Abstract
In recent years, lost foam casting technology has rapidly developed in China due to its advantages such as low pollution, flexible processing, low labor intensity, and good repeatability. It demonstrates significant advantages over other casting processes, especially in the production of box and shell products. This paper focuses on the analysis of casting defects such as burning-on, porosity, and sand wash that occur during the lost foam casting process of products like flywheel housings. By adjusting the combination process method of the pattern, optimizing the pouring system, arranging exhaust and slag discharge risers, and ensuring the rationality of the layout of the inner gate inlet, measures to solve the casting defects of related products are proposed based on production situations. After the production and machining process, the goal of solving casting defects in the product is achieved.
1. Introduction
Lost foam casting technology has seen rapid development worldwide since the patent expired in 1980. Compared to traditional sand-mold casting, lost foam casting offers advantages such as precise casting dimensions, good repeatability, flexible production, and high internal quality. It is particularly suitable for complex shell components. This paper analyzes the application of lost foam casting technology in shell castings, points out the main issues in producing shell products using this technology, and highlights several casting defects, their causes, and corrective measures.
2. Analysis of Casting Defects and Their Causes
2.1 Burning-On Defects
2.1.1 Description and Manifestation
Burning-on defects refer to the adhesion of sand grains and metal mechanical mixtures to the casting surface during the pouring process. This often occurs due to unreasonable placement, structural design, or process design of the flywheel housing, causing the mold cluster to be unable to vibrate and compact properly within the sand box.
2.1.2 Case Study: 9661 Flywheel Housing
- Material: HT250
- Weight: Approximately 22 kg
- Dimensions: 440 mm × 440 mm × 220 mm
- Wall Thickness: 5 mm
The main defect is sand inclusion at the top of the flywheel housing’s inner cavity, with a defect rate of 20%.
2.1.3 Cause Analysis
The primary causes of burning-on defects in the 9661 flywheel housing include:
- Insufficient sand filling or inability to compact the sand at the top of the flywheel housing.
- Small clearance between two flywheel housings, resulting in weak sand strength.
2.1.4 Control Measures
To address these issues, the following measures were taken:
- Adjust the placement of the white mold so that the motor hole of the flywheel housing is upward, facilitating sand filling and ensuring sufficient sand at the top.
- Increase the distance between two flywheel housings from 80 mm to 120 mm to ensure sufficient space between the mold clusters, thereby ensuring sand compactness.
2.1.5 Production Verification
After implementing these measures, production verification was conducted from small batches to large batches. With other variables such as dipping process, pouring temperature, and vacuum remaining unchanged, the burning-on defect rate was reduced to 0%.
Table 1: Comparison of Burning-On Defect Rates Before and After Measures
| Measure | Before Implementation | After Implementation |
|---|---|---|
| Defect Rate | 20% | 0% |
2.2 Porosity Defects
2.2.1 Description and Manifestation
Porosity defects occur when the molten metal enters the mold cluster, and the foam pattern gasifies and decomposes, generating a large amount of gas and residues that cannot be timely removed, forming pores on the casting surface.
2.2.2 Case Study: SAIC MAXUS Flywheel Housing
- Defect: Mainly pores at the top of the motor hole, with a defect rate of 30%.
2.2.3 Cause Analysis
The main causes of porosity defects include:
- Low pouring temperature, resulting in incomplete foam combustion and gas not fully discharged, forming subcutaneous pores.
- Excessive coating thickness at the motor hole, preventing gas from escaping after foam combustion.
- Insufficient vacuum, preventing timely gas extraction.
- Unreasonable process design, lacking exhaust vents at the top of the flywheel housing.
2.2.4 Control Measures
To address these issues, the following measures were taken:
- Increase the pouring temperature from 1430-1440°C to 1450-1460°C.
- Reduce the coating thickness at the motor hole from 2.0 mm to 0.5 mm.
- Increase the vacuum level from -0.025 MPa to -0.045 MPa.
- Add an exhaust vent at the motor hole, with dimensions of 50 mm (length) × 30 mm (height) × 5 mm (width).
2.2.5 Production Verification
After implementing these measures, production verification was conducted using the control variable method. Among the four measures tested, adding an exhaust vent achieved the best result, with a defect rate of 0%. Subsequent verification from small batches to large batches confirmed normal processing.
Table 2: Comparison of Porosity Defect Rates for Different Measures
| Measure | Defect Rate |
|---|---|
| Increased Pouring Temperature | 20% |
| Reduced Coating Thickness | 25% |
| Increased Vacuum Level | 15% |
| Added Exhaust Vent | 0% |
2.3 Sand Wash Defects
2.3.1 Description and Manifestation
Sand wash defects occur when the mold cluster’s pouring system, particularly the sprue, runner, and ingate, is not completely sealed, leading to the formation of a siphon effect and causing sand to enter the mold cavity with the molten metal.
2.3.2 Case Study: Connecting Rod Rest
- Material: HT200
- Weight: Approximately 50 kg
- Dimensions: 572 mm × 380 mm × 348 mm
- Base Thickness: 12 mm
- Defect: Mainly sand wash at the bottom near the ingate, with a defect rate of 20%.
2.3.3 Cause Analysis
The main causes of sand wash defects include:
- Low coating strength at the ingate, causing coating rupture due to molten metal erosion.
- High pressure at the ingate, leading to coating rupture.
2.3.4 Control Measures
To address these issues, the following measures were taken:
- Increase the coating dipping process at the ingate by one additional dip, increasing the coating thickness to 2.2 mm.
- Add one more ingate of the same size at the bottom.
2.3.5 Production Verification
After implementing these measures, production verification was conducted with other parameters such as pouring temperature and vacuum level remaining unchanged. Among the two measures tested, adding an additional ingate achieved the best result, with a defect rate of 0%. Subsequent verification from small batches to large batches confirmed normal processing.
Table 3: Comparison of Sand Wash Defect Rates for Different Measures
| Measure | Defect Rate |
|---|---|
| Increased Coating Thickness | 12% |
| Added Additional Ingate | 0% |
3. Conclusion
In the development of new products, casting defects should be avoided through clever process design. During the process of promoting process verification, the product quantity should gradually increase from small batches to large batches to avoid significant losses due to inadequate consideration. The process verification should follow five aspects: understanding the current situation, analyzing causes, formulating plans, implementing countermeasures, and confirming effects, ultimately achieving the goal of solving problems completely.