Fiberglass Cap Filter for Molten Aluminum — Dross Removal & Laminar Flow

Why Aluminum Foundries Rely on Cap Filters?

Dross, folded oxide skins, and non-metallic inclusions are the most common sources of scrap in aluminum casting. They are introduced into the melt during melting, transfer, and pouring, and if they reach the mold cavity, they become slag holes, sand inclusions, pressure leaks, and machinability problems that appear at X-ray inspection or on the customer's machining line.

A flat mesh screen inside the runner offers some protection, but it depends on a tightly machined seat — and that seat wears as sand shifts and the cope settles. Before long, aluminum finds its way around the filter, and all that dross passes directly into the casting.

A cap filter solves that installation problem. It slips over the outside of the pouring cup or riser neck and seals mechanically, so 100 percent of the metal is forced through the woven mesh — no bypass, no worn seat, no operator-dependent placement. The result is consistent, predictable filtration on every pour.

Specifications

How It Works?

As molten aluminum passes through the mesh, three filtration mechanisms act simultaneously:

1. Barrier filtration. Any particle larger than the mesh opening — dross, oxide skin, sand, refractory wash, flux lumps — is retained on the upstream surface.

2. Depth adsorption. Finer inclusions small enough to pass through the openings are adsorbed onto the fiber surface as the metal flows through the woven mesh.

3. Flow straightening. The mesh divides the falling jet into many thin streams, lowers the Reynolds number, and allows the cavity to fill in laminar fashion rather than folding over and reoxidizing. This also permits light inclusions to float out rather than being trapped in the casting.

Typical Applications

1. Casting processes

● Sand casting (green sand, resin sand)

● Gravity die casting / permanent mold

● Low-pressure die casting (LPDC) — standard on aluminum wheel lines

2. Investment (lost-wax) casting

● Common aluminum parts

● Aluminum wheels (LPDC)

● Engine blocks, cylinder heads, pistons

● Transmission housings, suspension knuckles, EV motor housings

● Pump bodies, valve bodies, hydraulic components

● Cookware, electrical housings, hardware

3. Compatible alloys

● A356, A380, ADC12, 3xx.x series, and general aluminum foundry alloys.

How to Choose the Right Size?

1. Grade

● Standard brown — general-purpose aluminum castings.

● Carbonized black — pistons, premium wheels, investment casting, EHS-sensitive shops.

2. Mesh opening

● 1.0 – 1.2 mm — aluminum wheels and thin-wall precision castings.

● 1.5 – 1.7 mm — general-purpose aluminum castings.

● 2.0 – 2.5 mm — large ingots, billets, heavy sections.

3. Cap diameter

The outside diameter of the pouring cup or riser neck should be measured, and a cap with an inside diameter roughly 2–3 mm larger should be selected for a snug slip fit.

4. Preheat(Optional)

A short preheat to 100–150 °C drives off stored moisture. Filters should be stored in a dry bin.

Manufacturing Process

Hot Press Forming      &      Edge Trimming

Frequently Asked Questions

1. Can the caps be reused?

No. They are single-use consumables. After pouring, the mesh is brittle and may hold trapped inclusions — the used cap should be discarded with the gating system.

2. Do I need to preheat them for aluminum?

It is Optional. A 100–150 °C preheat removes stored moisture. Room-temperature filters stored in a dry bin are suitable for use.

3. What temperature do they handle?

Standard E-glass is rated to approximately 800 °C, which covers the 680–800 °C aluminum pouring window.

4. Why use a cap instead of a flat mesh disc?

A flat disc sits inside the runner and relies on a tight seat. That seat wears, and aluminum flows around the filter. The cap slips over the outside of the pouring cup and seals mechanically — no bypass, no machined seat.

5. Does the fiberglass add iron to the melt?

No. E-glass is inert and iron-free. This is why it is preferred over stainless-steel wire mesh in aluminum foundries — stainless dissolves Fe into the melt, and 3xx.x and ADC12 alloys have tight iron content limits.