In the field of rail transit, which encompasses traditional railways, intercity rail systems, and urban metro networks, the demand for high-performance and reliable components is paramount. Safety is the top priority, with a “zero-defect” mindset driving the need for superior quality in critical parts. Among these, motor base castings serve as essential structural elements in traction systems, requiring excellent mechanical properties, manufacturability, and integrity. To enhance the quality, reduce costs, and improve economic efficiency of such casting parts, I have explored and implemented the sand-lined metal mould casting process, also known as coated permanent mould casting. This technique involves covering a metal mould (iron mould) cavity with a thin layer of resin-coated sand to form a mould, optimizing heat dissipation and solidification conditions. Through extensive finite element analysis and small-batch trials, I have observed significant improvements in nodularization grade, grain refinement, density, and overall mechanical performance of the casting part, while also increasing yield, reducing environmental impact, and achieving green foundry practices.
The motor base casting part, as a key component, typically has a rough weight of 130–150 kg, with uneven wall thicknesses ranging from 15 mm in cylindrical sections to 48 mm at lug areas. These lugs are stress-bearing zones that must be free from shrinkage porosity and require X-ray inspection. The internal structure is complex, demanding dimensional accuracy of CT9 grade and surface roughness of Ra25 µm. The material is often low-temperature ductile iron EN-GJS-400-18U-LT, with mechanical properties including tensile strength ≥400 MPa, yield strength ≥240 MPa, hardness 130–150 HBW, elongation ≥18%, and impact energy >12 J at -40°C. These stringent requirements make the choice of casting process critical for producing high-integrity casting parts.
In designing the sand-lined metal mould casting process for this casting part, I focused on several key aspects: gating system type, parting method, pouring time design, choke section calculation, and sand core design. Based on the structural characteristics and equilibrium solidification principles, I adopted a semi-closed bottom gating system to enhance slag trapping, minimize molten metal erosion, and ensure stable filling. For the specific motor base casting part example, a one-cavity-per-mould layout was selected due to equipment constraints.
The parting surface was determined along the mid-plane of the cylindrical section, as shown in the design, to balance mould and tooling dimensions, reducing interference with machinery. The pouring time, defined as the duration from metal entry into the cavity until the highest contour is filled, was calculated using the following formula:
$$ t = f(\sqrt{G_{\text{part}}}) \cdot (\delta \cdot \sqrt{G_{\text{part}}} + 53) / 1000 $$
where \( t \) is the effective pouring time in seconds, \( f \) is a material coefficient (typically 0.8 for ductile iron), \( G_{\text{part}} \) is the weight of the casting part in kg (excluding gating and risers), and \( \delta \) is the main wall thickness in mm (taken as the thinnest section). For the motor base casting part with \( G_{\text{part}} = 140 \) kg and \( \delta = 15 \) mm, the pouring time was computed as 15 seconds. This parameter is crucial for controlling the filling behavior and minimizing defects in the casting part.
The choke section area of the gating system was designed using the restricted cross-section method, with the formula:
$$ \sum S_{\text{choke}} = \frac{G_{\text{part}}}{\rho \cdot t \cdot \sqrt{2g \cdot H \cdot \omega}} $$
where \( \sum S_{\text{choke}} \) is the total choke area in cm², \( \rho \) is the density of the material (0.0072 kg/cm³ for ductile iron), \( g \) is gravitational acceleration (980 cm/s²), \( H \) is the effective metallostatic head in cm, and \( \omega \) is a material index (0.23 for ductile iron). For this casting part, the calculated choke area was 15.7 cm². Based on experience with sand-lined metal mould casting, I designed a semi-closed gating system with area ratios \( \sum S_{\text{sprue}} : \sum S_{\text{runner}} : \sum S_{\text{choke}} : \sum S_{\text{ingate}} = 1.13 : 2.29 : 1 : 1 \), resulting in a sprue diameter of 45 mm. The gating system layout ensures smooth metal flow and reduces turbulence for the casting part.
| Parameter | Value | Unit |
|---|---|---|
| Casting Part Weight (\(G_{\text{part}}\)) | 140 | kg |
| Main Wall Thickness (\(\delta\)) | 15 | mm |
| Pouring Time (\(t\)) | 15 | s |
| Choke Area (\( \sum S_{\text{choke}} \)) | 15.7 | cm² |
| Sprue Diameter | 45 | mm |
| Gating System Ratio | 1.13:2.29:1:1 | – |
Sand core design is a critical challenge due to the complexity and multiplicity of cores in this casting part. All cores were designed with independent positioning to ensure accuracy and precision. To optimize performance, cores were lightweighted while maintaining strength, with enhanced venting gaps, convex venting bosses at core prints, and enlarged vent pins to facilitate gas evacuation during pouring. This design minimizes gas-related defects and ensures the integrity of the casting part. The cores are strategically placed to form internal passages and reinforce structural features of the casting part.
To validate the casting process before tooling investment, I conducted finite element analysis simulations for filling and solidification. The sand-lined metal mould casting parameters, such as heat transfer coefficients, sand coating thickness, pouring temperature, and nodularization grade, were set based on practical production conditions. The filling simulation showed that the gating system remained full throughout, with metal flowing uniformly and rising steadily, completing in 15 seconds. The solidification simulation indicated isolated liquid metal zones at thick lug sections after 75% solidification, posing a risk of shrinkage porosity. Therefore, I added feeding risers to these areas to ensure soundness of the casting part. The simulation predicted minor shrinkage tendencies at a 5% porosity rate, which aligned with X-ray inspection results meeting acceptance standards.
| Parameter | Value | Unit |
|---|---|---|
| Pouring Temperature | 1420–1450 | °C |
| Sand Coating Thickness | 5–10 | mm |
| Iron Mould Thickness | 20–30 | mm |
| Heat Transfer Coefficient | 500–800 | W/(m²·K) |
| Predicted Shrinkage Porosity | <5% | – |
| Filling Time | 15 | s |
The solidification process can be modeled using the Chvorinov’s rule for approximation, though modified for sand-lined metal mould conditions:
$$ t_s = B \cdot \left( \frac{V}{A} \right)^n $$
where \( t_s \) is the solidification time, \( V \) is the volume of the casting part, \( A \) is the surface area, \( B \) is a mould constant dependent on thermal properties, and \( n \) is an exponent (typically around 2 for sand moulds, but adjusted for metal moulds). For this casting part, the rapid cooling due to the iron mould reduces \( t_s \), promoting finer grains and higher density.
In production, the sand-lined metal mould casting process was applied with controlled molten metal composition: C 3.6–3.8%, Si 1.6–2.0%, Mn <0.4%, P <0.04%, S <0.025%, and Mg 0.02–0.05%. Using a 0.75-ton medium-frequency induction furnace, the metal was tapped at 1520°C, treated with 1.4% nodularizer and 0.8% inoculant, achieving a nodularization grade of 2 before pouring. The pouring temperature was maintained at 1420–1450°C, with a pouring time of 15 seconds per mould. Twenty trial moulds were produced, and after shot blasting, heat treatment, and machining, all casting parts passed X-ray inspection, meeting technical requirements for surface quality, internal density, dimensional accuracy, and mechanical properties sampled from the casting part itself.
| Property | Requirement | Measured Value |
|---|---|---|
| Tensile Strength | ≥400 MPa | 420–450 MPa |
| Yield Strength | ≥240 MPa | 250–270 MPa |
| Elongation | ≥18% | 20–22% |
| Hardness (HBW) | 130–150 | 135–145 |
| Impact Energy (-40°C) | >12 J | 14–16 J |
For batch production, the process demonstrated stability with a yield of 90.8% and a scrap rate below 5%. The iron-to-sand ratio reached 1:6, significantly reducing resin sand consumption and lowering costs. This efficiency stems from the reusable iron mould and thin sand layer, which minimize material waste for each casting part. The economic benefits are substantial, with reduced cleaning and grinding efforts due to minimal flash and excellent surface finish on the casting part.
The advantages of sand-lined metal mould casting for such casting parts are multifaceted. By adjusting sand coating and iron mould thicknesses, the cooling rate is controlled, leveraging the high thermal conductivity of the metal mould to accelerate solidification. This results in a dense, fine-grained microstructure in the casting part, enhancing mechanical properties. The process capitalizes on the graphitization expansion of ductile iron during solidification to mitigate shrinkage defects, though risers are added for non-self-feeding thick sections. Moreover, the dimensional accuracy and surface quality of the casting part reduce post-processing, contributing to lower labor costs and faster throughput.
From a broader perspective, the application of sand-lined metal mould casting extends beyond motor base castings to other rail transit components like traction motor end covers and brake discs. The process supports the production of high-integrity casting parts that meet the stringent demands of modern transportation systems. Future work could involve optimizing sand formulations, integrating real-time monitoring, and expanding the process to other alloys. The success with this casting part underscores the viability of sand-lined metal mould casting for high-value applications, offering a competitive edge through improved quality and sustainability.
In conclusion, the sand-lined metal mould casting process has proven effective for producing rail transit motor base casting parts, achieving superior quality, high yield, and significant cost savings. The combination of simulation-driven design and practical optimization ensures reliable performance of the casting part under operational stresses. This approach not only meets current industry standards but also paves the way for innovative foundry solutions in the rail sector, reinforcing the role of advanced casting techniques in manufacturing critical components.