Abstract
The lost foam casting process of nodular iron vacuum pump housing through a comprehensive analysis. By employing 3D simulation for the casting method, a rational gating system was designed. Foam cutting technology was utilized to create the vacuum pump housing white pattern, ensuring smooth pattern assembly. The liquid iron was nodularized using a cored wire injection nodularization process, and the casting quality met the technical requirements. The results indicated the feasibility of producing thick and large nodular iron vacuum pump housings using the lost foam casting process. The white pattern, cut into pieces via numerical control and manually adhered, provided high casting dimensional accuracy. The liquid iron’s nodularization via a tundish cover ladle process with double wires ensured wire injection quality, meeting the material requirements of the casting. The casting method adopted top gating pouring and riser feeding, ensuring stable and orderly mold filling and directional solidification. The casting was dense, free from shrinkage and leakage.
Keywords: nodular iron; vacuum pump housing; lost foam casting
1. Introduction
Our company previously utilized steel plate welding for forming water-ring vacuum pump housings. However, prolonged exposure to water and sand abrasion led to leakage, causing a drop in vacuum system pressure and affecting casting production efficiency and quality. To address this, nodular iron was selected to produce vacuum pump housings, enhancing their lifespan.
2. Casting Method
The vacuum pump housing casting is characterized by large dimensions, with a maximum size of φ864mm × 637mm, a wall thickness of 35mm, and a weight of 425kg. The material grade is QT600-3, and the casting must be leak-free. The sand box dimensions are 1200mm × 1000mm × 1300mm, accommodating one casting per box. A 700kg ladle was used for iron pouring, with wire injection for nodularization. The pouring temperature was set at 1430~1450°C, and the vacuum pressure was controlled at -0.05 to -0.06 MPa. To prevent deformation, the pressure holding time after pouring was set to 1 hour.
The casting method employed top gating and a semi-open gating system with a ratio of ΣFstraight: ΣFtransverse: ΣFwithin = 1:1.4:1.2. The dimensions of the sprue, runner, and ingate were φ50mm, 75mm × 75mm, and 40mm × 140mm, respectively. Risers, sized 120mm × 120mm × 14mm, were uniformly distributed at the top of the casting. The casting method.
Due to the challenges of placing external chills in lost foam casting, and potential fusion issues with internal chills, the riser feeding process was adopted to eliminate shrinkage pores or shrinkages. Six risers, including two hot risers, were placed at the top of the casting. The liquid iron entered the mold cavity from the hot risers, filling the cavity bottom first, followed by a rise in the iron level. As the iron filled the cavity, the leading edge entered the other four risers. Once full, all six risers provided feeding to the casting.
3. Simulation Analysis
To verify the rationality of the top gating process and ensure smooth mold filling and effective feeding of the casting’s shrinkage position, MAGMA numerical simulation software was used to simulate the filling and solidification processes.
The mold filling simulation showed that with top gating, the iron entered the mold cavity through the gating system, settling at the bottom and gradually rising from the bottom. The filling was stable, with the lower iron cooling as the filling progressed, and the incoming hot iron compensating for the solidification of the lower iron. The upper part of the casting solidified last, fed by the six risers, ensuring orderly feeding.
The solidification simulation revealed that the risers solidified last, effectively feeding the casting. No significant isolated liquid regions were observed. Minor isolated liquid regions could be compensated by graphitization expansion to mitigate slight shrinkage.
4. Lost Foam Casting Process and In-situ Control
4.1 White Pattern Production
Due to the large size of the vacuum pump housing casting, the white pattern could not be made in one piece. Instead, it was produced through splicing. The vacuum pump housing was divided into several pieces using CAD software, and then cut using a fully automatic numerical control cutting platform. The cylinder was divided into 12 pieces, and the flanges into 8 pieces, with rounded corners of R10mm, bonded using fine foam strips.
4.2 White Pattern Assembly
The cut foam pieces were manually assembled. First, the center position was marked on a glass plate, and the foam pieces were aligned and fixed with fiber rods. Glue and paper tape were applied to the seams to prevent paint from entering during dipping. To prevent deformation during dipping, two wooden strips, each with a cross-sectional dimension of 15mm × 15mm, were bonded at the top and bottom of the white pattern for reinforcement. Risers were bonded for additional reinforcement.
4.3 Paint Dipping
Due to the increased size of the assembled vacuum pump housing, the paint needed to reach all parts of the white pattern during dipping. Operators slowly rotated the white pattern in the paint tank and used a small container to apply paint to hard-to-reach areas. The dipped pattern was placed on a specialized drying rack and pushed into the drying room for dehumidification and drying. Four dips were required, with a paint degree of Baumé controlled at 69~71°Bé.
4.4 Molding and Pouring
The bottom sand was leveled, and two people worked together to place the yellow pattern into a sand box . Flexible sand was added manually to fill the box, with three-dimensional vibration time controlled at 90s.
5. Melting Process and In-situ Control
The chemical composition of the iron is shown in Table 1. The tundish cover ladle nodularizing process with double wires was adopted, with FeSiMg25Re3 nodularizer added at 0.7%. In-ladle inoculation involved 0.3% pretreatment agent and 0.2% 75FeSi composite inoculation. Encapsulated in steel, the nodularizer avoided oxidation. PLC controlled the wire feeding length and speed, directly entering the bottom of the iron, ensuring high alloy absorption and effective nodularization. After nodularization, the iron was quickly skimmed and transported to the pouring station with a forklift.
Table 1. Chemical Composition of Liquid Iron
| Element | Content (w B/%) |
|---|---|
| C | 3.7 |
| Si (after nodularization) | 2.4 |
| Mn | 0.5 |
| S | 0.015 |
| P | 0.05 |
| Mg (residual) | 0.05 |
| Sn | 0.06 |
To minimize shrinkage defects, the pouring temperature was set at 1430~1450°C. Clean cold iron blocks were placed at the pouring site to prevent temperatures above 1450°C. If exceeded, clean cold iron blocks were immediately added to the ladle for cooling. Additional iron was poured into the mold cavity after the initial pour. The vacuum pressure was maintained at -0.05 to -0.06 MPa, with a post-pouring pressure holding time of 1 hour.
6. Production Results
The 5810 shot blasting equipment was utilized to remove sand from the vacuum pump housing castings after sand dropping. The risers and gating systems were removed using gas cutting. Observations made at the riser necks revealed no shrinkage pores or porosity within the castings. Measurements of the castings’ outer dimensions confirmed compliance with the designed size requirements for the vacuum pump housing castings.
The vacuum pump housing castings were sent to the processing unit for machining. Upon inspection of the machined surfaces, no shrinkage pores or slag inclusion pores were found, satisfying the operational requirements of the vacuum pump.
The conclusions drawn from this production process are as follows:
- The implementation of the lost foam casting process for producing thick and large nodular iron vacuum pump housings is feasible. The white pattern, cut into pieces via numerical control and manually adhered and assembled, results in high casting dimensional accuracy, making it suitable for small-batch production.
- The adoption of the tundish cover ladle nodularizing process with double wires for the liquid iron ensures the quality of wire injection, fulfilling the material requirements of the casting.
- The casting process employs a top gating pouring system combined with riser feeding, achieving a stable and orderly mold filling process and directional solidification. The casting is dense, free from shrinkage porosity, and leakage, fully meeting customer specifications.