1. Introduction
Lost foam casting technology has witnessed remarkable growth in China in recent years, attributed to its numerous advantages such as low pollution, flexible processing, reduced labor intensity, and excellent repeatability. When it comes to manufacturing box – shaped and shell – like products, this technology holds a distinct edge over other casting methods. However, during the lost foam casting process of products like flywheel housings, several casting defects frequently occur, including sand adhesion, porosity, and sand wash. These defects not only compromise the quality of the castings but also lead to increased production costs and waste. Therefore, a comprehensive understanding of these defects, their causes, and effective solutions is of great significance for the foundry industry.
2. Sand Adhesion Defect in Flywheel Housing Castings
2.1 Causes of Sand Adhesion
Sand adhesion is a defect where the molten metal adheres to the molding sand on the casting surface during the pouring process. For flywheel housing castings, improper placement, structural design, or process design can result in the inability to compact the mold cluster in the sand box, thus causing sand adhesion.
Take the 9661 flywheel housing as an example. Its material is HT250, weighing approximately 22 kg, with a contour size of 440 mm x 440 mm x 220 mm and a wall thickness of 5 mm. The existing process has an inner gate size of 50mm (length) x 30mm (height) x 6 mm (width), with the molten iron tapping temperature ranging from 1460 – 1470°C (using an electric furnace for smelting), a pouring temperature of 1430 – 1440°C, 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, accounting for 20% of the rejects.
The root cause of the sand adhesion in this case is that the existing process places the motor hole at the bottom. The top angle of the product is greater than 90°, making it impossible for the molding sand to fill and compact the mold cluster during vibration. As a result, the sand at the top is loose, and during the filling of the molten iron, local molten metal adheres to the molding sand, forming sand adhesion.
| Defect | Product Details | Process Parameters | Defect Location | Reject Rate |
|---|---|---|---|---|
| Sand Adhesion | 9661 Flywheel Housing, HT250, 22kg, 440x440x220mm, 5mm wall thickness | Inner gate: 50x30x6mm, Tapping temp: 1460 – 1470°C, Pouring temp: 1430 – 1440°C, Vacuum: – 0.025 MPa, No film, No pressure – holding | Inner cavity top | 20% |
2.2 Control Measures for Sand Adhesion
- Adjusting the White Mold Placement: By changing the placement of the flywheel housing’s white mold, with the motor hole facing upward, it becomes easier for the molding sand to fill. This ensures that there is sufficient sand at the top of the flywheel housing, enhancing the compactness of the sand.
- Increasing the Distance between Two Flywheel Housings: The original distance between two flywheel housings was 80mm. By increasing it to 120mm, there is enough space between the two mold clusters. This guarantees the strength of the molding sand, preventing sand adhesion caused by weak sand strength due to insufficient distance.
2.3 Production Verification of Sand Adhesion Control
After analyzing the causes of the sand adhesion defect in the flywheel housing and implementing the two measures of adjusting the product placement and combination spacing, production verification was carried out. During the normal production process, while keeping variables such as dip – coating process, pouring temperature, and vacuuming unchanged, small – batch to large – batch production tests were conducted. The results showed that the sand adhesion defect was completely eliminated. The final process is shown in Figure 1.
[Insert Figure 1: Final process of the flywheel housing]
3. Porosity Defect in Flywheel Housing Castings
3.1 Causes of Porosity
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, pores will form on the casting surface. The occurrence of pores is related to factors such as pouring temperature, coating permeability, and pouring speed.
For the SAIC MAXUS flywheel housing, the molten iron tapping temperature is 1460 – 1470°C, the pouring temperature is 1430 – 1440°C, the vacuum is – 0.025 MPa, without film coating and pressure – holding. The main defect is pores in the motor hole at the top of the product, accounting for 30% of the rejects.
The pores in this product are subcutaneous pores, which are characterized by normal – looking surfaces during external inspection, but smooth holes of varying sizes are found after processing, with an oxidized luster on the pore walls. The main influencing factors are as follows:
- Pouring Temperature: When the pouring temperature is low, the foam does not burn completely, and the gas cannot be fully discharged, resulting in pores under the skin.
- Local Coating Thickness: If the coating thickness around the motor hole is too thick, the gas generated after the foam burns cannot escape, forming pores.
- Vacuum Degree: Insufficient vacuuming means that the gas cannot be quickly evacuated, leading to pore formation.
- Process Design: The lack of vents at the top of the flywheel housing causes gas to accumulate at the top of the body, unable to be completely discharged, thus forming pores.
| Defect | Product Details | Process Parameters | Defect Location | Reject Rate |
|---|---|---|---|---|
| Porosity | SAIC MAXUS Flywheel Housing | Tapping temp: 1460 – 1470°C, Pouring temp: 1430 – 1440°C, Vacuum: – 0.025 MPa, No film, No pressure – holding | Motor hole at the top | 30% |
3.2 Control Measures for Porosity
- Increasing the Pouring Temperature: The pouring temperature was raised from 1430 – 1440°C to 1450 – 1460°C, and 10 groups of castings were poured.
- Reducing the Coating Thickness: The original coating thickness around the motor hole was 2.0mm, which was reduced to 0.5mm, and 10 groups of castings were poured.
- Increasing the Vacuum Degree: The vacuum degree was increased from – 0.025MPa to – 0.045MPa, and 10 groups of castings were poured.
- Adding Exhaust Sheets: An exhaust sheet with dimensions of 50 mm (length) x 30mm (height) x 5mm (width) was added at the motor hole, and 10 groups of castings were poured.
3.3 Production Verification of Porosity Control
After analyzing the causes of the porosity defect in the flywheel housing and implementing four measures of adjusting the pouring temperature, coating thickness, vacuum degree, and adding exhaust sheets, process tests were carried out. Using the method of controlling variables, each measure was tested separately while keeping the other three parameters unchanged.
| Test Scheme | Adjusted Parameter | Number of Castings Produced | Number of Castings with Pores | Pore Proportion in Motor Hole |
|---|---|---|---|---|
| One | Increase pouring temperature | 20 | 4 | 20% |
| Two | Reduce coating thickness | 20 | 5 | 25% |
| Three | Increase vacuum degree | 20 | 3 | 15% |
| Four | Add exhaust sheet | 20 | 0 | 0% |
The results showed that the fourth scheme was the best. After small – batch to large – batch verification, all the castings were processed normally. The final process is shown in Figure 2.
[Insert Figure 2: Schematic diagram of the air outlet piece at the position of the motor hole]
4. Sand Wash Defect in Connecting Rod Bracket Castings
4.1 Causes of Sand Wash
During the pouring process, if the sprue, runner, and inner gate in the mold cluster pouring system are not completely sealed, especially the sprue, siphoning is likely to occur, resulting in sand wash defects. In addition, an unreasonable product pouring system design can lead to unsmooth filling, high local pressure in the inner gate, and the rupture of the coating due to the scouring of the molten iron. This causes the molding sand to enter the mold cavity with the molten iron, also resulting in sand wash defects.
For the connecting rod bracket casting, its material is HT200, weighing about 50 kg, with a contour size of 572mm x 380mm x 348 mm and a bottom plate thickness of 12mm. The existing process introduces molten iron through 3 – point inner gates on the side, with the inner gate size of 60mm (length) x 30mm (height) x 6mm (width). The process parameters include a molten iron tapping temperature of 1460 – 1470°C (smelted in an electric furnace), 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 wash, concentrated near the bottom inner gate, accounting for 20% of the rejects.
| Defect | Product Details | Process Parameters | Defect Location | Reject Rate |
|---|---|---|---|---|
| Sand Wash | Connecting Rod Bracket, HT200, 50kg, 572x380x348mm, 12mm bottom plate thickness | Inner gate: 60x30x6mm, Tapping temp: 1460 – 1470°C, Pouring temp: 1430 – 1440°C, Vacuum: – 0.03 MPa, No film, No pressure – holding | Near bottom inner gate | 20% |
The main influencing factors for the sand wash defect are as follows:
- Low Inner Gate Coating Strength: The scouring of the molten iron causes the coating to rupture.
- High Inner Gate Pressure: High pressure in the inner gate leads to coating rupture. Although there is no severe back – splash of molten iron during the pouring process of this product, these two factors are still the main considerations.
4.2 Control Measures for Sand Wash
- Increasing the Coating Thickness of the Inner Gate: The original process had the coating dipped twice with a thickness of 1.5mm. One more dipping process was added to the inner gate, increasing the coating thickness to 2.2mm.
- Increasing the Number of Inner Gates: Another inner gate of the same size was added at the bottom two – point position.
4.3 Production Verification of Sand Wash Control
After analyzing the causes of the sand wash defect in the connecting rod bracket and implementing two measures of adjusting the coating thickness of the water inlet and the number of water inlets, production verification was carried out. During the process test, while keeping the pouring temperature, vacuum degree, and other influencing parameters unchanged, small – batch to large – batch production verification was carried out.
| Test Scheme | Adjusted Parameter | Number of Castings Produced | Number of Castings with Sand Wash | Sand Wash Proportion |
|---|---|---|---|---|
| One | Increase inner gate coating thickness | 50 | 6 | 12% |
| Two | Add an inner gate at the bottom | 50 | 0 | 0% |
The results showed that the second scheme was the best. After small – batch to large – batch verification, all the castings were processed normally. The final process adopted 3 inner gates at the bottom and 1 inner gate at the top. The three inner gates can play a role in shunting and pressure reduction. The cross – sectional area of a single inner gate is 60 mm x 8 mm; the area of the sprue (50mm diameter pipe), runner (50 mm x 40 mm), and inner gate is approximately \(1960 ~mm^{2}: 2000 ~mm^{2}: 1920 ~mm^{2}\), basically meeting the 1:1:1 condition, completely solving the sand wash defect of the connecting rod bracket. The final process is shown in Figure 3.
[Insert Figure 3: Final process of the connecting rod rest (four points of water entry on the side)]
5. Comprehensive Optimization of the Lost Foam Casting Process
5.1 Importance of Process Optimization
Addressing individual casting defects is crucial, but comprehensive process optimization is essential for the overall improvement of the lost foam casting process. By optimizing the combination process of the mold, the pouring system, the layout of exhaust and slag – discharging risers, and the rationality of the inner gate inlet layout, the quality of castings can be enhanced, production efficiency can be increased, and production costs can be reduced.
5.2 Optimization Strategies
- Mold Combination Process Optimization: According to the product structure, carefully design the combination method of the white mold to ensure good filling of the molding sand and uniform distribution of the sand’s compactness. For example, for complex – shaped products, the combination method should be adjusted to avoid areas where sand cannot be compacted.
- Pouring System Optimization: Design a reasonable pouring system to ensure smooth filling of the molten iron, uniform distribution of pressure, and minimize the scouring of the molten iron on the coating. This can be achieved by optimizing the size, shape, and position of the sprue, runner, and inner gate.
- Exhaust and Slag – Discharging Riser Layout: Properly arrange the exhaust and slag – discharging risers to ensure the timely discharge of gas and slag during the pouring process. This can prevent the formation of pores and slag inclusions in the castings.
- Inner Gate Inlet Layout Rationality: Ensure that the layout of the inner gate inlet is reasonable to achieve uniform filling of the molten iron in the mold cavity. This can help avoid defects such as shrinkage cavities and porosity caused by uneven filling.
6. Conclusion
- Defect Prevention through Process Design: In the process of developing new products, it is necessary to carefully analyze the possible causes of casting defects and use process design to avoid them. By comprehensively considering factors such as product structure, material properties, and process parameters, a scientific and reasonable casting process can be designed to minimize the occurrence of defects.
- Step – by – Step Process Verification: During the process verification, it is advisable to start with a small number of products and gradually increase to large – batch production. This approach can help identify potential problems in a timely manner and avoid significant losses caused by insufficient consideration in the process. By carefully observing and analyzing the production results at each stage, the process can be continuously optimized.
- Systematic Problem – Solving Approach: In the process of process verification, a systematic approach should be adopted, including understanding the current situation, analyzing the causes, formulating plans, implementing countermeasures, and verifying the effects. By following these steps one by one, casting defects can be effectively solved, and the quality of castings can be continuously improved.
Lost foam casting technology has broad application prospects in the foundry industry. By effectively solving casting defects and optimizing the process, the quality and competitiveness of castings can be enhanced, promoting the healthy development of the foundry industry.
