Foundry Filtration Terminology: A Complete Glossary

Every industry has its own language. Metal casting is no exception. And if you’re new to foundry filtration – or even if you’ve been in the business for years – you’ve probably run across terms that make you pause. PPI. CFF. NMIs. Breaker core. Washburn core. Exogenous vs. endogenous inclusions.

This glossary is for procurement professionals, foundry engineers, and anyone who wants to speak the same language as their filter supplier – clearly, accurately, and without confusion.

We’ve organized the terms from macro to micro – starting with the big concepts (what is a filter, what does it do) and working down to the specific mechanisms, product types, and quality terminology you’ll encounter when specifying or troubleshooting filters.

Use it as a reference. Bookmark it. Or just read through once to fill in the gaps.

Foundry Filtration Basics – Why Filters Exist

These are the foundational terms every foundry professional should know.

Casting Filter

A refractory component placed in the gating system to remove non-metallic inclusions from molten metal before it enters the mold cavity. Filters also modify metal flow, reducing turbulence and promoting laminar flow.

Non-Metallic Inclusions (NMIs)

Unwanted particles in molten metal that are not part of the intended alloy composition. Common examples include oxides, slag, dross, eroded sand, and refractory debris. Inclusions act as stress concentrators, leading to reduced mechanical properties, poor surface finish, and leak paths in pressure-tight components. Also known as non-metallic inclusions or simply “inclusions.”

Inclusion

General term for any unwanted solid particle trapped in a casting. Non-metallic inclusions originate from multiple sources: slag or dross from the furnace or ladle, sand from mold or core erosion, oxides formed during pouring (reoxidation), or reaction products from nodularization in ductile iron. Inclusions can be classified by origin:

• Exogenous Inclusions: Originate from outside the melt – for example, eroded sand, slag carried from the furnace, or refractory particles.

• Endogenous Inclusions: Form within the melt itself – for example, oxide films formed during turbulent pouring or reaction products from magnesium treatment in ductile iron.

Porosity

Voids or cavities within a casting. Two common types:

• Gas porosity: Small, round, smooth-walled voids caused by dissolved hydrogen coming out of solution during solidification.

• Shrinkage porosity: Irregular, rough-surfaced cavities caused by insufficient feed metal during solidification.

Filters address inclusion-related porosity but do not remove dissolved gases – that requires degassing.

Dross

A mixture of oxides and other non-metallic materials that forms on the surface of molten metal. In ductile iron, magnesium reaction products create a particularly sticky and damaging dross. Fiberglass mesh often fails to capture fine dross; ceramic foam filters are far more effective. Not to be confused with slag (furnace or ladle by-products, typically more fluid).

Slag

A vitreous (glassy) by-product of the melting process, consisting primarily of oxidized impurities and flux residues. Slag is typically less dense than molten metal and floats on the surface. If not properly skimmed before pouring, slag can be carried into the mold and become trapped as inclusions.

Reoxidation

The formation of new oxide inclusions after the melt has been filtered or during pouring. Turbulent flow entrains air, and the oxygen reacts with the metal to form oxides. A well-designed gating system with a ceramic foam filter minimizes reoxidation by promoting laminar flow.

Casting Filter Types – The Main Categories

Understanding these product types is the first step in specifying the right filter for your application.

Ceramic Foam Filter (CFF)

A three-dimensional, open-cell reticulated foam structure made from high-temperature ceramic (alumina, silicon carbide, or zirconia). Provides depth filtration – inclusions are trapped throughout the filter thickness, not just on the surface. Porosity typically 75–90%. Available in pore sizes from 10 PPI (coarse) to 40+ PPI (fine). Also referred to in the industry as reticulated ceramic foam filter.

Manufacturing process: A polyurethane foam template is impregnated with a ceramic slurry, then dried and fired. The organic foam burns away, leaving a ceramic copy of the foam structure – a three-dimensional network of interconnected struts and pores.

Cellular Ceramic Filter / Extruded Filter

A filter with straight, parallel channels (like a honeycomb) produced by extrusion. Pore size is expressed in CPSI (cells per square inch) or cell diameter (e.g., 2.5 mm). Unlike foam filters, extruded filters do not create a tortuous path – they provide a simple linear flow path, which means they do not capture inclusions as deeply. However, they have high mechanical strength and are often used as flow diffusers. Also known as honeycomb ceramic filter or straight‑channel filter.

Manufacturing process: A ceramic “dough” is extruded through a die, forming a continuous “log” with honeycomb cross-section. The log is sliced into individual filters, then dried and fired.

Pressed Filter

A filter made by pressing refractory powders into a complex mold containing many pins. After forming, the pins are extracted, leaving holes in the filter. Characterized by a honeycomb design with round holes. Made primarily from mullite or alumina with a clay bond. Generally less refractory than foam filters and rarely used for steel applications, but widely used for gray and ductile iron casting.

Fiberglass Mesh Filter

A two-dimensional woven screen made of high-temperature glass fibers. Provides surface straining, not depth filtration. Captures only particles larger than the mesh opening. Cost-effective and easy to cut and place, but limited to aluminum and low‑temperature alloys. High‑silica grades can be used for iron, but with significant limitations – including a maximum pour time of less than 10 minutes at typical iron pouring temperatures.

Strainer

A general term for a device that separates solids from liquids by mechanical sieving. In foundry terms, fiberglass mesh and wire screens are strainers, not true filters, because they lack depth filtration capability. A strainer captures only particles larger than the opening; finer contaminants pass through. By contrast, a true depth filter (such as CFF) captures particles throughout its internal structure. The distinction matters – many foundry professionals use “filter” loosely, but the filtration mechanism is fundamentally different.

CFF (Ceramic Foam Filter)

Abbreviation for Ceramic Foam Filter – the industry standard for high‑efficiency molten metal filtration.

Filter Materials – The Building Blocks

Different alloys require different filter materials. Here’s what each material is used for.

Alumina (Al₂O₃)

White or off-white ceramic material. Maximum service temperature ~1100°C. Standard choice for aluminum and magnesium casting because it is chemically inert in aluminum melts. Most economical of the three major filter materials. Porosity typically 80–90%. Also known as aluminum oxide ceramic.

Silicon Carbide (SiC)

Gray to dark gray ceramic material. Maximum service temperature ~1500–1560°C. Excellent thermal shock resistance and high-temperature strength. Standard choice for gray iron, ductile iron, malleable iron, and copper alloys. Also available in carbon‑bonded formulations for steel applications (Capital Refractories’ next‑generation filters).

Zirconia (ZrO₂)

White to tan/orange ceramic material. Maximum service temperature ~1700°C. Excellent thermal shock resistance (≥7 cycles ΔT > 1000°C). Highest filtration efficiency for sub‑10µm inclusions. Standard choice for steel, stainless steel, and superalloy casting. Highest cost of the three major materials.

Magnesia (MgO)

Filter material specifically used for magnesium and magnesium alloys. Provides chemical stability in aggressive magnesium melts.–

Carbon-Bonded Alumina

A next‑generation filter material combining alumina with a carbon bond. Offers superior refractoriness (suitable for steel) and provides a non‑wetting surface (high contact angle) that resists slag adhesion. Used for high‑temperature steel and heavy iron filtration.

Refractory

General term for materials that retain strength and chemical stability at very high temperatures. Casting filters are classified as refractory components because they must withstand molten metal without softening, melting, or reacting.

Pore Size & Specifications – How Filters Are Rated

These technical specifications determine how a filter performs.

PPI (Pores Per Inch)

The number of pores in one linear inch of filter material. Lower PPI = larger pores, higher flow rate, coarser filtration. Higher PPI = smaller pores, lower flow rate, finer filtration.

• 10 PPI – Large castings, high flow rate priority, coarse inclusions

• 15 PPI – Ductile iron, intermediate

• 20 PPI – General purpose, most common starting point for aluminum and gray iron

• 25–30 PPI – Fine filtration, thin‑wall castings, aluminum

• 30–40 PPI – High‑purity / critical applications

The number on a PPI rating is a nominal value, not an absolute count. A “20 PPI” filter typically contains 18–22 pores per inch in practice. Classification has traditionally been performed visually by specially trained personnel, which introduces some subjectivity, though more advanced suppliers use optical or image‑based measurement. Also referred to as pore density.

Mesh Size (for Fiberglass Filters)

The opening size between fibers, measured in millimeters. Typical mesh openings: 0.8–1.2 mm (fine), 1.5–2.0 mm (medium), 2.5–3.0 mm (coarse). Coarser mesh allows higher flow but captures fewer inclusions; finer mesh captures more inclusions but restricts flow more.

CPSI (Cells Per Square Inch)

Unit of measurement for honeycomb (extruded) filters. Represents the number of cells in a square inch of filter face. Higher CPSI means finer channels, higher filtration efficiency, but greater flow resistance. Typical range: 100–400 CPSI.

Porosity

The percentage of open pore volume in a filter. Ceramic foam filters typically have porosity of 75–90% – meaning 75–90% of the volume is open space, and only 10–25% is ceramic material. Also referred to as open porosity or void fraction.

Bulk Density

The density of the filter including its pores, measured in g/cm³. Lower bulk density = more porous = lighter weight but potentially lower mechanical strength. Typical range for ceramic foam filters: 0.35–0.55 g/cm³.

Filtration Efficiency

The percentage of inclusions removed from the molten metal by the filter. Ceramic foam filters typically achieve 80–95% efficiency for particles larger than the nominal pore size. The exact efficiency depends on PPI, melt cleanliness, and operating conditions. In one study, ceramic foam filters achieved 95% efficiency while two‑layer fiberglass mesh achieved only 67% for the same application.

Filtration Mechanisms – How Filters Actually Work

The science behind why filters work – explained without the textbook jargon.

Depth Filtration

A filtration mechanism in which particles are captured throughout the entire thickness of the filter medium, not just on the surface. Ceramic foam filters achieve depth filtration because their tortuous, three‑dimensional pore structure forces metal to change direction many times; inclusions collide with ceramic struts and adhere inside the filter. Depth filters can hold 3 to 20 times more contaminant than surface filters of the same size.

Surface Filtration (Straining)

A filtration mechanism in which particles are captured only on the outer surface of the filter medium. Fiberglass mesh and wire screens operate by surface filtration – they act like a sieve. Once the surface is blocked, flow stops. Depth filtration is always superior for fine inclusion removal.

Screening (Mechanical Sieving)

The simplest filtration mechanism: particles larger than the filter pore openings are physically blocked from passing through. Works for both foam filters and mesh. Also known as mechanical interception or sieving.

Cake Filtration (Cake Filtering)

After the filter captures enough large inclusions, they accumulate on the inlet surface, forming a “filter cake.” This cake is finer than the filter itself and can trap particles smaller than the original pore openings. The filter becomes more efficient as it clogs – up until the point where flow is completely blocked. Also referred to as filter cake formation or cake layer filtration.

Deep Bed Filtration (Deep Bed Filtering)

Similar to depth filtration; particles become trapped within the porous structure through collisions with the ceramic struts. The complex, winding path through a foam filter greatly increases the probability that an inclusion will contact the ceramic surface and adhere.

Adsorption

The adherence of fine oxide films and sub‑micron inclusions to the ceramic strut surfaces through physical and chemical forces. Even particles much smaller than the pore openings can be captured by adsorption. The high specific surface area of ceramic foam (due to its reticulated structure) makes this mechanism highly effective.

Rectification (Flow Rectification)

The conversion of turbulent molten metal flow into laminar flow after passing through the filter. The filter breaks a single turbulent stream into many small, parallel streams, reducing the Reynolds number and creating smoother, quieter flow. This reduces reoxidation and helps keep the metal clean downstream of the filter.

Measurement & Quality Control Terminology

Terms related to testing and qualifying filters – and to measuring melt quality before casting.

C.E. Cup (Carbon Equivalent Cup)

Another name for a thermal analysis cup. Used to measure carbon equivalent (CE%), carbon (C%), and silicon (Si%) in molten iron.

CE (Carbon Equivalent)

A calculated value that expresses the combined effect of carbon and silicon on the solidification behavior of cast iron. CE = %C + (%Si / 3). Higher CE promotes graphite formation and reduces chill; lower CE promotes carbide formation and increases hardness. Critical parameter for gray and ductile iron quality control.

Tellurium Cup

A thermal analysis cup containing tellurium. The tellurium stabilizes the formation of cementite (white iron structure) during cooling, making the cooling curve easier to interpret for CE, C, and Si calculation.

Nodularity Cup

A thermal analysis cup designed specifically for ductile iron. The cooling curve shape indicates nodularization quality (how spherical the graphite nodules are) and can predict shrinkage tendency.

LiMCA (Liquid Metal Cleanliness Analyzer)

An instrument that measures inclusion content in molten aluminum in real time by passing an electric current through a small orifice and counting voltage pulses as non-conductive particles pass through. Used as the primary tool for assessing ceramic foam filter performance.

RPT (Reduced Pressure Test)

A simple test for hydrogen content in aluminum. A molten aluminum sample is poured into a small cup and allowed to solidify under reduced pressure (a vacuum chamber). The solidified sample is sectioned and examined for porosity. Higher porosity indicates higher dissolved hydrogen.

NDT (Non-Destructive Testing)

Testing methods that examine castings without damaging them. Common NDT methods include radiography (X-ray), ultrasonic testing, dye penetrant inspection, and magnetic particle inspection. Distinguished from destructive testing, which involves sectioning or mechanically testing a casting to destruction.

Destructive Testing (DT)

Testing methods that require the casting to be cut, sectioned, or mechanically tested to failure. Provides precise data on internal structure and mechanical properties but consumes the test piece. Includes tensile testing, hardness testing, metallographic examination, and impact testing.

Feeding & Riser Terminology

Filters remove inclusions; risers prevent shrinkage. These terms describe the feeding side of the equation.

Riser

A reservoir of molten metal connected to the casting cavity. As the casting solidifies and contracts, the riser provides additional liquid metal to fill the void, preventing shrinkage defects. Also called feeder, feeder head, sink head, or metal head.

Riser Sleeve

The refractory sleeve placed around the riser cavity. It slows heat loss (insulating sleeve) or generates additional heat (exothermic sleeve) to keep the riser molten longer, improving feeding efficiency. Available in three types:

• Insulating sleeve: Slows heat loss through low thermal conductivity

• Exothermic sleeve: Generates additional heat through aluminothermic reaction

• Hybrid sleeve: Combines both mechanisms for maximum efficiency

For a detailed comparison, see Exothermic vs. Insulating Riser Sleeves: Which One Do You Need?.

Breaker Core (Washburn Core)

A constricted, often friable section at the neck of a riser that makes it easy to knock the riser off the casting during finishing. After solidification, a light tap breaks the casting free at the breaker core, reducing grinding and finishing labor. Also referred to as knock‑off core or break-off core.

Feed Metal

The volume of liquid metal required from the riser to compensate for solidification contraction. Must be calculated to ensure the riser contains enough metal to feed the casting without running dry.

Modulus (Casting Modulus)

A measure of a casting section’s cooling rate, calculated as Volume ÷ Surface Area (V/A). Sections with higher modulus cool more slowly and require larger or more efficient risers to feed them. The modulus method is a simple, reliable way to size risers without simulation software.

Gating System Terminology

Where the filter lives – and where inclusions are born if the design is poor.

Gating System

The complete set of channels that carry molten metal from the pouring cup to the casting cavity. Includes sprue, runner, gates, and (optionally) filter and slag trap. The gating system controls flow rate, reduces turbulence, and – when properly designed – helps trap inclusions.

Choke (Choke Area)

The smallest cross‑sectional area in the gating system. The choke controls the flow rate of molten metal. The filter should never be the choke – the choke should be elsewhere in the system.

Sprue (Downsprue)

The vertical channel that carries molten metal from the pouring cup downward into the runner system. Proper sprue design (tapered shape, rounded bottom) minimizes turbulence and air entrainment.

Runner

The horizontal channel that distributes molten metal from the sprue well to the ingates. Filters are typically placed in the runner, where flow is more stable than at the sprue base.

Ingate (Gate)

The final channel through which molten metal enters the casting cavity. Ingate design – size, shape, location, and angle – strongly influences flow characteristics and defect formation.

Application & Alloy Terminology

Which filter for which metal? These alloy designations tell you what the filter was designed to handle.

LPDC (Low Pressure Die Casting)

A casting process in which the die sits above a sealed furnace. Regulated pressure (0.5–1 bar) pushes molten aluminum upward through a riser tube into the die. Widely used for aluminum wheels and structural components. Filtration is typically placed in the launder before the shot sleeve – not in the runner system.

HPDC (High Pressure Die Casting)

A casting process in which molten metal is injected into a steel die at high velocity (30–100 m/s) and high pressure (up to 2,000 bar). Traditional runner filters cannot be placed in HPDC because there is no runner – metal is injected directly from the shot sleeve. Filtration must occur in the launder before the shot sleeve.

Investment Casting (Lost-Wax Casting)

A precision casting process in which a wax pattern is coated with ceramic slurry to form a shell, then the wax is melted out and the shell is fired. Filters must survive the shell‑building and dewax processes, so filter cups (pre‑assembled pouring cup with integrated filter) are commonly used. Zirconia foam is typical for high‑temperature alloys.

Sand Casting

The most versatile casting process, in which a mold is formed from compacted sand around a pattern. Filters are placed in the runner system, horizontally or vertically, and must be properly sealed to prevent bypass.

Breaker Core

Described in Part VII above – a constricted neck on a riser that allows easy knock‑off. Also used as a term for the same feature on filter cups and other gating components.–

Dross Defects (Dross Related Defects)

Defects caused by the entrapment of dross (magnesium reaction products in ductile iron, or oxide films in aluminum) in the casting. Dross is typically finer and stickier than slag, making it harder to remove with fiberglass mesh. Ceramic foam filters (particularly SiC for ductile iron) are the standard solution.

Emerging Technology Terminology

What’s next in foundry filtration.

Additive Manufacturing (3D Printing)

A manufacturing process that builds objects layer by layer from a digital CAD file. Used to produce ceramic foam filters with exact pore geometries, eliminating the random structure of traditional foam filters. 3D-printed filters offer exact reproducibility, no filter bits, and design freedom impossible with conventional foam impregnation methods.

EXACTPORE

A registered trademark for ASK Chemicals’ 3D‑printed ceramic foam filter technology. Achieves precise pore size control and consistent flow properties from filter to filter.

3D Printed Filters

Additively manufactured ceramic filters that offer exact pore geometries, eliminating the random structure of traditional foam filters. Benefits: no “filter bits” (detached particles), design freedom for custom geometries, consistent flow properties from filter to filter.

Functionalized Filters (Intelligent Filters)

An emerging generation of ceramic foam filters with reactive coatings or functionalized surfaces designed to chemically remove specific types of inclusions (e.g., deoxidation products, sulfides, nitrides) rather than just physically trapping them. Also called smart filters or reactive filters.

Quick Reference Table

Summary of Key Filter Specifications by Alloy

This glossary covers the terms foundry professionals encounter most often when specifying, using, or troubleshooting ceramic foam filters. But the foundry industry is full of jargon, and every shop has its own local terminology.

If you come across a term that’s not listed here – or if you’d like us to expand the glossary in a future update – please let us know. We’re happy to help.

Contact SF-Foundry Technical Support:

Email: info@sf-foundry.com
Phone / WhatsApp: +86 18636913699
Website: www.sf-foundry.com