Q.27 Explain the desirable characteristics of core sand. What are the normal binders used in core sand ? Types of cores used in foundry.
Ans. Core Sand : The core and mainly consists of silica sand and an organic binder, with very little, if any, clay content. The presence of clay in core sand reduced its permeability and collapsibility. The core sand may contain small percentages of other constituents also, to enhance its properties. The common organic binders used in core sands include core oils, dextrin, cereals, sulphite liquor, molasses, resins and other commercial binding materials. The main ingredients of core oils are vegetable oils for example linseed oil and corn oil. To these may be added; Fish oil, petroleum oils and coal tar. Core oils impart sufficient strength and good collapsibility. In the CO2 method of core making, water glass (sodium silicate) is used as the binder. The resin binders are discussed ahead.
Core binder : Since pure sand used for preparing core has not natural bond, binder is used. A core binder is an agent added to sand to produce a good 'green' bond and also hardness after baking. A typical natural binder is linseed oil, a typical synthetic binder is a resin of the thermosetting type.
Generally core oils (composed of 50 to 60% linseed oil, 25% resin and balance mineral oil) are used as they are very economical to produce better cores.
In the case of thermoplastic binders (resin and pitch), the powered binder is mixed with core sand and n heating the binder liquefies and coats the sand grains. On cooling the dispersed liquid binds the sand grains together to form a united mass.
Use of synthetic resins is limited as they are costly. Addition of resin to core oil speeds up drying of core and reduces the volume of core gas. Pitch compounded with dextrin and steam coal is used for large cores. Phenol and due to their high strength, low gas formation, collapsibility and resistance to moisture absorption.
Molasses adds hardness to the core but lacks in strength. On the core, molasses water may also be sprayed. Protein binders (gelatine, glue, caserium etc.) are used where collapsibility is the main criterion.
Types of Cores : The cores are broadly classified into the following two classes : (i) According to the material used for cores.
The cores, according to the material used, are classified as follows :
(a) Green-sand cores : The cores made with a sand in moist state are called green sand cores. These cores have relatively low strength.
(b) Dry-sand cores : The cores made with a sand and special binders which develop strength when backed, are called dry sand cores.
(ii) According to the position of the cores.
The cores, according to their position, are classified as follows :
(a) Horizontal Cores : These cores are place horizontally in core prints at the parting line of the mould as shown in figure. This type of core is often used on a split or two-piece pattern.
(b) Vertical Cores : These cores are placed vertically in the mould as shown in figure. The upper end of the core requires a considerable taper so as not to tear the sand in the cope when the flask is assembled.
(c) Balanced Cores : These are similar to horizontal cores, but these cores are supported at one end only, as shown figure. Therefore, the core print should be made of sufficient length in order to prevent the core from falling nto the mould.
(d) Drop Cores : The drop cores, (also known as wing core of stop off core), as shown in figure are used when a hole or cavity to be cored is not in line with the parting surface i.e. it is either above or below the parting line.
(e) Cover Cores or hanging cores : The cover cores, as shown in figure are used when the entire pattern is rammed in the drag and the core is required to be supported from top of the mould. This type of core usually requires a hole through the upper part to permit the metal to reach the mould.
If the core bangs from the cope and does not have any support at the bottom in the drag, then it is called hanging core.