I. Introduction
The casting industry has been constantly evolving to meet the growing demands of various industries. Among the numerous casting processes, the lost foam casting process has gained significant attention due to its unique advantages. This article focuses on the lost foam casting process for motor shell castings, exploring its key aspects, including mold design, pouring system design, casting process, melting and pouring process, production results, and its application in comparison to other casting methods.
II. Mold Design
A. Overall Molding Approach
For medium and large motor shell molds with heat dissipation ribs, a common practice is to divide them into multiple parts. However, this can lead to difficulties in controlling the dimensions and stability of the assembled white mold. In contrast, the approach of integral molding adopted by the author’s company offers several benefits. By using slider structures for both the outer shape and inner cavity, it ensures the smooth demolding of the entire white mold, effectively addressing issues related to casting deformation and dimensional accuracy.
B. Shrinkage Rate Selection
The selection of shrinkage rate parameters is crucial in mold design. For large lost foam motor shell castings, a shrinkage rate of 1.35% in the length direction and 1.15% in the height direction has been determined. This careful consideration of shrinkage rates helps to achieve more accurate casting dimensions.
III. Pouring System Design
A. Influence of Pouring System on Pattern Vaporization
The form of the pouring system in lost foam casting differs from that of traditional processes. The size, dimensions, shape, and direction of the ingate directly impact the vaporization of the pattern. If the size of the ingate is calculated based on traditional sand casting processes, the reduced amount of molten iron and increased heat dissipation area can cause a rapid drop in the temperature of the molten iron entering the mold cavity, thereby affecting the vaporization speed of the pattern.
B. Adjustment of Ingate Size
Based on the calculations of the traditional sand casting process, the total cross-sectional area of the ingate is generally increased by 10% to 25%. After testing, appropriate adjustments are made to optimize the pouring system.
C. Ladder Pouring Method and Ingate Location
Given the relatively high height of the casting, a ladder pouring method is employed. The location of the ingate is critical for ensuring a smooth pouring process. As shown in Figure 2, a multi-point iron liquid inlet ingate is used. The design of the pouring system and its parameters includes a straight runner with a cross-sectional dimension of φ48 mm, and the cross runner and ingate are arranged in 4 layers, evenly distributed in the height and diameter directions. There are 8 runners on the top layer and 6 runners on each of the other 3 layers, totaling 26 runners. Since the straight runner is hollow, the molten iron directly enters the lowest layer of the runner. As the liquid level rises to the height of the second layer, the second layer begins to receive the molten iron. Therefore, when calculating the pouring system ratio based on a single layer, it is ∑F_straight: ∑F_cross: ∑F_ingate = 1.25:2.5:1.
IV. Lost Foam Casting Process
A. Pre-foaming and Molding
- EPS Bead Selection
Due to the 5 mm thickness of the heat dissipation fins of the motor shell, smaller EPS original bead particle sizes are preferred. The author’s company uses the Longwang p-s/p-4s model. - Pre-foaming Density Control
To ensure uniform pre-foamed beads and low moisture content, the pre-foaming density is controlled within the range of 25 to 26 g/L through electric heating pre-foaming. - Foam Molding Equipment and Process
A dedicated hydraulic semi-automatic molding machine for motor shells is used for foam molding. The equipment has dimensions of 2.6 m in length, 2.6 m in width, and 9 m in height, and can accommodate molds with a maximum size of 2.3 m in length and 2.3 m in width. After automatic feeding, the remaining processes are completed automatically by the equipment control system, ensuring the consistency of the white mold quality.
B. Drying
- Moisture Evaporation and Inspection
After the model is produced, it is not immediately placed in the drying oven. Instead, it is left on the storage rack in the molding area to allow the surface moisture to evaporate approximately and undergo inspection. This step helps to avoid high humidity in the drying oven and identify defective molds for early disposal, reducing the occupation of drying oven space. - Drying Temperature and Time
The white mold is dried at a temperature of 40 to 45 °C for 48 hours to ensure thorough drying.
C. Assembly
- Weight Check and Defect Repair
The dried foam patterns are weighed individually, and those outside the specified range are isolated. The heat dissipation ribs are inspected, and defects within 10 mm × 10 mm are repaired using a special white mold repair paste. - Pouring Process and Bonding
A single-piece pouring process is adopted, with the non-driving end on top. The pouring system is bonded using hot melt adhesive, with the amount of adhesive minimized while ensuring a firm bond. The bonding surfaces are strictly sealed with masking tape.
D. Coating
- Coating Requirements
The coating for motor shell castings has specific requirements in addition to general performance requirements. It should have good anti-cracking and coating properties at room temperature, high temperature anti-cracking properties, and good peeling performance. - Coating Application Process
The author’s company uses a dip coating process. Before dip coating, the coating is stirred for 1 hour to ensure uniformity. The assembled white mold is slowly immersed in the coating pool to a depth of 200 to 300 mm, rotated 90 degrees after 1 minute, and repeated until the coating is evenly applied. The coated mold is then placed on a rack, and any missed areas are brushed with a paintbrush. After 2 hours, it is transferred to the drying room.
V. Melting and Pouring Process
A. Chemical Composition and Heat Treatment
The chemical composition of the motor shell is carefully controlled within the following ranges: w(C) 3.15% to 3.25%, w(Si) 1.6% to 1.8%, w(Mn) 0.7% to 0.9%, w(P) ≤ 0.05%, and w(S) ≤ 0.1%. The casting undergoes stress relief treatment to achieve a hardness of 180 to 230 HB.
B. Pouring Temperature and Vacuum Degree
The pouring temperature is maintained between 1490 and 1500 °C, and the pouring vacuum degree is controlled within the range of -0.065 to -0.07 MPa to ensure a smooth pouring process and good casting quality.
C. Pouring Time and Solidification
The pouring time is approximately 50 seconds, and the pressure is stopped 10 minutes after pouring. The box opening time is controlled at 4 hours to allow for proper solidification and cooling of the casting.
VI. Production Results
A. Casting Quality
The production practice shows that after switching to the lost foam casting process, the finished product rate of the motor shell casting is ≥ 97%. The metallographic structure and mechanical properties of the casting meet the standards, and the appearance quality is superior to that produced by the original furan resin sand process.
B. Cost Comparison
In terms of cost, the lost foam casting process offers significant savings. The raw material cost for the lost foam white mold per ton of casting is 33 yuan, and the coating cost is 30 yuan. Compared to the furan resin sand process, which requires 1.1% resin and 0.5% curing agent with a sand-iron ratio of 5:1, the lost foam casting process can save 600 yuan in raw material costs per ton of casting. Additionally, the labor efficiency of the lost foam process is twice that of the resin sand process, saving 800 yuan in labor costs per ton of casting. Overall, the comprehensive cost per ton of casting is reduced by 1400 yuan.
VII. Application and Comparison with Other Casting Methods
A. Application of Lost Foam Casting Process in Motor Shell Production
The successful application of the lost foam casting process in the production of medium and large motor shells demonstrates its feasibility and advantages. It provides a viable alternative to traditional casting methods, especially for complex castings with heat dissipation ribs.
B. Comparison with Furan Resin Sand Process
Compared to the furan resin sand process, the lost foam casting process not only offers better casting quality but also significantly reduces production costs. The lower raw material and labor costs make it a more competitive option in the market.
VIII. Conclusion
In conclusion, the lost foam casting process for motor shell castings, with its unique mold design, pouring system, and strict control of various process parameters, can produce high-quality castings with good surface finish, accurate dimensions, and satisfactory mechanical properties. The significant cost reduction compared to the furan resin sand process makes it an attractive option for the production of medium and large motor shells. The successful application of this process in practice validates its feasibility and potential for wider application in the casting industry.
