Molding materials play an important role in casting production. According to statistics, one ton of new sand is required for each ton of steel castings, and the consumption of molding sand is very large. In the process of box casting, the proportion of castings scrapped accounts for 30.50% of the total waste products due to improper use of casting materials or low quality of casting materials. In China, resin sand and sodium silicate sand are generally used as casting materials for box castings.
1. Characteristics of resin sand
Resin sand is made of furan resin and catalyst. In this process, the sand mold can solidify itself at room temperature without baking and hardening gas treatment.
1) Advantages of resin sand:
(1) The molding sand has good fluidity and simple molding operation. There is no need for molding and baking, so as to reduce the deformation caused by molding and baking.
(2) High surface precision and high strength of box casting mold, which can reduce the damage of mold in the process of lower core and mold matching.
(3) The box casting mold has high hardness, which can effectively resist the concession of the mold wall and reduce the thermal shock deformation.
(4) Molding sand has good collapsibility, easy sand falling and easy recovery of old sand.
2) Disadvantages of resin sand mold:
(1) The requirements for the quality of raw sand (particle shape, silica content, micro powder content, etc.) are high.
(2) Compared with the box type casting mold with inorganic binder, it has large air generation and is easy to produce pore defects.
2. Characteristics of sodium silicate sand
Water glass is a general term for all kinds of polysilicate brine solutions. Sodium (Na20 · msi02) water glass is commonly used in box casting, followed by potassium (K20 · msi02) water glass, lithium (li20 · msi02) water glass, potassium sodium (MK20 · Na20 · msi02) water glass, etc. Among them, sodium silicate is the most widely used.
1) Advantages of sodium silicate sand:
(1) Fast hardening, no drying, shorten the production cycle, reduce the process and improve the labor efficiency.
(2) It is convenient to simplify the production process and obtain high mechanical strength.
(3) The raw material cost of sodium silicate sand is low and the concession is good. It is suitable for thin-wall castings. The unique softening phenomenon of sodium silicate sand mold between 600-800 ℃ can prevent the hot crack defect of box castings.
(4) The precision and contour of water glass precision casting are relatively good, the amount of sand is small, and the complex box castings are easy to form.
(5) Sodium silicate sand has little environmental pollution. Sodium silicate is non-toxic and tasteless. It is recognized as an environment-friendly adhesive at present.
2) Disadvantages of sodium silicate sand:
(1) The collapsibility is not good, the sand falling is difficult, and the recovery and regeneration of old sand is difficult. Similarly, the dimensional accuracy is not as high as that of resin sand. It is generally used for cast steel.
(2) Although the sodium silicate sand mold has high strength, the thermal insulation effect after pouring is poor, which will lead to insufficient feeding capacity of the riser in the same box casting process, resulting in shrinkage.
To sum up, for box castings, the main defects of box castings are invasive pores, shrinkage porosity and cracks. Especially for thin-walled box castings, the crack, shrinkage cavity and porosity defects of castings are mainly considered. Therefore, sodium silicate sand casting mold is recommended.