This article focuses on the lost foam casting process, which has seen rapid development in recent years due to its numerous advantages such as low pollution, flexibility, and good repeatability. However, during the casting of products like flywheel housings, common defects such as sand adhesion, porosity, and sand washing occur. Through in – depth analysis of the causes of these defects and a series of experiments and improvements, effective solutions are proposed. The results of this study can provide valuable references for improving the quality of lost foam casting products and optimizing the casting process.
1. Introduction
Lost foam casting technology, since the patent expired in 1980, has developed rapidly worldwide. It has become a mature casting method, especially suitable for manufacturing complex shell – like components. Compared with traditional sand casting, it offers advantages such as high – precision casting dimensions, good repeatability, flexible production, and excellent internal quality. However, in the production of components like flywheel housings, several casting defects frequently appear, which not only affect the quality of the products but also increase production costs. Therefore, it is of great significance to study and solve these problems.
2. Defects in Lost Foam Casting of Flywheel Housings and Their Causes
2.1 Sand Adhesion
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 9661 Flywheel Housing
The 9661 flywheel housing, made of HT250 with a weight of about 22 kg and dimensions of 440 mm x 440 mm x 220 mm and a wall thickness of 5 mm, has a large – sized and thin – walled structure that is prone to deformation. In the existing process (as shown in Figure 1), with an inner gate size of 50mm (length) x 30mm (height) x 6 mm (width), the molten iron is discharged from the furnace at 1460 – 1470°C, poured at 1430 – 1440°C, with a vacuum degree of – 0.025 MPa, without film – coating and pressure – holding. The main defect is sand inclusion on the top of the inner cavity of the flywheel housing, with a scrap rate of 20%.
| Process Parameter | Value |
|---|---|
| Material | HT250 |
| Weight | About 22 kg |
| Dimensions | 440 mm x 440 mm x 220 mm |
| Wall Thickness | 5 mm |
| Inner Gate Size | 50mm (length) x 30mm (height) x 6 mm (width) |
| Furnace Discharge Temperature | 1460 – 1470°C |
| Pouring Temperature | 1430 – 1440°C |
| Vacuum Degree | – 0.025 MPa |
| Film – coating and Pressure – holding | None |
| Main Defect | Sand inclusion on the top of the inner cavity |
| Scrap Rate | 20% |
2.1.2 Analysis of Sand Adhesion in Flywheel Housing
The sand adhesion of the 9661 flywheel housing is characterized by the adhesion of a mechanical mixture of sand grains and metal on the shell surface. After cleaning, the surface shows a metallic luster, which is a typical feature of mechanical sand adhesion. The main influencing factors include the compactness of the molding sand during molding, the refractoriness of the coating, the pouring temperature, and the coating thickness. Since only the top of the casting has sand adhesion and the rest of the flywheel housing is normal, the main reason is considered to be the compactness of the molding sand. The specific reasons are as follows:
- The top of the flywheel housing cannot be filled with enough molding sand, or the existing molding sand cannot be compacted.
- The gap between two flywheel housings is too small, resulting in weak molding sand strength.
2.2 Porosity
Porosity occurs when the large amount of gas and residues generated by the gasification and decomposition of the white mold cannot be discharged from the casting body in time after the molten iron enters the mold cluster, forming pores on the surface of the casting.
2.2.1 Case Study of SAIC MAXUS Flywheel Housing
For the SAIC MAXUS flywheel housing, as shown in Figure 5, with a furnace discharge temperature of 1460 – 1470°C, a pouring temperature of 1430 – 1440°C, a vacuum of – 0.025 MPa, without film – coating and pressure – holding, the main defect is pores in the motor hole at the top of the product, with a scrap rate of 30%.
| Process Parameter | Value |
|---|---|
| Furnace Discharge Temperature | 1460 – 1470°C |
| Pouring Temperature | 1430 – 1440°C |
| Vacuum | – 0.025 MPa |
| Film – coating and Pressure – holding | None |
| Main Defect | Pores in the motor hole at the top of the product |
| Scrap Rate | 30% |
2.2.2 Influencing Factors of Porosity
The porosity defect is characterized by normal appearance inspection on the surface or near – surface of the casting, but there are smooth holes of different sizes on the surface after processing, and the pore walls have an oxidized luster. The pores are mainly concentrated at the top of the product, which is a typical feature of subcutaneous pores. The main influencing factors are:
- Pouring temperature: When the pouring temperature is low, the foam burns incompletely, and the gas cannot be completely discharged, forming pores under the skin.
- Local coating thickness: If the coating thickness at the motor hole is too thick, the gas generated after the foam burns cannot be discharged, forming pores.
- Vacuum degree: If the vacuum degree is too small, the gas cannot be evacuated in time, forming pores.
- Unreasonable process design: The lack of vents at the top of the flywheel housing causes the gas to concentrate at the top of the body and not be completely discharged, forming pores.
2.3 Sand Washing
Sand washing occurs when the pouring system of the mold cluster (such as the straight runner, horizontal runner, and inner gate) is not completely closed during the pouring process, especially the straight runner is prone to siphoning, resulting in sand washing. In addition, an unreasonable product pouring system design, unsmooth filling, and high local pressure in the inner gate can cause the coating to break due to the scouring of the molten iron, allowing the molding sand to enter the mold cavity with the molten iron.
2.3.1 Case Study of Connecting Rod Bracket Casting
The connecting rod bracket casting, made of HT200 with a weight of about 50 kg and dimensions of 572mm x 380mm x 348 mm and a bottom plate thickness of 12mm, has an existing process of introducing molten iron through 3 – point inner gates on the side. The inner gate size is 60mm (length) x 30mm (height) x 6mm (width). With a furnace discharge temperature of 1460 – 1470°C, a pouring temperature of 1430 – 1440°C, and a vacuum of – 0.03 MPa, without film – coating and pressure – holding, the main defect is sand washing, concentrated near the bottom inner gate, with a scrap rate of 20%.
| Process Parameter | Value |
|---|---|
| Material | HT200 |
| Weight | About 50 kg |
| Dimensions | 572mm x 380mm x 348 mm |
| Bottom Plate Thickness | 12mm |
| Inner Gate Size | 60mm (length) x 30mm (height) x 6mm (width) |
| Furnace Discharge Temperature | 1460 – 1470°C |
| Pouring Temperature | 1430 – 1440°C |
| Vacuum | – 0.03 MPa |
| Film – coating and Pressure – holding | None |
| Main Defect | Sand washing near the bottom inner gate |
| Scrap Rate | 20% |
2.3.2 Influencing Factors of Sand Washing
The sand washing defect is characterized by the appearance of lumps of sand – metal mixture at the bottom of the mold cavity along the straight – line of the gate and at the position where the molten iron enters the mold cavity from the inner gate. The main influencing factors are:
- Low coating strength of the inner gate: The scouring of the molten iron causes the coating to break.
- High pressure in the inner gate: Leading to the breakage of the coating. Although there is no severe back – spray of molten iron during the pouring of this product, the first two factors are mainly considered.
3. Control Measures and Production Verification
3.1 Control Measures for Sand Adhesion
- Adjust the placement of the white mold according to the product structure. Place the flywheel housing 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 from 80mm to 120mm to ensure a sufficient distance between the two mold clusters and the compactness of the molding sand.
3.1.1 Production Verification of Sand Adhesion Control Measures
After analyzing the reasons for the compactness problem of the flywheel housing molding sand and improving the product placement method and combination spacing, a production verification from small – batch to large – batch was carried out under the premise of keeping variables such as dip – coating process, pouring temperature, and vacuum pumping unchanged. The result showed that the sand adhesion defect was eliminated, achieving the goal of completely solving the sand adhesion defect at the top of the flywheel housing.
| Improvement Measure | Effect |
|---|---|
| Adjusting white mold placement | Facilitated sand filling and improved sand compactness at the top of the flywheel housing |
| Increasing the distance between two flywheel housings | Ensured molding sand compactness between mold clusters |
| Production Verification Result | Zero sand adhesion defect |
3.2 Control Measures for Porosity
- Increase the pouring temperature from 1430 – 1440°C to 1450 – 1460°C and pour 10 groups.
- Reduce the coating thickness at the motor hole from 2.0mm to 0.5mm and pour 10 groups.
- Increase the vacuum degree from – 0.025MPa to – 0.045MPa and pour 10 groups.
- Add an exhaust piece at the motor hole with dimensions of 50 mm (length) x 30mm (height) x 5mm (width) and pour 10 groups.
3.2.1 Production Verification of Porosity Control Measures
By analyzing the reasons for the porosity problem of the flywheel housing and improving the product pouring temperature, product coating thickness, product pouring vacuum degree, and adding exhaust pieces, a control – variable experiment was carried out. The results showed that the fourth plan (adding exhaust pieces) had the best effect, and subsequent small – batch to large – batch verification showed normal processing, completely solving the porosity problem of the motor hole.
| Improvement Measure | Number of Castings Produced | Number of Castings with Pores | Pore Proportion |
|---|---|---|---|
| Increasing pouring temperature | 20 | 4 | 20% |
| Reducing coating thickness | 20 | 5 | 25% |
| Increasing vacuum degree | 20 | 3 | 15% |
| Adding exhaust pieces | 20 | 0 | 0% |
3.3 Control Measures for Sand Washing
- Add one more dip – coating process for the inner gate. The original coating was dipped twice with a thickness of 1.5mm, and after the improvement, the coating thickness at the inner gate is 2.2mm.
- Increase the number of inner gates. Add an inner gate of the same size at the bottom two points.
3.3.1 Production Verification of Sand Washing Control Measures
After analyzing the reasons for the sand – washing problem of the connecting rod bracket and improving the coating thickness of the water inlet and the number of water inlets, a production verification from small – batch to large – batch was carried out under the condition of keeping parameters such as pouring temperature and vacuum degree unchanged. The results showed that the second plan (adding an inner gate at the bottom) had the best effect, and the final process with 3 inner gates at the bottom and 1 inner gate at the top could completely solve the sand – washing defect of the connecting rod bracket.
| Improvement Measure | Number of Castings Produced | Number of Castings with Sand – washing Defect | Sand – washing Defect Proportion |
|---|---|---|---|
| Adding one more dip – coating process for the inner gate | 50 | 6 | 12% |
| Adding an inner gate at the bottom | 50 | 0 | 0% |
4. Conclusions
- In the process of developing new products, it is necessary to avoid casting defects through ingenious process design according to the causes of the defects.
- During the process of process verification, the number of product verifications should increase gradually from small – batch to large – batch to avoid significant losses caused by insufficient consideration in the process.
- In the process of process verification, it is necessary to promote step by step from five aspects: grasping the current situation, analyzing the causes, formulating a plan, implementing countermeasures, and confirming the effects, so as to achieve the goal of completely solving the problem.
5. Future Research Directions
Although the above – mentioned methods can effectively solve some common defects in lost foam casting, there is still room for improvement in the lost foam casting process. Future research can focus on the following aspects:
- Further optimize the coating material and process to improve the coating’s performance in terms of refractoriness, permeability, and strength, which may help prevent multiple casting defects simultaneously.
- Study the influence of different molding sand properties on the casting quality, such as particle size distribution, clay content, and moisture content, to find the optimal molding sand formula for different casting products.
- Use numerical simulation technology to predict casting defects more accurately before actual production, which can guide the improvement of the casting process and reduce the cost of trial – and – error.
