12 Sep How To Make Ceramic Foam Filter
How To Make Ceramic Foam Filter
Adtech will let you know how to make a ceramic foam filter.
Preparation of a slip for impregnation of polyurethane foam: potassium salt and phosphoric acid are mixed in a container until completely neutralized.
Then an aluminum-chromophosphate binder is introduced and then, with constant stirring, loose components are introduced: melt, aluminum hydroxide, filler, and fine-ground foam ceramic filter waste.
Mixing the suspension continues for 30-40 minutes until complete homogenization.
How To Make Ceramic Foam Filter?
Billets made of polyurethane foam with an open-porous elastic structure are impregnated with a finished ceramic slip, and the excess suspension is squeezed out.
The semi-finished product is dried at a temperature of 100-130 ° C to constant weight and then fired at a maximum temperature of 1280-1290 ° C.
Finished foam filters were tested for resistance by filtration during casting in the form of aluminum alloys.
The compositions of the proposed masses and prototype Physico-technical characteristics were obtained during the testing of foam filters.
The proposed solution allows for increasing the mechanical strength and heat resistance of ceramic foam filters.
The ceramic foam filters from the proposed mass compare favorably with higher rates of mechanical strength and heat resistance and, very importantly, due to the absence of cracks in the ceramic body, have high bending fracture resistance, which favorably affects the quality of the metal being cleaned.
In addition, the introduction of fine-ground wastes from the PCF allows the organization of waste-free production of ceramic foam filters.
Ceramic foam filters are used to eliminate impurities in metal castings.
PPU-based filters remove impurities through their openly porous structure. The paths for molten metal pass through the pores during the casting process. In the direction of ceramic foam filters, PUF serves as a matrix for the further catalytic process. The foam is coated with a ceramic suspension and dried. Then the blanks of various shapes are launched into the furnace, where the matrix of the reticulated polyurethane foam burns out, leaving behind a ceramic mesh frame of the finished filter. Ceramic filter pore sizes range from 10 to 45 PPI / linear inch.
The mesh structure of the ceramic foam filter provides deep filtration, which carefully removes a large number of impurities without impurities
The filter can withstand significant metallostatic pressure while maintaining effective filtration. Ceramic foam filters are usually used for primary and secondary aluminum smelting, as well as for filters of aluminum smelting furnaces.
Ceramic foam filters have the following advantages
1 Filtration of metallic and non-metallic impurities from molten metal streams.
2 Reduction of oxide during turbulent flow formation.
3 Increased metal flow.
4 Elimination of inclusions and various metal defects.
5 Improving the quality of casting, PPU filters have good mechanical properties.
6 Reduction of scrap and additional work in production.
7 The ceramic foam filter is available in a wide variety of sizes and shapes.
8 All ceramic foam filters are characterized by a mesh matrix, spherical pores of uniform size and volume are connected throughout their volume, and form windows through their walls. The winding paths of this structure pass and simultaneously capture molten slag and other contaminants. As a result, this unique PPU-based filter structure shows superior metal filtration efficiency compared to other structures and filters in the foundry market.
9 Ceramic foam filters have high heat resistance and are easily impregnated with molten metal. The superior durability of PPU-based filters eliminates the need for redundant metal screens.
10 Improving the quality of casting occurs when filtering slags that are formed in the molten metal during the casting process. With the help of PPU-based filters, the quality of the final casting is improved, the workability of the metal is improved, the casting waste is reduced at the same time, and the production speeds of the lines are increased.
11 Pores in ceramic foam filters act as “traps” for various impurities. This filter structure compares favorably with conventional standard ceramic filters. When the molten mass of ferrous metals comes into contact with the surface of the ceramic foam filter, the special additives that make up the filter coating begin to decompose and instantly form a continuous film of fayalite. This coating becomes soft and viscous at high temperatures, the fayalite film retains even micron-sized inclusions as they pass through the filter. Conventional ceramic filters are not able to capture so many impurities of molten metal in full.
When using filters from polyurethane foam in the production, you will achieve cleaner metal, improve its fluidity, and in general the quality of the final casting, which will lead to an increase in the profits of the plant and foundry.
1. What is a Ceramic Foam Filter and How Does It Work?
A Ceramic Foam Filter (CFF) is a porous, highly refractory ceramic material characterized by a three-dimensional open-cell network (reticulated structure). Designed to withstand extreme temperatures, thermal shock, and molten metal erosion, CFFs are predominantly used in continuous, semi-continuous (DC casting), and shape casting of aluminum alloys.
The Three Filtration Mechanisms
- Mechanical Straining (Screening): Large oxide skins and refractory slag particles larger than the filter pore opening are mechanically trapped on the entry face of the filter plate.
- Cake Filtration: As trapped particles accumulate at the filter surface, they form a “filter cake.” This layer subsequently acts as a secondary filter medium, capturing particles smaller than the initial pore diameter.
- Deep-Bed Adsorption: Microscopic non-metallic inclusions pass into the tortuous 3D network of the reticulated ceramic structure. Due to inertia, thermal convection, and capillary action, these inclusions contact the ceramic strut surfaces and adhere to them via physical adsorption.
Core Benefits for Aluminum Cast Houses
- Dross & Inclusion Reduction: Removes non-metallic particles down to a few microns, dramatically lowering rejection rates during X-ray testing.
- Laminar Flow Stabilization: Reduces turbulence during mold filling, minimizing secondary oxidation and air entrapment.
- Improved Mechanical Properties: Enhances yield strength, fatigue resistance, and surface finish of extruded profiles, aluminum foils, and aerospace-grade sheets.
- Machining Tool Longevity: Decreases hard spot density, significantly reducing tool wear during CNC milling and cutting operations.
2. Step-by-Step Ceramic Foam Filter Manufacturing Process
The dominant industrial method for producing high-porosity reticulated ceramic foam filters is the Organic Sponge Impregnation Method (also known as the Schwartzwalder Process). Below is the precise 6-step manufacturing workflow.
Alumina Ceramic Foam Filter manufacturing process including mixing, molding, sintering, inspection and packaging.
Step 1: Polyurethane Sponge Selection and Pre-treatment
- Base Material: Flexible, fully reticulated open-cell polyurethane (PU) foam serves as the sacrificial template.
- Pore Uniformity: The PU foam must feature uniform pore size distribution, specified in PPI (Pores Per Inch).
- Thermal Pre-treatment: The foam template is heat-treated or chemically etched to remove residual cell membranes, ensuring 100% open porosity and optimal slurry adhesion.
Step 2: Ceramic Slurry Preparation & Impregnation
- Slurry Formulation: High-purity alumina (Al2O3), silica (SiO2), binder agents (e.g., aluminum phosphate or colloidal silica), rheological additives, and anti-foaming agents are blended in a high-shear mixer.
- Viscosity Control: Slurry rheology must exhibit thixotropic behavior—flowing easily under stress while maintaining a stable structure at rest to coat foam struts evenly without clogging pores.
- Dipping Process: The pre-cut PU foam matrices are submerged in the ceramic slurry, ensuring complete wet-out of all internal struts.
Step 3: Mechanical Rolling and Excess Slurry Removal
- Squeezing Action: The impregnated foam passes through precision-calibrated roller presses to squeeze out excess slurry.
- Web Thickness Retention: Controlled clearance settings on the rollers maintain optimal ceramic strut coating thickness while preserving open passage channels.
Step 4: Controlled Drying Phase
- Moisture Evaporation: The coated foam blocks undergo drying in continuous tunnel dryers or microwave drying systems operating between 100°C and 150°C.
- Crack Prevention: Humidity and temperature profiles are carefully regulated to prevent shrinkage cracks, warpage, or structural deformation.
Step 5: High-Temperature Sintering
- Polyurethane Burn-out: As temperature reaches 300°C to 600°C in the sintering kiln, the internal polyurethane foam template burns out completely without leaving toxic carbon residue.
- Ceramic Bonding: The kiln ramps up to sintering temperatures between 1100°C and 1500°C (depending on binder and alumina purity). Thermal sintering fuses ceramic grains together, developing high mechanical strength and thermal shock resistance.
Step 6: Edge Gasket Application & Quality Inspection
- Thermal Expansion Gasket: Expandable ceramic fiber or refractory gasket strips are fitted around the edges of the filter plate to ensure a tight seal inside the filter box, preventing molten metal bypass.
- Dimensional & Visual QC: Each filter plate undergoes dimensional tolerance verification, pore density checks, and structural integrity testing.
3. Key Technical Specifications & Material Comparison
Selecting the correct ceramic material matrix is crucial to withstand thermal stress and chemical reaction with specific molten alloys.
| Property / Parameter | Alumina (Al2O3) Ceramic Filter | Silicon Carbide (SiC) Filter | Zirconia (ZrO2) Ceramic Filter |
|---|---|---|---|
| Primary Alloy Application | Aluminum, Al-Mg Alloys, Zinc | Cast Iron, Ductile Iron, Copper Alloys | Carbon Steel, Stainless Steel, Superalloys |
| Max Operating Temp | Up to 1150°C (2102°F) | Up to 1500°C (2732°F) | Up to 1700°C (3092°F) |
| Porosity Rate | 80% – 90% | 80% – 85% | 75% – 85% |
| Flexural Strength | ≥ 0.8 MPa | ≥ 1.5 MPa | ≥ 2.0 MPa |
| Thermal Shock Resistance | Excellent | Superior | Outstanding |
| Chemical Inertness | Highly Resistant to Molten Al | Highly Resistant to Slag | High Resistance to Corrosion |
Standard Physical Specifications (Alumina CFF for Aluminum)
- Chemical Composition: Al2O3 ≥ 85%, SiO2 ≤ 10%
- Bulk Density: 0.40 – 0.55 g/cm³
- Cold Compressive Strength: ≥ 0.5 MPa
- Thermal Expansion Coefficient: 7.5 × 10⁻⁶ /K
4. How to Choose the Right PPI for Aluminum Casting
Pores Per Inch (PPI) defines the pore density of a ceramic foam filter. Selecting the appropriate PPI depends on your casting method, alloy cleanliness requirements, and molten metal flow rate goals.
Ceramic foam filter PPI selection guide showing 10–60 PPI pore size, filtration efficiency and aluminum casting applications
PPI Selection Matrix
10 PPI to 20 PPI (Coarse Filtration)
- Primary Uses: Primary aluminum remelt, foundry ingot production, scrap recycling, and gravity casting.
- Key Characteristics: High volumetric flow rate, high dirt-holding capacity, low initial priming head requirement.
- Target Inclusions: Removes large dross particles, furnace lining flakes, and coarse oxide skins.
30 PPI to 40 PPI (Medium-Fine Filtration)
- Primary Uses: Direct Chill (DC) casting of extrusions, aluminum billets, rolling slabs, and automotive component foundries.
- Key Characteristics: Balanced performance offering efficient micro-inclusion removal alongside stable flow velocity.
- Target Inclusions: Traps intermediate non-metallic inclusions down to 30 microns.
50 PPI to 60 PPI (Ultra-Fine / Precision Filtration)
- Primary Uses: High-precision aluminum foil stock, aerospace alloys, lithographic sheet production, and ultra-thin wire drawing.
- Key Characteristics: High filtration efficiency with tight pore channels. Requires preheating and higher molten metal priming head pressure.
- Target Inclusions: Captures micro-inclusions down to 5–10 microns.
5. Application Scenarios & Filter Box Installation Best Practices
Correct installation and preheating of ceramic foam filters in casting operations are critical to preventing molten metal bypass, thermal shock breakage, and cold folds.
Step-by-Step Installation Guidelines
- Inspect Filter Box & Seating: Clean the filter bowl or launder box thoroughly. Remove residual aluminum, oxide scale, or damaged lining refractory.
- Apply Expansible Gasket: Ensure the expandable ceramic fiber sealing gasket wraps tightly around all four sides of the filter plate.
- Position the Filter: Place the filter plate flush into the filter bowl seat. Press down evenly to verify there are no gaps along the perimeter where un-filtered liquid metal could bypass.
- Preheating Sequence: Preheating the filter plate using burner flames or electric heating elements is mandatory before casting starts:
- Target Preheat Temperature: 300°C to 500°C (572°F to 932°F).
- Purpose: Prevents thermal stress cracking upon contact with molten metal and ensures instant priming without freezing.
- Priming & Flow Rate Control: Maintain an adequate metal head height (typically 50–150 mm) during initial pour to overcome surface tension and prime the filter pores.
6. Troubleshooting Common Filtration Failures
| Issue / Symptom | Root Cause Analysis | Corrective Action Plan |
|---|---|---|
| Filter Floating / Displacement | Missing or incorrectly sized edge gasket; improper filter seating inside the bowl. | Use correct ceramic fiber gasket thickness to ensure tight mechanical wedge fit. |
| Metal Priming Failure (No Flow) | Filter temperature too low during pour; selected PPI too fine for available liquid metal head pressure. | Increase filter box preheat temperature; switch to a lower PPI filter (e.g., from 50 PPI to 30 PPI). |
| Filter Structural Cracking | Severe thermal shock due to non-uniform or insufficient preheating; physical impact during loading. | Standardize burner preheat sequence; handle filter plates carefully using thermal protective gear. |
| Inclusion Bypass / Downstream Defect | Damaged edge gasket allowing molten metal to bypass filter; cracks inside filter matrix. | Inspect gasket seal prior to casting; verify cold compressive strength specifications from supplier. |
7. Frequently Asked Questions (FAQ)
Q1: What is the standard lifespan of an alumina ceramic foam filter?
Alumina ceramic foam filters are designed primarily for single-cast usage in continuous and semi-continuous casting runs. They should be replaced after every drop/cast or once maximum inclusion load capacity is reached to avoid channel clogging and pressure drops.
Q2: Why is the expandable gasket around the filter plate critical?
The ceramic fiber gasket serves two vital functions: first, it seals the perimeter to prevent liquid aluminum from leaking past the sides without being filtered; second, it expands under high casting temperatures to securely lock the filter plate in position, preventing it from floating.
Q3: How do I determine whether to use a 30 PPI or 50 PPI filter?
Base your selection on your end product quality specifications. If you are manufacturing standard industrial extrusions or billets, 30 PPI provides excellent flow efficiency and clean melt quality. If you produce foil stock, lithographic plates, or aerospace components, 50 PPI or 60 PPI is necessary to remove fine micro-inclusions.
Q4: What is the main difference between Alumina (Al2O3) and Silicon Carbide (SiC) foam filters?
Alumina (Al2O3) filters are specifically formulated for aluminum and non-ferrous alloy casting up to 1150°C. Silicon Carbide (SiC) filters exhibit much higher mechanical strength and temperature resistance up to 1500°C, making them suitable for iron, brass, and bronze foundry applications.
Technical Consultation & Procurement Solutions
Are you seeking to improve your casting yield, reduce dross rejection rates, or custom-engineer ceramic foam filter sizes for your aluminum furnace launder system?
Contact the metallurgical engineering team at AdTech today for technical datasheets, custom PPI configurations, and factory-direct volume quotes.
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