Coated Sand. Molding sand or core sand with a solid resin film applied to the surface of the sand grains before shaping. There are two types of coating processes: cold and hot. In the cold process, the resin is dissolved in ethanol, and melamine is added during the mixing process to coat both on the sand grains. As the ethanol evaporates, coated sand is obtained. In the hot process, the sand is preheated to a certain temperature, the resin is added to melt it, and it is stirred to coat the sand grains. Melamine aqueous solution and lubricants are then added, and the coated sand is cooled, crushed, and sifted. Used for casting steel and cast iron parts.

Coated Sand; Resin-Coated Sand; Coating Process: Cold and Hot Methods; Main Raw Material: Natural Quartz Sand

Fused sand was invented by Dr. Klonning of Germany during World War II. The process involves mechanically mixing powdered thermosetting phenolic resin with sand and solidifying it upon heating. It later evolved into a coated sand product by adding thermoplastic phenolic resin, latent hardener (urotropine), and lubricant through a specific coating process. When heated, the resin on the sand particles melts, and under the action of the methyl groups released from the decomposition of urotropine, the molten resin rapidly transforms from a linear structure to an irreversible, solid structure, thereby solidifying and shaping the coated sand.

China's coated sand technology originated in the mid-1950s and was temporarily discontinued in the 1960s. By the 1980s, with the rapid development of the automotive industry and the need for mechanical product exports, higher quality requirements for castings were put forth, which in turn accelerated the rapid development of coated sand production and application technologies. In 1986, the Jinan Foundry and Forging Technology Research Institute was the first to develop a new coated sand production technology, reducing the resin addition rate from 6-10% in the early 1980s to 3-4%, approaching the international advanced level of the time. In the early 1990s, coated sand factories sprang up across the country, with new products and technologies emerging continuously. By the 21st century, China's coated sand industry had achieved an annual sales volume of over 900,000 tons, widely used in the production of automotive cylinder blocks, cylinder heads, intake manifolds, various pipe fittings, pump bodies, and other complex castings.

Primary Raw Materials Editor: Reporting

Coated Sand Raw Material

Coated Sand Raw Material

Coated sand primarily uses selected natural quartz sand as the raw material, along with thermoplastic phenolic resin, melamine, and reinforcing agents. The ratio is adjusted appropriately based on the varying technical requirements of customers, including curing speed, stripping properties, fluidity, breakability, surface finish of castings, and storage. It is one of the molding materials used in automotive, tractor, and hydraulic components.

Key Features Editor: Broadcast

1. Vibration sand cleaning, high strength, low gas emission, low expansion.

2. Good dispersion, high surface finish of castings.

3. Shell type without peeling, good thermal stability, excellent thermal conductivity, and good fluidity; casting surface is smooth.

4. High strength, high-temperature resistance, low expansion, good demolding properties, and excellent anti-adhesion to sand.

5. Uniform shell layer, no peeling, and fast curing speed.

Raw and Auxiliary Materials Editor Reporting

Coated Sand

Coated Sand

1. Raw sand is the main component of coated sand, with the requirements being: high refractoriness, low volatile content, and high particle strength. Generally, natural silicon sand is chosen due to its abundant reserves, low cost, and ability to meet casting needs. Only castings with special requirements use special sand (such as zircon sand, chromite, ceramic sand, magnetite sand, etc.) as the raw material for coated sand:

General requirements for silica sand are:

a. SIO2: The requirement for sand used in cast iron and colored casting is above 90%, and for steel castings, it is above 98%.

b. Acidic sand is superior to alkaline sand.

c. Silt content <0.3%.

d. Particle size distribution: Preferably use 3-sieve concentration and 5-sieve dispersion.

e. Granularity: Preferably use roundness good silica sand, the angular coefficient should generally be less than 1.3.

Typically, silica sand can meet requirements after secondary washing, but for improved performance of coated sand, it can be acid-washed or subjected to high-temperature activation treatment (calcined at 900-1200°C).

2. Adhesive

Raw Materials

Raw Materials

Typically, thermoplastic phenolic resin is used as the binder. The performance requirements for the thermoplastic phenolic resin are:

a. Aggregate Speed (Hot Press Method): 25-75s

b. Softening Point (G环球法): 90-105℃

c. Liquidity (Slant Plate Method): 60-110mm

d. Free Phenol Content (Bromination Method): ≤4%

The performance of bonding agents significantly impacts the quality of coated sand. Efforts have been ongoing to enhance the properties of phenolic resins, with various high-performance resins developed abroad, such as high-strength low-gassing resins and easy-dispersing resins. There have also been reports of using modified polyester resins as alternatives to phenolic resins, but this has not gained widespread adoption, and modified phenolic resins are still predominantly used. In recent years, China has made rapid progress in the research and development of coated sand phenolic resins, with companies like Shandong Chemical Plant, Qingdao Chemical Plant, and Jinan Shengquan Co., Ltd. developing high-strength phenolic resins, rapid-polymerizing phenolic resins, and easy-dispersing phenolic resins, among others. However, there is still a gap compared to developed countries.

3. Binder, lubricant, additive

The conventional curing agent for coated sand is melamine, with an addition of 5%-20% of the resin. To meet some special requirements (such as anti-peeling, rapid curing, etc.), some companies have developed special curing agents.

The function of the lubricant is to prevent caking and enhance the fluidity and demolding properties of the coating sand, mainly including calcium stearate, ammonium stearate, etc. It is recommended to choose products with high melting points for the lubricant to prevent the coating sand from shedding during use.

The primary function of additives is to enhance the performance of coated sand. Commonly used additives include high-temperature-resistant additives, easy-dispersible additives, reinforcing and toughening additives, anti-stick sand additives, and wet-state additives, among others. Many coated sand factories have unique additive formulas. The performance of additives plays a crucial role in the production of special coated sand and is one of the core competitive advantages in the coated sand market.

The mixing methods for coated sand have evolved through four stages: dry mixing, cold coating, warm coating, and hot coating methods. The production of coated sand predominantly employs the hot coating method.

The hot-molding film process involves first heating the raw sand to a certain temperature, then mixing it separately with resin, a solution of Urotropine, and calcium stearate, followed by cooling, crushing, and sieving. Due to variations in the formula, the mixing process differs accordingly.

There are many types of domestic film-coated sand production lines, with approximately 2,000 to 2,300 semi-automatic lines that are manually fed, and nearly 50 fully automatic lines controlled by computers, which have significantly improved production efficiency and product stability.