Causes of Gas Porosity in V-Process Casting and Countermeasures

V-process casting, also known as vacuum-sealed molding, negative-pressure molding, or reduced-pressure molding, originated in Japan. The V-process casting system adopts dry sand without any binder for molding. Plastic film is used to seal the mold, and a vacuum pump creates a pressure difference between the inside and outside of the mold, compacting the dry sand to form the mold cavity. Molten metal is then poured to obtain high-precision castings.

Due to its high reclaimed sand utilization rate, clean working environment, and high metal yield, the V-process has been listed by the state as a clean production technology, and it has been widely applied in the foundry industry.

It is generally believed that during pouring, the mold cavity in V-process casting is always under vacuum negative pressure, allowing gases to be easily discharged through connecting passages and open risers, thus minimizing the occurrence of gas porosity. However, in actual production, gas-related defects remain one of the most prominent defects in V-process castings.

Practice has shown that gas porosity is not unavoidable. As long as the formation mechanisms are correctly identified, effective measures can be taken to control and eliminate such defects.

1. Causes of Gas Porosity in V-Process Casting

Gas porosity defects are formed when gas is entrapped in the molten metal and fails to escape before solidification. On the casting surface, they usually appear as round, oval, or teardrop-shaped holes.

1. Entrapped porosity caused by improper gating system design

If the gating system is unreasonable, severe turbulence or air entrainment may occur during pouring. When the gas cannot be discharged from the molten metal before solidification, entrapped gas porosity is formed.

2. Reaction porosity caused by coating and EVA film gas generation

The coating is not fully dried, containing excessive moisture and organic matter, resulting in high and rapid gas evolution;

The EVA plastic film is too thick or has poor gasification performance;

EVA film at core fixing positions is not slit or perforated to guide gas discharge.

These factors lead to gas accumulation from coating decomposition or film gasification, forming reaction porosity.

3. Invasive porosity caused by sand cores

Water-glass sand cores, resin sand cores, or shell sand cores that are insufficiently dried or excessively thick generate a large amount of gas. This gas may block venting paths and invade the molten metal, forming invasive gas porosity.

4. Entrapped porosity caused by improper pouring conditions

Low pouring temperature or excessively fast pouring speed may cause gas to be trapped in the molten metal during solidification, resulting in entrapped gas porosity.

2. Countermeasures for Gas Porosity in V-Process Casting

Long-term production practice has proven that different combinations of gating systems, sand grain size, coating properties, and EVA film parameters have a significant influence on gas porosity defects. Scientific and rational gating system design is the key to preventing gas porosity and other casting defects.

1. Rational design of the V-process gating system

The gating system should avoid turbulence and vortex formation to prevent air entrainment;

A proper pouring speed should be ensured so that the sprue remains fully filled with molten metal throughout pouring.

2. Strict control of melting and core-making processes

Optimize melting practice to reduce gas content in the molten metal;

Minimize the thickness of EVA film;

Strictly control V-process core-making parameters to prevent invasive porosity.

3. Optimization of coating performance and reduction of gas evolution

The coating should have good adhesion to ensure firm bonding to the EVA film. While maintaining excellent refractory and strength properties, the gas evolution of the coating should be minimized as much as possible.

Based on repeated trials, the gas evolution of V-process special coatings should be controlled within

≤10 mL/g (at 100 ± 5 °C).

3. Key Control Points for V-Process Coating Application

V-process casting generally uses alcohol-based fast-drying coatings, with ethanol as the solvent and resin added to improve coating strength and adhesion. However, ethanol and resin are hydrocarbons, flammable and prone to gas generation, so strict control is required.

1. Strict control of solvent and additive quality

Ethanol purity should be ≥0.95;

The addition amounts of ethanol and resin must be strictly controlled;

Moisture and impurity contents should be minimized;

Coating Baumé degree should generally be maintained at about 75 °Bé.

2. Proper control of coating thickness

On the premise of ensuring casting quality, coating usage should be minimized;

The coating thickness in general areas should be controlled at approximately 1.5 mm;

At hot spots, to prevent sand adhesion, the thickness may be increased to approximately 2.0 mm;

The coating must be thoroughly dried. If fast-drying performance is insufficient, forced hot-air circulation drying should be adopted.