Abstract:
This article introduces the lost foam casting process for medium and large Siemens motor shell castings. Integral molding is employed, with different shrinkage rates applied in the radial and height directions. A stepped pouring system is adopted, and the drying temperature and time of the lost foam pattern are strictly controlled. The coating’s sagging, normal/high temperature crack resistance, exuviation, and other properties are also focused on. The pouring temperature is maintained at 14901500°C, and the pouring vacuum is controlled at -0.065-0.07 MPa. Production results show that the motor casing castings produced by the lost foam process exhibit good surface quality, qualified dimensions, and metallographic structure and mechanical properties that meet technical requirements. Moreover, the production cost per ton of castings is 1400 yuan lower than that of the furan resin sand process.
1. Introduction
The lost foam casting process is a advanced casting technology that has been widely applied in various fields due to its advantages such as high dimensional accuracy, good surface quality, and low production cost. In this paper, we introduce the lost foam casting process for medium and large Siemens motor shell casting, which have dimensions of 1240mm × 800mm × 700mm and a mass of approximately 780 kg. The material grade is ASTM A48 Class 30C (equivalent to HT200~HT250).
2. Mold Design
The mold for medium and large motor shell casting with heat dissipation fins is generally divided into four parts, which are then combined after molding. However, the size of the combined white mold is difficult to control, and its stability is poor. In our company, we adopt integral molding, with both the outer shape and inner cavity using slider structures to ensure smooth demolding of the integral white mold, fundamentally solving the issues of casting deformation and dimensional accuracy.
Table 1. Comparison of Mold Design Methods
| Method | Advantages | Disadvantages |
|---|---|---|
| Combination of multiple parts | Easy to manufacture and assemble | Poor dimensional stability, high deformation risk |
| Integral molding | High dimensional accuracy, good stability | More complex manufacturing process |
The selection of shrinkage rate parameters is critical in the mold design. For large-scale lost foam motor shell casting, the shrinkage rate in the length direction is set to 1.35%, and that in the height direction is set to 1.15%.
3. Gating System Design
The form of the gating system in lost foam casting is different from traditional processes. The size, shape, and direction of the internal gating channel directly affect the vaporization of the pattern. If the size of the internal gating channel is calculated based on traditional sand casting processes, the molten iron capacity will be low, and the surrounding heat dissipation area will be large, which will rapidly reduce the temperature of the molten iron entering the cavity and affect the vaporization speed of the pattern.
Based on the calculation of traditional sand casting processes, the total cross-sectional area of the internal gating channel is generally increased by 10%~25%. Adjustments are made after testing. Due to the high height of the casting, a stepped pouring method is adopted, and the location of the internal gating channel for the casting is crucial. A multi-point iron inlet design is used for the internal gating channel.
Table 2. Pouring System Design Parameters
| Gating Channel Type | Cross-Sectional Size (mm) | Number |
|---|---|---|
| Sprue | φ48 | 1 |
| Runner | Evenly arranged | 4 layers, 26 in total |
| Internal Gating Channel | Evenly arranged | 4 layers, 26 in total |
The pouring system and parameter design are as follows: the cross-sectional size of the sprue is φ48 mm, with four layers of runners and internal gating channels evenly arranged in the height and diameter directions. There are 8 channels on the top layer and 6 channels on each of the remaining three layers, totaling 26 channels. Since the sprue is hollow, the molten iron directly enters the lowest layer of the gating channel. As the molten iron level rises, it reaches the height of the second layer, and the second layer begins to fill with molten iron. Therefore, the ratio of the pouring system is calculated as ∑Fsprue:∑Frunner:∑Finternal=1.25:2.5:1 for a single layer.
4. Lost Foam Casting Process
4.1 Pre-foaming and Molding
Due to the 5 mm thickness of the heat dissipation fins of the motor shell casting, the particle size of the EPS raw beads should also be on the smaller side. Our company uses the Dragon King p-s/p-4s model. Due to the many and thin heat dissipation fins, the pre-expansion density should be controlled at 25~26 g/L. Electric heating is used for pre-expansion to ensure uniform bead size and low moisture content.
The foam molding process uses a special hydraulic semi-automatic molding machine for motor shell casting. The equipment dimensions are 2.6 m long, 2.6 m wide, and 9 m high, with a maximum installed mold size of 2.3 m long and 2.3 m wide. After automatic feeding is completed, the remaining processes are automatically completed by the equipment control system, which ensures consistent white mold quality.
4.2 Drying
The model cannot be immediately placed in the drying room after being produced. It should be placed on the storage rack in the molding area and allowed to dry naturally until the surface moisture has evaporated and it has been inspected before entering the drying room. The benefits of this approach are: firstly, the water content of the freshly produced white mold is high, and if it is directly placed in the drying room, it will cause the humidity in the drying room to remain high; secondly, during inspection, defective products can be identified and discarded directly, reducing the space occupied in the drying room. The drying temperature for the white mold is 40~45°C, and the drying time is 48 hours.
4.3 Combination
(1) The foam pattern is fully dried and weighed individually, with those outside the specified range being isolated.
(2) Each heat dissipation fin is inspected, and those not meeting the appearance requirements should be repaired or isolated. Defects within 10 mm × 10 mm are generally repaired using a special repair paste for white molds.
(3) A single-piece pouring process is adopted, with the non-driving end positioned at the top.
(4) The pouring system is bonded using hot melt glue at the joint surfaces. While ensuring adhesion, the amount of glue should be minimized, and it should be securely bonded and sealed with masking paper.
4.4 Coating
There are three specific points to consider for the coating requirements of motor shell castings, beyond general performance requirements:
(1) The coating must have good crack resistance and sagging performance at room temperature. Since almost 95% of the surface of the motor shell is vertical, if the sagging performance is poor, it is difficult to ensure the coating thickness for each layer. The motor shell casting has many heat dissipation fins and thin walls, with threading grooves on the inner wall. The coating is prone to poor sagging in these areas, easily resulting in uneven coating. Excessively thick water-based coatings are prone to cracking during the drying process, and these cracks are difficult to detect. If not properly handled, sand burning will occur during pouring, often leading to casting scrap. Our company adopts a dipping process. Before dipping, the coating is stirred for 1 hour using a mixer to ensure uniformity. The assembled white mold is placed into the coating pool with a uniformly stirred coating and a Baume degree that meets the process requirements. The white mold is manually pressed down to a depth of 200~300 mm with uniform force. After stopping for 1 minute, it is rotated 90 degrees, stopped for another 1 minute, and rotated again until evenly coated. It is then lifted out and placed upright on a rack, and any missed areas are brushed with a brush. After 2 hours, it is transferred to the drying room.
(2) High-temperature crack resistance. Due to the numerous heat dissipation fins, there will be many deep and narrow areas on the outer wall of the motor housing. These areas will become hot spots after pouring. Therefore, these areas have very high requirements for the coating. If the coating cannot resist the impact of molten iron during pouring, sand burning defects will occur. If the coating’s refractoriness is insufficient or the binder is used unreasonably, scabbing and roughness will appear on the surface after shot blasting.
(3) Coating stripping performance. The threading grooves in the inner cavity of the motor housing make it difficult for steel shots to normally enter during shot blasting, requiring high stripping performance of the coating. Our company added additives to the coating and, after multiple adjustments, finally added 2% Fe2O3 (iron oxide red) as aggregate to meet the requirements.