Furan resin can be divided into urea formaldehyde furan resin, phenol urea formaldehyde furan resin, formaldehyde sugar alcohol resin and high sugar alcohol resin. The price of UF resin is low, because of its high nitrogen content (5% – 11%). Although the strength of resin sand is high, the probability of nitrogen porosity is high, and the surface of the casting is easy to be passively nitrided, resulting in poor surface quality of the casting. Therefore, this resin is often used for some castings with low requirements on the surface of castings and some castings prone to sand sticking and bulging. However, the price of high sugar alcohol resin is higher, because of its low nitrogen content (0.3% – 2%), the probability of nitrogen porosity in the casting is reduced, and it is not easy to make the passive Nitriding on the casting surface, so the surface quality of the resin sand is better. However, considering the cost, it is mainly used for some castings with high surface quality requirements. Although phenolic resin sand does not contain nitrogen and has better high temperature strength, its other properties are worse than the two mentioned above. In the actual production, the resin used is the mixture ratio of sugar alcohol, urea aldehyde and phenol formaldehyde.
According to the nitrogen content, furan resin can be divided into the following four categories: 1. Nitrogen free resin, nitrogen content ≥ 0.3%; 2. Low nitrogen resin, nitrogen content < 0.3% – 2.0%; 3. Medium nitrogen resin, nitrogen content > 2.0% – 5.0%; 4. High nitrogen resin, nitrogen content > 5.0% – 13.5%. However, in recent years, with the increasing market demand for the surface quality of castings, coupled with the decline in the price of low nitrogen resin, low nitrogen resin has gradually replaced high nitrogen resin, so the furan resin mentioned below is low nitrogen resin.
It is a common method to improve the strength of molding sand by adjusting resin content, but the more resin content is not the better. Under the condition of high humidity (850 ℃), the gas generation of resin is more than twice that of common clay sand. Although the current selection of low nitrogen resin gas generation is relatively small, but if the resin content is high, the problem of porosity still has to be considered. When we study the relationship between the strength of resin sand and the thickness of resin film, we also think that when too much resin sand is added, the resin bonding efficiency is very low and the waste cost is very serious.
At the same time, for some castings with uneven wall thickness, resin sand is required to have good collapsibility and yield at high temperature. Although the mold still requires a certain rigidity at high temperature, if the strength of resin sand is too high, the collapsibility at high temperature is poor, and the probability of hot cracking tendency of castings increases greatly.
In addition, due to the use requirements and wall strength requirements, it is necessary to pay attention to the amount of resin. For example, the tensile strength of the sand core in the mold is higher than that of the outer mold due to the use requirements. For the consideration of the requirements of mold wall strength, for some castings with large mold wall pressure, such as some castings with large wall thickness and some castings with high parting surface, the requirements of mold tensile strength are relatively high.
Based on the above factors, there is a certain range for the control of resin blending. We have measured the relationship between the amount of resin added and the strength of sand mold when the temperature is 15’c and the amount of curing agent is 60%.
According to the actual situation, the tensile strength of most casting sand molds is required to be no less than 6kg / cm2, but no more than llkg / cm2 according to the corresponding mold conditions, so the resin content is generally 0.9% – 1.3%, while the actual production must be adjusted according to the corresponding mold conditions.
Although, according to the curing characteristics of resin sand, the tensile strength of resin sand is close to 24 hours after 6 hours
Tensile strength, but in fact most of the casting demoulding time is basically within 3 hours. Therefore, the curing speed of resin sand must be limited. According to the author’s own work experience, if the solidification speed is too fast, the uneven surface strength of the sand mold will lead to poor surface quality of the casting, and the tendency of shrinkage cavity and porosity on the surface of the casting will increase; if the solidification speed is too slow, it may cause rework of the sand mold operation, which will bring great waste.
Through the study of the curing reaction of resin sand, it is known that temperature and curing agent play an important role in improving the reaction speed of resin sand. Therefore, adjusting the sand temperature and the content of curing agent is the main means to control the curing reaction of resin sand.