This paper focuses on the application of lost foam casting in automobile engine cylinder block castings. It introduces the characteristics of cylinder block castings, the design and implementation process of the casting scheme in detail, and analyzes the advantages and key points of lost foam casting technology in engine cylinder block production. Through practical production data and case studies, it demonstrates the feasibility and superiority of this technology in improving casting quality, production efficiency, and realizing green casting.
1. Introduction
Lost foam casting is an advanced manufacturing process that has attracted increasing attention in the foundry industry. It simplifies the traditional casting process by using foam patterns that are vaporized during casting, allowing liquid metal to fill the mold cavity to form the final casting. In the production of automobile engine cylinder blocks, which are key components reflecting the development level of the automotive industry, lost foam casting shows unique advantages.
The traditional manufacturing process of engine cylinder blocks involves multiple complex procedures such as core making, molding, and core setting. In contrast, lost foam casting reduces these steps significantly. Each casting in lost foam casting requires a foam pattern, adding pre-foaming and foam molding steps but eliminating many traditional steps related to sand mold preparation. This not only shortens the production process but also saves raw and auxiliary materials and is conducive to the recycling of used sand, reducing environmental pollution.
2. Characteristics of Cylinder Block Castings
The material of the cylinder block casting studied in this paper is HT250 low alloy casting. The chemical composition control range is as follows: 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%), Cr (0.3% – 0.5%). The tensile strength is not less than 250 MPa. The casting undergoes stress relief treatment, and the hardness is in the range of 187 – 255 HBS with a hardness difference not exceeding 40 HBS.
| Element | Composition Range |
|---|---|
| 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% |
| Cr | 0.3% – 0.5% |
The structure of the engine cylinder block is complex, usually consisting of cylinders, cylinder liner cooling water jackets, cylinder head joint surface threaded holes, valve tappet holes, main oil passage systems, oil circuit holes, oil pump holes, camshaft holes, crankshaft holes, crankcases, oil pan flanges, filter flanges, flywheel housing flanges, cooling water pump flanges, oil cooler flanges, and various reinforcing ribs and webs. These components require high precision and quality in the casting process to ensure the performance of the engine.
3. Casting Scheme Design and Implementation
3.1 Design of Cylinder Block Mold
In the design of the foam model structure process of diesel and gasoline engine cylinder blocks, different schemes exist at home and abroad, but the treatment of the parting scheme is generally the same. The parting scheme mainly considers the forming quality of the foam mold pieces, the convenience of demolding, and the gluing quality of the overall model. For the parting scheme of the engine cylinder block foam model, the common practice is horizontal layer-by-layer cutting. The principle is to ensure that the mold pieces containing the intake and exhaust ports can be smoothly demolded in a two-part mold structure. Based on several typical engine cylinder block lost foam process schemes at home and abroad and years of production experience, a horizontal parting scheme is adopted, and the crankcase is partially sealed along the demolding direction, and an equal-wall thickness hollowed-out concave treatment is made from the outer wall of the crankcase at the partially sealed part. This parting and mold splitting scheme can ensure the quality of the foam model and facilitate the subsequent casting process.
The quality of the foam pattern has a significant impact on the casting quality. In lost foam casting, 50% – 60% of the casting forming quality depends on the quality of the foam pattern, and the quality of the mold directly affects the quality of the foam pattern. Therefore, it is necessary to select a mold manufacturer with strong technical force, advanced equipment, and good reputation to ensure the production of high-quality molds.
3.2 Pre-foaming and Curing of Pattern Material
Several domestic manufacturers produce expandable polystyrene beads suitable for lost foam casting. After preliminary trials, the Jiachang brand B107 model EPS material is selected as the raw material for making foam patterns according to the characteristics of the engine cylinder block casting. To ensure that the density of the EPS foam pattern is within the range of 23 – 24 g/L, the pre-foaming bulk density of the EPS beads must be strictly controlled within the range of 20 – 21 g/L. After the EPS material is foamed by the pre-foaming machine, it needs to be cured in the curing bin for 4 – 8 hours before use. This curing process can make the foam structure more stable and improve the quality of the foam pattern.
3.3 Foam Molding and Curing
The foam molding process uses a hydraulic semi-automatic molding machine. By optimizing the mold design, the cylinder liner pattern and the crankcase can be molded at one time, which solves the problems of deformation and dimensional accuracy and minimizes the impact of adhesive on the casting quality. To reduce the adverse effects of residual water evaporation and blowing agent diffusion in the foam pattern on the casting process, the foam pattern is required to be naturally aged for 20 days at room temperature. This natural aging process can make the internal structure of the foam pattern more stable and reduce the potential defects in the casting process.
3.4 Drying of Foam Patterns
Before assembling and bonding the complete foam pattern, the foam pattern and the forming gating system need to be dried in an independent drying chamber at 55 °C ± 5 °C and a relative humidity of less than 30% until they are completely dry. This drying process is crucial to ensure the quality of the foam pattern and prevent defects such as gas holes in the casting caused by moisture in the foam pattern during the casting process.
3.5 Finishing and Bonding Assembly of Foam Patterns
After sufficient aging, the foam patterns need to be carefully trimmed to remove burrs and flash, repair damaged surfaces, and flatten the joint surfaces. Then, the dimensions and quality of the foam patterns are inspected. The qualified foam patterns and foam gating systems are bonded into model groups using cold glue and hot melt glue. For the complex structure of the engine cylinder block foam pattern, the current manual bonding method is adopted. To ensure sufficient operation time, cold glue is used to bond the parting surfaces of the cylinder block foam pattern, and the hot melt glue is used to bond the joint surfaces of the gating system. The glue should be evenly applied, and the amount of glue used should be minimized while ensuring a firm bond. After bonding, the joint is strictly sealed with double-sided tape.
3.6 Dipping Coating and Drying
The success rate of casting in lost foam casting is about 30% dependent on the lost foam coating and the application process. In the experiment, the lost foam coating produced by Sanmenxia Yangguang Foundry Materials Co., Ltd. is used for cylinder block castings. The coating is applied twice, and the foam pattern groups are dried separately according to the number of coatings. The coating thickness is strictly controlled within the range of 1.0 – 1.5 mm. The coating can play a role in protecting the foam pattern, preventing the penetration of liquid metal during casting, and improving the surface quality of the casting.
3.7 Gating System
For the structurally complex and thin-walled engine cylinder block, the design of the gating system is extremely important. The gating system of the cylinder block casting adopts a closed type, that is, , and the ratio of (1.3 – 2):(1 – 1.5):1 is selected here. One set of gating system is designed to pour two cylinder block castings, and the pouring time is controlled within 35 – 40 s. The position of the ingate is also crucial. A multi-point water inlet ingate scheme is adopted, as shown in Figure 1. This design can ensure the smooth filling of liquid metal into the mold cavity, avoid defects such as misruns and cold shuts, and improve the quality of the casting.
[Insert Figure 1: Pouring System of Cylinder Block Casting]
3.8 Molding
The molding of the cylinder block casting uses 40 – 70 mesh dry sand. Before packing the foam model, the coating needs to be carefully inspected. If any small cracks are found, they must be repaired with quick-drying coating. At the same time, the deformation of the model is checked. If there is deformation, it must be returned. A five-drawer negative pressure special sand box is used, and four foam models are buried in each box. The jolting table uses an airbag frequency modulation locking jolting table. After the sand box is locked, a bottom sand layer with a thickness of 120 mm is added. After jolting, the sand surface is scraped to form an inclined angle before placing the foam model. When placing the model, the pouring cup should be as close to the edge of the box as possible to facilitate the pouring operation. The sand filling is carried out in two times. The height of the first sand filling is at the same level as or slightly higher than the end of the cylinder block. The vibration frequency is adjusted appropriately, and the vibration time is controlled within 10 – 20 s. The second sand filling uses cover sand, and the cover sand should have sufficient thickness to ensure sufficient sand coverage and prevent the box from expanding. The height of the sand filling is such that the sand plane after jolting is 15 mm lower than the end face of the pouring cup. After the sand is compacted, the sand surface should be scraped flat. A plastic film is covered by the sand filling and burying personnel, and a protective sand layer is added after the film is covered. When the protective sand layer thickness is greater than 20 mm, it also needs to be scraped flat. The pouring cup should be fully exposed. During the burying process, the burying personnel should operate according to the process requirements. The sand box after burying should be inserted and hung with a process card according to the process requirements and moved to the pouring station.
3.9 Pouring and Cooling Shakeout
Before pouring, the pouring personnel need to check whether the protective sand layer has sufficient thickness, whether the position of the pouring cup is appropriate and aligned, whether the vacuum pump is operating normally, and whether the negative pressure is stable. A teapot ladle is selected for pouring, and the ladle needs to be baked to a dark red color before use. The pouring personnel and crane operators need to be trained and obtain certificates before taking up their posts, and the pouring is carried out by a dedicated person. Before pouring the molten iron, the ladle must be lowered to the best height and position, and the ladle spout should be as close as possible to the pouring cup to ensure that the first drop of molten iron can be accurately poured into the center of the pouring cup. When starting to pour, a small flow rate is used for trial pouring. After the pouring cup burns and emits black smoke and the sound of the molten iron absorbing water is heard, the flow rate is increased. When the sound of water absorption decreases, the flow rate is controlled in advance to change from a large flow rate to a small flow rate to ensure that the pouring cup is filled without overflowing. A 1.5-ton intermediate frequency electric furnace is used as the pouring equipment, and the pouring vacuum degree is controlled within -0.035 – -0.040 MPa. The pouring temperature requires the tapping temperature to be controlled within 1600 – 1620 °C. Currently, in the on-site pouring situation, 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 greater than 1480 °C. The casting is shaken out 1.5 hours after cooling in the sand box.
3.10 Implementation Results
The engine cylinder block castings produced by the lost foam casting process have a yield of more than 95%. The machining qualification rate of the inspected and qualified cylinder block castings reaches 99%, and the process yield of the castings is as high as 91%. These data show that the lost foam casting process has high production efficiency and good casting quality in the production of engine cylinder block castings.
4. Advantages and Key Points of Lost Foam Casting in Cylinder Block Production
4.1 Process Simplification and Cost Reduction
Compared with traditional casting processes, lost foam casting reduces many steps such as core making, molding, and core setting. Although an additional pre-foaming and foam molding process is added, the overall process is still simplified. This reduces labor costs and production time. At the same time, the reduction of raw and auxiliary materials consumption and the recycling of used sand also contribute to cost reduction.
4.2 High Casting Quality
The use of foam patterns can ensure the accuracy of the casting shape. Through reasonable design of the gating system and strict control of the process parameters, defects such as misruns, cold shuts, and gas holes in the casting can be effectively reduced. The stress relief treatment of the casting also ensures its mechanical properties.
4.3 Green Casting
Lost foam casting is a relatively environmentally friendly casting process. The use of dry sand and the recycling of used sand reduce the generation of dust and waste sand. The reduction of chemical binder usage also reduces environmental pollution.
However, there are also some key points in the application of lost foam casting in cylinder block production. For example, the quality control of foam patterns is crucial. Any defect in the foam pattern may lead to casting defects. The design of the gating system needs to be carefully considered according to the characteristics of the cylinder block to ensure the smooth filling of liquid metal. In addition, the control of process parameters such as pouring temperature and vacuum degree also requires strict attention.
5. Conclusion
Lost foam casting has shown great potential in the production of automobile engine cylinder block castings. Through reasonable design of the casting scheme and strict control of the process, high-quality cylinder block castings can be produced with high yield and process yield. This technology not only meets the requirements of the development of the automotive industry for engine cylinder block manufacturing but also promotes the development of green casting. In the future, with the continuous improvement of technology and the deepening of research, lost foam casting is expected to play a more important role in the field of engine cylinder block production.
It should be noted that in the actual production process, continuous optimization and improvement are needed according to specific production conditions and product requirements to ensure the stable operation of the process and the continuous improvement of product quality. At the same time, strengthening the training of technical personnel and improving their technical level can also better promote the application and development of lost foam casting technology.
In conclusion, the application of lost foam casting in automobile engine cylinder block castings is a promising research and development direction, which has important significance for promoting the development of the automotive industry and the foundry industry.
