Knowledge

Section 1: Permeability

Permeability refers to the ability of molding sand to allow gases to pass through its pores. It is crucial in the casting process because good permeability enables the smooth escape of gases generated from the evaporation of moisture, combustion of organic matter, and decomposition and volatilization of certain substances within the molding sand to the outside of the mold, thereby preventing the formation of gas holes in the castings.

 

The magnitude of permeability is mainly related to the type and particle size composition of the raw sand. The smaller the particle size, the poorer the permeability. At the same time, the type and addition amount of resin and catalyst also affect permeability. For instance, the permeability of various furan resin self-hardening sands is usually higher than that of palm oil sands. Additionally, factors such as the tightness of the molding sand, clay content, moisture content, shape, and uniformity also influence its permeability.

 

Poor permeability can lead to the inability of gases inside the castings to escape smoothly, resulting in defects such as gas holes and shrinkage cavities. These defects can seriously affect the mechanical properties and surface quality of the castings. Therefore, a series of measures need to be taken during the casting process to improve the permeability of resin sands, such as selecting appropriate raw sands, adjusting the addition amounts of resin and catalyst, and optimizing the sand mixing process.

 

1. Properties of raw sand:

The size of sand grains, particle size distribution, particle shape, and clay content all affect permeability. Raw sand with round, coarse particles, uniform particle size, and low clay content has better permeability. The finer the particle size of the raw sand, the more dispersed the particle size, and the higher the content of fine powder, the worse the permeability.

 

2. Molding (core) sand compactness:

The higher the compactness of the molding (core) sand, the smaller the voids and the lower the permeability. However, once the compactness reaches a certain level, the permeability does not change much.

 

3. Types and amounts of binders:

The greater the amount of resin added, the worse the permeability. This is because excessive resin will reduce the voids in the molding (core) sand.

 

4. Additives:

The addition of additives such as iron oxide powder will also have a certain impact on the permeability.

 

5. Sand Mixing Process:

When mixing sand, evenly coating the binder on the surface of the raw sand, as well as conducting uniform infiltration treatment and loose sand treatment on the molding (core) sand, can enhance its permeability.

 

6. Coating condition and coating layer thickness:

Core (mold) sand without coating has high permeability. A thick coating layer results in poor permeability. However, certain additives in the coating leave voids after high-temperature combustion, which can improve permeability.

 

Section 2: Membrane Removal Rate

 

The demolding rate refers to the degree of removal of the film material in resin sand, usually expressed as a percentage. Its calculation formula is as follows:

Demolding rate (%) = (Initial weight - Weight after demolding) / Initial weight × 100%.

Here, the initial weight refers to the initial weight of the resin sand during the preparation process, and the weight after demolding refers to the weight of the resin sand after demolding treatment.

The higher the demolding rate, the more thoroughly the film material in the resin sand is removed, and the better the quality of the resin sand. Good quality resin sand can improve the surface quality and dimensional accuracy of the casting, reduce the occurrence rate of sand mold breakage and casting defects, and increase the yield of the casting.

 

Section 3: High Temperature Disintegration Resistance

 

High-temperature collapsibility is influenced by multiple factors, mainly including resin type, binder content, raw sand characteristics, and casting process conditions. Different types of resins have varying properties such as thermal decomposition temperature, residual carbon rate, and bonding strength, which leads to differences in collapsibility. Excessive binder content can result in overly high sand mold strength and poor collapsibility; while insufficient binder content may cause insufficient sand mold strength, failing to meet casting requirements. The characteristics of raw sand, such as particle size, shape, surface condition, and mineral composition, also affect the collapsibility of resin sand. Additionally, process conditions during casting, such as mixing method, compaction degree, baking temperature and time, also have an impact on the collapsibility of resin sand.

 

In production, a comprehensive analysis and optimization adjustment should be made based on specific circumstances to achieve the best collapsibility effect. For instance, it can be improved by choosing the appropriate type of resin, adjusting the binder content, optimizing the selection of original sand, and enhancing the casting process conditions.

 

Vigor team have rich experience on different castings. If any question or any demands we can help, please feel free to contact us at info@castings-forging.com