(1) The guarantee ability of melting equipment to molten iron composition is poor and the stability of sand mixing equipment is poor. The composition of molten iron is restricted by coke, furnace type, air volume, raw material condition and other factors; Resin sand is affected by temperature, resin and acid addition. For example, the sand often does not go through the regeneration and cooling bed, so that the sand temperature is very high, which seriously affects the sand mold strength, leads to serious sand expansion of valve castings, and increases the tendency of shrinkage cavity and shrinkage porosity defects of valve castings.
(2) The loose sand in the mold cavity and the impact of iron liquid in the pouring process directly lead to sand holes and sand inclusion defects.
(3) Molten iron always generates slag in the melting equipment. During pouring, the solid and liquid slag in the molten iron enter the mold cavity together with the molten iron to form slag holes.
(4) In the production process, the nitrogen content in molten iron increases with the increase of temperature and decreases with the increase of carbon equivalent. When nitrogen and hydrogen are together, it is easy to form pores, which is the main source of pores.
(5) The rigidity of the mold base plate is poor. It is placed unevenly and deformed before modeling, resulting in uneven parting surface of the sand mold. When closing the mold, the gap between the upper and lower parting surfaces is large, resulting in poor size and shape at the parting surface.
(6) The sand core at the casting foot of 2.2m valve body drifts downward during pouring, which is the main reason for the uneven wall thickness at the foot.