During pouring, molten slag flows into the mold cavity along with molten metal, resulting in casting defects known as slag holes. In foundry production, numerous castings are scrapped due to slag holes, which have become a major cause of rejection, especially for high-precision machine tool hydraulic components. Slag holes stem from various factors including smelting conditions, pouring operations, molding processes and gating system design.

1. Slag Removal Measures in Smelting Process

A large amount of slag is generated during cast iron melting. Complete separation of molten iron and slag before molten iron enters the ladle is essential. Slag entering the ladle increases the risk of flowing into the cavity, and slag skimming inside the ladle will lower molten iron temperature. Therefore, timely slag tapping during smelting is critical.

In cupola furnaces, viscous molten slag forms from coke ash, sand grains and peeled refractory lining materials, which is hard to discharge. Appropriate limestone flux shall be added during melting. Limestone decomposes into calcium oxide under high temperature and reacts with furnace slag to form low-melting-point slag. Such slag flows into the forehearth together with molten iron and stratifies by density, with molten iron at the bottom and slag floating on top. Open the slag outlet of the forehearth regularly for slag tapping to realize iron-slag separation. Insufficient flux leads to thick slag that cannot flow out smoothly.

Properly raise molten iron temperature. Low temperature makes slag viscous and difficult to discharge, and also increases resistance for suspended fine slag to float upward, hindering effective removal.

2. Slag Removal Before and During Pouring

Certain amount of slag inevitably flows into the ladle when molten iron is transferred from forehearth. Complete slag skimming in the ladle is required before pouring to avoid slag holes.

Spray slag coagulant evenly on the surface of molten iron in the ladle to bind floating slag together for easy removal with iron rods. Common slag coagulants include perlite and volcanic ash, among which perlite delivers the best effect. Rice husk ash can keep molten iron warm but has poor slag gathering performance; dry sand almost has no slag collecting effect.

Assign dedicated workers to block floating slag with slag stoppers during pouring. Avoid pouring interruption, which may fail to fully fill the sprue, damage the slag retaining function of the gating system and allow retained slag in the pouring cup to enter the cavity and form defects.

3. Slag Prevention in Molding Operation

Visible slag can be blocked manually, while invisible fine slag must be trapped by well-designed gating systems.

• Optimize pouring cup design to prevent liquid vortex and retain floating slag inside the cup.

• Standardize cross-section shape of ingates and runners. The main function of runners is slag retention. Runners adopt high trapezoidal or round-top trapezoidal sections, enabling slag to float upward and stay at the upper part while clean molten iron flows into the cavity from the bottom. Flat trapezoidal ingates reduce suction range and strengthen slag retaining effect.

• Keep sufficient distance between the first row of ingates and the sprue to ensure enough time for fine slag to float upward.

4. Special Gating Systems for Enhanced Slag Removal

Conventional gating systems have limited slag separating capacity. Specialized structures are adopted for castings with strict quality requirements.

• Slow-flow runners are widely used for mass production of medium and small important castings; choke runners are applied for products requiring high slag exclusion.

• Centrifugal slag traps are installed between runners and ingates for large and medium-sized key castings. Molten iron flows tangentially into the trap and forms swirling flow. Under centrifugal force and buoyancy, slag gathers at the top center for excellent slag removal effect.

• Integrate centrifugal slag traps with risers for high-standard thick-walled small castings to achieve both feeding and slag removal, improving process yield.

Implement timely slag tapping in smelting, thorough slag skimming before pouring, effective slag blocking during pouring and rational gating & riser system design. Strictly control every process link to prevent molten slag from entering mold cavities and eliminate slag hole defects fundamentally.