When selecting a filter, the first step is to consider its shape, material, and pore size, as these directly impact its performance in different casting scenarios.
Two main types: straight hole ceramic filters and foam ceramic filters.
Recommendation: Foam ceramic filters are generally preferred, though the specific choice may depend on the casting requirements (e.g., flow rate, slag content).
Common materials include aluminum oxide, silicon carbide, zirconia, etc.
Selection criteria: Match the filter material to technical indicators such as the casting material (e.g., iron, steel), pouring temperature, high-temperature continuous pouring time, and application range. For example, higher-temperature castings may require more heat-resistant materials like zirconia.
Pore size determines the filter’s ability to trap slag without getting clogged. It varies by casting type:
Ductile iron castings: Use 10–15 PPI filters. Larger apertures are needed because ductile iron melts often contain more slag; smaller pores would clog easily.
Gray iron castings: Use 10–20 PPI filters. Smaller apertures are suitable here since gray iron melts typically have less slag, allowing for finer filtration.
The filtering capacity refers to the amount of molten metal a filter can handle per unit area, which varies by casting material:
Gray iron: 4–6 kg of molten iron per 1 cm² of filter area.
Ductile iron: 2–4 kg of molten iron per 1 cm² of filter area.
Safety tip: To avoid overloading and clogging, it’s advisable to use the lower limit of these ranges in practice.
The effective area (considering the filter’s porosity/opening rate) must be sufficient to ensure smooth flow and efficient filtration.
Installation note: Filters work best in open pouring systems; avoid placing them in closed systems if possible, as closed systems may restrict flow and increase pressure.
Porosity variation: Different manufacturers produce filters with varying porosities (e.g., 40–60%, 50–60%, 80–90%). This affects the actual effective area.
Design requirement: The total effective filtration area of all filters in the system must be 2–5 times larger than the flow-blocking cross-sectional area (the narrowest point in the pouring system). This prevents flow restrictions and ensures proper filtration.
Proximity to the mold cavity: Place the filter as close to the casting mold as possible to maximize its ability to trap slag before the molten metal enters the mold.
Avoid direct impact: Molten metal should not hit the filter directly, as this can damage it or reduce efficiency. If direct impact is unavoidable, limit the pouring height to ≤ 300 mm.
Before installation, check the filter for:
Cracks or damage: These can cause leaks or failure during pouring.
Moisture: Moisture in the filter can react violently with molten metal, leading to splattering or defects in the casting.
By following these guidelines, you can select and install filters that effectively remove impurities, improve casting quality, and prevent process disruptions. The key is to match the filter’s properties to the specific casting material, process conditions, and system design.
