In this study, I focus on optimizing the lost wax investment casting process for ductile iron protective components used in oil drilling machinery. These parts play a critical safety role, and defects such as shrinkage porosity and gas entrapment during casting can compromise their integrity. Through a combination of three-dimensional modeling and numerical simulation, I aim to identify the root causes of these issues and refine the process parameters to enhance product quality. The lost wax investment casting method is particularly suitable for complex geometries, but it requires precise control to minimize defects. By leveraging advanced simulation tools, I systematically analyze the effects of various factors on casting outcomes, ensuring that the final components meet stringent industrial standards.
The protective part is designed as a hollow cylinder with an average wall thickness of 20 mm. Its external dimensions are 148.49 mm in diameter and 71.1 mm in height, featuring asymmetric slots at one end and eight through-holes between the end faces. The outer surface includes helical grooves, which add to the complexity of the lost wax investment casting process. The material used is QT600 ductile iron, with a chemical composition (in weight percentage) of 3.0–3.5% C, 2.4–2.8% Si, 0.3–0.5% Mn, 0.03–0.035% S, less than 0.1% P, and 0.045–0.050% Mg. Key properties include a tensile strength of at least 600 MPa, hardness in the range of HB 190–270, a solidification temperature interval of 1,129–1,194 °C, and a density of 7,300 kg/m³. These characteristics necessitate a carefully controlled lost wax investment casting process to achieve defect-free parts.
To design the gating system for the lost wax investment casting process, I analyzed the part’s structural features, which include a straightforward cylindrical shape with uniform thickness. For efficiency, I adopted a cluster arrangement with four parts per mold. This setup promotes sequential solidification, reducing the risk of shrinkage defects. The gating system consists of a sprue, runners, ingates, and vents, all configured to ensure smooth metal flow and effective feeding during solidification. The initial design was based on standard practices for lost wax investment casting, but simulations revealed areas for improvement.
The filling speed is a critical parameter in lost wax investment casting, as it influences defect formation. I calculated the minimum allowable rise velocity of the molten metal in the mold cavity using the empirical formula derived from industry standards. The equation is given by:
$$ v_{fill} = \frac{h \cdot \delta}{T \cdot k} $$
where \( v_{fill} \) is the filling speed in cm/s, \( h \) is the height of the casting in cm, \( \delta \) is the wall thickness in cm, \( T \) is the pouring temperature in °C, and \( k \) is a constant dependent on the alloy and mold material. For QT600 ductile iron in a ceramic shell mold, \( k \) is typically 0.5. Substituting the values—\( h = 7.11 \, \text{cm} \), \( \delta = 2.0 \, \text{cm} \), and \( T = 1,300 \, \text{°C} \)—I computed \( v_{fill} = 0.4577 \, \text{m/s} \). After rounding, I set the initial filling speed to 0.458 m/s for simulation purposes.
Key process parameters for the lost wax investment casting were defined based on material properties and experimental data. The liquidus and solidus temperatures for QT600 are 1,194 °C and 1,129 °C, respectively. The ceramic shell mold comprised six layers with a total thickness of approximately 6 mm, made from refractory quartz sand bonded with silica sol. Pouring was conducted in air using a top-gating approach, with a shell preheat temperature of 900 °C. The casting orientation was set along the negative X-axis to simulate gravity effects, with a pouring temperature of 1,300 °C and a filling speed of 0.458 m/s. Cooling occurred naturally under ambient conditions.
I developed a three-dimensional model of the protective part and its gating system using CAD software, which was then imported into simulation tools for mesh generation. The finite element model included 67,010 nodes, with element sizes of 8 mm for the sprue and pouring cup and 5 mm for the casting and ingates. This discretization ensured accurate representation of thermal and fluid dynamics during the lost wax investment casting process. Boundary conditions were applied to reflect real-world scenarios: the heat transfer coefficient between the shell and casting was set to 1,000 W/(m²·K), between the casting and air to 1,000 W/(m²·K), and between the shell and air to 50 W/(m²·K). Gravity was defined as 9.8 m/s² in the negative X-direction to model the pouring process accurately.
Simulation of the initial lost wax investment casting process revealed the filling sequence: molten metal entered the mold steadily, with minimal turbulence. The sprue filled within 0.7 seconds, the first two castings were fully filled by 2.5 seconds, and the remaining two reached 50% fill by 3.4 seconds, completing at 7.2 seconds. Solidification analysis showed that the casting began to solidify from the outer surfaces inward, with 10–20% solid fraction at 944 seconds and near-complete solidification of the main body by 1,774 seconds. The entire system solidified by 2,704 seconds. However, shrinkage porosity defects were identified, primarily in the sprue, ingates, and their junctions, with a shrinkage rate of 13.2603%. These defects arose because the ingates solidified early, blocking the feeding paths and leading to volumetric shrinkage in the casting body. This issue is common in lost wax investment casting when the gating design does not support adequate补缩.
To address these defects, I optimized the gating system for the lost wax investment casting process. Two modified schemes were proposed: Scheme A added side ingates on the rectangular sections of the casting, while Scheme B incorporated additional ingates along the tubular portions. Both designs aimed to improve metal distribution and feeding during solidification. After remodeling and meshing, simulations were run under identical conditions to the initial setup. The results indicated that Scheme B outperformed Scheme A, reducing the shrinkage rate to 1.3675% compared to 6.9025% for Scheme A and 13.2603% for the original. In Scheme B, the casting body was free of shrinkage defects, validating the effectiveness of the modified gating in the lost wax investment casting process.
Further optimization involved studying the impact of key process parameters on casting quality in lost wax investment casting. I selected pouring temperature, filling speed, and shell preheat temperature as variables, as they significantly influence fluidity, solidification, and defect formation. Based on industry guidelines, pouring temperature was varied between 1,250 °C and 1,300 °C (50–100 °C above the liquidus), filling speed between 0.450 m/s and 0.460 m/s (around the calculated value), and shell preheat temperature between 800 °C and 1,000 °C (typical for ductile iron). An orthogonal experimental design was employed to efficiently explore these factors, with shrinkage rate as the response variable. Lower shrinkage rates indicate better casting quality in lost wax investment casting.
The table below outlines the factors and levels used in the orthogonal array for the lost wax investment casting optimization:
| Level | A: Pouring Temperature (°C) | B: Filling Speed (m/s) | C: Shell Preheat Temperature (°C) |
|---|---|---|---|
| 1 | 1,250 | 0.450 | 800 |
| 2 | 1,280 | 0.455 | 900 |
| 3 | 1,300 | 0.460 | 1,000 |
I conducted nine simulation trials based on the orthogonal array, recording filling times and shrinkage rates for each combination in the lost wax investment casting process. The results are summarized in the following table:
| Trial | A: Pouring Temperature (°C) | B: Filling Speed (m/s) | C: Shell Preheat Temperature (°C) | Filling Time (s) | Shrinkage Rate (%) |
|---|---|---|---|---|---|
| L1 | 1,250 | 0.450 | 800 | 4.881 | 1.451 |
| L2 | 1,250 | 0.455 | 900 | 3.925 | 1.433 |
| L3 | 1,250 | 0.460 | 1,000 | 3.869 | 1.442 |
| L4 | 1,280 | 0.450 | 900 | 3.961 | 1.551 |
| L5 | 1,280 | 0.455 | 1,000 | 3.918 | 1.452 |
| L6 | 1,280 | 0.460 | 800 | 3.869 | 1.402 |
| L7 | 1,300 | 0.450 | 1,000 | 3.820 | 1.589 |
| L8 | 1,300 | 0.455 | 800 | 3.771 | 1.370 |
| L9 | 1,300 | 0.460 | 900 | 3.858 | 1.321 |
Analysis of the orthogonal experiments for the lost wax investment casting process showed that Trial L9 achieved the lowest shrinkage rate of 1.321%, with a pouring temperature of 1,300 °C, filling speed of 0.460 m/s, and shell preheat temperature of 800 °C. The filling time for this combination was 3.858 seconds. Range analysis confirmed that this parameter set (A3B3C1) is optimal for minimizing defects in lost wax investment casting. Simulations under these conditions demonstrated a significant reduction in shrinkage porosity, with defects confined to non-critical areas. Practical casting trials validated these findings, resulting in components with improved integrity and lower rejection rates.
In conclusion, this study demonstrates the effectiveness of numerical simulation in optimizing the lost wax investment casting process for ductile iron protective parts. By refining the gating system and process parameters, I achieved a substantial decrease in shrinkage defects. The optimal parameters—pouring temperature of 1,300 °C, filling speed of 0.460 m/s, and shell preheat temperature of 800 °C—provide a reliable framework for industrial applications of lost wax investment casting. This approach not only enhances product quality but also reduces material waste, underscoring the value of integrated modeling and experimentation in advancing lost wax investment casting techniques for complex components.