Production Technology of Alumina Ceramic Foam Filter for Casting

1. Ceramic Foam Filters

Ceramic foam is a porous material with a three-dimensional network structure and high porosity. Its unique structure provides several benefits, including low density, high porosity, high specific strength, excellent thermal shock resistance, and high-temperature tolerance. Due to these properties, ceramic foam is extensively used in applications such as gas and liquid filtration, purification, separation, chemical catalysis, sound and shock absorption, advanced insulation materials, biological implants, and specialized materials and sensors.

In the casting industry, ceramic foam filters play a crucial role. They help to smooth, uniform, and purify molten metal by allowing it to pass through the foam ceramic honeycomb structure. This process significantly reduces casting defects like non-metallic inclusions, thus lowering the rejection rate and saving production costs.

2. Production Process of Foam Ceramics

The production of ceramic foam filters (CFF) begins with polyurethane foam as the carrier. This foam is immersed in a slurry composed of ceramic powder, binder, sintering aid, and suspending agents. The excess slurry is then squeezed out, leaving a uniform coating of the ceramic slurry on the foam's skeleton. This coated foam, or "green body," is subsequently dried and subjected to high-temperature sintering. This method, known as the organic foam impregnation technique, is a well-established production process in China.

2.1 Process Flow

2.1.1 Sponge Processing

The organic foam used in ceramic foam filters primarily consists of polyurethane porous sponges, which come in various pore sizes, such as 10 PPI, 15 PPI, 20 PPI, and 30 PPI. Note that sponge mesh classifications differ from product mesh classifications; generally, a higher PPI (Pores Per Inch) value indicates smaller pore sizes and reduced inclusion sizes in the filtration process.

The sponge processing is a crucial initial step. It begins with selecting the appropriate sponge, as different sponges may have varying mesh sizes. Even within the same batch, it's essential to verify mesh standards before proceeding. Accurate cutting of the sponge to the desired size is also important, and the final sponge products must be ensured to be free of tilting or deformities.

2.1.2 Selection of Pulp Mixing Process Formula

To ensure even sizing and achieve the desired weight, it is crucial to maintain the optimal consistency and fluidity of the slurry. This consistency is essential for meeting the product's strength and porosity requirements. The performance of the slurry is assessed based on its specific gravity and consistency.

2.1.3 Sponge Modification Process

Sponge modification prepares the sponge for the sizing process by enhancing its ability to hold the slurry, ensuring even coating. This step improves the uniformity of the sizing application.

2.1.4 Impregnation and Sizing Process

In this process, the modified dry sponge is evenly coated with the adjusted slurry on a roller press to form a green body. The slurry consistency must be tailored to the mesh size of the sponge; otherwise, the sizing effect may be compromised.

2.1.5 Drying Process

The primary goal of the drying process is to evaporate the water from the semi-finished products, typically reducing moisture content to below 1.0%. For larger specifications, it is essential to carefully control the drying environment—such as temperature and humidity—to prevent deformation and cracking defects during the drying process.

2.1.6 Firing Process

The firing process is the final stage of production, focusing on improving the formula while managing production costs. Most SiC foam ceramics produced by foam ceramic companies do not require atmospheric protection during firing. The typical firing temperature ranges from 1350°C to 1450°C.

2.1.7 Quality Inspection Process

Given their porous structure, foam ceramics often experience some degree of slag shedding after firing. For foundries, this is a significant concern as it impacts both the strength and mesh of the filter. Slag shedding can counteract the filter's purpose, leading to casting defects and potential scrapping. During quality inspection, it's crucial to assess not only the appearance and internal quality of the foam ceramics but also to thoroughly clean any residual slag to ensure optimal performance.

3 Filtration Mechanism and Usage Suggestions of Foam Ceramics for Casting

3.1.1 Filter Cake Effect

The intricate structure of foam ceramics facilitates effective mechanical slag blocking. As molten metal flows through the foam ceramic filter, the filter medium captures and removes inclusions larger than its pore size through mechanical separation. Larger particles are filtered out and collect at the inlet end of the filter. Over time, the accumulation of inclusions on the filter surface forms a layer of "filter cake," which further narrows the flow channel for the molten metal. This filter cake effect allows the newly exposed surface of the filter to capture smaller inclusions. Additionally, the filter medium itself continues to perform filtration through its numerous small pores, some of which have tiny slits or dead ends. These variations in the pore structure provide additional opportunities for intercepting inclusions, enhancing the overall filtration efficiency.

3.1.2 Surface Effect

As molten metal flows through a ceramic body with a complex structure, it is divided into numerous small streams. This increases the contact area between the molten metal and the filter medium, enhancing the likelihood of inclusion capture. The filter's surface is uneven, with concave blocks ranging from 1 to 10 μm. This micro-texture provides electrostatic adsorption and adhesion, which are crucial for trapping inclusions.

3.1.3 Rectification Effect

When molten metal passes through the porous ceramic filter, it is divided into smaller unit streams, reducing the Reynolds number (Re_vd/r) and promoting laminar flow. In a laminar flow state, the much higher density of the molten metal compared to the inclusions allows the inclusions ample time to float and be removed. By installing a filter in the gating system, the resistance to molten metal flow increases, causing the metal to slow down and allowing inclusions to rise and accumulate on the surface of the runner. This rectification effect aids in slag prevention by enhancing inclusion removal.

3.2 Recommendations for the Use of Alumina Ceramic Foam Filters

1. Material Selection: Choose filters made from materials that match the alloy's melting point to prevent excessive temperatures from damaging the filter and compromising its effectiveness.

2. Mesh Selection: Select the appropriate mesh size to ensure the purification effect meets the specific requirements of the casting process.

3. Casting Temperature: Maintain a high casting temperature to increase the fluidity of the metal, enhancing the filter's performance.

4. Filter Placement: When placing the filter horizontally beneath the pouring cup or on the parting surface, ensure the casting height does not exceed 20 cm. The molten metal should flow onto the wall of the pouring cup rather than directly onto the filter.

5. Handling and Storage: Handle the filter with care. When not in use, store it in a dry and well-ventilated area to prevent moisture absorption, which can weaken the filter.

4 Conclusion

The organic foam impregnation method remains the most widely used production process for foam ceramic filters. However, with increasing market competition and rising raw material costs, the focus of current research has shifted to enhancing the process. The key objectives are to improve production efficiency, elevate product quality, and reduce production costs.