1. Introduction
Lost mold casting is a widely used manufacturing process with several advantages, such as good surface quality, high dimensional accuracy, and high process yield. It has been applied in the production of ductile iron components, including reducer housings. However, like any casting process, it is not without its challenges. In this study, we focus on the wrinkling and shrinkage hole defects that occur during the lost mold casting of ductile iron reducer housings and explore effective solutions.
1.1 The Importance of Reducer Housings
Reducer housings play a crucial role in mechanical systems. They are required to have high strength, toughness, wear resistance, and anti-seismic properties. The quality of the reducer housing directly affects the performance and reliability of the entire mechanical system.
1.2 The Application of Lost Mold Casting in Reducer Housings
Given the characteristics of lost mold casting and the requirements of reducer housings, this process has been chosen for production. However, as we will see, certain defects have emerged that need to be addressed.
2. The Original Process and Defects of the Ductile Iron Reducer Housing
2.1 The Original Casting Process
The original pouring system of the reducer housing combination process is shown in Figure 1. The key parameters of the original process are as follows:
| Parameter | Value |
|---|---|
| End face design process allowance | 4 mm |
| Furnace outlet temperature | 1580 – 1600 °C |
| Pouring temperature | 1370 – 1440 °C |
| Pouring negative pressure | -0.06 – 0.04 MPa |
| Negative pressure holding time | 900 s |
The chemical composition of the reducer housing is presented in Table 1.
| Main Element | Content |
|---|---|
| C | 3.5 – 4.0 |
| Si | 2.0 – 3.0 |
| Mn | ≤ 0.45 |
| P | ≤ 0.05 |
| S | ≤ 0.025 |
| Mg | 0.02 – 0.06 |
| RE | 0.015 – 0.04 |
The Y-type specimen mechanical properties meet the requirements of QT450 – 10, and the spheroidization rate of 2 – 3 levels also meets the technical requirements of the ductile iron reducer housing.
2.2 Defects in the Original Process
Despite meeting some of the basic requirements, the original process resulted in several defects:
- Wrinkling Defects: Wrinkling was observed on the end face of the reducer housing during production. These wrinkling defects presented as an uneven surface, similar to the texture of an orange peel.
- Shrinkage Hole Defects: Shrinkage holes were found in the geometric hot spots of the reducer housing. These defects affected the integrity and quality of the casting.
3. The Formation Mechanisms of Wrinkling and Shrinkage Hole Defects
3.1 The Formation of Wrinkling Defects
- The Role of Polymeric Materials: In lost mold casting, the polymeric materials used for shaping, such as the copolymer materials, play a crucial role. During the heating decomposition process, these materials produce a large amount of gaseous, liquid, and solid products. The complex nature of these decomposition products and their transport processes is the root cause of the wrinkling defects.
- The Influence of the Pouring System: The original pouring system, which was a top pouring system in essence (although it was a kind of middle-bottom pouring process using the cavity as a runner), led to an improper filling pattern. The high-temperature iron liquid entered the top surface of the white mold from the inner gate and did not fill the mold gradually from top to bottom. Instead, it directly penetrated the thick wall area at the top, using the cavity top as a runner, and then fanned down to the thin wall area and finally up again, resulting in a turbulent filling pattern. This caused the formation of “cold ends” in the thick and dead corner areas at the top and sides, leading to wrinkling defects.
3.2 The Formation of Shrinkage Hole Defects
- The Influence of Carbon Equivalent: Although the chemical composition of the ductile iron reducer housing met the requirements, and there were no cold shuts or sand sticking problems, the fundamental cause of shrinkage holes was related to the liquid contraction and solidification of the alloy. During solidification, some parts of the casting (usually the hot spots that solidify last) could not receive timely compensation of liquid metal, resulting in irregularly shaped holes with rough hole walls.
- The Role of Geometric Hot Spots: The shrinkage hole defects in the reducer housing were mainly located in the thick positions of the side processing holes, which were caused by geometric hot spots. Therefore, to solve the shrinkage hole defect, it is necessary to change the casting process to address the geometric hot spots.
4. Solutions and Verifications for the Defects
4.1 Solutions for Wrinkling Defects
- Redesign of the Pouring System: The wrinkling defects were mainly caused by the unreasonable design of the pouring system, which led to turbulent flow of the iron liquid during pouring. To solve this problem, the pouring system was redesigned from a top pouring system to a bottom pouring system (as shown in Figure 2). In the new bottom pouring system, the high-temperature iron liquid fills the mold gradually from the bottom up, ensuring a smooth filling process. The front-end low-temperature iron liquid and the products of insufficient gasification of the white mold are left at the processing allowance position on the top surface of the mold cavity, resulting in a surface-healthy casting.
- Calculation and Design of the New Pouring System: The new pouring system was designed through theoretical calculations. The pouring time was calculated as . The height of the average static pressure head for bottom injection was calculated as . The minimum cross-sectional area of the inner runner was calculated as . According to production experience and relevant charts, the inner runner cross-sectional area was determined to be between 3.5 – 12 cm². For the new design, considering the casting structure, four inner gates were set, each with a cross-sectional dimension of , and the total cross-sectional area of the four inner gates was . The length of the straight runner was designed to be 480 mm according to a pressure head of 200 mm.
- Verification of the Solution: A batch production of 2000 pieces was carried out using the new casting process. The results showed that the surface quality of the castings was qualified, and no wrinkling defects occurred on a large scale, as shown in Figure 3.
4.2 Solutions for Shrinkage Hole Defects
- Analysis of Traditional Solutions and Their Limitations: Traditional solutions for shrinkage hole defects include setting risers at hot spots and using chilling systems. However, for lost mold casting of reducer housings, these methods have limitations. Setting risers reduces the process yield of the casting and increases the overall process difficulty. Using cold irons is difficult due to the characteristics of lost mold casting, as the cold irons are prone to falling off during molding and may cause deformation of the casting, increasing the process difficulty and affecting the quality stability of the reducer housing, as well as increasing the casting cost.
- The Development of a New Heat Dissipation Process: A new heat dissipation process was developed to solve the shrinkage hole defect. The core idea of this process is to change the casting structure, increase the heat dissipation surface area, and reduce the modulus of the casting at the hot spots. During the pouring and solidification processes, negative pressure gas takes away a large amount of heat, achieving a chilling effect.
- The Implementation of the Heat Dissipation Process: The specific method is to bond foam sheets (referred to as “heat sinks”) at the hot spots of the casting (as shown in Figure 4). After coating, drying, boxing, and pouring, qualified castings are produced. During the casting process, negative pressure is continuously pumped. When the negative pressure pump is working, cold air enters from the upper surface of the sand box, flows through the casting and the heat sinks, and exchanges heat to take away heat. The heat sinks have a large specific surface area, which reduces the local modulus of the casting. The cold air takes away a large amount of heat, and a micro-channel flow heat exchange is formed between the local molding sand in contact with the casting and the heat sinks, creating a chilling zone with a large temperature difference. The heat sinks play the role of cold irons, changing the local solidification mode of the casting to a similar sequential solidification, eliminating shrinkage holes and shrinkage porosity defects, as shown in Figure 5.
- Verification of the Solution: For the original process-produced reducer housing castings, shrinkage holes were found in the bolt holes. After analyzing the casting structure, it was determined that the shrinkage hole positions belonged to geometric hot spots. Twelve heat sinks with dimensions of were bonded in the hot spot area during the cutting and bonding process. After trial production and machining verification, the bolt holes of the reducer housing were normal and had no quality problems (as shown in Figure 7). A batch production of 2000 pieces of reducer housings was carried out using the heat dissipation process, and no shrinkage hole defects occurred in the bolt holes after machining.
5. Conclusion
- Summary of the Solutions: Through optimizing the pouring system, the wrinkling defect of the casting was solved. By changing the filling pattern to a smooth bottom-up filling process, the iron liquid turbulence was eliminated, and the surface quality of the casting was improved. For the shrinkage hole defect, a new heat dissipation process was developed. By setting heat sinks at the hot spots of the casting, the local heat dissipation speed was increased, the geometric hot spots were eliminated, and the shrinkage hole defect was solved.
- Significance of the Study: This study provides practical solutions for the wrinkling and shrinkage hole defects in lost mold casting of ductile iron reducer housings. The new methods not only improve the quality of the castings but also have advantages such as simplicity and high process yield, which have important guiding significance for the production of reducer housings and other similar castings.
In conclusion, the research on defect prevention and solution in lost mold casting of ductile iron reducer housings is an important area that requires continuous exploration and improvement to meet the increasing demands for high-quality castings in various industries.
6. Future Research Directions
6.1 Optimization of the Heat Dissipation Process
Although the newly developed heat dissipation process has effectively solved the shrinkage hole defect, there is still room for further optimization. Future research could focus on exploring different materials and geometries for the heat sinks to enhance their heat dissipation efficiency. For example, experiments could be conducted with heat sinks made of different polymers or with modified surface textures to improve the heat transfer rate. Additionally, the optimal number and placement of heat sinks could be further investigated to achieve even better results in eliminating geometric hot spots and preventing shrinkage holes.
6.2 Study of the Pouring System under Different Casting Conditions
The redesigned bottom pouring system has proven successful in eliminating wrinkling defects for the current reducer housing casting. However, different casting conditions, such as varying casting sizes, shapes, and alloy compositions, may require further adjustments to the pouring system. Future studies could explore how the pouring system parameters need to be modified under these different conditions to ensure a smooth filling process and high-quality castings. This could involve simulations and experimental studies to understand the fluid dynamics of the iron liquid during pouring and to develop more adaptable pouring system designs.
6.3 Investigation of the Interaction between Different Defect Prevention Measures
In this study, the wrinkling and shrinkage hole defects were addressed separately through different measures. However, in actual casting processes, these defects may interact with each other, and the implementation of one defect prevention measure may have an impact on the effectiveness of another. Future research could focus on understanding these interactions and developing integrated strategies that take into account multiple defect prevention mechanisms simultaneously. This would require a comprehensive understanding of the casting process as a whole and the ability to balance different factors to achieve the best overall casting quality.
7. The Impact of Defect Prevention on the Quality and Performance of Reducer Housings
7.1 Quality Improvement
By effectively preventing wrinkling and shrinkage hole defects, the surface quality and internal integrity of the reducer housing castings are significantly improved. The absence of wrinkling ensures a smooth and even surface, which is not only aesthetically pleasing but also important for proper fitting and functioning of other components that interact with the housing. The elimination of shrinkage holes reduces the risk of cracks and fractures during operation, increasing the mechanical strength and reliability of the housing.
7.2 Performance Enhancement
The improved quality of the reducer housing directly impacts its performance in a mechanical system. A defect-free housing provides better support and protection for the internal components, ensuring smooth operation of the reduction gears. It also reduces the likelihood of vibration and noise during operation, as there are no internal defects to cause irregularities in the movement of the parts. This leads to a more efficient and reliable mechanical system, which is crucial for applications where precision and durability are required.
8. The Economic Significance of Defect Prevention in Lost Mold Casting
8.1 Cost Reduction in Production
The prevention of defects in lost mold casting of reducer housings can lead to significant cost savings in production. By eliminating the need for rework or rejection of defective castings, the production efficiency is increased, and the cost of raw materials and labor associated with defective parts is reduced. Additionally, the use of more efficient defect prevention measures, such as the optimized pouring system and heat dissipation process, can reduce the complexity and cost of the overall casting process, further contributing to cost savings.
8.2 Market Competitiveness
Producing high-quality reducer housings without defects gives a company a competitive edge in the market. Customers are more likely to choose products that have a proven track record of quality and reliability. This can lead to increased market share and higher profits for the company. Moreover, the ability to produce defect-free castings can also attract more business opportunities, as it demonstrates the company’s technical expertise and commitment to quality.
9. Comparison with Other Casting Methods
9.1 Advantages over Traditional Sand Casting
Lost mold casting offers several advantages over traditional sand casting when it comes to producing reducer housings. The surface quality achieved in lost mold casting is generally higher, with fewer surface imperfections such as sand inclusions and roughness. The dimensional accuracy is also better, as the use of a polymeric pattern allows for more precise replication of the desired shape. Additionally, the process yield in lost mold casting is often higher, as there is less waste material generated compared to sand casting.
9.2 Comparison with Investment Casting
Compared to investment casting, lost mold casting has its own set of advantages. While investment casting can produce very high-quality and complex parts, it is a more expensive and time-consuming process. Lost mold casting, on the other hand, offers a more cost-effective solution for producing reducer housings with good quality. It also has a relatively shorter production cycle, which is beneficial for meeting production deadlines and reducing inventory costs.
10. Conclusion
In conclusion, the research on wrinkling and shrinkage hole defects in lost mold casting of ductile iron reducer housings has led to the development of effective solutions. The optimization of the pouring system and the introduction of the heat dissipation process have significantly improved the quality of the castings, both in terms of surface appearance and internal integrity. These defect prevention measures also have important economic implications, reducing production costs and enhancing market competitiveness. Future research directions have been identified to further optimize the processes and understand the interactions between different defect prevention mechanisms. The study also highlights the advantages of lost mold casting over other casting methods for producing reducer housings. Overall, this research provides valuable insights and practical guidance for the production of high-quality ductile iron reducer housings in the lost mold casting industry.
