In the context of increasing global emphasis on energy conservation and emissions reduction, the automotive industry is accelerating the development of new powertrain systems. The hybrid electric drive transmission represents a significant innovation, integrating electric motors, mechanical gears, and hydraulic systems into a single, complex housing. For the prototype and low-volume production phases of such components, sand casting offers a uniquely advantageous manufacturing solution due to its flexibility, relatively low tooling cost, and ability to produce intricate geometries. This article details a self-developed sand casting process specifically designed for an aluminum alloy hybrid power transmission case, covering the analysis, design, simulation, and validation of the entire manufacturing route.
The component in question consists of two primary housings: a starter motor casing and a drive motor casing. Both parts feature highly complex internal geometries with segregated cavities for electrical components, mechanical gearing, and hydraulic fluid, demanding high integrity to prevent cross-contamination, especially between coolant and electrical systems. The initial design was intended for high-pressure die casting. However, for development and testing phases requiring only a few hundred units, the lead time and cost associated with die-casting tooling were prohibitive. Therefore, adapting the design for sand casting became the optimal strategy. The primary challenges included accommodating the original thin walls, managing the extensive open-face structure, ensuring the stability of deep, narrow sand cores, and achieving soundness in isolated thick sections.
The material specified for the casting is an Al-Si-Mg (Cu) alloy, equivalent to something like A356 or a similar secondary alloy, requiring a T6 heat treatment to meet the final mechanical properties. The key specifications for the drive motor casing are summarized below:
| Parameter | Specification |
|---|---|
| Alloy Type | Al-Si-Mg (Cu) (e.g., A356) |
| Heat Treatment | T6 (Solutionized, Quenched, Aged) |
| Tensile Strength (min) | 220 MPa |
| Yield Strength (min) | 180 MPa |
| Elongation (min) | 1% |
| Hardness, HB (min) | 75 |
| Leak Test Pressure | 1.0 MPa |
| Allowable Leakage | ≤ 20 mL/min |
The first step in process design was a thorough structural analysis. The original design had a nominal wall thickness of 3.5 mm, typical for die casting. For sand casting, which generally deals with slower cooling rates and different fluidity characteristics, this thickness was increased to 4.5 mm to ensure proper mold filling and reduce the risk of mistruns. The component’s geometry presented several critical areas: large, open faces that complicate dimensional control during solidification and shakeout, and isolated thick sections (like bosses and rib intersections) prone to shrinkage porosity. Furthermore, the presence of deep, recessed cavities necessitated the use of strong, reliable sand cores.
The selection of the molding medium is critical in any sand casting operation. For this application, furan resin-bonded sand was chosen. This no-bake binder system provides excellent dimensional accuracy, high green strength to support the complex cores and overhangs, good collapsibility to minimize hot tearing and casting stress, and a relatively favorable environmental profile compared to some other chemically bonded sands. The high strength of the cured sand is essential for maintaining the integrity of the cores defining the internal water jackets and cavities during metal pouring.
The fundamental casting layout was established with the large, open face oriented downward in the drag portion of the mold. This positioning provides a stable base and allows for a clean, flat reference surface. A bottom-gating system was designed to introduce molten metal into the mold cavity. This approach promotes tranquil filling, minimizes turbulence and associated oxide film entrainment, and supports a favorable temperature gradient for directional solidification. The gating system itself was designed as an open, pressurized system with specific area ratios to control flow velocity and pressure. The sprue acts as the flow restrictor. The initial design was based on empirical formulas for aluminum sand casting. The pouring weight of the drive motor casing is approximately 13.2 kg.
The required sprue cross-sectional area ($A_{sprue}$) can be estimated using the formula related to pouring time and head height:
$$ A_{sprue} = \frac{W}{\rho \cdot t \cdot C \cdot \sqrt{2gH}} $$
Where $W$ is the casting weight, $\rho$ is the liquid aluminum density (~2500 kg/m³), $t$ is the desired pouring time, $C$ is a discharge coefficient (~0.8), $g$ is gravity, and $H$ is the effective metallostatic head. For this weight and geometry, a sprue with a top diameter of 25 mm (area ~4.9 cm²) was selected to prevent aspiration and vortex formation. Based on this, the area ratios for the open system were set as:
$$ \sum A_{sprue} : \sum A_{runner} : \sum A_{ingate} = 1 : 4 : 4 $$
This yielded a total runner cross-sectional area ($\sum A_{runner}$) of approximately 19.6 cm² and a total ingate area ($\sum A_{ingate}$) of 19.6 cm². Runners and ingates were sized and positioned to distribute metal evenly to the bottom of the casting cavity.
To manage shrinkage in the isolated thick sections, blind risers (feeders) were strategically placed. The size of a riser can be approximated using Chvorinov’s Rule, ensuring its solidification time is longer than that of the casting section it is intended to feed:
$$ t_{riser} > t_{casting} $$
where solidification time $t = K \left( \frac{V}{A} \right)^2$, with $V$ being volume, $A$ being cooling surface area, and $K$ being the mold constant. Risers were designed with a modulus (V/A ratio) about 1.2 times greater than the modulus of the hot spot they were feeding.
The extreme complexity of the part geometry required a multi-part mold assembly. The sand casting mold was not built as a traditional two-part cope and drag box. Instead, it was constructed from five individual sand cores assembled with precise location pins and prints. This “core assembly” or “segment mold” approach is highly effective for complex box-like structures. The breakdown was as follows:
- Drag Side Core: Forms the major exterior surface (oil cavity side).
- Cope Side Core: Forms the opposite exterior surface (motor mount side).
- Motor Cavity Core: A large, complex core creating the internal void for the electric motor.
- Oil Gallery Core: Another intricate core forming the internal hydraulic passages.
- Various Small Cores: For bolt bosses, coolant channels, and other detailed features.
These cores were produced using aluminum pattern plates, which offer good durability and surface finish for a low-to-medium volume production run. The patterns were machined with the necessary draft, shrinkage allowance (typically ~1.3% for aluminum in furan sand), and core prints to ensure accurate assembly of the final sand mold package.
Prior to committing to tooling and physical trials, the entire sand casting process was simulated using dedicated foundry simulation software (e.g., MAGMAsoft, ProCAST, or equivalent). The 3D CAD models of the casting, gating system, risers, and sand cores were imported. The simulation parameters included the thermo-physical properties of the Al-Si-Mg alloy and the furan sand.
Filling Analysis: The mold filling sequence was simulated to assess flow characteristics. The results showed a calm, progressive fill from the bottom gates upward. No significant jetting, splashing, or early entrapped air pockets were predicted. The metal front advanced smoothly, validating the bottom-gating design for minimizing turbulence in this sand casting process.
Solidification Analysis: This was the most critical phase of simulation. The software calculated the evolution of the solid fraction over time. The goal was to achieve directional solidification, where the thinner sections of the casting solidify first, and the thicker sections solidify last, being fed by the liquid metal in the risers until they are completely solidified. The results indicated that the thermal gradients were correctly oriented. The casting sections solidified progressively toward the risers, which remained liquid longest, acting as effective feed metal reservoirs. No major isolated hot spots, which would lead to macro-shrinkage porosity, were identified in the critical areas.
The simulation provided confidence in the sand casting process design. Following the simulation-based optimization, the aluminum patterns were manufactured, and core boxes were fabricated. The furan sand was mixed, and the five major cores were produced. After curing, the cores were assembled in a steel fixture to create the complete mold cavity. The assembly was then coated with a refractory wash to improve surface finish and prevent metal penetration.
The alloy was melted in a resistance or gas-fired furnace, degassed using rotary argon or nitrogen, and cleaned of oxides. The melt temperature was carefully controlled, typically between 710°C and 730°C for this alloy, to ensure adequate fluidity for the sand casting process without excessive gas pickup. The mold was poured using a hand ladle. The actual pouring time was recorded and matched well with the calculated and simulated predictions.
After cooling, the sand mold was broken away, and the casting was shaken out. The initial visual inspection showed complete filling with no obvious surface defects like cold shuts or misruns. The gating and risering system was removed via sawing and grinding.
The castings underwent a full T6 heat treatment cycle: solution heat treatment at around 540°C, followed by rapid quenching in water, and finally artificial aging. After heat treatment, the castings were shot blasted to clean the surface and improve appearance.
To validate the internal soundness of the sand casting, particularly in the previously identified critical thick sections, 100% of the prototype castings were inspected using X-ray radiography. The radiographs revealed no detectable shrinkage cavities or gas pores in the areas of concern. The internal quality met the stringent requirements for a pressure-tight housing.
Finally, all castings were subjected to a pressure tightness test. Each housing was sealed and pressurized with air or another medium to 1.0 MPa, held for 20 seconds, and the leakage rate was measured. All prototype sand castings passed this test, with leakage rates well below the maximum allowable limit of 20 mL/min, confirming the integrity of the sand casting process in producing leak-free components.
This successful development project demonstrates the enduring capability and relevance of sand casting for manufacturing complex, high-integrity aluminum components in a low-volume, rapid-prototyping context. The keys to success were:
- Adapting the Design: Modifying wall thickness for the sand casting process.
- Strategic Process Design: Implementing a bottom-gated, riser-fed system to promote soundness.
- Advanced Mold Making: Utilizing a segmented core assembly approach to manage extreme complexity.
- Process Simulation: Leveraging software to predict and eliminate defects before tooling.
- Material and Process Control: Careful handling of melt quality, pouring, and heat treatment.
The sand casting process proved to be an ideal manufacturing solution for the development phase of this hybrid transmission component, providing the necessary flexibility, quality, and cost-effectiveness to support the product’s journey through design validation and initial road testing stages. This case study underscores that even for highly advanced automotive powertrains, traditional sand casting, when applied with modern engineering tools and expertise, remains a vital and competitive manufacturing technology.