Selection of raw materials for coated sand

Coated sand is generally composed of aggregate, binder, curing agent, lubricant and special additives.

1. Aggregate

Aggregate is the main component of coated sand. The requirements for aggregate are: high refractoriness, low volatile matter, relatively round particles and high strength. Natural scrubbed silica sand is generally used, mainly because of its abundant reserves, low price and ability to meet casting requirements. Only steel castings or iron castings with special requirements use zircon sand or chromite sand. The general requirements for silica sand are:

(1) High SO2 content. The requirements for SO2>90% for cast iron and nonferrous casting sand and>97% for steel castings;

(2) Mud content ≤0.2%;

(3) The particle size distribution should adopt 3-5 sieve dispersion;

(4) AFS fineness: Different fineness should be selected according to the surface roughness requirements of the casting, generally AFS50-65;

(5) Particle shape: Use silica sand with good roundness as much as possible, and the angular factor should be <1.3;

(6) pH value <7;

(7) Silica sand needs to be scrubbed with water. If there are special requirements, silica sand can be acid-washed or subjected to high-temperature activation treatment (900℃ roasting)

2. Binder

Currently, phenolic resin is widely used as a binder. Phenolic resins are divided into solid and liquid, thermosetting and thermoplastic. At present, thermoplastic solid phenolic resin is usually used to make coated sand. The performance requirements are:

(1) Polymerization speed (hot plate method): 25-27s;

(2) Softening point (ring and ball method): 90-105℃;

(3) Fluidity (inclined plate method): 60-110mm;

(4) Free phenol content (bromination method): ≤4%.

The performance of the binder has a great influence on the quality of coated sand. People have been committed to studying how to improve the performance of phenolic resin and finding substitutes for phenolic resin. Foreign countries have developed special resins with different properties, such as high-strength low-gassing resins and easy-to-disintegrate resins. There are also reports on the use of modified polyester resins, but they have not been fully promoted. Modified phenolic resins are still widely used. In recent years, China has made rapid progress in the research and development of special phenolic resins for coated sand, and has developed different modified phenolic resins for production use.

3. Curing agent, lubricant, additives

The curing agent usually uses hexamethylenetetramine, which has the molecular formula (CH2) 6N4 and is required to be an industrial grade one product. The lubricant generally uses calcium stearate. Its function is to prevent the coated sand from agglomerating, increase fluidity, make the mold and core surface dense, and improve the demoulding property of the sand mold (core). The main function of the additive is to improve the performance of the coated sand. The additives currently used mainly include: high temperature resistant additives (such as carbonaceous materials or other inert materials), Yiguisan additives (such as manganese dioxide, potassium dichromate, potassium permanganate, caprolactam, etc.), strengthening and toughening additives (such as ultra-short glass fiber materials, organic silane KH-550, etc.), anti-sand additives and anti-aging additives, etc.