Abstract: According to the structure and internal quality requirements of shell castings, the difficulties of casting process design were analyzed. The initial casting process is implemented by gating system, feeding riser, process subsidy design, shell preparation, smelting and pouring process control, etc. According to the test results, the process was optimized. After X-ray and fluorescence inspection, no loose or shrinkage defects were found in the castings cast according to the optimized process plan, which exceeded the requirements of the acceptance standard, proving that the optimized process plan was reasonable and feasible.

Keywords: hot spot; heat dissipation; process subsidy; temperature gradient; investment casting
1. Introduction
Investment casting, also known as lost-wax casting, is a precision casting method that involves creating a mold from a wax pattern, encasing it in a refractory material, melting out the wax, and then pouring molten metal into the void to form the casting. This method is particularly suited for producing complex and intricate shapes with high dimensional accuracy and surface finish. In this paper, we will discuss the design and optimization of an investment casting process for a large complex stainless steel shell, addressing the challenges encountered during the casting process and the solutions implemented to ensure the quality of the final product.
2. Casting Information Overview
The stainless steel shell casting in question has a structure that consists of a bearing hole, a mounting flange, and a flange edge. The casting is designed to meet the internal quality standards of Class I, Grade B, and it is subjected to high loads during operation, with gears assembled within its internal cavity to transmit power.
The maximum outer contour dimensions of the casting are ϕ330 mm × 186 mm, with a maximum wall thickness of 20 mm and a minimum wall thickness of 3 mm, resulting in a significant difference in wall thickness. The casting features 50 bosses (or ears) and nearly 60 casting hot spots formed at the intersections of arc plates, rib plates, and oil passages.
Table 1. Casting Dimensions and Wall Thickness
Casting Dimension | Maximum Outer Contour (mm) | Maximum Wall Thickness (mm) | Minimum Wall Thickness (mm) |
---|---|---|---|
Value | ϕ330 × 186 | 20 | 3 |
3. Analysis of Casting Process Challenges
The casting process for this complex stainless steel shell presents several challenges:
- Hot Spot Complications: The casting features multiple hot spots, including those at the junctions of reinforcing ribs, flange edges, and bearing hole walls. These hot spots are difficult to feed due to limited space or heat dissipation area, increasing the risk of shrinkage and porosity defects.
- Difficult Feeding Locations: Some hot spots, such as those on the flange bosses or within the bearing hole oil passages, are not easily accessible for feeding due to their location or the interference from adjacent structures.
- Heat Dissipation Issues: The casting has a large number of bosses, ribs, and oil passages, which create complex heat dissipation paths. The limited heat dissipation space in some areas can lead to slow solidification and increased defects.
4. Casting Process Design and Verification
To address the challenges outlined above, a comprehensive casting process design was developed, including a gating system, feeding risers, process subsidies, shell preparation, and melting and pouring process control.
4.1 Gating System Design
The hot spots are concentrated on the mounting flange, with a total of 25 hot spots. These hot spots have a “cylinder + conical bevel” structure, and a gating system was designed to effectively feed these hot spots.
Table 2. Gating System Design Overview
Component | Description |
---|---|
Hot Spot Location | Mounting flange (25 hot spots) |
Gating System Type | Side-top injection |
Riser Design | Annular riser for sequential feeding |
A side-top injection gating system was adopted, with the risers designed in a circular annulus to effectively feed the hot spots in a sequential manner from top to bottom. The size and location of the risers were calculated based on the casting structure and the size of the hot spots.
4.2 Feeding Channel Design
4.2.1 Boss ① Feeding Channel Design
Due to the interference from slot A, the riser on the mounting flange cannot feed boss ①, and the riser on boss ① cannot feed the mounting flange boss. To address this, process subsidies were designed on the flange edge to create a feeding path for boss ①.
The riser, positioned on these process subsidies, then effectively feeds boss ①, ensuring a robust feeding mechanism despite the spatial constraints posed by slot A. This design circumvents the direct feeding challenges associated with boss ① and the mounting flange boss, facilitating improved molten metal distribution and reducing the risk of shrinkage and porosity defects.
4.2.2 Feeding Channel Design for Oil Passage ②
Addressing the complex geometry of oil passage ②, which comprises multiple hot spots, designing a riser directly on the outer spherical surface is impractical due to difficulties in subsequent cleanup. Instead, a riser is strategically placed at positions c and d, focusing on hot spot “②-2”. This design leverages the feeding capacity of the riser at “②-2” to indirectly feed hot spot “②-3”, which has a spherical outer surface. The narrow space within the bearing hole necessitates careful consideration of heat dissipation and solidification rates. By positioning the riser at c, despite further restricting heat dissipation area, this approach carefully balances the need for feeding against the constraints of the bearing hole’s confined space.
4.2.3 Feeding Channel Design for Boss ③ (8 locations)
Boss ③, located between reinforcing ribs and within the bearing hole, faces feeding challenges due to interference from slot A and necking B. Direct riser placement on boss ③ is infeasible. Therefore, process subsidies are incorporated on the mounting boss D and the conical slope, allowing the riser on the mounting boss D to feed boss ③ through these subsidies. This indirect feeding method ensures that all eight boss ③ locations receive adequate molten metal, mitigating the risk of shrinkage and porosity defects.
4.2.4 Feeding Channel Design for Oil Passage ④ (2 locations)
Similarly, oil passage ④, influenced by reinforcing ribs, has limited space for direct riser placement. To address this, other boss risers are utilized for feeding, though the compact arrangement surrounded by reinforcing ribs significantly hampers heat dissipation. Enhanced heat dissipation strategies, such as modifying the shell preparation process and adjusting melting and pouring parameters, are essential to mitigate the high risk of shrinkage and porosity in this area.
In summary, the feeding channel designs for boss ①, oil passage ②, boss ③, and oil passage ④ incorporate process subsidies, strategic riser placements, and modifications to heat dissipation pathways, collectively ensuring adequate feeding and minimizing defects in these critical hot spot regions.