1. Why shouldn't paint be applied immediately to newly molded cores and repaired areas?

When molds and cores are newly molded or repaired, the resin curing reaction is still in its early stages: With water-based paints, the water in the paint will interfere with the normal curing of the resin; unreacted polyisocyanates in phenol-urethane self-curing resins will also react with water and become ineffective; with alcohol-based paints, immediate ignition after application can cause unreacted resin to overheat; ultimately, all of these will damage the surface stability of the mold and core.

2. Why does furan self-curing resin containing 70%-80% furfuryl alcohol have the highest final strength at room temperature?

The final strength of furan self-hardening resin at room temperature is affected by the ratio of furfuryl alcohol to urea-formaldehyde resin and the water content: A higher proportion of urea-formaldehyde resin results in a higher final strength at room temperature. When the furfuryl alcohol content exceeds 80%, the nitrogen content decreases, and the final strength at room temperature decreases with increasing furfuryl alcohol content. An increase in the proportion of urea-formaldehyde resin (and a decrease in furfuryl alcohol content) leads to an increase in the resin's water content, thus reducing the curing speed and final strength. When the furfuryl alcohol content is below 70%, the decreasing trend in final strength due to water content exceeds the increasing trend due to the increasing proportion of urea-formaldehyde resin; therefore, the final strength at room temperature reaches its maximum when the furfuryl alcohol content is between 70% and 80%.

3. Why does excessive catalytic activity/excessive addition of curing agent lead to a decrease in the final strength of resin sand?

Excessive catalytic activity or excessive addition of curing agent results in an excessively short pre-curing stage for the resin sand: the irregularly curled, clump-like prepolymers in the resin are not fully extended and ordered, and are blocked in the three-dimensional structure by cross-linked prepolymers. A large number of active groups cannot participate in the reaction, preventing the formation of a high-degree-of-polymer. The final result is rapid hardening and high initial strength, but a significant decrease in final strength.

4. Why is phosphoric acid not suitable as a curing agent for the recycling of self-hardening resin sand?

When phosphoric acid is used as a curing agent, after the sand mold and core are poured, the phosphoric acid will not decompose under the heat of the molten metal. Instead, it will form phosphate deposits on the surface of the sand particles. These phosphates are difficult to remove through regeneration, leading to a significant decrease in the strength of the resin sand prepared from the recycled sand. It also increases the mold's expansion and the probability of sand inclusion defects.

5. Why is it advisable to choose organic acids with low free acid content and high total acid content as curing agents for acid-hardening methyl phenolic resin self-hardening sand, instead of inorganic acids?

Acid-cured methyl phenolic resins have a high water content. During the crosslinking reaction, in addition to the water produced by the resin's own polymerization, a large amount of water is released. This water dilutes the hardener, slowing down the reaction. Simply increasing the free acid content of the hardener can accelerate the curing speed, but it will significantly reduce the strength of the resin sand. Increasing the total acid value can ensure the curing speed while avoiding a significant decrease in strength. Inorganic acids generally have a high free acid content, resulting in poor compatibility. It is advisable to choose organic acids with high total acidity and low free acid content.

6. Why should the amount of hardener added to acid-cured methyl phenolic resin sand be expressed as a percentage of the resin?

The core reason is the inherent properties of the resin and the dilution effect of the cross-linking reaction, requiring the amount of curing agent added to be adjusted synchronously with the amount of resin: the resin is neutralized by acid before leaving the factory and is weakly acidic, making it much less sensitive to acidic curing agents than furan resin, requiring a higher acid concentration to occur for the cross-linking reaction to take place; the resin has a high water content (approximately 15% or higher), and during the cross-linking reaction, in addition to producing water through its own condensation, it also releases a large amount of miscible water, which dilutes the curing agent; the more resin added, the stronger the dilution effect of water on the curing agent, requiring a synchronous increase in the amount of curing agent added to maintain the same curing speed; therefore, the amount of curing agent added should be calculated as a percentage of the resin, and the amount added should increase as the amount of resin added increases.