Key points of foundry technology of coated sand (1)
Resin coated sand casting has a long history in the field of casting, and the output of casting is quite large, but there are many problems when using resin coated sand casting to produce precision steel castings, such as sand burn, cold shut, and air hole. How to solve these problems remains to be explored.
KEYWORDS: coated sand, steel casting, product quality.
Awareness and understanding of coated sand (coated sand belongs to organic binder, core sand)
(1) resin coated sand is characterized by proper strength, good flowability, clear profile of sand mold and sand core, compact structure, being able to make complex sand core, good surface quality of sand mold (core), and up to Ra 6.3-12.5 M in Surface roughness The dimensional accuracy can reach CT7 ~ CT9, collapsibility is good, the casting is easy to clean.
(2) the scope of application: Resin coated sand can be used as both mold and core. Resin coated sand can be used with each other and with other sand molds (cores) ; not only can it be used in metal mold gravity casting or low pressure casting, it can also be used in sand-lined iron mold casting and thermocentrifugal casting, not only in the production of cast iron and non-ferrous alloy castings but also in the production of steel castings.
PREPARATION OF COATED SAND
1.coated sand generally consists of Refractory, binder, curing agent, lubricant and special additives.
- Refractory is the main component of coated sand. The requirements for Refractory are high refractory, less volatile matter, more round particles, solid, etc. General selection of natural silicon sand scrub. The requirements for Silica Sand are as follows: SIO2 content is high (more than 90% for cast iron and non-iron alloy castings and more than 97% for steel castings); The shape of grain should be round, the angle factor should not be larger than 1.3, the acid consumption should not be less than 5 ml.
- Binders are generally made of Phenol formaldehyde resin.
- The hardener is usually Hexamethylenetetramine and the lubricant is Calcium stearate. Its function is to prevent the coated sand from caking and increase its flowability. The main function of additives is to improve the performance of coated sand.
- The basic proportion of resin coated sand Original Sand 100%
Phenol formaldehyde 1.0 ~ 3.0 %, occupies the original Sand Weight
Hexamethylenetetramine (aqueous solution) : 10 ~ 15%, occupies resin weight
Calcium stearate: 5~7 %, occupies resin weight
The additive: 0.1 ~ 0.5%, occupies the original Sand Weight
2.The production technology of coated sand
There are mainly three kinds of coated sand preparation technology: cold coated, warm coated and hot coated. At present, the production of coated sand almost adopts the hot coated method. The thermal film covering process is made by heating sand to a certain temperature, then mixing with resin, Hexamethylenetetramine solution and Calcium stearate respectively, then cooling, crushing and sieving. The mixing technology is different because of the difference of the formula. At present, there are many kinds of resin coated sand production lines in China. There are about 2000 ~ 2300 semi-automatic production lines for manual feeding, and about 50 automatic production lines controlled by the computer. For example, the automatic visual production line of XX Foundry Co. , Ltd. , its feeding time control is accurate to 0.1 second, heating temperature control is accurate to 1 / 10, and can be observed through the video time mixed sand state, production efficiency of 6 tons per hour.
3. The main product types of resin coated sand
- Ordinary coated sand, brief introduction: ordinary coated sand is the traditional coated sand, its composition usually consists of quartz sand, thermoplastic Phenol formaldehyde resin, Hexamethylenetetramine and Calcium stearate, no relevant additives Its Resin content is usually relatively high under certain strength requirements, does not have high temperature, low expansion, low gas characteristics, suitable for the production of low-demand castings.
- High strength and low gassing coated sand, features: High Strength, low expansion, low gassing, slow gassing, anti-oxidation The strength of resin is 30% higher than that of ordinary coated sand, the gas emission rate is significantly reduced, and the gas emission rate can be delayed Can better adapt to the needs of casting production. This kind of coated sand is mainly applicable to the production of middle and small cast steel and alloy steel castings in iron castings. At present, there are three series of this kind of coated sand: gd-1 high-strength low-gassing coated sand; gd-2 high-strength low-expansion low-gassing coated sand; GD-3 high-strength low-expansion low-gassing anti-oxidation coating.
- High temperature resistant coated sand (ND type) features: High temperature resistant, high strength, low expansion, low gas evolution, slow gas evolution, easy breakdown, anti-oxidation Brief introduction: high temperature coated sand is a new type of resin coated sand with excellent high-temperature properties (high strength at high temperature, long heat resistance time, small thermal expansion and low gas evolution) and comprehensive casting properties. This kind of coated sand is especially suitable for the production of complicated and thin-wall precision iron castings (such as automobile engine block, cylinder head, etc.) and high-quality Steel Castings (such as container corner, train brake-slow, middle-device Shell, etc.) It can effectively eliminate sand burning, deformation, hot cracking, and air hole and other casting defects. At present, there are four series of resin coated sand: VND-1 high-temperature resistant resin coated sand. Nd-2 resin coated sand ND-3 resin coated sand ND-3 resin coated sand ND-4 resin coated sand
- Easily collapsible coated sand with good strength and excellent collapsibility at low temperature, suitable for the production of Non-ferrous metal castings.
- In order to meet the needs of different products, a series of specially coated sand has been developed, such as coated sand for centrifugal casting, coated sand for cooling, coated sand for the wet state, coated sand for preventing sticking, and coated sand for preventing vein and Orange Peel. At the same time, the fast curing coated sand introduced in 2004 can increase the curing speed by 30-70 %, effectively improve production efficiency, and save energy.