Where Are the Problems of Sand Adhesion, Slag Inclusion, and Porosity?

Looking at casting defects both domestically and internationally, the main issues boil down to porosity, slag inclusion, and shrinkage. The core causes are air entrapment and the use of cores. Consequently, vacuum casting and coreless casting methods have emerged as solutions, leading to the development of vacuum lost-foam casting (V-LFC).

Here, I emphasize the word “vacuum”, because it must be fully utilized to address the defects encountered in casting. The advantages of casting in a vacuum or negative-pressure environment are as follows:

1. Reduces porosity defects in castings.
2. Extracts slag inclusions from castings.
3. Improves mold filling ability.
4. Reduces heat loss during pouring.

These benefits are specific to vacuum lost-foam casting, but the key lies in the permeability and slag-passability of the coating.

Over the years, the domestic vacuum lost-foam casting industry has not addressed the concept of slag-passability. Therefore, I propose a technical approach using fine sand and thin coatings to solve various problems currently faced in China. This approach is not theoretical—it has been applied successfully for many years.

• Fine sand specifications: 80–100 mesh
• Thin coating thickness: 0.3–0.6 mm

Using fine sand as the mold material and thin coatings in aluminum alloy vacuum lost-foam casting, one can observe during mold break-out that the mold surface has a tar-like residue. This residue forms because the aluminum alloy pouring temperature is not high enough to fully vaporize the foam pattern, creating a gelatinous substance. With vacuum suction and slag-passable coating, this gelatinous material is drawn out of the mold cavity and absorbed into the sand, effectively addressing porosity and slag issues in aluminum castings.

From my years of experience in vacuum lost-foam aluminum casting, the same principle applies to vacuum lost-foam cast iron and steel casting. Therefore, I advocate using fine sand and thin coatings in vacuum lost-foam casting. The advantages are:

1. High permeability, effectively reducing porosity.
2. High slag-passability, removing carbonaceous slag and reducing defects.
3. High flowability of fine sand, improving casting precision and reducing sand adhesion.
4. Reduced coating usage, lowering material costs.
5. Faster drying, reducing drying costs.
6. Minimized casting backspray, preventing foam wetting and backspray defects.
7. Low gravitational impact of fine sand, reducing damage to foam patterns and avoiding deformation.
8. Thin coating during slurry application, preventing deformation or breakage of foam patterns.

In contrast, many domestic lost-foam casting factories use thick coatings and coarse sand, sometimes even pea-sized sand. Because coarser sand requires stronger coatings to prevent sand adhesion, thicker coatings are used. However, thicker coatings reduce permeability, and coarse sand further worsens airflow. This creates a vicious cycle:

• Foam patterns easily break during slurry application.
• Multiple slurry coats are needed, allowing moisture to re-enter the foam pattern and causing backspray during pouring.
• Thick, poorly permeable, low-flow coatings lead to porosity and slag defects.

To compensate, pouring temperature is increased and gating is enlarged, which raises costs. Consequently, many lost-foam casting operations are inefficient and costly. Some even resort to creating ceramic shells like traditional lost-wax casting, burning out the foam first, and then pouring, completely defeating the purpose of vacuum lost-foam casting.