Several Problems in the Foundry Technology of Sodium Silicate Sand

Release time: 2020-06-22 11:19:00.000


The application practice of resin bonded sand casting process shows that the resin bonded sand has many advantages such as high dimensional precision, smooth surface, high molding efficiency, capable of producing castings with complex shape and strict internal quality, easy reclamation and so on. However, the production cost of resin sand is high, and the environmental pollution is serious. Under the increasingly stringent requirements of people's living conditions and environment, the application of resin sand is limited because of the great investment in labor protection and production environmental sanitation. But water glass is colorless and odorless, and there is no gas leakage in the process of mixed sand modeling, hardening and casting. Therefore, in recent years, many places pay attention to sodium silicate sand again.
The hardening method of sodium silicate sand can be divided into hot hardening method, air hardening method and self hardening method, including many methods. However, at present, the commonly used hardening methods mainly include the following two kinds:
1. Ordinary CO2 gas hardening method
This method is a kind of earlier forming technology in the field of sodium silicate binder. Because of simple equipment, easy operation, flexible use and low cost, it has been widely used in the production of most steel castings.
The main advantages of CO2 gas hardening water glass sand are: high hardening speed, high strength; After hardening die, casting precision is high.
The disadvantages of common CO2 hardening sodium silicate sand are: mold (core) sand strength is low, the amount of sodium silicate (by mass) often reaches 7-8% or more; Water content is high and easy to absorb moisture; The hardness and permeability are poor in winter; Because of the poor collapsibility, the used sand is difficult to be recycled and a large number of used sand is abandoned, which causes alkaline pollution to the environment.

2. Organic ester self-hardening method
In this process, liquid organic ester is used as the hardener of sodium silicate instead of CO2.
The advantages of this hardening process are: mold (core) sand has high strength, the amount of sodium silicate can be reduced to less than 3.5%; The hardening speed can be adjusted by changing the kinds of binder and curing agent (5 ~ 150 min) in accordance with the production and environmental conditions; The mould (core) sand has good collapsibility, the casting sand is easy to clean up, the used sand is easy to be recycled by dry method, and the recycling rate is more than 80%, the environmental pollution of sodium silicate alkaline waste sand is reduced, the transportation and land occupation cost of waste sand are saved, and the silica sand resources are saved;The molding sand has good hot plasticity and low gas evolution, and can overcome the defects of cracks and blowholes which are easy to occur in the production of steel castings by furan resin sand. The technological problems of CO 2 sodium silicate sand, such as poor stability of sand mould surface and easy overblow, can be overcome, and the quality and dimensional accuracy of castings can be compared with those of resin sand. The production cost of the no-bake sand process used is relatively low and the working conditions are good.
The main disadvantages of the hardening process are slow hardening speed and poor fluidity of core sand.
At present, in that production of cast sodium silicate, the composite harden technology is sometimes adopted, for example, the mould is firstly lifted aft CO2 is blown for a short time to reach the lifting strength, and then hot air is blown or dried


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