How to use fiberglass filtration mesh for metal casting

In cast iron, aluminum, steel castings, the slag contained in the metal solution is not easy to clean, it will cause slag holes, pores and other defects and waste, so how to use the casting filter mesh?

The filter mesh can be cut into any specification, placed on the sprue parting surface of the box pressing, such as the combination of filter net and slag bag, its effect will be better.

However, simply placing a piece of filter mesh in the mold does not guarantee better casting quality. The filter material, mesh opening, filtration area, installation location and support structure must all match the alloy and casting process.

This guide explains how to use fiberglass filtration mesh correctly and avoid common problems such as filter movement, metal bypass, slow mold filling and incomplete castings.

Quick Guide: How to Use Fiberglass Filtration Mesh

The basic process includes eight steps:

1. Identify the molten metal and pouring temperature.

2. Select the correct fiberglass filter material.

3. Choose a suitable mesh opening and filter area.

4. Determine the filter location in the gating system.

5. Prepare a secure filter pocket or support.

6. Install the mesh flat without gaps or wrinkles.

7. Pour the molten metal without excessive direct impact.

8. Inspect the casting result and adjust the system if necessary.

Each step affects filtration efficiency and mold-filling performance.

Step 1: Identify the Molten Metal

The first step is to confirm which metal or alloy will pass through the filter.

Standard treated fiberglass filtration mesh is primarily used for molten aluminum and aluminum alloys. High-silica fiberglass mesh is designed for higher-temperature applications and may be used for selected cast iron, copper-alloy and small steel-casting processes when the product is specifically rated for those metals.

Do not use a standard aluminum fiberglass mesh for molten iron or steel simply because the mesh dimensions appear suitable. An incompatible filter can soften, break down or contaminate the casting system.

When requesting a filter recommendation, provide:

• Metal or alloy grade

• Normal pouring temperature

• Maximum pouring temperature

• Casting weight

• Pouring time

• Casting process

• Current defect type

Step 2: Select the Correct Filter Type

Fiberglass casting filters are available in several forms.

Flat Fiberglass Mesh

Flat mesh can be cut into round, square, rectangular or custom shapes.

It is commonly installed:

• Under the pouring cup

• At the top or bottom of the sprue

• On the mold parting surface

• Inside a runner

• Before an ingate

Flat sheets are economical and suitable for many conventional sand and gravity-casting systems.

Rigid Fiberglass Filters

Rigidized fiberglass filters maintain their shape more effectively than loose mesh.

They can be produced in shapes designed to fit existing filter pockets and reproduce the required metal-flow rate. Pyrotek notes that both the weave and filter shape affect flow performance, so dimensional fit and flow matching are important.

Rigid filters are suitable for:

• Permanent mold casting

• Repetitive sand casting

• Automotive aluminum castings

• Wheel and suspension components

• Low-pressure casting

• Applications where loose mesh may move

Fiberglass Cap Filters

Cap filters are formed into cup, hat, cone or box shapes.

They provide:

• A larger effective filtration surface

• Improved stability in the mold

• Better fit in selected sprue or filter-chamber designs

• More slag-holding area than a small flat sheet

Cap filters are commonly used for aluminum pistons, wheels, housings and other repeated production castings.

Fiberglass Filter Bags

Filter bags are used mainly in molten metal transfer systems, aluminum ingot casting and selected launder or outlet applications.

They may be installed at:

• Furnace outlets

• Ladle outlets

• Transfer funnels

• Launders

• Ingot or slab casting systems

The required shape should be selected according to the metal-flow path rather than convenience alone.

Step 3: Choose the Mesh Opening and Filtration Area

A finer mesh can capture smaller inclusions, but it also creates greater resistance to molten metal flow.

A filter that is too fine or too small may cause:

• Slow mold filling

• Reduced metal temperature

• Cold shuts

• Misruns

• Incomplete cavities

• Excessive pressure before the filter

• Filter deformation or displacement

A filter that is too open may allow larger inclusions to pass through.

The correct selection depends on:

• Alloy

• Casting weight

• Pouring rate

• Inclusion size

• Required cleanliness

• Metal temperature

• Filter shape

• Effective open area

• Gating-system dimensions

The filtration area should normally be larger than the original runner cross-section because only part of the mesh surface consists of open flow area. For high-silica mesh applications, one supplier recommends calculating filter area according to mesh porosity and filtration efficiency rather than matching the runner area directly.

For production castings, the filter size should be confirmed through process calculations, supplier flow data, casting simulation or controlled foundry trials.

Step 4: Choose the Best Filter Location

Filter location determines how early inclusions are captured and how the filter affects metal velocity.

Common positions include the top of the sprue, the runner overlap and the connection between the runner and ingate.

Under the Pouring Cup

Placing the mesh below the pouring cup filters molten metal at the beginning of the gating system.

Advantages:

• Early removal of larger slag particles

• Easy installation

• Easy visual inspection before mold closing

• Protection for the downstream runner system

Potential problems:

• Strong direct metal impact

• Premature blockage when the melt contains excessive dross

• Insufficient filter support

The pouring stream should not strike unsupported loose mesh directly.

At the Bottom of the Sprue

A filter at the sprue base removes inclusions before the molten metal enters the horizontal runner.

This position can also help reduce the energy of the descending metal stream.

It is commonly used for:

• Aluminum wheels

• Pistons

• Automotive components

• General sand castings

• Gravity castings

The mold should include a stable filter pocket and sufficient flow area.

Inside the Runner

Runner placement filters the metal before it is divided between different ingates.

This is useful for:

• Multi-cavity molds

• Long runner systems

• Castings with several ingates

• Large or complex components

Filtration in the runner can also create back pressure, help the runner fill more evenly and reduce the energy of the metal entering the ingates. Vesuvius testing shows that a filter installed in the runner can produce calmer and more regular mold filling when the filter and gating system are correctly designed.

Before the Ingate

Installing the filter close to the mold cavity provides final filtration before metal enters the casting.

This position is suitable for:

• Precision castings

• Thin-wall components

• Machined castings

• Products requiring high internal cleanliness

• Castings with strict surface requirements

Because the filter is close to the cavity, insufficient flow capacity may directly cause incomplete filling.

In a Low-Pressure Casting Sprue

Rigid fiberglass sprue filters can be used in selected low-pressure aluminum-casting systems.

The filter must:

• Fit the sprue accurately

• Maintain its shape during the pressure cycle

• Provide the required metal-flow rate

• Remain securely positioned

• Avoid releasing fragments into the metal

Rigid fiberglass filters have been developed specifically to maintain structural integrity through low-pressure casting cycles.

Step 5: Prepare a Secure Filter Pocket

A filter must not allow molten metal to flow around its edges.

Prepare a filter pocket or molded support that:

• Holds the filter flat

• Prevents sideways movement

• Prevents metal bypass

• Supports the filter against flow pressure

• Does not crush or damage the mesh

• Keeps loose sand away from the metal stream

For foam-filter installations, Vesuvius recommends holding the filter securely in a filter print and providing a downstream ledge so the pouring pressure forms a positive seal against metal bypass. The exact dimensions should be adapted to the selected fiberglass filter and mold design.

The filter should cover the entire flow path. A small gap around one side can allow unfiltered metal to enter the casting.

Step 6: Install the Filter Correctly

Before closing the mold:

1. Check that the filter matches the approved material and size.

2. Inspect it for tears, broken yarns, deformation or coating damage.

3. Remove loose dust and damaged filters from the work area.

4. Place the filter in the prepared pocket.

5. Keep the mesh flat and free from wrinkles.

6. Ensure the edges are fully supported.

7. Confirm that molten metal cannot bypass the filter.

8. Check that the filter will not move when the mold is handled.

9. Verify the correct orientation for shaped or curved filters.

10. Close the mold without crushing the filter.

For horizontally installed mesh, the surface should be flattened from the center toward the edges. Loose folds or wrinkles can reduce the effective flow area and produce uneven filtration.

Step 7: Prepare and Pour the Molten Metal

A fiberglass filter removes inclusions, but it does not replace correct melt treatment.

Before pouring:

• Remove furnace dross.

• Complete the required degassing process.

• Control the alloy temperature.

• Clean ladles and transfer tools.

• Minimize unnecessary turbulence.

• Prevent oxide skin from being folded into the melt.

• Keep the pouring stream stable.

The molten metal must have enough head pressure to pass through the filter, but the stream should not strike loose mesh with excessive force.

Where possible:

• Direct the initial stream toward a pouring-cup wall.

• Avoid uncontrolled free fall onto unsupported mesh.

• Maintain a continuous pour.

• Do not interrupt and restart the stream unnecessarily.

• Keep the pouring cup sufficiently full.

• Avoid excessive pouring height.

The filter should regulate the flow without becoming the only restriction controlling the entire mold-filling process.

Step 8: Evaluate the Casting Results

After introducing fiberglass filtration mesh, compare the filtered and unfiltered casting results.

Check:

• Pouring time

• Mold-filling time

• Filter condition

• Slag captured by the mesh

• Inclusion defects

• Oxide-film defects

• Surface finish

• Internal porosity

• Machining performance

• Rejection rate

• Metal yield

• Runner and ingate behavior

If the mold fills too slowly, possible causes include:

• Filter area is too small

• Mesh is too fine

• Metal temperature is too low

• Filter is partially blocked

• Pouring head is insufficient

• The gating system is too restrictive

If inclusions remain in the casting, possible causes include:

• Mesh opening is too large

• Filter bypass is occurring

• The filter is installed too early in the system

• Inclusions are generated downstream of the filter

• Melt treatment is inadequate

• The filter area is overloaded

Changes should be made one variable at a time so the effect can be measured.

Note

1. Use area of filter net

The relationship between the cross-sectional area F1 of the casting system and the original cross-sectional area F2 of the casting system without putting the filter net can be expressed as follows: 

F1 = F2 / AxB; 

A: the porosity of the filter screen, generally 50-60%. 

B: the filtration rate of filter screen, generally 60-80%.

Therefore, the cross-sectional area of the casting system is generally 2-4 times that of the casting system without the casting net.

2. Position of filter screen

A. Place the filter under the sprue cup;

B. Placed on the parting surface under the sprue;

C. Three different positions on the lap surface of the runner.

3. Precautions for use of filter screen:

When the filter is used, it is necessary to guard against oblique pulling, wear and break. The size of the filter block should be greater than the size of the gate area 20-30mm. Handling, handling should be careful to handle, prevent extrusion, moisture.

Casting filter screen for metal solution filtration purification treatment, remove the slag and part of the gas in the metal solution, improve its quality, reduce the scrap rate of high-quality products. It not only brings objective economic benefits to the majority of producers but also reduces energy consumption, improves labor productivity and product quality.

Fiberglass Mesh vs Ceramic Foam Filter

Fiberglass mesh is generally:

• Thinner

• Easier to cut into custom shapes

• Economical

• Suitable for many conventional aluminum castings

• Available as flat, rigid or shaped products

Ceramic foam filters generally provide:

• Depth filtration

• Greater internal inclusion-capture capacity

• Different flow-control characteristics

• Options for demanding aluminum, iron or steel applications

Rigid fiberglass filters can be designed to match the flow rate of existing ceramic foam filter systems in selected sand and permanent mold aluminum applications.

The best choice depends on casting quality requirements, alloy, inclusion load, flow rate and production cost.

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