Abstract
The manufacturing level of engine blocks is one of the important indicators to measure a country’s manufacturing industry, representing the development level of its automobile industry. Continuously improving engine efficiency, reducing fuel consumption, and minimizing exhaust emissions are both intrinsic demands for the automotive industry’s self-development and objective requirements from the external environment. Lost foam casting is one of the near-net-shape and precision forming processes, easily enabling cleaner production. The basic principle of lost foam casting involves using a foam plastic pattern that is completely identical in shape to the required casting, with an allowance for alloy contraction, to replace the molding for shaping. The foam pattern is not removed, forming solid casting, and metal liquid is poured in to vaporize it, forming the casting. The lost foam casting process uses dry sand without binders and additives (such as coal dust, bentonite, water, etc.), which saves a significant amount of raw and auxiliary materials, facilitates the recycling of used sand, and reduces environmental pollution. Lost foam casting technology is praised as the “green casting engineering.”
Introduction
Lost foam casting is a novel casting method where foam models similar in size and shape to the castings are bonded and assembled into clusters, coated with refractory paint, dried, buried in dry sand, and vibrated for shaping. Under atmospheric or negative pressure, the models are poured to gasify, and liquid metal takes the place of the models, solidifying and cooling to form castings. For each casting produced by lost foam casting, a foam model is consumed, adding pre-foaming and foaming molding processes while reducing many cumbersome processes such as core sand preparation, core making, molding, and core setting. Engine blocks typically consist of cylinders, cylinder block cooling jackets, strong bolt holes for cylinder head mating surfaces, 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, various reinforcing ribs and plates, etc. The manufacturing level of engine blocks is a vital indicator for measuring a country’s manufacturing industry and belongs to high-end casting products. Chengdu Kastine Company has provided a full set of lost foam casting production technology for engine blocks to three casting production enterprises nationwide, with normal mass production and a first inspection pass rate of over 95%.
Table 1: Key Process Steps and Descriptions in Lost Foam Casting of Engine Blocks
| Step Number | Process Step | Description |
|---|---|---|
| 1.1 | Cylinder Block Mold Design and Manufacturing | Foam model structural design differs for diesel and gasoline engine blocks, with consistent treatment of parting schemes. The preferred approach is horizontal layer-by-layer cutting, ensuring smooth ejection of mold pieces containing intake and exhaust ports within a two-part mold structure. The optimal parting and molding scheme involves horizontal parting and partially sealing the crankcase along the ejection direction with equal-wall-thickness hollowed-out concave areas on the crankcase outer wall. |
| 1.2 | Model Material Pre-foaming and Aging | EPS (expandable polystyrene) beads of type B107 from Jiachang are selected as raw materials for foam models, with a strict density control of 23-24g/L. Pre-foamed EPS beads must have a stacking density of 20-21g/L to achieve the desired foam model density. After pre-foaming, EPS beads are aged in a warehouse for 4-8 hours before use. |
| 1.3 | Foam Molding and Aging | Foam molding employs a hydraulic semi-automatic molding machine, achieving one-time molding of cylinder liners and crankcase boxes through optimized mold design, fundamentally solving deformation and dimensional accuracy issues while minimizing the impact of glue on casting quality. Foam models are aged naturally at room temperature for 20 days. |
| 1.4 | Foam Model Drying | Before assembly and bonding, foam models and pouring systems are dried in an independent drying room at 55±5°C and relative humidity below 30% until completely dry. |
| 1.5 | Foam Model Finishing and Bonding Assembly | After aging, foam models are carefully trimmed, removing flashes and burrs, repairing damaged surfaces, leveling mating surfaces, and checking for quality and critical dimensions. Qualified foam models and pouring systems are bonded using cold glue and hot-melt glue to form model assemblies. |
| 1.6 | Coating Application and Drying | The success of lost foam casting depends about 30% on the coating and application process. Commercial lost foam coating from Sanmenxia Yangguang Casting Materials Co., Ltd. is used, with two coats applied and dried separately. Coating thickness is strictly controlled at 1.0-1.5mm. |
| 1.7 | Gating System | For complex and thin-walled engine blocks, gating system design is crucial, involving gating system type and position of ingates. A closed gating system (F horizontal > F internal > F vertical) with a ratio of generally (1.3-2):(1-1.5):1 is used, pouring two cylinder block castings per system in 35-40 seconds. |
| 1.8 | Molding | 40-70 mesh dry sand is used for molding. Coatings are carefully inspected before boxing, and cracks are repaired with quick-drying coating. Models are checked for deformation. A five-drawer negative pressure special sandbox is used, with four foam models buried per box. |
| 1.9 | Alloy Iron Melting | The engine block material is HT250 low-alloy casting (chemical composition: 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%), with tensile strength not less than 250 MPa and hardness of 187-255HBS, processed to relieve internal stresses. A 1.5-ton medium-frequency induction furnace is used, with a tapping temperature controlled at 1600-1620°C. |
| 1.10 | Pouring and Cooling Sand Removal | Pouring personnel check the protective sand layer thickness and pouring cup position. The vacuum pump is checked for normal operation and stable negative pressure. Pouring is done using a kettle ladle heated to a dull red color. Pouring temperature should be above 1480°C. Castings are cooled in the sandbox for 1.5 hours before sand removal. |
Conclusion
- Advantages of Lost Foam Casting for Engine Blocks: The cylinder block casting blanks produced by adopting the lost foam process have a finished product rate of over 95%, a machining qualified rate of 99% for inspected castings, and a casting process yield rate of 91%.
- Optimized Mold Design: Based on the structural characteristics of cylinder block castings, different mold structures are adopted for different cylinder blocks. Mold optimization is employed to solve some process issues in lost foam casting.
- Process Analysis by Casting Technicians: If certain areas of the casting cannot meet the requirements of the lost foam casting process, other casting processes can be used for compensation to ensure that the casting process parameters meet the requirements of lost foam casting.