Counter gravity casting has very high requirements for molding, core making technology and sand fluidity.
(1) Sufficient strength and rigidity: the core must have enough strength and rigidity to avoid deformation or fracture in the process of forming and pouring;
(2) Low gas generation: because the slender sand core is completely surrounded by aluminum liquid after pouring, and the heating state is bad, the volatile substances in the core are fully volatilized, and the volatile gases are difficult to be discharged, resulting in the phenomenon of “choking fire”;
(3) Good sand resistance and surface finish: the surface of the inner cavity of the casting is difficult to clean and machining, the core must have enough sand resistance and good surface finish;
(4) Good collapsibility: good collapsibility of core is helpful to clean up slender core.
At present, furan resin self hardening sand is generally used for molding, and PEPSET series resin self hardening sand is used for core making; while the original sand is generally washed sand with good high temperature stability, with particle size of 40-70 mesh, SiO2 mass fraction greater than 95%, and mud mass fraction less than 2%.
When furan resin self hardening sand is used for molding, furan resin with high strength at room temperature, low viscosity, good filling property and easy to mix is used as binder, and matching acid is used as hardener, which hardens itself at room temperature. In production, two important factors need to be considered in the molding process are the usable time and mold starting time of furan resin self hardening sand. When the sand is mixed, the furan resin and the hardener start to harden once contacted. If it is not used immediately, the hardening strength will decrease with the prolongation of the retention time of resin sand, or even it will be dry and can not be used. The factors that affect the usable time include the amount of resin and hardener, temperature and humidity, among which temperature and hardener are the most important factors; the mold strength generally depends on the size, height, angle and complexity of the sand mold.
PEPSET resin self hardening sand has been widely used in foreign countries. In addition to the advantages of furan resin self hardening sand, such as low energy consumption, accurate size and good fluidity, PEPSET resin self hardening sand also has the characteristics of delayed reaction, rapid overall hardening. It can be poured into the sand core in a short time, with the advantages of high core strength, good collapsibility and small gas evolution, It is especially suitable for the production of aluminum castings with complex structure. The factors that have obvious influence on the usable time of PEPSET resin sand are the amount of catalyst and the ambient temperature of core making. When the amount of catalyst is reduced, the usable time of PEPSET resin sand increases, but the strength of sand sample will be significantly reduced if the amount of catalyst is too low. The environmental temperature of core making has a great influence on the usable time of PEPSET resin sand. The high environmental temperature will make the core sand hardened in advance and shorten the usable time. Therefore, it is necessary to determine the suitable resin ratio and core sand mixing process according to the specific requirements of castings.