Lost Foam Casting: Analysis and Improvement of “Coating Sheet” Slag Inclusion Defect

1. Introduction

Lost foam casting is a widely used casting process with many advantages, such as wide process adaptability, precise dimensional structure, and small machining allowance. However, it also faces some challenges, one of which is the “coating sheet” slag inclusion defect. This defect not only affects the quality of castings but also increases the production cost due to rework or rejection. Therefore, it is essential to analyze the causes of this defect and find effective improvement measures.

1.1 The Process of Lost Foam Casting

Lost foam casting involves several steps. First, a foam plastic model (usually made of expandable polystyrene, EPS) is fabricated according to the desired shape of the casting. This model is then coated with a special coating. After that, the coated model is placed in a flask and surrounded by sand. When the molten metal is poured into the flask, the foam model vaporizes, and the molten metal takes its place, forming the casting.

1.2 The Significance of Studying “Coating Sheet” Slag Inclusion Defect

The “coating sheet” slag inclusion defect can lead to various problems in the casting. It can reduce the mechanical properties of the casting, such as strength and toughness. Moreover, it can also cause surface defects, which may require additional machining or surface treatment to correct. By studying this defect, we can improve the quality of castings, reduce production costs, and enhance the competitiveness of the casting industry.

2. Causes of “Coating Sheet” Slag Inclusion Defect

2.1 Pouring System Design

Influence on Metal Flow: The design of the pouring system has a significant impact on the flow of molten metal. In an open pouring system, the slag may float on the surface of the molten metal and enter the casting cavity with the flow. In a closed pouring system, the rapid flow of molten metal after leaving the ingate can cause turbulence and splashing, increasing the oxidation of the molten metal and resulting in secondary slag inclusion. A semi-closed pouring system can achieve a better slag-blocking effect by maintaining a full state and reducing turbulence. However, in lost foam casting, factors such as the gasification speed of the model and the pressure head also affect the flow velocity, making it more complex to control the flow of molten metal solely by adjusting the cross-sectional area of the pouring system components.

Pouring System Type	Advantages	Disadvantages
Open Pouring System	Simple structure	Poor slag-blocking effect
Closed Pouring System	High filling speed	Turbulence and secondary slag inclusion
Semi-Closed Pouring System	Good slag-blocking effect, reduced turbulence	Complex design

Effect on Coating Integrity: An improper pouring system design can cause the molten metal to flow unevenly and scour the coating. This can lead to the breakdown of the coating and the entry of slag into the casting. For example, if the ingate is not properly designed, the molten metal may impact the coating with a high velocity, causing damage.

2.2 Coating Properties

Low Temperature Strength: The coating used in lost foam casting needs to have sufficient strength at both low and high temperatures. If the low-temperature strength of the coating is insufficient, it may develop microcracks during drying or handling. These microcracks can act as channels for slag to enter the coating and eventually the casting.
Permeability and Gas Generation: The coating should have good permeability to allow the decomposition products of the foam plastic to escape from the cavity. At the same time, it should have a low gas generation rate to avoid gas bubbles that could cause the coating to detach and affect the smoothness of the metal filling.

Coating Property	Requirement	Impact on Casting if Not Met
Low Temperature Strength	Sufficient strength to avoid microcracks	Microcracks can lead to slag inclusion
Permeability	Good permeability for foam decomposition products	Poor quality casting if products cannot escape
Gas Generation	Low gas generation rate	Coating detachment and uneven metal filling

2.3 Sand Compactness

The compactness of the sand around the coated model is crucial. If the sand is not compacted enough, it cannot provide sufficient support to the coating. During the filling process of the molten metal, the coating may rupture due to the lack of support, allowing slag to enter the casting.

3. Analysis of Molten Metal Filling Characteristics in Lost Foam Casting

3.1 Comparison with Traditional Cavity Casting

In traditional cavity casting, the molten metal fills the cavity from the bottom up in a relatively straightforward manner. The liquid metal first fills the bottom plane of the mold under the influence of gravity and then gradually rises along the vertical direction until it reaches the top of the casting.
In lost foam casting, the flow of molten metal is affected by the vaporization and retreat of the foam plastic model. The metal flow is multidirectional and is also influenced by the structure of the model and the change in gas pressure. It does not follow a simple vertical filling pattern as in traditional cavity casting.

3.2 Pulsating Filling Pattern

In lost foam casting, a pulsating filling pattern is often observed. Taking a bottom-return type ingate as an example, the molten metal spreads and fills in various directions such as upward, left, right, front, and back. Although affected by gravity, it is more influenced by gas resistance. Outside the filling range of a single ingate, when two streams of molten metal meet and all the inlet water from the ingates is connected, the filling gradually progresses from bottom to top. During this process, the metal near the inlet is noticeably thicker, and the filling of the entire molten metal shows a peak shape that decreases with the distance from the ingate.

3.3 “Pressure Loss” Phenomenon

During the filling process in lost foam casting, there is a phenomenon of “pressure loss.” When the cross-sectional area of the flow path suddenly decreases, the flow velocity of the molten metal increases sharply, causing turbulence and inevitably entrapping slag and gas. When the cross-sectional area suddenly increases, the flow velocity decreases, and the gas gap in front of the molten metal is in an unbalanced state, resulting in a momentary pressure loss. This can cause the coating layer to be scoured off by the molten metal, and slag and bubbles can accumulate at this location.
The gasification process of the lost foam creates a significant resistance to the filling process, while the changes in the runner cross-section and the frictional resistance along the path become negligible. The resistance caused by the foam disappearance process dominates the entire filling process.

4. The Role of Coating in Lost Foam Casting

4.1 Function Requirements

Good Permeability: The decomposition products of the foam plastic must be able to escape from the cavity through the coating layer. This requires the coating to have good permeability.
Low Gas Generation: After drying, the coating should not produce additional gases when in contact with the molten metal. Otherwise, it can cause the coating to detach and affect the stability of the metal filling.
Easy to Peel and Sinter: The coating should be easy to peel off after casting and have a certain sintering property, which is beneficial for the removal of sand from the casting and subsequent cleaning.
Good Infiltration and Adhesion: Since the foam plastic is a non-polar material and is not easily wetted and penetrated by water-based coatings, surface active agents need to be added to the coating to improve its wettability and adhesion.
Sufficient Strength: The coating should have sufficient strength at room temperature to protect the foam model during handling and shaping. It also needs to withstand the high-temperature scouring and erosion of the molten metal during pouring.

4.2 Possible Causes of Coating Cracking

Drying Process: If the amount of suspending agent added is too large or the coating layer is too thick, drying cracks may occur.
Shaping Process: Improper handling during the shaping process can cause microcracks in the coating, especially near the load-bearing points such as the flange of a mold product or the reverse side of a weight reduction hole.
Filling Process: The scouring of the molten metal during the filling process can damage the coating.
Heating during Filling: Due to the different thermal physical parameters of the coating components, cracks may occur under the intense heat of the molten metal during the filling process.
Solidification Process: The static pressure of the molten metal during the solidification process can also cause damage to the coating.

5. Case Study of “Coating Sheet” Slag Inclusion Defect

5.1 Product Structure Influence

For a body cover stamping die casting, the product structure has an impact on the occurrence of defects. The die base products often have an “I” or “Z” shape structure. The defect-prone areas are usually concentrated on the back of the junction between the bottom flange and the rib plate or at the reverse structure position. When the molten metal fills from the bottom working surface to the rib plate, the cross-sectional area decreases rapidly, causing the flow velocity to increase and turbulence to intensify, with slag and gas being entrapped. When the molten metal fills to the top flange surface, the cross-sectional area increases, the flow velocity decreases, and a momentary pressure loss occurs, which may cause the coating layer to be scoured off.

5.2 Pouring System Design

An improperly designed pouring system can cause molten metal turbulence and a momentary pressure loss, destroying the integrity of the coating. Under the scouring of the high-temperature molten metal, the coating layer breaks and enters the molten metal, and slag, bubbles, etc. gather together to form a complex defect. For example, when a reverse rain-type hollow pouring system is used, the molten metal starts to fill through each ingate almost simultaneously, leading to the convergence of multiple streams of molten metal, which is a defect-prone area.

5.3 Coating Parameters

The foam plastic (EPS or STMMA) is a non-polar material with low surface tension, and water-based coatings are not easily wetted and penetrated. If the surface active agent added to the coating fails, the coating may have poor adhesion, uneven thickness, or even be discontinuous. Under the scouring of the high-temperature molten metal, the coating in this area can enter the casting and cause a coating sheet defect.

5.4 Coating Drying and Spraying Process

If the coating ratio is inappropriate or the binder is not suitable, the coating may have insufficient low-temperature strength. During the drying process, microcracks may occur due to the unsuitable binder, which can lead to slag inclusion.

5.5 Shaping Process

Although the lost foam casting process simplifies the shaping operation, it does not mean that the operation requirements are reduced. On the contrary, due to the generally higher pouring temperature in lost foam casting, higher requirements are placed on the shaping process. For some reverse holes, they must be filled firmly. Sufficient sand strength can provide support and anti-scouring ability to the coating layer. A well-compacted sand mold has a lower probability of coating sheet defects.

6. Improvement Measures for “Coating Sheet” Slag Inclusion Defect

6.1 Optimization of Pouring System Design

The design of the pouring system should be optimized to ensure a smooth filling of the molten metal and reduce turbulence and slag entrapment. The cross-sectional area changes should be carefully considered to avoid the “momentary pressure loss” phenomenon. By improving the design of the pouring system, the filling speed and pressure of the molten metal can be increased, enabling it to fill the mold more smoothly.

6.2 Understanding the Filling Law of Molten Metal

It is crucial to understand the filling law of molten metal in lost foam casting. This includes the pulsating filling pattern and the influence of various factors such as the model structure and gas pressure on the metal flow. By understanding these laws, better control over the casting process can be achieved.

6.3 Selection of Appropriate Coating and Application Process

The coating should be selected according to the specific requirements of lost foam casting. It should have good properties such as sufficient strength, good permeability, and low gas generation. The application process of the coating should also be carefully controlled to ensure a uniform and intact coating layer.

6.4 Use of Slag Collection Devices

In areas where slag is likely to accumulate, such as at the top dead corners of the casting, local thick parts, and the convergence of molten metal, slag collection devices such as slag collection risers and exhaust pipes should be set up to reduce the occurrence of “coating sheet” slag inclusion defects.

7. Conclusion

Lost foam casting has many advantages, but the “coating sheet” slag inclusion defect is a significant challenge. By analyzing the causes of this defect from multiple aspects such as the pouring system design, coating properties, and sand compactness, and understanding the filling characteristics of molten metal in lost foam casting, we can take effective improvement measures. These measures include optimizing the pouring system design, understanding the filling law of molten metal, selecting appropriate coatings and application processes, and using slag collection devices. Through these efforts, the quality of castings can be improved, and the occurrence of “coating sheet” slag inclusion defects can be reduced, thereby enhancing the competitiveness of the casting industry.