Lost foam casting is an advanced manufacturing process that has gained significant attention in the automotive industry, particularly for the production of engine cylinder block castings. This technique offers several advantages over traditional casting methods, including near-net shape forming, reduced material consumption, and improved environmental sustainability. In this article, we will explore the application of lost foam casting in automotive engine cylinder block castings in detail.
Introduction
Lost foam casting is a novel casting method where a foam pattern similar in size and shape to the desired casting is bonded together to form a pattern cluster. The pattern is then coated with a refractory coating, dried, and buried in dry sand for vibration molding. During the pouring process, the foam pattern is vaporized under normal pressure or negative pressure, allowing the liquid metal to occupy the pattern’s position. Upon solidification and cooling, the casting is formed. This process eliminates the need for many cumbersome procedures such as core making, molding, and core setting, as each casting consumes one foam pattern.
The cylinder block of an engine is a complex component consisting of various parts such as cylinders, cooling water jackets, strong screw holes for the cylinder head joint surface, valve tappet holes, main oil passage system, oil return holes, oil pump holes, camshaft holes, crankshaft holes, crankcase, oil pan flange, filter flange, flywheel housing flange, cooling water pump flange, oil cooler flange, and various reinforcing ribs and webs. The manufacturing level of the engine cylinder block reflects the development level of a country’s automotive industry, and cylinder block castings are considered high-end casting products.
Characteristics of Cylinder Block Castings
The material of the cylinder block casting is HT250 low-alloy cast iron, with the chemical composition controlled within the following ranges: C (3.10% – 3.30%), Si (1.60% – 1.80%), Mn (0.60% – 0.75%), P (0.040% – 0.050%), S (0.050% – 0.060%), Cu (0.6% – 1.0%), and Cr (0.3% – 0.5%). The tensile strength should be no less than 250 MPa. The casting undergoes stress relief treatment, with a hardness of 187 – 255 HBS and a hardness difference of no more than 40 HBS.
Casting Scheme and Implementation
- Design of the Cylinder Block Mold
- Different schemes exist for the structural process design of the foam model of diesel and gasoline engine cylinder blocks worldwide, but the handling of the parting scheme is consistent. The parting scheme is mainly considered for the molding quality, demolding convenience, and overall model gluing quality of the foam mold. For the parting scheme of the engine cylinder block foam model, a common practice is horizontal layer-by-layer cutting. The principle is to ensure that the mold piece containing the intake and exhaust channels can be demolded smoothly in the two-opening mold structure. Based on several typical and representative lost foam process schemes for engine cylinder blocks at home and abroad, combined with years of production practice, the horizontal parting, and the local sealing of the crankcase along the demolding direction, with an equal wall thickness hollow concave treatment on the local sealing part, is the preferred parting and mold division scheme. Figure 1 shows the parting and mold division effect of the engine cylinder block casting according to this scheme.
[Insert Figure 1: Parting and Mold Division Effect of the Cylinder Block]- When the equipment for lost foam casting meets the casting process requirements, 50% – 60% of the casting molding quality depends on the quality of the foam pattern, and the mold manufacturing quality directly affects the quality of the foam pattern. Therefore, in the production of foam molds, it is essential to choose a mold manufacturer with strong technical strength, advanced equipment, and good faith.
- Pre-expansion and Curing of Mold Material
- Several domestic manufacturers produce expandable polystyrene beads that meet the requirements of lost foam casting. Based on the characteristics of the engine cylinder block casting, after preliminary trials, Jiachang brand B107 EPS material is selected as the raw material for making the foam pattern, and the density of the EPS foam pattern is strictly controlled within the range of 23 – 24 g/L. To achieve this pattern density, the pre-expansion bulk density of the EPS beads must be strictly controlled at 20 – 21 g/L. After pre-expansion by the pre-expansion machine, the EPS material is cured in the curing warehouse for 4 – 8 hours before use.
- Foam Molding and Curing
- A hydraulic semi-automatic molding machine is used for foam molding. The optimized mold design enables one-time molding of the cylinder liner pattern and the crankcase, solving the problems of deformation and dimensional accuracy while minimizing the impact of adhesive on the casting quality. To reduce the adverse effects of the evaporation of residual moisture in the foam pattern and the diffusion and evaporation of the foaming agent on the casting process, the foam pattern is required to undergo natural aging for 20 days at room temperature.
- Drying of the Foam Pattern
- Before assembling and bonding the complete pattern, the foam pattern and the forming pouring system need to be dried in an independent drying room at 55°C ± 5°C and a relative humidity of less than 30% until they are completely dry.
- Finishing and Bonding Combination of the Foam Pattern
- After sufficient aging, the foam pattern needs to be carefully trimmed to remove flash and burrs, repair the damaged surface, and smooth the joint surface. The trimming quality and key geometric dimensions of the foam pattern should be checked. The fully qualified foam pattern, foam pouring system, etc., are bonded into a model group using cold glue and hot melt glue. The foam pattern structure of the engine cylinder block is complex, and currently, manual bonding is adopted. To ensure sufficient operating time, cold glue is used to bond the parting surfaces of the cylinder block foam pattern, and the pouring system is bonded using the hot melt glue. When operating, the glue should be applied evenly. Under the premise of ensuring a firm bond, as little glue as possible should be used, and after bonding, it should be tightly sealed with double-sided adhesive tape.
- Coating and Drying
- About 30% of the success rate of casting molding in lost foam casting depends on the lost foam coating and the coating process. In this experiment, the lost foam coating produced by Sanmenxia Sunshine Casting Materials Co., Ltd. is used for the cylinder block casting, and two coats are applied. The foam pattern group is dried separately according to the number of coats. The coating thickness is strictly controlled within 1.0 – 1.5 mm.
- Pouring System
- For the complex and thin-walled engine cylinder block, the design of the pouring system is particularly important. Two factors need to be considered in the design of the pouring system: one is the form of the pouring system, and the other is the setting position of the inner gate. The pouring system of the cylinder block casting adopts a closed type, that is, F_loss > F_inner > F_straight. Here, a ratio of (1.3 – 2) : (1 – 1.5) : 1 is selected, and one set of the pouring system is designed to pour two cylinder block castings. The pouring time is controlled within 35 – 40 seconds. The position design of the inner gate of the cylinder block casting is very important, and a multi-point inlet inner gate scheme is adopted, as shown in Figure 2.
[Insert Figure 2: Pouring System of the Cylinder Block Casting] - Molding
- 40 – 70 mesh dry sand is selected for the molding of the cylinder block casting. Before packing the foam model into the box, a careful inspection of the coating should be conducted. Any small cracks must be repaired with quick-drying coating, and at the same time, the model should be checked for deformation. If there is any deformation, it must be returned. A five-pull negative pressure special sand box is used, and four foam models are buried in each box. An airbag frequency-modulated locking vibration table is used for the vibration table. After the sand box is locked, the bottom sand is added with a thickness of 120 mm. After vibration, the inclined angle is scraped out, and then the foam model is placed. When placing the model, the pouring cup should be placed as close to the box edge as possible to facilitate the pouring operation. The sand filling is carried out in two steps. In the first step, the filling height is level with or slightly higher than the end of the cylinder block. The appropriate frequency is adjusted for vibration, and the vibration time is controlled within 10 – 20 seconds. In the second step, the covering sand is used, and the covering sand should have sufficient thickness to ensure sufficient sand consumption and prevent the box from expanding. The pouring system of the cylinder block casting has been determined, and the filling height is based on the sand plane being 15 mm below the end face of the pouring cup after vibration. After the molding sand is vibrated, the sand surface should be scraped flat and not be in a hilly shape. The plastic film is covered by the sand filling and burying personnel, and after the film is covered, protective sand should be added. The thickness of the protective sand layer should be > 20 mm and also scraped flat. The pouring cup should be fully exposed. During the burying process, the burying personnel should operate according to the process requirements. After the buried sand box is completed, the process card should be inserted and hung according to the process requirements and moved to the pouring station.
- Pouring, Cooling, and Shakeout
- The pouring personnel should check whether the protective sand layer has sufficient thickness, whether the position of the pouring cup is suitable, whether the position is aligned, whether the vacuum pump is operating normally, and whether the negative pressure is stable. A teapot ladle is selected for the pouring ladle, and the ladle should be baked to a dark red color before use. The pouring personnel and the crane operator should be trained and certified to operate. During the pouring of molten iron, the ladle must be lowered to the optimal height and position, and the nozzle of the ladle should be as close to the pouring cup as possible to ensure that the first drop of molten iron can accurately pour into the center of the pouring cup. At the beginning of pouring, a small flow rate is used for a trial pour. After the pouring cup starts to burn and emits black smoke, and a sound of molten iron absorption is heard, the flow rate is increased. When the sound of absorption decreases, it is judged by experience that the cup is almost full, and the flow rate is controlled in advance, changing from a large flow rate to a small flow rate to ensure that the pouring cup is filled without overflowing.
A 1.5-ton medium-frequency electric furnace is used as the pouring equipment, and the vacuum degree during pouring is controlled within -0.035 to -0.040 MPa. The pouring temperature requires the tapping temperature to be controlled within 1600 – 1620°C. Currently, the on-site pouring situation is that each ladle of molten iron pours 4 boxes, and each box contains four cylinder blocks. The final pouring temperature of the cylinder block casting should be > 1480°C, and the casting starts to be shaken out after cooling in the sand box for 1.5 hours. Figure 3 shows the cleaned cylinder block casting.[Insert Figure 3: Cleaned Cylinder Block Casting] - Implementation Results
- Using the lost foam casting process, the yield rate of the engine cylinder block castings is > 95%, the processing qualification rate of the inspected qualified cylinder block castings is 99%, and the process yield rate of the castings is as high as 91%, as shown in Figure 4.
[Insert Figure 4: Cylinder Block Parts after Machining]
Conclusion
- Through the optimization design of the pattern 分片 process and the verification of production practice, analyzing the structural characteristics of the cylinder block castings, different mold structures are adopted for different structures of the cylinder block. By optimizing the design of the mold pieces, some process problems in lost foam casting can be solved.
- Foundry technicians must conduct process analysis. If the lost foam casting process cannot meet the requirements in some local areas of the casting, other casting process methods can be used to make up for it to ensure that the technical parameters of the casting meet the requirements of the lost foam casting process.
In summary, lost foam casting offers great potential for the production of high-quality automotive engine cylinder block castings. By carefully controlling each step of the process, from mold design to pouring and cooling, manufacturers can achieve excellent results in terms of yield rate, casting quality, and process efficiency. Continued research and innovation in this field will further enhance the competitiveness of the automotive industry and promote sustainable manufacturing practices.