In the field of industrial manufacturing, sand casting has long been a fundamental method for producing complex metal components. However, traditional sand casting often relies on wooden patterns, which can be time-consuming and costly, especially for intricate parts like pulping rotors used in papermaking equipment. This study explores the application of 3D printed sand molds in the sand casting process for pulping rotors, aiming to overcome the limitations of conventional methods. By integrating advanced simulation techniques, we optimize the casting parameters to enhance efficiency and quality. The focus is on leveraging 3D printing technology to streamline the sand casting workflow, reduce defects, and improve the overall performance of the rotor. Throughout this research, the term ‘sand casting’ is emphasized as a key enabler of innovation in foundry processes.
The pulping rotor is a critical component in papermaking, responsible for disintegrating pulp fibers through high-speed rotation. Traditionally, sand casting with wooden patterns has been employed, but this approach faces challenges such as long lead times and high costs. For instance, wooden pattern fabrication can take over 20 days and cost thousands of dollars, whereas 3D printed sand molds can be produced in just one day by directly importing digital models. This not only accelerates the sand casting process but also eliminates the need for draft angles, allowing for more complex geometries. In this work, we adopt 3D printed sand molds for gravity sand casting of the pulping rotor, utilizing ZG06Cr13Ni4Mo martensitic stainless steel to meet stringent requirements for corrosion resistance and mechanical strength. The integration of Procast simulation software enables us to predict and mitigate defects like shrinkage and porosity, thereby refining the sand casting工艺.
To provide a comprehensive overview, this article is structured as follows: First, we outline the technical specifications and structural characteristics of the pulping rotor. Next, we detail the sand casting process design, including aspects like shrinkage allowance, gating system, and riser optimization. We then describe the 3D printing of sand molds, followed by melting, pouring, and heat treatment procedures. Simulation results from Procast are presented to validate the process, and finally, we discuss the verification of cast quality through various tests. Throughout, tables and mathematical models are used to summarize key data and relationships, reinforcing the importance of sand casting in modern manufacturing.
Technical Requirements for the Pulping Rotor
The pulping rotor must adhere to specific material standards to ensure durability and performance in demanding environments. The chosen material, ZG06Cr13Ni4Mo, is a martensitic stainless steel known for its excellent combination of strength, toughness, and corrosion resistance. This makes it ideal for sand casting applications where components are subjected to high stresses and abrasive conditions. The chemical composition requirements are summarized in Table 1, which dictates the elemental limits to achieve the desired metallurgical properties. Controlling carbon content is particularly crucial in sand casting to prevent excessive brittleness and ensure weldability.
| Element | Min | Max |
|---|---|---|
| C | – | 0.06 |
| Si | – | 0.8 |
| Mn | – | 1.0 |
| S | – | 0.035 |
| P | – | 0.025 |
| Cr | 11.5 | 13.5 |
| Ni | 3.5 | 5.0 |
| Mo | 0.4 | 1.0 |
In addition to chemical composition, the mechanical properties of the pulping rotor are critical for its operational reliability. These properties are achieved through precise control of the sand casting and heat treatment processes. Table 2 outlines the minimum requirements for yield strength, tensile strength, elongation, reduction of area, impact energy, and hardness. The sand casting process must ensure that these targets are met consistently, as deviations could lead to premature failure in service. For example, the high tensile strength and hardness are essential for withstanding the centrifugal forces and abrasive wear encountered during pulp processing.
| Property | Value |
|---|---|
| Yield Strength (Rp0.2, MPa) | ≥550 |
| Tensile Strength (Rm, MPa) | ≥750 |
| Elongation (A5, %) | ≥15 |
| Reduction of Area (Z, %) | ≥35 |
| Impact Energy (KV, J) | ≥50 |
| Hardness (HBW) | 221-294 |
Non-destructive testing, such as ultrasonic inspection, is also mandated to verify internal soundness. The standard ASTM A609-12:2018 is applied to detect any internal flaws that could compromise the integrity of the sand casting. This emphasizes the need for a robust sand casting process that minimizes defects like shrinkage cavities and inclusions.
Structural Characteristics of the Pulping Rotor
The pulping rotor features a complex geometry designed for efficient pulp disintegration. As shown in the provided illustration, it has an outer diameter of 945 mm and a height of 158 mm, with a mass of approximately 156 kg. The central disk is surrounded by eight aerodynamically shaped blades that are critical for dynamic balance and performance. These blades have streamlined profiles to reduce turbulence and energy loss during operation. The hollow internal structure, as depicted in the cross-section, serves to reduce weight without compromising strength, which is a common design consideration in sand casting to optimize material usage and minimize inertial forces.
The intricate blade geometry poses significant challenges for traditional sand casting with wooden patterns, as it requires precise core assembly and can lead to dimensional inaccuracies. However, 3D printed sand molds excel in such scenarios by enabling direct fabrication of complex cavities without the constraints of pattern withdrawal. This advantage is particularly beneficial in sand casting for components with undercuts and fine details, as it ensures higher dimensional fidelity and reduces the risk of mold-related defects.
Sand Casting Process Design
The sand casting process for the pulping rotor was meticulously designed to achieve high-quality results. A key parameter is the shrinkage allowance, which accounts for the volumetric contraction of the metal during solidification. For ZG06Cr13Ni4Mo stainless steel in gravity sand casting, a linear shrinkage factor of 2.0% was applied. This is derived from the material’s solidification characteristics and can be expressed mathematically as:
$$ L_f = L_i \times (1 + \alpha) $$
where \( L_f \) is the final dimension after accounting for shrinkage, \( L_i \) is the initial pattern dimension, and \( \alpha \) is the shrinkage allowance (0.02 in this case). This equation ensures that the cast component meets the required dimensions after cooling.
The gating system was designed as a bottom-fed arrangement to promote smooth filling of the mold cavity. The inner gate diameter was set at 60 mm, allowing molten metal to enter from the bottom and rise steadily, thereby minimizing turbulence and oxide formation. This is crucial in sand casting to reduce slag inclusion and gas porosity. The total pouring weight was calculated to be 312 kg, including the gating and risering systems. The pouring time was optimized to approximately 6 seconds, with a pouring rate of 50 kg/s, to ensure complete filling without excessive thermal gradients.
Machining allowances were incorporated to facilitate post-casting processing. The top surface had an allowance of 8 mm, the outer circumference 10 mm, and the bottom surface 6 mm. The central shaft hole was not cast, reducing the need for extensive machining and aligning with sand casting best practices for cost efficiency. The final rough casting weight was 187 kg, accounting for these allowances.
Riser design is a critical aspect of sand casting to compensate for solidification shrinkage. A cylindrical riser with dimensions of 200 mm diameter and 280 mm height was placed at the center, with a modulus of 4.5 cm to ensure adequate feeding. Additionally, eight insulating risers, each 70 mm in diameter and 100 mm high, were positioned at the tooth roots to address localized shrinkage. The modulus for these risers was calculated as 1.8 cm. The riser design can be evaluated using Chvorinov’s rule for solidification time:
$$ t = k \left( \frac{V}{A} \right)^2 $$
where \( t \) is the solidification time, \( k \) is a constant dependent on the mold material, \( V \) is the volume, and \( A \) is the surface area. By optimizing the riser dimensions, we ensured that the risers solidify after the casting, thereby promoting directional solidification and reducing shrinkage defects in the sand casting process.
3D Printing of Sand Molds
The adoption of 3D printing for sand molds revolutionizes the sand casting approach by enabling rapid prototyping and production. The sand molds were fabricated using a VX2000 3D printer, which employs GS15 silica sand as the base material. The binder system consisted of a specialized 3DP resin and curing agent, with a cleaning agent to ensure print quality. The printer parameters included a layer thickness of 0.3 mm and a maximum build volume of 2000 mm × 1000 mm × 1000 mm, achieving a dimensional accuracy of ≤0.3 mm. The resin and curing agent were added at 1.2% and 0.3% by weight, respectively, resulting in a mold strength of 1.4 MPa, which is sufficient to withstand the pressures of molten metal during sand casting.
The 3D printing process involves layer-by-layer deposition of sand and binder, which hardens to form the mold. This method eliminates the need for patterns and allows for complex internal geometries, such as the hollow core of the pulping rotor. The upper and lower sand molds were printed separately and assembled for casting. The absence of draft angles in 3D printed molds simplifies the design and reduces the risk of damage during mold assembly, which is a significant advantage over traditional sand casting with wooden patterns.
To quantify the benefits, the cost and time savings are substantial. For a single pulping rotor, the 3D printed sand mold volume was 0.48 m³, with a material cost of approximately $0.35 per m³, leading to a total mold cost of around $0.168 thousand. In contrast, a wooden pattern could cost up to $3 thousand and take over 20 days to produce. This makes 3D printed sand molds highly economical for small-batch production in sand casting, especially for complex components.
Melting, Pouring, and Heat Treatment
The melting process involved a dual-system approach using an alkaline electric arc furnace followed by AOD (Argon Oxygen Decarburization) refining. This combination ensures precise control over composition, particularly for carbon content, which was maintained below 0.03% to enhance corrosion resistance. Impurity elements like phosphorus and sulfur were strictly limited to the specified ranges. After tapping, the molten metal was held in a ladle for more than 5 minutes to allow slag and inclusions to float to the surface, a common practice in sand casting to improve metal cleanliness.
Pouring was conducted at a temperature between 1560°C and 1580°C using a bottom-pour ladle with a 50 mm nozzle diameter. This temperature range was selected to balance fluidity and solidification characteristics, reducing the risk of cold shuts or hot tearing in the sand casting. The pouring rate was controlled to maintain a steady flow, minimizing turbulence and air entrapment.
Heat treatment is essential to achieve the desired mechanical properties in ZG06Cr13Ni4Mo steel. The process consisted of normalizing followed by tempering. The normalizing cycle involved heating to 1020°C ± 10°C, holding for 2 hours, and then air cooling. The tempering cycle was performed at 600°C ± 10°C for 4 hours. The heating rates were carefully controlled: ≤100°C/h between 300°C and 600°C, and ≤80°C/h between 600°C and 1020°C. This gradual heating prevents thermal stresses and ensures uniform microstructure development. The heat treatment can be modeled using the Larson-Miller parameter for creep and stress relaxation:
$$ P = T (\log t + C) $$
where \( P \) is the parameter, \( T \) is the temperature in Kelvin, \( t \) is the time in hours, and \( C \) is a constant specific to the material. This helps in optimizing the heat treatment parameters for the sand casting component.
Simulation of Sand Casting Process
Procast software was employed to simulate the filling and solidification stages of the sand casting process. The simulation setup included meshing the geometry, defining material properties for ZG06Cr13Ni4Mo, and setting boundary conditions such as a heat transfer coefficient of 500 W/(m²·K) at the mold-metal interface. The initial conditions were a pouring temperature of 1580°C and an ambient temperature of 25°C. The filling time was set to 6 seconds, and cooling was modeled as air cooling.
The temperature distribution during solidification, as shown in the simulation results, indicated progressive cooling from the outer surfaces toward the risers. This confirms the effectiveness of the riser design in promoting directional solidification. The simulation also predicted the locations of potential shrinkage porosity and cavities, which were primarily concentrated in the thicker sections and near the riser junctions. By analyzing these results, we optimized the riser sizes and placements to minimize defects, achieving a casting yield of 60% in the sand casting process.
The solidification simulation can be described using the Fourier heat conduction equation:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where \( T \) is temperature, \( t \) is time, and \( \alpha \) is the thermal diffusivity. This equation helps in predicting the temperature gradients and solidification fronts in sand casting, enabling better control over the process.
Verification of Cast Quality
After casting, the pulping rotor underwent rigorous testing to verify its quality. Dimensional inspection using 3D scanning confirmed that all eight blades met the specified tolerances, with uniform height and profile. This demonstrates the precision achievable with 3D printed sand molds in sand casting. Ultrasonic testing revealed no internal defects exceeding the ASTM A609-12:2018 Level 2 requirements, indicating sound internal integrity.
Chemical analysis and mechanical tests were conducted on samples extracted from the casting. The results, summarized in Table 3, show compliance with the required specifications. The tensile strength, yield strength, and impact values all met or exceeded the standards, affirming the effectiveness of the sand casting and heat treatment processes.
| Property | Measured Value |
|---|---|
| Yield Strength (Rp0.2, MPa) | 580 |
| Tensile Strength (Rm, MPa) | 780 |
| Elongation (A5, %) | 18 |
| Reduction of Area (Z, %) | 38 |
| Impact Energy (KV, J) | 55 |
| Hardness (HBW) | 230 |
The overall quality of the sand casting was deemed satisfactory, with the rotor ready for further assembly and use. The use of 3D printed sand molds not only reduced production time and cost but also enhanced dimensional accuracy and repeatability, making it a viable alternative for similar sand casting applications.
Conclusion
This research demonstrates the successful integration of 3D printed sand molds into the sand casting process for pulping rotors. By leveraging simulation tools like Procast, we optimized the riser design and achieved a high casting yield of 60%. The 3D printing approach significantly reduced lead times and costs compared to traditional wooden patterns, while improving dimensional precision and mold consistency. The verified mechanical and chemical properties confirm that the sand casting process meets all technical requirements. Future work could explore the application of this methodology to other complex components, further advancing the capabilities of sand casting in modern manufacturing. Overall, the combination of 3D printing and sand casting offers a robust solution for producing high-integrity castings with enhanced efficiency and economy.