Knowledge

Section 1: Strength ​​​​​​​

The strength of resin sand is often classified into room temperature strength and high-temperature strength. Room temperature strength is further divided into initial strength and final strength. Initial strength refers to the strength of the molding sand immediately after mixing, while final strength refers to the strength of the molding sand after 24 hours.

The influence of resin addition on strength

In the production of resin sand, strength increases with the increase of resin addition, but not in a direct proportion. When the resin addition is less than 1.0%, the strength increases rapidly with the increase of resin addition. However, when the resin content is greater than or equal to 1.0%, the growth of strength slows down. When the resin addition is too high, the resin bonding efficiency is very low.

2. The influence of temperature, humidity and curing agent on strength

The curing process of resin sand belongs to condensation reaction, which releases water. The timely discharge of condensation water has a significant impact on the bonding strength of resin sand. High environmental humidity hinders the escape of reaction water, affecting the curing reaction and resulting in low initial strength and reduced final strength of resin sand.

Strength does not always increase with the increase of curing agent dosage. When the dosage of curing agent is 0.4% to 1.2% of the original sand, the strength reaches a maximum value. However, with the increase in temperature and decrease in humidity, the initial and final strength of resin sand decreases when the humidity is high, even if the temperature is similar or increases. Both too fast and too slow curing are detrimental to strength. If curing is too fast, the resin film becomes brittle, reducing strength. If curing is too slow, the bonding force of the resin is not fully exerted. An appropriate curing speed is necessary to ensure the strength of the sand mold. The content of free acid in the curing agent has a significant impact on strength, especially for recycled sand. When the free acid content is greater than 10%, even if the curing time is appropriate, the strength will decrease. Therefore, the free acid content in the curing agent must be less than 10%.

3. The Influence of Resin Properties on Strength

A small amount of free formaldehyde in the resin helps to increase the initial strength of the resin sand and also enhances the final strength. Especially in high humidity conditions, a small amount of free formaldehyde can accelerate the curing reaction, increase the initial strength, and shorten the mold removal time. Increasing the nitrogen content in the resin can improve the initial strength of the resin sand, but the final strength will decrease. Resin with high furfural content and low viscosity has both high initial and final strength. The water content has a significant impact on strength. When the water content in the resin is ≥5%, the curing speed slows down, the mold removal time is prolonged, and the final strength is significantly reduced.

4. The influence of raw sand properties on strength:

The influence of raw sand particle size, shape and micro-powder content on strength: The moisture content and particle size distribution of raw sand have a direct impact on the strength of the sand mold. For resin sand, four-sieve sand is recommended. In new sand, the proportion of 150-mesh sand particles should not exceed 5%, and the micro-powder content should not exceed 0.4%. However, for recycled sand, overly fine sand particles can increase the curing speed, which is different from new sand. Fine sand particles in recycled sand have a larger specific surface area and contain more resin and curing agent than coarse sand particles. The loss on ignition of fine sand is significantly higher than that of coarse sand. When resin is added again, the residual curing agent continues to act as a catalyst, increasing the curing speed.

Resin sand has very strict requirements for the moisture content of raw sand because the moisture in the raw sand not only affects the bonding strength between the resin film and the sand particles but also slows down the hardening speed, resulting in a "crispy outside, tender inside" sand mold and making the casting prone to gas hole defects.

Generally, the moisture content should be less than 0.2%. For recycled sand, if water spraying is used for cooling in summer, the moisture content must be checked and should not exceed 0.3%.

5. The final strength standard should vary from factory to factory. After the resin sand is mixed, it can reach its maximum strength, or final strength, after about 24 hours of self-hardening. Due to the different production conditions and scales of each foundry, the time interval between molding and pouring may not exceed 24 hours. Therefore, the determination of the final strength should be based on the specific foundry.

Section Two: Surface Stability

The surface stability of resin sand refers to its ability to withstand wear during handling, the scouring of molten metal during pouring, and baking without causing defects such as sand flushing, sand holes, and mechanical sand sticking. The surface stability of the sand mold is consistent with the high or low room temperature strength: common influencing factors include:

1. Resin type and addition amount;

2. Quality and particle size of the original sand;

3. Selection of curing agent and additives;

4. Process parameters and operation;

5. Environmental factors: temperature, humidity and other conditions also affect the production of resin sand.

The main factors affecting the gas generation and permeability of resin sand can be analyzed in detail from the following aspects:

Section 3: Gas Generation Capacity

Gas evolution volume refers to the volume of gas in milliliters produced by a sample of molding sand (or other materials) after being heated within a specified time, also known as gas evolution volume. It can be expressed as the volume of gas produced per gram of sample.

During the casting process, when molten metal is poured into the sand mold, the sand mold and sand core are subjected to the thermal effect of the high-temperature metal, causing the moisture and various additives (such as clay, coal powder, and resin, etc.) in the sand mold to generate a large amount of gas. If these gases cannot be discharged in time, they will form gas hole defects in the casting, thereby affecting the quality and performance of the casting.

1. Resin composition and addition amount:

The gas evolution volume is mainly related to the composition and addition amount of the resin. For example, the main components of furan resin are furfural alcohol, urea-formaldehyde and formaldehyde, among which urea-formaldehyde has a relatively large gas evolution volume, while furfural alcohol and formaldehyde have relatively low gas evolution volumes. When the addition amount of resin is large, the gas evolution volume also increases, but not proportionally.

The higher the resin content, the more organic compounds in the sand mold, and the more gases produced by the decomposition of resin at high temperatures. Excessive gases can lead to gas holes or abnormal microstructure defects on the surface of the casting.

2. Raw sand factors:

The loss on ignition of raw sand, that is, the amount of impurities such as organic matter and carbonates in silica sand that are burned off at high temperatures, will increase the gas evolution of the molding sand and be detrimental to the surface quality of the casting.

When the particle size of the raw sand is too fine, the permeability decreases, which also affects the release of gas evolution.

3. Sand Mixing Process and Reclamation of Used Sand:

Excessive addition of resin during sand mixing can lead to a large amount of gas evolution from the sand core during casting. Poor reclamation of used sand can cause the loss on ignition of sand in the system to be out of control, with excessive fine powder content, resulting in increased gas evolution and reduced permeability.

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