Influence of resin sand raw material properties on the strength of fused cast AZS block sand pattern

I. Basic characteristics of resin sand

Resin sand is mainly composed of resin, curing agent, raw sand, and other auxiliary materials, of which resin and curing agent are the binder, and raw sand is the main skeleton of the sand type. The main characteristics of resin sand include:

♦Strong bonding: The resin component in resin sand makes it highly bondable, which can effectively bond the sand particles and form a strong sand mold system. This bonding ensures the stability of the mold and prevents castings from producing defects during the casting process.

♦Fast drying: With the addition of a hardener, resin sand reaches the desired strength quickly, increasing productivity. This characteristic gives resin sand a significant advantage in fast-paced production environments.

♦High strength: Resin sand has high strength, which can withstand all kinds of stresses and impacts during the casting process, preventing mold breakage or deformation, and guaranteeing the precision and quality of castings.

♦Good dispersibility: after the casting is cooled and solidified, the resin sand can be easily dispersed, which makes it easy to take out the casting, simplifies the cleaning work, and improves the production efficiency.

                    

II. Influence of raw material characteristics of resin sand on sand strength

Raw material characteristics of resin sand, including raw sand grain size, grain size distribution, mud content, water content, chemical composition, etc., have an important impact on the strength of the sand type.

1. Raw sand grain size

♦ Fine sand: The larger specific surface area of fine sand means that the resin film on the surface of fine sand is relatively thin for the same amount of resin and hardener added. This may result in insufficient bonding strength between the sand grains, affecting the overall strength of the sand pattern.

Although fine sand has a large surface area, the number of fine sand particles per unit mass is high, and there are many contact points (i.e., bonding bridges) between sand particles. However, these bond bridges may not be strong enough due to the thin resin film.

♦ Rough sand: The surface area of rough sand is small, and the resin film on the surface of rough sand is relatively thick for the same amount of resin and hardener added. This helps to increase the bonding strength between the sand grains, thus improving the overall strength of the sand pattern.

The number of particles per unit mass of rough sand is small, and the contact points between sand particles are relatively few. However, as mentioned earlier, these fewer bond bridges may be stronger due to the thicker resin film, thus increasing the strength of the sand pattern to some extent.

It was found through experiments that for 40-70 mesh raw sand, when the particle size is within this range, the influence of bond bridge and surface area play a comparable role, and there is no significant change in the strength of resin sand. This result indicates that when preparing fused cast AZS block sand patterns, choosing raw sand with a particle size distribution within this range can result in better sand strength.

2. Grain size distribution

Grain size distribution also has an important effect on the strength of resin sand. Under the condition of a certain amount of binder addition, if the original sand is equipped with a certain amount of fine sand, the fine sand can fill in the gaps of the tightly arranged rough sand and increase the number of bond bridges, thus improving the strength of the sand type. Although the larger specific surface area of the fine sand will reduce the thickness of the bonding film of the sand, the combined effect will still lead to an increase in the strength of the sand.

In the case of resin sands, the amount of binder is very small, so the effect of increasing the number of bond bridges is prominent. Therefore, when preparing fused cast AZS block sand patterns, raw sand of a certain grain size should be used so that it can be better aligned and will not have large gaps, which will result in a sand pattern with high strength.

            

3. Mud content

Mud content refers to the original sand particle size that is much smaller than the sand particles, and gives the surface of the sand particles or mixed in between the sand particles of various trace particles. Mud content directly affects the cost of recycled sand and casting quality. In casting production, mud content is too high not only affects the working environment, and pollution of the air, more importantly, the impact of recycled sand micro-powder content, increasing the amount of resin added to the mixing of sand and castings due to poor permeability caused by an increase in the rate of waste products.

In the case of a certain amount of resin and hardener, the higher the mud content, the lower the strength value of the sand type. This is because the mud particles will dilute the resin and curing agent, so that their concentration decreases, thus prolonging the curing time and reducing the strength of the sand. Therefore, in the preparation of fused cast AZS block sand type, the mud content of the raw sand should be strictly controlled to ensure the strength of the sand type.

4. Water content

The water content in the original sand seriously affects the curing strength of the resin and solid permeability. High water content will dilute the resin and curing agent, so that its concentration decreases, thereby extending the curing time and reducing the
strength of the sand. In addition, moisture will also form air bubbles in the sand, affecting the denseness and strength of the sand.

Therefore, in the preparation of fused cast AZS block sand, the original sand should be dried and its water content should be strictly controlled to ensure the strength and quality of the sand.
                                                        

5. Chemical composition

The chemical composition of the raw sand also has an effect on the strength of resin sand. If the quartz content in the raw sand is high, the refractoriness and reusability of the raw sand are good. This is because quartz has a high melting point and high hardness, which can improve the refractoriness and strength of the sand. Silicate minerals such as feldspar and mica, on the other hand, have a low melting point and hardness, which reduces the reusability and refractoriness of resin sand.

When selecting silica sand, the SiO2 content should be increased as much as possible and the impurity content should be reduced. In addition, the alkaline substance in the raw sand will consume extra acid curing agents and affect the hardness of resin sand. Therefore, the raw sand should be chemically analyzed and its chemical composition should be strictly controlled to ensure the strength and quality of the sand.

III. The resin sand preparation process on the strength of the sand type influence

In addition to the characteristics of raw materials, the preparation process of resin sand also has an important impact on the strength of the sand. The preparation process includes sand mixing, molding, curing, and other steps.

1. Sand mixing

Sand mixing is the process of mixing resin, hardener, raw sand, and other raw materials in a certain proportion. The quality of sand mixing directly affects the strength and quality of sand. In the process of sand mixing, the amount of resin and hardener should be strictly controlled to ensure the strength and hardening speed of the sand.

              

The amount of resin added is generally 0.9% to 1% of the original sand, and the amount of curing agent added is directly related to the total acid content of the curing agent, the ambient temperature, and the temperature of the sand, generally 30% to 65% of the amount of resin added. In the case of high external temperature, the amount of curing agent should be appropriately reduced, in order to prevent the hardening reaction is too fast, resulting in the resin film and binder bridge becoming brittle, reducing the strength of the sand type.

2. Moulding

Molding is the process of filling the mixed resin sand into the sandbox or mold to form the required shape of the sand mold. In the process of molding, attention should be paid to controlling the firmness and density of the sand pattern to ensure the strength and stability of the sand pattern. Too little compactness will lead to loose sand and strength reduction; too much compactness will make the sand too dense, increasing the risk of defects in castings.

              

3. Curing

Curing is the process of leaving the molded sand at a certain temperature for a certain period so that the resin and curing agent react chemically to form a strong sand. The curing temperature and time have an important effect on the strength of the mold. Curing temperature is too high or too long will lead to excessive cross-linking of resin, so that the sand becomes brittle; curing temperature is too low or too short a period will make the resin curing incomplete, reducing the strength of sand.

Therefore, during the curing process, the curing temperature and time should be strictly controlled to ensure the strength and quality of the sand mold. Generally speaking, the curing temperature is controlled between room temperature and 60℃, and the curing time depends on the size and thickness of the sand mold, which is generally from several hours to tens of hours.

IV. Conclusion