In the field of heavy machinery manufacturing, the production of large-scale shell castings, such as the upper housing for crushers, presents significant challenges due to their complex geometry, stringent quality requirements, and the need for cost-effective manufacturing. As a casting engineer involved in product development, I have focused on refining the casting process for these critical shell castings to enhance quality, reduce defects, and improve operational efficiency. The original process for the crusher upper shell castings utilized a template and core assembly molding method, which frequently led to severe dimensional deviations, wide sand core gaps, and sand drop defects. To address these issues, my team and I redesigned the process by adopting a hollow natural pattern molding approach, integrating computer simulation for validation, and implementing external chills. This article details the comprehensive process design, emphasizing key aspects like material selection, gating and risering systems, and thermal management, all aimed at optimizing the production of high-integrity shell castings.
The crusher upper shell casting is a annular-shaped component with an outer diameter of 6020 mm, a height of 2500 mm, and a main wall thickness ranging from 110 to 130 mm. It is manufactured from GS20Mn5N steel, with a net weight of 49.5 tons and a gross weight of 62 tons. Shell castings of this size require precise control over dimensional accuracy and internal soundness to ensure performance in crushing operations. The material specifications for GS20Mn5N are critical, as outlined in the tables below, which define the chemical composition and mechanical properties that must be achieved post-heat treatment.
| Element | Composition Range (wt.%) |
|---|---|
| C | 0.17–0.23 |
| Si | 0.30–0.60 |
| Mn | 1.00–1.50 |
| S | ≤0.015 |
| P | ≤0.020 |
| Cr | ≤0.30 |
| Ni | ≤0.40 |
| Mo | ≤0.15 |
| Property | Requirement |
|---|---|
| Yield Strength (ReH) | ≥280 MPa |
| Tensile Strength (Rm) | 500–600 MPa |
| Elongation (A) | ≥20% |
| Impact Energy (Akv at 20°C) | ≥40 J |
The original casting process for these shell castings involved a combination of template molding and core assembly. While this method reduced pattern-making costs and saved wood, it introduced several quality issues. Dimensional inaccuracies arose from the cumulative errors in core placement, and the gaps between cores led to sand incursions and potential leakage during pouring. Additionally, the use of chaplets to support cores added post-casting cleanup work. To overcome these drawbacks, we transitioned to a hollow natural pattern molding technique. This approach involves creating a full-scale, hollow wooden pattern that accurately replicates the casting’s external and internal geometries, thereby eliminating core assembly and reducing dimensional variability. Although pattern-making costs increase initially, this method streamlines foundry operations, minimizes defects, and lowers overall production costs for shell castings, especially in batch production where patterns are reusable.
In the redesigned process, the molding method employs pit molding with a hollow natural pattern. The pattern is placed on a leveled pit floor equipped with venting channels, and the mold is built around it. An outer jacket is used to contain the sand, and the cope is formed using a pre-made flask. This method ensures better dimensional stability and reduces the risk of sand-related defects. The parting line is set with the smaller opening of the shell castings facing upward and the larger opening downward, facilitating the creation of a directional solidification gradient from the bottom to the top, which is essential for effective riser feeding. The gating system is designed with a sprue in the inner cavity, a circular runner, and three-tier step gates to promote smooth metal flow.
Key casting parameters were established based on industry standards and empirical data. The casting shrinkage allowance was set at 1.8% to account for the uniform wall thickness of the shell castings. This is derived from the linear shrinkage formula: $$ \Delta L = L_0 \cdot \alpha $$ where \( \Delta L \) is the dimensional change, \( L_0 \) is the nominal dimension, and \( \alpha \) is the shrinkage factor (0.018 for this steel). Machining allowances were assigned as 25 mm on the top surfaces and 20 mm on the side surfaces, with additional allowances on flange areas to serve as process margins. Draft angles of 2–3 degrees were incorporated on external ribs, which were made as removable pattern sections to ease pattern withdrawal.
Pattern construction required meticulous attention to detail. The hollow wooden pattern was built with a robust structure to maintain dimensional accuracy under foundry conditions. The core box was designed as a collapsible type to ensure easy removal. All pattern surfaces were coated with a release agent, and fillets with radii of 20–30 mm were added to sharp corners to reduce stress concentrations. Location lines were engraved on the pattern to position risers, gates, feeders, and external chills accurately.
The molding material selection is crucial for shell castings to achieve good surface finish and dimensional precision. We used ester-hardened sodium silicate sand for both mold and core due to its high strength and low gas generation. The mold surfaces were coated with alcohol-based zirconium silicate paint applied by brushing in three layers, each 0.5–1.0 mm thick. After each coat, the paint was ignited to dry, and after the final coat, the mold was dried with gas burners until the coating turned orange-yellow. In areas prone to sand burning, such as corners and thick sections, chromite sand was placed to enhance refractoriness.
The risering system is designed to ensure soundness in shell castings by providing adequate feed metal and delaying solidification. We employed the modulus method combined with computer simulation to optimize riser design. The modulus \( M \) of a casting section is defined as the ratio of volume \( V \) to cooling surface area \( A \): $$ M = \frac{V}{A} $$ For effective feeding, the riser modulus \( M_r \) should satisfy: $$ M_r \geq 1.2 \cdot M_c $$ where \( M_c \) is the modulus of the casting section. Using SolidWorks 3D software, we modeled the shell castings and divided it into independent feeding zones, each served by a dedicated riser. Volume and surface area measurements were taken directly from the software to compute moduli. Based on this, we selected eight open top risers and two blind risers made from insulating materials. Common riser parameters are summarized below:
| Riser Type | Dimensions | Modulus | Volume |
|---|---|---|---|
| Spherical | H = D | D/6 | 0.533D³ |
| Cylindrical Open | H = D | D/6 | 0.785D³ |
| Cylindrical Open | H = 1.2D | 0.176D | 0.942D³ |
| Cylindrical Open | H = 1.5D | 0.188D | 1.178D³ |
| Cylindrical Blind | H = D | D/5 | 0.654D³ |
| Cylindrical Blind | H = 1.2D | 0.178D | 0.812D³ |
| Cylindrical Blind | H = 1.5D | 0.190D | 1.047D³ |
| Cylindrical Blind | H = 2D | 0.204D | 1.440D³ |
To promote directional solidification and avoid hot spots at riser junctions, the riser necks were tapered. External chills were strategically placed around the lower periphery of the shell castings to accelerate cooling in thick sections and enhance feeding efficiency. The simulation software was then used to verify the thermal behavior and feeding effectiveness, allowing us to fine-tune riser sizes and chill placements to minimize material usage while ensuring quality. The simulation results confirmed that the designed riser system could adequately compensate for shrinkage in the shell castings.
The gating system for these large shell castings was designed as a step-gate system to ensure平稳的金属填充 and minimize turbulence. Given the height of the castings, a single bottom gate could cause excessive erosion, so we adopted a tangential entry with three levels of ingates. This promotes a gradual rise of molten metal, reducing the risk of defects like sand inclusion and gas porosity. The gating is open-type with a cross-sectional area ratio defined as: $$ F_{\text{sprue}} : F_{\text{runner}} : F_{\text{ingate}} = 1 : 2 : (3 \text{ to } 4) $$ All gating components were made from ceramic tubes to withstand thermal shock and maintain dimensional stability. The final casting layout included risers, gates, and chills arranged to optimize feeding and minimize thermal stresses.
Dimensional control is paramount for shell castings to meet machining tolerances. The hollow natural pattern inherently ensures accurate wall thickness and overall dimensions. To counteract potential distortion during solidification, we added tie bars at the inner diameter near blind risers and placed sand blocks at the core center to accommodate circular contraction. This helps the casting shrink uniformly according to the 1.8% allowance, ensuring that final dimensions align with design specifications.
Pouring operations require precise temperature control and adequate metal supply. For the upper shell castings, a total of 93 tons of molten steel was prepared using a 75-ton LF refining furnace. The ladle was preheated to reduce heat loss, which typically averages 0.7–1.0°C per minute during holding. The pouring temperature was maintained between 1540°C and 1560°C to ensure fluidity while minimizing shrinkage. During pouring, when the metal reached half the riser height, the flow was slowed, and exothermic compounds were added to enhance riser feeding capacity.
Post-casting, the shell castings undergo stress relief annealing before riser removal. After cutting the risers, the castings are subjected to normalizing and tempering heat treatments. Normalizing is conducted at 920–940°C to refine the grain structure, followed by tempering at 530–550°C to achieve the desired toughness. The resulting mechanical properties consistently meet or exceed the requirements for GS20Mn5N shell castings, as shown in the table below from actual production tests.
| Property | Value 1 | Value 2 |
|---|---|---|
| Yield Strength (ReH) | 365 MPa | 322 MPa |
| Tensile Strength (Rm) | 531 MPa | 531 MPa |
| Elongation (A) | 30.5% | 31.5% |
| Impact Energy (Akv at 20°C) | 72 J | 120 J |
| Impact Energy (Akv at 20°C) | 110 J | 127 J |
| Impact Energy (Akv at 20°C) | 76 J | 116 J |
The optimized casting process has significantly improved the quality of crusher upper shell castings. Defects such as dimensional inaccuracies, sand drops, and core gaps have been substantially reduced, eliminating the need for chaplets and reducing finishing labor. The use of computer simulation allowed for precise riser design and chill placement, enhancing yield and reliability. This approach not only benefits the production of these specific shell castings but also serves as a valuable reference for other large, annular castings in heavy machinery. Future work may explore advanced simulation techniques and alternative materials to further optimize the process for shell castings.
In conclusion, the redesign of the casting process for crusher upper shell castings demonstrates how integrating traditional foundry methods with modern simulation tools can lead to substantial quality enhancements. By adopting hollow natural pattern molding, optimizing risering and gating, and implementing external chills, we have achieved a robust process that ensures dimensional integrity and internal soundness in these critical shell castings. The success of this project underscores the importance of continuous process improvement in the casting industry, particularly for complex components like shell castings that are essential to industrial equipment performance.