Penetrating gas porosity accounts for more than 60% of all gas pores in iron castings, making it the most prevalent and troublesome type of porosity in casting production. Characterized by large size and smooth inner walls, it mostly appears on the surface layer of castings and is difficult to remedy once formed. Starting from root causes, this paper systematically sorts out the core strategies to prevent penetrating gas porosity, helping you reduce the porosity reject rate to below 8%.

I. Basic Principles: How Does Penetrating Gas Porosity Penetrate into Castings?

The essence of penetrating gas porosity lies in the gas generated by sand molds or cores under the action of high-temperature molten metal. When the gas pressure at the mold-metal interface exceeds the flow resistance of the molten metal, the gas forcibly wedges into the liquid metal. Failing to float up and escape in time, the gas is finally trapped inside the casting during solidification.

II. Cut Off Gas Sources – Reduce Gas Evolution of Molding & Core Sand

• Strictly control the moisture content of molding sand

  The moisture of green sand shall not be excessive, especially for aluminum alloy casting; the moisture content of green sand shall be controlled below 6.0%. Once water vaporizes, its volume expands thousands of times, causing an instantaneous surge of gas pressure at the mold-metal interface. The moisture content of molding sand for steel castings shall not exceed 5.5%, and stricter control is required for ductile iron castings.

• Restrict gas-generating substances

  The content of gas-evolving materials such as coal powder and heavy oil shall be properly controlled, as excessive addition brings adverse effects instead of benefits.

  Minimize water brushing during pattern drawing and mold repairing to avoid local excessive moisture.

  Nitrogen and hydrogen decomposed gases from furan resin sand are the primary culprit for penetrating porosity in aluminum alloy castings. Low-nitrogen binders can be adopted as an alternative.

• Guarantee thorough drying quality

  Dry molds and surface-dried molds must be fully dried. Molds shall be assembled and poured promptly after drying, with no prolonged storage to prevent moisture reabsorption and gas adsorption. This requirement is particularly critical for the production of large castings — moisture reabsorption will render all previous work futile.

• Chills and chaplets must meet the "Three No’s" standard

  Free of rust, oil contamination and moisture, and kept thoroughly dry. 

III. Unblock Venting Channels – Improve Mold Permeability

1. Reasonably Control Mold Compaction Degree

   The higher the compaction degree, the poorer the permeability and the greater the tendency to form penetrating gas porosity. Reduce the compaction degree as much as possible while guaranteeing sufficient mold strength.

   Position	Compaction Requirement	Reason
   Mold wall, mold bar	Relatively dense	Ensure sufficient strength to prevent mold collapse during lifting and handling
   Lower part of sand mold	Denser than the upper part	Resist erosion impact from molten metal
   Mold cavity surface	Relatively dense	Resist scouring by molten metal
   Areas far from mold cavity	Relatively loose	Facilitate gas venting

2. Drill abundant vent holes – a simple yet most effective measure

   After ramming and leveling the cope mold, pierce vent holes with vent needles. The needle diameter ranges from 2 mm to 8 mm, with no less than 4–5 holes per square decimeter.

   The hole depth shall be 2–10 mm away from the pattern surface.

   Blind vents shall be pierced above casting sections, and open vents shall be arranged at the highest points of the mold cavity.

   The total cross-sectional area of all vents shall be greater than or equal to the total cross-sectional area of all ingates to guarantee unobstructed gas discharge.

3. Core Venting – Top Priority

   Core Type	Venting Scheme
   Simple small core	Pierce vent holes at the central position
   Complex / curved core	Embed wax threads or straw ropes, which burn out under high temperature to form gas passages
   Heavy rectangular core	Fill coke / slag inside to reduce sand layer thickness, and connect vent holes drilled on core prints to the inner cavity
   Long cylindrical core	Adopt iron tube as core frame, drill radial small holes on the tube, wrap straw rope externally

   
   

1. Appropriately increase pouring temperature

   After raising the pouring temperature: the viscosity of molten metal decreases with improved fluidity; the crust formation time is prolonged, giving penetrating gas sufficient time to float upward and escape. Aluminum alloys are particularly sensitive; low-temperature pouring almost inevitably causes gas porosity defects.

2. Reduce pouring speed and realize smooth mold filling

   Excessively fast pouring will entrain gas and generate trapped air, while sand cores cannot vent gas in time. Recommendations:

   Slow down the speed at which molten aluminum covers sand cores;

   Minimize the distance between the ladle and the sprue cup;

   Replace conical sprue cups with flat oval ones to avoid vortex formation;

   Prevent sharp bends in the runner to ensure the molten metal fills the mold cavity smoothly without impact.

3. Increase the height of the sprue

   This raises the static pressure of molten metal and increases the resistance against gas penetration. It is a simple adjustment that is often overlooked in production.

Coat the surface of sand molds with coatings featuring low gas evolution and low permeability, which form a "gas barrier" between molten metal and molding sand to block gas from penetrating into the mold cavity.

For the prevention of subsurface pinholes in steel castings, an appropriate amount of coal powder or heavy oil can be added to molding sand to form a reducing isolating gas film at the mold-metal interface. Meanwhile, the moisture content of molding sand shall be controlled at ≤5% and the permeability maintained above 200. On this basis, adding Xinda casting coating will bring an obvious upgrade to your anti-porosity effect.

Xinda series mold coatings are customized for anti-pore production demand. With ultra-low gas evolution and compact film-forming property, they build a denser gas isolation layer than ordinary coatings, effectively suppressing the generation of penetrating gas pores and subsurface pinholes of steel castings. Compatible with clay sand, furan resin sand and cold box core processes, Xinda coating can perfectly cooperate with coal powder and heavy oil to cut reject rate caused by gas defects, greatly improving the surface finish and yield of iron & steel castings.