casting part. This approach eliminates separate components, reduces material usage, minimizes machining surfaces, and cuts down on assembly time. Data indicates that integral casting parts can save over 12% in material costs and more than 8% in machining costs compared to split designs. However, these casting parts often face deformation issues during precision casting, particularly in structures like 4-inch 150LB American standard gate valve covers and supports, where the complex geometry leads to significant distortion. This article, from my perspective as a casting engineer, explores the deformation causes and effective solutions for such integral casting parts, focusing on practical experiments and innovative gating system designs.
The integral casting part for valve cover and support typically has dimensions of 260 mm × 160 mm × 220 mm (length × width × height), with a minimum wall thickness of 11.2 mm and flange thickness of 22 mm. Key machined surfaces include the bearing housing, stuffing box, and flange faces, with a machining allowance of 1.5 mm. The structure combines a bulky valve cover section with a slender support end, making it prone to deformation during solidification. In precision casting, a ceramic shell mold with high strength is used for shaping casting parts. To ensure efficient feeding and minimize defects like shrinkage porosity, a top-gating system is commonly employed. This creates a temperature gradient from top to bottom, enhancing the feeding efficiency of the gating and feeding system. The initial gating design involved attaching the ingate at the longitudinal top of the flange on the valve cover side, with a dedicated feeder above to provide adequate feeding. However, this setup led to deformation issues in the first trial batch of 50 casting parts.
During machining trials, where the bearing housing outer diameter was used as a reference for alignment, the flange face exhibited longitudinal deformation. After removing 1.5 mm of machining allowance at the ingate side, the lower part of the flange remained uncut, indicating deformation exceeding 2 mm in many casting parts. Out of 50 casting parts, 15 showed deformation over 2 mm, and 21 had deformation above 1.5 mm, resulting in a defect rate exceeding 40%. This deformation was systematic, ruling out random operational errors like premature wax pattern removal, improper handling, or heat treatment issues. Analysis revealed that the feeder, designed to be bulky for effective feeding, solidified last and created significant inward shrinkage stress. The ceramic shell mold could not withstand this stress, causing the upper flange to contract inward. The shrinkage stress ($\sigma$) can be approximated by the formula: $$ \sigma = E \cdot \alpha \cdot \Delta T $$ where $E$ is Young’s modulus, $\alpha$ is the coefficient of thermal expansion, and $\Delta T$ is the temperature drop during solidification. For casting parts, high $\Delta T$ in the feeder region amplifies stress, leading to deformation.
To address this, I conducted a series of experiments to mitigate deformation in these casting parts. The initial approach involved lowering the pouring temperature to reduce shrinkage stress. The standard pouring temperature range was 1,590–1,610°C, but I reduced it to 1,570–1,590°C. Pouring was done at 1,575°C for 10 casting parts (5 groups). After shell removal, deformation was measured, as summarized in Table 1. The results showed limited improvement: only 3 casting parts had deformation within 0.5 mm, while others ranged from 1 to 2.5 mm. Additionally, issues like cold shuts and insufficient filling of markings appeared, indicating that lower temperatures compromised fluidity and feeding capability. This highlighted the trade-off between reducing stress and maintaining casting quality for these integral casting parts.
| Test Number | Flange at Feeder End (mm) | Flange at Bottom End (mm) | Deformation Difference (mm) |
|---|---|---|---|
| 1 | 241.0 | 239.5 | 1.5 |
| 2 | 241.0 | 239.0 | 2.0 |
| 3 | 240.5 | 239.5 | 1.0 |
| 4 | 241.0 | 239.0 | 2.0 |
| 5 | 242.0 | 239.5 | 2.5 |
| 6 | 240.0 | 240.0 | 0.0 |
| 7 | 240.0 | 239.5 | 0.5 |
| 8 | 241.0 | 239.0 | 2.0 |
| 9 | 240.0 | 239.0 | 1.0 |
| 10 | 240.0 | 240.0 | 0.0 |
Next, I focused on reducing the size of the gating and feeding system to minimize its shrinkage impact. The feeder was thinned at its lower section, as illustrated in the modified design. This trial involved 10 casting parts (5 groups) poured under standard conditions. Deformation measurements, shown in Table 2, indicated better results: 8 casting parts had deformation within 1.5 mm, but 2 still exceeded this limit. However, upon cutting the feeders, shrinkage porosity was observed at the ingate roots in 4 groups, due to inadequate feeding from the reduced feeder size. The feeding efficiency ($\eta$) can be expressed as: $$ \eta = \frac{V_{\text{feed}}}{V_{\text{shrinkage}}} $$ where $V_{\text{feed}}$ is the volume of feeding metal and $V_{\text{shrinkage}}$ is the shrinkage volume. Thinning the feeder decreased $V_{\text{feed}}$, lowering $\eta$ and causing porosity. This required manual topping-up during pouring, which increased workload and variability, making it unsuitable for consistent production of casting parts.
| Test Number | Flange at Feeder End (mm) | Flange at Bottom End (mm) | Deformation Difference (mm) |
|---|---|---|---|
| 1 | 241.0 | 240.5 | 0.5 |
| 2 | 241.0 | 239.0 | 2.0 |
| 3 | 240.5 | 239.0 | 1.5 |
| 4 | 242.0 | 239.5 | 2.5 |
| 5 | 240.0 | 239.0 | 1.0 |
| 6 | 241.0 | 240.0 | 1.0 |
| 7 | 240.0 | 240.0 | 0.0 |
| 8 | 240.0 | 239.0 | 1.0 |
| 9 | 241.0 | 239.5 | 1.5 |
| 10 | 241.5 | 240.0 | 1.5 |
Given these challenges, I developed an innovative gating system with a U-shaped groove beneath the feeder. The U-shaped groove is filled with refractory sand during shell making, forming a rigid sand block that resists shrinkage. This design not only hinders contraction but also enhances feeding by slowing heat dissipation, prolonging the liquid state of the feeder. The U-shaped structure also dampens molten metal turbulence, reducing shell erosion and sand inclusion defects. For casting parts, this approach balances stress reduction and feeding effectiveness. The principle can be modeled using thermal resistance concepts: $$ R_{\text{thermal}} = \frac{L}{kA} $$ where $R_{\text{thermal}}$ is thermal resistance, $L$ is thickness, $k$ is thermal conductivity, and $A$ is area. The sand block increases $R_{\text{thermal}}$, maintaining higher feeder temperatures. I tested this on 10 casting parts (5 groups) under standard pouring conditions. As shown in Table 3, all casting parts exhibited deformation below 1.5 mm, with most showing negligible distortion. No shrinkage defects were found after cutting, confirming the viability of this method for producing high-quality casting parts.
| Test Number | Flange at Feeder End (mm) | Flange at Bottom End (mm) | Deformation Difference (mm) |
|---|---|---|---|
| 1 | 240.0 | 240.0 | 0.0 |
| 2 | 240.5 | 240.0 | 0.5 |
| 3 | 240.5 | 240.5 | 0.0 |
| 4 | 240.5 | 240.5 | 0.0 |
| 5 | 240.5 | 240.0 | 0.5 |
| 6 | 241.0 | 240.5 | 0.5 |
| 7 | 241.0 | 240.0 | 1.0 |
| 8 | 240.0 | 240.0 | 0.0 |
| 9 | 240.0 | 240.0 | 0.0 |
| 10 | 240.5 | 240.0 | 0.5 |
To ensure consistent quality, I implemented a final inspection step using dedicated templates for longitudinal and transverse checks on casting parts. This proactive measure prevents defective casting parts from moving to machining, reducing rework costs and improving efficiency. The templates verify deformation limits, aligning with the improved process stability from the U-shaped groove system.
In conclusion, for integral casting parts like valve covers and supports, traditional methods such as lowering pouring temperature or reducing feeder size are insufficient, as they introduce new defects like cold shuts and shrinkage porosity. The U-shaped groove gating system effectively addresses deformation by providing shrinkage resistance and enhanced feeding. The rigid sand block within the groove counteracts contraction stress, modeled as: $$ F_{\text{resistance}} = k_{\text{sand}} \cdot \Delta L $$ where $F_{\text{resistance}}$ is the resisting force, $k_{\text{sand}}$ is the stiffness of the sand block, and $\Delta L$ is the shrinkage displacement. This innovation significantly improves the yield of casting parts, offering a reliable solution for similar integral casting applications. Moreover, the U-shaped design promotes smoother metal flow, reducing defects related to shell integrity. Through these experiments, I have demonstrated that a tailored gating approach is crucial for overcoming deformation challenges in precision casting of complex casting parts, ultimately enhancing productivity and cost-effectiveness in valve manufacturing.