1. Why is phosphoric acid mostly used as a curing agent for high-nitrogen furan self-hardening resins, and rarely used for low-nitrogen furan self-hardening resins?

Because when low-nitrogen high-furfuryl alcohol resins use acid as a curing agent, the curing speed is slow, the demolding time is long, and the strength is very low. High-nitrogen low-furfuryl alcohol resins, using phosphoric acid as a curing agent, can achieve the necessary curing speed. Moreover, high-nitrogen low-furfuryl alcohol resins, using phosphoric acid as a catalyst, can achieve very good final strength.

This is because phosphoric acid has poor miscibility with furfuryl alcohol and a high affinity for water. This makes it difficult for water contained in the resin and catalyst, as well as water generated during the resin condensation reaction, to diffuse out. Instead, water droplets form around the phosphoric acid core and remain in the resin film, damaging the resin film's density, hence the lower strength. High-nitrogen resins, on the other hand, have good miscibility with water, and various water molecules do not easily concentrate into water droplets around the phosphoric acid core, resulting in a better resin film structure and higher strength.

2. Why is the hardening permeability of phenol-urea self-hardening resin better than that of furan self-hardening resin sand?

Because the curing process of phenolic urea resin is a polymerization reaction, it does not produce small-molecule water during curing. Therefore, there is no issue of inconsistent curing speeds due to different evaporation rates of moisture inside and outside the mold. In contrast, the curing process of furan self-hardening resin is a condensation polymerization reaction, which produces moisture. Due to the different evaporation rates of moisture inside and outside the mold core, the curing speeds differ, resulting in lower hardness and penetration. This is also why the curing speed of furan self-hardening resin sand is significantly affected by relative humidity.

3. Why can high-nitrogen furan resin be used for producing cast aluminum and cast copper parts?

Because molten aluminum and copper are almost insoluble in nitrogen, even when using high-nitrogen resin, the nitrogen produced by resin decomposition during casting will not be absorbed by the molten aluminum and copper, thus preventing nitrogen pores from forming during solidification. High-nitrogen resin can be chosen to meet the requirements of collapsibility in the production of cast aluminum and cast copper parts.

4. Why is it advisable to use ceramic tubes for the gating system when producing heavy castings with self-hardening resin sand?

When using self-hardening resin sand for molding, especially for heavy castings, the long pouring time and prolonged exposure to high-temperature molten metal can cause the resin sand to prematurely collapse and lose strength, leading to sand-washing defects. Therefore, for heavy castings, the gating system should ideally be made of ceramic tubing. This also solves the problem of difficulty in applying coatings to the gating system, particularly the sprue.

5. When using self-hardening resin sand for molding and core making, how do you determine if the usable time of the resin sand meets the requirements?

When using an intermittent sand mixer, the usable time of the resin sand must be longer than the time from when it is mixed and discharged until it is used up. When using a continuous sand mixer for molding and core making, the usable time of the resin sand must be longer than the time from when the sand is first discharged from the mixer's outlet until it returns to that position for continuous sand discharge.

6. Why should the draft angle of the pattern made with self-hardening resin sand be larger than that for clay sand? 

Resin sand cores have already hardened to a certain strength when demolded and have no yielding properties. Generally, the sand mold or core wall cannot be loosened by tapping the pattern and core box frame. The friction is relatively large when demolding. The repairability of resin sand molds and cores is poor. Once the sand mold or core is incomplete when demolding, it is difficult to repair.