Binders And Additives In Casting

Casting involves the use of various materials at several stages throughout the process. Two such materials which are crucial for enhancing the strength of the casts are binders and additives. Since sand casting is the predominantly adopted technique, foundries in India principally use sand binders and sand additives for moulds and cores.

Binders:

A good binder is one which is effective even when added in low quantities, has good reusability, quick speed of curing, is both environment and pocket friendly. Iron foundries in India use organic and inorganic sand binder systems, the former being exclusively synthetic resins while the latter is based mostly on sodium silicates.

Organic binders have various types, all of which are either heat curing or no-bake. Based on whether they are modified with silicate or not, inorganic binders may be self-setting or gassing/gas setting. Self-set inorganic binders are silicate modified, whereas gassing inorganic binders may be silicate modified or unmodified.

In the Indian market, binder systems marked with deep brown colour are chiefly in use, but the usage of light brown binders are steadily rising as they offer a better working environment in foundries. Commonly used sand binder systems include Furan No-Bake (FNB), Modern FNB, Alkaline Phenolic No-Bake (APNB) and Phenolic Urethane Cold Box (PUCB) / Amine Cured Cold Box among others. Cast iron foundries always prefer FNB.

Additives:

Additives, when added to the components of molding, improve the surface finish, refractoriness and cushioning of the castings as well as its dry strength. But, research has revealed that on increasing the volume fraction of additives in mechanical and microstructures of cast iron, the tensile strength and hardness increases while the resistance to shock loads is reduced. Hence, adding the right quantity of additives is highly imperative in ensuring good quality.

Iron Oxides were among the first sand additives and are the most preferred till date. Based on the types and usage they are classified into -red iron oxides which are most frequently used, followed by black iron oxides. Alternatively, there are organic additives like starch, wood flour and dextrin, which also serve the same purpose. However, at present, Engineered Sand Additives (ESAs) are under development to overcome the deficiencies of these existing additives.

Most foundries in India like Grey iron foundries or SG iron foundries have a major concern in terms of maintaining the capacity and quality of the casting process while remaining sustainable and environment friendly. Selecting the right additives and binders is one of the major decisions that act as the first step to resolving this dilemma.