In the modern metal forming industry, the sand casting foundry remains one of the most versatile and widely adopted manufacturing methods for producing complex ferrous components. The present study focuses on the optimization of the sand casting foundry process for a QT500-7 ductile iron fixed base, a critical structural component used in industrial robotic systems. This component serves as a supporting and fixing foundation for rotating bases, requiring high wear resistance, axial load-bearing capacity, and strict dimensional accuracy. Any internal defects such as shrinkage porosity, gas holes, or cold shuts would compromise the structural integrity and safety of the entire assembly. Therefore, a systematic numerical simulation approach was employed using ProCAST software to analyze mold filling, solidification, and defect formation, followed by targeted process modifications including riser and chill placement. The entire investigation was conducted from the perspective of a foundry engineer working in a sand casting foundry, aiming to deliver a robust, cost-effective, and defect-free casting solution.
This paper presents a comprehensive account of the casting process design, simulation, and optimization for the QT500-7 fixed base. The work begins with a detailed analysis of the casting geometry and technical requirements, followed by the initial process design including gating system, mold layout, and core configuration. Numerical simulation parameters are then defined, and the initial simulation results are critically evaluated. Based on the identified shrinkage defects in the thick sections, the process was optimized by adding cylindrical blind risers at the four corners of the base plate and applying gray cast iron chills at the side protrusions. The optimized design was re-simulated, confirming the complete elimination of internal defects. The final process yield reached 66.2%, demonstrating both economic and technical feasibility. The methodology and findings presented here provide valuable references for engineers working in any sand casting foundry dealing with similar ductile iron components.
1. Introduction and Background
The fixed base component is a quintessential structural part used extensively in mechanical engineering, construction machinery, and automotive systems. Its primary function is to provide stable support and fixation for rotating or moving assemblies, ensuring that the overall structure remains rigid and secure during operation. The QT500-7 ductile iron grade was selected for this application due to its excellent combination of strength, ductility, and wear resistance. The designation QT500-7 indicates a minimum tensile strength of 500 MPa and an elongation of at least 7%, making it suitable for components that experience both static and dynamic loads.
In the context of a modern sand casting foundry, numerical simulation has become an indispensable tool for process design and defect prediction. Traditional trial-and-error methods are time-consuming, costly, and often fail to identify internal defects before actual casting production. With the advent of computational simulation software such as ProCAST, foundry engineers can now visualize the entire mold filling and solidification process, predict the formation of shrinkage cavities, gas porosity, and hot spots, and iteratively optimize the process parameters before making any physical molds. This approach significantly reduces development lead time, material waste, and manufacturing costs.
The present study was motivated by the need to develop a reliable sand casting foundry process for the QT500-7 fixed base, which was selected as a competition entry for the 2024 China University Mechanical Engineering Innovation and Creativity Competition. The component geometry features a complex combination of thick and thin sections, multiple through-holes, side protrusions, and a large central bore, all of which pose challenges for directional solidification and defect-free casting. The work encompasses the entire process chain from three-dimensional modeling and gating system design to numerical simulation parameterization, defect prediction, process optimization, and final validation.
2. Casting Geometry and Technical Requirements
The QT500-7 fixed base has an overall envelope dimension of 760 mm × 760 mm × 270 mm, with a maximum wall thickness of 40 mm and a minimum wall thickness of 20 mm. The largest through-hole diameter is 450 mm, while the smallest is 140 mm. The net weight of the finished casting is 174.31 kg, classifying it as a medium-small component in the context of sand casting foundry production. The casting features a supporting base plate, a central main through-hole, multiple smaller through-holes, two side-extending bosses, a lateral rectangular through-hole, and a small top-side protrusion.
The technical specifications for the casting are stringent. All surface contaminants such as adhered sand, mold flash, burrs, riser remnants, and oxide scales must be completely removed. No cracks, cold shuts, shrinkage cavities, slag inclusions, or penetrating gas holes are permitted, as these would directly impair the mechanical performance and service safety of the component. Dimensional tolerances follow the GB/T 6414-2017 standard, with a selected grade of DCTG13 for small-batch production. The chemical composition of QT500-7 is specified in Table 1.
| Element | C | Si | Mn | P | S |
|---|---|---|---|---|---|
| Content (wt%) | 3.5 – 3.8 | 2.5 – 2.8 | ≤ 0.3 | ≤ 0.07 | ≤ 0.02 |
The structural analysis revealed that the supporting base plate corners and the side-extending bosses are relatively thick sections, which are prone to forming hot spots during solidification. These hot spots, if not properly addressed, would lead to shrinkage porosity or macro-shrinkage cavities. The central main bore and the lateral rectangular through-hole are the primary structural features that influence the stability and assembly accuracy of the final product. The smaller threaded holes and reamed holes on the base plate and side bosses are less than the minimum castable hole diameter and were therefore not directly cast but left for subsequent machining.
3. Design of the Initial Sand Casting Foundry Process
The design of a sand casting foundry process must adhere to three fundamental principles: practical applicability, process simplicity, and economic and environmental sustainability. The initial process design involved determining the casting orientation, parting surface, core layout, gating system, and key process parameters. Each of these elements is discussed in detail below.
3.1 Casting Orientation and Parting Surface
The casting orientation was selected based on several established guidelines: critical machined surfaces should be placed at the bottom or in the lower mold, large thin-walled sections should be oriented downward or laterally, and thick sections should be positioned at the top or side to facilitate riser feeding. Based on these considerations, the fixed base was oriented in an inverted position, with the supporting base plate facing upward. This orientation ensures that the central mounting face and the main bore, which directly interface with the robotic rotating base, are cast in the lower mold and thus achieve superior surface quality and dimensional accuracy. Additionally, this orientation simplifies mold removal and core placement, and allows risers to be conveniently positioned on the thick base plate sections.
The parting surface was selected to coincide with the maximum cross-sectional area of the casting, which is also consistent with the casting orientation. This arrangement facilitates mold removal, reduces the risk of mold shift, and ensures that the majority of the casting is contained in the lower mold. The resulting parting surface is a flat plane that bisects the casting at its widest horizontal section.
3.2 Mold and Core Design
Given the small-batch production requirement, manual mold making and core making were selected, using a two-part mold with one casting per mold. The mold material was furan resin-bonded sand, which offers high binder strength, excellent thermal stability, and good rigidity, all of which contribute to high casting yield and dimensional accuracy. Two separate sand cores were designed:
Core No. 1: This core was designed to form the lateral rectangular through-hole. It uses a bottom vertical locating core print to ensure precise positioning during mold assembly and casting. The core geometry is a simple rectangular prism with appropriate draft angles.
Core No. 2: This core was required to form the two side-extending bosses. During mold removal, these bosses would otherwise obstruct the pattern withdrawal, making it impossible to obtain an accurate external shape without a core. The core was therefore designed to envelop the boss geometry and provide a clean mold cavity. The top small-side protrusion was formed using a loose piece (live pattern) rather than a separate core, to simplify core making and reduce cost.
The internal cavity of the casting, including the central main bore and the smaller through-holes, was formed directly by the upper and lower mold halves, without requiring additional cores. This approach minimizes core complexity and reduces the risk of core shift and gas evolution.
3.3 Gating System Design
The gating system for ductile iron in a sand casting foundry must deliver molten metal rapidly and smoothly into the mold cavity while effectively retaining slag and dross. An open-top gating system was selected, which is characterized by a progressive increase in cross-sectional area from the sprue to the runner to the ingates. This configuration ensures low metal velocity at the mold entrance, reduced turbulence, minimized oxidation, and improved slag floating. The top-pouring orientation was chosen to enhance filling capability, facilitate riser feeding, and simplify mold assembly.
The cross-sectional area ratio for the open gating system was determined based on established foundry practice for ductile iron:
$$ \sum A_{\text{ingate}} : \sum A_{\text{runner}} : \sum A_{\text{sprue}} = 3 : 2.5 : 1 $$
Based on theoretical calculations considering the casting weight, pouring time, and metal flow rate, the sprue cross-sectional area was set to 11.88 cm², the runner area to 29.69 cm², and the total ingate area to 35.63 cm² distributed across four ingates. The actual dimensions of each gating element are summarized in Table 2.
| Gating Element | Number | Cross-sectional Area (cm²) | Shape |
|---|---|---|---|
| Sprue | 1 | 11.88 | Circular |
| Runner | 1 | 29.69 | Trapezoidal |
| Ingate | 4 | 35.63 (total) | Rectangular (short, thin, wide) |
The runner was designed with a large cross-section to prevent backflow of molten metal, and a filter screen was placed at the junction between the pouring cup and the sprue to enhance slag retention. The ingates were designed in a “short, thin, and wide” configuration to maximize their feeding effect and minimize localized overheating.
4. Numerical Simulation Parameterization
The three-dimensional model of the fixed base, along with the gating system and cores, was created in SolidWorks and exported in IGES format. The geometry was then imported into ProCAST for mesh generation. The mesh consisted of 21,244 two-dimensional elements and 178,308 three-dimensional elements, providing sufficient resolution for accurate flow and thermal analysis.
The critical simulation parameters are summarized in Table 3. These parameters were selected based on the material properties of QT500-7 ductile iron and the characteristics of the furan resin-bonded sand mold.
| Parameter | Value | Unit |
|---|---|---|
| Pouring temperature | 1360 | °C |
| Pouring time | 19.6 | s |
| Mold material | Furan resin-bonded sand | — |
| Casting material | QT500-7 ductile iron | — |
| Heat transfer coefficient (metal-sand) | 500 | W/(m²·K) |
| Heat transfer coefficient (sand-chill) | 500 | W/(m²·K) |
| Heat transfer coefficient (metal-chill) | 2000 | W/(m²·K) |
| Cooling condition (mold exterior) | Air cooling | — |
The liquidus temperature of QT500-7 was calculated to be approximately 1168°C, and the pouring temperature of 1360°C provided a superheat of about 192°C, which is typical for ductile iron castings in a sand casting foundry. The pouring time of 19.6 seconds was determined based on the gating system design and the desired filling rate. Continuous gravity pouring was assumed throughout the simulation.
5. Initial Simulation Results and Defect Analysis
The initial casting process was simulated using the parameters described above. The filling time predicted by the simulation was 18.7 seconds, which is in good agreement with the designed pouring time of 19.6 seconds. This confirms that the gating system design is hydraulically consistent and suitable for the intended application.
5.1 Mold Filling Behavior
The mold filling process was analyzed at various stages. During the initial phase, molten metal entered the mold cavity from the top and flowed downward toward the base. The flow was characterized by relatively high velocity and some degree of turbulence, particularly as the metal descended through the sprue and entered the runner system. This turbulence, if not controlled, could lead to sand erosion and the entrapment of air and mold gases. However, the overall filling pattern was stable, and no mistun or cold shut defects were observed. The temperature distribution across the molten metal front was relatively uniform, which is beneficial for promoting directional solidification and exploiting the self-feeding effect of graphite expansion in ductile iron.
Despite the generally acceptable filling behavior, the simulation revealed that the metal velocity at the ingate exits was relatively high, causing localized jetting and splashing. This phenomenon can entrain air bubbles and form oxide films, which may subsequently become trapped in the solidifying casting and manifest as porosity or inclusions. The uneven velocity distribution also indicated that the gating system could be further refined to promote smoother and more quiescent filling.
2.2 Solidification and Defect Prediction
The solidification simulation predicted the formation of shrinkage defects in the thick sections of the casting. Specifically, shrinkage porosity and macro-shrinkage cavities were observed at the four corners of the supporting base plate and within the side-extending bosses. These are precisely the regions where the local wall thickness is greatest and where heat dissipation is slowest, leading to the formation of hot spots. The defects are summarized in Table 4.
| Defect Location | Defect Type | Severity | Probable Cause |
|---|---|---|---|
| Base plate (four corners) | Macro-shrinkage cavity | High | Thick section, hot spot, no feeding |
| Side-extending bosses | Shrinkage porosity | Moderate | Localized thickening, slow cooling |
These defects are critical because they would directly compromise the mechanical integrity and sealing requirements of the fixed base. Macro-shrinkage cavities at the base plate corners would reduce the load-bearing area and create stress concentration points, while shrinkage porosity in the side bosses would weaken the bolted connections. Therefore, process optimization was necessary to eliminate these defects.
6. Process Optimization for the Sand Casting Foundry
Based on the defect analysis, two complementary optimization strategies were implemented: the addition of risers to feed the thick sections at the base plate corners, and the placement of chills to accelerate cooling of the side-extending bosses. These are standard techniques in any sand casting foundry for promoting directional solidification and eliminating shrinkage defects.
6.1 Riser Design for the Base Plate Corners
Cylindrical blind risers with a single neck were selected for the four corners of the base plate. These risers are positioned at the highest and thickest sections of the casting, which is consistent with the principle of directional solidification. The riser modulus was calculated using the modulus method:
$$ M_{\text{riser}} = \frac{V_{\text{riser}}}{A_{\text{riser}}} $$
where \( V_{\text{riser}} \) is the volume of the riser and \( A_{\text{riser}} \) is its surface area. The riser modulus must be greater than the modulus of the casting section it is intended to feed, typically by a factor of 1.2 to 1.3. Based on the casting modulus at the base plate corners and referring to standard foundry handbooks for machine tool blind risers, the final riser dimensions were determined as shown in Table 5.
| Parameter | Value | Unit |
|---|---|---|
| Riser diameter | 80 | mm |
| Riser height | 120 | mm |
| Neck diameter | 35 | mm |
| Neck height | 15 | mm |
| Number of risers | 4 | — |
These risers were designed to solidify after the casting section they feed, ensuring that liquid metal is available to compensate for volumetric contraction during solidification. The blind riser design also minimizes the need for extensive riser removal and reduces the overall material consumption.
6.2 Chill Design for the Side-Extending Bosses
To eliminate shrinkage porosity in the side-extending bosses, external chills were applied. Gray cast iron was selected as the chill material due to its high thermal conductivity and compatibility with ductile iron. The chill dimensions were determined based on the rule that the chill thickness should not be less than half the thickness of the hot spot section. To facilitate handling and reduce process complexity, the final chill dimensions were set to 130 mm × 130 mm × 10 mm, with two chills used in total.
The chills were placed inside Core No. 2, which forms the side-extending bosses. This required a slight modification of the core geometry to accommodate the chills. The core was redesigned with a recessed pocket on its cavity-facing surface, into which the chill was embedded. During pouring, the chill rapidly extracts heat from the adjacent molten metal, accelerating solidification of the boss section and eliminating the hot spot that would otherwise cause porosity.
7. Simulation Results of the Optimized Process
The optimized process, incorporating both risers and chills, was re-simulated using the same boundary conditions and simulation parameters as the initial case. The results were analyzed in terms of mold filling, solidification sequence, and defect formation.
7.1 Mold Filling in the Optimized Process
The filling behavior of the optimized process showed significant improvement compared to the initial design. The metal flow was smoother and more uniform, with reduced velocity at the ingate exits. As the molten metal rose through the mold cavity, it reached the chill locations at approximately 50% fill, and the chills began to absorb heat immediately upon contact with the metal. By 75% fill, the main body of the casting was nearly complete, and the chills had already started to cool the adjacent metal. At 100% fill, the entire cavity was completely filled without any evidence of mistun, cold shut, or excessive turbulence. The chills remained effective throughout the filling and subsequent solidification stages.
7.2 Solidification and Defect Elimination
The solidification simulation of the optimized process revealed a fundamentally different thermal history compared to the initial design. The risers at the four corners of the base plate remained liquid for a longer duration than the casting sections they were feeding, ensuring that liquid metal was available throughout the solidification of these thick sections. As a result, the macro-shrinkage cavities that were previously observed at these locations were completely eliminated, with all shrinkage defects transferred into the risers.
Similarly, the chills at the side-extending bosses accelerated the local solidification rate, causing the metal in these sections to solidify before the surrounding thinner sections. This reversed the local thermal gradient and prevented the formation of shrinkage porosity. The final defect prediction for the optimized process showed no detectable shrinkage cavities or porosity within the casting body. The results are summarized in Table 6.
| Location | Initial Process | Optimized Process |
|---|---|---|
| Base plate corners | Macro-shrinkage cavities | No defects (transferred to risers) |
| Side-extending bosses | Shrinkage porosity | No defects (chill accelerated cooling) |
| Main body | No defects | No defects |
| Risers | — | Shrinkage cavities confined within risers |
7.3 Process Yield Calculation
The process yield is a key economic indicator for any sand casting foundry. It is defined as the ratio of the casting weight to the total weight of metal poured, expressed as a percentage:
$$ \eta = \frac{W_{\text{casting}}}{W_{\text{casting}} + W_{\text{riser}} + W_{\text{gating}}} \times 100\% $$
For the optimized process, the casting weight is 174.31 kg, the total riser weight (four risers) is 28.6 kg, and the gating system weight is 60.2 kg. The process yield was calculated as follows:
$$ \eta = \frac{174.31}{174.31 + 28.6 + 60.2} \times 100\% = \frac{174.31}{263.11} \times 100\% \approx 66.2\% $$
This yield of 66.2% is considered acceptable for a ductile iron casting of this complexity in a sand casting foundry. It indicates that approximately two-thirds of the poured metal ends up as the finished casting, while the remaining one-third is recycled as scrap. Further optimization could potentially improve the yield, but the current value represents a good balance between technical performance and economic viability.
8. Discussion
The numerical simulation approach adopted in this study proved to be highly effective for the design and optimization of the sand casting foundry process for the QT500-7 fixed base. The initial process, while adequate in terms of mold filling, suffered from predictable shrinkage defects in the thick sections. The root cause of these defects was the lack of directional solidification: the thick sections at the base plate corners and side bosses cooled slowly and became hot spots, drawing liquid metal from the surrounding areas and ultimately forming cavities when the metal supply was exhausted.
The addition of risers at the base plate corners successfully provided a reservoir of liquid metal that could feed the solidification shrinkage. The key design parameter was the riser modulus, which must exceed the casting modulus to ensure that the riser solidifies after the casting. The cylindrical blind riser design was chosen for its simplicity, ease of mold assembly, and efficient metal usage. The simulation confirmed that the shrinkage defects were completely transferred to the risers, leaving the casting body sound.
The use of chills at the side-extending bosses addressed a different type of defect: localized shrinkage porosity in moderately thick sections that were not severe enough to form macro-cavities but still contained dispersed porosity. The chills accelerated the local cooling rate, effectively eliminating the hot spot and promoting a more uniform solidification front. The chill dimensions were selected based on practical experience and the geometric constraints of the core, and the simulation confirmed their effectiveness.
It is worth noting that the optimized process also exhibited improved mold filling characteristics compared to the initial design. The smoother filling profile and reduced velocity at the ingates are likely attributable to the presence of the risers and chills, which alter the flow dynamics within the mold cavity. Although the gating system itself was not modified, the overall flow behavior was positively influenced by the changes in the mold geometry.
From a practical perspective, the optimized process is well-suited for implementation in a typical sand casting foundry. The risers and chills are standard items that can be easily incorporated into the mold assembly. The core modification to accommodate the chills is straightforward and does not significantly increase core-making cost or complexity. The process yield of 66.2% is economically viable, and the elimination of internal defects ensures that the casting will meet the stringent technical requirements specified for the application.
9. Conclusions
This study presents a comprehensive numerical simulation-based optimization of the sand casting foundry process for a QT500-7 ductile iron fixed base. The following conclusions can be drawn from the work:
1. Process Design: A robust initial casting process was designed, incorporating an inverted casting orientation, a flat parting surface at the maximum cross-section, two sand cores for the lateral through-hole and side bosses, and an open-top gating system with a cross-sectional area ratio of ΣAingate:ΣArunner:ΣAsprue = 3:2.5:1. The design was hydraulically sound and capable of filling the mold completely without mistun or cold shuts.
2. Defect Identification: The initial simulation predicted macro-shrinkage cavities at the four corners of the supporting base plate and shrinkage porosity within the side-extending bosses. These defects were attributed to the formation of hot spots in the thick sections, which solidified last and lacked sufficient liquid metal feeding.
3. Optimization Strategy: Two complementary optimization measures were implemented: cylindrical blind risers at the base plate corners to provide feeding during solidification, and gray cast iron external chills embedded in Core No. 2 to accelerate cooling of the side bosses. The riser dimensions were determined using the modulus method, and the chill dimensions were selected based on the hot spot geometry.
4. Defect Elimination: The optimized process simulation demonstrated complete elimination of all internal shrinkage defects. The risers effectively concentrated the shrinkage cavities within themselves, while the chills eliminated the thermal hot spots at the side bosses. The casting body was entirely free of defects, satisfying the technical requirement that no cracks, cold shuts, shrinkage cavities, slag inclusions, or penetrating gas holes are permitted.
5. Process Yield: The final process yield was calculated to be 66.2%, which is economically viable for a ductile iron casting of this complexity in a sand casting foundry. The yield can be further improved in future iterations by optimizing the riser size and gating system design.
6. Practical Applicability: The optimized process is readily implementable in a real sand casting foundry environment. The modifications are simple, cost-effective, and do not require specialized equipment or extensive retooling. The methodology and results presented here serve as a valuable reference for foundry engineers dealing with similar ductile iron components that require high internal quality and dimensional accuracy.
In summary, this work demonstrates the power of numerical simulation as a tool for process development in the sand casting foundry industry. By enabling engineers to visualize and predict the behavior of molten metal during filling and solidification, simulation allows for targeted and efficient process optimization, reducing the need for costly and time-consuming physical trials. The QT500-7 fixed base casting produced using the optimized process will meet the demanding requirements of industrial robotic applications, providing reliable and long-lasting service.