Lost Foam Casting of Oil Pan: Analysis of Defects and Optimization Measures

1. Introduction

Lost foam casting is a modern casting process that has gained significant popularity due to its numerous advantages. It offers reduced machining allowances, precise forming, simplified production procedures, shortened process flows, increased production efficiency, and decreased labor intensity. In the case of oil pan production, this process has been adopted to replace traditional sand casting methods. However, like any manufacturing process, lost foam casting of oil pans is not without its challenges. This article focuses on the analysis of casting defects encountered during the lost foam casting of oil pans and the corresponding optimization measures implemented to improve the quality of the castings.

2. Oil Pan and Lost Foam Casting Process Overview

2.1 Oil Pan Characteristics

The oil pan is an essential component of the crankcase, serving as a reservoir for engine oil. It is typically made of HT250 with a wall thickness ranging from 6 to 30 mm and has a contour size of (601.5×32×163) mm. It plays a crucial role in the engine’s lubrication system, storing oil when the engine is not running and supplying it to the lubricated parts when the engine is in operation. It also helps in preventing impurities from entering the crankcase and dissipates heat to some extent.

2.2 Lost Foam Casting Process for Oil Pan

The lost foam casting process for the oil pan in this study involves several key steps:

White Model Production: H – S beads with a density controlled between (22 – 26) g/L are used. The beads are heated in a molding machine at (40 – 50) °C for 3 minutes for proper maturation. Before drying, necessary corrections are made to the white model. For thin – walled parts, fiber rods and coated sand gaskets are used for support, and a shaping card board is made to ensure uniform machining allowances.
Pouring System Design: A closed pouring system is adopted for thin – walled parts. Foam boards are cut for the pouring system, which has a hollow straight runner and is a top – pouring type with two inlet points.
Coating Application: A water – based coating specific to lost foam casting is used. The coating layer is required to be uniform, without any missed spots or accumulations. The model (including the riser, pouring system, and casting) is placed in a container, and a combination of dipping and brushing methods is used to apply the coating. The riser is wrapped with fiber cloth to thicken the coating layer. After coating, the model is baked at different positions for each coating layer, and a shaping support is made to prevent baking deformation.
Molding: Sand with a particle size of (0.4 – 0.8) mm is used for molding. The yellow model is placed in the sand box, and the bottom sand is first spread to fix the model. The sand box is then clamped on a vibrating table for bottom vibration. Sand is added while vibrating until it reaches three rib positions, then the vibration and sand addition are paused for a while before continuing. The sand addition stops when it covers the top of the oil pan by about 80 mm, and the total vibration time is not less than 360 s with a vibration frequency of (40 – 45) Hz.
Melting and Pouring: The melting charging sequence is waste steel first, then return material, and finally ferrosilicon. Manganese iron is added before tapping, and a carburizing agent (low sulfur) is added during the process. After all the furnace materials are melted into a liquid, a covering agent (slag remover) is added to protect the molten iron from oxidation. The temperature of the molten iron in the furnace is sampled and analyzed for chemical composition when it reaches the sampling temperature. After the composition is qualified, inoculation treatment is carried out. The molten iron is poured into a ladle and further inoculated in the ladle. The pouring temperature is controlled within (1380 ± 20) °C, and the time from tapping to pouring is (2 – 3) minutes for standing and slag removal again. The single casting time is controlled within 30 s, and the single box pouring time is within 3 minutes. After pouring, the pressure is maintained for 3 minutes.
Post – processing: The pouring riser is cut using a diamond cutting blade, and the casting is subjected to shot blasting treatment using a “Q3716 hook – type shot blasting machine” with 4 pieces processed at a time until the surface is clean and free of debris.

3. Casting Defects and Their Analysis

3.1 Deformation Defects

Defect Description: Deformation is a significant problem in the casting of oil pans, especially in the thin – walled and large – cavity structure. The deformation mainly occurs at the two wide edges, and during the trial production, the rejection rate due to excessive deformation reached 50%.
Causes:
- Model Processing: The operation methods in the processes of foam model forming, drying, assembling, coating, and vibrating and burying the box are not reasonable or not strictly controlled.
- Molding Pressure: During vibrating and burying the box, the pressure exerted by the molding sand on the model in different directions is not uniform, leading to deformation.

3.2 Sand Adhesion Defects

Defect Description: The sand adhesion defect occurs when the molten iron penetrates the coating and adheres to the sand during the casting process, affecting the surface quality of the casting.
Causes:
- Sand Quality and Flowability: The distribution of reinforcing ribs on the oil pan is uneven, and the particles in the molding sand are not uniform with a high ash content. This results in poor flowability of the sand into the corner areas during molding, reducing the compactness of the sand.
- Coating Quality: The thickness and strength of the coating layer are insufficient, and it is easily broken through by the high – temperature and high – pressure molten iron, leading to sand adhesion.

3.3 Slag Inclusion Defects

Defect Description: Slag inclusion defects are manifested as foreign substances (slag) being trapped inside the casting after solidification.
Causes:
- Pouring Process: During the pouring process, the molten iron may infiltrate into the model or impact the molding sand, mixing the washed – off coating or sand into the metal liquid.
- Slag Removal Process: The slag removal, slag blocking, and slag discharging processes are not effective, resulting in a large amount of solid and liquid products in the model that cannot be discharged in time and remain in the casting as slag inclusions.

3.4 Cold Shut and Shrinkage Porosity Defects

Defect Description: Cold shut occurs when the molten iron does not fully fuse at the end of the pouring process due to a slow flow rate and rapid temperature drop. Shrinkage porosity occurs when the molten iron in the area that needs to be fed does not receive sufficient replenishment before solidification.
Causes:
- Pouring System and Molten Iron Flow: The flow rate of the molten iron in thin – walled shell castings is slower than that in thick and large castings, and the temperature drops faster. An unreasonable pouring system fails to provide sufficient molten iron for the areas that need to be fed in a timely manner.

3.5 Metallographic Structure Defects

Defect Description: The metallographic structure of the casting does not meet the required standards, mainly manifested as an insufficient content of pearlite.
Causes:
- Raw Material Proportioning: The on – site batching is not strictly weighed, and the control of the raw material ratio is not strict, resulting in unstable chemical composition.
- Inoculation Effect: The inoculation effect is not good, and the carbon equivalent is low, with the Si/C ratio not meeting the requirements.

4. Optimization Measures for Casting Defects

4.1 Deformation Defect Improvement Measures

White Model Quality Control: After the white model is formed, it is first inspected for flatness on an inspection platform and then measured for dimensions. The allowable deviation of the white model is – 1 mm to + 3 mm.
Drying and Bonding: The white model is dried flat with multiple points of force application. After drying, the humidity is measured using a hygrometer. Once it meets the requirements, the next process can be carried out. A shaping card board is used to shape the dimensions, and then fiber rods are used for bonding. After bonding, the dimensions are measured again to ensure compliance before coating.
Coating and Drying Adjustment: The coating’s Baumé degree and thickness are ensured, and the drying process parameters are adjusted. When the starting drying temperature is low, the drying time is appropriately extended to ensure shrinkage balance.
Molding Improvement: During molding, the bottom sand is manually spread evenly, and the sand addition process is also carried out evenly to ensure uniform force application. A positioning device for molding the buried box is made to ensure the angle of the buried box. The box is buried in a staggered manner, and the sand is added and vibrated in layers. At the corner positions, the sand is manually built up. These measures significantly reduce the occurrence of deformation defects, reducing the rejection rate due to deformation to less than 3%.

4.2 Sand Adhesion Defect Improvement Measures

Coating Enhancement: To ensure that the coating adheres firmly to the white model, 3% bentonite, 15% graphite powder, and 15% quartz powder are added to the coating to enhance its strength and flowability. The coating thickness is required to be uniform and not less than 1.6 mm.
Molding Parameter Optimization: During molding, the compaction force should not be too large to prevent damage to the coating. The vibration time should also not be too long to avoid coating cracking. At a vibration frequency of 40 Hz, the total vibration time is not less than 360 s, and at 50 Hz, it is 280 s. At the corner positions, manual sand filling is carried out to ensure high compactness of the sand in the dead corners.
Sand Selection and Screening: The selection of molding sand must be appropriate. For dry sand molding, sand with a specified mesh size is used, and circular or approximately circular sand is preferred. For general cast iron and ordinary cast steel parts, (0.4 – 0.8) mm sand is used. A screening machine is used to screen the used molding sand to remove larger and smaller particles as well as internal dust, and new sand is replaced in a timely manner. Negative Pressure and Pouring Temperature Control: An appropriate negative pressure is selected. If the negative pressure during pouring is too high, it will cause severe sand adhesion, and if it is too low, the casting will collapse. For cast iron, the negative pressure is controlled between (0.04 – 0.05) MPa. During burying the box, a staggered burying method is used to avoid the influence of overlapping surfaces on the negative pressure distribution. The pouring temperature is controlled within (1420 – 1460) °C, and only one box is poured per ladle.

4.3 Slag Inclusion Defect Improvement Measures

Furnace Maintenance: The furnace table, furnace surface, and around the furnace body are regularly swept. The furnace nozzle is repaired in a timely manner.
Slag Removal Process Optimization: The slag removal process in the furnace is divided into four steps, with two steps of slag removal at a high power state in the furnace and two steps of static slag removal at the furnace mouth. The furnace nozzle and ladle nozzle are repaired, swept, and painted with repair materials. Before the water is discharged, the ladle is cleaned. The slag removal in the ladle is divided into three steps: (1) Slag is removed once when the molten iron is poured into the ladle; (2) A slag remover is used to cover the molten iron ladle and it is transported to the pouring area for static slag removal again; (3) During pouring, the slag remover covers the molten iron, only reserving one flowing opening. The position of the flowing opening is blocked with a fiber blanket, and at the same time, an exhaust and slag discharge opening is added on the upper processing surface.

4.4 Cold Shut and Shrinkage Porosity Defect Improvement Measures

Improving Molten Iron Parameters: The tapping temperature, pouring temperature, and pouring speed of the molten iron are increased. The tapping temperature is increased to 1540 °C (1560 °C for a cold ladle), and a rapid pouring operation method is adopted.
Pouring System Optimization: The design of the pouring system is a key process in lost foam casting. The cross – sectional areas of each component of the pouring system are calculated and determined, and they are improved and optimized through process experiments to make them reasonable. The shape of the straight runner is optimized, and a cylindrical straight runner is adopted. A top – pouring inclined pouring process is used. According to the design principles, the cross – sectional size of the pouring system is selected, and the area ratio of the pouring system is controlled according to the ratio of straight runner : cross runner : inner runner = 7 : 1 : 0.4. A middle – pouring type pouring system is adopted, with two inlet points, which can both block slag and ensure the feeding amount. The pouring process follows the “slow – fast – slow” principle. First, the channel is opened slowly, with an expected pouring time of 2 s. Then, it is poured rapidly, with an expected pouring time of 20 s. Finally, at the position of the cross runner, it is poured slowly until it is full, with an expected time of 3 s. The total single pouring time does not exceed 25 s. A feeding blind riser is set at the position where cold shut is concentrated.

4.5 Metallographic Structure Defect Improvement Measures

Optimizing Chemical Composition: The chemical composition of the oil pan is optimized, and the proportion of components is adjusted to control the carbon equivalent within 3.8% – 4.1%. The Si/C ratio is controlled at about 0.65. The moisture content of the molding sand is controlled within 3.2% – 3.5%.
Raw Material Control and Inoculation Improvement: The raw materials and auxiliary materials are strictly weighed and added according to the proportion. The inspection intensity of the chemical composition before the furnace is increased, and the addition proportion is adjusted in real time to ensure that the composition is qualified before tapping. The process is optimized to determine the addition time and temperature of each alloy element. The carburizing agent is added at 1/4, 1/2, and 3/4 of the molten iron, with a single addition amount not exceeding 40 kg. Manganese iron is added at 3/4 of the molten iron, and ferrosilicon is added 10 minutes before tapping. A feeding device is made to change the inoculation in the furnace to surface inoculation, improving the inoculation effect. A heat preservation board is used to improve the heat preservation effect of the ladle, ensuring that the inoculation time is not less than 5 minutes, and the inoculation agent is added according to the proportion and requirements. The inoculation process requires standing to allow it to react fully.

5. Implementation Effects

After implementing the above improvement measures, a process standard specification was formulated. Six batches of oil pans were trial – produced strictly according to the formulated process standard, with 32 pieces in each batch. The batch production tests proved that:

The use of support ribs can prevent the model from deforming.
The selection of a cylindrical straight runner effectively solves the slag inclusion defect.
Adjusting the vibration parameters and manual auxiliary sand burying measures solve the iron – coated sand defect.
Increasing the tapping temperature, pouring temperature, and adopting a rapid pouring method effectively control the cold shut defect.
Improving the pouring system and adjusting the component proportion effectively improve the internal defects and structure of the casting. The casting qualification rate can reach 96%.

6. Conclusion

In conclusion, through the analysis of the defects in the lost foam casting process of the oil pan and the implementation of corresponding optimization measures, the following conclusions can be drawn:

The casting deformation can be controlled by standardizing the white model baking method and the buried box molding process, making a shaping card board, and using fiber rods for fixing and bonding.
The sand adhesion defect can be improved by using a coating with good flowability, selecting (0.4 – 0.8) mm molding sand, ensuring a vibration frequency of ≥40 Hz during molding, a casting negative pressure of ≥0.04 MPa, and a pouring temperature of (1420 – 1460) °C.
The surface slag inclusion defect can be reduced by solidifying the slag removal process and using a large – particle slag remover. The cold shut and shrinkage porosity defects can be effectively controlled by setting a feeding blind riser at the position where cold shut is concentrated, adopting a closed pouring system, and using a cylindrical straight runner.
The metallographic structure can meet the standard by adjusting the raw material addition proportion, determining the addition time and temperature of alloy elements, and controlling the Si/C ratio within 0.6 – 0.7 to ensure the stability of the pearlite content. These results provide valuable references for the lost foam casting production of oil pans and contribute to improving the quality and production efficiency of castings.