Coated sand generally consists of aggregates, binders, curing agents, lubricants, and additives.
Aggregate is the main material of coated sand. The requirements for aggregate (original sand) are: high refractoriness, less volatile matter, relatively round particles, high compressive and impact resistance, generally scrubbed silica sand, which is mainly due to Its reserves are abundant and the price is cheap. Under normal circumstances, it can meet the requirements of customers. Only special sands such as zircon sand, chromite ore, or artificial sands such as Baozhu sand can be used only when there are special requirements for steel castings and cast irons. The general requirements for silica sand are:
(1) SiO2 content: >90% for cast iron and colored alloy parts, >97% for cast steel parts;
(2) Mud content: <0.2%;
(3) Particle size distribution: 3-5 sieve distribution should be adopted;
(4) AFS fineness: different fineness should be selected according to the casting size and surface roughness requirements, generally AFS50-65;
(5) Grain shape: Raw sand with a relatively round shape should be used as much as possible, and the angular factor is <1.3;
(6) Neutral material, pH value about 7;
(7) Silica sand needs to be scrubbed with water. If there are special requirements, it needs acid washing or high temperature activation treatment (900 °C roasting)
Since the new century, many artificial foundry sands have been rapidly used in the coated sand industry. Among them, the comprehensive performance is better, and the application is common.
Baozhu Sand is a new type of foundry sand developed by Luoyang Kailin Casting Co., Ltd. in the late 1990s. Its main chemical composition is Al2O3 and SiO2. The main mineral phases are mullite and corundum.
The orb sand has a round shape and a small amount of resin. Compared with the same size of silica sand, the amount of resin is reduced by about 40% under the same strength, the fluidity is good and the gas permeability is good;
Orb sand is neutral and has high refractoriness. It is suitable for casting of cast iron and cast steel (carbon steel, medium and low alloy steel, stainless steel, chrome steel, manganese steel) and non-ferrous alloys;
Baozhu sand particles have high hardness and strength, low breaking rate, high recycling rate and less old sand emissions;
Baozhu sand has small thermal expansion, which can significantly reduce the tendency of casting veins;
As artificial sand, Baozhu sand has a wide particle size distribution and is suitable for various casting processes and casting process requirements. When using fine sand, it still has high gas permeability, which is beneficial to improve the surface quality of castings.
When the orb sand coated sand is used in cast iron and non-ferrous alloy parts, the Al2O3 content is required to be not less than 70%. When used in steel castings, especially stainless steel, Al2O3 is not less than 75%.
Regarding the particle size of the Baozhu sand coated sand, in general, like the silica sand, the AFS 50-65 raw sand, 4 mesh or 5 mesh is used. However, due to the advantages of Baozhu sand, in recent years, 100/200, 140/200 and other finer bead sands have been used successfully on small steel castings, especially the surface quality of castings is close to precision castings. The table below shows the performance of 50/100 Azurite sand coated sand of Kailin Casting Co., Ltd., which reflects the higher specific strength of the resin and the excellent high temperature performance.
2, the binder
Phenolic resins are commonly used as binders, and phenolic binders are solid and liquid, thermosetting and thermoplastic. Commonly used are solid thermoplastic phenolic resins which are converted to thermoset by the action of a catalyst. The performance requirements are:
(1) polymerization rate: 25s-27s (plate method);
(2) Softening point: 95-102 ° C (ring and ball method);
(3) Fluidity: 60-110mm (slanting plate method);
(4) Free phenol content: <4% (bromination method);
The properties of the binder have a great influence on the quality of the coated sand. Casting workers have been studying how to improve the performance of phenolic resins and find alternatives to phenolic resins. Foreign countries have developed special resins with different properties, such as high-strength low-gas resins, easy-to-break resins, and reports on modified polyester resins, but they have not been widely promoted. In recent years, research and development of phenolic resin for coated sand has progressed rapidly in recent years, and different modified phenolic resins have been developed for use.
3, curing agent, lubricant, additives
The curing agent is usually urotropine, the scientific name of hexamethylenetetramine, and the molecular formula is (CH2)6N4, which is required to be a first-grade industrial product. Lubricating oil generally uses calcium stearate, which is used to prevent the agglomeration of the coated sand, increase the fluidity of the coated sand, prevent the type and core surface from being dense, and improve the mold release property. The role of various additives is mainly to improve the performance of coated sand. Currently, the additives used mainly include: high temperature resistant additives (such as carbon materials or other inert materials), and easily broken additives (such as manganese dioxide, potassium permanganate, heavy chromium). Potassium acid, etc., reinforced toughening additives (such as ultra-short glass fiber materials, organosilane KH-550, etc.), in addition to anti-adhesive additives, aging additives, veining additives.