Abstract: The various defects encountered during the production of oil pans using the lost foam casting process, including deformation, metal penetration, dross inclusion, shrinkage porosity, and substandard metallographic structure. Through optimization measures targeting the oil pan’s structural design, gating system, smelting and compounding, as well as the casting process, we have achieved significant improvements. The results indicate that by standardizing the white mold baking method and buried box molding process, using fiber sticks for fixation and bonding, and adhering to specific process parameters, we can effectively control casting deformation. Selecting molding sand with particle sizes ranging from 0.4 to 0.8 mm, maintaining a vibration frequency of ≥40 Hz during molding, and ensuring a casting negative pressure of ≥0.04 MPa, along with a pouring temperature between 1420 and 1460 °C, helps improve metal penetration defects. Additionally, employing large particle size fluxing media reduces surface dross. By setting blind risers at locations prone to cold shuts and adopting a closed gating system with cylindrical straight sprues, we can minimize cold shuts and shrinkage porosity defects. Controlling the Si/C ratio in raw materials between 0.6 and 0.7 ensures stable pearlite content and metallographic structure compliance.
1. Introduction
The oil pan, also known as the lower crankcase, serves as the lower half of the crankcase in engines. It stores engine oil, which returns to the oil pan due to gravity when the engine is not running. Once the engine starts, the oil pump transports the oil to various lubricating parts of the engine. The oil pan is a crucial component for enclosing the crankcase as an oil reservoir, preventing impurities from entering, collecting and storing lubricating oil that flows back from various friction surfaces of the diesel engine, dissipating heat, and preventing lubricant oxidation.
The material of the oil pan casting is HT250, with a wall thickness ranging from 6 to 30 mm and contour dimensions of 601.5 × 32 × 163 mm. The structural shape and machining allowances for various surfaces are specified as per design requirements.
Conventionally, the oil pan was produced using sand-mold casting, but due to high production costs and significant environmental pollution, it is gradually being replaced by lost foam casting. Lost foam casting is an emerging casting process that offers advantages such as small machining allowances, precise molding, simplified production steps, shortened process flows, improved production efficiency, and reduced labor intensity.
Discusses and analyzes the lost foam casting process technology for a specific type of oil pan produced by our company.
2. Industrial Test Scheme
2.1 Process Challenges
The oil pan belongs to small-to-medium-sized thin-walled shell components. Casting molding and shape and size inaccuracies often lead to defects such as deformation and inconsistent flange widths. The selection of models, coatings, molding, and process methods are crucial in determining the casting molding performance and stability of shape and size. Based on structural size analysis, the main casting defects identified are dross inclusion, metal penetration, and cold shuts.
2.2 Lost Foam Casting Process Design
The process principle of lost foam casting involves a double-cycle separation line design layout, comprising mold making, mold bonding and coating, sand preparation, casting cleanup, electric furnace melting, and machining processes. This paper focuses on mold making and pouring procedures.
3. Existing Issues
During production trials, castings exhibited surface defects such as deformation (insufficient machining allowance), dross inclusion, metal penetration, and cold shuts, as well as internal defects such as porosity and shrinkage, and substandard metallographic structure, severely impacting the product qualification rate.
4. Analysis of Oil Pan Lost Foam Casting Defects
4.1 Deformation Defect Analysis
The oil pan is a thin-walled shell component with a large internal cavity and thin walls, prone to deformation during the lost foam casting process, mainly concentrated on the wide edges on both sides. During trial production, the waste rate due to excessive deformation reached 50%.
4.2 Metal Penetration Defect Analysis
Poor pouring practices allow molten iron to penetrate the model or impact the molding sand, mixing the washed-off coating or molding sand into the molten metal, forming metal penetration defects after solidification. Inadequate slag removal, shielding, and discharging processes result in the formation of dross defects when solid and liquid phase products cannot be timely removed and remain inside the casting.
4.3 Cold Shut and Shrinkage Defect Analysis
Thin-walled shell castings exhibit slower molten iron flow during pouring compared to thick castings, with longer flow durations and faster temperature drops. In areas where molten iron arrives last, excessive heat consumption prevents complete fusion, leading to cold shuts. An unreasonable gating system results in insufficient molten iron replenishment to areas requiring feeding before internal solidification, causing porosity and shrinkage defects.
5. Improvement Measures for Oil Pan Lost Foam Casting Defects
Based on the analysis of causes for deformation, dross inclusion, metal penetration, cold shuts, porosity, and shrinkage defects in oil pan castings, process experiments were conducted for improvement and optimization, perfecting production processes and procedures.
5.1 Deformation Defect Improvement Measures
After white mold forming, inspect flatness on the inspection platform and measure dimensions. Allowable deviations for white molds are -1 mm and +3 mm, with specific main control dimensions detailed in Table 1.
| Item | Drawing Final Size | Casting Required Size | Mold Size | White Mold Inspection |
|---|---|---|---|---|
| Inner Cavity Total Length | 551 | 551 | 556 | 556 |
| Flywheel Housing Total Length | 601.5 | 604 | 611 | 611~611.5 |
| Connecting Piece Tower Opening | 304 | 308 | 312 | 312 |
| Combination Surface to Bottom Tower | 163 | 168 | 171 | 171 |
| Block Side Width at Machine Surface | 287 | 287 | 291 | 291 |
Table 1: Model Size Checklist
Dry white molds flatly with multiple points of support. Measure humidity after drying using a hygrometer before proceeding to the next step. Use sizing templates for dimensional stability and fiber sticks for bonding, as illustrated. Re-measure dimensions after bonding to ensure compliance before coating.
| Drying Step | Drying Temperature (°C) | Drying Time (h) |
|---|---|---|
| First Coat | 45 | 10 |
| Second Coat | 40 (50 for corner touch-ups) | 14 |
Table 2: White Mold Drying Temperature and Time
During box burial and molding, spread the base sand evenly to fix the model. Clamp the sand box on the vibrating table for bottom vibration before adding sand layer by layer, compacting uniformly. Fabricate positioning devices for box burial and molding to ensure the burial angle. Misalign the burial positions, add sand in layers, compact uniformly, and manually fill and compact sand at corners. These measures significantly reduced the occurrence of deformation defects, lowering the waste rate due to deformation to within 3%.
5.2 Metal Penetration Defect Improvement Measures
The coating should adhere firmly to the white mold, be dense with sufficient strength to prevent peeling during operations and cracking during vibration due to sand molding. Incorporate 3% bentonite, 15% graphite powder, and 15% quartz powder into the coating to enhance strength and fluidity, ensuring a uniform coating thickness of no less than 1.6 mm.
Select molding sand suitable for dry sand molding, preferably circular or nearly circular grains of the specified mesh size. The compaction force should not be excessive to prevent coating damage during molding, and vibration time should not be too long to avoid coating cracking. Experiments determined that the total vibration time should be no less than 360 s at a frequency of 40 Hz and 280 s at 50 Hz. Additionally, manually fill and compact sand at corners to ensure high compaction in dead spaces.
5.3 Dross Inclusion Defect Improvement Measures
Regularly blow down furnace platforms, surfaces, and surrounding areas; repair furnace nozzles promptly; perform slag removal frequently; conduct slag removal in four steps, including two steps under high power and two steps at the furnace mouth during static conditions; repair and clean furnace nozzles and pouring nozzles, apply repair materials, and clean the ladle before tapping; perform slag removal in three steps within the ladle: once after tapping, once after adding a fluxing agent and transporting the ladle to the pouring area for static conditions, and once more during pouring while covering with a fluxing agent, leaving only one pouring port with fiber blanket for slag shielding, and adding a vent and slag discharge port at the upper machined surface.
5.4 Cold Shut and Shrinkage Defect Improvement Measures
Increase the tapping temperature, pouring temperature, and pouring speed of molten iron, raising the tapping temperature to 1540 °C ( 1560 °C when the ladle is cold). Adopt the operation method of rapid pouring. The design of the gating system is a critical process in lost foam casting, and the cross-sectional areas of the various components of the gating system need to be calculated and determined. Improvements and optimizations are made through process experiments to make it reasonable [8]. Optimize the shape of the sprue, adopt a cylindrical sprue, and use a top-pouring inclined pouring process. The cross-sectional dimensions of the gating system are selected according to the design principles, and the cross-sectional area ratio of the gating system is controlled at direct sprue: cross sprue: ingate = 7:1:0.4. A medium-pour gating system is adopted, with water entering at two points, which can not only skim but also ensure the amount of feeding.
By implementing the above improvement measures and formulating process standards and specifications, six batches of oil pans were trial-produced according to the formulated process standards, with 32 oil pans per batch. Batch production tests have proven that: using support ribs can prevent mold deformation; selecting a cylindrical sprue effectively solves the problem of slag inclusion; adjusting the vibration parameters and manual auxiliary sand burial measures solve the problem of iron-coated sand defects; increasing the tapping temperature and pouring temperature of molten iron and adopting rapid pouring methods have effectively controlled the cold shut defects; improving the pouring system and adjusting the composition ratio effectively improve internal defects and structure of the castings. The castings have a finished product rate of up to 96%, with qualified castings after pouring and processing.