1. Introduction
Lost foam casting technology has witnessed remarkable growth in China in recent years, attributed to its numerous advantages such as low pollution, flexibility in 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 common defects frequently occur, including sand adhesion, porosity, and sand washing. These defects can significantly undermine the quality of the castings, resulting in increased production costs and potential performance issues in the final products. Therefore, a thorough understanding of these defects, their causes, and effective solutions is of utmost importance for the foundry industry.
2. Sand Adhesion Defects in Flywheel Housings
2.1 Analysis of Sand Adhesion in Flywheel Housings
Sand adhesion is a defect where the molten metal bonds with the molding sand on the surface of the casting during the pouring process. Take the 9661 flywheel housing as an example. It is made of HT250, weighs approximately 22 kg, and has a contour size of 440 mm x 440 mm x 220 mm with a wall thickness of 5 mm. This product has issues such as a large surface area and a thin base wall, making it prone to deformation.
The existing process has an inner gate size of 50 mm (length) x 30 mm (height) x 6 mm (width). The molten iron is discharged from the furnace at a temperature of 1460 – 1470°C (using an electric furnace as the melting equipment), and the pouring temperature is 1430 – 1440°C. The vacuum degree is – 0.025 MPa, without film coating and pressure – maintaining. The main defect is the inclusion of sand on the top of the inner cavity of the flywheel housing, with a scrap rate of 20%.
The current process places the motor hole at the bottom. Due to the large angle (greater than 90°) at the top of the inner cavity, the molding sand cannot be compacted during the vibration process, leaving the sand at the top loose. As a result, during the filling of the molten iron, local molten metal adheres to the molding sand, causing sand adhesion. The sand adhesion on the 9661 flywheel housing is characterized by a mechanical mixture of sand grains and metal adhering to the surface of the shell. When cleaned, the surface shows a metallic luster, which is a typical feature of mechanical sand adhesion. The main influencing factors are 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 issue is likely related to the compactness of the molding sand. The 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 by vibration.
- The gap between two flywheel housings is too small, weakening the strength of the molding sand.
| Defect | Product | Material | Weight | Dimensions | Wall Thickness | Process Parameters | Defect Location | Scrap Rate |
|---|---|---|---|---|---|---|---|---|
| Sand Adhesion | 9661 Flywheel Housing | HT250 | 22 kg | 440 mm x 440 mm x 220 mm | 5 mm | Inner gate size: 50 mm x 30 mm x 6 mm; Furnace temperature: 1460 – 1470°C; Pouring temperature: 1430 – 1440°C; Vacuum degree: – 0.025 MPa | Top of inner cavity | 20% |
2.2 Control Measures for Sand Adhesion
- Adjust the placement of the white mold according to the product structure. By placing the motor hole of the flywheel housing upward, it is easier to fill the molding sand, ensuring that there is sufficient molding sand at the top of the flywheel housing.
- Increase the distance between two flywheel housings. Change the original distance of 80 mm to 120 mm to ensure that there is enough space between the two mold clusters, thereby enhancing the compactness of the molding sand.
2.3 Production Verification of Sand Adhesion Control
After analyzing the reasons for the low compactness of the molding sand in the flywheel housing and implementing two improvement measures – adjusting the product placement method and the combination spacing – production verification was carried out. During the normal production process, with other variable factors such as the dip – coating process, pouring temperature, and vacuum extraction remaining unchanged, production verification was conducted from small – batch to large – batch production. The result showed that the sand adhesion defect was eliminated. Through these measures, the final process successfully resolved the sand adhesion defect at the top of the flywheel housing.
3. Porosity Defects in Flywheel Housings
3.1 Analysis of Porosity in Flywheel Housings
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 surface of the casting. The presence of pores is related to factors such as the pouring temperature, the permeability of the coating, and the pouring speed.
Take the SAIC MAXUS flywheel housing as an example. The pores are mainly located in the motor hole at the top of the product. The molten iron is discharged from the furnace at a temperature of 1460 – 1470°C (using an electric furnace as the melting equipment), and the pouring temperature is 1430 – 1440°C. The vacuum degree is – 0.025 MPa, without film coating and pressure – maintaining. The main defect is the pores in the motor hole at the top of the product, with a scrap rate of 30%.
The pores in this product are characterized by normal appearance on the surface or near – surface of the casting during visual inspection, but after processing, there are smooth holes of various sizes on the surface. The pore walls have an oxidized luster, and the pores are mainly concentrated at the top of the product, which is a typical feature of subcutaneous pores. 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 completely discharged, resulting in pores under the skin.
- Local coating thickness: If the coating on the motor hole is too thick, the gas generated after the foam burns cannot be discharged, forming pores.
- Vacuum degree: If the vacuum extraction is too small, the gas cannot be removed in time, causing pores.
- Unreasonable process design: The lack of vents at the top of the flywheel housing leads to gas accumulation at the top of the casting body, which cannot be completely discharged, forming pores.
| Defect | Product | Material | Weight | Dimensions | Wall Thickness | Process Parameters | Defect Location | Scrap Rate |
|---|---|---|---|---|---|---|---|---|
| Porosity | SAIC MAXUS Flywheel Housing | – | – | – | – | Furnace temperature: 1460 – 1470°C; Pouring temperature: 1430 – 1440°C; Vacuum degree: – 0.025 MPa | Motor hole at the top | 30% |
3.2 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. Currently, the coating thickness is 2.0 mm, and it is reduced to 0.5 mm. 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) x 30 mm (height) x 5 mm (width) and conduct 10 sets of pouring tests.
3.3 Production Verification of Porosity Control
After analyzing the reasons for the porosity in the flywheel housing and implementing four improvement measures – adjusting the pouring temperature, coating thickness, vacuum degree, and adding an exhaust piece – process tests were carried out. Using the control variable method, when other three process parameters were kept unchanged for each test:
- For the first plan of increasing the pouring temperature, 20 pieces were produced and processed, with 4 pieces having pores, and the proportion of pores in the motor hole was 20%.
- For the second plan of reducing the coating thickness, 20 pieces were produced and processed, with 5 pieces having pores, and the proportion of pores in the motor hole was 25%.
- For the third plan of increasing the vacuum degree, 20 pieces were produced and processed, with 3 pieces having pores, and the proportion of pores in the motor hole was 15%.
- For the fourth plan of adding an exhaust piece, 20 pieces were produced and processed, with 0 pieces having pores, and the proportion of pores in the motor hole was 0%.
The fourth plan showed the best results. Subsequent small – batch to large – batch verification also indicated normal processing. Through these measures, the pores in the motor hole were completely resolved.
4. Sand Washing Defects in Connecting Rod Rests
4.1 Analysis of Sand Washing in Connecting Rod Rests
During the pouring process, if the pouring system of the mold cluster, including the straight runner, cross – runner, and inner gate, is not completely sealed, especially the straight runner, siphoning is likely to occur, leading to sand washing defects. In addition, if the product pouring system is not designed reasonably, the filling is not smooth, the local pressure in the inner gate is high, and the iron – liquid scouring causes the coating to crack, which will also make the molding sand enter the mold cavity with the iron liquid, resulting in sand washing defects.
The connecting rod rest is made of HT200, weighs about 50 kg, and has a contour size of 572 mm x 380 mm x 348 mm with a bottom plate thickness of 12 mm. The existing process introduces iron liquid through 3 – point inner gates on the side, and the inner gate size is 60 mm (length) x 30 mm (height) x 6 mm (width). The process parameters are: the molten iron is discharged from the furnace at a temperature of 1460 – 1470°C (using an electric furnace as the melting equipment), the pouring temperature is 1430 – 1440°C, the vacuum degree is – 0.03 MPa, without film coating and pressure – maintaining. The main defect is sand washing, which is concentrated near the bottom inner gate, with a scrap rate of 20%.
The sand washing defect is located at the bottom of the mold cavity along the straight line of the gate and the area where the iron liquid flows into the mold cavity from the inner gate. The presence of lumps formed by a mixture of sand grains and metal in this area is a typical feature of sand washing. The main influencing factors are:
- Low coating strength of the inner gate: The scouring of the iron liquid causes the coating to crack.
- High pressure in the inner gate: This leads to the cracking of the coating.
- Although there is no severe back – splash of the iron liquid during the pouring of this product, the first two factors are still the main considerations.
| Defect | Product | Material | Weight | Dimensions | Wall Thickness | Process Parameters | Defect Location | Scrap Rate |
|---|---|---|---|---|---|---|---|---|
| Sand Washing | Connecting Rod Rest | HT200 | 50 kg | 572 mm x 380 mm x 348 mm | 12 mm | Inner gate size: 60 mm x 30 mm x 6 mm; Furnace temperature: 1460 – 1470°C; Pouring temperature: 1430 – 1440°C; Vacuum degree: – 0.03 MPa | Near bottom inner gate | 20% |
4.2 Control Measures for Sand Washing
- Add one more dip – coating process to the inner gate. Currently, the coating is dipped twice with a thickness of 1.5 mm. After adding one more dip – coating, the thickness becomes 2.2 mm.
- Increase the number of inner gates. Add another inner gate of the same size at the bottom.
4.3 Production Verification of Sand Washing Control
After analyzing the reasons for the sand washing in the connecting rod rest and implementing two improvement measures – adjusting the coating thickness of the inlet and the number of inlets – production verification was carried out. During the process test, with other influencing parameters such as the pouring temperature and vacuum degree remaining unchanged, production verification was conducted from small – batch to large – batch production.
- For the first plan of adding one more dip – coating to the inner gate, 50 pieces were produced, with 6 pieces having sand washing, accounting for 12%.
- For the second plan of adding an inner gate at the bottom, 50 pieces were produced, with 0 pieces having sand washing.
The second plan was proven to be the best. The subsequent small – batch to large – batch verification also showed normal processing. The optimized process adopted 3 inner gates at the bottom and 1 inner gate at the top. The 3 inner gates can play a role in flow – dividing and pressure – reducing. The cross – sectional area of a single inner gate is 60 mm x 8 mm; the area of the straight runner (pipe with a diameter of 50 mm), the cross – runner (50 mm x 40 mm), and the inner gate area are approximately \(1960~mm^{2}:2000~mm^{2}:1920~mm^{2}\), basically meeting the 1:1:1 condition, which completely solved the sand – washing defect of the connecting rod rest.
5. General Conclusions
- In the process of developing new products, it is essential to carefully consider the potential causes of casting defects and avoid them through reasonable process design. By understanding the characteristics of different products and the influencing factors of defects, appropriate measures can be taken in advance to prevent defects from occurring.
- When promoting process verification, the number of products should be gradually increased from small to large, starting from small – batch production and then moving to large – batch production. This approach can help to identify and address potential issues in a timely manner, avoiding significant losses caused by insufficient consideration in the process.
- During the process verification, it is necessary to follow a systematic approach, including understanding the current situation, analyzing the causes, formulating plans, implementing countermeasures, and confirming the effects. By proceeding step – by – step in these five aspects, problems can be thoroughly resolved, and the quality of the casting products can be effectively improved.
In conclusion, the lost foam casting technology has great potential in the production of various products. However, to fully utilize its advantages and ensure high – quality casting products, it is necessary to have a deep understanding of the common defects, take effective control measures, and continuously optimize the production process. This will not only improve the competitiveness of products in the market but also promote the sustainable development of the foundry industry.
