Introduction
The water-ring vacuum pump housing used in our company was previously made by welding and processing steel plates. However, during use, it was subjected to long-term erosion by water and sand particles, resulting in water leakage in the vacuum pump housing, which caused the pressure of the entire vacuum system to be low, affecting the production efficiency and quality of the castings. Therefore, it was decided to use nodular cast iron to produce the vacuum pump housing castings to improve the service life of the vacuum pump housing.
- Casting Process
- Casting Dimensions and Requirements: The vacuum pump housing casting has a large outer dimension, with a maximum outer dimension of φ864 mm × 637 mm, a wall thickness of 35 mm for the casting cylinder, and a single casting weight of 425 kg. The material grade is QT600 – 3, and the technical requirement is that the casting must not leak. The sand box size is 1200 mm × 1000 mm × 1300 mm, with one casting per box. The 700 kg nodularizing ladle is used to pour the molten iron, and the wire feeding method is used for the nodularization treatment. The pouring temperature of the molten iron is set at 1430 – 1450 °C, and the vacuum negative pressure is controlled at -0.05 to -0.06 MPa. To prevent the casting from deforming, the pressure holding time after pouring the molten iron is set at 1 hour.
- Gating System: A top-pouring, semi-open gating system is adopted, with a pouring ratio of ∑F 直 : ∑F 横 : ∑F 内 = 1 : 1.4 : 1.2. The cross-sectional dimension of the straight runner is φ50 mm, the cross-sectional dimension of the horizontal runner is 75 mm × 75 mm, and the cross-sectional dimension of the inner runner is 40 mm × 140 mm. The riser size is 120 mm × 120 mm × 140 mm, and the risers are evenly distributed on the top of the casting. Since it is inconvenient to arrange external chill in the lost foam casting process and using internal chill may cause poor fusion, the riser feeding process is adopted to eliminate shrinkage holes or porosity in the casting. Six risers are placed on the top of the casting, of which two are hot risers. The molten iron enters the cavity from the hot risers and first falls to the bottom of the cavity. Then, as the liquid level of the molten iron rises, when the molten iron fills the top of the cavity, the leading edge molten iron enters the other four risers. After the molten iron is filled, the six risers jointly feed the casting.
- Simulation Analysis
- Mold Filling Simulation: To verify the rationality of the top-pouring process and whether the molten iron filling can ensure the smooth pouring and the effective feeding of the shrinkage position of the casting by the gating system, the MAGMA numerical simulation software is used to simulate the filling and solidification processes of the pouring process. From the simulation process of molten iron filling, it can be seen that when the top-pouring process is used for pouring, the molten iron enters the cavity from the gating system, falls to the bottom of the cavity, and then rises gradually from the bottom. The filling is stable, and the lower molten iron gradually cools down during the filling process, and the hot molten iron entering the upper part feeds the solidification of the lower molten iron. Finally, the solidification of the upper molten iron of the casting is fed by the six risers at the top, realizing the orderly feeding of the molten iron.
- Solidification Simulation: From the solidification simulation process, it can be seen that the risers solidify last during the solidification process of the casting, playing a good feeding role, and there is no large isolated liquid phase area in the entire process. Local tiny isolated liquid phase areas can compensate for slight porosity through graphitization expansion.
- Lost Foam Casting Process and On-site Control
- White Pattern Making: Due to the large size of the vacuum pump housing casting, it is impossible to make the white pattern of the casting completely at one time, and it can only be made by splicing. First, the vacuum pump housing casting is decomposed into several pieces using CAD software (as shown in Figure 5), and then cut using a fully automatic numerical control cutting platform. Among them, the cylinder part of the vacuum pump housing is decomposed into 12 pieces, and the flanges at both ends of the vacuum pump housing are decomposed into 8 pieces. The fillet size of the cylinder is R10 mm, and the fine foam strips are used for bonding.
- White Pattern Assembly: The cut foam blocks are manually spliced. First, the center position of the circle is found on the glass plate, and then the cut foam blocks are aligned, and the fiber rods are used to bond the foam to fix the position. Then, the glue is applied to the gap of the white pattern and the paper tape is wrapped to eliminate the gap of the white pattern to prevent the coating from entering the gap during the dipping of the white pattern. To prevent the white pattern of the vacuum pump housing from deforming during the coating dipping, two wooden strips with a cross-sectional dimension of 15 mm × 15 mm are bonded to the upper and lower ends of the white pattern for reinforcement. To prevent shrinkage holes or porosity in the casting, the gating system is used in the process to feed the casting. Finally, the fiber rods are used to bond and reinforce the riser of the white pattern to prevent the white pattern from being damaged during the production process. Figure 6 shows the white pattern assembly.
- Coating Dipping: Due to the increased size of the vacuum pump housing after the assembly, it is required to dip the coating to every position of the white pattern during the dipping. First, the operator slowly rotates the white pattern when dipping the white pattern in the coating pool, and then uses a small container to pour the coating onto the places where the white pattern is not easy to be dipped. After the coating is dipped, the yellow pattern is placed on a special drying rack and pushed into the drying room for dehumidification and drying. To ensure that the molten iron is not damaged during the pouring process, it is required to dip the coating four times, and the Baume degree of the coating is controlled at 69 – 71 °Be. The state of the yellow pattern is shown in Figure 7.
- Packing and Pouring: First, the bottom sand is scraped flat, and then two people cooperate to load the yellow pattern into a sand box (as shown in Figure 8), and the sand box is filled with artificial flexible sand. The three-dimensional vibration time of the sand box is controlled at 90 seconds.
- Melting Process and On-site Control
- Chemical Composition of Molten Iron: The chemical composition of the molten iron is shown in Table 1. The ladle cover double-wire feeding nodularization process is adopted, and the addition amount of the FeSiMg25Re3 nodularizer is 0.7%. The in-ladle inoculation adopts the method of 0.3% pretreatment agent and 0.2% 75FeSi compound inoculation. Since the nodularizer is wrapped in the steel skin to avoid oxidation, the length and speed of the wire feeding are controlled by the PLC to directly enter the bottom of the molten iron, resulting in a high alloy absorption rate and a good nodularization effect. After the molten iron is nodularized, the slag is quickly skimmed off and transported to the pouring station by forklift.
- Pouring Temperature and Vacuum Control: To reduce the shrinkage holes or porosity defects in the casting, the pouring temperature of the molten iron is set at 1430 – 1450 °C. Clean cold iron blocks should be placed at the pouring site to prevent the influence of pouring when the temperature of the molten iron is higher than 1450 °C. During pouring, the clean cold iron blocks should be immediately thrown into the ladle for cooling. After pouring, the cavity is supplemented with molten iron, and the vacuum negative pressure at the pouring site is controlled at -0.05 to -0.06 MPa. The pressure holding time after pouring is set at 1 hour.
- Production Results
- Metallurgical Structure and Mechanical Properties: According to the above process for trial production, the metallurgical structure and mechanical properties test results of the attached test specimen are shown in Table 2, and the metallurgical structure photo is shown in Figure 9.
- Casting Inspection: After the vacuum pump housing casting is shot blasted by the 5810 shot blasting equipment, the risers and runners are removed by gas cutting. From the observation at the riser neck, there are no shrinkage holes or porosity in the casting. The outer dimension of the casting is measured, which meets the dimension requirements of the designed vacuum pump housing casting. The vacuum pump housing casting is sent to the processing unit for processing, and no shrinkage holes or sand slag holes are found on the processing surface, which can meet the use requirements of the vacuum pump.
- Conclusions
- Feasibility of Lost Foam Casting Process: It is feasible to produce the thick and large nodular cast iron vacuum pump housing by the lost foam casting process. The white pattern is cut into pieces by numerical control and manually assembled and bonded, resulting in high dimensional accuracy of the casting, which is suitable for small batch production.
- Quality of Molten Iron Treatment: The ladle cover double-wire feeding nodularization process ensures the quality of the wire feeding of the molten iron and meets the material requirements of the casting.
- Effectiveness of Casting Process: The top-pouring and riser feeding casting process realizes the smooth and orderly filling process and the directional solidification, resulting in a dense casting without shrinkage or leakage, meeting the customer’s requirements.
| Item | Spheroidization Grade | Graphite Size | Volume Fraction of Pearlite | Volume Fraction of Cementite | Volume Fraction of Phosphide Eutectic | Tensile Strength | Elongation |
|---|---|---|---|---|---|---|---|
| Value | [Specific Grade] | [Specific Size] | 6% | 1.0% | 0.5% | 641 MPa | 3.5% |
In summary, the lost foam casting process provides a viable solution for the production of nodular cast iron vacuum pump housing castings. By carefully designing the casting process, controlling the molten iron treatment, and ensuring the quality of the white pattern and coating, high-quality castings can be obtained. This not only improves the performance and service life of the vacuum pump housing but also demonstrates the advantages of the lost foam casting process in the production of complex castings. Further research and optimization can continue to enhance the process and expand its application in various casting fields.
- Further Considerations and Improvements
- Material Selection and Optimization: Although nodular cast iron has been successfully used in this application, further research could be conducted to explore alternative materials or material compositions that may offer better performance or cost-effectiveness. This could involve investigating different grades of nodular cast iron or incorporating alloying elements to enhance specific properties.
- Process Parameter Optimization: Continued optimization of the lost foam casting process parameters, such as the coating properties, pouring temperature, and vacuum level, can lead to further improvements in the quality and consistency of the castings. Fine-tuning these parameters based on further experimentation and simulation could result in reduced defects and improved mechanical properties.
- Quality Control and Inspection: Implementing more stringent quality control measures and advanced inspection techniques can help ensure that each casting meets the required standards. This could include the use of non-destructive testing methods, such as ultrasonic testing or X-ray inspection, to detect any internal defects that may not be visible during visual inspection.
- Design Optimization: Collaborating with designers to optimize the design of the vacuum pump housing can potentially lead to improved casting performance and reduced production costs. For example, considering factors such as wall thickness uniformity, part geometry, and gating system placement during the design stage can have a significant impact on the casting process and the final product quality.
- Energy Efficiency and Sustainability: Exploring ways to reduce the energy consumption and environmental impact of the casting process is becoming increasingly important. This could involve adopting more energy-efficient equipment, optimizing the production workflow to minimize waste, and exploring the use of recycled or sustainable materials.
- Market Trends and Applications
- Increasing Demand for High-Performance Castings: As various industries continue to demand more reliable and efficient equipment, the need for high-quality castings like the nodular cast iron vacuum pump housing is expected to grow. This presents an opportunity to further expand the market for lost foam casting and related technologies.
- Advancements in Manufacturing Technologies: The continuous advancements in manufacturing technologies, such as 3D printing and simulation software, can be leveraged to enhance the design and production processes of castings. This can lead to more efficient and precise manufacturing, as well as the development of more complex and customized castings.
- Applications in Diverse Industries: Nodular cast iron castings produced by the lost foam casting process have applications in a wide range of industries, including automotive, aerospace, and industrial machinery. Exploring new applications and markets can help drive the growth and development of this casting technology.
- Case Studies and Success Stories
- Presenting case studies of successful implementations of the lost foam casting process for producing nodular cast iron vacuum pump housings or similar castings can provide valuable insights and inspiration for other manufacturers. These case studies could highlight the benefits and challenges encountered, as well as the solutions and strategies adopted to achieve successful outcomes.
- Sharing success stories of how these castings have improved the performance and reliability of vacuum pump systems or other equipment can demonstrate the practical value of this casting technology and encourage its wider adoption.
- Conclusion
- The production of nodular cast iron vacuum pump housing castings using the lost foam casting process has shown great potential and has achieved satisfactory results. However, there is always room for further improvement and optimization to meet the evolving needs of the market and customers.
- By continuously exploring new materials, optimizing process parameters, enhancing quality control, and considering design and sustainability factors, the performance and competitiveness of these castings can be further enhanced.
- The future looks promising for the lost foam casting process in the production of nodular cast iron castings, and its applications in various industries are expected to continue to expand, contributing to the advancement of manufacturing technology and the development of high-quality products.
