Porosity defects and countermeasures of lost foam steel castings

Porosity is a common defect in lost foam steel castings. It is necessary to analyze the causes of its occurrence and propose corresponding preventive measures to provide guidance for the better application of lost foam casting technology in the production of steel castings.

1. Causes of invasive pores

Invasive pores are the main form of pores in lost foam steel castings. The causes of their formation are as follows.

(1) During the pouring process, the pouring time is too short and the filling is too fast. The foam plastic pattern cannot be quickly vaporized, and the cracked liquid product enters the molten metal. After the casting solidifies, the gas cannot be discharged, and pores are formed in the casting.

(2) The gas emission of the foam plastic pattern increases sharply with the increase of pouring temperature. Figure 2 shows the change of gas pressure with pouring time during pouring. It can be seen that the instantaneous gas pressure increases sharply during pouring. Due to the high pouring temperature of cast steel, the instantaneous gas emission during pouring is very large, and the gas cannot be discharged in time, especially when the molten steel is in a turbulent state, the gas is entrained in it, forming pores.

(3) Improper use of adhesives for bonding foam plastic patterns is an important cause of invasive pores. In actual production, it is found that a large number of pores are often generated at the bonding point of foam plastic patterns in steel castings. If the foam plastic pattern is connected by plugging or nailing, there will be no pores in the casting. This is because more adhesive is used in the connection. Whether it is 851 super glue or AB glue, its density is several times or even dozens of times that of foam plastic. Its vaporization speed is much slower than that of foam plastic and its vaporization temperature is higher. When the glue has not yet vaporized, the metal liquid rises and wraps the glue. It continues to vaporize during the subsequent cooling process, thus forming pores in the casting.

(4) Moisture contributes to pore formation in steel castings. Its sources during pouring include:

Incompletely dried paint or water-based coatings containing organic adhesives (CMC, polyvinyl alcohol, latex, starch). If a coated pattern is left unused, it absorbs ambient moisture. Additionally, coating agents like bentonite retain structural and bound water, which vaporizes under molten steel heat as drying at ~40°C is insufficient.
Foam plastic patterns with residual moisture due to inadequate drying.
Water content in molding sand.

2. Causes of precipitation pores

Precipitation pores form when molten metal absorbs air during smelting and releases it during solidification. Water vapor in the metal decomposes under high temperatures, producing hydrogen, which dissolves into atoms and re-polymerizes into gas during solidification. Trapped hydrogen forms pore defects. These pores have smooth walls with a metallic luster, are small in size but numerous.

3. Measures to solve the pores in steel castings

From the above analysis, it can be seen that the main causes of pores in lost foam steel castings are that the pouring time is too short, the filling is too fast, the foam plastic pattern cannot be quickly vaporized, and the adhesive used to bond the foam plastic pattern is improperly used during the pouring process. In addition, the pouring system is not designed reasonably, and there are factors such as the pattern, coating and pouring.

Reduce the amount of gas and reduce the gas emission rate

(1) To reduce gas amount while maintaining pattern strength and rigidity, make the sprue, riser, and thick parts hollow. The foam plastic used in our factory has a density of 0.018–0.020g/cm³.

(2) Reduce the pouring system size, adjusting the inner runner from 20mm×15mm to 15mm×15mm, and extend pouring time to ~20s. Lower pouring speed to slow polystyrene gas emission, allowing gas to escape through the coating without entering the molten steel. Adequate vaporization time enhances vaporization rate, minimizing liquid residue at the molten steel-coating interface and preventing rim-side pores.

Use a binder with low gas emission

The less the amount of binder used, the better. It is best to form the riser and the product as a whole at one time, or use plug-in or steel nail connection, or use a combination of binder and steel nails.

Ensure the quality of the coating

(1) Use a high-temperature permeable coating to enhance permeability and wettability, reducing air pressure in the gap and ensuring smooth metal flow. In dry sand lost foam casting, dry sand has much higher permeability than the coating. The permeability of the coating mainly depends on the size of the permeability of the coating. Low coating permeability, much lower than the pattern’s decomposition rate, causes high air gap pressure and unstable molten metal flow, leading to air entrainment and inhalation.

(2) The coating should be evenly applied and have high high-temperature strength to reduce the coating thickness, which is generally controlled at 0.5~1mm.

(3) The coating should be fully dried, and the dried mold should be cast in time to avoid moisture.

We improved the coating by adjusting three factors: silica sand size from 0.071mm (200 mesh) to 0.080mm (180 mesh), latex from 25kg to 20kg, and cellulose from 4kg to 5kg. The improved coating has enhanced high-temperature strength and air permeability, and is not easy to crack.

Set up slag collection and exhaust risers

The gas decomposition products of polystyrene foam plastic molds must escape from the mold cavity through the coating. Fast metal filling prevents full pyrolysis and vaporization, causing liquid residues to accumulate in casting corners or tops, leading to pores. To prevent this, small risers should be placed at these points, with the coating on the riser top pierced with small holes for gas release during vaporization. Riser size should match the product, and smaller risers are easier to clean. Practice has proved that these small risers can effectively eliminate slag inclusions and pore defects in castings.

Choose the correct pouring process

(1) Position the casting so that key large surfaces are vertical or inclined, ensuring smooth metal flow, gradual pattern vaporization, and balanced air gap pressure.

(2) The pouring system should preferably use closed bottom pouring, step pouring or side pouring to ensure the smooth flow of the molten metal and avoid a large amount of air entering the cavity during the pouring process.

(3) Increasing the pouring temperature of the molten metal and reducing the pouring speed are conducive to the full vaporization and rapid discharge of the foam plastic pattern. Practice has proved that increasing the pouring temperature can effectively reduce a series of casting defects such as pores. The pouring temperature of steel castings should be above 1600℃. At this time, the coating should have a high high temperature strength, otherwise it will cause serious sand sticking.

(4) Follow the “slow-fast-slow” pouring principle: start with a small flow to pass through the sprue, then increase flow to quickly fill the sprue cup and seal the sprue, maintaining vacuum until the molten metal stabilizes. As the liquid level nears the casting top, slow the pouring speed to allow gas to escape through the coating or precipitate from the molten metal.

The molten steel should be fully degassed before pouring

(1) Clean the charge, especially the charge with severe rust should be derusted.

(2) Rapid smelting to shorten the high temperature smelting time.

(3) Fully deoxidize, the amount of aluminum is generally 0.04% to 0.06%, not more than 0.15%.

In addition, the molding sand should be dry, and do not use water-containing or wet molding sand. The vacuum negative pressure of steel castings should generally be controlled at 0.05-0.06MPa.

yvonne@nydcasting.com +86 13365137968