Lost foam casting is an advanced manufacturing process that has gained significant attention in the field of automotive engine casting. This technique offers several advantages over traditional casting methods, including near-net shape forming, precise dimensional accuracy, and the ability to produce complex geometries. In this article, we will explore the application of lost foam casting in the production of automotive engine cylinder block castings, focusing on its benefits, process steps, and key considerations.
I. Introduction
The cylinder block is a critical component of an automotive engine, and its manufacturing quality directly affects the performance and reliability of the engine. Traditional casting methods often involve multiple complex steps and can result in material waste and environmental pollution. Lost foam casting, on the other hand, provides a more efficient and environmentally friendly alternative.
II. Characteristics of Cylinder Block Castings
Cylinder block castings are typically made of materials such as HT250 low-alloy cast iron. The chemical composition of the material is carefully controlled to ensure the required mechanical properties, such as a tensile strength of not less than 250 MPa and a hardness of 187 – 255 HBS with a hardness difference of no more than 40 HBS. The castings also undergo stress relief treatment to improve their dimensional stability.
III. Lost Foam Casting Process for Cylinder Block Castings
- Cylinder Mold Design
- Different design schemes exist for the foam model structure of diesel and gasoline engine cylinder blocks, but the 分型方案 is generally handled in the same way.
- The horizontal layering cutting is commonly adopted to ensure smooth demolding of the mold pieces containing the intake and exhaust channels.
- Choosing a reliable mold manufacturer with strong technical capabilities and advanced equipment is crucial for ensuring the quality of the foam patterns.
- Mold Material Pre-expansion and Curing
- Several domestic manufacturers produce expandable polystyrene (EPS) beads suitable for lost foam casting. After pre-expansion in a pre-expansion machine, the EPS beads are cured in a curing chamber for 4 – 8 hours before use.
- The density of the EPS foam pattern is strictly controlled within the range of 23 – 24 g/L by controlling the pre-expansion bulk density of the EPS beads.
- Foam Molding and Curing
- Hydraulic semi-automatic molding machines are used to optimize the mold design and achieve one-time molding of the cylinder liner pattern and the crankcase, minimizing deformation and ensuring dimensional accuracy.
- To reduce the adverse effects of residual moisture and foaming agent evaporation, the foam patterns are naturally aged for 20 days at room temperature.
- Drying of Foam Patterns
- Before assembling the foam patterns and the gating system, they are dried in an independent drying room at a temperature of 55°C ± 5°C and a relative humidity of less than 30% until they are completely dry.
- Finishing and Bonding of Foam Patterns
- After sufficient aging, the foam patterns are carefully trimmed to remove burrs and repair damaged surfaces.
- Cold glue is used to bond the 分型分模 surfaces of the cylinder block foam patterns, while the hot melt glue is used for the bonding surfaces of the gating system. Minimal glue is used to ensure a strong bond while minimizing its impact on the casting quality.
- Coating and Drying
- The success rate of lost foam casting depends largely on the quality of the coating and the coating process. In this case, a coating from Sanmenxia Yangguang Casting Materials Co., Ltd. is used for the cylinder block castings, with two layers of coating applied and dried separately.
- The coating thickness is strictly controlled within the range of 1.0 – 1.5 mm.
- Gating System
- For complex and thin-walled engine cylinder blocks, the design of the gating system is of utmost importance.
- A closed gating system is used, with a ratio of (1.3 – 2) : (1 – 1.5) : 1. One set of the gating system is designed to cast two cylinder block castings, and the pouring time is controlled within 35 – 40 seconds.
- A multi-point inlet gating scheme is adopted for the inner gates, as shown in the following table:
| Gating Scheme | Details |
|---|---|
| Inner Gate Location | Multiple points around the cylinder block to ensure even filling of the mold. |
| Pouring Time Control | 35 – 40 seconds to prevent cold shut and other defects. |
- Molding
- 40 – 70 mesh dry sand is used for the molding of the cylinder block castings.
- Before packing the foam patterns into the sand box, the coating is carefully inspected for any cracks, which are repaired with quick-drying coating. The models are also checked for deformation.
- A five-drawer negative pressure sand box is used, with four foam models placed in each box.
- The vibration table is a balloon-frequency-modulated locking vibration table. After locking the sand box, a bottom layer of sand with a thickness of 120 mm is added, and then the foam models are placed with the pouring cup as close to the box edge as possible for easy pouring.
- The filling of sand is done in two steps. In the first step, the sand is filled to a height equal to or slightly higher than the end of the cylinder block, and the vibration time is controlled within 10 – 20 seconds. In the second step, covering sand is used to ensure sufficient sand consumption and prevent mold expansion.
- The height of the sand filling is determined such that the sand surface is 15 mm below the end face of the pouring cup after vibration.
- A plastic film is used to cover the sand, and a protective sand layer with a thickness of more than 20 mm is added and flattened. The pouring cup should be fully exposed.
- Pouring and Cooling
- The pouring personnel check the thickness of the protective sand layer, the position of the pouring cup, and the operation of the vacuum pump.
- A teapot-shaped ladle is used for pouring, and the ladle is heated to a dark red color before use.
- The pouring is controlled to ensure that the first drop of molten iron accurately falls into the center of the pouring cup. The pouring starts with a small flow rate, and then the flow rate is increased after the pouring cup starts to burn and emit black smoke. The flow rate is reduced before the cup is filled to prevent overflow.
- A 1.5-ton medium-frequency electric furnace is used as the pouring equipment, and the vacuum degree is controlled within -0.035 to -0.040 MPa.
- The pouring temperature is controlled at 1600 – 1620°C, and the final pouring temperature of the cylinder block castings should be higher than 1480°C. The castings are cooled in the sand box for 1.5 hours before being removed.
- Implementation Results
- The use of lost foam casting for engine cylinder block castings results in a high yield rate of more than 95%, a processing qualification rate of 99% for the inspected castings, and a high process yield rate of 91%.
IV. Benefits of Lost Foam Casting in Cylinder Block Casting
- Simplified Process Flow
- Lost foam casting eliminates many cumbersome steps such as core making, molding, and core setting, reducing the overall process complexity.
- Material Savings
- This technique allows for more efficient use of materials, reducing material waste compared to traditional casting methods.
- Improved Casting Quality
- The precise molding capability of lost foam casting results in better dimensional accuracy and surface finish of the castings.
- Environmental Friendliness
- It is a cleaner production process that reduces the generation of waste and emissions, contributing to sustainable manufacturing.
V. Key Considerations in Lost Foam Casting
- Foam Pattern Quality
- The quality of the foam pattern directly affects the casting quality. Factors such as pattern density, dimensional accuracy, and surface finish need to be carefully controlled.
- Coating Selection and Application
- The choice of coating and its proper application are crucial for ensuring the success of the casting process. The coating should have good thermal stability, permeability, and adhesion properties.
- Pouring Parameters Control
- Parameters such as pouring temperature, pouring time, and vacuum degree need to be accurately controlled to prevent defects such as cold shut and porosity.
- Equipment and Process Stability
- Reliable equipment and a stable process are essential for ensuring consistent casting quality and productivity.
VI. Future Trends and Developments in Lost Foam Casting
- Advanced Materials and Coatings
- The development of new materials and coatings will further improve the performance and quality of lost foam castings.
- Process Optimization and Automation
- Continued efforts in process optimization and the introduction of automation will increase the efficiency and reliability of the casting process.
- Integration with Other Technologies
- Lost foam casting is likely to be integrated with other advanced manufacturing technologies, such as 3D printing and simulation software, to achieve more precise and efficient production.
VII. Conclusion
Lost foam casting offers significant advantages in the production of automotive engine cylinder block castings. By simplifying the process flow, saving materials, improving casting quality, and reducing environmental impact, it has become an increasingly attractive option for the automotive industry. However, to fully realize its potential, careful attention must be paid to the quality of the foam patterns, the selection and application of coatings, the control of pouring parameters, and the stability of the equipment and process. With continued research and development, lost foam casting is expected to play an even more important role in the future of automotive manufacturing.