1. Introduction
Lost foam casting (LFC) is a modern casting process that has gained significant attention in the manufacturing industry due to its numerous advantages. This process involves the use of a polystyrene foam pattern that is coated with a refractory material and then surrounded by unbonded sand. When the molten metal is poured into the mold, the foam pattern vaporizes, leaving behind a cavity that is filled with the metal to form the final casting.
In recent years, there has been an increasing demand for the production of large – sized and thin – walled shell parts. These parts are widely used in various industries such as automotive, aerospace, and machinery. However, the traditional casting methods such as sand casting and die casting often face challenges in producing these parts with high quality and low cost. Lost foam casting, on the other hand, offers a promising solution to these challenges.
This article focuses on the development of lost foam casting technology for large – sized and thin – walled shell parts. The article begins with an analysis of the product structure and processability of a tractor transmission box shell, followed by the design of the mold and casting process. Numerical simulation is used to optimize the process, and experimental verification is carried out to demonstrate the feasibility of the proposed process. The article also discusses the advantages and limitations of lost foam casting for large – sized and thin – walled shell parts and provides some suggestions for future research.
2. Product Structure and Processability Analysis
2.1 Product Structure
The part under study is a tractor transmission box shell. The initial design of the part was available only in 2D drawings, and a 3D model was created for the purpose of process design and numerical simulation. The part is made of HT250 material and has a theoretical mass of 265.1 kg. The maximum outer dimensions are 816 mm × 530 mm × 578 mm, and the minimum wall thickness is about 14 mm, while the maximum wall thickness is about 50 mm.
2.2 Processability Analysis
The large size and thin – walled structure of the part pose several challenges for the casting process. The main issues are deformation, cold shut, and sand sticking. Deformation can occur due to various factors such as thermal stress, shrinkage, and gravity during the casting process. Cold shut can result from improper pouring temperature or gating system design, while sand sticking can be caused by insufficient coating thickness or improper sand properties.
To address these issues, a detailed analysis of the casting process was carried out. The following factors were considered:
- Mold design: The mold was designed to have a two – piece structure with an intermediate plane as the parting line. The mold material was GBZL106, and the mold surface was coated with TEFLON to reduce friction and improve the surface finish of the casting.
- Pouring system design: Three different pouring system designs were considered, namely, flat – top pouring, inclined – side pouring, and vertical – top pouring. The pouring system parameters such as the diameter of the sprue, runner, and gate, as well as the pouring head height, were optimized based on numerical simulation results.
- Sand filling and compaction: The sand filling and compaction process was carefully controlled to ensure that the sand was evenly distributed and compacted around the foam pattern. The sand properties such as the grain size, shape, and moisture content were also optimized to reduce sand sticking and improve the dimensional accuracy of the casting.
3. Mold Design
3.1 Mold Structure
The mold was designed as an automatic machine mold with a contour size not exceeding 1000 mm × 900 mm × 600 mm. The mold body wall thickness was controlled between 12 – 15 mm to ensure mold strength and uniform heating and cooling. The outer frame wall thickness was controlled between 18 – 20 mm and was provided with reinforcing ribs.
3.2 Mold Material and Surface Treatment
The mold material was GBZL106, and the mold surface was free from casting defects that could affect its performance. The mold body surface was smooth with a surface roughness of Ra ≤ 1.6 μm. The cavity, core, and inserts were coated with TEFLON to reduce friction and improve the surface finish of the casting.
3.3 Mold Assembly and Fixing
The mold modules were connected using 定位销 or 止扣镶嵌方式 and were fixed using stainless steel bolts and spring washers. The mold surface steam plugs were evenly and reasonably distributed, and aluminum steam plugs with a diameter of about 12 mm were used. The machining allowance was set between 3 – 5 mm.
4. Casting Process Design
4.1 Process Schemes
Three different lost foam casting process schemes were designed, namely, flat – top pouring (Scheme 1), inclined – side pouring (Scheme 2), and vertical – top pouring (Scheme 3). The details of each scheme are as follows:
| Scheme | Sprue Diameter (mm) | Runner Section (mm × mm) | Gate Section (mm × mm) | Pouring Head (mm) |
|---|---|---|---|---|
| 1 | 50 | 40 × 20 | 40 × 7.5 | 210 |
| 2 | 50 | 60 × 55 | 55 × 15 | 190 |
| 3 | 50 | 45 × 50 | 60 × 15 | 320 |
4.2 Numerical Simulation
Numerical simulation was carried out using the Huazhu CAE casting simulation system lost foam simulation module to analyze the advantages and disadvantages of each process scheme. The simulation results are as follows:
- Scheme 1: In the later stage of solidification, slag inclusion and shrinkage cavity defects were observed at the marked positions in the figure, and an overflow structure needed to be added. In addition, due to the flat placement, the plane part of the part was at the bottom, which was prone to collapse of the box. Therefore, during the vibration compaction process, it was necessary to pay attention to adding sand and vibrating compaction multiple times, and during the pouring process, it was necessary to pour quickly. And due to the size influence during the molding process, only one piece could be placed in one box, and the box weight was small.
- Scheme 2: Compared with Scheme 1, it had a smaller tendency of slag inclusion and shrinkage cavity, and also reduced the risk of box collapse. However, although the inclined placement was beneficial for sand filling, it also caused difficulties in the sand adding and vibration compaction process, requiring multiple sand additions and low – amplitude high – frequency vibration compaction. Similarly, due to the size influence during the molding process, only one piece could be placed in one box, and the box weight was small.
- Scheme 3: It had the same advantages as Scheme 2 and could achieve pouring of two pieces in one box, with a large box weight and high efficiency. However, the filling process was disordered, and it was prone to cold shut and slag inclusion defects. And during the sand filling process, the sand was not easy to fill, and it was necessary to pre – fill self – hardening sand in some parts.
4.3 Process Optimization
Based on the numerical simulation results, Scheme 2 was selected as the base process, and some improvements were made. The number of gates was increased to increase the filling speed and reduce the temperature gradient during the solidification process of the molten iron. The optimized process scheme is shown in Figure 10. The numerical simulation results of the optimized scheme show that the filling speed is fast, the filling process is stable, and the temperature gradient during the solidification process is small, which is beneficial to reducing the risk of shrinkage porosity.
5. Process Experiment
5.1 Pattern Making
The pre – foaming density of the beads was required to be (25 ± 1) g/L. The process parameters for white pattern forming were as follows: cooling water ≤ 40 °C, compressed air 0.45 – 0.6 MPa, and system hydraulic pressure fixed at 0.5 MPa.
5.2 Assembly Scheme
The process scheme adopted was a 1 – box – 1 – piece scheme, with the molten iron entering from the upper part and an overflow block added at the top.
5.3 White Pattern Coating and Yellow Pattern Drying
The white pattern was coated using an immersion coating process. The Baume degrees of the coating were as follows: the first pass was 69 – 71, the second pass was 67 – 69, and the third pass was 65 – 67. The yellow pattern was heated and dried. When the drying room temperature reached 40 °C, the drying time was recorded, and the drying time was required to be ≥ 12 h. The drying room temperature was 40 – 55 °C. After the yellow pattern was dried, the exposed white on the surface of the yellow pattern was brushed with coating again, and the parts where the molding sand was not easy to fill, such as the gate and the concave on the surface of the yellow pattern, were brushed with coating again to increase the coating strength. The quality change of the finished yellow pattern was ≤ 5 was considered as dried.
5.4 Molding, Pouring, and Cleaning
The molding method is shown in Figure 14. The number of boxes was 1 – box – 1 – piece, the bottom sand thickness was 100 mm, the surface sand thickness was 30 mm, the pouring temperature was 1490 – 1510 °C, the negative pressure strength was 5.5 – 6 MPa, the single – box design pouring time was 90 s, and the holding pressure time was ≥ 20 min. After shot blasting cleaning, the surface quality of the casting was good, and no obvious defects were observed. The end face hardness was HB180 – 190.
5.5 Model Comparison
A 3D scanner was used to scan the sample blank to obtain the 3D model of the blank. The deformation and machining allowance of the blank were detected by comparing the 3D model of the blank with the theoretical 3D model. The results show that the 3D model of the blank and the theoretical model almost completely matched, and no part with a large deformation amount was found, and the deformation amount was within the process control range.
5.6 Machining Identification
To further verify the deformation and machining of the sample blank, the sample blank was processed and identified. The results show that the sample deformation amount was small, within the process control range, and no other defects were found, and the sample was qualified. The processed product is shown in Figure 17.
6. Advantages and Limitations of Lost Foam Casting for Large – Sized and Thin – Walled Shell Parts
6.1 Advantages
- Cost reduction: Lost foam casting can significantly reduce the cost of casting large – sized and thin – walled shell parts compared to traditional sand casting. This is mainly due to the simplification of the process, reduction in energy consumption, high recycling rate of dry sand, and reduction in the number of molds required.
- Dimensional accuracy: The use of a foam pattern allows for better control of the dimensional accuracy of the casting. The foam pattern can be precisely molded to the desired shape, and the shrinkage of the pattern during the casting process can be predicted and compensated for.
- Complex shape casting: Lost foam casting is suitable for casting parts with complex shapes. The foam pattern can be easily molded into any shape, and the molten metal can fill the cavity formed by the vaporized pattern without any restrictions.
6.2 Limitations
- Deformation control: Deformation is a major challenge in lost foam casting of large – sized and thin – walled shell parts. The deformation can occur due to various factors such as thermal stress, shrinkage, and gravity during the casting process. Controlling deformation requires careful control of the process parameters and the use of appropriate anti – deformation measures.
- Cold shut and slag inclusion: Cold shut and slag inclusion are also common problems in lost foam casting. These problems can be caused by improper pouring temperature, gating system design, or sand filling process. To address these issues, it is necessary to optimize the pouring system and sand filling process.
- Surface quality: The surface quality of the casting may not be as good as that of some other casting methods. The surface of the casting may have some roughness or porosity due to the vaporization of the foam pattern and the filling of the molten metal. To improve the surface quality, it is necessary to optimize the coating process and the pouring temperature.
7. Conclusions and Future Research
7.1 Conclusions
This article has presented a comprehensive study on the development of lost foam casting technology for large – sized and thin – walled shell parts. The following conclusions can be drawn:
- The product structure and processability of a tractor transmission box shell were analyzed, and three different lost foam casting process schemes were designed. Numerical simulation was used to optimize the process, and experimental verification was carried out to demonstrate the feasibility of the proposed process.
- The mold design and casting process were carefully considered to address the challenges of large – sized and thin – walled shell parts. The mold was designed to have a two – piece structure with an intermediate plane as the parting line, and the casting process was optimized to control deformation, cold shut, and sand sticking.
- The advantages and limitations of lost foam casting for large – sized and thin – walled shell parts were discussed. Lost foam casting offers several advantages such as cost reduction, dimensional accuracy, and complex shape casting, but also has some limitations such as deformation control, cold shut, and slag inclusion.
7.2 Future Research
Based on the conclusions of this study, the following areas of future research are suggested:
- Process optimization: Further research is needed to optimize the lost foam casting process for large – sized and thin – walled shell parts. This includes optimization of the mold design, pouring system, sand filling process, and coating process to improve the quality of the casting and reduce the cost.
- Deformation control: Deformation is a major challenge in lost foam casting of large – sized and thin – walled shell parts. Future research should focus on developing more effective anti – deformation measures to control deformation during the casting process.
- Surface quality improvement: The surface quality of the casting is an important aspect of the quality of the final product. Future research should focus on improving the surface quality of the casting by optimizing the coating process and the pouring temperature.