This article comprehensively explores the lost foam casting technology, focusing on the common casting defects such as sand adhesion, porosity, and sand wash in the production of components like flywheel housings and connecting rod brackets. Through in – depth analysis of the causes of these defects and the presentation of corresponding control measures and production verification results, it provides a detailed understanding of how to optimize the lost foam casting process. Additionally, it emphasizes the importance of a systematic approach in process design and verification to ensure high – quality casting production.
1. Introduction
Lost foam casting technology has witnessed rapid development in recent years due to its numerous advantages over traditional sand – casting processes. These advantages include low pollution, high precision in casting dimensions, good repeatability, flexibility in production, and excellent internal quality of castings. It is particularly suitable for manufacturing complex shell – shaped components. However, during the casting process, various defects may occur, which can significantly affect the quality and performance of the castings. Therefore, it is crucial to understand these defects, analyze their causes, and develop effective solutions.
2. Common Defects in Lost Foam Casting and Their Causes
2.1 Sand Adhesion Defects
Sand adhesion is a defect where the molten metal adheres to the molding sand on the surface of the casting during the pouring process.
2.1.1 Case Study of Flywheel Housing Sand Adhesion
Take the 9661 flywheel housing as an example. Its material is HT250, with a mass of about 22 kg and contour dimensions of 440 mm×440 mm×220 mm, and a wall thickness of 5 mm. The existing process has an inner runner size of 50 mm (length)×30 mm (height)×6 mm (width), with the molten iron pouring temperature ranging from 1430 – 1440 °C and a vacuum degree of – 0.025 MPa. The main defect is the sand inclusion at the top of the inner cavity of the flywheel housing, with a scrap rate of 20%.
The sand adhesion of the 9661 flywheel housing is characterized by a mechanical mixture of sand grains and metal adhering to the shell surface. When cleaned, the surface shows a metallic luster, which is a typical feature of mechanical sand adhesion. The main influencing factors are as follows (Table 1):
| Influencing Factor | Explanation |
|---|---|
| Molding sand compactness | The top of the flywheel housing cannot be filled with enough molding sand, or the existing molding sand cannot be compacted properly. The small gap between two flywheel housings leads to weak sand strength. |
| Coating refractoriness | |
| Pouring temperature | |
| Coating thickness |
In this case, since the casting only has sand adhesion at the top and is normal in other positions, the main cause is considered to be the problem of molding sand compactness.
2.1.2 Control Measures for Sand Adhesion
- Adjust the placement of the white mold according to the product structure. For the flywheel housing, place the motor hole upwards to facilitate the filling of molding sand and ensure that there is enough molding sand at the top of the flywheel housing.
- Increase the distance between two flywheel housings. Change the original 80 mm to 120 mm to ensure that there is sufficient distance between the two mold clusters, thereby guaranteeing the compactness of the molding sand.
2.1.3 Production Verification of Sand Adhesion Control
After analyzing the causes of the molding sand compactness problem of the flywheel housing and making adjustments to the product placement method and combination spacing, production verification was carried out from small – batch to large – batch production. Under the premise that variables such as the dipping process, pouring temperature, and vacuum pumping remained unchanged, the sand adhesion defect rate dropped to 0, indicating that the sand adhesion defect at the top of the flywheel housing was completely resolved.
2.2 Porosity Defects
When the molten iron enters the mold cluster, the white mold gasifies and decomposes, generating a large amount of gas and residues. If these cannot be discharged from the casting body in a timely manner, porosity will form on the surface of the casting.
2.2.1 Case Study of Flywheel Housing Porosity
For the SAIC Maxus flywheel housing, the porosity defect is shown in Figure 1. The molten iron pouring temperature is 1430 – 1440 °C, and the vacuum degree is – 0.025 MPa. The main defect is the porosity in the motor hole at the top of the product, with a scrap rate of 30%.
The porosity defect is characterized by normal appearance detection on the surface or near – surface of the casting, but there are smooth holes of different sizes on the surface after processing, and the hole walls have an oxidized luster. The porosity is mainly concentrated at the top of the product, which is a typical feature of subcutaneous porosity. The main influencing factors are as follows (Table 2):
| Influencing Factor | Explanation |
|---|---|
| Pouring temperature | When the pouring temperature is low, the foam does not burn completely, and the gas cannot be completely discharged, forming porosity under the skin. |
| Local coating thickness | The coating on the motor hole is too thick, and the gas cannot be discharged after the foam burns, resulting in porosity. |
| Vacuum degree | The vacuum pumping is too small, and the gas cannot be sucked away in time, forming porosity. |
| Process design | The top of the flywheel housing lacks vents, and the gas accumulates at the top of the body and cannot be completely discharged, forming porosity. |
2.2.2 Control Measures for Porosity
- Increase the pouring temperature from 1430 – 1440 °C to 1450 – 1460 °C and conduct 10 sets of pouring tests.
- Reduce the coating thickness at the motor hole from 2.0 mm to 0.5 mm and conduct 10 sets of pouring tests.
- Increase the vacuum degree from – 0.025 MPa to – 0.045 MPa and conduct 10 sets of pouring tests.
- Add an exhaust piece at the motor hole with dimensions of 50 mm (length)×30 mm (height)×5 mm (width) and conduct 10 sets of pouring tests.
2.2.3 Production Verification of Porosity Control
After analyzing the causes of the porosity problem of the flywheel housing and making adjustments to the process parameters such as pouring temperature, coating thickness, vacuum degree, and adding exhaust pieces, a control – variable method was used for process testing. The results are shown in Table 3:
| Scheme | Adjustment Measure | Number of Castings Produced | Number of Porous Castings | Proportion of Porous Castings in Motor Hole |
|---|---|---|---|---|
| 1 | Increase pouring temperature | 20 | 4 | 20% |
| 2 | Reduce coating thickness | 20 | 5 | 25% |
| 3 | Increase vacuum degree | 20 | 3 | 15% |
| 4 | Add exhaust piece | 20 | 0 | 0% |
It can be seen that Scheme 4 has the best result. After small – batch to large – batch verification, all castings are processed normally, indicating that the porosity problem of the motor hole has been completely resolved.
2.3 Sand Wash Defects
During the pouring process, if the pouring system of the mold cluster, especially the straight runner, is not completely closed, it is easy to form a siphon, resulting in sand wash defects. In addition, unreasonable design of the product pouring system, unsmooth filling, and high local pressure in the inner runner can cause the coating to crack, allowing the molding sand to enter the mold cavity with the molten iron, also leading to sand wash defects.
2.3.1 Case Study of Connecting Rod Bracket Sand Wash
Take the connecting rod bracket as an example. Its material is HT200, with a mass of about 50 kg and contour dimensions of 572 mm×380 mm×348 mm, and a bottom plate thickness of 12 mm. The existing process uses 3 – point side – inlet inner runners to introduce molten iron, with an inner runner size of 60 mm×30 mm×6 mm. The pouring temperature is 1430 – 1440 °C, and the vacuum degree is – 0.03 MPa. The main defect is sand wash, which is concentrated near the bottom inner runner, with a scrap rate of 20%.
The sand wash defect is characterized by the appearance of sand – metal mixed nodules at the bottom of the mold cavity along the straight – line direction of the gate and at the part where the molten iron enters the mold cavity from the inner runner. The main influencing factors are as follows (Table 4):
| Influencing Factor | Explanation |
|---|---|
| Inner runner coating strength | The coating strength of the inner runner is low, and the molten iron scouring causes the coating to crack. |
| Inner runner pressure | High pressure in the inner runner leads to coating cracking. |
| Molten iron back – spray | Severe back – spray of molten iron during pouring causes coating cracking. In this case, the molten iron pouring is stable without back – spray, so the first two factors are mainly considered. |
2.3.2 Control Measures for Sand Wash
- Increase the number of dipping processes for the inner runner coating. The original coating is dipped twice with a thickness of 1.5 mm, and now an additional dipping process is added to make the coating thickness 2.2 mm.
- Increase the number of inner runners. Add a same – sized inner runner at the bottom two – point position.
2.3.3 Production Verification of Sand Wash Control
After analyzing the causes of the sand wash problem of the connecting rod bracket and making adjustments to the coating thickness and the number of inner runners, production verification was carried out from small – batch to large – batch production under the condition that the pouring temperature and vacuum degree remained unchanged. The results are shown in Table 5:
| Scheme | Adjustment Measure | Number of Castings Produced | Number of Sand – Washed Castings | Proportion of Sand – Washed Castings |
|---|---|---|---|---|
| 1 | Increase the dipping process of the inner runner coating | 50 | 6 | 12% |
| 2 | Add an inner runner at the bottom | 50 | 0 | 0% |
It can be seen that Scheme 2 has the best result. The final process adopts 3 inner runners at the bottom to achieve the function of flow – splitting and pressure – reducing, and 1 inner runner at the top. The cross – sectional area of a single inner runner is 60 mm×8 mm. The area ratio of the straight runner (a 50 – mm – diameter round pipe), the cross – runner (50 mm×40 mm), and the inner runner is approximately 1960 \(mm^{2}\):2000 \(mm^{2}\):1920 \(mm^{2}\), which basically meets the 1:1:1 condition, completely solving the sand wash defect of the connecting rod bracket.
3. Importance of Process Design and Verification
3.1 Process Design
In the development of new products, it is necessary to carefully design the process according to the possible causes of casting defects. This includes choosing the appropriate white mold placement method, optimizing the pouring system design, and reasonably arranging exhaust and slag – discharging risers. For example, in the case of the flywheel housing, adjusting the white mold placement can improve the filling of molding sand and avoid sand adhesion defects; for the connecting rod bracket, optimizing the pouring system can reduce the occurrence of sand wash defects.
3.2 Process Verification
During the process verification, it is necessary to gradually increase the number of product launches from a small number to a large number, from small – batch production to large – batch production. This can avoid significant losses caused by insufficient consideration in the process. The process verification should be carried out step by step from five major aspects: understanding the current situation, analyzing the causes, formulating a plan, implementing countermeasures, and confirming the effects. Only in this way can the casting defects be completely resolved and high – quality casting production be ensured.
4. Conclusion
Lost foam casting technology has broad application prospects, but it is necessary to pay attention to the common casting defects in the production process. By analyzing the causes of defects such as sand adhesion, porosity, and sand wash in detail and taking corresponding control measures, and through strict production verification, these defects can be effectively resolved. At the same time, in the process of product development, a scientific and reasonable process design and verification system should be established to continuously improve the quality of castings and promote the development of the lost foam casting industry.
