Lost foam casting is a casting process that has gained significant popularity in recent years due to its advantages such as low pollution, flexible processing, low labor intensity, and good repeatability. This technique is particularly suitable for the production of complex shell-like components. In this article, we will focus on the application of lost foam casting in shell castings, identify the main issues encountered in the production of such products, and discuss several casting defects, their causes, and corrective measures.
Introduction to Lost Foam Casting
Lost foam casting, also known as expendable pattern casting, is a modern casting process that offers precise dimensional accuracy, good repeatability, and high internal quality of castings. Since the patent expired in 1980, this technology has developed rapidly worldwide and has become increasingly mature. It is widely used in various industries, especially for producing complex shell parts.
Casting Defects in Lost Foam Casting
During the lost foam casting process, several defects may occur, such as burning-on, porosity, and sand wash. These defects can significantly affect the quality and performance of the castings. Understanding the causes of these defects is crucial for implementing effective corrective measures.
Defect 1: Burning-on
Reason Analysis
Burning-on is a defect where the molten metal adheres to the sand mold, resulting in a rough surface on the casting. In the case of the flywheel housing, the placement method, structural design, or process design can lead to inadequate compaction of the mold cluster in the sand box, causing the top of the mold cluster to have loose sand. When the molten iron fills the mold, the local metal liquid and sand can bind together, resulting in burning-on.
For example, the 9661 flywheel housing, made of HT250 with a weight of about 22 kg and dimensions of 440 mm × 440 mm × 220 mm, has a large area, thin base wall, and is prone to deformation. The existing process had an inner gate size of 50 mm (length) × 30 mm (height) × 6 mm (width). The tapping temperature of the molten iron was 1460 – 1470 °C (using an electric furnace for melting), and the pouring temperature was 1430 – 1440 °C, with a vacuum of -0.025 MPa, no film coating, and no pressure holding. The main defect was the inclusion of sand at the top of the flywheel housing cavity, with a rejection rate of 20%.
The analysis of the burning-on defect in the 9661 flywheel housing revealed that the top angle was greater than 90°, making it difficult for the molding sand to fill the mold cluster during the vibration process. As a result, the top molding sand was relatively loose, and the local metal liquid and sand adhered to each other during the iron filling process, causing the burning-on defect.
Control Measures
To address the burning-on defect, the following measures were taken:
- Adjust the placement of the white pattern. By placing the motor hole of the flywheel housing upwards, it facilitated the filling of molding sand and ensured sufficient sand at the top of the flywheel housing.
- Increase the distance between the two flywheel housings. The distance was changed from 80 mm to 120 mm to ensure adequate space between the mold clusters, thereby enhancing the compactness of the molding sand.
Production Verification
After analyzing the causes of the molding sand compactness issue in the flywheel housing and implementing the above two measures to rectify the product placement method and combination spacing in the existing combination process, production verification was conducted from small batches to large batches. During the normal production process, while ensuring that variables such as the dipping coating process, pouring temperature, and vacuum pumping remained unchanged, the burning-on defect was eliminated, achieving the goal of completely resolving the burning-on defect at the top of the flywheel housing.
Measure | Description | Result |
---|---|---|
Adjust placement of white pattern | Place the motor hole of the flywheel housing upwards to facilitate sand filling | Ensured sufficient sand at the top of the flywheel housing |
Increase distance between flywheel housings | Change the distance from 80 mm to 120 mm | Enhanced the compactness of the molding sand |
Defect 2: Porosity
Reason Analysis
Porosity occurs when the gas and residues generated during the gasification and decomposition of the white pattern in the mold cluster cannot be discharged from the casting in time, resulting in the formation of pores on the surface of the casting. The appearance of porosity is related to the pouring temperature, coating permeability, pouring speed, and other factors.
For instance, in the case of the flywheel housing for SAIC Maxus, the porosity defect was observed as smooth holes of varying sizes on the surface after machining, with an oxidized color on the hole walls. The pores were mainly concentrated at the top motor hole of the product, with a rejection rate of 30%.
The main influencing factors for this porosity defect were as follows:
- Pouring Temperature: When the pouring temperature is low, the foam does not burn completely, and the gas is not fully discharged, leading to the formation of subcutaneous pores.
- Local Coating Thickness: Excessive coating thickness at the motor hole prevents the gas from being discharged after the foam burns, resulting in the formation of pores.
- Vacuum Degree: Insufficient vacuum makes it difficult for the gas to be pumped away in time, causing the formation of pores.
- Unreasonable Process Design: The lack of exhaust ports at the top of the flywheel housing causes the gas to accumulate at the top of the casting and not be fully discharged, leading to the formation of pores.
Control Measures
Based on the mechanism of porosity formation and its influencing factors, the following measures were implemented:
- Increase the pouring temperature from 1430 – 1440 °C to 1450 – 1460 °C and pour 10 groups.
- Reduce the coating thickness at this location from 2.0 mm to 0.5 mm and pour 10 groups.
- Increase the vacuum from -0.025 MPa to -0.045 MPa and pour 10 groups.
- Add an exhaust piece at the motor hole with dimensions of 50 mm (length) × 30 mm (height) × 5 mm (width) and pour 10 groups.
Production Verification
To address the porosity issue in the flywheel housing, the existing process parameters were adjusted by implementing the above four measures, including changing the pouring temperature, coating thickness, vacuum during pouring, and adding an exhaust piece. During the process experiment, a control variable method was adopted, ensuring that the other three process parameters remained unchanged in each case. Specifically:
- Option 1: Increase the pouring temperature. After producing and processing 20 pieces, 4 pieces had porosity, with the porosity at the motor hole accounting for 20%.
- Option 2: Reduce the coating thickness. After producing and processing 20 pieces, 5 pieces had porosity, with the porosity at the motor hole accounting for 25%.
- Option 3: Increase the vacuum. After producing and processing 20 pieces, 3 pieces had porosity, with the porosity at the motor hole accounting for 15%.
- Option 4: Add an exhaust piece. After producing and processing 20 pieces, there was no porosity, and the porosity at the motor hole was 0%.
The results showed that Option 4 was the most effective. Subsequent verification through small-scale to large-scale production confirmed that the processing was normal. Through these measures, the goal of completely resolving the porosity at the motor hole was achieved.
Measure | Description | Result |
---|---|---|
Increase pouring temperature | Raise the pouring temperature from 1430 – 1440 °C to 1450 – 1460 °C | Reduced the formation of subcutaneous pores |
Reduce coating thickness | Decrease the coating thickness from 2.0 mm to 0.5 mm | Facilitated the discharge of gas after foam burning |
Increase vacuum | Enhance the vacuum from -0.025 MPa to -0.045 MPa | Improved the removal of gas |
Add exhaust piece | Add an exhaust piece at the motor hole | Ensured the complete discharge of gas |
Defect 3: Sand Wash
Reason Analysis
Sand wash occurs when the gating system of the mold cluster, including the sprue, runner, and ingate, is not completely sealed during the pouring process, especially the sprue, which can easily form a siphon, leading to the defect. Additionally, an unreasonable design of the product’s gating system, resulting in an unsmooth filling process and high local pressure at the ingate, can cause the coating to rupture due to the scouring of the molten iron, allowing the molding sand to enter the mold cavity along with the molten iron.
For example, in the case of the connecting rod rest, the gating system shown in Figure 7 had a material of HT200, a weight of about 50 kg, and dimensions of 572 mm × 380 mm × 348 mm, with a base plate thickness of 12 mm. The existing process involved introducing the molten iron through three ingates on the side, with each ingate having dimensions of 60 mm × 30 mm × 6 mm. The process parameters included a tapping temperature of the molten iron of 1460 – 1470 °C (using an electric furnace for melting), a pouring temperature of 1430 – 1440 °C, a vacuum of -0.03 MPa, no film coating, and no pressure holding. The main defect was sand wash, which was concentrated near the bottom ingate (as shown in Figure 8), with a rejection rate of 20%.
The analysis of the sand wash defect in the connecting rod rest revealed that the main influencing factors were as follows:
- Low Coating Strength at the Ingate: The scouring of the molten iron can cause the coating to rupture.
- High Pressure at the Ingate: This can also lead to the rupture of the coating.
- In this case, the molten iron pouring process was stable without any backspray phenomenon, so the focus was on the first two factors.
Control Measures
To address the sand wash defect, the following measures were taken:
- Increase the number of dipping coatings for the ingate. The existing process had a coating thickness of 1.5 mm after two dips. An additional dip was added for the ingate to increase the coating thickness to 2.2 mm.
- Increase the number of ingates. Add one more ingate of the same size at the bottom two points.
Production Verification
To address the sand wash issue in the connecting rod rest, the existing process parameters were adjusted by implementing the above two measures, including increasing the coating thickness at the inlet and the number of inlets. During the process experiment, while ensuring that the pouring temperature, vacuum, and other influencing parameters remained unchanged, production verification was conducted from small batches to large batches. Specifically:
- Option 1: Add one more coating dip for the ingate. After producing 50 pieces, 6 pieces had sand wash, accounting for 12%.
- Option 2: Add one more ingate at the bottom. After producing 50 pieces, there was no sand wash.
The results showed that Option 2 was the best. Subsequent verification through small-scale to large-scale production confirmed that the processing was normal. By adopting three ingates at the bottom and one at the top, with a single ingate cross-sectional size of 60 mm × 8 mm, and a ratio of the sprue area (a 50 mm diameter circular tube), runner area (50 mm × 40 mm), and ingate area of approximately 1960 mm² : 2000 mm² : 1920 mm², which basically met the 1:1:1 condition, the goal of completely resolving the sand wash defect in the connecting rod rest was achieved.
Measure | Description | Result |
---|---|---|
Increase coating thickness at inlet | Add one more dip for the ingate to increase the coating thickness to 2.2 mm | Enhanced the resistance to molten iron scouring |
Increase the number of inlets | Add one more ingate of the same size at the bottom two points | Reduced the pressure at the ingate |
Conclusion
In the process of developing new products, casting defects should be cleverly avoided through process design based on their causes. During the process of promoting process verification, the number of products should be gradually increased from a small scale to a large scale to avoid significant losses due to insufficient consideration. In the process of verifying the process, it is essential to advance step by step from understanding the current situation, analyzing the causes, formulating plans, implementing countermeasures, and confirming the effects to ultimately achieve the goal of completely solving the problem.
Lost foam casting is a promising technology, but it requires careful attention to detail and continuous improvement to ensure the quality of the castings. By understanding and addressing the common casting defects, we can further enhance the reliability and performance of the lost foam casting process.